One Dimensional Coordination Polymers Based on
Bridging N,N Donor Ligands
Der Naturwissenschaftlichen Fakultät
der Friedrich-Alexander-Universität Erlangen-Nürnberg
zur
Erlangung des Doktorgrades Dr. rer. nat.
vorgelegt von
Nico Fritsch aus Forchheim
Als Dissertation genehmigt von der Naturwissen-
schaftlichen Fakultät der Friedrich-Alexander-Universität
Erlangen-Nürnberg
Tag der mündlichen Prüfung: 13. Juni 2014
Vorsitzender des Promotionsorgans: Prof. Dr. Johannes Barth
Gutachter: Prof. Dr. Nicolai Burzlaff
Prof. Dr. Julien Bachmann
Die vorliegende Arbeit entstand in der Zeit von Juni 2011 bis März 2014 im Department für
Chemie und Pharmazie (Lehrstuhl für Anorganische und Metallorganische Chemie) der
Friedrich-Alexander-Universität Erlangen-Nürnberg unter der Anleitung von Prof. Dr.
Nicolai Burzlaff.
Teile dieser Dissertation wurden bereits veröffentlicht:
“trans-1,2-Bis(N-methylimidazol-2-yl)ethylene: Towards Building Blocks of 2D Fabrics and
MML-Type 2D Molecular Strands”, N.V. Fischer, M. S. Alam, I. Jum´h, M. Stocker, N.
Fritsch, V. Dremov, F.W. Heinemann, N. Burzlaff, P. Müller, Chem. – Eur. J., 2011, 17,
9293-9297.
“Battlement-shaped 1D coordination polymer based on bis(N-methylimidazole-2yl)butadiyne
ligand”, T. Waidmann, N. Fritsch, J. Tucher, M. Rudolf, F. Glaser, D.M. Guldi, N. Burzlaff,
CrystEngComm, 2013, 10157-10160. – Reproduced by permission of The Royal Society of
Chemistry.
You can´t always get what you want.
But if you try sometimes you might find,
You get what you need. (Sir Mick Jagger)
Table of contents
I
1! GENERAL INTRODUCTION ............................................................................................ 1!1.1! From metal organic frameworks to 1D coordination polymers ........................................... 2!1.2! Different concepts of molecular wires .................................................................................... 5!
1.2.1! Organic based molecular wires ........................................................................................... 5!1.2.2! Inorganic based molecular wires ......................................................................................... 9!
1.3! Synthesis of trans-bis(N-methylimidazol-2-yl)ethylene (trans-bie) ..................................... 20!1.4! Synthesis of the first 1D coordination polymer with trans-bie ........................................... 21!1.5! Scanning tunneling microscopy ............................................................................................. 25!
1.5.1! Theory ............................................................................................................................... 25!1.5.2! Function and surface property ........................................................................................... 27!
2! AIMS AND OBJECTIVES ................................................................................................ 31!
3! RESULTS AND DISCUSSION ......................................................................................... 33!3.1! From single to quadruple bond metal(II) acetate complexes with trans-bie ..................... 34!
3.1.1! Preparation and Characterisation of [Rh2(OAc)4(trans-bie)]n (4) ...................................... 34!3.1.2! Preparation and Characterization of [Ru2(OAc)4(trans-bie)]n (5) ..................................... 38!3.1.3! Preparation and Characterization of [Mo2(OAc)4(trans-bie)]n (6) ..................................... 41!3.1.4! Preparation and Characterization of [Cr2(OAc)4(trans-bie)]n (7) and
[Cr2(OAc)4(trans-bie)2] (8). ............................................................................................... 43!3.1.5! Preparation and Characterisation of [Zn3(OAc)6(trans-bie)]n (9) ..................................... 47!3.1.6! Electronic structure and bond orders of binuclear units within polymers ......................... 50!3.1.7! Comparison of the metal(II) acetate compounds .............................................................. 56!
3.2! Battlement shaped 1D coordination polymer based on bmib ............................................. 58!3.2.1! Synthesis and Characterization of bmib (3) ...................................................................... 58!3.2.2! Preparation and Characterization of [Zn5(OAc)10(bmib)2]n (10) ....................................... 64!
3.3! Sawhorse-type diruthenium tetracarbonyl polymers with N,N ligands ............................ 68!3.3.1! Preparation and Characterisation of [Ru2(OAc)2(CO)4(L)]n .............................................. 68!3.3.2! Comparison of sawhorse-type compounds ....................................................................... 73!3.3.3! STM images on HOPG ..................................................................................................... 75!
4! SUMMARY ....................................................................................................................... 79!
5! ZUSAMMENFASSUNG ................................................................................................... 84!
6! EXPERIMENTAL SECTION ............................................................................................ 90!6.1! Physical part ............................................................................................................................ 91!
6.1.1! Experimental ..................................................................................................................... 91!6.2! Chemical part .......................................................................................................................... 92!
6.2.1! General remarks ................................................................................................................ 92!
Table of contents
II
6.2.1.1! Working techniques ................................................................................................................. 92!6.2.1.2! Instrumentation ........................................................................................................................ 92!6.2.1.3! Chemicals ................................................................................................................................. 93!
6.2.2! Synthesis of ligands .......................................................................................................... 94!6.2.2.1! Synthesis of rac-1,2-Hbmie (1)[145] .......................................................................................... 94!6.2.2.2! Synthesis of trans-bie (2)[145] ................................................................................................... 94!6.2.2.3! Synthesis of bmib (3)[219] .......................................................................................................... 95!
6.2.3! Synthesis of paddlewheel polymers ................................................................................. 96!6.2.3.1! Synthesis of [Rh2(OAc)4(trans-bie)]n (4) ................................................................................. 96!6.2.3.2! Synthesis of [Ru2(OAc)4(trans-bie)]n (5) ................................................................................. 97!6.2.3.3! Synthesis of [Mo2(OAc)4(trans-bie)]n (6) ................................................................................ 97!6.2.3.4! Synthesis of [Cr2(OAc)4(trans-bie)]n (7) .................................................................................. 98!6.2.3.5! Synthesis of [Cr2(OAc)4(trans-bie)2] (8) .................................................................................. 99!
6.2.4! Synthesis of zinc-polymers ............................................................................................... 99!6.2.4.1! Synthesis of [Zn3(OAc)6(trans-bie)]n (9) ................................................................................. 99!6.2.4.2! Synthesis of [Zn5(OAc)10(bmib)2]n (10)[219] ............................................................................ 100!
6.2.5! Synthesis of sawhorse-type polymers ............................................................................. 101!6.2.5.1! Synthesis of [Ru2(OAc)2(CO)4(trans-bie)]n (11) ................................................................... 101!6.2.5.2! Synthesis of [Ru2(OAc)2(CO)4(pyz)]n (12) ............................................................................ 102!6.2.5.3! Synthesis of [Ru2(OAc)2(CO)4(4,4’-bipy)]n (13) ................................................................... 102!6.2.5.4! Synthesis of [Ru2(OAc)2(CO)4(bpe)]n (14) ............................................................................ 103!6.2.5.5! Synthesis of [Ru2(OAc)2(CO)4(DABCO)]n (15) .................................................................... 104!
6.3! Computational details .......................................................................................................... 104!
7! APPENDIX ...................................................................................................................... 106!7.1! Details of the structure determinations .............................................................................. 107!7.2! Powder X-ray diffraction patterns ..................................................................................... 112!7.3! UV-Vis spectra ..................................................................................................................... 115!7.4! Computational details .......................................................................................................... 120!
7.4.1! IR vibrational bands ........................................................................................................ 120!7.4.2! Active space and DFT Mayer Bond Order ..................................................................... 121!
7.5! List of abbreviations and symbols ...................................................................................... 132!7.6! List of compounds ................................................................................................................ 135!
8! BIBLIOGRAPHY ............................................................................................................ 136!
9! DANKSAGUNG ............................................................................................................. 148!
1
1 GENERAL INTRODUCTION
General introduction
2
1.1 From metal organic frameworks to 1D coordination polymers
In his classic talk of 1959, FEYNMAN pointed out that there is “plenty of room at the
bottom”.[1-2] He predicted exciting new phenomena that might revolutionise science and
technology and affect our everyday lives – if only we were to gain precise control over
matter, down to the atomic level.[3] The decades since then have seen the invention of the
scanning tunneling microscope (STM) that allows us to image and manipulate individual
molecules and atoms.[4-5] We also have access to nanostructured materials with extraordinary
functional properties, such as semiconductor quantum dots and carbon nanotubes,[6-7] and a
growing understanding of how structural features control the function of such small
systems.[3]
These complementary developments are different aspects of nanotechnology, which aim to
create and use structures, devices and systems in the size range of about 0.1-100 nm (covering
the atomic, molecular and macromolecular length scale).[3] Because of this focus on the
nanometre scale, nanotechnology might meet the emerging needs of industries that have
thrived on continued miniaturization and now face serious difficulties in upholding the trend,
particularly in microelectronics.[8] But even if nanosystems and nanodevices with suitable
performance characteristics are available, nanotechnological solutions will find practical use
only if they are economically viable.[3]
The two basic approaches to creating surface patterns and devices on substrates in a
controlled and repeatable manner are the ‘top-down’ and ‘bottom-up’ techniques (Figure 1).[9]
For top-down fabrication, methods such as lithography, writing or stamping are used to define
the desired features.[3] The bottom-up techniques make use of self controlled processes for
ordering supramolecular or solid-state architectures from the atomic to the mesoscopic scale.
Shown in Figure 1 (clockwise from top) are an electron microscopy image of a
nanomechanical electrometre obtained by electron-beam lithography,[10] patterned films of
carbon nanotubes obtained by microcontact printing and catalytic growth,[11] a single carbon
nanotube connecting two electrodes,[12] a regular metal-organic nanoporous network
integrating iron atoms and functional molecules,[13] and seven carbon monoxide molecules
forming the letter ‘C’ positioned with the tip of a scanning tunneling microscope.[3]
General introduction
3
Figure 1: Two approaches to control matter at the nanoscale. Reprinted by permission from Macmillan
Publishers Ltd: [Nature] (Barth, J. V.; Costantini, G.; Kern, K., Nature 2005, 437, 671-679), copyright (2005)[3]
The past years have witnessed a tremendous growth in coordination chemistry, leading to
advances in our understanding of the synthesis, structure, and reactivity of novel complexes,
organometallic catalysts and extended inorganic polymers.[14] In recent decades, two new
branches of coordination chemistry have emerged, metal-organic frameworks (MOFs)[15-16]
and supramolecular coordination complexes (SCCs).[14] The former are comprised of infinite
networks of metal centres or inorganic clusters bridged by simple organic linkers through
metal-ligand coordination bonds.[15] The latter encompass discrete systems in which carefully
selected metal centres undergo self-assembly with ligands containing multiple binding sites
oriented with specific angularity to generate a finite supramolecular complex.[14] On the most
basic level, both SCCs and MOFs share the design of metal nodes linked by organic ligands
and such constructs can be broadly defined as metal-organic materials (MOMs).[14]
Crystal engineering of coordination polymers (CPs) allows us to incorporate functional
properties at the metal centres or in the backbone of the organic linkers very easily to develop
strategies for engineering multifunctional polymeric materials.[17-18]
General introduction
4
In the first review on coordination polymers published in 1964, BAILAR JR. discussed a few
basic principles to synthesise polymers along with various inorganic polymers based on
cyanido, hydroxido, and halogenido bridging, which were mainly characterised by
noncrystallographic techniques.[17, 19] Thereafter several comprehensive reviews offered a wide
range of topics in the area of one dimensional (1D) coordination polymers over the past two
decades focusing either on metal ion, spacer ligands, or particular structures.[20-26] Since the
review on 1D coordination polymers by CHEN and SUSLICK in 1993 significant progress has
been made in the structural and functional aspects of coordination polymers or MOFs in
general.[17, 20] Research in coordination polymers with specific topologies is making excellent
progress by virtue of the possible design of materials with specific electronic, optic, magnetic
and catalytic properties.[17] Various structural motifs of 1D, 2D, and 3D polymers are known
to exist.[27] Moreover, noncovalent interactions between 1D infinite chains can lead to the
formation of interesting architectures. Some of the most common structural features are
shown in Figure 2.[17]
Figure 2: Various common conformations of 1D coordination polymers.[28]
At a glance, it may be realized that 1D coordination polymers can easily be synthesized by
design from a linear spacer ligand coordinated to metal ions in a straight forward fashion and
it is not surprising that there are numerous examples available in the literature on linear
coordination polymer structures.[17, 29-52] From this point the next logical step is to use 1D
coordination polymers as minimised wires in the field of electronics.
General introduction
5
1.2 Different concepts of molecular wires
A wire is the simplest electronic component, with the sole function of facilitating the passage
of current between two points.[53] The rapidly developing field of molecular electronics is one
of the driving forces behind the interest in molecular wires.[54-58] The limitation of the present
“top-down” method of producing semiconducting-based devices have been the subject of
debate and conjecture since MOORE´S prediction that the number of components per
integrated circuit would double every 18 months.[54] It was thought that the inherent limitation
of the present technology would lead to a dead-end in the next few years. For instance,
silicon´s band structure disappears when silicon layers are just a few atoms thick.[53]
Lithographic techniques that are used to produce circuitry on the silicon wafers are limited by
the wavelengths at which they work. However, leaders on the semiconductor-manufacturing
world are still making advances that appear to be pushing “MOORE´S LAW” beyond its prior
perceived limits.[54] AVIRAM and RATNER first suggested in 1974 that molecules could be used
as alternatives to silicon chips,[59] but only more recently significant progress towards this goal
could be realised through the introduction of techniques such as scanning tunneling
microscopy, able to image and address single molecules.[60] There are many different
electrical components that need to be considered, for example switches, logic gates, diodes
etc. and this area has attracted much interest in recent years.[56, 61] Wires are the simplest of all
electrical devices, and are as such particularly suited for the development of some
fundamental understanding and techniques required for the realisation of molecule-scale
electronics.[60]
1.2.1 Organic based molecular wires
During the 1940s it was widely believed that the π-π* energy gaps of long conjugated
polyenes of the type trans-H(CH=CH)nH would decrease continually with increasing chain
length (n), reaching zero in polyacetylene (n = ∞) and resulting in metallic conductivity along
the polymer backbone.[62-63] Enthusiasm for molecular wires was rekindled in the 1960s by the
discovery of metallic conductivity in polysulfurnitride (SN)n,[64-66] and by LITTLE´S proposal
that polyacetylenes substituted with cyanine-dyes might be superconductors.[63, 67] Two great
discoveries have shaped research on organic semiconductors. The first was the demonstration
of metallic conductivity in doped polyacetylene by SHIRAKAWA, MAC DIARMID, HEEGER and
co-workers in 1977, which was awarded with the Nobel Price for chemistry in 2000.[68-69] The
General introduction
6
carbocation formed in long conjugated polyenes can act as a charge carrier for electrical
conduction (Scheme 1).[69]
Scheme 1: Possible resonance structures for the intermediate in the chlorination of polyacetylene and the
expected chemical structures of partially chlorinated polyacetylene. [69]
The second discovery was the demonstration of electroluminescence in undoped conjugated
polymers by FRIEND, HOLMES and co-workers in 1990.[63, 70] The combination of good
structural properties of poly(p-phenylenevinylene) (PPV), which is shown in Scheme 2, its
easy fabrication and light emission in the green-yellow part of the spectrum with reasonably
high efficiency, suggest that the polymer can be used for the development of large-area light-
emitting displays.[70]
Scheme 2: Poly(p-phenylenevinylene) (PPV).[70]
One example using PPV as a molecular wire was presented by WASIELEWSKI in 1998. They
produced a family of donor-bridge-acceptor molecules using oligomers of the conducting
+ Cl2
Cl
Cl
+ Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
n
n n
n
General introduction
7
polymer PPV as the bridging motif (Scheme 3). It was shown that photo induced charge
separation in these systems is very weakly distance dependant, which indicates that the
unsaturated bridge acts as an incoherent molecular wire.[71]
Scheme 3: Molecular-wire behaviour in PPV oligomers.[71]
Today, conjugated polymers are gaining commercial importance in light-emitting diodes,[70, 72-
73] thin-film field effect transistors,[74-75] photovoltaic cells[76-77] and sensors.[78-79]
To get a basic understanding of conjugated polymers it is necessary to measure the current
through single molecule wires. LAFFERENTZ el al. performed a measurement of a long
π-conjugated oligomer absorbed on a surface. They used dibromoterfluorene (DBTF)
monomers, consisting of three fluorene units, carrying lateral methyl groups and a bromine
atom at each end (Figure 3A). These terminal groups are dissociated form the terfluorene (TF)
N N C8H17
O
O
O
O
DONOR ACCEPTOR
WIRE =
OR
RO
OR
RO
OR
RO
1.
2.
3.
4.
5.
WIRE
R = 2-ethylhexyl
General introduction
8
molecular core in the first activation step of the on-surface synthesis.[80] At a surface
temperature of 10 K, single DBTF molecules on a Au(111) surface appeared in constant
current STM as three intense lobes corresponding to the dimethyl groups (Figure 3B). This
result is in very good agreement with image calculations (Figure 3C). After heating the
Au(111) surface up to 520 K, the DBTF´s bromine atoms dissociate and covalent bonds are
formed between different activated TF monomers, which diffuse randomly on the surface
(Figure 3D).[80]
Figure 3: On-surface polymerization of DBTF to conjugated molecular chains. A) Chemical structure of DBTF. B) Calculation C) STM image of intact molecules. D) Overview STM image after on-surface polymerization. E) STM image of a single polyfluorene chain end with its chemical structure superimposed. From [Lafferentz, L.; Ample, F.; Yu, H.; Hecht, S.; Joachim, C.; Grill, L., Science 2009, 323, 1193-1197]. Reprinted with permission
from AAAS.[80]
The high mobility of the monomer being a prerequisite for polymerization is favoured by the
presence of lateral methyl groups on each monomer. Hence, this on-surface synthesis allows
the formation of long, well-defined chains on the Au(111) surface. Oligomer lengths greater
than 100 nm were observed. The homogeneous appearance of each molecular chain
demonstrates the extreme regularity of their chemical composition, meaning that the newly
formed covalent bonds are equivalent to the existing bond, connecting the three fluorene units
with each DBTF monomer, and that defect-free polymers were synthesized on the surface
General introduction
9
(Figure 3E).[80] The resulting polymers are mobile enough on the Au(111) surface to be
manipulated laterally with an STM tip for chains as long as 25 nm. Furthermore, the
manipulation proves their high flexibility, enabling different curvatures of the chain without
breaking the chemical bonds between the different TF monomers.[80]
After selecting an isolated oligomer chain on the surface on the basis of a first STM image,
this chain was pulled upward by the STM tip apex (Figure 4A). With this method it was also
possible to measure the electronic conductivity along one single strand.[80]
Figure 4: Lifting a single molecular chain with the STM tip. A) Scheme of the chain pulling procedure. From
[Lafferentz, L.; Ample, F.; Yu, H.; Hecht, S.; Joachim, C.; Grill, L., Science 2009, 323, 1193-1197]. Reprinted with permission from AAAS [80]
1.2.2 Inorganic based molecular wires
Another concept beside the already shown organic based molecular wires, are polymer
compounds including metal fragments. It is not possible to list all concepts currently pursued,
but to give a brief overview two examples of different possibilities are presented. Pyridyl
based bridging bidentate ligands are able to build up coordination networks with M(NO3)2
(M = Co(II), Ni(II), Cu(II), Zn(II), Cd(II)). The number of coordination modes depends on the
coordination geometry of the nitrate anions as well as bipyridyl functionality, which leads to
different coordination arrangements.[81]
HOSSEINI et al. reported in another example on a double linear strand of [AgL(ClO4)]. In these
strands two pyridyl groups of different ligands are arranged coplanar and coordinate to a
Ag(I) centre.[17]
The high levels of functionality that may be integrated into a molecular system through the
incorporation of a metal centre, permitting redox or photochemical addressing of a molecular
component, or modification of the electronic structure, suggests a promising role for metal
General introduction
10
complexes as components in a molecular-based electronic technology.[53, 82-84] In 1999
COTTON et al. published the communication “Getting the right answer to a key question
concerning molecular wires”, that addressed the fundamental principle how metal atoms are
spaced along the chain direction. Growing attention has been focused on molecular
compounds having linear chains of metal atoms in which there are bonding contacts between
some or all adjacent metal atoms.[85]
The complex Cr5(tpda)4Cl2 (Figure 5) is only one example where five atoms are arranged in a
chain. It was shown that the complex molecule has long and short Cr---Cr contacts. Two
longer bond lengths of 2.598(3) and 2.609(2) Å alternate with two shorter bond lengths of
1.872(2) and 1.963(3) Å.[85]
Figure 5: The molecular structure of Cr5(tpda)4Cl2.[85]
These complexes have shown the importance of metal-metal interaction, but the length of the
wire is relatively short and limited by the axial chloride ligands.[85]
Beside this complex there are also various other inorganic building block concepts reported in
literature. In the following part of this introduction a special type of dinuclear metal fragments
will be introduced.
Cr
Cl
N
C
H
O
General introduction
11
Dinuclear Paddlewheel Complexes
Inorganic based coordination polymers can be built up with a dinuclear metal unit. Especially,
several coordination polymers based on MMX units, which consist of a so-called paddlewheel
unit (MM) and a bridging ligand (X = Cl, Br, I) have been reported in recent years.[86]
There is a large number of paddlewheel complexes described in the literature, mostly reported
by COTTON.[87-101] The first paddlewheel unit in a complex was the molecular structure of
copper(II) acetate [Cu2(OAc)4] × 2H2O which has been described by BROOK (1823), SCHABUS
(1855), GROTH (1910) and HULL (1938). But the final structural clarification by X-ray crystal
structure determination was reported by NIEKERK and SCHOENING in 1952.[102] The molecular
structure is shown in Figure 6. Two copper(II) centres are centrosymmetric bridged by four
acetato groups to form a paddlewheel unit. Each copper centre coordinates a water molecule
in an axial position opposite to the Cu atom. The Cu-Cu distance reported by NIEKERK and
SCHOENING is 2.63 Å, which is slightly larger than the interatomic distance of 2.556 Å in
metallic copper at 20 °C.[103]
Figure 6: Molecular structure of [Cu2(OAc)4] × 2H2O.[104]
A possible bonding between the metals in the copper(II) acetate has been discussed in
detail.[105-107] Magnetic measurements on copper paddlewheel complexes indicated a very
weak antiferromagnetic coupling between the metals,[105, 108-109] and recent theoretical
investigations by GUIHÉRY, DE GRAAF and NEESE et al. and also from KLOPPER et al. reported
Cu
O
C
H
General introduction
12
on the difficulties in predicting the coupling constants.[110-111] The distance between the centres
in the paddlewheel units depends on the employed metal and the resulting bond order of such
paddlewheel complexes. Beside the copper(II) acetate there are different metal(II) acetate
complexes known to literature, for example rhodium(II) acetate, ruthenium(II) acetate,
chromium(II) acetate and molybdenum(II) acetate. To understand the bond order between the
metal centres it is important to understand the electronic configuration of each of these
complexes. Therefore, it is useful to get an access to this topic with the help of the molecular
orbital scheme. In Figure 7 the orbital diagram of the d-block of two ML5 or ML4 fragments
connected in a ecliptic arrangement is shown. The position of the σ-orbital can be localized
over the π-level, depending on the ligand field splitting, the axial ligand and the metal-metal
distance. For sake of clarity only one combination of the π-orbitals is shown. An intervention
of the empty s-and p-orbital is disregarded for clarity.[112] A key factor in stabilizing Rh24+
units is the formation of Rh-Rh single bonds, which generally show single bond lengths in the
range of 2.35-2.45 Å.[113] In terms of a simplified molecular orbital picture, eight of the 14
electrons are distributed in the σ-, π- and δ-orbitals and the remaining six electrons occupy
the π*- and δ*-orbitals, resulting in a net Rh-Rh bond order of one and no unpaired
electrons.[113] The d6-d6 Ru2+-Ru2+ unit with the electronic configuration of σ2π4δ2δ*2π*2 and
two unpaired electrons, results in a Ru-Ru bond order of two with a bond length around
2.28 Å.[113] Going to the transition metals like chromium and molybdenum (d4-d4) with the
configuration σ2π4δ2 only the binding orbitals are occupied which results in a metal-metal
quadruple bond with a bond length from 1.83 to 2.60 Å for Cr-Cr and about 2.10 Å for Mo-
Mo.[113] Of course in this series of compounds a d3-d3 configuration, which gives a triply
bonded dinuclear unit, is missing.[113] The only neutral complex would be a vanadium based
acetate complex of the formula [V2(OAc)4].[114] The calculations clearly show the possible
existence of paddlewheel molecules and predict a V-V- triple bond length between 2.0 to
2.1 Å.[115] However all reported efforts to synthesise [V2(OOCR)4] compounds failed so
far.[116]
General introduction
13
Figure 7: Orbital interaction of d-block of two ML5-or ML4 fragments eclipsed conformation.[112]
Over the years paddlewheel complexes with different bridging ligands have been synthesized.
An overview of the used ligand is given in Figure 8. A great variation of bridging ligands for
example with carboxylato (O,O), thiocarboxalato (S,O) donor atoms are used. There are also
(N,O), (N,N), (S,N), (S,S), (N,P) donor atoms able to bridge the respective metal atoms.[113]
M
LL
L L
LM
L L
LL
L
M
L L
LL
L M
LL
L L
L
Energy
δ1
σ
π1,2
δ1∗
δ2
δ2∗
σ∗
π1,2∗
b2
b1
a1
e
General introduction
14
Figure 8: Overview of some bridging ligands.[113]
Dinuclear Sawhorse Complexes
So far only symmetric complexes were described. By reducing the symmetry by exchanging
two ligands through four monodentate carbon monoxide ligands a new class of dinuclear
metal complexes can be obtained. LEWIS et al. first studied these so-called sawhorse-type
complexes for ruthenium in Cambridge. Refluxing [Ru3(CO)12] in acetic acid produced
polymeric materials to which the formula [Ru2(OAc)2(CO)4]n was assigned on the basis of
their infrared and micro-analytical data. These polymers were found to dissolve in
coordinating solvents such as tetrahydrofuran to form dinuclear complexes assumed to be
[Ru2(OAc)2(CO)4(THF)2] that lose the coordinated molecules upon evaporation of the solvent
and go back to the polymeric structure. The dinuclear nature of these solvent complexes was
deduced from their reaction with PPh3, leading to the corresponding phosphine complexes
[Ru2(OAc)2(CO)4(PR3)2], which have been isolated and fully characterized.[117-118] In this
pioneering study, the structure of these dinuclear complexes was proposed entirely on the
basis of mass, NMR and in particular, of infrared spectroscopy. The bridging µ2-η2-O,O
coordination mode of the of the carboxylate ligands was deduced from the two characteristic
IR absorptions between 1550 and 1400 cm–1, assigned to the symmetrical and asymmetrical
ν(OCO) vibrations. The Ru2(CO)4 backbone with four terminal all-cis carbonyl ligands was
presumed in the light of symmetry arguments (point group C2v) from the typical ν(CO) three-
band signature around 2000 cm–1.[117-120] The molecular structure of these dinuclear complexes
was confirmed in 1977, when SCHUMANN et al. performed a single-crystal X-ray structure
analysis of the tri(t-butyl)phosphine derivate of the butyrato complex
[Ru2(OOCnPr)2(CO)4(PtBu3)2] (Figure 9).[118, 120]
ZY
X
R
OC
O
R
OC
S
R
OC
R'N
R
N OX N N
R R
NC
N
RArAr
NN
N
RArAr
N N
N
(CH2)m
O
M M M M M M M M
M M M M M M
M M M M M M
General introduction
15
Figure 9: Molecular structure of [Ru2(OOCnPr)2(CO)4(PtBu3)2].
The molecular structure of the labile solvent complexes [Ru2(OOCR)2(CO)4L2]
(L = tetrahydrofuran, acetonitrile and pyridine), supposed to be comparable to that of the
isolated phosphine complexes, could also be confirmed by X-ray structure analysis.
BRUCE et al. first isolated the acetonitrile complex [Ru2(OOCCF3)2(CO)4(NCMe)2] in a
crystalline form. The structure analysis revealed indeed the expected sawhorse structure.[118,
121] Over the years different dinuclear ruthenium(I) complexes of the sawhorse structure have
been synthesized.[121-131] There are also examples for polymeric compounds known in
literature. The low solubility of different diphosphine containing complexes was the argument
for a polymeric structure.[132-134] In case of the dithioether derivative
[Ru2(OAc)2(CO)4(MeSCH2SMe2)]n, the first single-crystal X-ray structure analysis revealed
the polymeric nature of this compound (Figure 10).[135] Within a polymer chain the acetato
bridged dimeric unit have an alternating ”up-down” arrangement. This means that the acetato
bridges on one dimer unit are adjacent to the carbonyl groups in the units on either side. The
bidentate MeSCH2SMe ligands link the dimeric units through the axial sites trans to the
Ru-Ru bonds.[135]
Ru
P
O
C
General introduction
16
Figure 10: Molecular structure of [Ru2(OOCCMe)2(CO)4(MeSCH2SMe2)]n.[135]
Beside the shown polymer with the molecular formula [Ru2(O2CR)2(CO)4L]n, there is also the
possibility to connect the previously described paddlewheel units with a bridging ligand L, to
form 1D coordination polymers with the molecular formula [M2(OAc)4L]n. As bridging ligand
L organic molecules or ions can be used.
One example is the 1D chain system based on a mixed-valence dinuclear unit bridged by
halogen ligands, with the formula [Pt2(n-butylCS2)4I]n (Figure 11a).[136-137] In the coordination
polymer the oxidation number is +2.5 and the metal-metal bond order is ½. This is an
appealing material with unique properties. In particular, in the solid state the polymer behaves
as a metallic electric conductor at room temperature. This property suggests its potential use
as molecular wire.[138]
The atomic force microscopy (AFM) topography shown Figure 11b displays a 2.4 µm
platinum-based polymer fibre adsorbed on a highly oriented pyrolytic graphite (HOPG)
surface that is kept at room temperature during polymer sublimation. The figure shows a
strand of heights ranging from 0.6 to 2.5 nm. Since an individual chain in a monocrystal
shows a diameter of 1.2 nm, according with the X-ray data. The interaction with the surface
squeezes the molecule to almost half of this diameter. Fibres longer than 12 µm have been
measured by AFM (Figure 11c). The height of these fibres is about 35 nm, indicating that it is
a bundle of ∼850 individual polymers chains. When this polymer is adsorbed on a insulating
mica substrate at room temperature, during sublimation no observation of any 1D structure
but instead islands were recorded.[138]
General introduction
17
Figure 11: a) Representation of a single chain of [Pt2(n-butylCS2)4I]n and AFM topography images showing b) fibers of small diameter c)a micrometer-length bundle of this polymer on a HOPG substrate at RT. Height
profiles along lines drawn in the insets of b) and c) respectively.[138]
Some coordination polymers are soluble. For instance, the already mentioned MMX chains
with several metal ions (M = Ru, Rh, and Pt) are soluble in some polar solvents. In general,
the interaction of the MMX with the solvent molecules results in the cleavage of the most
labile coordination bonds leading to the formation of small entities. In any case the
solubilisation of the MMX polymer form entities in solution that under suitable conditions
(concentration, temperature) are able to reassemble again to form the MMX chains. Notice
that the reversibility in the M-X bond is a basic and interesting feature of coordination
polymers, and it is not typical of classic organic polymers. The self-assembly of these
polymers on a surface from solution is an easy way to build up these structures. Following the
route, isolation of different nanostructures have been characterized on mica by the controlled
deposition from solution of the MMX polymer [Pt2(n-pentylCS2)4I] (Figure 12) by the
ZAMORA group.[86, 139]
General introduction
18
Figure 12: a) AFM topographic image of [Pt2(n-pentylCS2)4I] adsorbed by drop-casting on a mica. b) A zoomed
area displaying the typical features of a selected fibre. c) Height profile along the line represented in b).[86, 139]
Due to its simplicity, this is probably a easy method to deposit and organize molecules on
surfaces. AFM images taken immediately after deposition of the complex solution showed
smooth and continuous structures. [139] The typical height of the formed fibers is ca. 1.5 to
2.5 nm (Figure 12c). According to the X-ray data (1.5 nm being the expected height of one
MMX chain), these fibers correspond to a few MMX chains.[86, 140]
In order to measure the electrical resistance in these fibers, a macroscopic gold electrode was
evaporated using a conventional mask technique.[141] A relevant example is the sublimation of
[Pt2(dta)4I]n (dta = dithioacetate) on mica. The resulting sample consists of two macroscopic
regions, one covered with gold and the other one free of gold. Along the boundary between
both regions many nanoribbons partially covered with gold could be easily located by AFM.
The uncovered part of the nanoribbons could then be contacted with a conductive AFM tip,
used as a second mobile electrode.[139] Figure 13a is a schematic representation of the
experiment consisting of a AFM topography (projection view) plus the electrical circuit used
to perform the electrical characterization. The measurement is achieved by applying bias
voltage between the electrode edge and the metallic tip that is connected to the polymer.
Figure 13b shows a line profile along the green line in Figure 13a. According to it the height
and width of the nanoribbons are about 5 nm and 100 nm, respectively. However, as
represented in Figure 13c the contact region of the tip is much smaller.[142] Figure 13d shows a
typical current vs. voltage chart (I-V) taken at a distance of 100 nm from the edge of the gold
electrode, that characterizes the electrical properties of the nanoribbon. At first glance, the I-V
dependence is clearly non linear suggesting the existence of a barrier between the
nanoribbons and the metal contacts and/or the presence of defects along the nanoribbons.[139]
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[Pt 2 ( n-pentyl CS 2 ) 4 I] showed smooth and continuous structures (Figure 3 ). [ 18 ] Further AFM images taken after contacting with a gold electrode in a vacuum chamber reveal the existence of craks across the structures. AFM conductance experiments demonstrate that the continuous regions near the macroscopic gold electrode present a high electrical conductance. The frac-tures are a direct consequence of the exposition of the sample
4. Electrical Features of Coordination Polymers on Surfaces A fi rst, but illustrative example of the problems observed to produce electrically conductive MMX system, is the forma-tion of nanocrystal by drop casting on a mica surface. AFM image taken immediately after deposition of a solution of
Figure 1 . Schematic representation of a chain of the [Pt 2 ( n-butyl CS 2 ) 4 I] n (a) and AFM topography images showing fi bers of small diameter (b) and a micron length bundle (c) of [Pt 2 ( n-butyl CS 2 ) 4 I] n adsorbed on a HOPG substrate at RT. Height profi les along lines drawn in the insets of (b) and (c), respectively. Reproduced with permission from [17]. Copyright 2009 Wiley.
Figure 2 . (a) AFM topographic image of [Pt 2 ( n-pentyl CS 2 ) 4 I] adsorbed by drop-casting on a mica showing over a micron length well-defi ned fi bers. (b) A zoomed area displaying the typical features of a selected fi ber. (c) Height profi le along the line represented in (b). Reproduced with permission from [18]. Copyright 2010 Wiley.
General introduction
19
Figure 13: a) Scheme of conductance AFM electrical experiment used for the characterization of [Pt2(dta)4I]n. b)
Height profile taken along the green line drawn in a). c) Schematic representation of a cross section of the tip MMX contact area. d) I-V characteristic.[139, 142]
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to vacuum suggesting the existence of solvent occluded within the structures that is sundenly realeased in the vacuum condi-tions resulting in electrical discontinuity of the nanocrystals.
Direct sublimation of polymer crystals is, according to the fi ndings described above, a suitable procedure to create robust coordination polymers nanoribbons. Since this method does not use a solvent there is no problem to expose the CPs nano-structures to vacuum. A relevant example is the sublimation of [Pt 2 ( dta ) 4 I] n ( dta = dithioacetate) on a insulating substrate (mica). [ 20 ] This MMX polymer is a room temperature metal conductor with conductivities of about 13 Scm − 1 . [ 21 ] Upon sublimation, further inspection with AFM showed a network of nanoribbons adsorbed on mica with dimensions: ∼ 5 nm thicknesses, 100–200 nm width and a quite uniform length of ∼ 2.5 µ m. Also from the AFM images it was stimated a density of ∼ 0.5 nanoribbons/ µ m 2 . For the electrical characterization, by means of conductance AFM, a macroscopic gold electrode was evaporated using an appropriated shadow mask. [ 22 , 23 ] The resulting sample consists in two macroscopic regions, one cov-ered with gold and other one free of gold. Along the boundary between both regions many nanoribbons partially covered with gold could be easily located by AFM. The uncovered part of the nanoribbons could then be contacted with a conductive AFM tip, used as a second mobile electrode. Figure 4 a is a sche-matic representation of the experiment consisting on an AFM topography (projection view) plus the electrical circuit used to perform the electrical characterization. The variation of the cur-rent as a function of the tip bias voltage, along with the loading force, was measured during the contact experiment. Figure 4 d
Figure 3 . AFM imaging and morphological/electrical characterization of nanostructures showing clear orientation along well defi ned directions (nano-crystals) of [Pt 2 ( n-pentyl CS 2 ) 4 I]. (a) Nanocrystals of [Pt 2 ( n-pentyl CS 2 ) 4 I] grown on a mica by drop-casting. (b) Zoom of one nanocrystal shown in (a). (c) Details of fractures through the nanocrystals made after vacuum exposition. (d) Gold electrode evaporated on mica (top of the image): A nanocrystal is partially covered by the gold electrode. (e) Height profi le of the nanocrystal shown in (d). (f) Current vs voltage features showing high conductance when the nanocrystal is contacted on the blue spot in (d). Reproduced with permission from [18]. Copyright 2010 Wiley.
60-6
40
20
0
-20
-40
V[V]
I[nA
]
(f)
800nm
(b)
1.2µm
(a)
460nm
(c)
1.20
2.5
0X[µm]
Z[n
m]
(e)
420nm
Gold(d)
60-6
40
20
0
-20
-40
V[V]
I[nA
]
(f)
60-6
40
20
0
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V[V]
I[nA
]
(f)
800nm
(b)
800nm
(b)
1.2µm
(a)
1.2µm
(a)
460nm
(c)
460nm
(c)
1.20
2.5
0X[µm]
Z[n
m]
(e)
1.20
2.5
0X[µm]
Z[n
m]
(e)
420nm
Gold(d)
420nm
Gold(d)
Figure 4 . Scheme of the conductance AFM electrical experiment used for the characterization of the [Pt 2 I( dta ) 4 ] n nanoribbons. (a) AFM topographic image showing a MMX nanoribbon on mica electrically connected to a gold electrode. The nanoribbon is partially covered with gold. (b) Height profi le taken along the green line drawn in (a). (c) Schematic showing a cross section of the tip [Pt 2 I( dta ) 4 ] n contact area. A qualitative stress distribution has been drawn using a pseudo color scale along the contact region. The geometrical parameters for the Hert’s model ( ρ , R, F) are also shown. (d) Current vs voltage characteristic taken by contacting the nanoribbon at 100 nm from the gold electrode. Reproduced with permis-sion from [20]. Copyright 2010 Nature Publishing Group.
2001000
6
4
2
0
∼100 nm
∼ 6 n
m
3002001000
6
4
2
0
6 nm
300
Z(nm
)
X(nm)
210-1-2
15
50
-5
- 15
- 25210-1-2
15
50
-5
- 15
- 25
(d)
I(nA)
V(V)
(b)(a)
(c)
AV
General introduction
20
1.3 Synthesis of trans-bis(N-methylimidazol-2-yl)ethylene (trans-bie)
Within the last two chapters the “bottom-up” technique has been introduced briefly and the
different concepts of molecular wires, the formation of 1D coordination polymers based on
metal organic compounds has been reviewed. Beside platinum dimer columns also
nanostructures of MMX chains exhibit electrical conductivity, in which a halogenido ligand
bridges other bimetal MM fragments, e.g. a Cu-Cu paddlewheel unit.[143-144] In theory,
replacing the bridging halogenido ligand X by a conjugated N,N donor ligand might result in
nanomaterials with related properties.[145]
N. V. FISCHER from the BURZLAFF group developed a new imidazole based N,N donor ligand
capable to form polymeric structure similar to pyrazine, 4,4´-bipyridine or trans-bis(pyridine-
4-yl)ethylene.[146-153] The starting point of this project was the optimization of the synthetic
route of the trans-bie ligand. Imidazoles exhibit σ-donor, π-acceptor but also π-donor
capacities, which makes them interesting for bridging ligands in the field of molecular
electronics.[145]
Scheme 4: General synthetic route, leading to rac-Hbmie (1) and trans-bie (2).[145]
The chiral but racemic ligand rac-1,2-bis(N-methylimidazole-2-yl)ethanol (rac-Hbmie) (1)
was obtained by a two step synthesis. Starting from N-methylimidazole, which was
deprotonated with n-BuLi and further reacted with DMF, N-methyl-2-
imidazolecarboxaldeyde was formed.[154] Formation of N-methyl-2-imidazolecarboxaldeyde
with in situ generated lithium(N-methylimidazol-2-yl)methanid led to rac-1,2-Hbmie (1). The
N,N,O ligand was then dehydrated with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and
trifluoroacetic anhydride (TFAA), which yields trans-bis(N-methylimidazol-2-yl)ethylene
(trans-bie) (2) (Scheme 4).[145]
N
N N
N
O
1) n-BuLi
2)
3) H2O, H+
N N
N N
HO
N N
N N
DBU,TFAA,THF
1 2
General introduction
21
1.4 Synthesis of the first 1D coordination polymer with trans-bie
The reaction of trans-bie (2) with CuCl led to a 2D sheet structure, where three ligands bind
to one Cu(I) centre via the nitrogen atom of the imidazole ring. Each of these three trans-bie
ligands binds to different Cu(I) centre via its second nitrogen donor. The more interesting
reaction of trans-bie (2) is with copper(II) acetate which resulted in a coordination polymer
[Cu2(OAc)4(trans-bie]n. A single crystal X-ray structure determination revealed 1D
coordination polymer strings. Cu2(OAc)4 moieties alternate with trans-bie ligands within the
polymer, which connect the dinuclear units (Figure 14).[145]
Figure 14: Cutout of the molecular structure of [Cu2(OAc)4(trans-bie)]n.[145]
Hence, low Cu-Cu distances of 2.6868 Å are observed within these dinuclear moieties
compared to rather long Cu-ligand-Cu distances of 8.4004 Å. Surprisingly, all trans-bie
ligands are oriented almost coplanar within one 1D coordination polymer string, a geometry
that resembles features of typical organic conducting polymers. The angle between the planes
formed by the acetato σ donors and the ligand plane is close to 45°.[145]
In order to understand the surface morphology of the 1D coordination polymer, an acetonitrile
solution was deposited on mica and investigated by non-contact mode atomic force
microscopy (AFM). The frequency modulation AFM technique obtained high-resolution
images but it was not possible to map a single strand. Figure 15 depicts kinds of arrangements
of the complex on mica surfaces. Figure 15a and b demonstrate the growth of the complex in
different patterns. In the background of Figure 15a residues of solvent seem to be present. The
observed polymer chains can be up to several hundred nanometres long. Figure 15d represents
a profile of the AFM image taken along the line in Figure 15c. This indicates that the polymer
O
O
Cu
Cu
O
O
N
General introduction
22
strand has a width of 12 nm and a height of 2.7 nm. Even when taking into account that the
image is widened by the convolution of the real topography with the AFM tip, the height of
the strand definitely indicates that we see a thick strand up of several polymer strands.[145]
Figure 15: AFM topographies of [Cu2(OAc)4(trans-bie)]n deposited on a mica surface. a) and b) large scale
images with non-uniform accumulations; c) an enlarged section in Figure 15b; d) profile taken along dotted line in Figure 15c.[145]
The interaction between the molecules and the substrate (mica) seems to be much weaker than
the molecule-molecule interaction, therefore leading to the formation of thick strands. The
molecule surface interactions play a vital role in the preparation of the samples.[145]
A detailed understanding of the electronic structure of the individual polymer strands is a
necessary step in any predictive polymer physics. Recent studies have shown that the
structural and electronic properties of metal complexes on solid surfaces can be investigated
by STM/CITS techniques.[155] For this measurements a drop of 10–9 M solution of the polymer
in acetonitrile was deposited on a highly oriented pyrolytic graphite (HOPG) surface. Figure
2
O1
N
N11
O2
C11
O3
Cu
O4
C1O4
CuCu C1
O3
O2Cu
O1
N
N
Each of these three trans-bie ligands binds to a different Cu(I) centre via its second nitrogen donor, which results in a 2D sheet structure of the coordination polymer 3. Top and side views of such a sheet are visualized in Figure 1a and 1b, respectively.
Scheme 2. Formation of 1D and 2D copper containing coordination poly-mers with trans-bis(N-methylimidazol-2-yl)ethylene (trans-bie) (2).
An analogous reaction of 2 with Cu(II) acetate resulted in a coordination polymer [Cu2(OAc)4(trans-bie)]n (4). A single crystal X-ray structure determination revealed one-dimensional coor-dination polymer strings which are embedded in several acetonitrile solvent molecules. Cu2(OAc)4 moieties of the Cu(II) acetate alternate with trans-bie ligands within the one dimensional coordination polymer strings of 4, which connect these dinuclear units (Figure 2).
Figure 2. Cutout of the molecular structure of 4; thermal ellipsoids are drawn at the 30% probability level; hydrogen atoms and acetonitrile molecules have been omitted for clarity; selected bond lengths (Å) and angles (°): C1-C1 1.342(4), C1-C11 1.455(3), Cu-N11 2.1540(16), Cu-O1 1.9811(13); Cu-O2 1.9790(13); Cu-O3 1.9882(14); Cu-O4 1.9845(14); Cu-Cu 2.6868(4); Cu-Cu 8.4004(5); N11-Cu-Cu 172.07(5), O1-Cu-N11-C11 136.12(18); C1-C1-C11-N11 161.4(3).
Hence, low Cu-Cu distances of 2.6868 Å are observed within
these dinuclear moieties compared to rather long Cu-trans-bie-Cu distances with 8.4004 Å. Surprisingly, all trans-bie ligands are oriented almost coplanar within one 1D coordination polymer string, a geometry that resembles features of typical organic conducting polymers. The angle between the planes formed by the acetato σ donors and the bie plane is close to 45°.
In order to understand the surface morphology of complex 4 on
insulating surfaces, an acetonitrile solution was deposited on mica and investigated by non-contact mode atomic force microscopy (AFM). The frequency modulation AFM technique allowed us to obtain high resolution images but we could not map a single polymer strand. Figure 3 depicts different kinds of arrangements of 4 on mica surfaces. Figures 3a and 3b demonstrate the growth of complex 4 in different patterns. In the background of Figure 3a residues of solvent seem to be present. Figure 3c shows a magnified section of Figure 3b. The observed polymer chains can be up to several hundred nanometers long. Figure 3d represents a profile of the AFM image taken along the line in Figure 3c. This indicates that the polymer strand has width of 12 nm and a height of 2.7 nm. Even when taking into account that the image is widened by the convolution of the real topography with the AFM tip, the height of the strand definitely indicates that we see a thick
strand made up of several polymer strands. The interaction between the molecule and the substrate (mica) seems to be much weaker than the molecule-molecule interaction, therefore leading to the formation of thick strands. The molecule-surface interactions play a vital role in the preparation of the samples.[17]
Figure 3. AFM topographies of complex 4 deposited on a mica surface. a) and b) large scale images with non-uniform accumulation of 4; c) an enlarged section in Figure 3b; d) profile taken along the dotted line in Figure 3c.
A detailed understanding of the electronic structure of the
individual polymers strands is a necessary step in any predictive polymer physics. Further, an understanding of specific polymer-surface interactions is necessary for many applications.[8a] However, by their very nature, studies of individual polymer strands or isolated clusters of several molecules are exceedingly difficult.[18,
19]
Figure 4. STM topographies on HOPG surfaces showing a) the aggregation of complex 4 as single strand (white arrow) and as a bundle of strands (yellow arrow); b) a high resolution scan of the marked area in Figure 4a. In the background graphite atoms are visible. For a) tip stabilization parameters were -100 mV and 10 pA and for b) -50 mV and 30 pA.
Our recent studies have shown that the structural and electronic properties of metal complexes on solid surfaces can be investigated by STM/CITS techniques.[1c] We successfully applied this technique on different type of metallocomplexes.[1c,12,20] For STM/CITS measurements we deposited a drop of acetonitrile 10-9 M solution of 4 on a highly oriented pyrolytic graphite (HOPG) surface. Figure 4a shows large scale STM topography of
a) b)
a) b)
c)
d)
0 10 20 30
0
1
2
3
!
!
Z(n
m)
X +(nm)
General introduction
23
Figure 16a shows the large scale STM topography of the polymer. Most of the deposited
strands observed were tangled and twisted with an occasional straight strand observed. In
Figure 16a a single strand (white arrow) as well as bundles of polymer strands (marked by a
yellow arrow) were mapped. Figure 16b represents a high resolution scan of the marked area
in Figure 16a. In this STM topography it was posible to visualize both, a single polymer
strand and surface atoms, in the same scan with the width of the line being about 1 nm. This
nicely matches the width of a single polymer strand obtained from X-ray crystallographic data
of the compound. As expected, the polymer can be found in the contours of the substrate
surface and trapped across steps of the substrate. The surprising fact that they can also be
found to lie on the flat parts of the surface of HOPG suggest that some kind of interaction
between the substrate and the polymer molecule exist.[156] In Figure 16b additional shadows
are visible next to the polymer line. This can be attributed to an accumulation of solvent
molecules.[145]
Figure 16: STM topographies on HOPG showing a) the aggregation of [Cu2(OAc)4(trans-bie)]n as single strand (white arrow) and as a bundle of strands (yellow arrow), b) a high resolution scan of the marked area in Figure
16a. In the background graphite atoms are visible.[145]
In a view cases it was possible to map single polymer strands and their electronic properties
by STM /CITS techniques. Figure 17 depicts the simultaneously recorded topography (Figure
17a) and CITS current image (Figure 17b) of an isolated polymer strand.[145]
The polymer strand is attached next to a monoatomic graphite step. From the topography
image it is found again that the width of the structure conforms to the diameter of a single
polymer strand. Additionally the CITS current image (Figure 17b) shows the distribution of
the tunneling current at a bias voltage of –0.8 V. In this image (Figure 17b, marked by an
2
O1
N
N11
O2
C11
O3
Cu
O4
C1O4
CuCu C1
O3
O2Cu
O1
N
N
Each of these three trans-bie ligands binds to a different Cu(I) centre via its second nitrogen donor, which results in a 2D sheet structure of the coordination polymer 3. Top and side views of such a sheet are visualized in Figure 1a and 1b, respectively.
Scheme 2. Formation of 1D and 2D copper containing coordination poly-mers with trans-bis(N-methylimidazol-2-yl)ethylene (trans-bie) (2).
An analogous reaction of 2 with Cu(II) acetate resulted in a coordination polymer [Cu2(OAc)4(trans-bie)]n (4). A single crystal X-ray structure determination revealed one-dimensional coor-dination polymer strings which are embedded in several acetonitrile solvent molecules. Cu2(OAc)4 moieties of the Cu(II) acetate alternate with trans-bie ligands within the one dimensional coordination polymer strings of 4, which connect these dinuclear units (Figure 2).
Figure 2. Cutout of the molecular structure of 4; thermal ellipsoids are drawn at the 30% probability level; hydrogen atoms and acetonitrile molecules have been omitted for clarity; selected bond lengths (Å) and angles (°): C1-C1 1.342(4), C1-C11 1.455(3), Cu-N11 2.1540(16), Cu-O1 1.9811(13); Cu-O2 1.9790(13); Cu-O3 1.9882(14); Cu-O4 1.9845(14); Cu-Cu 2.6868(4); Cu-Cu 8.4004(5); N11-Cu-Cu 172.07(5), O1-Cu-N11-C11 136.12(18); C1-C1-C11-N11 161.4(3).
Hence, low Cu-Cu distances of 2.6868 Å are observed within
these dinuclear moieties compared to rather long Cu-trans-bie-Cu distances with 8.4004 Å. Surprisingly, all trans-bie ligands are oriented almost coplanar within one 1D coordination polymer string, a geometry that resembles features of typical organic conducting polymers. The angle between the planes formed by the acetato σ donors and the bie plane is close to 45°.
In order to understand the surface morphology of complex 4 on
insulating surfaces, an acetonitrile solution was deposited on mica and investigated by non-contact mode atomic force microscopy (AFM). The frequency modulation AFM technique allowed us to obtain high resolution images but we could not map a single polymer strand. Figure 3 depicts different kinds of arrangements of 4 on mica surfaces. Figures 3a and 3b demonstrate the growth of complex 4 in different patterns. In the background of Figure 3a residues of solvent seem to be present. Figure 3c shows a magnified section of Figure 3b. The observed polymer chains can be up to several hundred nanometers long. Figure 3d represents a profile of the AFM image taken along the line in Figure 3c. This indicates that the polymer strand has width of 12 nm and a height of 2.7 nm. Even when taking into account that the image is widened by the convolution of the real topography with the AFM tip, the height of the strand definitely indicates that we see a thick
strand made up of several polymer strands. The interaction between the molecule and the substrate (mica) seems to be much weaker than the molecule-molecule interaction, therefore leading to the formation of thick strands. The molecule-surface interactions play a vital role in the preparation of the samples.[17]
Figure 3. AFM topographies of complex 4 deposited on a mica surface. a) and b) large scale images with non-uniform accumulation of 4; c) an enlarged section in Figure 3b; d) profile taken along the dotted line in Figure 3c.
A detailed understanding of the electronic structure of the
individual polymers strands is a necessary step in any predictive polymer physics. Further, an understanding of specific polymer-surface interactions is necessary for many applications.[8a] However, by their very nature, studies of individual polymer strands or isolated clusters of several molecules are exceedingly difficult.[18,
19]
Figure 4. STM topographies on HOPG surfaces showing a) the aggregation of complex 4 as single strand (white arrow) and as a bundle of strands (yellow arrow); b) a high resolution scan of the marked area in Figure 4a. In the background graphite atoms are visible. For a) tip stabilization parameters were -100 mV and 10 pA and for b) -50 mV and 30 pA.
Our recent studies have shown that the structural and electronic properties of metal complexes on solid surfaces can be investigated by STM/CITS techniques.[1c] We successfully applied this technique on different type of metallocomplexes.[1c,12,20] For STM/CITS measurements we deposited a drop of acetonitrile 10-9 M solution of 4 on a highly oriented pyrolytic graphite (HOPG) surface. Figure 4a shows large scale STM topography of
a) b)
a) b)
c)
d)
0 10 20 30
0
1
2
3
!
!
Z(n
m)
X +(nm)
General introduction
24
arrow) one can recognize the periodicity of a repeated segment along the strand which
conforms to the length of a monomer obtained from the X-ray structure of the complex.
Figure 17: a) and b) simultaneously recorded STM topography and CITS current image at -0.8 V. c) I-V
characteristics recorded at three different locations in Figure 17b (on strand: blue & red; apart from the strand: green). d) the differential conductance calculated from the curves in Figure 17c.[145]
The observed repeat segments are approximately 12 Å in length. Figure 17c represents the
current-voltage (I-V) characteristics recorded at three different positions in Figure 17b. The
differential conductance (dI/dV) calculated from the measured I-V characteristics of the
polymer are presented in Figure 17d. Three main spectral features are observed approximately
at –0.35, –0.55 and 0.50 V. These signals can be assigned to the HOMO, HOMO–1 and
LUMO state. The signal in the differential conductance should offer information about the
effective HOMO–LUMO gap. Due to the measurements of the monomer section this gap is
roughly 0.8 V. Therefore, the polymer [Cu2(OAc)4(trans-bie)]n could be a potential candidate
for future semiconducting nanowires.[145]
3
complex 4. Most of the deposited polymer strands observed were tangled and twisted with an occasional straight strand observed. In Figure 4a a single strand (white arrow) as well as bundles of polymer strands (marked by a yellow arrow) were mapped. Figure 4b represents a high resolution scan of the marked area in Figure 4a. In this STM topography we were able to visualize both, a single polymer strand and surface atoms, in the same scan. The width of the line is about 1 nm. This nicely matches the width of a single polymer strand obtained from x-ray crystallographic data of complex 4. As expected, the polymer can be found in the contours of the substrate surface and trapped across steps of the substrate. The surprising fact that they can also be found to lie on the flat parts of the surface of HOPG suggests that there exist some kind of interaction between the substrate and the polymer molecule.[20] In Figure 4b additional shadows are visible next to the polymer line. This can be attributed to an accumulation of solvent molecules.
Figure 5. a) and b) simultaneously recorded STM topography and CITS current image at -0.8 V. c) I-V characteristics recorded at three different locations in Figure 5b (on strand: blue & red; apart from the strand: green). d) the differential conductance calculated from the curves in Figure 5c.
In few cases we were able to map single polymer strands and their electronic properties by STM/CITS techniques. Figure 5 depicts simultaneously recorded topography (Figure 5a) and CITS current image (Figure 5b) of an isolated polymer strand. The polymer strand is attached next to a monoatomic graphite step. From the topography image (see Figure 5a) it is found again that the width of the structure conforms to the diameter of a single polymer strand. Additionally the CITS current image (Figure 5b) shows the distribution of the tunnelling current at a bias voltage of -0.8V. In this image (Figure 5b, marked by an arrow) one can recognize the periodicity of a repeated segment along the strand which conforms to the length of a monomer obtained from the x-ray structure of complex 4. No features arising exclusively from the Cu ions were observed.[20] The observed repeat segments are approximately 12 Å in length. Figure 5c represents current-voltage (I-V) characteristics recorded at three different positions in Figure 5b. The differential conductance (dI/dV) calculated from the measured I-V characteristics of the polymer are presented in Figure 5d. Three main spectral features are observed approximately at -0.35, -0.55 and 0.50. These peaks should originate from the HOMO, HOMO-1 and LUMO state. The peak in the differential conductance should be the effective HOMO-LUMO gap from measurements of the monomer section is roughly 0.8 V. To elucidate the mechanism of the strand formation, ESI MS data were recorded from 1:1 mixtures of [Cu2(OAc)4] and trans-bie (2) dissolved in acetonitrile. So far no molecule ion peaks assignable to oligomeric fragments [Cu2(OAc)4(trans-bie)]n could be observed.
This implies that the polymer strands are formed on the surface. In conclusion, with trans-bis(N-methylimidazol-2-
yl)ethylene an alternative to trans-bis(pyrid-4-yl)ethylene was developed that might add interesting properties to MOFs and metal ligand derived molecular wires. Furthermore, we investigated complex 4 on two different substrates by AFM/STM. One is the insulating, polar mica (repulsive surface) and the other one is the non-polar (neutral surface) semiconductor HOPG. Deposition of the 1D structure was achieved from diluted solutions. On the mica surface, we were unable to map a single isolated strand of complex 4 but on the HOPG surface single isolated strands were mapped by STM and their electronic properties were subsequently characterised by CITS. Molecule–surface interactions play a central role in the successful surface deposition of supramolecular complexes.[21] On the polar surface (mica) the material shows a tendency to form bundles of strands. This can be attributed to the minimization of the contact area between the polymer and the substrate, resulting in the formation of thick polymer strands. On the other hand graphite is a non-polar surface, resulting in single isolated strands. The calculated differential conductance curve of a single polymer strand shows a HOMO-LUMO gap of 800 mV. Therefore, the complex 4 could be a potential candidate for future semiconducting nanowires. Experimental Section AFM measurements: For the purpose of AFM imaging on an insulating substrate, a drop- drying method was used. In this preparation method, a drop of 10-9 M solution of 4 in was deposited onto the surface of freshly-cleaved mica. After drying in air, the samples were loaded into the microscope. AFM images were recorded in ambient conditions using a home built combined tuning fork STM/AFM head operated in frequency modulation mode. A tuning fork with a resonance frequency of 32768 Hz, a spring constant of 1800 Nm-1, and an estimated Q factor of 3500 was used as a force sensor. The AFM tips were prepared from electrochemically etched Pt-Rh (90/10) wire which was then glued to the end of the tuning fork by silver epoxy. For amplitude, excitation and frequency control a PLL from Nanonics was used; distance control and scanning were carried out by a SPM 1000 system from RHK. All AFM images were recorded in frequency modulation mode with constant oscillation amplitude of 1 nm and a constant scan speed of 0.25 Hz. The images were recorded at frequency shifts ranging from 2 to 6 Hz with the PLL bandwidth limited to 15 Hz. Resolution for topography measurements was 256 х 256 points. STM measurements: The STM investigations were performed using a home-built, low-drift STM head interfaced with a home-developed controller and software. Like AFM measurements, prior to imaging, a droplet of the acetonitrile solution (10–9 M) of complex 4 was applied to a freshly cleaved HOPG surface. Distances in the STM images were calibrated by observing the lattice constant of HOPG. All topography images were recorded in constant current mode. Typically, for the STM measurements, tunneling currents between 5 and 100 pA were employed. The bias voltage was ± 50 mV to ± 100 mV for topography measurements. The scan frequency was varied between 2 and 5 Hz. Resolution was 256 × 256 points for topography, and 128 × 128 in the CITS measurements. CITS measurements were performed simultaneously with topographic imaging, using the interrupted feedback loop technique.[12] This was achieved by opening the feedback loop at a fixed separation of tip and sample, and ramping the bias voltage over the range of interest. I/V curves were acquired at every pixel of the topography image. This produces a three-dimensional map of the current as a function of position and voltage. The data set is then usually decomposed into a set of current maps, i.e. current I vs. position, for any measured value of the bias voltage. The scan range of voltages was typically from –0.8 V to 0.8 V relative to the tip potential for approximately 100 discrete voltage steps. Typically, tunneling resistances of the order of 2 GΩ were set. We used Pt-Ir (90/10) tips mechanically cut from wires with a diameter of 0.25 mm. Figs. (3-5) were produced using the program WSxM.[22]
Acknowledgment
Financial support by the Deutsche Forschungsgemeinschaft DFG (SFB 583) is gratefully acknowledged. Special thanks to Mr. v. Gernler for recording ESI MS data.
a) b)
!1.0 !0.5 0.0 0.5 1.0!1.2
!0.8
!0.4
0.0
0.4
!
!
H O P GP olymerP olymer
I(nA)
U (V)!1.0 !0.5 0.0 0.5 1.00
2
4
!
!
!H O P G!P olymer!P olymer
(dI[nA]/dU[V
])
U (V)
c) d)
a) b)
!1.0 !0.5 0.0 0.5 1.0!1.2
!0.8
!0.4
0.0
0.4
!
!
H O P GP olymerP olymer
I(nA)
U (V)!1.0 !0.5 0.0 0.5 1.00
2
4
!
!
!H O P G!P olymer!P olymer
(dI[nA]/dU[V
])
U (V)
c) d)
General introduction
25
1.5 Scanning tunneling microscopy
The starting point of scanning probe microscopy (SPM) was the invention in 1982 of the
scanning tunneling microscope (STM) by BINNIG and ROHRER from the IBM Zurich Research
Laboratory, rewarded with the Nobel Prize for physics in 1986.[4, 157-158] STM is nowadays
used routinely as a surface imaging technique providing atomic resolution.[159-160]
1.5.1 Theory
The basic physical principle of any scanning probe method is the interaction between the
scanning probe and the sample. In the STM, a sharp metallic needle is scanned over the
surface at a distance of less than 1 nm. This distance is controlled by the tunneling current
between the tip and the conducting surface. The tunneling current is a quantum mechanical
effect, with two properties important for STM. Which is the flow between electrodes even
through a thin insulator of vacuum gap and decays on the length scale of one atomic radius. In
the STM the tunneling current flows from the very last atom of the tip apex to single atoms of
the surface, inherently providing atomic resolution.[158]
Tunnel effect
Between two regional surfaces opposite each other, two such steps combine to make a
potential barrier. Classically, particle can be at either side of this barrier, but the barrier region
is inaccessible. Quantum-mechanically, electrons are described by wavefunctions, which do
not drop to zero abruptly at the surface but extend into the barrier, a process known as
tunneling.[161] We differ areas I,II and III with the potential ! at different positions 0, ! and !
within the potential well:[162]
I ! ≤ 0 mit ! = 0
II 0! ≤ !! ≤ ! mit ! = !!
III !! ≤ ! mit ! = !0
a0 x
V0
V I II III
0
General introduction
26
The interesting part is, ! < !!!. Application of the Schrödinger equation leads to wave
functions for areas I to III, with the constants A-G and the rate constant !! and !!.[162]
!! = !! ∙ !!!!!!! + ! ∙ !!!!!!!! !! = !! ∙ !!!!!!! + ! ∙ !!!!!!!! !! = !! ∙ !!!!!!! + ! ∙ !!!!!!!!
The transmission coefficient as a function of ! und !! with h = Planck constant, ℏ = reduced
Planck constant and m = mass of an electron.
! = ! 1+ 14!!!
!(!! − !)!sinℎ! 2!!!(!! − !)
ℏ
! !!!
!
For ! < !!! the transmission coefficient results in a finite value, if ! and !! are not ∞. For
better estimation, we observe the situation where 2!!!(!! − !) ℏ! ≫ 1, so we achieve for
!.
!!− !16!(!! − !)!!!!!!!(!!!!(!!!!) ℏ!)! !
This function clearly shows the dependency of the transmission coefficient ! on the energy
difference !! − ! and the width of the potential barrier !. ! decreases with an increasing
!!!and !. The tunnel probability decreases according to the size and the broadness of the
barrier.
In STM the barrier is given by the vacuum gap between the sample and tip.[161-162] Then the
tunneling current !! can be calculated by taking into account the density of states of the
sample, !!(!!), at the Fermi edge:
!! ∝ !!! !! !!! !! !!! !!
ℏ
where the barrier height ! is in eV and the gap z in angstrom units.
General introduction
27
1.5.2 Function and surface property
The tunneling electrons constitute a current that depends exponentially on the distance
between the sample and the tip. It is this sensitivity that gives the STM its unique resolution.
A few angstrom extra separation leads to a decrease in the tunnel current by a factor ten. In
order to control the barrier width and the lateral position of the tip, it is mounted onto an
actuator consisting of piezoelectric elements. A contact voltage is established and
maintained.[161]
Constant Height Mode (CHM)
Primarily there are two methods that are used to obtain STM images. The first is constant
height STM. Here the tip is brought within close proximity of the sample, then held at fixed z-
position while the tip is scanned over the surface.[163] The current between the tip and the
sample, a function of position on the surface, visualizes the sample relief, which is shown in
Figure 18.[161] It is visible in the zoom (Figure 18 b & c) that the current level depends on
distance between the tip and the surface.[161] This technique is often preferred when imaging
very flat samples that have no terraces, because surface corrugations higher than 5-10 Å will
cause the tip to crash.[161, 163]
Figure 18: a) Schemetic representation of the constant height mode (CHM), b & c) enlargement of a) at different
positions.[161]
General introduction
28
Constant Current Mode (CCM)
The second, more common method is constant current imaging. Instead of fixing the distance
between the tip and the sample, a fixed sample-tip bias voltage, and a fixed current are
required. Then, as the tip is rastered across the surface, the distance between the tip and the
sample adjusted by a feedback loop to maintain the set point tunneling current. The voltage
applied to the piezo tube will change depending on the structural and electronic features on
the surface. This voltage is then recorded as a function of position on the surface of the
sample, which reflects the surface topography, shown in Figure 19. Equal to the change of the
z-position the current level remains constant (Figure 19 b & c).[161]
Figure 19: a) Schemetic representation of the constant current mode (CCM), b & c) enlargement of a) at
different positions.[161]
Current Imaging Tunneling Spectroscopy (CITS)
Topographical images of the surface of a metal or semiconductor sample are actually a
combination of the topographical and electronic characteristics of the system under study.
Determination of the contributions of the electronic structure versus the topographical
features is difficult. As further understanding of STM was developed, it became clear that the
dependence of the image on the electronic characteristics of the system could be used to
extract information on the electronic structure of the materials being imaged. Researchers
found that by making minor mechanical or software changes, much more information than the
topography could be derived about the physical system.[163] The field of scanning tunneling
spectroscopy (STS) was thus developed.[163-164] There are four common methods: conductance
General introduction
29
spectroscopy,[165] constant separation STS,[166] constant voltage STS,[164] and current imaging
tunneling spectroscopy (CITS).[166] In order to achieve a better understanding of how STM
images are related to geometric and electronic structure of the deposited samples, spatially
resolved CITS technique has been performed. Figure 20 shows the schematic of CITS mode
operation.[161] This method involves the acquisition of an I-V curve at every pixel within the
topographic image. The position of the tip is held stationary and the tunneling current is
measured at various different bias voltages.[166] The tip to sample distance is defined by the
topography parameters. You can see the difference in the surface corresponding to the
difference in the I-V curves. The green part with the green curve stands for the surface, the
blue part with the corresponding blue curve represent the sample on the surface.[161]
Figure 20: Schemetic representation of the current imaging tunneling spectroscopy (CITS).[161]
Highly ordered pyrolytic graphite (HOPG)
As standard surface for STM a highly ordered pyrolytic graphite (HOPG) can be used, which
was also applied in the performed experiments of this work. Figure 21 displays the structure
of graphite, a layered structure with a hexagonal lattice of carbon atoms linked by strong sp2
bonds with a next-neighbour distance of only 142 pm. The layers are stacked in such a way
that three of the six atoms within the hexagon have direct neighbours in the layer underneath
at a distance of 334.8 pm. The electronic state of the valence electrons in graphite differs from
the ground state configuration in atomic carbon (1s22s22p2). One of the 2s electrons is
promoted to a 2p state and three electrons in the 2s, 2px and 2py states hybridize to sp2 states
lying in the xy plane. The fourth valence electron is in a 2pz-like state.[167-168] The 2pz states
have the lowest bonding energy. For a single sheet of graphite, the 2pz states at the α and β
General introduction
30
positions have the same energy. However, for crystalline graphite the overlap of the 2pz states
centred at the α sites leads to a lower bonding energy, leaving the 2pz states centred at the β
sites as a highest occupied (and lowest unoccupied) surface states. Adjacent atoms at α and β
sites are connected by extremely strong bonds, forming very durable layers in x-y plane.
However, in the z-direction these layers are weakly coupled.[167]
Figure 21: Crystal structure of graphite. The unit cell is shaded in green. (A) Top view on the surface layer. (B) Perspective view, showing the layered structure. "Copyright (2003) National Academy of Sciences, U.S.A."[167]
Figure 22 shows an STM image of graphite recorded in a constant height mode. In this STM
image, only the β atoms (red) are visible.[167-171] Theoretical investigations have shown that
only the β atoms, which have no direct neighbour in the subjacent layer, can be imaged by
STM and the α atoms remain hidden to STM. This is caused by the shift of the pz orbital to
lower energy.[159, 167]
Figure 22: STM image of graphite. β atoms (red) are visible, α atoms (white) are hidden. "Copyright (2003)
National Academy of Sciences, U.S.A."[167]
31
2 AIMS AND OBJECTIVES
Aims and Objectives
32
In the wide field of molecular electronics with its different concepts a lot of research has been
done during the last years. In 2011 N. V. FISCHER of the BURZLAFF group synthesized the new
N,N donor ligand trans-bis(N-methylimidazol-2-yl)ethylene (trans-bie) based on methyl-
imidazole and created a 1D coordination polymer with copper(II) acetate. The first aim was
an improvement of the synthetic rout to the trans-bie ligand. In cooperation with the MÜLLER
group of the physics department the properties as a molecular wire were investigated with
STM/CITS measurement, as described in the introduction.
One purpose of this work was to extend the scope on the coordination chemistry of trans-bie
in different dinuclear 1D coordination polymers. Therefore, the ligand should be reacted with
a number of paddlewheel complexes, bridged by four acetato groups. The influence of the
metal-metal bond order within a polymer linked with trans-bie, in dependence of the used
transition metal, was of interest. For further understanding theoretical calculations in
cooperation with C. WICK of the CLARK group from the Computer Chemie Centrum should be
performed.
In the second part of this work the synthesis of a new N,N donor ligand consisting of two
methylimidazole groups bridged by a bisacetylene unit was aimed in cooperation with T.
WAIDMANN. The ability of the ligand system to form a 1D coordination polymer should be
verified by single crystal X-ray structure determination. Furthermore, the photoluminescence
properties after excitation of the ligand system should be characterized in cooperation with
the GULDI group.
In the third part an alternative metal fragment, a so-called sawhorse type unit, should be
applied to study the coordination polymer properties in a wider context. These units consist of
a ruthenium(I) dinuclear unit, which is bridged by two acetato groups and contains two
terminal coordinated carbonmonoxide ligands on each ruthenium centre. Therefore, a number
of different N,N donor ligands should be used as linkers between these metal units. The
adsorption behaviour and electrical conductivity on a highly ordered pyrolytic graphite
(HOPG) surface should be extensively studied via STM/CITS measurements within this
chapter.
33
3 RESULTS AND DISCUSSION
Metal(II) acetate compounds
34
3.1 From single to quadruple bond metal(II) acetate complexes with trans-bie
3.1.1 Preparation and Characterisation of [Rh2(OAc)4(trans-bie)]n (4)
Starting from the previously described copper(II) acetate coordination polymer with trans-bie,
a comparable paddlewheel unit with a metal-metal bond should be applied to study the
coordination polymer properties in a wider context. Therefore, the isostructural rhodium(II)
acetate was used in a similar synthesis. Dirhodium carboxylate complexes in general are most
commonly obtained by reduction of Rh(III) compounds in alcohols which presumably act as
the reducing agent, but mechanistic details are unknown. The most efficient synthetic method
for dirhodium tetraacetate [Rh2(OAc)4] × 2 H2O involves heating of RhCl3 × 3 H2O to reflux
under N2 in a mixture of sodium acetate, acetic acid and EtOH.[113, 172-173] These complexes
generally are air-stable solids that readily form adducts with a variety of donor ligands, which
occupy the axial positions. A conspicuous feature of these compounds is the sensitivity of
their colours to the identity of the axial ligand. Blue or green products are usually obtained
with oxygen donor ligands, like [Rh2(OAc)4] × 2 H2O. Red or violet with nitrogen donor
ligands, like [Rh2(OAc)4(py)2], and orange for phosphorus donor ligands, like
[Rh2(OAc)4(PPh3)2], respectively.[174-175]
The polymer complex [Rh2(OAc)4(trans-bie)]n (4) was prepared by reacting of the trans-
bie (2)[145] ligand with rhodium(II) acetate in MeCN (Scheme 5).
Scheme 5: Synthesis of [Rh2(OAc)4(trans-bie)]n (4).
The elemental analysis indicates a 1:1 [Rh2(OAc)4] : trans-bie stoichiometry for the resulting
complex. In the IR spectrum of a powder sample (KBr pellet) of compound 4 the O-C-O
[Rh2(OAc)4] x 2 H2ORh Rh
n
O O
O OO O
OO
N N
N N
4
NN
N N
2
Metal(II) acetate compounds
35
vibrations (!asym(OCO) = 1592 cm–1, !sym(OCO) = 1420 cm–1) appear as a set of distinctive
two bands in a very similar energy region to the corresponding educt (!asym(OCO) = 1590 cm–
1, !sym(OCO) = 1414 cm–1).
For further investigation the solid UV/Vis absorption spectra was compared with the
corresponding educts. Therefore, a nujol mull of 4 was measured between two NaCl slides.
The band at 536 nm was assigned to be the π*(Rh–Rh) → σ*(Rh–Rh) transition, which is
strongly influenced by the axial ligand.[176] A hypsochromic shift of 63 nm in comparison to
[Rh2(OAc)4] × 2H2O (599 nm) is observed. The electronically allowed π(Rh–O) → σ*(Rh–O)
excitation appears at 456 nm.[176] The broad band around 330 nm should originate from the
coordinated trans-bie ligand (Appendix Figure 51).
The powder diffraction pattern of [Cu2(OAc)4(trans-bie)]n and [Rh2(OAc)4(trans-bie)]n (4)
look nearly congruent, which can be seen in Figure 23.
Figure 23: Comparison of the powder diffraction patter of [Cu2(OAc)4(trans-bie)]n and [Rh2(OAc)4(trans-bie)]n
(4).
10 20 30
0
100
200
300
Inte
nsity
[cts
]
! "#$%&'()*
+&t ra n s ,-./*0
n
! "12%&'()*
+&t ra n s ,-./*0
n
10 20 30 40 50 60 70 80
3*
43*
444*
44*
567.8.69! ":%Θ0
4*
! 567.8.69! ":%Θ0
[Cu2(OAc)4(trans-bie)]n [Rh2(OAc)4(trans-bie)]n
Metal(II) acetate compounds
36
Both spectra feature similar signals, which indicates that the [Cu2(OAc)4(trans-bie)]n and
[Rh2(OAc)4(trans-bie)]n (4) polymers are isostructural.
The conclusion of these findings is a preservation of the dimer skeletons upon a reaction with
the bidentate ligand trans-bie (2) to form linear coordination polymers comparable to the
already published [Cu2(OAc)4(trans-bie)]n.[95, 145]
Crystals of this complex suitable for X-ray diffraction determination were grown by layering
rhodium(II) acetate in THF with trans-bie in MeOH. The polymer [Rh2(OAc)4(trans-bie)]n (4)
crystallizes in the space group P 21/c. The asymmetric unit contains C27H36N6O12Rh3 × 2
MeOH, i.e. 1.5 paddlewheel and trans-bie units and two solvent molecules. A molecular
cutout of the polymer is illustrated in Figure 24 and selected bond lengths and angles are
given in Table 1.
Figure 24: Cutout of the molecular structure of [Rh2(OAc)4(trans-bie)]n (4); thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms and MeOH molecules have been omitted for clarity.
Table 1: Selected bond lengths and angles for [Rh2(OAc)4(trans-bie)]n (4).
Distances in Å Distances in Å
d (C1-C1) 1.345(6) d (Rh2-O1) 2.047 (2)
d (C11-C12) 1.336(4) d (Rh2-O3) 2.050(2)
d (Rh1-N12) 2.224(2) d (Rh2-O5) 2.047(2)
d (Rh2-N31) 2.256(2) d (Rh2-O7) 2.037(2)
d (Rh3-N21) 2.253(3) d (Rh3-O11) 2.048(2)
d (Rh1-O2) 2.032(2) d (Rh3-O12) 2.039(2)
d (Rh1-O4) 2.039(2) d (Rh3-O13) 2.036(2)
C
H
N
O
Rh
Rh1
Rh2 Rh3
Rh3
N31
N32
N21
N22
C12 C11
N12
N12
N12
N11
C1 C1
O1
O2
O3
O4
O5
O6
O7
O8
O11 O12
O13
O14
O11 O12
O13
O14
Metal(II) acetate compounds
37
d (Rh1-O6) 2.046(2) d (Rh3-O14) 2.052(2)
d (Rh1-O8) 2.061(2) d (Rh1-Rh2) 2.4105(4)
d (Rh3-Rh3) 2.4119(5)
Angles in °
∡ (N12-Rh1-Rh2) 176.34(6) ∡ (Rh1-Rh2-N31) 175.89(7)
∡ (N21-Rh3-Rh3) 174.27(7)
In the structure an infinite chain of Rh2(OAc)4 units bridged by the N,N donor ligand trans-bie
(2) can be observed. The paddlewheel unit consists of two rhodium centres, which are µ-
bridged by four acetato ligands. Each metal atom in this unit has a distorted square pyramidal
environment, with four oxygen atoms from acetates, to form the equatorial plane and one
nitrogen atom of the trans-bie ligand. In compound 4 in every second ligand an inversion
centre occurs between the double bond, which connects the two methylimidazole groups. In
every second paddlewheel unit an inversion centre can be found in the middle of the Rh-Rh
bond. The Rh-Rh bond distance of d(Rh1-Rh2) = 2.4105(4) Å, d(Rh3-Rh3) = 2.4119(5) Å
respectively, indicate a metal-metal single bond, similar to comparable dinuclear rhodium
complexes, such as 2.400(1) Å for [Rh2(OAc)4(py)2] or 2.384(1) Å for
Rh2(OAc)4(NCCH3)2].[113] Bond distances of the rhodium paddlewheel and the trans-bie
ligand have lengths of d(Rh1-N12) = 2.224(2) Å, d(Rh3-N21) = 2.253(3) Å and d(Rh2-
N31) = 2.256(2) Å. The Rh-O bond lengths of the polymer complex are almost identical in
the range of d(Rh1-O2) = 2.032(2) Å to d(Rh3-O14) = 2.052(2) Å. The Rh-Rh-N bond angles
of ∡ (N21-Rh3-Rh3) = 174.27(7)°, ∡ (Rh1-Rh2-N31) = 175.89(7)° and ∡ (N12-Rh1-
Rh2) = 176.34(6)°are indicating an almost ideal linearity of the chain structure.
Similar to the previously described [Cu2(OAc)4(trans-bie)]n we also tried to deposit the
[Rh2(OAc)4(trans-bie)]n (4) on a HOPG surface. However, it was possible to locate the
rhodium(II)-polymer 4 on the surface, but only bundles of strands were detected on the
surface so far , which are shown in Figure 25. The diameter of an observable strand is about
3 nm, which is considerable wider than the diameter of a single strand in the X-ray crystal
structure determination of about 1 nm. A lower concentration of the solution applied in the
deposition experiment did not lead to the favoured single strands. The influence of the metal-
metal bond in the rhodium paddlewheel unit is relatively low in comparison to the copper
polymer. A reason for the different adsorption behaviour between these isostructural polymers
Metal(II) acetate compounds
38
on the HOPG surface is so far not comprehensible. Probably this can be explained by a better
interaction between the single strands, compared to copper(II)-polymer on surface.
Nevertheless, it looks like the polymer is located at a flat area of the HOPG surface between
two edges and not along an edge of a defect line.
Figure 25: a) STM topographies of [Rh2(OAc)4(trans-bie)]n (4) on a HOPG surface. b) Enlargement of the area
in a).
3.1.2 Preparation and Characterization of [Ru2(OAc)4(trans-bie)]n (5)
After the copper(II) acetate and the rhodium(II) acetate polymers of trans-bie with no metal-
metal and a single metal-metal bond, respectively, we decided to diversify the paddlewheel
unit ever further. There we chose a dinuclear unit with a double bond between the metal
centres. This can be found in ruthenium(II) acetate compounds. Despite the fact that the mild
reduction potentials for Ru25+ (i.e. RuII/RuIII) tetracarboxylates indicated that the one-electron
reduced Ru24+ analogues were chemically accessible, it was not until 1984 that WILKINSON et
al. reported the synthesis of the first Ru24+ (i.e. RuII/RuII) tetracarboxylate
[Ru2(OAc)4] × 2 THF. An efficient method to synthesize this type of complexes is the
reaction of Na+ or Li+ salts of the appropriate carboxylic acids with a “blue solution of
RuCl3”, which is a MeOH solution of RuCl3 × H2O that has been reduced with H2.
A number of [Ru2(O2CR)4L2] complexes were made following this synthetic method, like
[Ru2(OAc)4(H2O)2] or [Ru2(O2CEt)4(acetone)2].[177] The Ru-Ru bond lengths of
[Ru2(O2CR)4L2] compounds are in the range of 2.252-2.311 Å. They do not show any
significant dependence on the substituent R of the carboxylate bridge or the type of the axial
donor ligand L.
88nm 3.0nm
a) b)
Metal(II) acetate compounds
39
By the reaction of ruthenium(II) acetate ([Ru2(OAc)4] × 2 THF ) with trans-bie (2) in THF the
coordination polymer [Ru2(OAc)4(trans-bie)]n (5) was obtained (Scheme 6).
Scheme 6: Synthesis of [Ru2(OAc)4(trans-bie)]n (5).
The elemental analysis confirmed that the resulting compound has a stoichiometry of
[Ru2(OAc)4] : trans-bie of 1:1. In the IR spectrum of a powder sample (KBr pellet) of
compound 5 the O-C-O vibrations appear as a set of distinctive two bands
(!asym(OCO) = 1569 cm–1, !sym(OCO) = 1429 cm–1). The electronic configuration in the d6-d6
Ru2+-Ru2+ unit should exhibit two unpaired electrons resulting in a Ru–Ru bond order of two.
For further understanding a magnetic susceptibility measurement was performed with a
Faraday Balance at room temperature. The result of 2.58 BM (Bohr Magnetone) indicates the
presence of two unpaired electrons.[113]
In the solid UV-Vis absorption spectra of 5 the band localised at 442 nm should originate
from the π(Ru–Ru) → π*(Ru–Ru) transition, with a small blue shift of 23 nm in comparison
to [Ru2(OAc)4] × 2 THF (465 nm).[178] The bands around 330 nm are assigned to the
coordinated trans-bie ligand (Appendix Figure 52).
Crystals of complex 5 suitable for a single-crystal structure determination were grown by
layering ruthenium(II) acetate in THF with trans-bie in MeCN. The substance crystallises in
the space group P1. The asymmetric unit consists of C9H12N2O4Ru × 0.5 THF. A molecular
cutout of the polymer is illustrated in Figure 26. Selected bond lengths and angles are given in
Table 2.
[Ru2(OAc)4] x 2 THFRu Ru
n
O O
O OO O
OO
N N
N N
5
NN
N N
2
Metal(II) acetate compounds
40
Figure 26: Cutout of the molecular structure of [Ru2(OAc)4(trans-bie)]n (5); thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms and THF molecules have been omitted for clarity.
Table 2: Selected bond lengths and angles of [Ru2(OAc)4(trans-bie)]n (5).
Distances in Å Distances in Å
d (C1-C1) 1.334(15) d (Ru-O1) 2.063(6)
d (C1-C11) 1.453(10) d (Ru-O2) 2.061(6)
d (Ru-N11) 2.325(7) d (Ru-O3) 2.074(5)
d (Ru-Ru) 2.2829(11) d (Ru-O4) 2.081(5)
Angles in °
∡ (N11-Ru-Ru) 173.89(17)
The structure of 5 consists of an infinite chain of [Ru2(OAc)4] units bridged by the trans-bie
ligand. The paddlewheel unit is constructed by two ruthenium centres, which are µ-bridged by
four acetato ligands. Each metal atom in this unit has a distorted square pyramidal
environment, with four oxygen atoms from the acetates, to form the equatorial plane, and one
nitrogen atom of the trans-bie ligand. The crystallographic inversion centres are located at the
midpoint of the Ru-Ru bond and the double bond, which connects the two imidazole groups.
The metal-metal bond distance of d(Ru-Ru) = 2.2829(11) Å is shorter than in the previously
shown rhodium(II) acetate polymer. The bond length can be compared with similar
complexes, like [Ru2(OAc)4(H2O)2] with 2.262(3) Å between the ruthenium centres.[113] The
distance between the paddlewheel unit and the trans-bie ligand is d(Ru-N11) = 2.325(7) Å.
The Ru-O bond lengths of the polymer complex are almost identical in the range of d(Ru-
a
bc
C
H
N
O
Ru
Ru
Ru O1
O1
O2 O2
O3
O3
O4
O4
N11
N11 N12
N12
C11
C1 C1
Metal(II) acetate compounds
41
O2) = 2.061(6) Å to d(Ru-O4) = 2.081(5) Å. The Ru-Ru-N bond angle of ∡ (N11-Ru-
Ru) = 173.89(17)° shows an already ideal linearity of the chain structure.
3.1.3 Preparation and Characterization of [Mo2(OAc)4(trans-bie)]n (6)
A database search e.g. Scifinder reveals that currently the total number of dinuclear Mo2n+
compounds amounts to approximately 1100, of which about 550 have been characterised
crystallographically. WILKINSION and co-workers first described the synthesis of the
carboxylates by heating the mononuclear starting material molybdenum hexacarbonyl with
carboxylic acid in diglyme.[179-184] Most of the research in the field followed closely on the
structural characterisation of molybdenum(II) acetate [Mo2(OAc)4] by COTTON et al.[185-186]
The metal-metal distance in [Mo2(OAc)4] is 2.093(1) Å, a typical value for Mo-Mo quadruple
bonds.[113] There is also a huge number of different axial coordinated paddlewheel complexes
known in literature.[113] One dimensional coordination polymers [Mo2(OAc)4L]n based on N,N
donors like pyrazine, 4,4´-bipyridine and 1,4-diazabicyclo[2.2.2]octane have also been
prepared and characterized.[95] This inspired us to start from a molybdenum(II) acetate unit as
a metal fragment for our polymer compounds.
The polymer complex [Mo2(OAc)4(trans-bie)]n (6) was prepared by the addition of the trans-
bie (2)[145] ligand with molybdenum(II) acetate in THF (Scheme 7).
Scheme 7: Synthesis of [Mo2(OAc)4(trans-bie)]n (6).
The elemental analysis of the resulting complex agrees well with a 1:1 stoichiometry
regarding [Mo2(OAc)4] : trans-bie. In the IR spectrum of a powder sample (KBr pellet) of
compound 6 the O-C-O vibrations (!asym(OCO) = 1527 cm–1, !sym(OCO) = 1433 cm–1) appear
Mo Mo
n
O O
O OO O
OO
N N
N N
6
NN
N N
2
[Mo2(OAc)4]
Metal(II) acetate compounds
42
as a set of distinctive two bands in a very similar energy region to these of the corresponding
educt (!asym(OCO) = 1520 cm–1, !sym(OCO) = 1443 cm–1).
In the solid UV/Vis absorption spectrum of 6 the lowest energy absorption band of weak
intensity is localised at 452 nm and was assigned to the δ(Mo–Mo) → δ*(Mo–Mo)
transition.[187] The more intense band around 343 nm might be caused by an overlap of the
trans-bie absorption band and the π(Mo–Mo) → π*(Mo–Mo) and the δ(Mo–Mo) → π*(Mo–
Mo) transition (Appendix Figure 53).[187]
Crystals of 6 suitable for X-ray diffraction determination were grown by layering
molybdenum(II) acetate in THF with trans-bie in MeCN. The substance crystallizes in the
space group P 21/c with the asymmetric unit C9H12N2O4Mo. A molecular cutout of the
polymer is depicted in Figure 27 and selected bond lengths and angles are given in Table 3.
Figure 27: Cutout of the molecular structure of [Mo2(OAc)4(trans-bie)]n (6); thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms have been omitted for clarity.
Table 3: Selected bond lengths and angles of [Mo2(OAc)4(trans-bie)]n (6).
Distances in Å Distances in Å
d (C1-C1) 1.339(3) d (Mo-O1) 2.1084(10)
d (C1-C11) 1.4482(17) d (Mo-O2) 2.1197(9)
d (Mo-N11) 2.7032(11) d (Mo-O3) 2.1129(9)
d (Mo-Mo) 2.1099(2) d (Mo-O4) 2.1243(9)
Angles in °
a
b
c
Mo
O
C
H
N
O2O2
O1
O1
O4
O4
O3
O3
Mo
Mo
N11
N11
N12
N12
C1
C1
C11
Metal(II) acetate compounds
43
∡ (N11-Mo-Mo) 163.32(2)
In the polymeric structure an infinite chain of [Mo2(OAc)4] units bridged by the N,N donor
ligand trans-bie (2) can be observed. The paddlewheel unit is formed by two molybdenum
centres, which are µ-bridged by four acetato ligands. Each metal atom in this unit has a
distorted square pyramidal environment, with four oxygen atoms from acetates, to form the
equatorial plane, and one nitrogen atom of the trans-bie ligand. In molecular structure of 6 the
crystallographic inversion centres are located at the midpoint of the Mo-Mo bond and the
double bond, which connects the two imidazole groups, just like in the previously described
polymer 5. The Mo-Mo bond distance d(Mo-Mo) = 2.1099(2) Å indicates a metal-metal
quadruple bond, similar to comparable complexes, such as 2.093(1) Å in [Mo2(OAc)4(pyz)]n
or 2.095(1) Å in [Mo2(OAc)4(DABCO)]n.[95] The bond lengths of the coordinating nitrogen
atom of the trans-bie ligand is d(Mo-N11) = 2.7032(14) Å. The Mo-O bond lengths of the
polymer complex varies within the range of d(Mo-O1) = 2.1084(10) Å to d(Mo-
O4) = 2.1243(9) Å. The Mo-Mo-N bond angle of ∡(N11-Mo-Mo) = 162.32(2)° indicates a
good linearity of the chain structure.
3.1.4 Preparation and Characterization of [Cr2(OAc)4(trans-bie)]n (7) and [Cr2(OAc)4(trans-bie)2] (8).
In 1844 PELIGOT and co-workers prepared and characterised chromium(II) acetate
[Cr2(OAc)4] for the first time. Dichromium tetracarboxylato compounds are generally air-
sensitive, especially in solution. The dinuclear paddlewheel complex was made by the
addition of NaOAc in approximately stoichiometric quantity to a freshly prepared aqueous
solution of CrCl2.[188] Chromium is unique among the elements of the first transition metal
series in its ability to form many compounds with multiple bonds in dinuclear Cr24+
complexes, like [Cr2(OAc)4(H2O)2], [Cr2(OAc)4(py)2] or [Cr2(OAc)4(pyz)2].[87, 189] While many
if not all of these can be formally called quadruple bonds, in the sense that they entail one σ ,
two π and one δ bond. The strengths of these bonds, as indicated by the Cr-Cr distances, vary
widely with Cr-Cr distances ranging from 1.83 Å to 2.60 Å. Nevertheless a general similarity
in geometrical paddlewheel arrangement of the ligands can be observed.[113] The axial
positions are occupied either by separate ligands L or by oxygen atoms of other Cr2(O2CR)4
molecules.
Metal(II) acetate compounds
44
This inspired us to use the [Cr2(OAc)4] metal fragment as a dinuclear unit for a comparable
polymer to the molybdenum(II) acetate polymer 6, although there are no polymeric structures
of chromium(II) acetate characterised by X-ray crystal structure determination known in
literature so far. The polymer complex [Cr2(OAc)4(trans-bie)]n (7) was synthesized by the
reaction of chromium(II) acetate with trans-bie in MeCN (Scheme 8).
Scheme 8: Synthesis of [Cr2(OAc)4(trans-bie)]n (7).
According to the elemental analysis the reaction product has a stoichiometry of
[Cr2(OAc)4] : trans-bie of 1:1. In the IR spectrum of a powder sample (KBr pellet) of
compound 7 the O-C-O vibrations appear as a set of distinctive two bands
(!asym(OCO) = 1597 cm–1, !sym(OCO) = 1437 cm–1) in a very similar energy region to their
educt [Cr2(OAc)4] × 2H2O (!asym(OCO) = 1550 cm–1, !sym(OCO) = 1453 cm–1).
The solid UV/Vis absorption spectrum of 7 shows a broad maximum around 330 nm which is
assigned to overlapping trans-bie and δ(Cr–Cr) → π*(Cr–Cr) transitions of the chromium(II)
acetate unit spectrum (Appendix Figure 54).[190]
No single crystals of the coordination polymer could be obtained so far. But instead the
dinuclear paddlewheel complex [Cr2(OAc)4(trans-bie)2] (8) was crystallized by layering
chromium(II) acetate in THF with trans-bie in MeCN (Scheme 9).
Cr Cr
n
O O
O OO O
OO
N N
N N
7
NN
N N
2
[Cr2(OAc)4] x 2 H2O
Metal(II) acetate compounds
45
Scheme 9: Synthesis of [Cr2(OAc)4(trans-bie)2] (8).
The elemental analysis backs that the complex has a [Cr2(OAc)4] : trans-bie stoichiometry of
1:2 in this case. The IR spectrum of the compound showed nearly identical O-C-O vibrations,
respectively ! asym(OCO) = 1597 cm–1, ! sym(OCO) = 1436 cm–1 for 8 compared to 7. The
substance crystallizes in the space group Pbca. The dinuclear compound C28H36Cr2N8O8 fills
the asymmetric unit. The molecular structure of the complex is depicted in Figure 28 and
selected bond lengths and angles are given in Table 4.
Figure 28: The molecular structure of [Cr2(OAc)4(trans-bie)2] (8); thermal ellipsoids are drawn at the 50%
probability level. Hydrogen atoms have been omitted for clarity.
[Cr2(OAc)4] x 2 H2ONN
N N
2
Cr Cr
O O
O OO O
OO
N N
N N
N N
N N
8
THF/MeCN
N11
N12
N21
N22
O2
O1
O1
O2
O3O3
O4
O4 Cr
Cr
C11
C1 C2
C21
Metal(II) acetate compounds
46
Table 4: Selected bond lengths and angles for [Cr2(OAc)4(trans-bie)2] (8).
Distances in Å Distances in Å
d (C1-C2) 1.338(2) d (Cr-O1) 2.0121(11)
d (C1-C11) 1.445(2) d (Cr-O2) 2.0337(11)
d (Cr-N11) 2.3243(12) d (Cr-O3) 2.0244(11)
d (Cr-Cr) 2.4517(5) d (Cr-O4) 2.0264(11)
Angles in ° Angles in °
∡ (N11-Cr-Cr) 178.56(3) ∡ (N11-C11-C21-N21) 9.5
The paddlewheel unit is formed by two chromium centres, which are κ2-bridged by four
acetato ligands. Each metal atom in this unit has a distorted square pyramidal environment,
with four oxygen atoms of the acetato ligands to form the equatorial plane, and one nitrogen
atom of the trans-bie ligand. The structure of [Cr2(OAc)4(trans-bie)2] (8) shows one inversion
centre located in the midpoint of the Cr-Cr bond. The Cr-Cr bond distance of d(Cr-
Cr) = 2.4517(5) Å indicates a metal-metal quadruple bond, comparable to similar complexes
like [Cr2(OAc)4(H2O)2], [Cr2(OAc)4(py)2] or [Cr2(OAc)4(pyz)2] (Table 5). The Cr-O bond
lengths of the complex varies within the range of d(Cr-O3) = 2.0244(11) Å to d(Cr-
O1) = 2.0121(11) Å. A comparison of the axial M-L bond length in 6 and 8 exhibited a
slightly longer bond in case of [Mo2(OAc)4(trans-bie)]n (6) compared to that in the dinuclear
complex [Cr2(OAc)4(trans-bie)2] (8) d(Cr-N11) = 2.3243(12) Å. This difference can be
explained best by the different interaction of [Cr2(O2CR)4] and [Mo2(O2CR)4] units with the
axial ligands respectively. The tendency of [Cr2(O2CR)4] to bind the axial ligand is much
stronger than that of [Mo2(O2CR)4], probably due to the difference between the 3d and 4d
orbital overlaps.[95] Another reason might be the more oxophilic chromium centre or the
difference in the atomic radii of the metal centres. Within the trans-bie ligand a slight
deviation of 9.5° from planarity is observed. To the best of our knowledge the crystallisation
of polymer complexes of the general form [Cr2(OAc)4(L)]n have never been described in
literature so far.[113] The Cr-Cr bond length (d(Cr-Cr) = 2.4517(5) Å) in complex 8 is the
longest in comparison to similar complexes like [Cr2(OAc)4(H2O)2], [Cr2(OAc)4(py)2] or
[Cr2(OAc)4(pyz)2].[87, 113, 189] The coordinated N-atom of the imidazole based ligand shows an
analogue interaction with the chromium centre, with distances around 2.3 Å, which is shown
in Table 5.[87, 113, 189]
Metal(II) acetate compounds
47
Table 5: Selected bond lengths of [Cr2(OAc)4(L)2] complexes.
Complex Cr-Cr distances in Å Cr-L distances in Å
[Cr2(OAc)4(H2O)2] 2.362(1)[189] 2.272(3)[189]
[Cr2(OAc)4(py)2] 2.369(2)[87] 2.335(5)[87]
[Cr2(OAc)4(pyz)2] 2.295 (5)[87] 2.314(10)[87]
[Cr2(OAc)4(trans-bie)2] (8) 2.4517(5) 2.3243(12)
3.1.5 Preparation and Characterisation of [Zn3(OAc)6(trans-bie)]n (9)
To compare the previously described paddlewheel based polymers with another species of
dinuclear complexes, zinc(II) acetate was used as a starting material. Several zinc
paddlewheel complexes with non-bridging and bridging N-donor ligands have been discussed
in literature, e.g. [Zn2(OAc)4(py)2] or the coordination polymer complex
[Zn2(OAc)4(bpe)]n.[191-192]
By layering trans-bie (2) in THF with Zn(OAc)2 × H2O in MeOH the coordination polymer
[Zn2(OAc)4(trans-bie)]n was aimed, but surprisingly the elemental analysis of the received
crystals suggested a polymer complex with the molecular formula of [Zn3(OAc)6(trans-bie)]n
(9) (Scheme 10).
In the IR spectrum of a powder sample (KBr pellet) of the compound 9 the O-C-O vibrations
appear as a set in the region on 1651 to 1422 cm–1, in a very similar energy region to its educt
Zn(OAc)2 × H2O (!asym(OCO) = 1559 cm–1, !sym(OCO) = 1448 cm–1). Nevertheless the number
of the signals in this region indicates different binding modes of the carboxylate groups,
which can be seen in the crystal structure.
Metal(II) acetate compounds
48
Scheme 10: Synthesis of [Zn3(OAc)6(trans-bie)]n (9).
Instead of the paddlewheel-polymer of zinc(II) acetate a polymer containing a trinuclear zinc
unit was formed. The substance crystallizes in the space group P1. The asymmetric unit is
filled by C11H15N2O6Zn1.5. A molecular cutout of the polymer is illustrated in Figure 29 and
selected bond lengths and angles are given in Table 6.
NZn Zn Zn
OOOO
O OO OO
O
O
O
N
N N
n
Zn Zn
n
O O
O OO O
OO
N N
N N
9
NN
N N
2
Zn(OAc)2 x 2 H2O
Metal(II) acetate compounds
49
Figure 29: Cutout of the molecular structure of [Zn3(OAc)6(trans-bie)]n (9); thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms have been omitted for clarity.
Table 6: Selected bond lengths and angles of [Zn3(OAc)6(trans-bie)]n (9).
Distances in Å Distances in Å
d (C15-C15) 1.335(3) d (Zn2-O1) 1.9609(12)
d (C15-C11) 1.443(2) d (Zn2-O3) 1.9376(13)
d (Zn2-N11) 2.3258(17) d (Zn2-O5) 1.9567(13)
d (Zn2-Zn1) 3.2272(14) d (Zn2-O6) 2.8300(15)
d (Zn1-O2) 2.0320(12)
d (Zn1-O4) 2.1275(13)
d (Zn1-O5) 2.1384(14)
The trinuclear Zn(II) acetate unit consists of six bridging acetato ligands and similar to
previously described trinuclear zinc carboxylates presented by KIM and KIM et al. with axial
N- or O-donor ligands.[193-196] There are inversion centres between the double bond of the
trans-bie ligand, which connects the imidazole groups and central zinc atom. Out of three Zn
centres, the central Zn is hexa-coordinated by six acetato groups forming a distorted
octahedral geometry. The other two Zn centres are symmetrical to each other and each has a
bridging trans-bie ligand and three oxygen donors from the acetato ligand to form a
tetrahedrally coordinated metal centre. Two different types of carboxylate coordination were
found in this unit. Four acetato ligands are κ2 coordinated by both oxygen atoms of a
carboxylate group, forming syn–syn bridges between central and terminal zinc ions. The Zn-
O bond lengths of the polymer variegate in the range of d(Zn-O1) = 1.9609(12) Å to d(Zn-
a
b c
C
H
N
O
Zn
N11N12
C15C15
Zn2
Zn2
Zn1O6O5
O1
O1
O3
O3
O4
O4
O2
O2
O5 O6
Metal(II) acetate compounds
50
O5) = 2.1384(14) Å. The other two acetato ligands are κ1 coordinated and function as
monatomic Zn–O–Zn bridges with distances of d(Zn1-O5) = 2.1384(14) Å and d(Zn2-
O5) = 1.9567(13) Å. One oxygen atom which remains uncoordinated interacts weakly with
the zinc atom (Zn2-O6 = 2.8300(15) Å).[197] The bond lengths of the coordinated nitrogen
atom of the trans-bie ligand is d(Zn2-N11) = 2.3258(17) Å, which is slightly longer in
comparison to trinuclear Zn(II) benzoate polymer linked with 1,2-di(4-pyridyl)ethylene (bpe)
with a lengths of d(Zn-N) = 2.036(2) – 2.075(2) Å.[198] The Zn…Zn distance of 3.2272(14) Å is
much longer than in similar dinuclear paddlewheel compounds, for example the dinuclear
Zn(II) benzoate complex coordinated with two trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene
ligands (d(Zn-Zn) = 2.971(8) Å).[193, 198-199] In comparison to similar trinuclear Zn complexes,
([Zn3(MeCH=CHCO2)6(C9H7N)2], d(Zn…Zn) = 3.264 Å) the Zn…Zn distance is nearly
identical.[200]
3.1.6 Electronic structure and bond orders of binuclear units within polymers
In cooperation with C. WICK from the CLARK group a number of theoretical calculations were
performed.
To obtain a more comprehensive description of the nature of the metal-metal bonds and the
electronic structure of the dinuclear units, we investigated single [M2(OAc)4(trans-bie)2] units
with DFT and multi-configuration state averaged CASSCF (SA-CASSCF) followed by
second-order perturbation theory (CASPT2) calculations. We first focus on the new Rh, Ru
and Mo complexes and then also give a short description of the bonding in the Cu complex
reported previously.
Metal(II) acetate compounds
51
Figure 30: Energy difference between the lowest singlet and triplet states of [M2(OAc)4(trans-bie)2] (M = Rh,
Ru, Mo).
Since in DFT the results depend strongly on the exchange and correlation functionals, we
compared three different functionals (BP86[201-202], B3LYP[203-205] and PBE0[206-207]). The first is
a pure density functional, while the other two are hybrid functionals and include a Hartree-
Fock (HF) exchange. DFT is a single determinantal method and the correlation energy of the
complexes is included by the correlation functional and, to a certain amount, by the HF
exchange in hybrid functionals. CASSCF calculations define the multi-determinantal
character and therefore the static correlation, while additional CASPT2 calculations are
needed to capture dynamic correlation. The choice of the active space is crucial in CASSCF.
The bonding and anti-bonding M-M 4d σ, δ and two sets of π orbitals were chosen as active
orbitals. Together with four carboxylate-metal σ(M-O) and σ*(M-O) orbitals the active space
Metal(II) acetate compounds
52
consists of n electrons in 12 orbitals (Mo: n = 12; Ru: n = 16; Rh: n = 18) and will be
referenced as CASSCF(n,12). The active orbitals are visualized in the Appendix.
The energy differences between the lowest spin states (singlet and triplet states) are
summarized in Figure 30. The three functionals give consistent results for all three complexes
with a well-defined triplet ground state for 5 (M = Ru), around 19 kcal mol–1 below the singlet
and an even clearer singlet ground state for 4 (M = Rh), which is calculated to lie between 41
and 49 kcal mol–1 below the triplet state and for 6 (M = Mo), whose singlet ground state lies
19 to 34 kcal mol–1 lower in energy than the triplet.
CASSCF/CASPT2 agrees with the DFT results that the Rh and Mo complexes 4 and 6 have a 1Ag ground state, and the Ru complex 5 has 3Ag. The singlet-triplet gap obtained with CASPT2
is reduced to 10 kcal mol–1 for complex 5 (Figure 30). For complexes 4 and 6 the BP86 results
agree better with the CASPT2 energies than the two hybrid functionals. However, both
methods predict the expected singlet ground states for dinuclear Mo(II) carboxylate and
Rh(II) carboxylate complexes and a triplet ground state for a dinuclear Ru(II) carboxylate
complex.[113]
Table 7: CASSCF configurations of the lowest singlet and triplet states of [M2(OAc)4(trans-bie)2] (M = Rh, Ru,
Mo)
complex weight configuration State
[Rh2(OAc)4(trans-bie)2] (4) 88 % σ(M-O)4 σ2 π4 δ2 δ*2π*4 1Ag
59 % σ(M-O)4 σ2 π4 δ2 δ*2 π*3 σ*(M-O)1 3Au
19 % σ(M-O)4 σ2 π3 δ2 δ*2 π*4 σ*(M-O)1
[Ru2(OAc)4(trans-bie)2] (5) 83 % σ(M-O)4 σ2 π4 δ2 δ*2 π*2 3Ag
51 % σ(M-O)4 σ2 π4 δ2 δ*2 1π*2 2π*0 1Ag
16 % σ(M-O)4 σ2 π4 δ2 δ*2 1π*0 2π*2
9 % σ(M-O)4 σ2 π4 δ2 δ*2 1π*1 2π*1
[Mo2(OAc)4(trans-bie)2] (6) 73 % σ(M-O)4 σ2 π4 δ2 1Ag
8 % σ(M-O)4 σ2 π4 δ*2
80 % σ(M-O)4 σ2 π4 δ1 δ*1 3Au
The configurations with the highest contributions to the CASSCF wavefunctions for the
lowest singlet and triplet states are summarized in Table 7. The CASSCF(12,12)
Metal(II) acetate compounds
53
wavefunction of the 1Ag ground state of the Mo complex 6 is dominated to 73 % by a
σ(M-O)4σ2π4δ2 configuration, followed by the double excitation σ(M-O)4 σ2π4 δ*2 with 8 %.
The 3Ag state corresponds to a δ → δ* excitation.
The 3Ag state of the Ru complex 5 is dominated by a σ(M-O)4 σ2 π4 δ2 δ*2π*2 configuration
(83%). This is consistent with structural considerations and theoretical evidence on different
dinuclear Ru(II) carboxylate complexes, as summarized in ref [113]. The first singlet state has
strong multi-configurational character and the electrons occupy either one (51%) or the other
(16%) of both π* orbitals. The lowest singlet and triplet states differ in configurations of the
same sets of orbitals in both complexes 4 and 5. Furthermore, the triplet states have only one
dominant configuration with weights above 80 % in both cases. This indicates that the triplet
high-spin states should be well described by a single determinant DFT wavefunction.
However, the multi-configurational low-spin states may be better described by the broken-
symmetry (BS) solution as introduced by NOODLEMAN within the DFT framework.[208-209] The
idea behind this approach is that the variationally determined single-determinant BS state is
not a pure spin state but a mixture of “ionic” and “neutral” contributions as pointed out by
NEESE.[210] The (intended) spin-contamination problem of such wavefunctions can be
addressed by a spin projection technique introduced by NOODLEMAN[208] for the weak
coupling limit and improved by YAMAGUCHI[211-212] for the complete coupling regime (see
computational details section). The singlet-triplet splittings obtained with the BS approach
(YAMAGUCHI corrected) for the Ru complex 5 are lowered and in good agreement with the
CASPT2 results (Figure 30). For complex 6 (M = Mo) the broken symmetry results with the
hybrid functionals are also closer to the CASPT2 result, confirming that the closed-shell
singlet is too high in energy. However, in the case of the pure functional BP86 functional, the
BS method converged to the closed-shell wavefunction. Additional tests were performed to
confirm the stability of this wavefunction (Appendix). This behaviour may be attributable to
the overly delocalized nature of pure DFT and the overly local nature of HF, which makes a
BS solution more likely with increasing HF exchange in hybrid functionals.[213]
Metal(II) acetate compounds
54
Table 8: The MBO between the metal centres in [M2(OAc)4(trans-bie)2] (4-6) calculated with different functionals compared to the EBO from CASSCF. In case of Mo the MBO obtained from the closed shell
wavefunction is given in parenthesis.
CASSCF EBO MBO
σ π δ total BP86 B3LYP PBE0
Rh-Rh 0.82 0.00 0.01 0.83 0.76 0.77 0.80
Ru-Ru 0.87 0.81 0.00 1.68 1.70 1.73 1.76
Mo-Mo 0.90 1.74 0.72 3.36 3.32 3.30 (3.43) 3.16 (3.48)
Based on the wavefunctions obtained, we can now proceed to analyse the different metal-
metal bonds. The metal-metal bond orders obtained at different levels of theory are
summarized in Table 8. The MBOs all lie below the maximum possible formal metal-metal
bond orders. A theoretical single bond in the Rh complex 4 and a double bond in the Ru
complex 5 are consistently reduced to bond orders of 0.8 and 1.7, respectively. However, the
bond order of the Mo-Mo bond in complex 6 is slightly more dependent on the functional, but
in all cases much lower than the maximum formal bond order of four in a full quadruple bond.
Interestingly, the bond orders of the BS wavefunctions for the hybrid functionals are the
lowest ones obtained, while their closed shell counterparts have higher MBO.
The MBO agrees well with the CASSCF EBO for all three complexes, with bond orders of
approximately 0.8 for 4 (M = Rh), 1.7 for 52 (M = Ru) and 3.4 for complex 6 (M = Mo). The
closed-shell singlet states calculated with DFT give MBOs higher than the EBO values, while
those calculated from the BS wavefunctions are in good agreement.
The EBO can also be calculated for individual bonding and anti-bonding orbital pairs. In the
case of the Mo complex, σ, π and δ orbital interactions all contribute to the total bond order
with decreasing values. The additional four electrons in the case of the Ru complex 5 lead to
zero bonding between δ orbitals because the anti-bonding orbital is filled completely. Only σ
contributions can be found for the Rh complex 4, because all π and δ bonding and anti-
bonding orbitals are filled.
The metal-to-ligand bond orders are summarized in Table 9. The M-O bond orders do not
vary much between the different transition metals, however the Mo-O bonds gave the highest
values, despite their bond lengths being marginally longer (about 0.06 Å) than Ru-O and Rh-
O. The M-N bond orders show a clear trend. The MBO decreases in the order Rh > Ru > Mo,
Metal(II) acetate compounds
55
in agreement with the trends inferred from geometrical criteria; the bond lengths increase and
the N-M-M angles deviate more from linearity in the same order.
Table 9: Mayer bond orders calculated with different functionals for metal-ligand interactions in
[M2(OAc)4(trans-bie)2] (4-6). For 6 (M = Mo), the MBO obtained from the closed-shell wavefunction is given in parentheses.
BP86 B3LYP PBE0
Mo Ru Rh Mo Ru Rh Mo Ru Rh
M-N 0.20 0.29 0.33 0.18 (0.18) 0.26 0.29 0.20 (0.20) 0.28 0.32
M-Oa 0.60 0.51 0.52 0.52 (0.57) 0.45 0.46 0.55 (0.57) 0.48 0.49 a Because the complex has Ci symmetry, the values for four slightly different M-O interactions were averaged.
A possible bonding between the metals in the copper(II) acetate, which is the starting material
of the earlier described Cu(II) polymer,[145] has been discussed in detail.[105-107] Magnetic
measurements on Cu paddlewheel complexes indicated a very weak antiferromagnetic
coupling between the metals,[105, 108-109] and recent theoretical investigations by GUIHÉRY, DE
GRAAF and NEESE et al. and also from KLOPPER et al. showed the difficulties in predicting the
coupling constants or zero-field splitting.[110-111] We use the same minimum active space of
two electrons in two σ*(M-O) orbitals for our CASSCF/CASPT2 calculations on the Cu
complex as used in these recent investigations. BS DFT calculations and CASPT2(2,2) both
predict a weak antiferromagnetic coupling (Table 10). The difference of 0.6 to 0.9 kcal mol–1
between CASPT2 and the hybrid functionals and a 3.0 kcal mol–1 difference to BP86
resembles the one reported for the hydrated and anhydrous Cu(II) acetate and formate
complexes.[111] All bond orders obtained indicate zero bonding between the metal atoms.
CASSCF(2,2) shows two configurations with almost equal weights (0.49 and 0.51) for the 1Ag
state. Both electrons occupy either one or the other of both σ*(M-O) orbitals.
The metal-nitrogen coordination is the strongest in this series of trans-bie complexes for Cu
with an MBO of 0.5.
Table 10: Triplet-singlet gap (ΔE) and bond orders (BO) for [Cu2(OAc)4(trans-bie)2]. The EBO is obtained from
CASSCF and the MBO from DFT.
CASPT2 BP86 B3LYP PBE0
DE [kcal mol–1] 0.5 3.5 1.4 1.1
EBO/MBO Cu-Cu –0.02 0.12 >0.10 >0.10 MBO Cu-N 0.51 0.49 0.51
Metal(II) acetate compounds
56
3.1.7 Comparison of the metal(II) acetate compounds
By the reaction of trans-bie (2) with different metal(II) acetates, six different complexes were
obtained. 1D coordination polymers of rhodium-, ruthenium-, chromium-, molybdenum- and
zinc(II) acetate were build. The structures are nearly identical, except for two complexes. A
dinuclear metal unit with four bridging acetato group forms the so-called paddlewheel unit.
The IR spectra of the powder samples (KBr) pellets) of the polymers exhibits the O-C-O
vibrations as a set of two bands in a very similar region to the corresponding starting material.
Table 11: Summary of observed vibrational bands (KBr) of the polymeric complexes 4 - 6 with their educts.
Complex !asym (OCO) [cm–1] !sym (OCO) [cm–1] Δ [cm–1]
[Rh2(OAc)4(H2O)2] 1590 1414 176
[Rh2(OAc)4(trans-bie)]n (4) 1592 1420 172
[Ru2(OAc)4(THF)2] 1560 1440 120
[Ru2(OAc)4(trans-bie)]n (5) 1569 1429 140
[Mo2(OAc)4] 1520 1443 77
[Mo2(OAc)4(trans-bie)]n (6) 1527 1433 94
The Δ values of the asymmetric and symmetric O-C-O vibrations indicate the presence of
bridging acetato groups.[214] The Δ values of the educts and the corresponding polymers are
nearly the same. Furthermore, they are all in the range proposed for bridged compounds,[214]
also for molybdenum, which is known to have smaller values.[95]
Scheme 11: General synthetic route to build 1D coordination polymers with trans-bie (2).
M M
n
O O
O OO O
OO
N N
N N
NN
N N
2
[M2(OAc)4]
M = Rh, Ru, Cr, Mo
Metal(II) acetate compounds
57
This suggests that the dinuclear skeletons are preserved on the reaction with the bidentate
ligand trans-bie (2). These paddlewheel units are linked with the N,N donor ligands trans-bie
(2) to build up the polymer (Scheme 11). In all polymers a correlation between the M-M bond
and the axial coordination can be shown. All M-M distances increase with a coordination of
the trans-bie (2) ligand compared to their educts. [Cu2(OAc)4(trans-bie)]n shows the longest
distance between the metal centres with d(Cu-Cu) = 2.6143(17) Å. From [Rh2(OAc)4(trans-
bie)]n (4) with a single bond between the metal centres over [Ru2(OAc)4(trans-bie)]n (5) with a
double bond to the quadruple bond in [Mo2(OAc)4(trans-bie)]n (6), the length decreases to
d(Mo-Mo) = 2.1099(2) Å. It was also found that a strong M-M bond leads to a weaker
coordination of the nitrogen donor which can be seen in the longest M-N distance in polymer
6 d(M-N) = 2.7032(11) Å, as previously described.
Table 12: Selected bond lengths of [M2(OAc)4(trans-bie)]n (M = Cu, Rh, Ru, Mo) and their educts.
Complex M-M distances in Å M-N distances in Å
[Cu2(OAc)4(H2O)2] 2.6143(17)[103] ---
[Cu2(OAc)4(trans-bie)]n 2.6868(4)[145] 2.1540(16)[145]
[Rh2(OAc)4(H2O)2] 2.386(1)[97, 189] ---
[Rh2(OAc)4(trans-bie)]n (4) 2.4105(4), 2.4119(5) 2.224(2), 2.256(2), 2.253(3)
[Ru2(OAc)4(THF)2] 2.260(2)[215] ---
[Ru2(OAc)4(trans-bie)]n (5) 2.2829(11) 2.325(7)
[Mo2(OAc)4] 2.091(1)[216] ---
[Mo2(OAc)4(trans-bie)]n (6) 2.1099(2) 2.7032(11)
There are two exceptions. On the one hand the polymer [Cr2(OAc)4(trans-bie)]n (7) could not
be crystallized, instead the complex [Cr2(OAc)4(trans-bie)2] (8) was obtained. Here the
chromium(II) acetate paddlewheel unit is coordinated by two trans-bie ligands, one on each
metal centre. On the other hand the reaction of zinc(II) acetate resulted in a 1D coordination
polymer with a trinuclear zinc unit, which is linked with trans-bie (2), to form
[Zn3(OAc)6(trans-bie)]n (9). Of course in this series of complexes a paddlewheel polymer of
d3-d3 atoms, which gives a triply bounded dinuclear unit, is missing.[113] The only neutral
complex would be a V2(O2CH)4.[114] The calculations clearly show the possible existence of
paddlewheel molecules and predict a V-V- triple bond length between 2.0 and 2.1 Å.[115]
However, all efforts to synthesise V2(carboxylato)4 compounds failed, hence, there is no
suitable triple bounded paddlewheel precursor available.[116]
1D coordination polymer of bmib
58
3.2 Battlement shaped 1D coordination polymer based on bmib
3.2.1 Synthesis and Characterization of bmib (3)
Based on the N,N donor ligand trans-bie (2) in cooperation with T. WAIDMANN the synthetic
route leading to another ligand was developed. The idea was to get a methylimidazole based
precursor where the two imidazole subunits should be connected via a bridge of carbon
atoms, with alternating single and triple bonds. These requirements were given by the linear
ligand bis(N-methylimidazol-2-yl)butadiyne (bmib) (3). T. WAIDMANN developed two
synthetic routs towards bmib (3). The relevant route for this work was the second approach,
which will be introduced in detail later.
The first approach to get hands on the bmib (3) ligand started with 1-methylimidazole-2-
carbaldehyde (II) that is accessible from 1-methylimidazole (I).[217] A Wittig reaction of II
and chloromethylen-(triphenyl)phosphine, resulted in 2-(2’-chlorovinyl)-N-methylimidazole
which was obtained as a 2:1 mixture of E/Z isomers III a,b. Treating 2-(2’-chlorovinyl)-N-
methylimidazole (III a,b) with KOtBu yielded the 2-ethynyl-N-methylimidazole (VI) suitable
for a Glaser coupling (Scheme 12).
Scheme 12: General synthetic route, leading to bmib (3). a) n-BuLi, DMF, THF, b) n-BuLi,
(chloromethyl)triphenylphosphonium chloride, THF, c) KOtBu, NH4Cl, THF, d) n-BuLi, I2, THF, e) Pd(PPh3)2Cl2, CuI, Et3N, DMF, TMS-acetylene, f) KF, MeOH, g) CuCl, O2, pyridine.[218]
Due to a rather cumbersome purification of III a,b, mostly caused by unreacted Wittig
reagent, we decided to follow another route to 2-ethynyl-N-methylimidazole (VI).
N
N
N
NI
N
N O
N
NTMS
N
N
N
N Cl
N N
N N
I
3
II III a,b
IV V
VI
a)
b)
c)
d)
e)
f)
g)
1D coordination polymer of bmib
59
This second approach involved 2-iodo-1-methylimidazole (IV). 1-methylimidazole (I) was
deprotonated with n-BuLi at –90 °C in THF, then warmed to 0 °C. After 5 min the reaction
mixture was cooled to –60 °C and treated dropwise with a solution of I2 in THF. After 20 min
at 0 °C the mixture was quenched by adding a Na2S2O3 solution.[219] The resulting imidazole
IV was treated under Sonogashira coupling conditions to yield 2-trimethylsilyethynyl-1-
methylimidazole (V).[220] 2-iodo-1-methylimidazole (IV), [Pd(PPh3)2Cl2] and CuI dissolved in
NEt3 reacted with trimethylsilylacetylene for 3.5 h at 70 °C. After workup the protecting
group was removed with KF in MeOH over a period of 2 h. In the final step bis(N-
methylimidazol-2-yl)butadiyne (bmib) (3) was obtained by a homocoupling Glaser reaction
of VI. Therefore, two equivalents were reacted in pyridine catalysed by CuCl under O2-
atmosphere at 40 °C for 4 h (Scheme 12). In the 1H NMR spectrum the methyl groups appear
at 3.70 ppm, the imidazole protons are located at 6.89 and 7.03 ppm. All signals appear as
singlets. In the 13C NMR spectrum the methyl groups can be found at 33.9 ppm, the acetylenic
carbon atoms at 73.7 and 76.5 ppm and the imidazole carbon atoms at 124.6, 131.0 and
132.2 ppm.[218]
bmib (7) crystallizes in the space group P21/c and the result of an X-ray single crystal
structure determination shows the presence of intermolecular π-stacking (Figure 31).[218]
Figure 31: Molecular structure of bmib (3).[218]
The average distance between the centres of the stacking imidazoles is approximately 3.6-
3.7 Å. Bond distances of the acetylene chain show prevalent values with a triple bond
distance of 1.202 Å.[218, 221-222]
In cooperation with M. RUDOLF from the GULDI group fluorescence measurement were
performed. Similar to related compounds,[223] the absorption spectrum (Figure 32) of bmib (3)
ab
c
C
H
N
C1C2C3C4
C24
C22
C23C21 C11
C14
C12
C13N11
N12
N21
N22
1D coordination polymer of bmib
60
gives rise to vibronic transitions in the 280 to 350 nm range with maxima at 292, 308, 328,
350 nm with an energetic spacing of 2000 cm–1. These are assigned to the triple bond centered
π-π* excited state. Following excitation at a wavelength of 300 nm an emission spectrum
evolves that gives rise to a 115 nm redshifted maximum at 415 nm. Emission quantum yields
were as high as 1.5 × 10–3 and 1.1 × 10–3 in CHCl3 and THF, respectively.[218]
Figure 32: The absorption (solid line) and emission (dotted line) spectra for bmib (3) in CHCl3 – 300 nm
excitation wavelength.[218]
Further insights into the excited state deactivation of bmib (3) came from transient absorption
measurements following femtosecond and nanosecond excitation in THF (Figure 33 and 34).
Upon 258 nm excitation we note intense transient absorption features throughout the visible
and near-infrared region, which maximize at 490, 555 and 939 nm. We interpret these
differential absorption changes as the population of the singlet excited state of bmib (3).
1D coordination polymer of bmib
61
Figure 33: Differential absorption spectra (visible and near-infrared) obtained upon femtosecond pump probe experiments (258 nm) of bmib (3) in argon-saturated THF with several time delays between 0 and 150 ps at
room temperature.
Figure 34: Time-absorption profiles of the spectra shown in Figure 32 at 555 and 939 nm monitoring the
intersystem crossing.
400 600 800 1000 1200
0.00
0.01
0.02
0.03
))) ) )0))1.5)) ) ) )2))2.5)) ) ) )5)) ) ) )9)) )11)))13)))15)))20)))25)))37)))50)))75)150Δ
A"/"a.u.
λ"/"nm
0 30 60 90 120 150
0.00
0.01
0.02
0.03
)555)nm)939)nm
ΔA"/"a.u.
t"/"ps
1D coordination polymer of bmib
62
Figure 35: Differential absorption spectra (visible and near-infrared) obtained upon nanosecond flash photolysis
(266 nm) of bmib (3) in argon-saturated THF at 50 ns after excitation at rt.
Figure 36: Time-absorption profile of the spectrum shown in Figure 35 at 450 nm monitoring the excited state
decay.[218]
400 600 800 1000 1200'0.01
0.00
0.01
0.02
0.03
0.04
ΔA"/"a.u.
λ"/"nm
0 10 20 30 40 50
0.000
0.005
0.010
0.015
ΔA"/"a.u.
t"/"µs
1D coordination polymer of bmib
63
On a rather short timescale, that is, 25 ± 5 ps, these transient absorption features decay. At the
end of this decay we note a broad transient absorption throughout the visible and near-infrared
region with maxima at 450 and around 1100 nm. The latter are stable and are still discernable
at the end of the experimental time scale of 7.5 ns. Tentatively, we ascribe these transients to
the triplet excited state of bmib (3). To explore the triplet excited state deactivation
nanosecond transient absorption measurements with 266 nm laser pulses were performed
(Figure 35 and 36). The nanosecond transient absorption spectrum is in perfect agreement
with the observation at the end of our femtosecond transient absorption measurements. In
particular, transient absorption features with maxima at 450 and around 1100 nm were noted.
A kinetic analysis revealed two lifetimes – a short lived component with 1.7 ± 0.2 µs and a
long lived component with 7.9 ± 0.5 µs. The two components are likely to arise from triplet-
triplet annihilation and ground-state quenching. Support for the assignment that the transient
features correlate with the triplet excited state of bmib (3) came from analogous experiments
in oxygen saturated solutions. In the latter, the triplet excited lifetime is reduced to 80 ±
5 ns.[218]
1D coordination polymer of bmib
64
3.2.2 Preparation and Characterization of [Zn5(OAc)10(bmib)2]n (10)
Inspired by the coordination polymer of trans-bie (2) with zinc(II) acetate, which resulted in a
trinuclear based bridged polymer (9) (chapter 3.1.5), we decided to investigate the
coordination behaviour of the new N,N donor based ligand bmib (3) with zinc(II) acetate.
The reaction of bmib (3) in MeCN with Zn(OAc)2 × 2 H2O in THF yielded a 1D polymeric
compound, by slow evaporation of the solvent (Scheme 13). The X-ray structure analysis
revealed a coordination polymer [Zn5(OAc)10(bmib)2]n (10) consisting of alternating dinuclear
[Zn2(OAc)4] paddlewheel units and trinuclear [Zn3(OAc)6] units ‘clamped’ together by bmib
ligands.
Scheme 13: Synthesis of [Zn5(OAc)10(bmib)2]n (10).
This rather unusual coordination mode of the bmib ligand causes a one-dimensional sequence
that resembles the battlements of a fortress (Figure 37). Zinc acetate is capable of forming
coordination polymers with N,N donor ligands by either forming a paddlewheel or trinuclear
inorganic units.[191, 193-196, 200, 224-225] To the best of our knowledge such alternating paddlewheel
and trinuclear units have only been reported once in literature before. KWAK et al. described a
zinc(II) benzoate linked with 1,2-di(4-pyridyl)ethylene (bpe).[198]
N N
N N
Zn(OAc)2 x 2 H2O
3
10
N
N
Zn
Zn
Zn
O
OO
O
O
OO
O
O
OO
O
N
N
N
N
N
N
ZnO
OZn O
OO
O
O
O
n
1D coordination polymer of bmib
65
Figure 37: Cutout of the molecular structure of [Zn5(OAc)10(bmib)2]n (10).[218]
Table 13: Selected bond lengths and angles of [Zn5(OAc)10(bmib)2]n (10).
Distances in Å Distances in Å
d (Zn2-O61) 2.035(3) d (Zn1-O31) 2.129(3)
d (Zn2-O62) 2.036(4) d (Zn1-O41) 2.118(3)
d (Zn2-O71) 2.077(4) d (Zn1-O51) 2.052(3)
d (Zn2-O72) 2.036(3) d (Zn3-O31) 1.980(3)
d (Zn2-N11) 2.034(4) d (Zn3-O42) 1.938(4)
d (Zn2-Zn2) 2.9489(10) d (Zn3-O52) 1.971(3)
d (Zn1-Zn3) 3.3287(6) d (Zn3-N21) 2.002(4)
d (Zn3-O32) 2.645(4)
Angles in ° Angles in °
∡ (Zn2-Zn2-N11) 178.24(12) ∡ (Zn3-Zn1-N21) 167.06(12)
The trinuclear Zn(II)-acetate unit is constructed by six bridging acetato ligands and similar to
previously described trinuclear zinc carboxylates with axial N- or O-donor ligands.[193-196]
There is an inversion centre at the central Zn atom. Out of three Zn atoms the central Zn is
hexacoordinated from six acetato groups to form a distorted octahedron. The two terminal Zn
centres are coordinated tetrahedrally by three oxygen atoms of the acetato groups and by the
a
bc
C
H
N
O
Zn
N11
Zn2
Zn2
O72
O71
O62
O61
N11
N21
Zn3
Zn1
O42
O41 O51
O52Zn3
O31
O32
N21
1D coordination polymer of bmib
66
nitrogen atom of the bmib ligand. Two different types of carboxylate coordination were found
in this unit. Four acetato ligands show κ2-coordination, forming syn–syn bridges between
central and terminal zinc ions. The other two acetato ligands are κ1-coordinated and function
as Zn–O–Zn µ-bridges with d(Zn1-O31) = 2.129(3) Å and d(Zn3-O31) = 1.980(3) Å. The
other uncoordinated oxygen atom is slightly involved in a weak interaction with the terminal
zinc atom (d(Zn3….O32) = 2.645(4) Å).[197]
The paddlewheel unit is constructed by two symmetrically equivalent Zn atoms, which are
µ-bridged by four acetato ligands. Each Zn atom in this unit has distorted square pyramidal
environment, with four oxygen atoms from acetates, to form the equatorial plane, and one
nitrogen atom of the bmib ligand. To describe the coordination geometry of the five-
coordinated Zn centre of the paddlewheel unit the tau (τ) parameter, defined as the difference
between the two largest angles divided by 60° was determined. The ideal value of 0 is
assigned to a square pyramidal conformation and the ideal value of 1 to a trigonal bipyramidal
conformation.[226-228] The τ value calculated from the angle ∡(O61-Zn2-O62) = 158.97°and
∡ (O72-Zn2-O71) = 159.35° is 0.006, indicating a nearly ideal square pyramid. In the
paddlewheel unit, the Zn-O bond distances range from d(Zn2-O61) = 2.035(3) Å to d(Zn2-
O71) = 2.077(4) Å and the Zn-N bond distance amounts to d(Zn2-N11) = 2.034(4) Å, which
compare well to values of related compounds like the dinuclear zinc(II) benzoate complex
[Zn2(C7H5O2)4(C12H10N2)2] coordinated by two terminal trans-1-(2-pyridyl)-2-(4-
pyridiyl)ethylene ligands .[191, 193, 199-200, 224-225] In the trinuclear unit, the Zn-O bond distances of
the κ2-OAc range from 1.938(4) to 2.118(3) Å. The κ1-OAc shows a bond distance of
2.129(3) Å for Zn1-O31 and 1.980(3) Å for Zn3-O31. The Zn-N bond distance of 2.002(4) Å
is slightly shorter than in the dinuclear unit. The Zn-Zn distances of dinuclear and trinuclear
units are 2.9489(10) and 3.3287(6) Å, which are similar to literature values of the zinc(II)
benzoate polymer with bpe.[198, 229] The bond distances of the bmib ligand do not vary
significantly from those of the free ligand. It is worthwhile mentioning that obviously bmib
(7) acts as some sort of molecular clip for metal ions with almost fixed angles ∡(M-N11-N21)
close to 90° and torsion angles ∡(M-N11-N21-M’) close to 0°. A similar coordination
property has recently been reported for the slightly shorter 1,4-bis(N-methylimidazol-2-
yl)benzene ligand coordinated to a palladium(II) centre.[229]
1D coordination polymer of bmib
67
Figure 38: Side view of four polymer strands of [Zn5(OAc)10(bmib)2]n (10).
In Figure 38 the side view of four polymer strands of [Zn5(OAc)10(bmib)2]n (10) is shown. It
can be seen, that the bmib ligands arranged parallel to each other. The distances between the
ligands are about 8.52 Å, so there is no intermolecular π-stacking, but nevertheless the
assembly results in channels between the strands.
8.53 Å
Sawhorse-type polymers
68
3.3 Sawhorse-type diruthenium tetracarbonyl polymers with N,N ligands
3.3.1 Preparation and Characterisation of [Ru2(OAc)2(CO)4(L)]n
Based on the previously described paddlewheel polymer of ruthenium(II) acetate
[Ru2(OAc)4(trans-bie)]n (5) with a double bond between the metal centres, we were also
interested in a dinuclear system with a different oxidation state of the ruthenium centre. This
can be found in a so-called sawhorse unit. The central metal is ruthenium(I), which is bridged
by two acetato groups and both are coordinated by two CO ligands. Surprisingly, so far only
one coordination polymer [Ru2(OAc)2(CO)4(MeSCH2SMe2)]n with a bridging dithioether
ligand, is known in literature, as described in chapter 1.2.2.[118] Thus, we focused on the
synthesis of N,N donor based coordination polymers with this sawhorse type diruthenium
tetracarbonyl unit.
The metal precursor [Ru2(OAc)2(CO)4]n was synthesized by the reaction of [Ru3(CO)12] in
acetic acid as described in literature (Scheme 14).[230] Syntheses of compounds 11-15 are
shown in Scheme 15.
Scheme 14: Synthesis of [Ru2(OAc)2(CO)4]n.[230]
Scheme 15: Syntheses of 1D coordination polymers 11-15.
2n Ru3(CO)12+ 6n HOAc, Δ
+ 12n CO + 3n H23 Ru Ru
n
C C
O OO O
OCOC
OO
1. THF, Δ2. ligand LRu Ru
n
C C
O OO O
OCOC
OO
Ru Ru
n
C C
O OO O
OCOC
OO
L
Sawhorse-type polymers
69
The coordination polymers [Ru2(OAc)2(CO)4L]n based on carbonyl ruthenium(I) were
obtained by the reaction of a mixture of [Ru2(OAc)2(CO)4]n in THF and the addition of
various N,N donor ligands. With pyrazine (pyz), 1,4-diazabicyclo[2.2.2]octane (DABCO),
4,4´-bipyridine (4,4´-bipy), trans-1,2-bis(N-methylimidazol-2-yl)ethylene (trans-bie) (2) and
1,2-di(4-pyridyl)ethylene (bpe), five representative ligands were chosen for these structures,
which are shown in Figure 39.
Figure 39: N,N donor ligands suitable to form one dimensional coordination polymers.
The elemental analysis showed that all of the resulting coordination polymers have a ligand to
[Ru2(OAc)2(CO)4] stoichiometry of 1:1. In the IR spectra of powder samples (KBr pellet) of
all compounds the ν(OCO) vibrations appear as a set of distinctive two bands between 1580
and 1400 cm–1 (educt: ν(OCO) = 1556, 1402 cm–1) and the typical ν(CO) three-band signature
around 2000 cm–1 (educt: ν(CO) = 2054, 1964, 1950 cm–1) (Table 14).[119-120] The Δ values of
the asymmetric and symmetric O-C-O vibrations indicate the presence of bridging acetato
groups.[214] The vibrational bands of polymer 11 and 12 were also calculated and are in good
agreement with the experimental data (Appendix).
N
NN
N
N N
NN
N NNN
pyz DABCO 4,4´-bipy
trans-bie (2) bpe
Sawhorse-type polymers
70
Table 14: Summary of observed vibrational bands (KBr) of polymer complexes 11-15.
Complex ! (CO)
[cm–1]
! (CO)
[cm–1]
! (CO)
[cm–1]
! (OCO)
[cm–1] Δ (OCO)
[cm–1]
[Ru2(OAc)2(CO)4(trans-bie)]n (11) 2022 1972 1939 1574, 1442 132
[Ru2(OAc)2(CO)4(pyz)]n (12) 2029 1972 1953 1564, 1436 128
[Ru2(OAc)2(CO)4(4,4’-bipy)]n (13) 2024 1969 1938 1571, 1440 131
[Ru2(OAc)2(CO)4(bpe)]n (14) 2022 1968 1934 1571, 1435 136
[Ru2(OAc)2(CO)4(DABCO)]n (15) 2027 1965 1937 1564, 1441 123
The solid UV/Vis absorption spectra of all polymers were recorded by the same method
described for the paddlewheel polymers 4-6. The absorption bands localised in the region
between 220 to 430 nm are very similar to each other. The maxima at 322 and 445 nm in the
[Ru2(OAc)2(CO)4]n precursor can also be found in the polymers (Appendix Figure 55-54).
To get a better understanding of the formation mechanism of the polymers, we tried to detect
larger fragments in mass spectrometry experiments. Therefore, Ru2(OAc)2(CO)4(trans-bie)]n
(11) was suspended in MeOH and ultrasonificated over night. The ESI MS data of polymer
11 shows a 100 % signal at m/z 751.02 assigned to the [Ru2(OAc)(CO)4(trans-bie)2]+
fragment. The same procedure was also implemented for the polymers 12-15, but the
corresponding mass spectra showed no significant signal. This can be explained either by
fragmentation or with a trace of contamination of polymer 11.
Based on these findings, it was expected that the sawhorse unit was preserved upon a reaction
with the bidentate N,N donor ligands to form linear coordination polymers comparable to the
unique dithioether compound.[135] It was possible to obtain crystals of compounds 11 and 12
suitable for single crystal X-ray structure analysis. Crystals of [Ru2(OAc)2(CO)4(trans-bie)]n
(11) were grown by layering [Ru2(OAc)2(CO)4]n in THF with trans-bie in MeOH. The
substance crystallizes in the space group C 2/c as the polymeric compound [C16H18N2O9Ru2]n.
A molecular cutout of the polymer is illustrated in Figure 40 and selected bond lengths and
angles given in Table 15.
Sawhorse-type polymers
71
Figure 40: Cutout of the molecular structure of [Ru2(OAc)2(CO)4(trans-bie)]n (11); thermal ellipsoids are drawn
at the 50% probability level. Hydrogen atoms and THF molecules have been omitted for clarity.
Table 15: Selected bond lengths and angles of [Ru2(OAc)2(CO)4(trans-bie)]n (11).
Distances in Å Distances in Å
d (Ru-C1) 1.823(3) d (Ru-N11) 2.235(2)
d (Ru-C2) 1.834(3) d (Ru-Ru) 2.6749(8)
d (Ru-O3) 2.112(2) d (C1-O1) 1.154(4)
d (Ru-O4) 2.126(2) d (C2-O2) 1.152(4)
Angles in °
∡ (N11-Ru-Ru) 165.34(6)
In the polymeric structure an infinite chain of [Ru2(OAc)2(CO)4] units bridged by the N,N
donor trans-bie can be observed. The sawhorse-type unit is constructed by two ruthenium(I)
centres, which are κ2-coordinated by two acetato ligands. Each metal centre is all-cis
coordinated by two carbonyl ligands to build a distorted octahedral environment with the
coordinated nitrogen atom of the N,N donor ligand. In compound 11 a C2 rotation axis
perpendicular to the middle of the Ru-Ru bond is present. An inversion centre is located in the
middle of the double bond of the trans-bie ligand, which connects the two imidazole groups.
Within a polymer chain the acetato bridged dimeric units have an alternating “up-down”
arrangement in which the acetato bridges on one dimer unit are adjacent to the carbonyl
groups on the units on either side. The metal-metal bond distance exhibits a length of d(Ru-
ab
c
C
H
N
O
Ru
N11N12
C15C15
Ru Ru
O3
O3
O4
O4
C1
C1
O1
O1O2
O2
C2C2
Sawhorse-type polymers
72
Ru) = 2.6749(8) Å. The Ru-O distances of the bridging acetato ligand are d(Ru-
O3) = 2.112(2) Å, d(Ru-O4) = 2.126(2) Å respectively. The coordinated carbonyl ligands
have a distance of d(Ru-C1) = 1.823(3) Å and of d(Ru-C2) = 1.834(3) Å. The bond of the
coordinating nitrogen atom of the trans-bie ligand is d(Ru-N11) = 2.235(2) Å. The Ru-Ru-N
bond angle is ∡(N11-Ru-Ru) = 165.34(6)°.
Crystals of [Ru2(OAc)2(CO)4(pyz)]n (12) suitable for single crystal X-ray structure
determination were grown by layering [Ru2(OAc)2(CO)4]n in THF with pyrazine in MeCN.
The substance crystallizes in the space group C 2/c as the polymeric compound
[C13H17N2O5Ru]n. A molecular cutout of the polymer is illustrated in Figure 41 and selected
bond lengths and angles given in Table 16.
Figure 41: Cutout of the molecular structure of [Ru2(OAc)2(CO)4(pyz)]n (12); thermal ellipsoids are drawn at the
50% probability level. Hydrogen atoms and THF molecules have been omitted for clarity.
Table 16: Selected bond lengths and angles of [Ru2(OAc)2(CO)4(pyz)]n (12).
Distances in Å Distances in Å
d (Ru-C1) 1.831(4) d (Ru-N11) 2.203(3)
d (Ru-C2) 1.843(4) d (Ru-Ru) 2.6370(8)
d (Ru-O3) 2.113(2) d (C1-O1) 1.152(5)
d (Ru-O4) 2.119(2) d (C2-O2) 1.140(5)
Angles in °
∡ (N11-Ru-Ru) 164.75(7)
a
bc
C
H
N
O
Ru
N11
N11
RuRu
O3
O3
O4
O4
C1
C2C1
C2 O2
O2 O1
O1
Sawhorse-type polymers
73
The sawhorse-type unit is constructed similar to the one in polymer 11 with two ruthenium(I)
centres κ2-coordinated by two acetato ligands. Each metal centre is all-cis coordinated by two
carbonyl ligands to build a distorted octahedral environment with the coordinated nitrogen
atom of the pyrazine ligand. In compound [Ru2(OAc)2(CO)4(pyz)]n (12) we have also a C2
rotation axis perpendicular to the middle of the Ru-Ru bond. An inversion centre is located in
the middle the pyrazine ligand. Within a polymer chain the acetato bridged dimeric units have
an alternating “up-down” arrangement in which the acetato bridges on one dimer unit are
adjacent to the carbonyl groups on the units on either side. The metal-metal bond distance
exhibits a length of d(Ru-Ru) = 2.6370(8) Å. The Ru-O distances of the bridging acetato
ligand are d(Ru-O3) = 2.113(2) Å, respectively d(Ru-O4) = 2.119(2) Å. The coordinated
carbonyl ligands have a distance of d(Ru-C1) = 1.831(4) Å and d(Ru-C2) = 1.843(4) Å. The
bond length of the coordinating nitrogen atom of pyrazine is d(Ru-N11) = 2.203(3) Å. The
Ru-Ru-N bond angle is ∡(N11-Ru-Ru) = 164.75(7)°.
3.3.2 Comparison of sawhorse-type compounds
By the reaction of different N,N donor ligands with [Ru2(OAc)2(CO)4]n five 1D coordination
polymers with the general formula [Ru2(OAc)2(CO)4(L1-5)]n were obtained. The structures are
very similiar. A dinuclear ruthenium(I) unit with two bridging acetato groups and two
carbonyl ligands on each metal centre forms the so-called sawhorse unit. These units are
linked with the nitrogen atoms of the ligand to form an infinite chain. The molecular structure
of compound [Ru2(OAc)2(CO)4(trans-bie)]n (11) and [Ru2(OAc)2(CO)4(pyz)]n (12) are nearly
identical in this regard.
Table 17: Selected bond lengths of different diruthenium(I) tetracarbonyl complexes.
Complex M-M distances in Å M-N distances in Å
[Ru2(OAc)2(CO)4(trans-bie)]n (11) 2.6749(8) 2.235(2)
[Ru2(OAc)2(CO)4(pyz)]n (12) 2.6370(8) 2.203(3)
[Ru2(OAc)2(CO)4(MeSCH2SMe)]n 2.682(1), 2.684(1)[135] 2.483(3), 2.496(3), 2.504(3),
2.508(3)[135]
[Ru2(OAc)2(CO)4(py)2] 2.672(1)[124] 2.197(4)[124]
[Ru2(OAc)2(CO)4(pz)2] 2.6746(6)[122] 2.202(2), 2.203(2)[122]
Sawhorse-type polymers
74
In comparison to polymer [Ru2(OAc)2(CO)4(MeSCH2SMe)]n and various dinuclear complexes
with N donor ligands, the Ru-Ru bond length of polymer 11 and 12 are nearly identical.[118, 122,
124, 135] The Ru-N bond lengths are similar to complex [Ru2(OAc)2(CO)4(py)2] with two axial
pyridine, respectively [Ru2(OAc)2(CO)4(pz)2] with two pyrazole ligands (Table 17).[122, 124] In
both polymers the bridging N,N ligands are twisted to each other. In compound 12 we see an
angle of 24° measured between the plane calculated for the pyrazine ligand. In compound 11
the angle of 44° between the two trans-bie planes is significantly larger. Furthermore the two
imidazole groups of one trans-bie ligand in polymer 11 are not located in the same plane.
In cooperation with C. WICK from the CLARK group the different bonds (M-M, M-L) were
analysed by means of DFT and complete active space self-consistent field (CASSCF)
calculations as described for the paddlewheel polymers recently. The results are summarized
in Table 18. The metal to metal Mayer Bond Orders (MBO)[231-232] of 0.66 according to DFT
and the effective bond order (EBO)[233-234] of 0.85 indicate a weakening of a possible single
bond (σ orbitals) due to ligand coordination. All other metal-metal bonding and anti-bonding
sets of 4d-orbitals are filled completely as expected for Ru(I) dinuclear complexes (CASSCF
active orbitals and their occupation numbers are given in the Appendix). The different metal
to ligand bond orders are similar in both polymers 11 and 12.
Table 18: Calculated (B3LYP) bond orders for 11 and 12.
Complex Ru-Ru M-N Ru-O Ru-C
[Ru2(OAc)2(CO)4(trans-bie)2] 0.85a / 0.66 0.43 0.45b 1.26b
[Ru2(OAc)2(CO)4(pyz)2] 0.85a / 0.66 0.42 0.49b 1.25b a) The EBO calculated form CASSCF natural orbital occupations. b) values are averaged over two (according to symmetry) nearly identical values.
Sawhorse-type polymers
75
3.3.3 STM images on HOPG
In comparison to literature known paddlewheel complexes, which have been tested regarding
to their properties in the field of molecular electronics, we also tried to image the coordination
polymer [Ru2(OAc)2(CO)4(trans-bie)]n (11) deposited on HOPG with scanning tunneling
microscopy (STM) and measure the electronic properties with current-imaging-tunneling
spectroscopy (CITS) maps.
Therefore, a 10–9 M solution of [Ru2(OAc)2(CO)4(THF)2] was mixed with a 10–9 M solution of
trans-bie (2) in THF. Afterwards a droplet of the mixture was deposited on a freshly cleaved
HOPG surface. After drying them under air the samples were loaded into the microscope and
investigated by STM/CITS. It was observed from the topography that the compound adsorbs
at defect sites of the substrate (Figure 42a &b).
Figure 42: Topography STM measurement of [Ru2(OAc)2(CO)4(THF)2] mixed with a 10–9 M solution of trans-
bie (2) dropcasted from THF on HOPG.
Figure 42a shows a Moiré pattern in the left bottom area caused by an overlap of graphite
layers. In the right upper area this effect is not visible, which suggests that the molecules
arrange themselves at the edge of a defect line. Figure 42b shows an enlargement of the same
area. The distance from the methyl group of the acetate moiety of the ruthenium sawhorse
unit to the edge of the trans-bie ligand in one monomer unit is about 1.1 nm, as can be
deduced from the molecular structure gained by X-ray structure determination. The diameter
of one bundle measured along the fast scan direction in the STM image is approximately 1.1
+/– 0.1 nm, which is in accordance to the X-ray crystallographic data of polymer 11. This
(a) (b)
Sawhorse-type polymers
76
might indicate a single strand. The periodicity between two bundle centres in the topography
has a size of 2.3 +/– 0.1 nm, which might reflect the up and down arrangement observed from
the X-ray single structure determination. On the other hand this periodicity might be caused
by arrangement of dinuclear fragments without intermolecular coupling. So far the resolution
of the STM topography images does not allow a distinct answer to that problem. Furthermore,
we consider that we see an arrangement of the compound on a defect line instead of a single
polymer strand, due to the observed Moiré pattern.
In order to understand the morphology of the adsorbed compounds on the HOPG surface the
electronic properties have been studied by CITS technique. The IV-ramps were conducted
between –1.0 V and +1.0 V since higher values for the voltage deliver no reliable
characteristics under ambient conditions. Figure 43a depicts the simultaneously recorded
topography while a CITS map was recorded at the molecule (red) and on the HOPG substrate
(blue).
Sawhorse-type polymers
77
-1.0 -0.5 0.0 0.5 1.0-3
-2
-1
0
1
2
I (nA
)
U (V)
Molecule HOPG
-1.0 -0.5 0.0 0.5 1.00.5
1.0
1.5
2.0
2.5
dI/d
U (a
rb.u
)
U (V)
Molecule
0
2
4
6
HOPG
dI/d
U (a
rb.u
)
Figure 43 a) STM topography; b) I-V characteristic recorded at different locations in Figure 3a (one the
molecule: red; apart of the molecule: blue); c) dI/dV calculated from the I-V characteristic.
4.0nm
(a)
(b)
(c)
Sawhorse-type polymers
78
The I-V diagram of the measurement shows a significant difference between the two
locations, which can be seen in Figure 43b. A lower conductivity at the molecule in
comparison to HOPG can be seen, which indicates an adsorbed molecule on top of the
surface. The differential conductivity dI/dV calculated from the measured I-V characteristics
of the polymer is presented in Figure 43c. An onset in the conductivity can be observed at
approximately –0.6 V and +0.6 V. These features should originate from the start and end of a
HOMO-LUMO gap, but can also be attributed to vibrational modes of the molecule since
they are more or less symmetrical around zero voltage. Furthermore, there are also regular
features visible both in the dI/dV of the deposied molecules and the HOPG substrate, probably
caused by 50 Hz noise. In general, gaps in the order of 1.0 V or more are typical for organic
semiconductors, and since no distinct HOMO or LUMO peak is visible in the range of +/–
1.0 V, the absorbed material seems to belong to that class of high-band-gap semiconductors.
79
4 SUMMARY
Summary
80
In previous work of the BURZLAFF group, which was also the starting point of this thesis, the
design of the new N,N donor ligand trans-1,2-bis(N-methylimidazol-2-yl)ethylene (trans-bie)
(2) suitable to form 1D coordination polymers with copper(II) acetate was achieved. The
absorption behaviour and electronic properties were investigated by scanning tunneling
microscopy (STM) measurements on a highly ordered pyrolytic graphite (HOPG). This
polymer exhibits interesting properties towards metal ligand derived semiconducting
nanowires.[145]
To get a better understanding of the coordination behaviour of the trans-bie (2) ligand, the
first part of this thesis (chapter 3.1) describes the synthesis and characterisation of a range of
dinuclear metal(II) acetato paddlewheel polymers with different metal-metal bond orders
linked by trans-bie (2). The reaction of trans-bie (2) with the metal(II) acetates of rhodium,
ruthenium, molybdenum and chromium led to the formation of the corresponding 1D
coordination polymer with the molecular formula [M2(OAc)4(trans-bie)]n (M = Rh (4), Ru (5),
Mo(6), Cr(7)) (Figure 44). These complexes have been characterised by elemental analysis,
infrared spectroscopy and UV/Vis absorption spectroscopy.
Figure 44: Synthesis of paddlewheel 1D coordination polymers with trans-bie.
It was possible to characterize the polymers [Rh2(OAc)4(trans-bie)]n (4), [Ru2(OAc)4(trans-
bie)]n (5) and [Mo2(OAc)4(trans-bie)]n (6) by single crystal X-ray structure determination. A
comparison of these polymer structures with the copper(II) acetate polymer
[Cu2(OAc)4(trans-bie)]n shows a correlation between the strength of the metal-metal and the
metal-nitrogen bond of the coordinating ligand. The stronger the metal-metal bond in the
paddlewheel unit the weaker the bond between the metal and the coordinating nitrogen donor.
In cooperation with C. WICK from the CLARK group theoretical calculations were performed.
CASSCF/CASPT2 and DFT calculations of a single periodic unit of [Mo2(Ac)4(trans-bie)2]n
M M
n
O O
O OO O
OO
N N
N N
NN
N N
2
[M2(OAc)4]
M = Rh, Ru, Cr, Mo
Summary
81
showed a weakening of the possible quadruple bond, which is mainly attributed to weakly
overlapping δ orbitals and a resulting effective bond order (EBO) of 3.3. The calculated
metal-metal bond orders in [Rh2(OAc)4(trans-bie)2] and [Ru2(OAc)4(trans-bie)2] indicate
single and double bonds with EBO of 0.8 and 1.7, respectively.
Furthermore, a complex [Cr2(OAc)4(trans-bie)2] (8) was crystallized, consisting of the
chromium paddlewheel unit, which is coordinated by two trans-bie ligands. By reacting
zinc(II) acetate with trans-bie (2) a polymer with a trinuclear metal unit was obtained. Here
the three zinc centres are bridged by six acetato ligands to form a polymer with the formula
[Zn3(OAc)6(trans-bie)]n (9).
Based on these results in cooperation with T. WAIDMANN of the BURZLAFF group a new N,N
donor based imidazole ligand was synthesized (chapter 3.2). Two methylimidazoles were
linked by a bisacetylene unit to obtain the new linear ligand bis(N-methylimidazol-2-
yl)butadiyne (bmib) (3). In cooperation with the GULDI group the fluorescence properties of
bmib (3) were investigated by time dependent absorption spectroscopy. By the reaction of
zinc(II) acetate with bmib (3) a 1D coordination polymer was obtained in which the ligand
coordinates to a zinc paddlewheel unit on the one side and to a trinuclear zinc unit on the
other side similar to zinc polymer 9 mentioned above. The molecular formula of the polymer
is [Zn5(OAc)10(bmib)2]n (10) (Figure 45).
Figure 45: Synthesis of [Zn5(OAc)10(bmib)2]n (10).
N N
N N
Zn(OAc)2 x 2 H2O
3
10
N
N
Zn
Zn
Zn
O
OO
O
O
OO
O
O
OO
O
N
N
N
N
N
N
ZnO
OZn O
OO
O
O
O
n
Summary
82
In the final part of this thesis (chapter 3.3) a range of polymers containing a so called
sawhorse unit were synthesized. The reaction of [Ru3(CO)12] with acetic acid led to the
formation of a polymeric precursor [Ru2(OAc)2(CO)4]n consisting of a dinuclear ruthenium(I)
unit. The metal centres are bridged by two acetato groups and each ruthenium is coordinated
by two carbonyl ligands. In the next step the reaction of the N,N donor ligands pyrazine (pyz),
1,4-diazabicyclo[2.2.2]octane (DABCO), 4,4´-bipyridine (4,4´-bipy), trans-1,2-bis(N-
methylimidazol-2-yl)ethylene (trans-bie) (2) and 1,2-di(4-pyridyl)ethylene (bpe)
corresponding 1D coordination polymers with the general molecular formula
[Ru2(OAc)2(CO)4(L)]n (11-15) were formed (Figure 46).
Figure 46: Synthesis of sawhorse type 1D coordination polymers with the N,N donor ligands L1-L5.
These complexes have been characterised by elemental analysis, infrared spectroscopy and
UV/Vis absorption spectroscopy. It was also possible to characterize the organometallic
coordination polymers 11 and 12 by single crystal X-ray structure determination. Within a
polymer chain the acetato bridged dinuclear units have an alternating “up-down” arrangement
in which the acetato bridges on one dimer unit are adjacent to the carbonyl groups on the units
on either side. In cooperation with the CLARK group the different bonds by means of DFT and
1. THF, Δ2. ligand LRu Ru
n
C C
O OO O
OCOC
OO
Ru Ru
n
C C
O OO O
OCOC
OO
L
N
NN
N
N N
NN
N NNN
pyz DABCO 4,4´-bipy
trans-bie (2) bpe
Summary
83
CASSCF calculations as described for paddlewheel polymers were analysed. The metal to
metal Mayer Bond Orders (MBO) of 0.66 according to DFT and the effective bond order
(EBO) of 0.85 indicate a weakening of a possible single bond (σ orbitals) due to ligand
coordination. For further investigations we imaged the coordination polymer
[Ru2(OAc)2(CO)4(trans-bie)]n (11) with STM. It was observed from the topography
measurements that the compound adsorbs at defect line of the substrate in a linear
arrangement (Figure 47). So far a distinct answer on the formation of a polymer or a dinuclear
complex during adsorption on surface cannot be given.
Figure 47: Topography STM measurement of [Ru2(OAc)2(CO)4(THF)2] mixed with a 10–9 M solution of trans-
bie dropcasted from THF on HOPG.
84
5 ZUSAMMENFASSUNG
Zusammenfassung
85
In vorangegangenen Arbeiten der Arbeitsgruppe BURZLAFF, die auch den Startpunkt dieser
Dissertation darstellten, wurde der neue N,N-Donorligand trans-1,2-bis(N-methylimidazol-2-
yl)ethylene (trans-bie) (2) hergestellt mit dem es möglich war, durch die Umsetzung mit
Kupfer(II)acetat, ein 1D-Koordinationspolymer aufzubauen. Dessen Adsorptionsverhalten
und elektronischen Eigenschaften auf einer highly ordered pyrolytic graphite (HOPG)
Oberfläche wurden mittels Rastertunnelmikroskopie untersucht. Dieses Polymer weißt
interessante Eigenschaften als metall-ligand-basierter halbleitender Nanodraht auf.[145]
Um ein besseres Verständnis über das Koordinationsverhalten des Liganden (trans-bie) (2) zu
erlangen, befasst sich der erste Teil dieser Arbeit (Chapter 3.1) mit der Synthese und
Charakterisierung einer Reihe von dinuklearen Metall(II)acetat Polymeren mit
unterschiedlichen Metall-Metall Bindungsordnungen. Diese so genannten Paddlewheel-
Einheiten (Schaufelrad-Einheit) werden durch den trans-bie Liganden verknüpft. Die
Reaktion von Rhodium(II)-, Ruthenium(II)-, Molybdän(II)- bzw. Chrom(II)acetat mit trans-
bie (2) führte zur Ausbildung der entsprechenden 1D-Koordinationspolymere mit der
allgemeinen Formel [M2(OAc)4(trans-bie)]n (M = Rh (4), Ru (5), Mo (6), Cr (7)) (Abbildung
1). Diese Verbindungen wurden mittels Elementaranalyse, Infrarotspektroskopie und UV/Vis
Absorptionsspektroskopie charakterisiert.
Abbildung 1: Synthese von 1D Koordiniatiospolymeren mit dem trans-bie Liganden.
Des weiteren war es möglich die Verbindungen [Rh2(OAc)4(trans-bie)]n (4),
[Ru2(OAc)4(trans-bie)]n (5) und [Mo2(OAc)4(trans-bie)]n (6) mittels der Einkristall
Röntgenstrukturanalyse zu charakterisieren. Ein Vergleich mit der bereits bekannten
Polymerstruktur des Kupfer(II)acetats [Cu2(OAc)4(trans-bie)]n zeigte eine Beziehung
zwischen der Bindungsstärke der Metall-Metall-Einheit und der Metall-Stickstoff-Bindung
des koordinierten trans-bie Ligandens. Je stärker die Metall-Metall-Bindung in der
M M
n
O O
O OO O
OO
N N
N N
NN
N N
2
[M2(OAc)4]
M = Rh, Ru, Cr, Mo
Zusammenfassung
86
Paddlewheel-Einheit desto schwächer ist die Bindung zwischen dem Metall und dem
koordinierten Stickstoffatom des Ligandens. Theoretische Berechnungen zu den gezeigten
Polymeren wurden in einer Kooperation mit C. WICK aus der Arbeitsgruppe CLARK
durchgeführt. CASSCF/CASPTS und DFT Berechnungen für eine einzelne periodische
Einheit des Polymers [Mo2(OAc)4(trans-bie)]n (6) zeigten eine Schwächung der möglichen
Vierfachbindung, welche maßgeblich auf geringer überlappende δ-Orbitale zurückzuführen
ist, was eine effektive Bindungsordnung (EBO) von 3.3 zur Folge hat. Die berechneten
Metall-Metall-Bindungsordnungen in den Polymeren [Rh2(OAc)4(trans-bie)]n (4) und
[Ru2(OAc)4(trans-bie)]n (5) weisen auf eine Einfach- bzw. Doppelbindungen mit EBO Werten
von 0.8 bzw. 1.7 hin.
Des weiteren konnte ein Chrom(II)acetat Komplex mit der Formel [Cr2(OAc)4(trans-bie)2] (8)
kristallisiert werden, der aus einer Chrom Paddlewheel-Einheit besteht, die von zwei trans-bie
Liganden koordiniert wird. Durch die Reaktion von Zink(II)acetat mit trans-bie (2) bildete
sich ein Polymer mit einer trinuklearen Metalleinheit. In diesem Fall werden drei Zinkatome
durch sechs Acetatgruppen verbrückt die ein Polymer mit der Formel [Zn3(OAc)6(trans-bie)]n
(9) bilden.
Aufbauend auf diese Ergebnisse wurde in Kooperation mit T.WAIDMANN aus der BURZLAFF-
Gruppe ein neuer N,N-Donor basierter Ligand hergestellt (Chapter 3.2). Dabei werden zwei
Methylimidazole durch eine Bisacetyleneinheit verbrückt um den linearen bis(N-
methylimidazol-2-yl)butadiyne (bmib) (3) Liganden zu erhalten. In einer Kooperation mit der
Arbeitsgruppe GULDI wurden die Fluoreszenzeigenschaften mittels zeitaufgelöster
Absorptionsspektroskopie untersucht. Durch die Umsetzung von Zink(II)acetat mit bmib (3)
konnte ein 1D-Koordinationspolymer mit der Formel [Zn5(OAc)10(bmib)2]n (10) synthetisiert
werden (Abbildung 2). Der Ligand koordiniert auf der einen Seite an eine Zink-Paddlewheel-
Einheit auf der anderen Seite an eine trinucleare Zinkeinheit, ähnlich dem oben beschriebenen
Polymer 9.
Zusammenfassung
87
Abbildung 2: Synthese von [Zn5(OAc)10(bmib)2]n (10).
Im letzten Teil dieser Arbeit (Chapter 3.3) wurden Polymere mit einer so genannten
Sawhorse-Einheit (Sägebock-Einheit) hergestellt. Die Reaktion von [Ru3(CO)12] mit
Essigsäure führt zur Ausbildung einer polymeren Vorstufe [Ru2(OAc)2(CO)4]n, die aus einer
zweikernigen Ruthenium(I)-Einheit besteht. Die Metallzentren sind hier durch zwei Acetato-
Liganden verbunden, wobei jedes Ruthenium zusätzlich durch zwei Carbonylliganden
koordiniert ist. Im nächsten Schritt wurde durch die Reaktion mit den N,N-Donorliganden
Pyrazin (pyz), 1,4-Diazabicyclo[2.2.2]octan (DABCO), 4,4´-Bipyridin (4,4´-bipy), trans-1,2-
bis(N-methylimidazol-2-yl)ethylene (trans-bie) (2) und 1,2-di(4-pyridyl)ethylene (bpe) das
entsprechende 1D Koordinationspolymer mit der Formel [Ru2(OAc)2(CO)4(L)]n (11-15)
hergestellt (Abbildung 3).
N N
N N
Zn(OAc)2 x 2 H2O
3
10
N
N
Zn
Zn
Zn
O
OO
O
O
OO
O
O
OO
O
N
N
N
N
N
N
ZnO
OZn O
OO
O
O
O
n
Zusammenfassung
88
Abbildung 3: Synthese von Sawhorse 1D-Koordiniationspolymeren mit den N,N -Donorliganden L1-L5.
Diese Verbindungen wurden mittels Elementaranalyse, Infrarotspektroskopie und UV/Vis-
Absorptionsspektroskopie charakterisiert. Es war des weiteren möglich die metallorganischen
Koordinationspolymere 11 und 12 durch Einkristall Röntgenstrukturanalyse zu
charakterisieren. Innerhalb eines Polymeres alternieren die dinuklearen Einheiten in einer
“up-down” Anordnung. Dabei sind die Acetatbrücken einer Einheit zu den Carbonylgruppen
der nächsten Einheit benachbart. In einer Kooperation mit der Arbeitsgruppe CLARK wurden
die unterschiedlichen Bindungsordnungen durch DFT und CASSCF, ähnlich wie für die
bereits beschriebene Paddlewheelpolymere, berechnet. Die Metall-Metall Mayer Bond Orders
(MBO) von 0.66 entsprechend für DFT und die Effektiven Bindungsordnung (EBO) von 0.86
zeigen eine Schwächung der möglichen Einfachbindung (σ-Orbitale) durch die
Ligandenkoordination. Für weitere Untersuchen wurde das Polymer [Ru2(OAc)2(CO)4(trans-
bie)]n (11) mittels Rastertunnelmikroskopie hin untersucht. Es konnte aufgrund der
Oberflächenaufnahmen gezeigt werden, dass die Verbindung sich an einer Defektlinie der
Oberfläche als lineare Stränge anlagert (Abbildung 4). Eine genaue Antwort über die
1. THF, Δ2. ligand LRu Ru
n
C C
O OO O
OCOC
OO
Ru Ru
n
C C
O OO O
OCOC
OO
L
N
NN
N
N N
NN
N NNN
pyz DABCO 4,4´-bipy
trans-bie (2) bpe
Zusammenfassung
89
Anordnung der Verbindung als Polymer oder als dinuklearer Komplex auf der Oberfläche,
kann aufgrund der bisher erhaltenen Aufnahmen nicht gegeben werden.
Abbildung 4: STM Topografieaufnahme von [Ru2(OAc)2(CO)4(THF)2] gemischt mit einer Lösung von trans-bie
in THF aufgebracht auf HOPG.
90
6 EXPERIMENTAL SECTION
Experimental section
91
6.1 Physical part
One part of this thesis was to perform scanning tunneling microscopy (STM) and current
imaging tunneling spectroscopy (CITS) images of different coordination polymers on an
highly ordered pyrolytic graphite (HOPG) surface under ambient conditions. The
experimental setup including the software, electronics and scan head were developed by the
physics department of the university of Erlangen-Nürnberg.[159]
6.1.1 Experimental
The measurement setup can be divided into three subareas:
• Computer and software
• Control- and amplifier electronics
• Scan head
Scheme 16 shows each subareas as well as measuring (tunneling current) and control (X, Y,
Z) signals. To control the STM device an Intel x86 compatible computer with Windows XP,
as operating system, was used. The measurement software is a self-developed software based
on the AFM (atomic force microscope) software MINIMA of Neuron Star Computing. The
analogue electronic part consists of two junction boxes, which allows better signals of the DA
(digital-to-analogue) and AD (analogue-to-digital) cards, a module for gradient compensation,
an analogue regulatory circuit to maintain the tunnel current and a high voltage amplifier. In
these experiments the scan head was moved over a fixed sample. In all measurements
platinum-iridium alloy wires (90/10) were used as tips. [159]
For the purpose of STM imaging a drop-drying method was used. In this preparation method,
a drop of 10–9 M solution of the polymer was deposited onto a freshly cleaved HOPG surface.
Distances in the STM images were calibrated by observing the lattice constant of HOPG. All
topography images were recorded in constant current mode. Typically, for the STM
measurements, tunneling currents between 5 and 100 pA were employed. The Bias voltage
was ± 50 mV to ± 100 mV for topography measurements. The scan frequency was varied
between 2 and 5 Hz. Resolution was 256 × 256 points for topography.
Experimental section
92
Scheme 16: Schematic measurement setup.[159]
6.2 Chemical part
6.2.1 General remarks
6.2.1.1 Working techniques
All air sensitive compounds were prepared under dry N2 atmosphere (or dry Ar where
mentioned) using conventional Schlenk techniques, unless noted otherwise. Purchased
solvents (p.a. grade, < 50 ppm H2O) were degassed prior to use and stored under N2 or Ar
atmosphere.
6.2.1.2 Instrumentation
Elemental analyses were determined with a Euro EA 3000 (Euro Vector) and EA 1108
(Carlo Erba) instrument (σ = ± 1 % of the measured content). IR spectra were recorded with
an Excalibur FTS-2500 FTIR as KBr pellets. KBr pellets were prepared using a Perkin-Elmer
Experimental section
93
hydraulic press (10 t/cm2). 1H and 13C NMR spectra were measured with a Bruker DPX300
Avance instrument. The δ values are given relative to tetramethylsilane or the deuterated
solvent. Mass spectra were recorded with a Bruker Daltonics maXis ultrahigh resolution
ESI-TOF MS. Peaks were identified by using simulated isotopic patterns created within the
Bruker Data Analysis software. Melting points were measured with an Electrothermal digital
melting point apparatus (capillary). X-ray structure determinations were carried out on a
Buker-Nonius Kappa-CCD diffractometer. Powder XRD analyses were measured with a
Philips X’Pert powder diffractometer with Cu Kα radiation (40 kV, 40 mA).
6.2.1.3 Chemicals
Following chemicals were used as purchased without further purification:
• [Rh2(OAc)4] × 2 H2O
• 1,2-Bis(4-pyridyl)ethylene (bpe)
• 1,2-Dimethylimidazole
• 1,4-Diazabicyclo[2.2.2]octane (DABCO)
• 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU)
• 4,4'-bipyridine (4,4'-bipy)
• n-Butyllithium (n-BuLi)
• Pyrazine (pyz)
• Trifluoroacetic anhydride (TFAA)
• Zn(OAc)2 × 2 H2O
Following chemicals were synthesized according to literature methods:
• [Cr2(OAc)4] × 2 H2O[188]
• [Mo2(OAc)4][181]
• [Ru2(OAc)2(CO)4]n[230]
• [Ru2(OAc)4] × 2 THF[235]
• 1-Methyl-2-trimethylsilylethynylimidazole[220]
• 2-Ethynyl-1-methylimidazole[220]
• 2-Iodo-1-methylimidazole[236]
• N-Methyl-2-imidazolecarboxaldehyde[154]
Experimental section
94
6.2.2 Synthesis of ligands
6.2.2.1 Synthesis of rac-1,2-Hbmie (1)[145]
1,2-Dimethylimidazole (15.0 mL, 16.3 g, 169 mmol) was mixed with THF (600 mL), and
cooled to –40 °C. After 15 min, n-BuLi (68.0 mL, 170 mmol, 2.5 M in hexanes) was added
dropwise while stirring. After 2 h the mixture was cooled to –80 °C and N-methyl-2-
imidazolecarboxaldehyde (18.6 g, 170 mmol) in THF (500 mL) was added dropwise over a
period of 1 h. The reaction mixture was allowed to warm slowly to ambient temperature while
stirring. After 72 h water (50 mL) was added and all volatiles were removed by rotary
evaporation. The resulting brown residue was recrystallized from toluene to yield an offwhite
powder (12.7 g, 61.7 mmol, 36 %). M.p. 134 °C (dec.). EA: C10H14N4O (206.24 g mol–1):
calcd. C 58.24, H 6.84, N 27.17; found C 58.25, H 6.97, N 27.22 %. 1H NMR (CDCl3):
δ = 3.29, 3.44 (ABX system, 2JAB = 16.1 Hz, 3JH,H 3.4 Hz, 3JH,H = 9.4 Hz, 2H, CH2), 3.58 (s,
3H, CH3), 2.73 (s, 3H, CH3), 5.33 (dd, 3JAX 3.6 Hz, 3JBX = 9.3 Hz), 5.40 (s, 1H, OH), 6.81 (d,
1H, 3JH,H 1.1 Hz, CHim), 6.84 (d, 1H, 3JH,H 1.0 Hz, CHim), 6.92 (s, 2H, CHim) ppm. 13C NMR
(CDCl3): δ = 31.2 (CH2), 32.8 (CH3), 33.2 (CH3), 64.6 (CHOH), 120.7 (CHim), 122.1 (CHim),
126.6 (CHim), 126.9 (CHim), 146.6 (Cim), 127.8 (Cim) ppm. IR (KBr): ! = 3107 (s), 2837 (m),
1532 (w), 1490 (s), 1415 (w), 1316 (w), 1281 (m), 1148 (w), 1120 (m), 1081 (w), 1054 (m),
1025 (m), 933 (w), 915 (w), 777 (w), 738 (s), 714 (m), 662 (w), 522 (w) cm–1.
6.2.2.2 Synthesis of trans-bie (2)[145]
NN
HO
N N
NN
N N
Experimental section
95
rac-1,2-Hbmie (1.00 g, 4.85 mmol) was dissolved in THF (100 mL) and reacted with TFAA
(2.02 mL, 3.05 g, 14.6 mmol). After stirring for 30 min at ambient temperature DBU
(4.35 mL, 4.43 g, 29.1 mmol) was added and the mixture was heated to 50 °C. After 1 h, all
volatiles were removed by rotary evaporation. The brown residue was suspended in water
(100 mL) and was extracted with CH2Cl2 (4 × 100 mL). The combined organic layers were
dried over Na2SO4 and the solvent was removed by rotary evaporation. Column
chromatography (silica, 3 × 20 cm, CH2Cl2 / MeOH / NEt3 = 1000 /50 / 1) and recrystalli-
zation from hot water yielded an offwhite powder (623 mg, 3.31 mmol, 68 %) M.p. 223 °C
(dec.). EA: C10H12N4 (188.23 g mol–1): calcd. C 63.81, H 6.43, N 29.77; found: C 63.91,
H 6.36, N 29.43 %. 1H NMR (CDCl3): δ = 3.73 (s, 6H, CH3), 6.90 (s, 2H, CHim), 7.09 (s, 2H,
CHim), 7.40 (s, 2H, CH2) ppm. 13C NMR (CDCl3): δ = 32.9 (CH3), 116.2 (CHim), 122.1 (CH2),
129.2 (CHim), 145.5 (Cim) ppm. IR (KBr): ! = 3432 (br), 3107 (m), 3098 (m), 2953 (w), 2883
(w), 1802 (w), 1521 (w), 1478 (s), 1455 (m), 1413 (s), 1289 (s), 1267 (m), 1131 (m), 1088
(w), 1043 (w), 962 (s), 770 (s), 729 (s), 658 (s), 541 (m) cm–1.
6.2.2.3 Synthesis of bmib (3)[218]
2-ethynyl-N-methylimidazole (530 mg, 5.00 mmol) was reacted with CuCl (49.0 mg,
0.500 mmol) in pyridine (3 mL) under oxygen atmosphere for 1.5 h at 45 °C. Pyridine was
removed in vacuum, the brown residue was washed with aqueous NH4Cl solution (10 %,
70 mL), and extracted with CH2Cl2 (3 × 100 mL). The combined organic phases were washed
(sat. NH4Cl solution), and dried (MgSO4). Solvent was removed and the product was purified
by column chromatography (silica, 3 × 30 cm, CHCl3 / MeOH / NEt3 = 12 /1 / 1, v/v/v) to
yield a yellow-brown powder. Crystals suitable for X-Ray diffraction analysis were obtained
by slow evaporation from a solution of bmib in acetone. Yield (490 mg, 2.33 mmol, 93 %). 1H NMR (CDCl3): δ = 3.70 (s, 3H, CH3), 6.89 (s, 1H, CHim), 7.03 (s, 1H, CHim) ppm. 13C NMR (acetone-d6): δ = 33.9 (CH3), 73.7 (-C≡C-C≡C-), 76.5 ((-C≡C-C≡C-), 124.6 (Cim),
NN
N N
Experimental section
96
131.0 (Cim), 132.2 (Cim) ppm. EA: C12H10N4 (210.09 g mol–1): calcd. C 68.56; H 4.79;
N 26.65; found C 68.96; H 4.43; N 27.21 %. IR (KBr): ! = 2153 (w), 1709 (w), 1619 (w),
1509 (s), 1477 (s), 1454 (m), 1445 (m), 1415 (m), 1401 (m), 1388 (m), 1355 (m), 1287 (s),
1192 (m), 1150 (m), 1139 (m), 1083 (w), 1046 (w), 921 (w), 911 (w), 863 (w), 771(w), 754
(s), 697(w), 691(w), 623 (w), 598 (w), 537 (w) cm−1.
6.2.3 Synthesis of paddlewheel polymers
6.2.3.1 Synthesis of [Rh2(OAc)4(trans-bie)]n (4)
In a test tube, under atmospheric conditions, a solution of ligand trans-bie (40.0 mg,
0.209 mmol) in MeCN (10 mL) was added to a solution of [Rh2(OAc)4] × 2 H2O (100 mg,
0.209 mmol) in MeCN (10 mL). Immediately a pink precipitate formed which was separated
by filtration, washed with MeCN (3 × 10 mL), and dried in vacuum to yield a pink powder
(56.0 mg, 0.0888 mmol, 42 %). Crystals of the compound, suitable for X-ray structure
determination, were obtained by layering [Rh2(OAc)4] × 2 H2O in THF with trans-bie in
MeOH. M.p. 284 °C (dec.). EA: C18H24Rh2N4O8 (630.22 g mol–1): calcd. C 34.30, H 3.84,
N 8.89; found C 34.36, H 3.67, N 9.05 %. IR (KBr): ! = 2927 (w), 1592 (s, CO2), 1530 (w),
1494 (w), 1420 (m, CO2), 1357 (w), 1342 (w), 1293 (w), 1277 (w), 1262 (w), 1223 (w), 1151
(m), 1047 (w), 1026 (w), 984 (w), 939 (w), 841 (w), 765 (w) 744 (w), 720 (m), 694 (m), 673
(w), 626 (w), 591 (w), 524 (w) cm–1.
Rh Rh
n
O O
O OO O
OO
N N
N N
Experimental section
97
6.2.3.2 Synthesis of [Ru2(OAc)4(trans-bie)]n (5)
In a schlenk tube, a solution of ligand trans-bie (37.0 mg, 0.200 mmol) in THF (5 mL) was
added to a solution of [Ru2(OAc)4] × 2 THF (115 mg, 0.200 mmol) in THF (5 mL). After 1 h,
a brown precipitate was separated by filtration, washed with THF (5 mL) and Et2O (5 mL),
and dried in vacuum to yield a brown powder (89.0 mg, 0.141 mmol, 71 %). Crystals of the
compound, suitable for X-ray structure determination, were obtained by layering
[Ru2(OAc)4] × 2 THF in THF with trans-bie in MeCN. M.p. 276 °C (dec.). EA: C18H24Ru2N4O8 (630.22 g mol–1): calcd. C 34.51, H 3.96, N 8.94; found C 34.35, H 3.60,
N 8.03 %. IR (KBr): ! = 3257 (w), 2961(w), 1569 (s, CO2), 1533 (w), 1493 (m), 1453 (m),
1429 (s, CO2), 1343 (w), 1292 (w), 1277 (w), m 1147 (w), 1045 (w), 982 (w), 937 (w), 764
(w), 745 (w), 721 (w), 684 (w), 672 (w), 620 (w), 573 (w) cm–1.
6.2.3.3 Synthesis of [Mo2(OAc)4(trans-bie)]n (6)
In a schlenk tube, a solution of ligand trans-bie (79.0 mg, 0.420 mmol) in THF (10 mL) was
added to a solution of [Mo2(OAc)4] (60.0 mg, 0.140 mmol) in THF (22 mL). After 20 min, a
Ru Ru
n
O O
O OO O
OO
N N
N N
Mo Mo
n
O O
O OO O
OO
N N
N N
Experimental section
98
yellow precipitate was separated by filtration, washed with THF (2 × 10 mL), and dried in
vacuum to yield a yellow powder (16.0 mg 0.0259 mmol, 19 %). Crystals of the compound,
suitable for X-ray structure determination, were obtained by layering [Mo2(OAc)4] in THF
with trans-bie in MeCN. M.p. 209 °C (dec.). EA: C18H24Mo2N4O8 (616.33 g mol–1): calcd.
C 35.08, H 3.92, N 9.09; found C 35.26, H 4.04, N 9.46 %. IR (KBr): ! = 3116 (w), 2928
(w), 1645 (s), 1527 (w, CO2), 1473 (w), 1459 (s), 1433 (w, CO2), 1420 (w), 1350 (s), 1286
(s), 1276 (m), 1130 (m), 1043 (s), 1020 (s), 960 (m), 928 (s), 874 (s), 772 (m), 727 (m), 672
(m), 663 (m), 631 (s), 572 (s), 518 (s) cm–1.
6.2.3.4 Synthesis of [Cr2(OAc)4(trans-bie)]n (7)
In a schlenk tube, a solution of ligand trans-bie (34.0 mg, 0.186 mmol) in MeCN (20 mL)
was added to a solution of [Cr2(OAc)4] × 2 H2O (70.0 mg, 0.186 mmol) in MeCN (50 mL).
Immediately a violet precipitate was separated by filtration, washed with MeCN (5 mL), and
dried in vacuum to yield a violet powder (42.0 mg, 0.0794 mmol, 44 %). M.p. 76 °C (dec.).
EA: C18H24Cr2N4O8 (528.40 g mol–1): calcd. C 40.91, H 4.58, N 10.60; found C 40.84, H 4.82,
N 10.67 %. IR (KBr): ! = 2924 (m), 1737 (w), 1597 (s, CO2), 1437 (s, CO2), 1344 (w), 1290
(w), 1149 (w), 1048 (w), 1031(w), 992 (w), 935 (w), 860 (w), 781 (w), 752 (w), 728 (m), 678
(m), 622 (w), 523 (w) cm–1.
Cr Cr
n
O O
O OO O
OO
N N
N N
Experimental section
99
6.2.3.5 Synthesis of [Cr2(OAc)4(trans-bie)2] (8)
In a schlenk tube, a solution of [Cr2(OAc)4] × 2 H2O (20.0 mg, 0.0532 mmol) in THF (5 mL)
was layered with MeCN (4 mL). A solution of trans-bie (10.0 mg, 0.0532 mmol) in MeCN
(5 mL) was deposited on top of it. After two days a few red crystals were collected and dried
in vacuum to give a red powder (11.0 mg, 0.0153 mmol, 40 %). Crystals of the compound,
suitable for X-ray diffraction determination, were obtained from the reaction solution. M.p. 63 °C (dec.). EA: C28H36Cr2N8O8 (716.63 g mol–1): calcd. C 46.93, H 5.06, N 15.64; found
(crystals suitable for X-ray structure determination): C 46.96, H 4.52, N 14.93 %. IR (KBr):
! = 3115 (w), 3053 (w), 2938 (w), 1597 (s, CO2), 1526 (w), 1514 (w), 1481 (w), 1473 (w),
1456 (m), 1436 (s, CO2), 1416 (m), 1352 (w), 1344 (m), 1308 (w), 1289 (w), 1268 (w), 1149
(w), 1132 (w), 1085 (w), 1051 (w), 1032 (w), 992 (w), 934 (w), 859 (w), 844 (w), 781 (w),
751 (w), 728 (m), 693 (w), 678 (w), 622 (w), 523 (w) cm–1.
6.2.4 Synthesis of zinc-polymers
6.2.4.1 Synthesis of [Zn3(OAc)6(trans-bie)]n (9)
Cr Cr
O O
O OO O
OO
N N
N N
N N
N N
NZn Zn Zn
OOOO
O OO OO
O
O
O
N
N N
n
Experimental section
100
In a test tube, under air, a solution of Zn(OAc)2 × 2 H2O (100 mg, 0.455 mmol) in MeOH
(2 mL) was layered with a solution of trans-bie (85.8 mg, 0.455 mmol) in THF (15 mL).
After two days white crystals were collected and dried in vacuum to give a white powder
(21.0 mg, 0.0284 mmol, 6 %). Crystals of the compound, suitable for X-ray structure
determination, were obtained from the reaction solution. M.p. 255 °C (dec.). EA:
C22H30Zn3N4O12 (738.63 g mol–1): calcd. C 35.77, H 4.09, N 7.59; found (measured crystals):
C 36.13, H 4.09, N 7.25 %. 1H NMR (CDCl3): δ = 1.79 (s, 18 H, OAc), 3.75 (s, 6H, CH3),
7.05 (s, 2H, CHIm), 7.29 (s, 2H, CHIm), 7.36 (s, 2H, CH2) ppm. IR (KBr): ! = 3435 (br), 3135
(w), 3115 (w), 2927 (w), 1651 (m), 1592 (s), 1493 (m), 1422 (m), 1342 (w), 1299 (m), 1158
(w), 1049 (w), 1023 (w), 966 (w), 957 (w), 925 (w), 793 (w), 768 (w), 730 (w), 672 (w), 617
(w), 525 (w) cm–1.
6.2.4.2 Synthesis of [Zn5(OAc)10(bmib)2]n (10)[218]
In a test tube, under air, a solution of Zn(OAc)2 × 2 H2O (104 mg, 0.472 mmol) in THF
(35 mL) was added to a solution of bmib (50.0 mg, 0.236 mmol) in MeCN (15 mL). After
three days of slowly evaporation of the solvent brown crystals, suitable for X-ray structure
determination, were obtained.
N
N
Zn
Zn
Zn
O
OO
O
O
OO
O
O
OO
O
N
N
N
N
N
N
ZnO
OZn O
OO
O
O
O
n
Experimental section
101
6.2.5 Synthesis of sawhorse-type polymers
6.2.5.1 Synthesis of [Ru2(OAc)2(CO)4(trans-bie)]n (11)
A suspension of [Ru2(OAc)2(CO)4]n (170 mg, 0.790 mmol) in THF (50 mL) was heated under
reflux for 6 h. Afterwards a solution of trans-bie (192 mg, 1.02 mmol) in THF (50 mL) was
added dropwise and the heating was continued for 1 h. The suspension was allowed to cool
down to room temperature. The precipitate was separated by filtration, washed with THF
(2 × 10 mL) and Et2O (5 mL) and dried in vacuum to yield a yellow powder (217 mg,
0.350 mmol, 89 %). Crystals of the compound, suitable for X-ray structure determination,
were obtained by a similar method. The metal precursor [Ru2(OAc)2(CO)4]n (50.0 mg,
0.231 mmol) was converted into a solvated complex by refluxing in THF (20 mL) for 6 h.
After cooling to room temperature the THF phase was layered with trans-bie (56.6 mg,
0.300 mmol) in MeOH (50 mL). M.p. 271 °C (dec.). EA: C18H18Ru2N4O8 (620.50 g mol–1):
calcd. C 34.84, H 2.92, N 9.03; found C 34.84, H 2.89, N 8.91 %. IR (KBr):!! = 3134 (w),
2927 (w), 2022 (s, CO), 1985 (w), 1972 (s, CO), 1939 (s, CO), 1932 (s), 1908 (w), 1574 (s,
CO2), 1474 (m), 1442 (m, CO2), 1347 (w), 1280 (w), 1149 (w), 1045 (w), 766 (w), 760 (w),
723 (w), 698 (w), 689 (w), 667 (w), 589 (w), 539 (w), 521 (w) cm–1. HRMS (ESI-TOF,
MeOH) m/z (%): 751.02 (100) [Ru2(OAc)(CO)4(trans-bie)2]+.
Ru Ru
n
C C
O OO O
OCOC
N N
N N
OO
Experimental section
102
6.2.5.2 Synthesis of [Ru2(OAc)2(CO)4(pyz)]n (12)
A suspension of [Ru2(OAc)2(CO)4]n (160 mg, 0.740 mmol) in THF (50 mL) was heated under
reflux for 6 h. Afterwards a solution of pyrazine (77.1 mg, 0.960 mmol) in THF (50 mL) was
added dropwise and the heating was continued for 1 h. The suspension was allowed to cool
down to room temperature. The precipitate was separated by filtration, washed with THF
(2 × 10 mL) and Et2O (5 mL) and dried in vacuum to yield a yellow powder (177 mg,
0.345 mmol, 93 %). Crystals of the compound, suitable for X-ray structure determination,
were obtained by a similar method. The metal precursor [Ru2(OAc)2(CO)4]n (50.0 mg,
0.231 mmol) was converted into a solvated complex by refluxing in THF (10 mL) for 6 h.
After cooling to room temperature the THF-phase was layered with pyrazine (24.0 mg,
0.300 mmol) in MeCN (20 mL). M.p. 262 °C (dec.). EA: C12H10Ru2N2O8 (512.36 g mol–1):
calcd. C 28.13, H 1.97, N 5.47; found C 28.43, H 1.72, N 5.33 %. IR (KBr): ! = 2924 (s),
2852 (s), 2029 (s, CO), 1972 (s, CO), 1953 (s, CO), 1923 (w), 1909 (w), 1564 (s, CO2), 1482
(w), 1436 (m, CO2), 1350 (w), 1159 (w), 1127 (w), 1055 (w), 812 (w), 805 (w), 701 (w), 690
(w), 667 (w), 622 (w), 589 (w), 537 (w), 525 (w), 473 (w) cm–1.
6.2.5.3 Synthesis of [Ru2(OAc)2(CO)4(4,4’-bipy)]n (13)
Ru Ru
n
C C
O OO O
OCOC
N
OO
N
Ru Ru
n
C C
O OO O
OCOC
N
OO
N
Experimental section
103
A suspension of [Ru2(OAc)2(CO)4]n (150 mg, 0.694 mmol) in THF (50 mL) was heated under
reflux for 6 h. Afterwards a solution of 4,4’-bipyridine (140 mg, 0.902 mmol) in THF
(50 mL) was added dropwise and the heating was continued for 1 h. The suspension was
allowed to cool down to room temperature. The precipitate was separated by filtration,
washed with THF (2 × 10 mL) and Et2O (5 mL) and dried in vacuum to yield a yellow
powder (182 mg, 0.309 mmol, 89 %). M.p. 251 °C (dec.). EA: C18H14Ru2N2O8
(588.45 g mol–1): calcd. C 36.74, H 2.40, N 4.76; found C 36.64, H 2.32, N 4.75 %. IR
(KBr): ! = 2928 (s), 2024 (s, CO), 1969 (m, CO), 1938 (s, CO), 1604 (m), 1571 (s, CO2),
1486 (w), 1440 (m, CO2), 1220 (w), 1069 (w), 813 (w), 698 (w), 689 (w), 671 (w), 632 (w),
591 (w), 541 (w) cm–1.
6.2.5.4 Synthesis of [Ru2(OAc)2(CO)4(bpe)]n (14)
A suspension of [Ru2(OAc)2(CO)4]n (208 mg, 0.962 mmol) in THF (50 mL) was heated under
reflux for 6 h. Afterwards a solution of 1,2-bis(4-pyridyl)ethylene (bpe) (235 mg, 1.25 mmol)
in THF (50 mL) was added dropwise and the heating was continued for 1 h. The suspension
was allowed to cool down to room temperature. The precipitate was separated by filtration,
washed with THF (2 × 10 mL) and Et2O (5 mL) and dried in vacuum to yield a yellow
powder (169 mg, 0.275 mmol, 57 %). M.p. 245 °C (dec.). EA: C20H16Ru2N2O8
(614.49 g mol–1): calcd. C 39.09, H 2.62, N 4.56; found C 38.82, H 2.59, N 4.51 %. IR
(KBr): ! = 2927 (s), 2022 (s, CO), 1968 (m, CO), 1934 (s, CO), 1607 (m), 1571 (m, CO2),
1500 (w), 1435 (m, CO2), 1348 (w), 1218 (w), 1204 (w), 1067 (w), 1016 (w), 973 (w), 828
(w), 698 (w), 689 (w), 670 (w), 591 (w), 552 (w), 522 (w) cm–1.
Ru Ru
n
C C
O OO O
OCOC
N
OO
N
Experimental section
104
6.2.5.5 Synthesis of [Ru2(OAc)2(CO)4(DABCO)]n (15)
A suspension of [Ru2(OAc)2(CO)4]n (182 mg, 0.842 mmol) in THF (50 mL) was heated under
reflux for 6 h. Afterwards a solution of 1,4-diazabicyclo[2.2.2]octane (123 mg, 1.09 mmol) in
THF (50 mL) was added dropwise and the heating was continued for 1 h. The suspension was
allowed to cool down to room temperature. The developed solid was separated by filtration,
washed with THF (2 × 10 mL) and Et2O (5 mL) and dried in vacuum to yield a yellow
powder (52.0 mg, 0.0955 mmol, 23 %). M.p. 288 °C (dec.). EA: C14H18Ru2N2O8
(544.44 g mol–1): calcd. C 30.88, H 3.33, N 5.15; found C 31.06, H 3.57, N 5.45 %. IR (KBr):
! = 2927 (s), 2027 (s, CO), 1965 (s, CO), 1937 (s, CO), 1903 (s), 1884 (w), 1564 (m, CO2),
1467 (m), 1441 (m, CO2), 1323 (w), 1058 (w), 998 (w), 794 (m), 700 (w), 690 (w), 592 (w),
540 (w) cm–1.
6.3 Computational details
The geometries of single [Ru2(OAc)2(CO)2L2] units obtained from the crystal structures were
used in all calculations.
DFT calculations were performed with the ORCA 3.0 program package.[237] The def2-TZVP
basis set (including effective core potentials on Ru) in combination with the def2-TZVP/J
Coulomb fitting basis for the resolution of identity (RI for the GGA) was used in all
calculations.[238-239] The GGA functional BP86[201-202] and two hybrid functionals, PBE0[206-207]
and B3LYP[203-205] (with parameters VWN-III[240] for the local part of the correlation energy)
were used. A very tight SCF convergence criteria (energy change of 10–9 Eh) and a larger
integration grid for the numerical integration (grid5) were chosen.
State-averaged CASSCF/CASPT2 calculations were performed with the MOLCAS 7.8
package[241]. The natural orbital type basis sets ANO-RCC-VTZP for the metal atoms and
Ru Ru
n
C C
O OO O
OCOC
N
OO
N
Experimental section
105
ANO-RCC-VDZP for all other atoms were used.[242-243] The Douglas-Kroll-Hess Hamiltonian
was used to include scalar relativistic effects. To speed up the calculation of the two-electron
integrals the Cholesky decomposition method with an atomic compact basis set was applied.
An imaginary shift of 0.1 units was added to the external part of the zero order Hamiltonian to
prevent intruder states in the CASPT2 calculations. The C2 symmetry of the complexes was
used in the calculations. The EBO were calculated from the sums of the occupation numbers
of bonding (!!) and antibonding (!!) orbitals, equation:
!"# = !!! − !!2
Broken Symmetry DFT Calculations
The BS calculations were performed in accordance to the phenomenological Heisenberg-
Dirac-van-Fleck Hamiltonian !!" = −2!!!!!!, with the spin angular momentum operators !
and the magnetic exchange coupling constant !. The energy of a state with spin !!"# is
!!"# = −! !!"# !!"# + 1 − !! !! + 1 − !! !! + 1 . The singlet-triplet splitting can then
be obtained by eq. (1):
!! − !! = −2! (1)
The exchange coupling constant ! was derived by spin projection according to Yamaguchi[211-
212], eq. (2):
! = − !!"!!!"!! !"! !! !"
(2)
106
7 APPENDIX
Appendix
107
7.1 Details of the structure determinations
Single crystals of the complexes have been obtained by solvent diffusion (layering technique)
as summarized in Table 19.
Table 19: Solvents for the crystallization of the synthesized complexes.
Complex Metal precursor
dissolved in
Layered with ligand
dissolved in
[Rh2(OAc)4(trans-bie)]n (4) THF MeOH
[Ru2(OAc)4(trans-bie)]n (5) THF MeCN
[Mo2(OAc)4(trans-bie)]n (6) THF MeCN
[Cr2(OAc)4(trans-bie)2] (8) THF MeCN
[Zn3(OAc)6(trans-bie)]n (9) MeOH THF
[Zn5(OAc)10(bmib)2]n (10) THF MeCN
[Ru2(OAc)2(CO)4(trans-bie)]n (11) THF MeOH
[Ru2(OAc)2(CO)4(pyz)]n (12) THF MeCN
A Bruker-Nonius Kappa CCD or a Bruker Smart APEXII diffractometer was used for data
collection Single crystals were coated with perfluoropolyether, picked with a glass fiber, and
immediately mounted in the nitrogen cold gas stream of the diffractometer. The structures
were solved by using direct methods and refined with full-matrix least squares against
F2(Siemens SHELX-97).[244] A weighting scheme was applied in the last steps of the
refinement with w = 1/[σ2(Fo2) + (aP)2 + bP] and P = [2Fc
2 + max(Fo2,0)]/3. Hydrogen atoms
were included in their calculated positions and refined in a riding model. All further details
and parameters of the measurements are summarized in Table 20 to Table 23. The structure
pictures were prepared with the program Diamond 2.1e.[245-246]
Appendix
108
Table 20: Details for the structure determination of 4 and 5.
4 5
empirical formula C27H36N6O12Rh3 × 2 MeOH C9H12N2O4Ru × 0.5 THF
formula weight [g mol–1] 1009.43 349.33
crystal color / habit red plate red block
crystal system monoclinic triclinic
space group, Z P 21/c, 4 P1
a [Å] 15.9732(17) 8.2774(8)
b [Å] 8.3335(9) 8.3739(17)
c [Å] 29.297(3) 10.5985(11)
α [°] 90 105.369(12)
β [°] 101.457(2) 107.452(7)
γ [°] 90 93.312(10)
V [Å3] 3822.1(7) 668.18(17)
θ [°] 2.5-28.7 6.25-26.5
h min, max – 14 to 21 – 10 to 10
k min, max – 11 to 11 – 10 to 10
l min, max – 39 to 38 – 13 to 13
F(000) 2028 352
μ(Mo-Kα) [mm–1] 1.35 1.186
crystal size [mm] 0.03 × 0.08 × 0.12 0.067 × 0.104 × 0.110
Dcalcd [g cm–3] 1.754 1.736
T [K] 100(2) 293(2)
reflections collected 37864 7188
indep. reflections 9844 2723
obs. reflections (>2σI) 8594 2193
parameter 504 193
wt. Parameter a 0.0349 0.1704
wt. Parameter b 10.9310 0.0000
R1, wR2 (obsd.) 0.0302, 0.0785 0.0770, 0.2060
R1, wR2 (overall) 0.0374, 0.0823 0.0924, 0.2228
Diff. Peak / hole [e/Å3] 2.036 / – 0.762 2.291 / – 3.417
Goodness-of-fit on F2 1.045 1.015
Appendix
109
Table 21: Details for the structure determination of 6 and 8.
6 8
empirical formula C9H12N2O4Mo C28H36Cr2N8O8
formula weight [g mol–1] 308.15 716.65
crystal color / habit yellow prism purple plate
crystal system monoclinic orthorhombic
space group, Z P21/c, 4 Pbca
a [Å] 9.8956(1) 8.2265(6)
b [Å] 13.8296(1) 18.7004(13)
c [Å] 8.6218(1) 20.32978(14)
α [°] 90 90
β [°] 107.01 90
γ [°] 90 90
V [Å3] 1128.27(2) 3127.5(4)
θ [°] 2.15-29.56 2.85-28.7
h min, max – 13 to 13 – 11 to 10
k min, max – 19 to 18 – 25 to 25
l min, max – 11 to 11 – 27 to 26
F(000) 616 1488
μ(Mo-Kα) [mm–1] 1.163 0.757
crystal size [mm] 0.18 × 0.22 × 0.40 0.03 × 0.16 × 0.20
Dcalcd [g cm–3] 1.814 1.522
T [K] 100(2) 100
reflections collected 21738 27618
indep. reflections 3087 4042
obs. reflections (>2σI) 2888 3426
parameter 148 212
wt. Parameter a 0.0232 0.0322
wt. Parameter b 0.5338 2.4388
R1, wR2 (obsd.) 0.017, 0.0439 0.0296, 0.0749
R1, wR2 (overall) 0.0184, 0.0446 0.0383, 0.0802
Diff. Peak / hole [e/Å3] 0.427 / – 0.555 0.356 / – 0.404
Goodness-of-fit on F2 1.058 1.076
Appendix
110
Table 22: Details for the structure determination of 9 and 10.
9 10
empirical formula C11H15N2O6Zn1.5 C22H25N4O10Zn2.5
formula weight [g mol–1] 369.3 668.89
crystal color / habit colourless prism brown block
crystal system triclinic triclinic
space group, Z P1, 2 P1, 2
a [Å] 8.5151(17) 8.0568(10)
b [Å] 9.4623(19) 10.3125(10)
c [Å] 10.267(2) 16.0151(16)
α [°] 68.04(3) 98.594(8)
β [°] 68.92(3) 94.875(9)
γ [°] 70.41(3) 93.583(9)
V [Å3] 696.3(2) 1307.1(2)
θ [°] 6.25-26.5 6.24-26.5
h min, max – 10 to 10 – 10 to 10
k min, max – 11 to 11 – 12 to 11
l min, max – 12 to 12 – 20 to 19
F(000) 376 680
μ(Mo-Kα) [mm–1] 2.631 2.344
crystal size [mm] 0.12 × 0.23 × 0.33 0.056 × 0.103 × 0.15
Dcalcd [g cm–3] 1.761 1.699
T [K] 150(2) 150(2)
reflections collected 10912 13144
indep. reflections 2831 5321
obs. reflections (>2σI) 2668 3632
parameter 191 356
wt. Parameter a 0.0207 0.0500
wt. Parameter b 0.4225 0.3497
R1, wR2 (obsd.) 0.0187, 0.049 0.0530, 0.1122
R1, wR2 (overall) 0.0207, 0.0503 0.0931, 0.1285
Diff. Peak / hole [e/Å3] 0.341 / – 0.294 0.618 / – 0.711
Goodness-of-fit on F2 1.071 1.015
Appendix
111
Table 23: Details for the structure determination of 11 and 12.
11 12
empirical formula C13H17N2O5Ru × THF C12H10N2O8Ru2 × THF
formula weight [g mol–1] 382.36 584.46
crystal color / habit yellow needle yellow prism
crystal system monoclinic monoclinic
space group, Z C 2/c (No. 15), 8 C 2/c (No. 15), 4
a [Å] 18.690(4) 13.096(3)
b [Å] 8.8273(18) 13.729(3)
c [Å] 19.094(4) 11.603(2)
α [°] 90 90
β [°] 106.09(3) 101.41(3)
γ [°] 90 90
V [Å3] 3026.7(11) 2044.9(7)
θ [°] 6.36-26.5 6.21-26.5
h min, max – 23 to 23 – 16 to 16
k min, max – 11 to 11 – 17 to 17
l min, max – 23 to 23 – 14 to 14
F(000) 1544 1152
μ(Mo-Kα) [mm–1] 1.058 1.527
crystal size [mm] 0.056 × 0.135 × 0.382 0.127 × 0.147 × 0.272
Dcalcd [g cm–3] 1.678 1.898
T [K] 150(2) 150(2)
reflections collected 16389 15963
indep. reflections 3084 2088
obs. reflections (>2σI) 2587 1929
parameter 192 151
wt. Parameter a 0.0194 0.0167
wt. Parameter b 8.6418 14.4923
R1, wR2 (obsd.) 0.0308, 0.06 0.026, 0.0629
R1, wR2 (overall) 0.0453, 0.0645 0.029, 0.0644
Diff. Peak / hole [e/Å3] 0.606 / – 0.479 1.16 / – 0.579
Goodness-of-fit on F2 1.056 1.118
Appendix
112
7.2 Powder X-ray diffraction patterns
Figure 48: Comparison of the powder X-ray diffraction patterns of paddlewheel polymers based on trans-bie: I) [Cu2(OAc)4(trans-bie)]n; II) [Rh2(OAc)4(trans-bie)]n (4); III) [Ru2(OAc)4(trans-bie)]n (5); IV) [Mo2(OAc)4(trans-
bie)]n (6); V) [Cr2(OAc)4(trans-bie)]n (7).
10 20 30 40 50 60 70 80
10 20 30 40 50 60 70 80
!"
#!"
###"
##"
$%&'('%)* +,-Θ.
#"
!"
#!"
###"
##"
* $%&'('%)* +,-Θ.
#"
Appendix
113
Figure 49: Powder X-ray diffraction pattern of [Zn3(OAc)6(trans-bie)]n (9).
10 20 30 40 50 60 70 80
!"#$%$"&' ()*Θ+
,-
Appendix
114
Figure 50: Powder X-ray diffraction patterns of sawhorse typ polymers: I) [Ru2(OAc)2(CO)4(trans-bie)]n (11); II)
[Ru2(OAc)2(CO)4(pyz)]n (12); III) [Ru2(OAc)2(CO)4(4,4´-bipy)]n (13); IV) [Ru2(OAc)2(CO)4(bpe)]n (14); V) [Ru2(OAc)2(CO)4(DABCO)]n (15).
10 20 30 40 50 60 70 80
!"
#!"
###"
##"
$%&'('%)* +,-Θ.
#"
Appendix
115
7.3 UV-Vis spectra
Figure 51: Qualitative UV/Vis absorption spectra of [Rh2(OAc)4(trans-bie)]n (4) and [Rh2(OAc)4] × 2H2O as nujol mull and trans-bie in MeOH.
Figure 52: Qualitative UV/Vis absorption spectra of [Ru2(OAc)4(trans-bie)]n (5) and [Ru2(OAc)4] × 2THF as nujol mull and trans-bie in MeOH.
200 300 400 500 600
Abs
orption0[a.u.]
W ave leng th0[nm]
0[R h2(O A c )
4]0x 02H
2O
0[R h2(O A c )
4(trans Dbie )]
n
0trans Dbie
200 300 400 500 600
Abs
orption0[a.u.]
W ave leng th0[nm]
0[R u2(O A c )
4]0x 02T H F
0[R u2(O A c )
4(trans Fbie )]
n
0trans Fbie
Appendix
116
Figure 53: Qualitative UV/Vis absorption spectra of [Mo2(OAc)4(trans-bie)]n (6) and [Mo2(OAc)4] as nujol mull and trans-bie in MeOH.
Figure 54: Qualitative UV/Vis absorption spectra of [Cr2(OAc)4(trans-bie)]n (7) and [Cr2(OAc)4] × 2H2O as nujol mull and trans-bie in MeOH.
200 300 400 500 600
Abs
orption0[a.u.]
W ave leng th0[nm]
0[Mo2(O A c )
4]
0[Mo2(O A c )
4(trans Bbie )]
n
0trans Bbie
200 300 400 500 600
Abs
orption0[a.u.]
W ave leng th0[nm]
0[C r2(O A c )
4]0x 02H
2O
0[C r2(O A c )
4(trans Dbie )]
n
0trans Dbie
Appendix
117
Figure 55: Qualitative UV/Vis absorption spectra of [Ru2(OAc)2(CO)4(trans-bie)]n (11) and [Ru(OAc)(CO)2]n as nujol mull and trans-bie in MeOH.
Figure 56: Qualitative UV/Vis absorption spectra of [Ru2(OAc)2(CO)4(pyz)]n (12) and [Ru(OAc)(CO)2]n as nujol mull and trans-bie in MeOH.
300 400 500 600 700
Abs
orption0[a.u.]
W ave leng th0[nm]
0[R u(O A c )(C O )2]n
0[R u2(O A c )
2(C O )
4(trans Dbie )]
n
0trans Dbie
200 300 400 500 600
Abs
orption0[a.u.]
W ave leng th0[nm]
0[R u(O A c )(C O )2]n
0[R u2(O A c )
2(C O )
4(py)]
n
0py
Appendix
118
Figure 57: Qualitative UV/Vis absorption spectra of [Ru2(OAc)2(CO)4(4,4´-bipy)]n (13) and [Ru(OAc)(CO)2]n as nujol mull and trans-bie in MeOH.
Figure 58: Qualitative UV/Vis absorption spectra of [Ru2(OAc)2(CO)4(bpe)]n (14) and [Ru(OAc)(CO)2]n as nujol mull and trans-bie in MeOH.
200 300 400 500 600
Abs
orption0[a.u.]
W ave leng th0[nm]
0[R u(O A c )(C O )2]n
0[R u2(O A c )
2(C O )
4(bipy)]
n
0bipy
200 300 400 500 600
Abs
orption0[a.u.]
W ave leng th0[nm]
0[R u(O A c )(C O )2]n
0[R u2(O A c )
2(C O )
4(bpe)]
n
0bpe
Appendix
119
Figure 59: Qualitative UV/Vis absorption spectra of [Ru2(OAc)2(CO)4(DABCO)]n (15) and [Ru(OAc)(CO)2]n as nujol mull and trans-bie in MeOH.
200 300 400 500 600
Abs
orption0[a.u.]
W ave leng th0[nm]
0[R u(O A c )(C O )2]n
0[R u2(O A c )
2(C O )
4(D AB C O )]
n
0D AB C O
Appendix
120
7.4 Computational details
In cooperation with C. WICK from the CLARK group from the computer chemie centrum,
different theoretical calculations were performed. In chapter 7.4 the results were summarised.
7.4.1 IR vibrational bands
To compare the Δ values of the OCO vibrational bands of the IR spectra, theoretical
calculations were performed (chapter 3.1 and 3.3).
Geometries of the model complexes of [M2(OAc)4(trans-bie)2] 4, 5, 6 and
[Ru2(OAc)2(CO)4(trans-bie)2] (11) and [Ru2(OAc)2(CO)4(pyz)2] (12) were optimized with
Gaussian 09[247]. The def2-TZVP[239, 248] basis set was obtained from the EMSL basis set
exchange[249-250]. A scaling factor of 0.959 was used for B3LYP calculations.[251]
Table 24: Experimental and calculated (models) vibrational bands of polymer 4.
[Rh2(OAc)4(trans-bie)]n (4) !asym (OCO) [cm–1]
!sym (OCO) [cm–1]
Δ [cm–1]
exp 1592 1420 172
B3LYP 1636 1436 200
B3LYP scaled 1569 1377 192
BP86 1579 1369 210
Table 25: Experimental and calculated (models) vibrational bands of polymer 5.
[Ru2(OAc)4(trans-bie)]n (5) !asym (OCO) [cm–1]
!sym (OCO) [cm–1]
Δ [cm–1]
exp 1569 1429 140
B3LYP* 1586 1440 146
B3LYP scaled 1521 1381 140
BP86 1557 1377 180 *nimag = 1 (-10.51 cm–1)
Appendix
121
Table 26: Experimental and calculated (models) vibrational bands of polymer 6.
[Mo2(OAc)4(trans-bie)]n (6) !asym (OCO) [cm–1]
!sym (OCO) [cm–1]
Δ [cm–1]
exp 1527 1433 94
B3LYP 1537 1436 101
B3LYP scaled 1473 1377 96
BP86 1483 1372 111
Table 27; Experimental and calculated (models) vibrational bands of polymer 11.
[Ru2(OAc)2(CO)4(trans-bie)2]
(11) !asym
(OCO) [cm–1]
!sym (OCO) [cm–1]
Δ
[cm–1]
! (CO)
[cm–1]
exp 1574 1442 132 2022 1972 1939
B3LYP 1610 1445,
1450
165,
160
2086 2034 2012
B3LYP scaled 1544 1386,
1391
158,
153
2000 1950 1930
pBP86 1552 1384,
1389
168,
163
2005 1964 1933
Table 28: Experimental and calculated (models) vibrational bands of polymer 12.
[Ru2(OAc)2(CO)4(pyz)2] (12) !asym (OCO) [cm–1]
!sym (OCO) [cm–1]
Δ
[cm–1]
! (CO)
[cm–1]
exp 1564 1436 128 2029 1972 1953
B3LYP 1615 1448,
1454
167,
161
2099 2049 2033
B3LYP scaled 1548 1389,
1394
160,
154
2012 1965 1950
BP86 1558 1387,
1392
171,
166
2017 1979 1956
7.4.2 Active space and DFT Mayer Bond Order
Appendix
122
[Rh2(OAc)4(trans-bie)2] CASSCF(18,12) 1Ag active orbitals (Occupation numbers of the ground state in parenthesis)
σ (1.82) σ* (0.18) δ (2.00) δ* (1.99)
1π (1.99) 1π* (1.99) 2π (1.99) 2π* (1.99)
1σ(RhO) (1.99) 2σ(RhO) (1.99) 1σ*(RhO) (0.03) 2σ*(RhO) (0.03)
Appendix
123
[Ru2(OAc)4(trans-bie)2] CASSCF(16,12) 3Ag active orbitals (Occupation numbers of the ground state in parenthesis)
σ (1.87) σ* (0.14) δ (1.99) δ* (1.99)
1π (1.90) 1π* (1.09) 2π (1.90) 2π* (1.09)
1σ(RuO) (1.99) 2σ(RuO) (1.99) 1σ*(RuO) (0.02) 2σ*(RuO) (0.03)
Appendix
124
[Mo2(O2CCH3)4(trans-bie)2] CASSCF(12,12) 1Ag active orbitals (Occupation numbers of the ground state in parenthesis)
σ (1.90) σ* (0.10) δ (1.72) δ* (0.28)
1π (1.87) 1π* (0.13) 2π (1.87) 2π* (0.13)
1σ(MoO) (1.99) 2σ(MoO) (2.00) 1σ*(MoO) (0.01) 2σ*(MoO) (0.01)
Appendix
125
Table 29: [Rh2(OAc)4(trans-bie)2] CASSCF(12,12)/CASPT2 excited states.
State
ΔE
[kcal mol-1]
ΔE
[eV] weight configuration
1Ag 0.0 0.00 88 % σ(M-O)4 σ2 π4 δ2 δ*2 π*4 3Au 40.7 1.77 59 % σ(M-O)4 σ2 π4 δ2 δ*2 π*3 σ*(M-O)1 19 % σ(M-O)4 σ2 π3 δ2 δ*2 π*4 σ*(M-O)1 3Au
41.6 1.80 33 % σ(M-O)4 σ2 π4 δ2 δ*2 π*3 σ*(M-O)1
23 % σ(M-O)4 σ1 π4 δ2 δ*2 π*4 σ*(M-O)1 19 % σ(M-O)4 σ2 π3 δ2 δ*2 π*4 σ*(M-O)1 3Ag
41.8 1.81 51 % σ(M-O)4 σ2 π4 δ2 δ*1 π*4 σ*(M-O)1
32 % σ(M-O)4 σ2 π4 δ1 δ*2 π*4 σ*(M-O)1 3Au
42.0 1.82 44 % σ(M-O)4 σ2 π4 δ2 δ*1 π*4 σ*(M-O)1
36 % σ(M-O)4 σ2 π4 δ1 δ*2 π*4 σ*(M-O)1 1Au
49.9 2.16 62 % σ(M-O)4 σ2 π4 δ2 δ*2 π*3 σ*1 14 % σ(M-O)4 σ2 π4 δ2 δ*2 π*3 σ*(M-O)1 50.5 2.19 54 % σ(M-O)4 σ2 π4 δ2 δ*2 π*3 σ*1 19 % σ(M-O)4 σ2 π4 δ2 δ*2 π*3 σ*(M-O)1 3Ag 50.6 2.19 57 % σ(M-O)4 σ2 π4 δ2 δ*2 π*3 σ*(M-O)1 15 % σ(M-O)4 σ2 π3 δ2 δ*2 π*4 σ*(M-O)1
Appendix
126
Table 30: [Ru2(OAc)4(trans-bie)2] CASSCF(12,12)/CASPT2 excited states.
State ΔE
[kcal mol-1] ΔE [eV] weight configuration
3Ag 0.0 0.00 83 % σ(M-O)4 σ2 π4 δ2 δ*2 π*2 1Ag 10.3 0.45 51 % σ(M-O)4 σ2 π4 δ2 δ*2 1π*2 2π*0
16 % σ(M-O)4 σ2 π4 δ2 δ*2 1π*0 2π*2 1Ag
10.6 0.46 68 % σ(M-O)4 σ2 π4 δ2 δ*2 1π*1 2π*1 3Au 12.3 0.53 77 % σ(M-O)4 σ2 π4 δ2 δ*1 π*3 3Au 14.1 0.61 77 % σ(M-O)4 σ2 π4 δ2 δ*1 π*3 1Ag
17.3 0.75 55 % σ(M-O)4 σ2 π4 δ2 δ*2 1π*0 2π*2 19 % σ(M-O)4 σ2 π4 δ2 δ*2 1π*2 2π*0
1Au 17.7 0.77 55 % σ(M-O)4 σ2 π4 δ2 δ*1 1π*1 2π*2
15 % σ(M-O)4 σ2 π4 δ2 δ*1 1π*2 2π*1 1Au
19.3 0.84 55 % σ(M-O)4 σ2 π4 δ2 δ*1 1π*2 2π*1
15 % σ(M-O)4 σ2 π4 δ2 δ*1 1π*1 2π*2 1Ag
35.0 1.52 47 % σ(M-O)4 σ2 π4 δ2 δ*0 π*4 20 % σ(M-O)4 σ2 π4 δ2 δ*1 π*3 14 % σ(M-O)4 σ2 π4 δ0 δ*2 π*4
3Ag 36.4 1.58 66 % σ(M-O)4 σ2 π4 δ1 δ*2 π*3 17 % σ(M-O)4 σ2 π3 δ2 δ*1 π*4
3Ag 38.1 1.65 65 % σ(M-O)4 σ2 π4 δ1 δ*2 π*3 17 % σ(M-O)4 σ2 π3 δ2 δ*1 π*4
Appendix
127
Table 31: [Mo2(OAc)4(trans-bie)2] CASSCF(12,12)/CASPT2 excited states.
State
ΔE
[kcal mol-1]
ΔE
[eV] weight configuration 1Ag 0.0 0.00 73 % σ(M-O)4 σ2 π4 δ2
8 % σ(M-O)4 σ2 π4 δ*2 3Au 30.6 1.33 80 % σ(M-O)4 σ2 π4 δ1 δ*1 3Au
67.9 2.94 81 % σ(M-O)4 σ1 π4 δ2 δ*1 3Ag 70.8 3.07 59 % σ(M-O)4 σ2 π4 δ1 π*1
21 % σ(M-O)4 σ2 π3 δ2 δ*1 3Ag
72.3 3.13 49 % σ(M-O)4 σ2 π4 δ1 π*1 33 % σ(M-O)4 σ2 π3 δ2 δ*1
1Au 73.3 3.18 71 % σ(M-O)4 σ2 π4 δ1 δ*1
1Au 75.5 3.28 81 % σ(M-O)4 σ1 π4 δ2 δ*1
3Ag 82.6 3.58 31 % σ(M-O)4 σ1 π4 δ2 π*1
30 % σ(M-O)4 σ2 π3 δ2 δ*1 11 % σ(M-O)4 σ2 π4 δ1 σ*(M-O)1
1Ag 84.4 3.66 60 % σ(M-O)4 σ2 π4 δ1 π*1
1Ag 85.5 3.71 58 % σ(M-O)4 σ2 π4 δ1 π*1
15 % σ(M-O)4 σ2 π3 δ2 δ*1
Appendix
128
[Cu2(OAc)4(trans-bie)2] CASSCF(2,2) 1Ag active orbitals (Occupation numbers in
parenthesis)
1σ*(CuO) (0.98) 2σ*(CuO) 1.02)
Appendix
129
[Ru2(OAc)2(CO)4(trans-bie)2] (11) CASSCF/CASPT2(14,8) active space
σ (1.85) σ* (0.15)
δ (2.00) δ* (2.00)
π (2.00) π* (2.00)
π (2.00) π* (2.00)
Appendix
130
[Ru2(OAc)2(CO)4(pyz)2] (12) CASSCF/CASPT2(14,8) active space
σ (1.85) σ* (0.15)
δ (2.00) δ* (2.00)
π (2.00) π* (2.00)
π (2.00) π* (2.00)
Appendix
131
DFT Mayer Bond Orders
Table 32: DFT Mayer Bond Orders calculated for complexes 11 and 12.
[Ru2(OAc)2(CO)4(pyz)2] (12) [Ru2(OAc)2(CO)4(trans-bie)2] (11)
BP86 B3LYP PBE0 BP86 B3LYP PBE0
Ru-Ru 0.66 0.66 0.67 0.66 0.65 0.67
Ru-C 1.25 1.19 1.17 1.26 1.19 1.17
Ru-N 0.42 0.37 0.40 0.43 0.39 0.42
Ru-O 0.49 0.45 0.48 0.45 0.41 0.45
Ru-O (CO) 0.19 0.17 0.16 0.21 0.19 0.17
Appendix
132
7.5 List of abbreviations and symbols
! wavenumber
1D one dimensional
4,4'-bipy 4,4'-bipyridine
AD analogue-to-digital
AFM atomic force microscopy
bmib bis(N-methylimidazol-2-yl)butadiyne
BO bond order
bpe 1,2-bis(4-pyridyl)ethylene
br broad
BS broken-symmetry
Bu butyl
calcd. calculated
CASPT2 complete active space second order perturbation
CASSCF complete active space self-consistent field
CCM constant current mode
CHM constant height mode
CITS current imaging tunneling spectroscopy
d doublet (NMR spectroscopy)
DABCO 1,4-diazabicyclo[2.2.2]octane
DBTF dibromoterfluorene
DBU 1,8-diazabicyclo[5.4.0]undec-7-ene
dd double doublet (NMR spectroscopy)
dec. decomposition
DFT density functional theory
DMF dimethylformamide
dta dithioacetate
EA elemental analysis
EBO effective bond order
Eh Hartree energy
Et ethyl
exp experiment
Appendix
133
GGA generalized gradient approximation
h hour
HOMO highest occupied molecular orbital
HOPG highly ordered pyrolytic graphite
im imidazole, imidazolyl
IR infrared
J coupling constant
LUMO lowest unoccupied molecular orbital
m medium (IR spectroscopy)
m multiplet (NMR spectroscopy)
M.p. melting point
MBO Mayer bond order
Me methyl
MeCN acetonitrile
MOF metal-organic frameworks
MOM metal-organic materials
n-BuLi n-butyllithium
NMR nuclear magnetic resonance
OAc acetate
ppm parts per million
PPV poly(p-phenylenevinylene)
Pr propyl
pyz pyrazine
rac racemic
rac-Hbmie rac-1,2-bis(N-methylimidazol-2-yl)ethanol
RI resolution of identity
s singlet (NMR spectroscopy)
s strong (IR spectroscopy)
SCC supramolecular coordination complex
SCF self-consistent field
SPM scanning probe microscopy
STM scanning tunneling microscopy
STS scanning tunneling spectroscopy
TF terfluorene
Appendix
134
TFAA trifluoroacetic anhydride
THF tetrahydrofuran
trans-bie trans-bis(N-methylimidazol-2-yl)ethylene
w weak (IR spectroscopy)
XRD X-ray diffraction
δ chemical Shift
Appendix
135
7.6 List of compounds
rac-1,2-bis(N-methylimidazol-2-yl)ethanol (rac-Hbmie) (1)
trans-bis(N-methylimidazol-2-yl)ethylene (trans-bie) (2)
bis(N-methylimidazol-2yl)butadiyne (bmib) (3)
[Rh2(OAc)4(trans-bie)]n (4)
[Ru2(OAc)4(trans-bie)]n (5)
[Mo2(OAc)4(trans-bie)]n (6)
[Cr2(OAc)4(trans-bie)]n (7)
[Cr2(OAc)4(trans-bie)2] (8)
[Zn3(OAc)6(trans-bie)]n (9)
[Zn5(OAc)10(bmib)2]n (10)
[Ru2(OAc)2(CO)4(trans-bie)]n (11)
[Ru2(OAc)2(CO)4(pyz)]n (12)
[Ru2(OAc)2(CO)4(4,4´-bipy)]n (13)
[Ru2(OAc)2(CO)4(bpe)]n (14)
[Ru2(OAc)2(CO)4(DABCO)]n (15)
136
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9 DANKSAGUNG
Mein besonderer Dank gilt meinem Doktorvater Prof. Dr. Nicolai Burzlaff für die Aufnahme
in seine Arbeitsgruppe, den großen akademischen Freiraum zur Bearbeitung des interessanten
Themas, seine Unterstützung während der Promotionszeit sowie für meine Empfehlung für
die Graduate School Molecular Science. Ganz herzlich möchte ich mich auch bei Prof. Dr.
Dr. h.c. mult. Rudi van Eldik und dessen Nachfolger Prof. Dr. Sjoerd Harder bedanken für die
Aufnahme in den Lehrstuhl und das positive Arbeitsklima. Ich danke der Graduate School
Molecular Science für die finanzielle Unterstützung, insbesondere Prof. Dr. Norbert Jux.
Weiterhin gebührt mein Dank allen Kooperationspartnern, mit denen ich während meiner
Promotionsarbeit zusammenarbeiten durfte. Ich danke Prof. Dr. Tim Clark, Christian Wick,
Dr. Tatyana E. Shubina und Dr. Pavlo Dral für die theoretischen Berechnungen zu meinen
Polymeren. Prof. Dr. Paul Müller und Dr. Michael Stocker, sowie Prof. Dr. Sabine Maier und
Christian Steiner danke ich für deren Unterstützung bei der Durchführung der STM
Aufnahmen. Prof. Dr. Dirk Guldi und Marc Rudolf für die Untersuchungen der Fluoreszenz-
eigenschaften des Liganden bmib.
Des Weiteren gebührt ein besonderer Dank allen Mitarbeitern des Departments für Chemie
und Pharmazie, die maßgeblich zum Gelingen dieser Arbeit beigetragen haben. Ich danke Dr.
Achim Zahl und Jochen Schmidt (NMR), Dr. Frank Heinemann und Susanne Hoffmann
(XRD), Christina Wronna (EA), Ursula Niegratschka (Sekretariat), Dr. Carlos Dücker-Benfer,
Dr. Jörg Sutter, Dr. Andreas Scheurer und Dr. Christian Färber (Akademische Räte), sowie
allen Mitarbeitern der Werkstatt, der Glasbläserei und der Chemikalienausgabe.
Ich danke allen Personen rund um die AC, die mich auf meinem Weg zur Promotion begleitet
haben, für die Hilfsbereitschaft und Freundschaft, insbesondere den “Harders“ vor allem
Harmen, Julia, Benni, Xian, Andrea und Johanne und den “Ivanas“, speziell Julia, Olli und
Max. Ein herzlicher Dank auch an Johannes Tucher für die Messung zahlreicher
Kristallstrukturen.
149
Ein Dank gilt meinen Laborpartnern Eva und Andy für die tolle Arbeitsatmosphäre, den Spaß
im A 0.2/Keller, die große Unterstützung in stressigen Phasen und zahlreichen fachlichen
Diskussionen. Ein besonderer Dank gilt vor allem Frank und Philipp, welche mich während
dieser Arbeit stets mit Rat und Tat unterstützt haben. Danken möchte ich auch Gazi, meinem
Vorgänger im A 0.2. Ebenso danke ich Fatima, Tom, Stefan, Sascha, Tobi, Thomas und Susy,
sowie den Neuen im Arbeitskreis, Barbara, Lisa, Marleen, Fabian und Stephan, für die
kollegiale Zusammenarbeit und das positive Arbeitsklima. Vielen Dank auch an meine
Mitarbeiterpraktikanten und Bachelorstudenten, die im A 0.2 ihr Unwesen getrieben haben.
Danken möchte ich auch allen Freunden außerhalb der Uni, die stets ein offenes Ohr für
meine Sorgen und Nöte hatten.
Der größte Dank gebührt jedoch meinen Eltern Carola und Klaus, meinem Bruder Ulf und
meiner Freundin Nina, die mich stets unterstützt haben und immer ein offenes Ohr für meine
Probleme hatten. Ohne euer Verständnis und eure Unterstützung wäre diese Arbeit so nicht
möglich gewesen.
− Danke −
!