Aspirin headache solved - Semantic Scholar · 2017-10-27 · Interaction anisotropy and shear instability of aspirin polymorphs established by nanoindentation† Sunil Varughese,a

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© 2011 RSC

DRUG POLYMORPHISM

Aspirin headache solved

Neil Withers

Nature Chemistry 3, 835 (2011) doi:10.1038/nchem.1186

Published online 24 October 2011

Chem. Sci. http://dx.doi.org/10.1039/c1sc00430a (2011)

Polymorphism — the existence of different crystal structures of the same compound

— is a problem in the pharmaceutical industry, because different polymorphs of the

same drug may have different physical properties. Monitoring these subtle differences

at each stage of a rigorous production process is a huge expense. Obtaining high-

quality crystal structures is not always possible or practical in these circumstances.

Now, Sunil Varughese and colleagues from the Indian Institute of Science and the

University of Southern Denmark have used a nanoindentation technique, which can

measure the mechanical properties of very small amounts of solids to extremely high

precision, to study and differentiate between the different polymorphs of aspirin.

Aspirin, although used and studied as a drug for more than a century, has only recently been revealed to have a

metastable polymorphic form, known as form II. The structures of the two polymorphs are very similar in two

dimensions, and form II has been observed to transform into the more stable form I at ambient conditions.

The different physical properties of the polymorphs — such as elastic modulus and hardness — mean that

nanoindentation can be used to differentiate between them, as form II is considerably softer than form I. Varughese

and colleagues discovered that what had appeared to be single crystals of form II in fact contained small domains of

form I — something that single-crystal diffraction had failed to detect.

MECHANOCHEMISTR…

Tearing aparttriazoles

DRUG POLYMORPHI…

Aspirin headachesolved

MAGNETIC MOLECU…

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Cite this: Chem. Sci., 2011, 2, 2236

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Interaction anisotropy and shear instability of aspirin polymorphs establishedby nanoindentation†

Sunil Varughese,a M. S. R. N. Kiran,b Katarzyna A. Solanko,c Andrew D. Bond,*c U. Ramamurty*b

and Gautam R. Desiraju*a

Received 6th July 2011, Accepted 5th August 2011

DOI: 10.1039/c1sc00430a

Nanoindentation is applied to the two polymorphs of aspirin to examine and differentiate their

interaction anisotropy and shear instability. Aspirin provides an excellent test system for the technique

because: (i) polymorphs I and II exhibit structural similarity in two dimensions, thereby facilitating

clear examination of the differences in mechanical response in relation to well-defined differences

between the two crystal structures; (ii) single crystals of the metastable polymorph II have only recently

become accessible; (iii) shear instability has been proposed for II. Different elastic moduli and hardness

values determined for the two polymorphs are correlated with their crystal structures, and the

interpretation is supported by measured thermal expansion coefficients. The stress-induced

transformation of the metastable polymorph II to the stable polymorph I can be brought about rapidly

by mechanical milling, and proceeds via a slip mechanism. This work establishes that nanoindentation

provides ‘‘signature’’ responses for the two aspirin polymorphs, despite their very similar crystal

structures. It also demonstrates the value of the technique to quantify stability relationships and phase

transformations in molecular crystals, enabling a deeper understanding of polymorphism in the context

of crystal engineering.

Introduction

Polymorphism in molecular crystals is an intensively studied

phenomenon.1 Its practical importance arises from the fact that

different crystal structures of the same compound can exhibit

significantly different materials properties. This has particular

implications, for example, in the pharmaceutical industry, where

polymorphism must be rigorously monitored at all stages of the

production process, because of chemical, regulatory and legal

issues.2,3 Current research in polymorphism spans a broad range

of disciplines, including for example crystal structure analysis

and prediction,4 crystallisation monitoring and control,5 and

structure-property correlation.6 Despite this extensive activity,

aSolid State and Structural Chemistry Unit, Indian Institute of Science,Bangalore, 560 012, India. E-mail: [email protected] of Materials Engineering, Indian Institute of Science,Bangalore, 560 012, India. E-mail: [email protected] of Physics and Chemistry, University of Southern Denmark,5230 Odense, Denmark. E-mail: [email protected]

† Electronic supplementary information (ESI) available: Experimentaldetails, including crystallisation protocols and a full description of thenanoindentation studies; tables of calculated attachment energies andintermolecular interaction energies; residual indent impressions for{100} of II. Crystallographic data for form II at 123 and 298 K havebeen deposited with the Cambridge Crystallographic Data Centre.CCDC reference numbers 820697 and 820698. For ESI andcrystallographic data in CIF or other electronic format see DOI:10.1039/c1sc00430a

2236 | Chem. Sci., 2011, 2, 2236–2242

however, quantitative understanding of the mechanical proper-

ties of polymorphic molecular crystals remains rare. This

knowledge gap has an obvious impact in applications where

polymorphs might be subjected to mechanical stress. For

example, stress-induced phase transformations are highly unde-

sirable for pharmaceutical ingredients during milling or tablett-

ing.7 A quantitative description of mechanical properties also has

relevance in computer modelling of molecular crystals, since

tensorial properties such as elastic stiffness or thermal expansion

provide a basis against which to evaluate and optimise atomistic

models in terms of derivatives of calculated energy, rather than

single-point calculations.

Nanoindentation is established as an effective method to

assess the mechanical response of solids with high precision, and

on extremely small volumes.8 A limited number of studies have

been made on organic crystals,9–18 including our own report on

saccharin,19 and there have been other attempts to link measured

values of elastic modulus and hardness to molecular and crystal

structures.20–23 However, very little has been done with poly-

morphs of molecular crystals—indeed, we are aware of only one

other published report where nanoindentation has been explicitly

applied to a polymorphic molecular system.24 Thus, there exists

a clear opportunity for advancement in this area, since poly-

morphic systems should provide an ideal basis to establish

correlations between crystal structure and mechanical response.

In this work, we apply nanoindentation to the two polymorphs

This journal is ª The Royal Society of Chemistry 2011

http://dx.doi.org/10.1039/c1sc00430a






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of aspirin (acetylsalicylic acid). Aspirin provides an excellent test

case for several reasons: (i) the two polymorphs (I and II) exhibit

structural similarity in two dimensions, thereby facilitating clear

examination of the differences in mechanical response in relation

to well-defined differences between the two crystal structures; (ii)

single crystals of aspirin form II have become accessible only

recently; (iii) the mechanical properties of form II are thought to

be implicated in its apparent metastability. In this paper, we use

the comparative data established by nanoindentation to suggest

a rational explanation for the observed phase stability and

transformation between the two crystal forms. Our study

establishes that nanoindentation can provide ‘‘signature’’

responses for the two aspirin polymorphs, despite their very

similar crystal structures, and it demonstrates also the value of

the technique for application to chemical problems associated

with polymorphism in the context of crystal engineering.

Aspirin polymorphism

Aside from its obvious chemical and pharmaceutical impor-

tance,25,26 aspirin exhibits a fascinating and perplexing structural

chemistry. The possible occurrence of aspirin polymorphs has

been debated since the 1960s.27 Recently, Ouvrard and Price28

predicted a new crystal form, referred to as form II, with the

long-known crystal form29 labelled accordingly as form I. Inde-

pendently, Munson and co-workers reported differences in solid-

state NMR spectra measured for freeze-dried aspirin,30 while

Vishweshwar et al.31 reported single-crystal X-ray diffraction

data that claimed32 to show the existence of form II. Some of us

went on to show that aspirin crystals can exist as intergrowths

containing domains of the two structure types,33 and a detailed

crystallographic investigation based on diffuse X-ray scattering

has been made subsequently by Chan et al.34

A brief summary of the structural situation is as follows:

aspirin molecules are linked into centrosymmetric dimers by O–

H/O hydrogen bonds between their carboxyl groups, and these

dimers are arranged into 2-dimensional layers parallel to the

(100) planes that are essentially identical in the two polymorphs.

The distinction between forms I and II is the manner in which the

layers are arranged relative to each other. The form I structure is

related to that of form II by a relative shift of adjacent layers

parallel to one of the crystallographic axes (specifically 1/2 c in

Fig. 1). Previous computational estimates of the lattice energies

at various levels of theory have all concluded that the energetic

difference between the two polymorphs is insignificantly

small.28,35,36

With regard to the crystallisation of aspirin, some of us have

reported that form II domains can be systematically introduced

into aspirin crystals by solution crystallization in the presence of

aspirin anhydride.37 By extension, single crystals were obtained

that appear to be structurally pure aspirin form II—that is,

without form I domains or any apparent disordered regions—

within the detection limits of laboratory CCDX-ray instruments.

The isolated single crystals were found to be stable under

ambient conditions for months, as well as under the application

of hydrostatic pressure up to 2.2 GPa.37 Nonetheless, it is

apparent that form II is metastable with respect to form I, since

we have observed transformation of form II bulk powders under

ambient conditions during our studies of the compound.‡ It has


been suggested that instability of form II may arise because of

a low shear modulus.28 This has been questioned by Bauer et al.

on the basis of experimentally measured elastic constants for

form I and predicted constants for form II,35 which showed no

reason to suspect shear instability. Recourse to predictions for

form II was a requirement in the absence of samples for experi-

mental study, and the question regarding shear instability

therefore remains open.

Results and discussion

Expectations from the crystal structures

Calculated attachment energies, Eatt, (Table S2 in ESI†) confirm

the expectation that the {100} planes, which are the most widely

separated and have the lowest Eatt, should be slip planes for both

polymorphs. The potential slip systems are therefore either {100}

<010> or {100}<001>, meaning slip parallel to the {100} plane

along either the b or c crystallographic axes. The latter is clearly

implicated in the transformation between the two forms. Our

analysis of pairwise intermolecular interaction energies38 (Tables

S3–S9 in ESI†) shows two distinct significantly stabilizing

interactions between molecules in neighbouring layers across

{100} in form I, denotedA and B in Fig. 1a. Both interactions are

formed around crystallographic inversion centres. Interaction A,

calculated to be the more stabilising of the two, involves C–H/O contacts between the aromatic ring and the acetyl carbonyl

group, while interaction B involves C–H/O contacts between

the methyl group and another acetyl carbonyl. By contrast, the

significant stabilizing interactions between layers across {100} in

form II are all of the same type, involving C–H/O contacts

between the acetyl substituents of molecules related by a crys-

tallographic 21 screw axis (Fig. 1b). Thus, although the structures

of the two polymorphs are closely related, a clear difference exists

in the intermolecular interactions across the expected slip planes,

specifically in the positions of the symmetry elements relative to

molecules involved in significantly stabilising interactions.

Nanoindentation

Nanoindentation analyses were carried out on the structurally

equivalent {001} face of I and {10�2} face of II. These faces

correspond to indentation approximately parallel to the crys-

tallographic direction of the interlayer shift relating forms I and

II (<001>, Fig. 1). To provide an indication of the mechanical

anisotropy for I, which could be compared to the values obtained

from previous studies on form I single crystals,39,40 indentation

was also performed on the {100} face. Representative load, P,

versus displacement, h, curves and the indent impressions on each

crystal face are shown in Fig. 2 and 3. Full experimental details

are provided in the ESI†. The maximum load applied in our

experiments was 6 mN, with accompanying indentation depths

of�1 mm. Indentation on {100} of II was complicated by the fact

that the face exhibited only a very small size on the crystals that

we could obtain, thereby making it difficult to hold rigidly while

indenting. Indentation left only featureless dents (see ESI†), and

we could not derive modulus or hardness values that we consider

to be sufficiently reliable for discussion.

All of the P–h curves (Fig. 3) show large residual depths upon

unloading, which is a manifestation of significant plastic

Chem. Sci., 2011, 2, 2236–2242 | 2237


Fig. 1 Crystal structures of the aspirin polymorphs: (a) form I, (b) form II. In the two figures, the grey slabs highlight planes parallel to {001} or {10�2}.

The slip planes are coloured blue. The projection drawings in the centre show molecules within a single grey slab that are involved in significant sta-

bilising interactions across the slip plane. In form I, these interactions (A andB) are across inversion centres. The figures to the right are projections down

the c axis (e3 in the Cartesian reference system), showing compression and elongation of the interactions during relative motion along b in the direction of

the arrow. Yellow and green molecules lie in adjacent planes parallel to {001} or {10�2} (i.e. in adjacent grey slabs in the left-hand figure). H atoms are

omitted. In I, movement of the upper molecules compresses interaction A1 and elongates interaction B1 in the front plane (yellow). For the rear plane

(green), movement in the same direction elongates A2 and compresses B2. In form II, however, all interactions shown by the arrows are equivalent and

21-related, so movement in either direction along e2 causes equivalent compression and elongation of interactions in the front and rear planes.

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deformation undergone by the crystals during indentation. The

loading parts of the P–h curves obtained for both {001} of I and

{10�2} of II are smooth, while distinct displacement bursts are

evident for {100} of I. These displacement bursts, which indicate

heterogeneous deformation, occur consistently at penetration

depths of 0.434, 0.564, 0.680 and 0.770 � 0.02 mm, with the

magnitude of the pop-in length being either 23 � 4 nm at lower

loads, or 35 � 4 nm at higher loads. The latter implies that the

pop-ins are related to the underlying crystal packing since they

are integral multiples of d100 (11.373 �A). Similar correlation of

pop-in lengths with dhkl was observed for saccharin.19 For I, the

indent impression on {100} shows significant ‘‘pile-up’’, which

arises from incompressible plastic deformation of material from

beneath the indenter to the top surface along the edges of the

indenter. Cracking is also observed specifically along <010> at

higher loads. Fracture or significant pile-up is not observed for

either {001} of I or {10�2} of II. The maximum penetration depth,

hmax, for {10�2} of II is higher than {001} of I, indicating a softer

nature for form II compared to I. The average values of hardness

(H) and elastic modulus (E) for the examined faces are shown in

Table 1. Most notably, the mechanical properties of the two

polymorphs are found to be significantly different for indenta-

tion along the potential shearing direction: {001} of I is stiffer

2238 | Chem. Sci., 2011, 2, 2236–2242

and harder than {10�2} of II, with a difference of 48% for the

elastic moduli and 37% for the hardness indices.

Recently, Haware et al. have examined elastic properties of

single crystals of aspirin form I in directions normal to the (100),

(010) and (001) planes using powder X-ray diffraction and an in

situ compression stage.39 Measurement of changes in d-spacing

gave values of 1.3(3), 1.6(6) and 4.6(8) GPa, respectively, for the

elastic modulus along each direction. The trend in these values

was in line with an accompanying computational estimation. Our

observation for I that {001} has a greater modulus than {100} is

consistent with these assessments. In another recent study, Olu-

sanmi et al. have reported nanoindentation analyses on aspirin

form I,40 which gave average elastic modulus values of 5.02 �0.57 GPa and 2.95 � 0.13 GPa for indentation on the {100} and

{001} faces, respectively. These measurements refer to indenta-

tion depths of �6 mm at a maximum load of 100 mN. The trend

in these values is neither consistent with the results of Haware

et al. nor our results. This could be due to several factors, most

notable being the following two: (i) Olusanmi et al. have used as-

received particles whereas we have used carefully grown single

crystals. Images given by them suggest that the particles they

have used may have experienced considerable surface damage

(Fig. 1b and 12 of their paper40) and also that they may contain



Fig. 2 Images of the nanoindentation indents: (a) {001} of I; (b) {10�2} of II; (c) {100} of I showing cracking along <010> at higher loads.

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significant porosity (Figs. 11 and 13); (ii) the indentation loads

applied by them are considerably higher that the loads that we

have applied, which led to cracking along <010> during inden-

tation on the {100} face. We have also observed cracking on

{100} of I at higher loads (Fig. 2c), but not under the conditions

from which our modulus values are derived. As is well known,

cracking makes a material considerably compliant and hence

tests for modulus measurements should be performed in a such

a way that cracking does not occur during loading. Certainly, we

note that the P–h curve reported by Olusanmi et al. for


indentation on the {100} face of I (Fig. 9 in ref. 40) appears

highly atypical compared to others reported in the literature.

Correlation of mechanical response with crystal structure

During indentation, molecules either stretch (elastic deforma-

tion) or slide relative to each other (plastic deformation). The

difference in the measured elastic moduli for I and II demon-

strates significant variation in the interaction characteristics

under the indenter. We consider the expected relative motion of

Chem. Sci., 2011, 2, 2236–2242 | 2239


Fig. 3 Representative P–h curves for all three faces examined, with pop-

ins indicated by arrows for the loading curve of {100} of I.

Table 1 Determined values of hardness (H) and elastic modulus (E) forthe three crystal faces examineda

H (GPa) E (GPa)

{100} of I 0.257 � 0.007 5.97 � 0.291{001} of I 0.240 � 0.008 9.57 � 0.201{10�2} of II 0.152 � 0.004 4.96 � 0.226

a See ESI† for details of the determination of these values.

Fig. 4 Thermal expansion measured for single crystals of aspirin I and

II. The expansion is referred to the Cartesian reference system e3 k c, e2 kb, e1 k e2� e3, and the lines drawn between data points provide a guide to

the eye. (a) The expansion in I is highly anisotropic, with expansion along

e2 approximately half as large as any other direction in either polymorph.

This result is in agreement with an assessment made previously by Bauer

et al.35 (b) The expansion in II is considerably less anisotropic.

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the molecules along different directions on the basis of the

intermolecular interactions in order to suggest some structural

basis for the deformation processes involved. For convenience,

we refer to a Cartesian axis system, ei, having e2 parallel to the

crystal b axis <010>, e3 parallel to the crystal c axis <001> and e1perpendicular to both (Fig. 1). Axes e2 and e3 lie parallel to the

planes of the hydrogen-bonded dimer layers, while e1 is parallel

to the layer stacking direction.

Since the hydrogen-bonded dimer layers within I and II are

identical, the interactions between molecules within these layers

should have closely comparable characteristics in the two forms.

The differences described for the crystal structures exist in the

regions between the layers, and mechanical differences should

therefore be expected when molecules in neighbouring layers

move relative to each other. Considering adjacent layers moving

relative to each other along e2: it is evident from Fig. 1a that one

of the two distinct stabilising interlayer interactions (say A1)

formed by a given molecule in I is compressed while the other

(B1) is elongated. For molecules in the next plane parallel to

{001}, the situation is reversed; motion in the same direction

along e2 serves to elongate interaction A2 and compress inter-

action B2. Since the interaction types (A and B) and associated

potential energy profiles are different, a given molecule must

favour one of the two possible circumstances, and there exists

some hindrance in I for relative motion of adjacent hydrogen-

bonded layers parallel to e2. In a typical nanoindentation

experiment, several thousands of such molecular layers are

involved. Thus, even if the hindrance between a pair of adjacent

layers is small, the cumulative effect has a significant influence on

2240 | Chem. Sci., 2011, 2, 2236–2242

the elastic modulus. The same hindrance does not exist for II

because the two significantly stabilising interlayer interactions

made by a given molecule are symmetrically equivalent (Fig. 1b),

and the same compression and elongation of the interlayer

interactions is therefore experienced by all molecules for relative

motion in either direction along e2. For interlayer stretching

parallel to e3 there is also little distinction between the two

structures so that no great difference should be expected for

relative interlayer motion in this direction.

The expectation of hindrance to the relative molecular motion

along e2 in I is corroborated by measurements of the thermal

expansion, derived from lattice constants of form I and II single

crystals at temperatures in the range 100–298 K (Fig. 4 and

Supplementary Tables S10–S13†). The anisotropy of the thermal

expansion is much more pronounced for I than for II, and the

thermal expansion parallel to e2 for I is about half that of any

other direction in the two polymorphs.

For indentation on the {001} face of I or the {10�2} face of II,

the established elastic moduli probe a combination of the elastic

deformation characteristics along the principal indentation

direction parallel to e3 and inclined to it by virtue of the



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pyramidal indenter shape. The latter directions can be decom-

posed into a component along e3 plus some component along e1and/or e2. On the basis of the preceding analysis, the two poly-

morphs would be expected to show minimal difference for elastic

deformation along e3 and for any component along e1, but any

component of the elastic deformation along e2 should be signif-

icantly more restricted for I. This is consistent with the measured

elastic moduli for indentation on the {001}/{10�2} faces, since II is

observed to have a smaller modulus than I. Thus, the elastic

moduli and thermal expansion coefficients of the two poly-

morphs reflect inherent differences between the nature and

symmetries of the intermolecular interactions in the two struc-

tures along the {100} planes; it is the component of the elastic

deformation along e2 that accounts principally for the signifi-

cantly different values. Both the elastic modulus and the coeffi-

cient of thermal expansion are directly dependent on the

intermolecular interactions and are inversely related to each

other. Hence, the observation that thermal expansion parallel to

e2 for I is about 50% lower than that of any other direction is

consistent with the elastic modulus of {001} of I being nearly

twice that of either {10�2} of II or {100} of I.

Plastic deformation and polymorph transformation

The observed differences in hardness for the faces examined

indicate an orientation and structure dependence for the micro-

mechanisms of plasticity. In molecular crystals (as in inorganic

crystals), plastic deformation occurs via slip, which is aided by

movement of dislocations under the influence of applied stress.41

Generally, higher mobility of dislocations within the crystal leads

tomore facile plastic deformation. Typically, slip occurs onplanes

that are most widely spaced, with the smallest Eatt (so that the

frictional resistance for slip is the least) and in directions that are

closest packed (so that the slip vector is the smallest). Roberts

et al., on the basis of a simple model that relates hardness to

cohesive energies of organic solids, have previously suggested

{100}<001> as a probable slip system in aspirin form I.22 The

smooth nature of theP–h curves obtained for nanoindentation on

{001} of I and {10�2} of II are consistent with facile and smooth

slip along the system {100}<001>, while the distinct pop-ins

observed on {100} of I occur because the applied stress

(compressive in nature) is normal to the slip planes.Consequently,

the resolved shear stress on the slip planes is zero, necessitating

alternative slipmechanisms.Upon the build upof sufficient stress,

slip occurs intermittently in multiples of d100.

Crucially, the larger magnitude of plastic deformation, and

hence lower hardness, measured for {10�2} of II compared to

{001}of I suggests that slip in IIoccursmore readily. The chemical

implications are that the transformation from form II to form I

takes place through slip along the system {100}<001>. We have

observed that the transformation can occur under stress-free and

ambient conditions, which implies that the activation energy

barrier for the II/ I transformation is small, although it proceeds

over a relatively long time scale—commonly, we observe periods

of several months for complete transformation to occur in crys-

talline powders.‡ Our nanoindentation results, however, imply

that the transformation should be rapid when shear stress is

applied. To verify this hypothesis, crystalline powders of form II

were subjected to mechanical grinding. Powder X-ray diffraction


(See SupplementaryFig. S2†) establishes that the diagnostic peaks

(19.9 and 25.5� 2q for Cu-Ka radiation) for form II are absent

after grinding, thereby demonstrating that II/ I transformation

does indeed take place quickly under the influence of shear stress.

In their study of form I, Olusanmi et al. also propose {100} as

the slip planes, but suggest that the likely direction of slip is along

<010>, since this has the shortest lattice translation (b z6.5 �A).40 However, it is crucial in the case of aspirin to realise that

slip along <001> provides a shorter translation (1/2 c z 5.75 �A).

Although this is not a lattice translation for form I, it is an

energetically viable translation that transforms form I to form II.

Thus, it is more reasonable that {100}<001> should be the most

probable slip system.

Regarding the composition of aspirin crystals

The intergrown aspirin crystals that we described previously33

could be viewed as snapshots along the pathway of the II / I

transformation process. In this current study, we have observed

single crystals of II that contain domains with a nanoindentation

signature corresponding to I. This is despite the fact that single-

crystal X-ray diffraction (using laboratory CCD instruments)

suggests the crystals to be structurally pure form II. This

provides a good illustration of the limits of diffraction methods

for characterising smaller domains within crystals and illustrates

a further valuable feature of nanoindentation as a local probe to

test for crystal homogeneity. In the pharmaceutical industry,

crystallographic techniques (both powder and single-crystal) are

the tools usually used to establish phase purity and thereby claim

intellectual property rights for new polymorphs. The presence of

form I and II domains beyond the recognition of crystallographic

tools, coupled with the view of a continuous slip-mediated

pathway between forms I and II, revives the question of principle

that we raised previously42 regarding the identity of the indi-

vidual aspirin forms.

Conclusions

Nanoindentation provides valuable information on the

mechanical properties of polymorphic molecular crystals, with

implications for the understanding of stability relationships and

phase transformations in the molecular solid state. The technique

removes the principal practical barrier associated with conven-

tional techniques for probing mechanical response, namely the

requirement to grow exceptionally large single crystals. The

polymorphs of aspirin have a close structural relationship, but

nanoindentation establishes that the metastable form II of

aspirin is considerably softer than the stable form I. Shear slip

along {100}<001> provides a mechanistic rationale for the

observed solid-state II/ I transformation. This transformation

takes place over months under ambient conditions but can be

accelerated by mechanical grinding. In general terms, our work

shows that nanoindentation has significant potential for the

study and understanding of polymorphism in the context of

crystal engineering.

Acknowledgements

S.V. thanks the Department of Science and Technology for

a Young Scientist fellowship. M.S.R.N.K. thanks University

Chem. Sci., 2011, 2, 2236–2242 | 2241


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Grants Commission for a D. S. Kothari postdoctoral research

fellowship. K.A.S. and A.D.B. acknowledge funding from

the Danish Council for Independent Research | Natural Sciences.

G.R.D thanks the Department of Science and Technology for

the award of a J. C. Bose fellowship.

Notes and references

‡ We have observed this transformation using powder X-ray diffractionfor numerous bulk samples stored under ambient conditions in ourlaboratories. The diagnostic peaks for form II diminish with time and aretypically absent after a period of months.

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Switching and tuning organic solid-state luminescence

via a supramolecular approachw

Savarimuthu Philip Anthony, Sunil Varughese and Sylvia M. Draper*

Received (in Cambridge, UK) 14th July 2009, Accepted 22nd October 2009

First published as an Advance Article on the web 4th November 2009

DOI: 10.1039/b914027a

Unusual intermolecular interactions of organic luminescent acid,

2-cyano-3(4-(diphenylamino)phenyl)acrylic acid (CDPA), with

amines lead to the formation of supramolecular luminescence

systems with switchable and tunable solid-state luminescence.

Organic solid-state luminescent materials have a potential role

in a wide range of high-technology applications such as

organic light emitting diodes (OLEDs),1 semiconductor lasers2

and fluorescent sensors.3 In particular the switching and

tuning of organic solid-state luminescence is of current interest

to fundamental research and practical applications. In solution,

solvatochromism,4,5 the addition of metal ions,5a,6 and the

variation of substitution5 or pH5a,6,7 lead to the tuning and

switching of the luminescence. However organic solid-state

luminescence, which is rare due to the aggregation quenching

effect, has mostly been tuned by the modification of substitutions

on single molecules8 or the exploitation of polymorphism.9,10

The latter approach is effective as the optical properties in the

solid are controlled by molecular organization but it offers

little predictability, the former requires interactive synthetic

improvement.

Supramolecular chemistry provides a versatile approach to

tuning and switching solid-state luminescence by controlling

the molecular organization through weak interactions.11 For

example, solvent dependent solid-state luminescence has been

demonstrated by supramolecular host systems generated from

the mixing of luminescent organic acids and amines.12 In

general, deprotonation and protonation provides a simple

strategy to tune or switch the emission of organic luminescent

acids across a wide wavelength range. Furthermore the amine

induced manipulation of the acid protons, via subtle variations

in H-bond formation and controlled deprotonation, creates an

opportunity to gradually tune the solid-state luminescence

and simultaneously provides a platform for amine sensing.

Herein, we report supramolecular luminescence systems based

on 2-cyano-3(4-(diphenylamino)phenyl)acrylic acid (CDPA)

and amines (pyridine (1), pyrrolidine (2), piperidine (3) and

morpholine (4)). These systems exhibit blue shifted luminescence

compared to the parent acid and in the case of 3 solvent

dependent solid-state luminescence was observed.

CDPA was synthesized following the reported procedure13

and crystallized from CH3CN by slow evaporation. The single

crystal X-ray structure revealed the formation of a helical

network generated via intermolecular H-bond interactions

(O–H� � �NC) involving the carboxylic proton and cyano

nitrogen atom (Fig. 1a).z Crystals 1–4 were grown from 1 : 1

molar solutions of the appropriate amine and CDPA in CH3CN.

1 : 0.5 and 1 : 1 CDPA–amine co-crystals were formed in 1 and

2–3, respectively. Fig. 1b–e show the selective H-bond inter-

actions of 1–4 in the crystal lattice. In 1, all five pyridine

(pKa = 5.14) protons are involved in intermolecular H-bond

interactions with different CDPAmolecules. The disordered para

carbon of pyridine is involved in C–H� � �O interactions with the

carbonyl oxygens of CDPA. The other four pyridine C–H form

C–H� � �NC H-bond interactions with the cyano nitrogen of

different CDPA molecules (Fig. 1b). The multiple pyridine

interactions coupled with the carboxylic O–H� � �O–H inter-

molecular H-bond interactions lead to the formation of CDPA

dimers. Thermogravimetric studies reveal the loss of pyridine

molecules at high temperature, in support of the presence of

multiple H-bond interactions in the lattice (Fig. S3, ESIw).2 (pyrrolidine pKa = 11.27) and 3 (piperidine pKa = 11.22),

containing stronger bases, form only intermolecular H-bonding

interactions without deprotonating the carboxylic acid. In 2

the hydrogen atoms of the meta carbons on the pyrrolidine

form strong C–H� � �O interactions with the carboxyl oxygens

(Fig. 1c). The CDPA carboxylic proton is involved in an

intramolecular H-bond interaction with the cyano nitrogen

atom (O–H� � �NC). The pyrrolidine nitrogen atoms, usually

active in H-bonding, do not take part in any supramolecular

interactions. In 3, however, the piperidine nitrogen atoms do

take part in H-bond interactions with the carboxylic acid: the

resulting N–H� � �O and O–H� � �N intermolecular H-bond inter-

actions lead to the formation of CDPA dimers. Unexpectedly

morpholine (pKa = 8.36), the weakest base of the alicyclic

amines used, deprotonates CDPA in 4. The ionic NH� � �OH-bond interactions between the morpholine nitrogen and the

carboxylate oxygens of CDPA lead to the formation of CDPA

dimers (Fig. 1d). These are further linked by C–H� � �O inter-

actions between morpholine CH and CDPA carboxylate

oxygens into tetramers. Clearly there is a complex balance of

forces at play in the solid-state structures which go beyond or

over-ride mere solution-based pKa considerations.

3 forms a supramolecular luminescent host system by

including two CH3CN molecule in the crystal lattice (Fig. 2);

one of which is disordered on single crystal X-ray analysis.

Attempts were made to crystallize 1–4 in various solvents to

check for the formation of other supramolecular luminescent

host systems. 2, 3 and 4 were found to produce crystals in

CH3CN and EtOAc; EtOAc producing very small crystals. 1,

however, formed a crystalline product only in CH3CN. The

School of Chemistry, Trinity College Dublin, Dublin-2, Ireland.E-mail: [email protected]; Fax: +353 16712826;Tel: +353 18962026w Electronic supplementary information (ESI) available: Experi-mental, crystallographic, PXRD and luminescence details. CCDC737399–737403. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/b914027a

7500 | Chem. Commun., 2009, 7500–7502 This journal is �c The Royal Society of Chemistry 2009

COMMUNICATION www.rsc.org/chemcomm | ChemComm

similar powder X-ray diffraction (PXRD) patterns of 2 and 4

obtained from CH3CN and EtOAc confirmed the same crystal

lattice for these systems irrespective of solvent (Fig. S1, ESIw).However in the case of 3 there were distinct differences (before

and after the removal of CH3CN and 3 obtained from EtOAc)

suggesting a difference in the crystal structures (Fig. S2, ESIw).This was anticipated given the host–guest inclusion of CH3CN

in the single crystal X-ray structure of 3 (Fig. 2) and was

implied by its solid-state luminescence properties (discussed

later, Fig. 3b). Thermogravimetric studies support the formation

of a supramolecular host system only in CH3CN and reveal

the loss of CH3CN from 3 and the appropriate amine from 1

to 4 at higher temperatures (Fig. S3, ESIw).The normalized solid-state luminescence spectra of CDPA

and 1–4 are shown in Fig. 3a. The quantum yield (Ff) of

CDPA, as determined by comparison with coumarin 6, was

0.165 in CH2Cl2. The intensity of this luminescence was

found to vary very little from that of the solid-state sample.

The presence of amines however enhanced the solid-state

luminescence intensity of CDPA, e.g. a 2-fold enhancement

was observed for 3 (Fig. S4, ESIw). This might be due to the

deaggregation of CDPA in the solid matrix. Powdered CDPA

shows solid-state luminescence at 587 nm which undergoes a

gradual blue shift to 494 nm in the presence of amines. The

subtle change of carboxylic acid H-bond interactions from

O–H� � �NC in CDPA to O–H� � �O–H (Fig. 1a and b) in 1 blue

shifts the luminescence from 587 nm to 565 nm and the

formation of H-bond interactions with pyrrolidine and piperidine

further blue shifts the luminescence to 531 and 536 nm

(2 and 3). The complete deprotonation of CDPA carboxylic

acid in 4 results in solid-state luminescence at 494 nm.

The supramolecular luminescent host system 3 might be

expected to exhibit solvent dependent solid-state luminescence

properties. The CH3CN was removed from 3 by drying under

vacuum for 24 h. The strong H-bond formation of CH3CN

necessitates this long drying time. The solvent dependent

change of luminescence of 3 is shown in Fig. 3b. 3 with

CH3CN exhibit luminescence (lmax) at 536 nm, whereas on

removing CH3CN luminescence (lmax) occurs at 507 nm.

Re-exposure to CH3CN switches the luminescence lmax back

to 536 nm. 3 obtained from EtOAc exhibits luminescence

(lmax) at 510 nm. Exposure of EtOAc, CH2Cl2, CHCl3,

MeOH, EtOH, toluene and H2O solvent vapor on powdered

3 for 3–5 min red shifts the luminescence to 518–522 nm

(Fig. S5, ESIw). This observation clearly supports the selective

inclusion of CH3CN in the crystal lattice of 3 as was confirmed

by single crystal investigation. No solvent dependence in the

solid-state luminescence of 2 or 4 was observed.

Importantly solid-state luminescent switching was demon-

strated by the cyclical exposure of powdered CDPA to amines

(pyrrolidine, morpholine) and then to immersion in 0.1 M HCl

solution for 2 h (Fig. 4). The amine exposure blue shifts the

CDPA solid-state luminescence from 587 nm to 531 nm

(pyrrolidine) and 494 nm (morpholine). These luminescence

lmax closely match those of 2 and 4 obtained from CH3CN

solution. The PXRD studies also confirm the conversion of

CDPA to 2 and 4 by pyrrolidine and morpholine vapor

exposure, respectively (Fig. S6, ESIw). The conversion in

morpholine takes considerably longer (2 h) which might be

due to the low volatility of morpholine. For both samples,

submersion in HCl solution for 2 h results in the reversal of the

luminescence signals back to those of powdered CDPA. The

PXRD pattern of the HCl solution immersed samples also

closely matches that of the simulated PXRD pattern of single

crystal CDPA (Fig. S7, ESIw).In conclusion we have used a supramolecular approach to

tune and switch the solid-state luminescence of CDPA. The

subtle variation in the H-bond interactions and deprotonation

Fig. 1 Selected H-bond interactions in the crystal lattice of (a)

CDPA, (b) 1, (c) 2, (d) 3 and (e) 4. Only H atoms involved in H-bond

interactions are shown; C (grey), N (blue), O (red), H (white); H-bonds

(broken line). dH� � �A distances (A) are marked.

Fig. 2 Supramolecular luminescent host structure of 3 with CH3CN.

CH3CN are shown in space filling mode. Only H atoms having

H-bond interactions are shown; C (grey), N (blue), O (red), H (white);

H-bonds (broken line).

This journal is �c The Royal Society of Chemistry 2009 Chem. Commun., 2009, 7500–7502 | 7501

leads to the gradual blue shift of CDPA solid-state

luminescence from 587 nm to 494 nm. 3 gives a CH3CN

selective supramolecular luminescent host system. Tuning

and indeed switching of the luminescence is demonstrated by

exposing powdered CDPA to amine vapor (pyrrolidine,

morpholine) and HCl solution. Developments of chiral supra-

molecular luminescent host systems based on CDPA are

underway.

This material is based upon works supported by EU FP6

[MKTD-CT-2004-014472] and Science Foundation Ireland

[05PICAI819].


z Crystal data: CDPA (CCDC: 737400): C22H16N2O2, M = 340.37,monoclinic, P21/n, a = 13.568(1), b = 9.459(1), c = 13.642(1) A, b =104.167(2), V = 1697.6(3) A3, Z = 4, T = 150 K, 9595 reflectionsmeasured, 2985 unique (Rint = 0.0248), final R values: 0.0330, wR:0.0893; 1 (CCDC 737402): 2(C22H16N2O2), C5H5N, M = 759.84,monoclinic, P21/c, a = 7.877(1), b = 9.424(1), c = 26.835(3) A, b =93.023(3), V = 1989.3(4) A3, Z = 2, T = 150 K, 11 236 reflectionsmeasured, 3497 unique (Rint = 0.0307), final R values: 0.0553, wR:0.1404; 2 (CCDC 737399): C22H16N2O2, C4H9N, M = 411.49, mono-clinic, P21/c, a = 9.461(1), b = 25.424(2), c = 11.161(1) A, b =125.704(5), V = 2180.0(3) A3, Z = 4, T = 150 K, 12 692 reflectionsmeasured, 3855 unique (Rint = 0.0402), final R values: 0.0611, wR:0.1660; 3 (CCDC 737403): 2(C22H16N2O2), 2(C5H11N), 3(C2H3N),

M = 971.17, orthorhombic, Pbcn, a = 32.166(2), b = 9.900(7), c =16.407(1) A, V = 5224.4(6) A, Z = 4, T = 150 K, 53 187 reflectionsmeasured, 4686 unique (Rint = 0.0287), final R values: 0.0490, wR:0.1398; 4 (CCDC 737401): C22H15N2O2, C4H10NO, M = 427.49,triclinic, P�1,a = 11.231(1), b = 16.368(1), c = 26.031(2) A, a =99.435(1), b = 102.219 (1), g = 99.471(2), V = 4514.0(6) A3, Z = 8,T = 150 K, 49086 reflections measured, 15976 unique (Rint = 0.0505),final R values: 0.0580, wR: 0.1282.

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Fig. 3 Normalized solid-state luminescence of (a) CDPA, 1–4 and (b) solvent dependent luminescence of 3 (excitation l = 370 nm).

Fig. 4 Switching of solid-state CDPA luminescence. Arrow indicates

the time required for the conversion (excitation l = 370 nm).

7502 | Chem. Commun., 2009, 7500–7502 This journal is �c The Royal Society of Chemistry 2009

DOI: 10.1002/chem.200500570

A Competitive Molecular Recognition Study: Syntheses and Analysis ofSupramolecular Assemblies of 3,5-Dihydroxybenzoic Acid and Its BromoDerivative with Some N-Donor Compounds

Sunil Varughese and Venkateswara Rao Pedireddi*[a]

Introduction

Molecular recognition,[1] the process of bringing differentchemical entities together through noncovalent forces, is apowerful tool for the development of novel targeted assem-blies with tailor-made properties.[2–4] Initial studies towardshost–guest-type assemblies, in which the guest molecules arecaptured in the cavities/channels formed by host molecules,such as crown ethers,[5] cryptands,[6] and so on, indeedbrought about dramatic changes in organic synthesis and ledto the development of novel synthetic strategies for thepreparation of complex molecules to be employed in molec-ular recognition studies. As a result, the synthesis of hoststructures[7,8] by using noncovalent bonds (e.g., hydrogenbonds), which are weaker than conventional covalent bonds(s and p bonds) and also have the advantage of flexibility

for fine-tuning to obtain the desired network or architecture,evolved as a general synthetic strategy for the creation ofexotic assemblies that can be utilized in molecular recogni-tion studies.[9] Numerous examples of host–guest systemsgenerated through noncovalent synthetic procedures haveappeared in the recent literature. For example, adducts oftrimesic acid,[10] 3,5-dinitrobenzoic acid,[11] 3,5-dinitrobenzo-nitrile,[12] cyanuric acid,[13] trithiocyanuric acid,[14] 1,2,4,5-ben-zenetetracarboxylic acid,[15] and so on, representing a varietyof host–guest systems of different architectures (Scheme 1),demonstrate the elegance and reliability of molecular recog-nition process.

In further developments aimed at increased knowledge ofhydrogen bonds,[16] attention was also directed towards uti-lization of hydrogen bonds for synthesis of targeted assem-blies to perform unusual chemical transformations that oth-erwise appear to be either infeasible or complex innature.[17,18] In this regard, the elegant studies by MacGilliv-ray et al. on the synthesis of ladderanes by cocrystallizationand subsequent photochemical reaction by irradiation of theadducts of resorcinol and unsaturated N-donor moleculessuch as 1,2-bis(4-pyridyl)ethene, prepared by inducing rec-ognition between the constituents through O�H···N hydro-

Abstract: A molecular recognitionstudy of 3,5-dihydroxybenzoic acid (1)and its bromo derivative 4-bromo-3,5-dihydroxybenzoic acid (2) with the N-donor compounds 1,2-bis(4-pyridyl)eth-ene (bpyee), 1,2-bis(4-pyridyl)ethane(bpyea), and 4,4’-bipyridine (bpy) is re-ported. Thus, the syntheses and structur-al analysis of molecular adducts 1a–1c(1 with bpyee, bpyea, and bpy, respec-tively) and 2a–2c (2 with bpyee, bpyea,and bpy, respectively) are discussed. Inall these adducts, recognition betweenthe constituents is established througheither O�H···N and/or O�H···N/C�

H···O pairwise hydrogen bonds. In allthe adducts both OH and COOH func-tional groups available on 1 and 2 inter-act with the N-donor compounds,except in 2a, in which only COOH(COO�) is involved in the recognitionprocess. The COOH moieties in 1a, 1b,and 2b form only single O�H···N hy-drogen bonds, whereas in 1c and 2c,

they form pairwise O�H···N/C�H···Ohydrogen bonds. In addition, subtle dif-ferences in the recognition patterns re-sulted in the formation of cyclic net-works of different dimensions. In fact,only 1c forms a four-molecule cyclicmoiety, as was already documented inthe literature for this kind of assemblies.All complexes have been characterizedby single-crystal X-ray diffraction. Thesupramolecular architectures are quiteelegant and simple, with stacking ofsheets in all adducts, but a rather com-plex network with a threefold inter-penetration pattern was found in 2c.

Keywords: crystal engineering ·hydrogen bonds · molecularrecognition · self-assembly ·supramolecular chemistry

[a] S. Varughese, Dr. V. R. PedireddiSolid State & Supramolecular Structural Chemistry LabDivision of Organic Chemistry, National Chemical LaboratoryDr. Homi Bhabha Road, Pune 411 008 (India)Fax: (+91)20-258-93153E-mail : [email protected]

Chem. Eur. J. 2006, 12, 1597 – 1609 D 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 1597

FULL PAPER

gen bonds, are superb examples that highlight the use ofmolecular recognition phenomenon for the generation ofcomplex chemical systems (Scheme 2).[19] From those stud-ies, it was further noted that the recognition pattern be-tween the OH groups in the meta-positions and aromatic N-donor compounds was not affected by the presence of differ-ent functional groups, as was shown recently in a case studyon a homologous series of phluroglucinols.[20] However,studies on the influence of functional group such as COOH,which is also capable of forming O�H···N hydrogen bonds,on the topological arrangement shown in Scheme 2, is notwell explored. Thus, studies towards understanding the com-petition of COOH and OH for N-donor compounds wouldprovide valuable information for the development of hither-to unknown assemblies.

In our continued exploration of utilization of O�H···Nand O�H···N/C�H···O pairwise hydrogen bonds[21] in supra-molecular synthesis and molecular recognition, we carriedout such competitive recognition studies employing molecu-lar entities having both OH and COOH groups by cocrystal-lization with different heteroaromatic compounds, whichcan exploit the robustness of the four-membered recognitionpattern (Scheme 2) or may lead to new assemblies generat-ed through different recognition schemes. Thus, we chose3,5-dihydroxybenzoic acid (1) for cocrystallization with 1,2-bis(4-pyridyl)ethene (bpyee), 1,2-bis(4-pyridyl)ethane,(bpyea), and 4,4’-bipyridine (bpy). Furthermore, the studywas extended to halo derivatives of 1, as such substitutiondid not have any effect on the basic molecular recognitionfeatures in the resultant assemblies, as known from the earli-er studies of MacGillivray et al.[22] Hence, cocrystallizationsof 4-bromo-3,5-dihydroxybenzoic acid with bpyee, bpyea,and bpy were also carried out. The synthetic strategies andthe nature of the products are illustrated in Scheme 3.

Results and Discussion

Cocrystallization of 3,5-dihydroxybenzoic acid (1) with theN-donor compounds 1,2-bis(4-pyridyl)ethene (bpyee), 1,2-bis(4-pyridyl)ethane (bpyea) and 4,4’-bipyridyl (bpy) fromCH3OH gave single crystals of 1a–1c, respectively. Similarly,4-bromo-3,5-dihydroxybenzoic acid gave cocrystals 2a–2cwith bpyee, bpyea, and bpy, respectively. Single-crystal X-ray diffraction revealed that the reactants recognize eachother by interaction of OH and/or COOH groups of 1 and 2with the N atoms of N-donor compounds. Each adduct isunique in aspects of structural arrangements with respect toScheme 1.

Scheme 2.

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the conformation and the nature of the hydrogen bondsformed by OH and COOH groups. However, collectively,they exhibit many common features, especially in the forma-tion of ladderlike structures. Thus, descriptions of theunique features of each adduct are followed by a compari-son to deduce common features which may be useful forevaluating literature examples and formulating new systems.

Molecular complex of 3,5-dihydroxybenzoic acid and 1,2-bis(4-pyridyl)ethene (1a): Cocrystallization of 3,5-dihy-

droxybenzoic acid (1) and bpyee in a 1:1 ratio from metha-nol gave single crystals suitable for X-ray diffraction. Struc-ture determination (Table 1)[23] revealed that 1 and bpyeeare present in a 2:3 ratio in the molecular complex 1a, andthe asymmetric unit is shown in Figure 1. The two OHgroups on 1 are arranged in a syn–syn orientation with re-spect to the H atom in the para position, and one of thebpyee molecules is disordered around the olefinic bridge ina 53:47 distribution. The ordered bpyee molecules are de-noted as B, and the disordered molecules as C. These mole-

Scheme 3.

Table 1. Crystallographic data for 1a, 1b and 2a–2c

1a 1b 2a 2b 2c

formula 2(C7H6O4):3(C12H10N2) (C7H6O4):(C12H12N2) 2(C7H4O4Br):(C12H12N2) 2(C7H5O4Br):2(C12H12N2) (C7H5O4Br):(C10H8N2)Mr 852.88 338.35 648.26 830.48 389.20crystal habit blocks blocks blocks blocks blockscrystal system triclinic triclinic monoclinic triclinic monoclinicspace group P1 P1 P21/n P1 P21/na [H] 9.035(8) 7.234(2) 6.693(2) 7.321(2) 10.247(4)b [H] 10.648(9) 13.914(3) 16.425(4) 8.047(2) 9.270(3)c [H] 12.813(9) 17.141(4) 11.197(3) 16.817(5) 17.130(6)a [8] 107.46(9) 78.54(4) 90 98.34(5) 90b [8] 102.50 (9) 82.50(4) 91.55(5) 90.74(5) 99.39(5)g [8] 109.12(9) 84.00(4) 90 114.91(4) 90V [H3) 1041.0(2) 1671.0(7) 1230.5(6) 886.0(4) 1605.4(10)Z 1 4 2 1 41calcd [g cm

�3] 1.360 1.345 1.750 1.556 1.610T [K] 298(2) 298(2) 298(2) 298(2) 298(2)l(MoKa) 0.71073 0.71073 0.71073 0.71073 0.71073m [mm�1] 0.094 0.095 3.350 2.347 2.5852q range [8] 46.74 46.66 46.58 46.62 46.56index ranges �10�h�10 �8�h�8 �7�h�7 �8�h�8 �11�h�10

�11�k�11 �15�k�15 �18�k�18 �8�k�8 �10�k�10�14� l�13 �18� l�19 �11� l�12 �18� l�18 �19� l�18

F(000) 446 712 648 420 784total reflns 6482 14115 5235 7360 9552unique reflns 3000 4816 1785 2554 2306reflns used 1924 2213 1399 2072 1939parameters 374 455 212 305 266GOF on F2 1.038 0.819 0.930 0.910 1.013R1 [I>2s(I)] 0.0527 0.0493 0.0267 0.0363 0.0275wR2 0.1069 0.0973 0.0623 0.0914 0.0740max./min. residual electrondensity [eH�3]

0.366/�0.144 0.194/�0.180 0.331/�0.404 0.492/�0.243 0.305/�0.399

Chem. Eur. J. 2006, 12, 1597 – 1609 D 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.chemeurj.org 1599

FULL PAPERSupramolecular Assemblies

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cules are arranged in the crystal lattice to yield a sheet struc-ture, stacked along the a axis (Figure 1b).

In each sheet, recognition between 1 and bpyee is estab-lished through O�H···N hydrogen bonds. The basic recogni-tion pattern is shown in Figure 2a. Each molecule of 1 isconnected one ordered and one disordered bpyee moleculeby O�H···N hydrogen bonds (H···N 1.77, 1.97 H) involvingthe OH groups. The characteristics of the hydrogen bondsare listed in Table 2. Furthermore, these two bpyee mole-cules in turn interact with a pair of molecules of 1, whichthemselves are held together by cyclic C�H···O hydrogenbonds (H···O 2.68 H), by formation of O�H···N hydrogenbonds (H···N 1.97 and 1.62 H, Table 2) involving OH andCOOH groups (see Figure 2a). Thus, a five-memberedsupramolecular entity is established in such a manner thatthe disordered bpyee forms O�H···N hydrogen bonds exclu-sively with OH groups, while the ordered bpyee moleculesinteract with both OH and COOH groups. As a result, it ap-pears that the presence of COOH group disturbed the rec-ognition pattern, which was otherwise expected be a four-membered unit, as shown in Scheme 2. However, adjacentsupramolecular ensembles interact with each other to forma ladder structure in which bpyee molecules are inserted asrungs between the rods of acid 1 (Figure 2b).

The distance between the rungs is 4.2 H (Figure 2b),which is a reactive distance for photodimerization. Thus,even though COOH is able to influence the fundamentalrecognition pattern, the gross structure still did not deviatefrom the required topological arrangement, and thus theproperties of the structures remain intact for utilization infurther reactions, such as [2+2] cycloaddition. Retention ofsuch three-dimensional packing irrespective of the nature ofsubstitutuents on the acid molecules is further reflected in amore elegant manner in adduct 1b, in which not only thebasic recognition interaction is totally different than that ob-served in 1a, but also from that of the pattern shown inScheme 2.

Molecular complex of 3,5-dihydroxybenzoic acid and 1,2-bis(4-pyridyl)ethane (1b): Complex 1b was prepared under

Figure 1. a) ORTEP plot of molecular entities in the asymmetric unit of 1a. b) Packing of molecules in stacked layers in the crystal lattice (view down caxis).

Figure 2. a) Recognition pattern between 1 and bpyee to give a five-membered cyclic moiety. b) and c) Ladderlike structure observed in 1aalong with a schematic representation. The different colors of the rungsrepresent ordered and disordered bpyee.


V. R. Pedireddi and S. Varughese

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the same conditions as 1a and is a 1:1 complex of 1 andbpyea (Table 1), but with two symmetry-independent mole-cules in the asymmetric unit (Figure 3a), without any abnor-mal features, such as the disorder that was observed in 1a.

The two symmetry-independent molecules of 1 are denot-ed as A and B, and those of bpyea as C and D. While themolecular geometries of A and B are more or less the same,the differences in C and D are mainly due to the variableconformational arrangement of methylene bridge; the twophenyl moieties in bpyea are twisted by 5.28 in C and 7.38 inD. The molecules form a stacked planar sheet structurealong the c axis. However, the interactions between the mol-ecules in each layer are quite intriguing.

Unlike in 1a, the recognition between 1 and bpyea is es-tablished such that each symmetry-related molecule ofbpyea is held by both symmetry-independent molecules (Aand B) of 1 with formation of O�H···N hydrogen bonds(H···N 1.73 and 1.90 H; 1.72 and 1.77 H; Table 2) involvingboth OH and COOH groups. This arrangement is shown inFigure 3b and c, respectively, for molecules C and D. Thus,an infinite one-dimensional crinkled tape is formed. In twodimensions, adjacent tapes are arranged in antiparallelmanner such that the two symmetry-independent moleculesof 1 interact with each other through an O�H···O hydrogen

bond (H···O, 1.81 and 1.85 H, Table 2) between OH andCOOH groups, which is supplemented by C�H···O hydro-gen bonds (H···O, 2.67–2.79 H, Table 2), as shown in Fig-ure 3d. Thus, a ladderlike network is established in whichthe rods are the two symmetry-independent molecules of 1,and the molecules of bpyea of particular symmetry (C or D)are rungs. Hence, two different types of ladders are formed,which are arranged in a crinkled manner, as shown schemat-ically in Figure 3d. Thus, although the basic recognition pat-tern in 1b is entirely different to either known pattern(Scheme 2) or that observed in 1a, retention of the globalpacking motif and formation of a ladderlike structure sug-gests its stabilization in the solid state. However, cocrystalli-zation of 1 and 4,4’-bipyridine gave an assembly (1c) that isclearly different from 1a and 1b.

Molecular complex of 3,5-dihydroxybenzoic acid and 4,4’-bi-pyridine (1c): Cocrystallization of 1 and bpy in a 1:1 ratiofrom methanol gave single crystals of 1c. However, a CSDsearch[24] revealed that the crystal structure of 1c with a 2:3ratio is known. Furthermore, the unit-cell dimensions of 1csynthesized by us (a=9.666, b=14.359 c=14.769 H, a=

63.21, b=83.22, g=80.148) were similar to those of the re-ported structure (a=9.683(1), b=14.378(3), c=14.797

Table 2. Characteristics of hydrogen bonds [bond lengths in H, angles in 8] in 1a–1c and 2a–2c.

1a 1b 1c 2a 2b 2c

O�H···O 1.812 2.631 177 1.778 2.595 169 2.034 2.708 150 1.888 2.668 1691.846 2.666 178 1.830 2.655 173

1.624 2.599 178 1.717 2.535 175 1.853 2.685 171 1.703 2.539 173 1.744 2.557 1711.773 2.692 179 1.729 2.547 175 1.882 2.721 177 1.919 2.633 174 1.917 2.741 172

O�H···N 1.965 2.814 174 1.767 2.681 163 1.886 2.720 1721.898 2.673 157 1.918 2.750 170

1.917 2.756 1781.976 2.814 176

N+�H···O� 1.667 2.608 1792.529 3.121 121

2.359 3.321 174 2.666 3.578 167 2.450 3.342 156 2.400 3.077 129 2.680 3.523 167 2.443 3.429 1752.567 3.431 144 2.674 3.329 128 2.496 3.393 157 2.482 3.281 149 2.791 3.755 172 2.709 3.305 1242.626 3.332 128 2.693 3.547 153 2.515 3.373 151 2.835 3.440 124 2.916 3.741 147 2.827 3.389 1212.669 3.473 143 2.708 3.339 126 2.569 3.236 128 2.874 3.636 141 3.018 3.871 1642.680 3.565 153 2.730 3.416 131 2.582 3.264 1292.711 3.594 147 2.730 3.463 136 2.618 3.486 152

C�H···O 2.936 3.857 166 2.781 3.688 165 2.655 3.572 1622.785 3.636 153 2.662 3.452 1412.831 3.740 156 2.676 3.429 1372.839 3.625 143 2.687 3.364 1292.841 3.454 125 2.748 3.414 1282.876 3.796 170 2.756 3.367 1232.910 3.634 136 2.764 3.479 133

2.792 3.403 1232.936 3.641 1322.972 3.628 1272.973 3.612 1262.980 3.568 122

2.701 3.342 128 2.913 3.676 136 2.522 3.254 134 2.882 3.473 123 2.910 3.455 1282.782 3.472 133 2.892 3.643 135 2.611 3.304 130

C�H···N 2.690 3.375 1302.737 3.679 1712.815 3.477 1282.911 3.837 1652.979 3.692 133



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(3) H, a=63.17(6), b=83.25(11), g=80.17(10)8), so we didnot proceed with the determination of the crystal structureof 1c.

However, the focus of the study was to compare the abili-ty of the COOH group to yield O�H···N hydrogen bondswith N-donor compounds in a series of carboxylic acids.Thus, emphasis on the three-dimensional networks was alto-gether different, and the competitive nature of differentfunctional groups was not addressed. Hence, we continuedour analysis using the data retrieved from the CSD, as it isan accurate structure with good R factor.

In 1c, the basic recognition pattern (Figure 4) is quite in-triguing, as the recognition feature shown in Scheme 2 isformed, with a network of cyclic tetramers comprising twomolecules each of 1 and bpy, formed through O�H···N hy-

drogen bonds (H···N, 1.89–1.98 H, Table 2) involving theOH groups. Such adjacent units are held together by an ad-ditional molecule of bpy by forming O�H···N/C�H···O pair-wise hydrogen bonds (H···N 1.85 H, H···O 2.97 H) betweenthe COOH group and the N atom (see Figure 4a). As aresult an infinite open braceletlike structure is formed,which is represented in a close-packing mode in Figure 4b.

Thus, the two functional groups OH and COOH interactwith bpy as if they were on two different molecules. Further-more, adjacent bracelets are held together differently in dif-ferent directions of packing. The two orientations are shownin Figure 4c and d. Along the a axis, adjacent bracelets areheld together by a combination of C�H···O hydrogen bondsand p–p interactions, as shown in Figure 4c. Along the baxis, however, a combination of O�H···N (H···O 1.89 H) and

Figure 3. a) ORTEP plot of asymmetric unit of 1b. b) and c) Different recognition patterns formed by the two symmetry-independent molecules ofbpyea. d) and e) Two-dimensional ladderlike arrangement in the crystal lattice and its schematic representation. The different colors of rungs in theladder represent different symmetry-independent molecules.



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C�H···O (H···O, 2.52 H) hydrogen bonds binds adjacentrings in a quartet manner. In fact, this quartet network is anovel pattern in supramolecular structures and may be uti-lized in strategic design to prepare novel assemblies infuture.

However, it is quite surprising that in the three-dimen-sional arrangement, despite its having the expected cyclicmolecular component, a ladderlike structure did not form.Since subtle variations have been observed among 1a–1c,the study was extended to further molecular complexeswhile keeping the OH and COOH groups intact. To thisend, we considered bulky halo substituents, with their anom-alous electronic effects on aromatic moieties and their abili-ty to form the pattern shown in Scheme 2, as exemplified bythe studies of MacGillivray et al.,[22] and attempted cocrys-tallization of halo derivatives of 1 with the N-donor com-pounds. However, we were successful only in obtainingsingle crystals of complexes of 4-bromo-3,5-dihydroxybenzo-ic acid with bpyee, bpyea, and bpy.

Molecular complex of 4-bromo-3,5-dihydroxybenzoic acid and1,2-bis(4-pyridyl)ethene (2a):Cocrystallization of 4-bromo-3,5-dihydroxybenzoic acid (2)and bpyee from methanol gavesingle crystals of (2a) with a 2:1ratio of 2 and bpyee in theasymmetric unit (Table 1). Thestructure is fully ordered (Fig-ure 5a).

Among the complexes stud-ied so far, deprotonation ofCOOH occurred only in 2a.Furthermore, recognition be-tween 2 and bpyee is establish-ed through the carboxylategroup and the protonated Natom of bpyee by formation ofN+�H···O� (H···O� 1.67 H) andC�H···O (H···O 2.40 H) pair-wise hydrogen bonds. The rec-ognition pattern is shown inFigure 5b. The resultant threemolecular ensembles are fur-ther held together in a perpen-dicular direction by an interac-tion between carboxylate andOH groups (Figure 5c) by for-mation of O�H···O hydrogenbonds (H···O 1.78 and 1.83 H).Thus, in the two-dimensionalarrangement, in each chain Bratoms on molecules of 2 lie onthe same side of the chain (Fig-ure 5d).

It is also evident from Fig-ure 5d that complex 2a also has

a ladder structure, but not exactly as was observed in 1aand 1b. All the acid molecules that constituted rods of theladders in 1a and 1b lie in the same plane, while they aretwisted by almost 908 in 2a. Also, the ladders are formed asdiscrete units, whereas in 1a and 1b adjacent ladders shareedges. It could be rationalized that the Br substituent couldperturb the basic recognition patterns, perhaps due to elec-tronic effects on the molecular structures of the reactants,without any significant dramatic changes in the three-dimen-sional arrangement in the crystal lattice. This is indeed wellreflected in the structure of 2b, which was synthesized toevaluate the influence of the heavy atom Br in the light ofthe unusual observations made in 2a, such as deprotonationand subtle variations in the formation of ladders.

Molecular complex of 4-bromo-3,5-dihydroxybenzoic acidand 1,2-bis(4-pyridyl)ethane (2b): Cocrystallization of 2 andbpyea resulted in formation of 2:2 complex 2b, with twomolecules of each reactant in the unit cell (Table 1). An

Figure 4. a) Basic recognition pattern and formation of molecular tape in 1c. b) Arrangement of adjacenttapes in a braceletlike network. c) Close-packed model of the bracelet (along a axis) shown in b). d) Arrange-ment of the tapes in the perpendicular direction forming an unusual tetrameric hydrogen-bonding network.



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ORTEP plot of 2b is shown in Figure 6a. The two moleculesof bpyea in 2b differ in the ethylene bridges, one of which isfully ordered, while the other is disordered in the ratio of68:32.

In the three-dimensional arrangement, these moleculespack to form sheets stacked along the b axis (Figure 6b).The interactions among the molecules in the sheets arequite intriguing and show many common features with thepacking observed in 1b.

As observed in 1b, the two molecules of bpyea interactwith 2 in different modes. In one case, a disordered mole-cule of bpyea forms O�H···N hydrogen bonds (H···N1.70 H) exclusively by interacting only with a COOH group.The second molecule of bpyea, with a perfectly ordered eth-ylene bridge, interacts with 2 exclusively by forming O�H···N hydrogen bonds (H···N 1.92 H) involving only OHgroups. Thus, an infinite chain of alternately ordered anddisordered bpyea molecules are separated by molecules of2, results. These chains, in two-dimensions, yield a sheetstructure by formation of O�H···O hydrogen bonds (H···O2.03 H) between OH and COOH groups (Figure 6d). As aresult, a ladderlike structure is formed, exactly as observedin 1b, in which alternating ladders have bpyea molecules ofdifferent orientation. Such similar global packing between1b and 2b further supports that the role of Br is limited tovariations in the molecular geometry and basic recognitionpatterns, without any influence on the ultimate three-dimen-sional packing features. However, the influence of the Bratom in a unilateral manner is observed in complex 2c,formed between 2 and bpy.

Molecular complex of 4-bromo-3,5-dihydroxybenzoic acidand 4,4-bipyridine (2c): Cocrystallization of acid 2 with bpyfrom methanol occurred in a 1:1 ratio to give 2c (Table 1).There are no anomalous features about the molecular geom-etry of the product, as the structure is well refined withoutany ambiguity. However, the packing arrangement in thecrystal lattice is fascinating in many aspects. The basic recog-nition between 2 and bpy involves both COOH and OHgroups, which form O�H···N hydrogen bonds as in 1c.

In 2c, each bpy interacts with two molecules of 2 formingO�H···N hydrogen bonds (H···N 1.92 H) with OH groupsand pairwise O�H···N (H···N 1.74 H) and C�H···O (H···O,2.71 H) hydrogen bonds formed by COOH groups. Thus, aone-dimensional crinkled tape (Figure 7a) is formed, whichis quite usual feature of this type of recognition process thatwas observed in earlier examples, too. However, the interac-tion between adjacent tapes and the resulting three-dimen-sional arrangement is quite fascinating.

The one-dimensional units are held together by forming afour-membered O�H···N and C�H···O hydrogen-bond cou-pling (inset of Figure 7b). The H···N and H···O distances are1.92 and 2.44 H, respectively. Interestingly, a similar networkwas observed in the crystal structure of complex 1c, which isformed by bpy with 1 instead of 2. Thus a huge void space,(12L29 H2) results, as observed in many other organic as-semblies (see Figure 7c). Since such void structures are quite

Figure 5. a) ORTEP plot showing deprotonation of acid molecules 2 inthe asymmetric unit of 2a. b) The basic recognition pattern, formed ex-clusively by interaction between COO� of 2 and protonated N atom ofbpyee. c) Arrangement of molecules of 2 showing cisoid orientation ofBr atoms. d) Two-dimensional arrangement of the molecular ensemblesin 2a (viewed along a axis).



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unstable, they are generally occupied by guest species if anappropriate molecule is available; otherwise, catenation orinterpenetration result through self-assembly. In complex2c, as no guest species is present, the void space is filled bya self-assembly process leading to exotic threefold inter-penetration (Figure 7d). The three units are shown in differ-ent colors for a better understanding of the interpenetration.Although the Br atom is not involved directly in any appre-ciable nonbonding interactions or hydrogen bonds, its effectis fully reflected in the formation of an interpenetrated net-work structure, despite the similar nature of interactions tothose observed in 1c, perhaps due to its bulky nature.

From ladders to interpenetration and host–guest networks :It is apparent from the study of molecular complexes 1a–1cand 2a–2c that ladderlike structure are predominatelyformed in 1a, 1b, 2a, and 2b, whereas 1c and 2c completelydeviate from this behavior. In particular, it is noteworthy

that bpy as spacer molecule did not yield ladderlike struc-tures with either 1 or 2. However, bpyea, irrespective of thenature of the acid, gave the same type of supramolecularstructure, whereas bpyee showed variations, and it is notpossible to draw firm conclusion on its behavior, as the de-protonation of the acid molecule in complex 2a can signifi-cantly change the overall packing pattern. However, the ob-served transition from ladders to interpenetration could berelated to the dimensions of the molecular components andtheir ability to form closed ensembles. In this process, if thedimension of the void space is within the van der Waalslimits, a regular ladderlike structure results, otherwise an in-terpenetrated or host–guest network would be formed. Aschematic representation of the relations among the globalarchitectures is shown in Scheme 4.

Conformational differences of the OH groups in complexes1a–1c and 2a–2c : A collective analysis of all the complexes

Figure 6. a) ORTEP plot of 2b showing disorder around the methylene bridge in one of the molecules of bpyea. b) and c) Different recognition patternsshown by the two molecules of bpyea with acid 2. d) Two-dimensional arrangement of the ensembles formed by the two different molecules of bpyeawith 2, yielding a ladderlike structure. e) Schematic representation of the ladder (viewed along c axis).



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reveal that the conformations of acids 1 and 2 in 1a–1c and2a–2c are different due to the differences in the arrange-ment of OH groups, and it appears that this is the primefactor in the formation of specific structural arrangements.The different conformations observed are shown in Figure 8.In 1a and 1c the OH group adopts a syn–syn arrangement,while in 2a and 2c the hydroxyl groups are in anti–anti ar-rangements. In 1b and 2b, the arrangement is syn–anti.

The above classification is with respect to the orientationof the H atom on the OH group towards the H/Br atom atthe para position on molecules 1 and 2. It is apparent thatbpy and bpyee directed the syn–syn conformation in 1,while they induced the anti–anti conformation in 2. In con-trast, bpyea always strongly favored the syn–anti conforma-tion. This could be the reason for the formation of same lad-

Figure 7. a) Recognition pattern between 2 and bpy through the formation of O�H···N and O�H···N/C�H···O pairwise hydrogen bonds between OH andCOOH, respectively, with N atoms to form a crinkled tape. b) Arrangement of the adjacent tapes, held together by a fourfold hydrogen-bonding patterninvolving O�H···N and C�H···O hydrogen bonds. c) Void space in tetrameric unit shown in b). d) Filling of the void space by interaction between the ad-jacent tapes and threefold interpenetration (viewed along a axis).

Scheme 4.



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derlike structure in 1b and 2b, whereas the nature of theglobal structure varied in the other complexes due to thevariations in the conformations of the OH groups. Further-more, the syn–anti arrangement is intact in both 1b and 2b,irrespective of the presence of a Br atom. Similarly, theanti–anti arrangements in 2a and 2c could also be attributedto the bulky nature of the Br atom. Although global packingand molecular interactions could not give conclusive infor-mation about the role of the Br atom, the conformationalanalysis more or less demonstrates its size effect, based onthe observed variations in the lattice arrangements between1a–1c and 2a–2c.

Conclusion

We have synthesized and structurally evaluated molecularcomplexes 1a–1c and 2a–2c to account for the basic recog-nition pattern between the constituents. It is evident thatthe affinities of COOH and OH groups towards N-donorcompounds are fairly competitive, and this is in a way re-flected in the formation of different recognition patterns.However, the global packing arrangement is not much per-turbed, perhaps due to the operation of same principles inall the complexes, that is, effective space filling in accord-ance with crystallographic symmetry rules. Thus, ladderlikestructures and interpenetrated networks appeared, depend-ing on the size of the available molecular components anddimensions of resultant void space. Furthermore, the role ofa Br substituent is enigmatic, as it appears to be dominatedby its electronic nature in the basic recognition aspect, butin a molecular analysis, the role of its bulky nature is morepredominant. Nevertheless, the packing arrangements in2a–2c are not that different from those of unsubstitutedstructures 1a–1c, except for 2c. We believe that a large

number of further examples are required to draw authenticconclusions on the effects of various functional groups onthe robustness of formation of four-membered cyclic unitsas depicted in Scheme 2. To this end, we are synthesizingand analyzing other halogen derivatives and also those withother functional groups such as nitrile, nitro, and amide moi-eties.

Experimental Section

Preparation of molecular complexes 1a–1c and 2a–2c : All chemicals, re-agents, and solvents were obtained from commercial suppliers and usedwithout further purification. We used spectroscopic-grade solvents in allcocrystallization studies. All cocrystals 1a–1c and 2a–2c were preparedby dissolving the respective reactants in a ratio of 1:1 in CH3OH and al-lowing the solvent to evaporate under ambient conditions. In all cases,single crystals suitable for X-ray diffraction analysis were obtained within3 d.

In a typical cocrystallization experiment 4-bromo-3,5-dihydroxybenzoicacid (2, 0.094 g, 0.400 mmol) and 1,2-bis(4-pyridyl)ethene (bpyea, 0.072 g,0.400 mmol) were dissolved in MeOH (8 mL) in a 25 mL conical flask bywarming on a water bath. The resultant solution was allowed to evapo-rate under ambient conditions, and colorless single crystals were obtainedin 2 d. The crystals were separated from the mother liquor by filtration,washed with ice-cold CH3OH, and dried under vacuum.

Crystal structure determination : Single crystals were analyzed under aLeica microscope equipped with a CCD camera, and good-quality crys-tals were chosen for structure determination by x-ray diffraction with aPolaroid detector. The crystals were mounted on a goniometer by gluingto a glass fiber with cyanoacrylate adhesive, and crystal data were collect-ed on a CCD diffractometer with APEX detector. The intensity datawere processed using SAINT[23] software of the Bruker suite of programs.The structures were solved and refined using the SHELXTL package,[23]

and no anomalies were observed at any stage of structure solutions. Thefinal crystallographic details and data collection strategies are given inTable 1. All calculations of intermolecular interactions listed in Table 2were done with PLATON.[25]

CCDC-278691–278695 contain the supplementary crystallographic datafor this paper. These data can be obtained free of charge from the Cam-bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgement

We thank Dr. S. Sivaram (Director, NCL) and Dr. K. N. Ganesh (Headof Organic Division, NCL) for their constant support and encourage-ment. S.V. thanks Council of Scientific and Industrial Research (CSIR)for the award of Senior Research Fellowship (SRF).

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Figure 8. Representation of different conformations of 1 in complexes1a–1c and 2 in complexes 2a–2c.



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Received: May 23, 2005Revised: July 18, 2005

Published online: October 26, 2005



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Hydrogen bond mediated open-frame networks in coordinationpolymers: supramolecular assemblies of Pr(III) and 3,5-dinitro-4-methylbenzoic acid with aza-donor compounds{

Sunil Varughese and V. R. Pedireddi*

Received (in Cambridge, UK) 23rd November 2004, Accepted 25th January 2005

First published as an Advance Article on the web 7th February 2005

DOI: 10.1039/b417754a

A coordination assembly of 3,5-dinitro-4-methylbenzoic acid

and Pr(III), synthesized by hydrothermal methods forms a host

structure, which is stable up to 300 uC, through C–H…O

hydrogen bonds and accommodates different types of guest

species varying from simple molecules like water to larger

molecules like trans-1,2-bis(4-pyridyl)ethene.

Naturally occurring inorganic minerals, like zeolites,1 with well-

defined open-frame networks, possessing different types of voids

and channels, are the source of inspiration for current research

activities in the design and synthesis of open-frame networks of

varied architectures.2,3 Metal–organic hybrids with distinctly

strong bonding properties between metal and organic ligands are

considered to be robust enough to mimic zeolite type structures.4

Several carboxylate mediated coordination polymers have

proved to be quite successful in forming self-assembled open-

frame networks that accommodate various guest species.5,6 In

most of these assemblies the host structures are designed to form

exclusively through dative bonds. However, taking into account

the nature of functional groups such as –NO2 and –NH2 to form

well defined robust hydrogen bond networks,7 but which have less

affinity towards metal ions, we have focused on the syntheses of

metal–organic supramolecular assemblies employing organic

ligands having –NO2 groups along with carboxylates in order to

evaluate the role of both types of bonds in forming the requisite

host structures, as only a few such structures are known in the

literature.8 For this purpose, syntheses of coordination assemblies

of 3,5-dinitro-4-methylbenzoic acid (DNMB) with Pr(III), con-

sidering the growing interest of evaluation of lanthanide ions in the

supramolecular studies,9 have been carried out.

Pale yellow single crystals obtained from the hydrothermal

reaction of an aqueous solution of DNMB and praseodymium

acetate (PA) gave a coordination complex, 1, [Pr2(C8H5N2O6)6-

(H2O)4]?6H2O, as characterized by X-ray diffraction techniques.{Analysis of the coordination geometry reveals that 1 is a

dinuclear metal complex with two Pr(III) ions being bridged by

the carboxylate groups of two DNMB molecules with an average

Pr–O distance of 2.400 A (Fig. 1(a)). Unique metal–ligand bond

distances are given in Table 1. In addition, each Pr(III) shows a

nine coordination environment with further bonding to three more

DNMB molecules and two water molecules in such a manner that

one of the DNMB molecules forms chelated Pr–O bonds, through

carboxylate groups. The average Pr–O distance is 2.665 A. The

remaining two DNMB molecules, however, act as bis-

monodentate ligands towards Pr(III), giving rise to one-

dimensional polymers, with Pr–O distances of 2.418 and 2.495 A.

The water molecules are coordinated to Pr(III) at distances of 2.563

and 2.610 A. The three-dimensional arrangement of these species,

however, is quite intriguing in the formation of an open-frame

network through the anticipated hydrogen bonding networks

{ Electronic supplementary information (ESI) available: Experimentaldetails, ORTEP diagrams and thermogravimetric data. See http://www.rsc.org/suppdata/cc/b4/b417754a/*[email protected]

Fig. 1 (a) Coordination of DNMB and water around Pr(III) forming a

dinuclear unit in the crystal structure of complex, 1. (b) Formation of

channels, occupied by six water molecules. C–H…O hydrogen bonds

between the units denoted A and B are shown in expansion.

COMMUNICATION www.rsc.org/chemcomm | ChemComm

1824 | Chem. Commun., 2005, 1824–1826 This journal is � The Royal Society of Chemistry 2005

between the –CH3 and –NO2 groups. A typical arrangement is

shown in Fig. 1(b).

Two different types of hydrogen bonding networks, as

represented by A and B in Fig. 1(b), connect four neighboring

units with the formation of C–H…O hydrogen bonds with H…O

distances in the range 2.43–2.95 A. Thus, the aggregation of the

coordination units through C–H…O hydrogen bonds, results in

the formation of channels (6 6 15 A2), which are occupied by six

water molecules (see Fig. 1(b)).

It is apparent from the thermogravimetric analysis that water

molecules in the channels are being evacuated at around 130 uCand the complex is found to be crystalline and stable as confirmed

by X-ray powder diffraction methods. We wished to insert

hydrocarbons like naphthalane and anthracene in such channels as

these hydrocarbons are well known to act as guests.7 But, our

experiments revealed that the components crystallized separately,

perhaps due to incompatibility between the dimensions of the

channels and the guests.

Hence, we carried out hydrothermal synthesis of DNMB and

PA in the presence of 4,49-bipyridyl (bpy) with a hope that bpy will

facilitate the creation of larger channels as it is well known to act as

a spacer in many other assemblies to accommodate large guest

molecules. To our surprise, crystal structure determination,

however, discloses that a complex, [Pr2(C8H5N2O6)6-

(H2O)4]?2C10H8N2, 2, was formed, in which bpy exists as a free

ligand in the asymmetric unit without coordinating to Pr(III).{Nevertheless, complexes 1 and 2 are isostructural, with similar

aggregation patterns and channels due to the association of the

neighboring coordination units through C–H…O hydrogen

bonds, except for the guest molecules. The C–H…O hydrogen

bonds observed in 2 are acyclic with H…O distances in the range

2.53–2.92 A as shown in Fig. 2(a). Such an arrangement ultimately

constituted channels (7 6 14 A2), in a three-dimensional

arrangement as shown in Fig. 3, which are occupied by two

molecules of bpy, replacing all six water molecules observed in

complex, 1.

The bpy molecules in the channels interact with the host

through the formation of O–H…N hydrogen bonds (N…O, 2.77

and 2.80 A). Furthermore, the open-frame network observed in

complexes 1 and 2 is quite stable even to perform guest-exchange

reactions, as we noted that a reaction between complex 1

(possessing water in the channels) and bpy gave exclusively

complex, 2 (channels filled with bpy) in 100% yield.

Similar observations were also noted for the guest-exchange

reaction between complex 1 and trans-1,2-bis(4-pyidyl)ethene

(bpyee), forming a complex, [Pr2(C8H5N2O6)6(H2O)4](C12H10N2)-

(4H2O), 3,{ except that four water molecules still remain in the

channels along with bpyee. The open-frame network, positions of

bpyee and water molecules within the channels (6 6 16 A2) are

shown in Fig. 2(b). In fact, complex 3 could not be synthesized by

a direct reaction between DNMB and PA in the presence of bpyee.

In contrast, 1,2-bis(4-pyridyl)ethane (bpyea), an analogue of

bpyee, forms the complex [Pr2(C8H5N2O6)6(H2O)4](C12H12N2)-

(4H2O), 4, directly from a reaction between DNMB and PA along

with bpyea.{ The molecular arrangement in complex 4 is shown in

Fig. 2(c). This complex also is isostructural to those of 1–3 but

bpyea molecules occupy the channels (5 6 16 A2) as guest

molecules. Thus, in addition to the robustness, the open-frame

network formed by DNMB and Pr(III) also shows the flexibility to

accommodate additional molecules. Perhaps interaction between

the coordination units through weak bonds (C–H…O hydrogen

bonds), which would reorganize with ease, rather than strong

bonds, facilitated the expansion of the channels.

In conclusion, we have demonstrated the utilization of both

coordinate bonds as well as hydrogen bonds to form host–guest

complexes. Further, the robustness of the host network formed by

DNMB and Pr(III) is comparable to those of similar structures

Fig. 2 Channels observed in the complexes, 2, 3 and 4 which are occupied by (a) 4,49-biprydine, (b) trans-1,2-bis(4-pyridyl)ethene and (c) 1,2-bis(4-

pyridyl)ethane, respectively. Arrows indicate the vivid description of C–H…O hydrogen bonding pattern between the adjacent units.

Table 1 Bond distances around the coordination sphere of Pr(III)

Compound

Pr–O(carboxylate)

Pr–O(water) Bridging Chelating m-oxo bridging

1 2.563(7) 2.391(6)a 2.559(6) 2.460(6)2.610(6) 2.408(6)a 2.772(6)

2.418(6)b

2.495(6)b

2 2.563(2) 2.420(2)a 2.567(2) 2.483(2)2.563(2) 2.421(2)a 2.828(2)

2.422(2)b

2.467(2)b

3 2.567(4) 2.393(4)a 2.572(4) 2.486(4)2.628(4) 2.406(4)a 2.708(4)

2.428(4)b

2.472(4)b

4 2.552(3) 2.405(3)a 2.577(3) 2.488(2)2.617(3) 2.414(2)a 2.730(2)

2.424(3)b

2.468(2)b

a Bridging within the bimetallic unit. b Bridging between bimetallicspecies.

This journal is � The Royal Society of Chemistry 2005 Chem. Commun., 2005, 1824–1826 | 1825

formed by purely dative bonds. We believe that the complexes

presented in this communication will emerge as leading examples

to explore numerous systems employing different types of organic

ligands and we are currently exploring many of these systems.

We thank Dr. S. Sivaram, Director, NCL, and Dr. K. N.

Ganesh, Head of the Division, NCL for their encouragement. One

of us (SV) thanks CSIR, New Delhi for the award of JRF.

Sunil Varughese and V. R. Pedireddi*Division of Organic Chemistry, National Chemical Laboratory,Dr. Homi Bhabha Road, Pune 411 008, India.E-mail: [email protected]; Fax: +91 20 25893153;Tel: +91 20 25893400 ext 2097


{ Crystal data for complex, 1: [Pr(C8H5N2O6)3(H2O)2]?3(H2O), Mr =906.41, yellow needles, 0.35 6 0.27 6 0.19 mm, triclinic, P1, a = 9.483(3),b = 12.593(3), c = 15.517(4) A, a = 76.89(9), b = 78.10(9), c = 81.33(9)u, V =1755.4(12) A3, Z = 2, rcalcd = 1.715 g cm23, m =1.488 mm21, 2hmax = 46.50,l(Mo Ka) = 0.71073 A, T = 298(2) K, 14587 total reflections, R1 = 0.0598and wR2 = 0.1884 for 4903 reflections (I . 2s(I)). CCDC 233738.

Crystal data for complex, 2: [Pr(C8H5N2O6)3(H2O)2]?(C10H8N2), Mr =1008.55, yellow needles, 0.37 6 0.23 6 0.18 mm, triclinic, P1, a = 9.774(4),b = 12.721(6), c = 16.355(7) A, a = 72.01(9), b = 82.90(9), c = 83.15(9)u,

V = 1912.2(18) A3, Z = 2, rcalcd = 1.752 g cm23, m = 1.372 mm21, 2hmax =46.60, l(Mo Ka) = 0.71073 A, T = 298(2) K, 16061 total reflections,R1 = 0.0255 and wR2 = 0.0661 for 5183 reflections (I . 2s(I)). CCDC233739.

Crystal data for complex, 3: [Pr(C8H5N2O6)3(H2O)2]?0.5-(C12H10N2)?2(H2O), Mr = 979.50, yellow needles, 0.41 6 0.29 60.22 mm, triclinic, P1, a = 9.510(9), b = 12.669(9), c = 16.430(2) A, a =72.28(2), b = 79.06(2), c = 79.72(2)u, V = 1836(2) A3, Z = 2, rcalcd =1.772 g cm23, m = 1.429 mm21, 2hmax = 46.70, l(Mo Ka) = 0.71073 A,T = 298(2) K, 15345 total reflections, R1 = 0.0345 and wR2 = 0.0711 for4253 reflections (I . 2s(I)). CCDC 233740.

Crystal data for complex, 4: [Pr(C8H5N2O6)3(H2O)2]?0.5-(C12H12N2)?2(H2O), Mr = 980.51, yellow needles, 0.39 6 0.26 60.20 mm, triclinic, P1, a = 9.517(2), b = 12.747(2), c = 16.366(3) A, a =72.21(2), b = 78.98(3), c = 79.84(2)u, V = 1840.8(6) A3, Z = 2, rcalcd =1.769 g cm23, m = 1.425 mm21, 2hmax = 46.54, l(Mo Ka) = 0.71073 A,T = 298(2) K, 11563 total reflections, R1 = 0.0283 and wR2 = 0.0768 for5016 reflections (I . 2s(I)). CCDC 233741. See http://www.rsc.org/suppdata/cc/b4/b417754a/ for crystallographic data in .cif or otherelectronic format.

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Fig. 3 Representation of channels (7 6 14 A2) observed in the three-

dimensional arrangement of complex, 2, (a) without guest molecules and

(b) with guest molecules 4,49-biprydine.

1826 | Chem. Commun., 2005, 1824–1826 This journal is � The Royal Society of Chemistry 2005

Solvent-Dependent Coordination Polymers: Cobalt Complexes of3,5-Dinitrobenzoic Acid and 3,5-Dinitro-4-methylbenzoic Acid with4,4′-Bipyrdine

V. R. Pedireddi* and Sunil Varughese

DiVision of Organic Chemistry, National Chemical Laboratory, Dr. Homi Bhabha Road,Pune 411 008, India

Received August 8, 2003

The synthesis, structure elucidation, and analysis of the self-assembly of Co(II) complexes of 3,5-dinitrobenzoicacid and 3,5-dinitro-4-methylbenzoic acid with 4,4′-bipyridine have been reported. Formation of the complexes andthe self-assembly in the three-dimensional structures have been found to be dependent on the solvents (such asacetone, dimethly sulfoxide, etc.) employed for the synthesis of the aggregates. 3,5-Dinitrobenzoic acid forms twocoordination polymers, 1a and 1b, from methanol and a mixture of methanol and acetone solvents, respectively,with entirely different recognition patterns. Similarly, 3,5-dinitro-4-methylbenzoic acid also forms two coordinationcomplexes, 2a and 2b, incorporating the solvent of the reaction medium into the crystal lattice. Complex 2a formsa solvated channel structure, whereas 2b gives a bilayered structure, with the layers being separated by solventof crystallization (dimethyl sulfoxide) molecules. All the complexes have been characterized by single-crystal X-raydiffraction studies. Complexes 1b, 2a, and 2b crystallize in a monoclinic lattice, but 1a adopts a tetragonal lattice.The unit cell dimensions are, for 1a, a ) 8.095(1) Å, b ) 8.095(1) Å, c ) 46.283 (6) Å, R ) 90°, â ) 90°, andγ ) 90° (space group P43212, Z ) 4), for 1b, a ) 22.774(2) Å, b ) 11.375 (1) Å, c ) 22.533(2) Å, R ) 90°,â ) 104.15(1)°, and γ ) 90° (space group P21/c, Z ) 4), for 2a, a ) 17.657(6) Å, b ) 18.709(4) Å,c ) 21.044(6) Å, R ) 90°, â ) 108.68(3)°, and γ ) 90° (space group, C2/c, Z ) 8), and, for 2b, a ) 11.025(5)Å, b ) 15.139(4) Å, c ) 11.443(4) Å, R ) 90°, â ) 97.48(3)°, and γ ) 90° (space group P2/n, Z ) 2). In allthe complexes 1a, 1b, 2a, and 2b, the basic interaction between Co(II) and 4,4′-bipyridine remains the same withthe formation of linear Co−N dative bonds, but the carboxylates display various modes of interaction with Co(II).The average Co−N and Co−O distances are 2.161 and 2.108 Å, respectively.

Introduction

The design and synthesis of coordination compounds datesback to the pioneering work of Cotton and others carriedout in the last few decades, in which emphasis was focusedon the creation of molecular entities to meet the requirementsof tailor-made applications such as catalytic activity, per-formance of selective chemical reactions, etc., exactly themanner in which conventional organic synthesis is beingcarried out to synthesize targeted compounds.1,2 However,the successful creation of supramolecular assemblies throughnoncovalent bonds such as hydrogen bonds3-5 employingorganic entities directed the utilization of highly directionalcoordinate bonds also to create functionalized solids in the

form of organic-inorganic hybrids6-8 with intriguing struc-tural motifs and potential applications.9,10The main advantageof these systems is the formation of rigid networks throughstrong coordinated bonds unlike the counterparts of purely

* Author to whom correspondence should be addressed. E-mail: [email protected].

(1) (a) Cotton, F. A.; Wilkinson, G.AdVanced Inorganic Chemistry: AComprehensiVe Text; Wiley: New York, 1979. (b) Cotton, F. A.;Hillard, E. A.; Murillo, C. A. J. Am. Chem. Soc.2002, 124, 5658. (c)Cotton, F. A.; Daniels, L. M.; Murillo, C. A.J. Am. Chem. Soc.2002,124, 2878. (d) Cotton, F. A.; Dikarev, E. V.; Petrukhina, M. A.Angew.Chem., Int. Ed. 2000, 39,2362. (e) Cotton, F. A.; Daniels, L. M.; Lu,J.; Ren, T.Acta Crystallogr.1997, C53, 714. (f) Cotton, F. A.; Dikarev,E. V.; Petrukhina, M. A.; Schmitz, M.; Stang, P. J.Inorg. Chem.2002,41, 2903.

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450 Inorganic Chemistry, Vol. 43, No. 2, 2004 10.1021/ic0349499 CCC: $27.50 © 2004 American Chemical SocietyPublished on Web 12/17/2003

organic based systems, which are being created by weakbonds such as hydrogen bonds. We find numerous examplesof hybrid structures in the literature,11 the majority of whichwere created by employing organic compounds with func-tional groups having affinity toward metal ions placed at therequired positions to give desired architectures. In thisrespect, a vast majority of studies are being carried out usingthe carboxylate group, and indeed, hybrid structures formedbetween trimesic acid and Co(II), reported by Yaghi and co-workers, could be regarded as representative examples forthe implication of dative bonds in supramolecular chemis-

try.12 Also, splendid contributions from several groups ofresearchers such as Zaworotko,13 Braga,14 Hosseini,15 Stang,16

etc., employing different types of ligands with multiplebridging moieties (in particular, 4,4′-bipyridyl), are quitenoteworthy. Nevertheless, the studies are more often focusedon using organic ligands having functional groups that areexclusively able to form dative bonds such that porousstructures could be synthesized which find applications incatalysis and molecular adsorption.9,10 However, instead,organic ligands with noncoordinated functional groups suchas-NO2, -NH2, etc. in conjunction with carboxylate groupsare considered for the creation of supramolecular assemblies.A variety of novel materials would result, due to thesimultaneous formation of dative bonds and noncovalentbonds, because these functional groups are well-known toform robust and strong hydrogen bonds, but such studiesare limited. In this direction, we are interested to utilize theknowledge of dative bonds as well as noncovalent bonds(for instance, hydrogen bonds of different strengths such asO-H‚‚‚O, C-H‚‚‚O, etc.) to create novel coordinationcomplexes that can yield different networks such as poly-mers, channels, layers, etc. in their three-dimensional struc-tures. Since tuning of hydrogen bonds can be done with achange of the solvent system of the reaction medium,17 onecould obtain different architectures at ease by carrying outa reaction in different solvents. As a result, either polymorphsor pseudopolymorphs can be obtained easily utilizing both

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(13) (a) Yoon, C. H.; Zaworotko, M. J.; Moulton, B.; Jung, K. W.Org.Lett. 2001, 3, 3539. (b) Fryzuk, M. D.; Duval, P. B.; Mao, S. S. H.;Zaworotko, M. J.; MacGullivray, L. R.J. Am. Chem. Soc.1999, 121,2478. (c) Bourne, S. A.; Lu, J.; Mondal, A.; Moulton, B.; Zaworotko,M. J. Angew. Chem., Int. Ed. Engl. 2001, 40, 2111. (d) Copp, S. B.;Holman, K. T.; Sangster, J. O. S.; Subramanian, S.; Zaworotko, M. J.J. Chem. Soc., Dalton Trans.1995, 2233. (e) MacGullivray, L. R.;Subramanian, S.; Zaworotko, M. J.; Biradha, K.Inorg. Chem.1999,38, 5078. (f) Rather, B.; Moulton, B.; Walsh, R. D. B.; Zaworotko,M. J. Chem. Commun.2002, 694. (g) Hennigar, T. L.; MacQuarrie,D. C.; Losier, P.; Rodgers, R. D.; Zaworotko, M. J.Angew. Chem.,Int. Ed. Engl.1997, 36, 972.

(14) (a) Braga, D.; Maini, L.; Grepioni, F.Chem. Commun. 1999, 937. (b)Braga, D.; Maini, L.; Grepioni, F.J. Organomet. Chem. 2000, 593,101. (c) Grepioni, F.; Cojazzi, G.; Braga, D.; Marseglia, E.; Scac-cianoce, L.; Johnson, B. F. G.J. Chem. Soc., Dalton Trans. 1999,553. (d) Braga, D.; Maini, L.; Cojazzi, G.; Polito, M.Chem. Commun.1999, 1949. (e) Braga, D.; Maini, L.; Grepioni, F.; Polito, M.Organometallics1999, 18, 2577. (f) Braga, D.; Maini, L.; Grepioni,F.; Cojazzi, G.; Emiliani, D.Chem. Commun. 2001, 2272.

(15) (a) Kaes, C.; Katz, A.; Hosseini, M. W.Chem. ReV. 2000, 100, 3553.(b) Grossham, P.; Jouaiti, A.; Hosseini, M. W.New J. Chem. 2003,793. (c) Jouaiti, A.; Hosseini, M. W.; Kyritsakas, N.Chem. Commun.2003, 472. (d) Jouait, A.; Hosseini, M. W.; Kyritsakas, N.Chem.Commun.2002, 1898. (e) Schmaltz, B.; Hosseini, M. W.; Cian, A. D.Chem. Commun.2001, 1242.

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SolWent-Dependent Coordination Polymers

Inorganic Chemistry, Vol. 43, No. 2, 2004 451

dative bonds and noncovalent bonds.18 To evaluate some ofthese features of coordination complexes, we have considered3,5-dinitrobenzoic acid,1, and its 4-methyl-substitutedderivative (toluic acid),2, to form complexes with cobaltnitrate, and the results are described in the following sections.The choice of these acids lies with the fact that numerousorganic assemblies of both acids1 and2 are well-known inthe literature,19 but only a few metal complex studies arereported.20 Further, in this process, 4,4′-bipyridine (bpy) hasbeen chosen as a coligand,21 because it is well-known forits robustness to act as a spacer to enhance the intermetaldistance, which may facilitate the coordination between themetal ion and bulky organic molecules such as1 and2 dueto the minimization of crowding. In addition, bpy also hasthe ability to form hydrogen bonds such as C-H‚‚‚O throughits acidic phenyl hydrogens with the-NO2 groups of acids1 and2.22

Results and Discussion

Cocrystallization of 3,5-dinitrobenzoic acid,1, or thecorresponding toluic acid,2, with 4,4′-bipyridine and cobaltnitrate gave different types of coordination polymers withvariation of the solvent of crystallization. Thus, complexes

1a, 1b, 2a, and2b were obtained as listed in Scheme 1. Thereactions were carried out at ambient conditions, simply byheating the reaction mixtures in a water bath to dissolve thereactants. Nevertheless, the colossal differences in theobtained products are quite intriguing.

In complexes1a and 2a, the solvent of crystallization(CH3OH) is coordinated to the metal ion. However, in1band 2b, the solvent molecules, acetone and dimethyl sul-foxide (DMSO), respectively, are being incorporated into thecrystal lattices as guest molecules. The salient feature of allfour complexes will be discussed in this paper, highlightingsimilarities and differences, independently and collectively,so that a wide range of new assemblies can be synthesizedemploying derivatives of dinitrobenzoic acid in conjunctionwith metal salts.

Complexes [Co(C7H3N2O6)2(C10H8N2)2(CH3O)2], 1a, and[Co4(C7H3N2O6)8(C10H8N2)8(CH3O)2]CH3COCH3, 1b. 1forms a 2:1:1 complex, which we label1a, with 4,4′-bipyridine (bpy) and cobalt nitrate, respectively, upondissolving the constituents in a methanol solution andsubsequent slow evaporation over a period of 2 days atambient conditions. The crystal structure determinationreveals that complex1acrystallizes in a noncentrosymmetricspace group,P43212. Complete details of the crystallographicinformation are given in Table 1.

In the asymmetric unit, both acid1 (in its carboxylateform) and bpy interact with Co(II), forming Co-O andCo-N dative bonds, respectively. Selected bonding param-eters of carboxylate1 and bpy are given in Tables 2 and 3,respectively. The molecular arrangement around Co(II) isshown in Figure 1a. Thus, each Co(II) is connected to twobpy molecules and two carboxylate molecules of1 andcompletes the 6-fold coordination with the aid of two CH3-OH molecules to yield an octahedral arrangement. The twonitrogen heteroatoms on bpy form two distinct Co-Ndistances, 2.170 and 2.187 Å. All the metal-ligand distancesare listed in Table 2. The carboxylate of1 interacts withCo(II), in a monodentate fashion, with a Co-O distance of2.070 Å (Table 2), and the uncoordinated oxygen atom formsweak intermolecular C-H‚‚‚O hydrogen bonds. A schematicrepresentation of the Co-O bonding pattern is shown inChart 1a. The observed pattern, indeed, is topologicallyidentical to that of the catemeric hydrogen bonding23 betweencarboxylic acid groups in organic crystal structures (Chart1a).

Thus, in a typical octahedron, while bpy molecules lie atthe axial positions, carboxylates and methanol molecules

(18) (a) Bernstein, J.Polymorphism in Molecular Crystals; OxfordUniversity Press: New York, 2002. (b) McCrone, W. C. InPhysicsand Chemistry of the Organic Solid state; Fox, D., Labes, M. M.,Weissberger, A., Eds.; Wiley-Interscience: New York, 1965; Vol. 2,pp 725-767. (c) Dunitz, J. D.; Bernstein, J.Acc. Chem. Res.1995,28, 193. (d) Byrn, S. R.; Pfeiffer, R. R.; Stowell, J. G.Solid StateChemistry of Drugs, 2nd ed.; SSCI Inc.: West Lafayette, IN, 1999;pp 489-498. (e) Bilton, C.; Howard. J. A. K.; Madhavi, N. N. L.;Nangia, A.; Desiraju, G. R.; Allen, F. H.; Wilson, C. C.Chem.Commun.1999, 1675. (f) Bernstein, J.; Davey, R. J.; Henck, J.-O.Angew. Chem., Int. Ed.1999, 111, 3646. (g) Henck, J.-O.; Bernstein,J.; Ellern, A.; Boese, R.J. Am. Chem. Soc.2001, 123, 1834. (h)Brittain, H. G.Polymorphism in Pharmaceutical Solids; Marcel DekkerInc.: New York, 1999. (i) Kumar, V. S. S.; Addlagatta, A.; Nangia,A.; Robinson, W. T.; Charlotte, K. B.; Mondal, R.; Evans, I. R.;Howard, J. A. K.; Allen, F. H.Angew. Chem., Int. Ed.2002, 41, 3848.

(19) (a) Peng, Z.-H.; Woerpel, K. A.Org. Lett.2000, 2, 1379. (b) Aakeroy,C. B.; Beatty, A. M.; Helfrich, B. A.Angew. Chem., Int. Ed.2001,40, 3240. (c) Evans, P. A.; Murthy, V. S.; Roseman, J. D.Angew.Chem., Int. Ed.1999, 38, 3175. (d) Gonzalez, C. C.; Kennedy, A. R.;Leon, E. I.; Fagundo, c. R.; Suarez. E.Angew. Chem., Int. Ed.2001,40, 2320. (e) Dambrin, V.; Villieras, M.; Janvier, P.; Toupet, L.; Amri,H.; Lebreton, J.; Villieras, J.Tetrahedron2001, 57, 2155. (f) Ermer,O.; Mason, S. A.Chem. Commun. 1983, 53.

(20) (a) Zhang, S.-L.; Tong, M.-L.; Fu, R.-W.; Chen, X.-M.; Seik-Weng,Ng. Inorg. Chem. 2001, 40, 3562. (b) Sudenberg, M. R.; Klinga, M.;Uggla, R.Inorg. Chim. Acta. 1994,216, 57. (c) Thahir, M. N.; Ulku,D.; Movsumov, E. M.Acta Crystallogr., Sect. C: Cryst. Struct.Commun.1996, 52, 1392. (d) Yang, G.; Zhu, H.-G.; Zhang, L, -Z.;Cai, Z.-G.; Chen, X.-M.Aust. J. Chem.2000, 53, 601.

(21) (a) Johnson, J. W.; Jacobson, A. J,; Rich, S. M.; Brody, J. F.J. Am.Chem. Soc. 1981, 103, 5246. (b) Stephens, F. S.; Vagg, R. S.Inorg.Chim. Acta1980, 42, 139. (c) Kim, H.-J.; Redman, J. E.; Nakash,M.; Feeder, N.; Teat, S. J.; Sanders, J. K. M.Inorg. Chem.1999, 38,5178. (d) Zhang, Y.; Jianmin, L.; Nishiuram, M.; Imamoto, T.J. Mol.Struct. 2000, 519, 219. (e) Nakajima, K.; Yokoyama, K.; Kano, T.;Kojima, M. Inorg. Chim. Acta1998, 282, 209. (f) Wisner, J. A.; Loeb,S. J.Chem. Commun.1998, 2757. (h) Zaworotko, M. J.; Biradha, K.;Bonasevitch, K. V.; Moutlon, B.; Seward, C.Chem. Commun.1999,1327. (g) Gudbjartson, H.; Biradha, K.; Poirier, K. M.; Zaworotko,M. J. J. Am. Chem. Soc. 1999, 121, 2599.

(22) (a) Rother, I. B.; Friesinger, E.; Erxleben, A.; Lippert, B.Inorg. Chim.Acta2000, 301, 339. (b) Friesinger, E.; Lippert, B.; Meier, S.J. Chem.Soc., Dalton Trans.2000, 3274. (c) Rother, I. B.; Lippert, B.;Willermann, M.Supramol. Chem.2002, 14, 189. (d) Hunks, W. J.;Jennings, M. C.; Puddephatt, R. J.Inorg. Chem.2002, 41, 4590. (e)Schneider, W.; Bauer, A.; Schmidbaur, H.Organometallics1996, 15,5445. (f) Raper, E. S.Coord. Chem. ReV. 1996, 153, 199.

(23) (a) Desiraju, G. R.; Murty, B. N.; Kishan, K. V. R.Chem. Mater.1990, 2, 447. (b) Hardy, G. E.; Kaska, W. C.; Chandra, B. P.; Zink,J. I. J. Am. Chem. Soc.1981, 103, 1074. (c) Thalladi, V. R.; Nusse,M.; Boese, R.J. Am. Chem. Soc.2000, 122, 9227. (d) Ermer, O.;Bell, P.; Mason, S. A.Angew. Chem., Int. Ed. Engl.1989, 28, 1239.

Scheme 1

Pedireddi and Varughese

452 Inorganic Chemistry, Vol. 43, No. 2, 2004

occupy the equatorial sites. A noteworthy feature is that thephenyl moieties of1 and methyl groups of CH3OH lie aboveand below the basal plane constituted by Co(II) and Co-O

bonds, and further, either the two carboxylates or the twoCH3OH molecules are situated attranspositions to each otherwith respect to Co(II); see Figure 1a. Further, coordinationcomplex1a forms infinite linear polymer chains, joining theadjacent octahedrons through bpy molecules. In three-dimensional arrangement, these polymer chains are arrangedin a crossed manner and interact with each other throughweak C-H‚‚‚O hydrogen bonds. The arrangement of thesepolymer chains, in a truncated form, is shown in Figure 1b.It is evident that the weak C-H‚‚‚O hydrogen bonds areformed between the-NO2 group and hydrogen atoms ofthe bpy and methanol molecules. The H‚‚‚O distancesinvolving bpy are 2.85 and 2.90 Å, whereas the similardistance formed by CH3OH molecules is 2.73 Å (Table 3).

However,1 forms an entirely different type of coordinationpolymer with cobalt nitrate and bpy upon crystallization froma methanol/acetone mixture. The crystal structure determi-nation (see Table 1) reveals the presence of the solvent of

Table 1. Crystal Data, Structure Determination, and Refinement Parameters of Coordination Complexes1a, 1b, 2a, and2b

1a 1b 2a 2b

chemcial formula (C7H3N2O6)2(C10H8N2)CoII-(CH3O)2

(C7H3N2O6)4(C10H8N2)2CoII2-

(CH3O)(C3H6O)(C8H5N206)2(C10H8N2)CoII (C8H5N2O6)2(C10H8N2)CoII-

(H2O)2(C2H6SO)2fw 699.41 1363.80 665.39 841.55cryst habit needlelike needlelike needlelike needlelikecryst color pale pink pale pink pale pink pale pinkcryst system tetragonal monoclinic monoclinic monoclinicspace group P43212 P21/c C2/c P2/na (Å) 8.095(1) 22.774(2) 17.675(6) 11.025(5)b (Å) 8.095(1) 11.375(1) 18.709(4) 15.139(4)c (Å) 46.283(6) 22.533(2) 21.044(6) 11.443(4)R (deg) 90.00 90.00 90.00 90.00â (deg) 90.00 104.15(1) 108.68(3) 97.48(3)γ (deg) 90.00 90.00 90.00 90.00V (Å3) 3033 (7) 5660.2(9) 6592(3) 1893.8(12)Z 4 4 8 2Dcalcd(g cm-3) 1.532 1.600 1.341 1.476T (K) 293 293 293 293λ(Mo KR) (Å) 0.71073 0.71073 0.71073 0.71073µ (mm-1) 0.646 0.687 0.586 0.6412θ range (deg) 46.52 46.60 46.36 46.60limiting indices -8 e h e +7 -25 e h e +23 -19 e h e +1 -12 e h e +12

-8 e k e +6 -12 e k e +12 -20 e k e +18 -16 e k e +16-51 e l e +51 -25 e l e +24 -10 e l e +21 -6 e l e +12

F(000) 1428 2780 2712 854no. of reflns measd 12826 23238 5005 7728no. of unique reflns 2169 8158 3085 2726no. of reflns used 1957 3933 2318 1972no. of params 250 830 420 262GOF onF2 1.229 1.121 1.338 1.061R1 [I > 2σ(I)] 0.050 0.104 0.099 0.052WR2 0.114 0.215 0.282 0.140final diff Fourier map

(e-‚Å-3), max, min0.270,-0.536 0.748,-0.812 1.251,-0.623 0.644,-0.403

Table 2. Details of the Coordinate Bond in the Complexes1a, 1b,2a, and2b

bond 1a 1ba 1bb 2a 2b

Co-N(1) 2.170 2.153 2.140 2.156 2.175Co-N(2) 2.187 2.143 2.153 2.157 2.174Co-O(1) 2.070 2.053 2.087 2.041 2.080Co-O(2) 2.070 2.040 2.094 2.068 2.080Co-O(3) 2.139c 2.103 2.106 2.217d 2.139e

Co-O(4) 2.139c 2.162c 2.119c 2.218d 2.139e

a Terminal metal.b Middle metal center.c Bonds from MeO.d Chelatedbonds.e Bonds from H2O.

Table 3. Characteristics of C-H‚‚‚O Hydrogen Bondsa Observed in Complexes1a, 1b, 2a, and2b

1a 1b 2a 2b

2.73 3.73 139.2 2.52 3.45 173.1 2.90 3.80 164.9 2.72 3.32 123.72.85 3.36 126.8 2.60 3.51 167.9 2.44 3.20 136.9 2.68 3.59 166.72.90 3.51 126.1 2.84 3.76 161.0 2.95 3.67 134.7

2.84 3.73 159.9 2.66 3.26 120.92.86 3.75 158.82.49 3.34 152.82.93 3.78 152.82.67 3.52 152.52.75 3.56 145.32.89 3.65 140.0

a Three columns for each complex represent the distances H‚‚‚O and C‚‚‚O (Å) and the angle C-H‚‚‚O (deg), respectively.



crystallization, acetone, in the crystal lattice. The constituents(excluding the solvent of crystallization) have a compositionof 4:2:2, which is different from that of1a. A molecularunit of the coordination complex is shown in Figure 2a.

Analysis of the molecular arrangement discloses a uniquepattern with four Co(II) atoms, eight molecules of1 and bpyeach, and two molecules of methanol in every tetramericunit. In a typical unit, all Co(II) atoms are attached tocarboxylate1 and bpy through Co-O and Co-N coordinatebonds, respectively. As in1a, all the Co-N bonds areidentical with an average distance of 2.147 Å. However,unlike in 1a, the carboxylate group in complex1b has twodifferent modes of interaction with Co(II), forming a singleCo-O bond (in a monodentate fashion) with terminal Co-

(II), but two Co-O bonds (as a bridged bidentate ligand)with the two intermediate Co(II) ions. Further, two terminalCo(II) ions have coordinated methanol molecules also, alongwith carboxylates and bpy, to fulfill the 6-fold coordination.As a result, although octahedral geometry is found aroundeach Co(II), the ligands around the terminal Co(II) aredifferent from the intermediate ones.

Thus, in the octahedral arrangement, around each Co(II),molecules of bpy are situated at axial positions, but depend-ing upon the placement of Co(II) (terminal or middle), eithercarboxylates alone or carboxylate and methanol together formthe basal plane. These quadrate cobalt-centered units con-stitute a three-dimensional arrangement in such a mannerthat the adjacent units form a polymer block-type structureby joining the adjacent tetramers through Co-N bonds along[010]. Packing of these blocks is shown in Figures 2b and3. It is evident that, along a lateral direction, the blocks areheld together by C-H‚‚‚O hydrogen bonds formed betweenthe-NO2 groups of1 and phenyl hydrogens of bpy (Figure2b). The H‚‚‚O distances are in the range 2.49-2.89 Å.However, acetone (solvent of crystallization) molecules areinserted as guest species between the adjacent blocks (Figure3), along the Co(II)-bpy bonding direction. Further, acetonemolecules interact with the cages by establishing C-H‚‚‚Ohydrogen bonds formed between the methyl groups of

Figure 1. (a, top) Coordination polymer unit observed in complex1a.(b, bottom) Interaction between the perpendicular polymer units throughC-H‚‚‚O hydrogen bonds. Dashed lines represent the hydrogen bonds. Colorcoding: green, Co(II); red, O; blue, N; gray, C; white, H.

Chart 1Figure 2. (a, top) Representation of the tetrameric polymer unit in thecrystal structure of1b, viewed along [010]. (b, bottom) Interaction betweenthe adjacent polymer blocks. Dashed lines represent C-H‚‚‚O hydrogenbonds.



acetone and the uncoordinated oxygen atom of a terminalcarboxylate. The H‚‚‚O distances are in the range 2.52-2.87 Å.

Complex [Co2(µ2-C8H5N2O6)2(µ2-C8H5N2O6)2(C10H8N2)4],2a. 2forms a 2:1:1 complex,2a, with cobalt nitrate and 4,4′-bipyridine upon crystallization from a methanol solution. Thestoichiometry was determined from the structure determi-nation by single-crystal X-ray diffraction methods. As wasobserved in1a and 1b, in the crystal structure of2a also,the bpy and carboxylate of2 interact with Co(II), formingCo-N and Co-O dative bonds. However, complex2a is adinuclear metal system with a novel coordination arrange-ment as shown in Figure 4a. The packing arrangement ofthe coordination units is shown in Figure 4b. In each unitthe two Co(II) metal centers are held together by twocarboxylates of2 through Co-O bonds in a bridging fashionwith distances of 2.041 and 2.068 Å (Table 2). The topologyof Co-O bonds around Co(II) is, in fact, similar to that ofthe bonds formed in complex1b, involving the middle Co-(II). Further, this arrangement is reminiscent of the hydrogen-bonded cyclic couplings that are formed between carboxylicacids (see Chart 1b).

Further, each Co(II) completes its 6-fold coordination byinteracting with two bpy molecules and one more carboxylateof 2. The Co-N bonds formed between Co(II) and bpy arelinear with distances of 2.156 and 2.157 Å (Table 2).However, the carboxylate interacts with Co(II) as a bidentateligand and forms chelated Co-O bonds with distances of2.217 and 2.218 Å (Table 2). A schematic representation ofthe chelated bonding pattern is shown in Chart 1c. Thetopological arrangement of organic ligands (2 and bpy)around Co(II) is very much similar to that of1a and 1b,with the carboxylates occupying equatorial positions and thebpy molecules lying at axial positions of the octahedron.Further, these coordination units form infinite polymer chainsthrough Co-N bonds formed between Co(II) and bpy. Thesepolymer chains in the three-dimensionsional arrangementconstitute a channel structure as shown in Figure 4c. Anoteworthy feature is that, in complex2a, the channels arethe result of interaction between the one-dimensional polymerchains connected together by C-H‚‚‚O hydrogen bonds(H‚‚‚O distance 2.44-2.95 Å, Figure 4b), unlike similarchannel structures known in the literature wherein thechannels are created exclusively due to the dative bondsformed between the metal and organic ligands.9,10,12However,

the occupants of the channels, most probably the solventmolecules (CH3OH), could not be determined by single-crystal X-ray diffraction methods, unequivocally, as complex2a is unstable. However, the calculations reveal that the voidspace in the channels is 170 Å3, suggesting the possiblepresence of methanol molecules. Further, another noteworthyand interesting feature of complex2a is that no solventmolecule is coordinated to Co(II), unlike in1a and1b. Thishas prompted us to carry out crystallization of2, bpy, andcobalt nitrate from different solvents with a hope thatpseudopolymorphs would result, replacing the solvent mol-ecules in the crystal lattice of2a. However, we were onlysuccessful in obtaining a complex from a DMSO solution.

Crystal Structure of [Co(C8H5N2O6)2(C10H8N2)2(H2O)2]-(CH3)2SO, 2b.The pale pink crystals of2b, obtained fromDMSO solution, in fact, are found to be quite stable, unlike2a. The crystal structure determination, however, reveals thepresence of DMSO molecules in the crystal lattice, in which2, bpy, and cobalt nitrate are in a 2:1:1 ratio. Following thesame trend as observed in1a, 1b, and2a, molecules of bpyinteract with Co(II) through Co-N bonds with a distanceof 2.174 Å and form infinite polymeric chains. We haveshown the arrangement of molecules around each metalcenter and their packing in the three-dimensional arrangementin Figure 5a.

It is evident that two carboxylates of2 interact with Co-(II) in a monodentate manner, with a distance of 2.080 Å.However, the 6-fold coordination around Co(II) is completedwith the coordination of two water molecules. So, as wasnoted in2a, no solvent of crystallization (DMSO) moleculescoordinated to Co(II). Thus, the octahedron geometry aroundeach Co(II) is achieved with bpy molecules at the apices,with acid molecules and water molecules occupying theequatorial positions.

An interesting feature is that, in contrast to the arrangementof carboxylates in1a, 1b, and2a, the two carboxylates in2b arrange in a cisoid manner in the basal plane. As aconsequence, a bilayered structure is obtained such thatcarboxylate molecules are embedded between the polymerchains constructed by Co(II) and bpy. This arrangement isshown in Figure 5b. These bilayers stack in three dimensionsseparated by solvent of crystallization (DMSO) molecules.Calculations of intermolecular interactions reveal that DMSOinteracts with the bilayers through O-H‚‚‚O hydrogen bondsformed between coordinated water molecules and DMSO.

Figure 3. Placement and interaction of acetone molecules between the coordination polymer units.



Thus, complex2b forms a host-guest-type assembly, whichis reminiscent of the well-known clay structures.

Similarities and Contrasts. It is evident from the abovediscussion of individual structures of complexes1a, 1b, 2a,

and2b that these are unique on their own, but a close lookat the structures in a collective manner indicates severalcommon features along with differences. The packingarrangement in the crystal structures is schematically rep-resented in Chart 2. First, all the complexes1a, 1b, 2a, and2b form infinite coordination polymer chains due to theinteraction between Co(II) and bpy. Further, in all thecomplexes, the major binding force between the polymerchains is the C-H‚‚‚O hydrogen bonds formed between the-NO2 groups and hydrogen atoms of bpy.

It is further evident from Chart 2 that in all the complexesthe adjacent polymer chains, parallel to each other, are heldtogether by C-H‚‚‚O hydrogen bonds, except in1a. In the

Figure 4. (a, top) Bimetal coordination polymer unit in2a. (b, middle)Packing of bimetal coordination polymers in the crystal structure of complex2a. (c, bottom) Representation of channels, along a crystallographicdirection, observed in2a.

Figure 5. (a, top) Three-dimensional arrangement of coordination polymersin the crystal structure of complex2b. (b, bottom) Representation of bilayerswith the coordinated acid molecules between the layers.

Chart 2. Representation of the Packing of Polymer Chains in theCrystal Structures of (a)1a, (b) 1b, (c) 2a, and (d)2b



crystal structure of1a, the C-H‚‚‚O hydrogen bonds areformed between the polymer chains perpendicular to eachother. Further, in all the complexes, octahedral geometry wasobtained with bpy molecules lying at the apices andcarboxylate molecules in the equatorial positions. Complexes1a, 1b, 2a, and2b, however, also differ from each other,especially in the way the polymer chains are arranged in thethree-dimensional space, which is of course influenced bythe carboxylate groups. Thus, while1a forms a square gridnetwork (Chart 2a), the other complexes,1b, 2a, and 2b,form host-guest-type structures with the incorporation ofsolvent molecules. Further, complexes1b and2b are closelyrelated to each other with the insertion of solvent moleculesbetween the polymer chains. In coordination complex2a,however, the solvent molecules occupy the channels createdalong a crystallographic axis.

In conclusion, we have reported four coordination poly-mers, 1a, 1b, 2a, and 2b, formed by 1 and 2 with4,4′-bipyridine and cobalt nitrate. All the complexes haveoctahedral arrangement at Co(II). 4,4′-Bipyridine serves asa bismonodentate ligand and bridges adjacent Co(II) metalcenters in one dimension to form infinite chains. Formationof different types of coordination linkages by carboxylategroups with the variation of solvent molecules leads to thecreation of different topologies such as square grids, chan-nels, and clay-type structures. Except1a, all other complexesform void structures, with the solvent molecules occupyingthe void positions. Also, the geometries of the voids aredifferent due to the differences in the arrangement of thepolymer chains. In addition, the observed topologies ofCo-O bonds have been correlated with the different typesof hydrogen bonds formed by carboxylic acids in organiccrystal structures.

Experimental Section

Synthesis of Complexes by Cocrystallization Methods.All thechemicals were obtained commercially, and the crystallizationexperiments were carried out at room temperature by dissolvingthe constituent reactants in spectroscopic-grade solvents, as the casemay be. The synthesis of each complex is described below.

[Co(C7H3N2O6)2(C10H8N2)2(CH3O)2], 1a.A solution of1 (0.106g, 0.5 mmol) in methanol (10 mL) was slowly added to a warmsolution of Co(NO3)2‚6H2O (0.146 g, 0.5 mmol) in methanol (10mL) with constant stirring over a period of 5 min. To this mixturewas added dropwise 4,4′-bipyridine (0.078 g, 0.5 mmol) in methanol(5 mL). The reaction mixture was warmed for a while and allowedto evaporate slowly under ambient conditions. Pale pink needlessuitable for X-ray analysis were obtained within 2 days.

[Co4(C7H3N2O6)8(C10H8N2)8(CH3O)2]CH3COCH3, 1b. To awarm solution of Co(NO3)2‚6H2O (0.146 g, 0.5 mmol) in methanol(10 mL) was added slowly with constant stirring the methanolicsolution (5 mL) of1 (0.106 g, 0.5 mmol). To the mixture was addeddropwise over a period of 5 min 4,4′-bipyridine (0.078 g, 0.5 mmol)in methanol (5 mL). Acetone (5 mL) was allowed to diffuse slowlythrough the reaction mixture. Pale pink needles of X-ray qualitywere obtained over a period of 3 days.

[Co2(µ2-C8H5N2O6)2(µ2-C8H5N2O6)2(C10H8N2)4], 2a.Co(NO3)2‚6H2O (0.146 g, 0.5 mmol) in methanol (15 mL) was added dropwiseto a solution of acid2 (0.113 g, 0.5 mmol) in methanol (5 mL). Tothis stirred solution was added slowly over a period of 5 min 4,4′-bipyridine (0.078 g, 0.5 mmol) in a 1:1 mixture of methanol andacetone (10 mL). Pale pink needles were obtained over a period of2 days and were found to be suitable for X-ray analysis.

Crystal Structure of [Co(C8H5N2O6)2(C10H8N2)2(H2O)2](CH3)2-SO, 2b.A solution of2 (0.113 g, 0.5 mmol) in methanol (10 mL)was added dropwise with constant stirring to a warm solution ofCo(NO3)2‚6H2O (0.146 g, 0.5 mmol) in methanol (5 mL). To thisreaction mixture was added over a period of 5 min 4,4′-bipyridine(0.078 g, 0.5 mmol) in methanol (10 mL). This reaction mixturewas warmed for a while, and DMSO (2 mL) was allowed to diffuseslowly to yield pale pink X-ray-quality crystals.

X-ray Crystallography. Good-quality single crystals of1a, 1b,2a, and2b were carefully chosen after they were viewed througha Leica microscope supported by a rotatable polarizing stage anda CCD camera. The crystals were glued to a thin glass fiber usingan adhesive (cyano acrylate) and mounted on a diffractometerequipped with an APEX CCD area detector. The X-ray intensitydata were collected into 2424 frames with varying exposure time(10 s,1a; 5 s,1b and2a; 10 s,2b) depending upon the quality andstability of the crystal(s). The data collection was smooth in all thecases, and no extraordinary methods were employed, except thatthe crystals were smeared in cyano acrylate to protect them fromambient laboratory conditions. The intensity data were processedusing Bruker’s suite of data processing programs24 (SAINT), andabsorption corrections were applied using SADABS. The structuresolution of all the complexes was carried out by direct methods,and refinements were performed by full-matrix least-squares onF2 using the SHELXTL-PLUS suite of programs. All the structuresconverged to goodR factors. All the non-hydrogen atoms wererefined anisotropically, and the hydrogen atoms obtained fromFourier maps were refined isotropically. All the refinements weresmooth in all the structures. Intermolecular interactions werecomputed using the PLATON program.25

Acknowledgment. We thank the Department of Scienceand Technology, New Delhi, for financial assistance. Also,we thank Dr. S. Sivaram, Director, National ChemicalLaboratory (NCL), and Dr. K. N. Ganesh, Head of theDivision, NCL, for their encouragement. S.V. thanks theCSIR for the award of a Junior Research Fellowship (JRF).

Supporting Information Available: X-ray data with detailsof the refinement procedures (CIF files), ORTEP plots, lists of bondparameters (bond lengths and angles), and structure factors ofmolecular complexes1a, 1b, 2a, and2b. This material is availablefree of charge via the Internet at http://pubs.acs.org.

IC0349499

(24) (a) Siemens.SMART System; Siemens Analytical X-ray InstrumentsInc.: Madison, WI, 1995. (b) Sheldrick, G. M.SADABS Siemens AreaDetector Absorption Correction Program, University of Gottingen:Gottingen, Germany, 1994. (c) Sheldrick, G. M.SHELXTL-PLUSProgram for Crystal Structure Solution and Refinement; Universityof Gottingen: Gottingen, Germany.

(25) Spek, A. L.PLATON, Molecular Geometry Program; University ofUtrecht: Utrecht, The Netherlands, 1995.



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