PAPER www.rsc.org/dalton | Dalton Transactions

A new series of dinuclear Au(I) complexes linked by diethynylpyridinegroups†‡

Peiyi Li,a Birte Ahrens,b Andrew D. Bond,a John E. Davies,a Olivia F. Koentjoro,b Paul R. Raithby*b andSimon J. Teatc

Received 30th October 2007, Accepted 18th December 2007First published as an Advance Article on the web 24th January 2008DOI: 10.1039/b716664h

A series of novel digold complexes incorporating ethynyl pyridine derivatives as a spacer unit,[(R3P)Au(C≡C)X(C≡C)Au(PR3)] (R = Ph, X = 2,5-pyridine (1); R = Cy (cyclohexane), X =2,5-pyridine (2); R = Ph, X = 2,6-pyridine (3); R = Ph, X = 2,5′-bipyridine (4); R = Ph, X =2,6′-bipyridine (5)), has been synthesised. All the complexes have been characterised spectroscopicallyand the structures determined by single-crystal X-ray crystallography. The central(C≡C)(X)(C≡C) unit is essentially linear for complexes 1, 2 and 4 and kinked for complexes 3 and 5,but only in 1, with the shortest spacer group and the less bulky phosphine ligand, is there evidence ofd10 · · · d10 Au · · · Au interactions (Au–Au 3.351(2) A). The solution UV/visible absorption and emissionspectra for all the complexes are similar to those of the free ligands suggesting that the spectra aredominated by p–p* ligand-centred transitions and this is confirmed by DFT calculations.

Introduction

The area of transition metal acetylide chemistry has seen increasedinterest because of the potential application of metal acetylidecomplexes, oligomers and polymers in the area of opto-electronicswhere these species1 could act as “molecular wires” in devicessuch as light emitting diodes, lasers, photocells, and field effecttransistors.2–8 Metal containing poly-ynes are also potential can-didates as low-dimensional conductors and non-linear opticalmaterials.9–11

Gold(I) acetylide complexes are of particular interest becauseof their luminescent properties,12 which are a consequence ofaurophilic d10 · · · d10 Au · · · Au interactions. Aurophilic d10 · · · d10

Au · · · Au interactions may contribute bonding interactions to thestructure of ca. 30 kJ mol−1, a value comparable to hydrogenbonds, when the Au · · · Au distance is in the range of 2.5–3.9 A.13,14 In addition, the linear arrangement of the acetylide unitand its proven ability to participate in extended, delocalisedp-bonding interactions has stimulated interest in the use ofgold acetylide in electronic applications.15 In this context, Wadeet al. have examined diethynyl-para-carboranes16 and carboranegold derivatives17 with a view to exploiting their physical andelectronic properties. More generally, gold acetylides have beenshown to exhibit a range of interesting properties including opticalnonlinearity, liquid crystallinity and electrical conductivity.18

aDepartment of Chemistry, University of Cambridge, Lensfield Road,Cambridge, UK CB2 1EWbDepartment of Chemistry, University of Bath, Claverton Down, Bath, UKBA2 7AY. E-mail: p.r.raithby@bath.ac.ukcSTFC Daresbury Laboratory, Daresbury, Warrington, UK WA4 4AD† Dedicated to Professor Ken Wade on the occasion of his 75th birthdayin recognition of his outstanding contribution to chemistry.‡ CCDC reference numbers 665741–665745. For crystallographic data inCIF or other electronic format see DOI: 10.1039/b716664h

Extensive studies have been carried out on linear gold(I)acetylides and isocyanide systems13 and on related polynuclear,phosphine-bridged complexes.15 The emission originating fromthese compounds is linked to the presence or absence of gold–goldinteractions in the solid state structures of these materials, and ishighly dependent on the acetylide chain length between adjacentgold centres, the bonding properties of the linking groups, and thenature of the phosphine ligands.19–21

The large role the spacer group plays on the luminescenceproperties of gold(I) acetylide complexes is highlighted in severalstudies involving a variety of aromatic rings such as 1,3-diethynyl-benzene, 1,3-diethynylmesitylene, 1,4-diethynylbenzene, 1,4-di-ethynylmesitylene, 1,4-diethynyldurene, 9,10-dithynylanthracene,diethynylthieno[3,2-b]thiophene, diethynylthieno[3,2-b:2′,3′-d]-thiophene and pyridine,19–26 and a number of metal-containingspecies.27 The emission in these complexes is attributed to a p–p*(acetylide):[r(Au–P)→p*(acetylide)] state. In view of the shortintermolecular Au · · · Au separation observed, a metal-centred3[(dd*)1(pr)1] excited state has been suggested to account for thered shift in emission energy observed in the solid state.23

Compared to benzene or thiophene, pyridine is electron defi-cient; and consequently the metal acetylide complexes should haveincreased electron affinity and improved electron-transportingproperties.28 This is borne out in a series of comparativestudies of platinum(II) poly-yne complexes and polymers thathave arene groups and heterocyclic spacer groups linking themetal acetylide units.29 In the platinum poly-yne complexes withbis(ethynyl)oligopyridine as the spacer group in the backbone, themetal complexes exhibit a decrease in thermal stability with anincreased number of pyridine units in the spacer group.30

Here we describe a detailed study on a series of digold diynecomplexes with pyridine or bipyridine spacer groups where notonly the chain length but also the geometry of the chain and thebulk of the substituent ligands has been altered systematically to

This journal is © The Royal Society of Chemistry 2008 Dalton Trans., 2008, 1635–1646 | 1635

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online / Journal Homepage / Table of Contents for this issue

Scheme 1

probe the effect on the formation of the intermolecular aurophilicAu · · · Au interactions.

Results and discussion

Synthesis of dinuclear gold(I) r-acetylides

The series of five gold(I) diyne complexes complexes[(R3P)Au(C≡C)X(C≡C)Au(PR3)] (R = Ph, X = 2,5-pyridine (1);R = Cy, X = 2,5-pyridine (2); R = Ph, X = 2,6-pyridine (3);R = Ph, X = 2,5′-bipyridine (4); R = Ph, X = 2,6′-bipyridine(5)) were prepared by the reaction of gold(I) phosphine chloridewith the appropriate terminal alkyne in the presence of methanolicNaOMe,31 which acts as both a base and as a chloride abstractor, toyield yellow to orange-yellow crystalline solids (Scheme 1). Sincethe terminal alkynes were not particularly stable in the presenceof light they were prepared immediately prior to the reactionvia treatment of the bis-trimethylsilyl derivatives with KOH indichloromethane.

The IR spectrum of all the dinuclear gold(I) r-acetylides com-pounds 1–5 show a fairly strong characteristic m(C≡C) absorptionband in the region of 2110–2120 cm−1 (see the Experimentalsection). The 1H NMR spectra for the five complexes showresonances in the aromatic region representative of the presenceof the appropriate pyridine or bipyridine spacer unit. The 13CNMR spectra of compounds 1–5 clearly show acetylene carbonresonances in the region of 100–104 ppm, as well as resonancesin the aromatic region. All the symmetric dinuclear gold(I) r-acetylides compounds, 2–5, display a singlet in their 31P{1H}NMRspectra, at d 57.22 for the cyclohexyl phosphine in 2 and in therange d 42.72–42.78 for the triphenylphosphine groups in 3–5. For

the asymmetric acetylide complex 1 the 31P{1H} NMR spectrumdisplays two signals at d 42.69 and 42.80. The position of thesesignals is similar to that in a range of related gold phosphineacetylenic complexes.20,21 In the mass spectra of the complexesa molecular ion peak [M]+ or [M + H]+ is present in all cases,however, the most intense peak in all the spectra is at m/z 459 (477),that can be attributed to the [AuPPh3]+ ([AuPCy3]+) fragment.Peaks related to [Au(PPh3)2]+ ([Au(PCy3)2]+) and [M + Au]+ arealso seen. It appears that the fragmentation favours the generationof mono-positive ions.

The 1H and 13C NMR spectra of gold(I) r-acetylides were verysimilar to that obtained for the trimethylsilyl-protected ligandsand the related platinum-containing complexes23 which indicatesthat there was little change to the ligand upon the coordinationto the gold moiety. This indicates that the interaction between theAu(I) centre and the acetylide ligand is not significant enough to bedissimilar from the trimethylsilyl group. It seems the large gold unitseparates the phosphine ligand and the acetylide ligand into twoalmost independent units. This is confirmed by the electronicstructure calculations performed on these molecules (vide infra).

Solid state molecular structure of dinuclear gold(I) r-acetylides

The structures of compounds 1–5 have been determined by singlecrystal X-ray crystallography. Selected bond lengths and bondangles of five complexes are listed in Table 1 and their molecularstructures are presented in Fig. 1–5. In agreement with thespectroscopy all the structures confirm that the complexes aredinuclear with the gold phosphine units linked by the diethynylspacer groups. To a first approximation the coordination geometryat the gold centres in all five complexes is linear and two-coordinate

1636 | Dalton Trans., 2008, 1635–1646 This journal is © The Royal Society of Chemistry 2008

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online

Fig. 1 The solid state structure of [(Ph3P)Au(C≡C)(2,5-pyridine)(C≡C)Au(PPh3)] 1 showing the Au · · · Au interactions between adjacent molecules.The pyridine nitrogen atom is disordered over two symmetry related sites. The atoms denoted “A” are related to those in the asymmetric unit by thesymmetry operation −1 − x, 2 − y, −z.

Table 1 Selected bond lengths (A) and bond angles (◦) of 1–5

Au–C(≡C) Au–P C≡C (C≡)C–Au–P C≡C–Au

1 2.11(5) 2.289(11) 1.10(5) 168.0(14) 162(5)2 1.998(7) 2.2823(19) 1.212(11) 175.7(2) 173.2(8)3 1.984(15) 2.279(4) 1.21(2) 168.6(5) 167.4(14)4 2.011(4) 2.2769(11) 1.197(6) 174.84(12) 169.2(4)5 2.013(5) 2.2738(12) 1.157(6) 177.68(12) 178.6(4)

Fig. 2 The molecular structure of [(Cy3P)Au(C≡C)(2,5-pyri-dine)(C≡C)Au(PCy3)] 2 showing the atom numbering scheme. Theatom denoted “A” is related to those in the asymmetric unit by thesymmetry operation 1 − x, 1 − y, 1 − z. The disordered dichloromethanemolecule has been omitted for clarity and the pyridine nitrogen isdisordered over the four equivalent positions on the pyridine ring andrefined with partial occupancy as are the related carbon atoms.

as would be expected for Au(I) systems. In the structures of 2, 4and 5 the C–Au–P and C≡C–Au angles do not show substantialdeviations from linearity. However, in the structures of 1 and 3there are larger deviations from linearity that can be attributedto intermolecular interactions. All Au–C(≡C) bond lengths areunexceptional when compared to those of published phosphine

gold(I) r-acetylide complexes,19,21,24–39 spanning the narrow rangeof 1.984(15)–2.013(5) A. Similarly, the Au–P bond distances arecomparable to those found in known phosphine gold(I) r-acetylidecomplexes (2.2736–2.290 A)23,25,31–56 but they are longer than thoseof chlorogold(I) phosphines,57,58 consistent with the stronger trans-influence of the alkynyl group than the chloride. The C≡C bondlengths are typical of di-substituted carbon–carbon triple bondsand fall in the reported range of 1.157–1.206 A.20,21,23,25,31–56 It isthus evident that significant changes in the spacer group have littleeffect on the intramolecular bond lengths of Au–C(≡C), Au–P andC≡C units but, on the other hand, these changes do govern thecrystal packing and the intermolecular interactions as discussedin the following paragraphs.

Aurophilicity is the result of relativistic effects on Au(I) causedby non-covalent attractions between these d10 metal ions.59–61

An Au · · · Au distance below 3.3 A (smaller than 3.6 A, thesum of Au(I) van der Waals radii62) is thought to provide anAu · · · Au interaction of similar strength to hydrogen bonds.Aurophilicity is responsible for various interesting molecularand supramolecular architectures. In the solid state structureof 1 short intermolecular Au · · · Au interactions are observedwith a separation of 3.351(2) A, and each [(Ph3P)Au(C≡C)(2,5-pyridine)(C≡C)Au(PPh3)] molecular unit sits on a crystallo-graphic centre of symmetry located at the centre of the pyridinering; the pyridine nitrogen atom was treated in the refinementas being disordered over two symmetry related positions as wasthe related carbon atom. The Au(I) centre is linked to anotherAu(I) centre from a neighbouring molecule via an Au · · · Auinteraction (Fig. 1) so that loose chain-like polymers are formed.The two Au · · · Au interactive {PPh3Au–C≡C–} vectors are anti-parallel with torsion angles (C≡C–Au · · · Au–C≡C) of 178.7◦.The formation of these chain-like polymers generated by theAu(I) · · · Au(I) interactions is a common feature of the chemistry ofmetal acetylides21 and isocyanides13 and is particularly prevalentwhen the spacer groups are relatively short and the auxiliaryphosphine ligands are not bulky.

This journal is © The Royal Society of Chemistry 2008 Dalton Trans., 2008, 1635–1646 | 1637

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online

Fig. 3 The molecular structure of [(Ph3P)Au(C≡C)(2,6-pyridine)(C≡C)Au(PPh3)] 3 showing the atom numbering scheme. The atoms denoted “A” arerelated to those in the asymmetric unit by the symmetry operation −1 − x, 2 − y, −z.

Fig. 4 The molecular structure of [(Ph3P)Au(C≡C)(2,5′-bipyridine)(C≡C)Au(PPhR3)] 4 showing the atom numbering scheme. The atoms denoted “A”are related to those in the asymmetric unit by the symmetry operation 1 − x, 2 − y, 2 − z.

Fig. 5 The molecular structure of [(Ph3P)Au(C≡C)(2,6′-bipyridine)(C≡C)Au(PPh3)] 5 showing the atom numbering scheme. The atoms denoted “A”are related to those in the asymmetric unit by the symmetry operation −x, −y, 1 − z. Only one orientation of the disordered phenyl group is shown forclarity and the disordered dichloromethane molecule is also omitted.

1638 | Dalton Trans., 2008, 1635–1646 This journal is © The Royal Society of Chemistry 2008

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online

Fig. 6 The crystal structure of [(Ph3P)Au(C≡C)(2,6-pyridine)(C≡C)Au(PPh3)] 3 showing the nature of the intermolecular interactions.

In [(Ph3P)Au(C≡C)(2,5-pyridine)(C≡C)Au(PPh3)] 2 the spacergroup is the same as in 1 but the triphenylphosphine groups,coordinated to the two gold centres, have been replaced bytricyclohexylphosphine groups. This change has a marked effecton the intermolecular interactions exhibited in the structure of2, although the situation is complicated in the solid state be-cause the molecule co-cystallises with a disordered molecule ofdichloromethane. While the molecule crystallises in the samespace group as 1, P1 (No. 2), and the centre of the pyridinering is again located on a crystallographic centre of symmetry(in this instance the pyridine nitrogen atom was refined as beingdisordered over the four possible sites on the ring with occupanciesfor nitrogen and carbon on each site summing to unity; oneach site the carbon and nitrogen were constrained to have thesame atomic coordinates), the packing arrangement is differentand there are no Au · · · Au interactions; the shortest intermolec-ular Au · · · Au contact being 5.545(1) A. A similar change inAu · · · Au distances has been observed previously in the relatedpair of molecules [(MeO)3PAu(C≡C)(C6H4)(C≡C)AuP(OMe)3]54

where the intermolecular Au · · · Au separations are 3.1733(2)and 3.5995(3) A, indicative of Au · · · Au interactions, and[Cy3PAu(C≡C)(C6H4)(C≡C)AuPCy3],56 where the Au · · · Au sep-aration of 5.648(1) A is significantly too long to be consideredan interaction. While this difference may be attributed to thedifference in steric bulk between the phosphorus donor ligandsin the two examples electronic factors cannot be excluded. TheTolman cone angles for the phosphines and phosphate are 170◦

(Cy), 145◦ (Ph) and 107◦ (OMe), respectively.63 In the case of2 compared to 1 the situation is further complicated as 2 co-crystallises with a molecule of dichloromethane solvent whichexhibits short contacts with the central pyridine ring (the carbon ofthe dichloromethane molecule has contacts of 3.108 and 3.446 Awith the N(4) and C(5) or their symmetry equivalents).

In complex 3 the 2,5-pyridine ligand present in 1 and 2 isreplaced by a 2,6-pyridine ligand which, of necessity, intro-duces non-linearity into the molecular backbone. The structureof [(Ph3P)Au(C≡C)(2,6-pyridine)(C≡C)Au(PPh3)] 3 is shown inFig. 3. In the crystal the molecule sits on a crystallographic 2-foldrotation axis that passes through N(1) and C(5) and the C(3)–(pyridine ring centroid)–C(3A) angle is 116.02◦. By comparison

to 1 which has the same auxiliary phosphine group as 3 theintermolecular interactions are significantly different. The shortestintermolecular Au · · · Au contact in 3 is at 3.677(1) A which isca. 0.32 A longer than in 1 and slightly longer than the sumof the van der Waals radii. In addition, there is a short contactbetween Au(1) and the a-carbon of the acetylene group C(1B),also related by the symmetry operation −x, −y, −z, at 3.55(2) A,which may be indicative of a favourable intermolecular interaction(Fig. 6). As with 1 the Ph3PAu–C≡C units are anti-parallel witha P–Au · · · Au–P torsion angle of −180◦. The intermolecularinteractions in the crystal are completed by a p–p stackinginteraction between the central pyridine ring on one molecule andone of the phenyl rings [C(21)–C(26)] on an adjacent molecule(again related by the symmetry operation −x, −y, −z) where theangle between the rings is 15.2◦ and the centroid–centroid distanceis 3.89 A.

The structure of 3 may also be contrasted with thatof [(Tol3P)Au(C≡C)(3,5-pyridine)(C≡C)Au(PTol3)]26 (Tol =C6H4CH3) in which there are intermolecular aurophilic Au · · · Aucontacts of 3.2265(5) A and the molecular chains are arrangedso that the Tol3PAu–C≡C units of two vicinal molecules areeffectively perpendicular (P–Au · · · Au–P 89◦). The C(acetylenica-carbon)–(pyridine centroid)–C(acetylenic a-carbon) angle is121.55◦. Thus, in digold complexes with relatively short di-ethynylpyridine spacer groups there is a propensity to form au-rophilic Au · · · Au contacts to produce molecular chains, althoughthe shape of the spacer group and the packing arrangement maydiffer, provided that the auxiliary phosphine groups are not toobulky to prevent a close approach between the gold centres.

In the structure of [(Ph3P)Au(C≡C)(2,5′-bipyridine)(C≡C)-Au(PPh3)] 4 the spacer group has been extended to include twoaromatic rings while the linear nature of the molecular backbonehas been retained. The molecular structure of 4 is displayed inFig. 4. The molecule sits on a crystallographic centre of symmetrylocated at the mid-point of the C(6)–C(6A) bond [where C(6A) isrelated to C(6) by the symmetry operation 1 − x, 2 − y, 2 − z]and, as a consequence, the two rings of the bipyridyl unit areprecisely co-planar. The bipyridine group adopts the expectedtrans configuration to minimise the steric congestion betweenaromatic protons. There are no short aurophilic Au · · · Au contacts

This journal is © The Royal Society of Chemistry 2008 Dalton Trans., 2008, 1635–1646 | 1639

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online

in the crystal structure with the shortest intermolecular Au · · · Audistance being 7.069(1) A. The absence of aurophilic interactionsin digold ethynyl complexes with longer spacer groups is a commonfeature of the structural chemistry of these systems even whenmoderately sized auxiliary phosphines are present.21 In 4 theshortest intermolecular contact to the gold centre is from a phenylhydrogen atom on an adjacent molecule (Au1 · · · H11B 3.30 A,H11B being related by the symmetry operation 3 − x, 0.5 + y,1.5 − z).

In the structure of [(Ph3P)Au(C≡C)(2,6′-bipyridine)(C≡C)-Au(PPh3)] 5 a kink has been introduced into the longer spacergroup in order to assess whether this makes a difference to thenature of the intermolecular interactions and mirrors the compar-ison between the structures of 1 and 3 discussed above. The discretemolecules of 5, illustrated in Fig. 5, sit on crystallographic centresof symmetry located at the mid-point of the C(2)–C(2A) bonds[where C(2A) is related to C(2) by the symmetry operation −x,−y, 1 − z] and, as in the case of 4, the bipyridine group is preciselyplanar by crystal symmetry. In 5 the shortest intermolecularAu · · · Au contact is 4.785(1) A, which is considerably shorter thanthe distance found in 4, but still much too long to be consideredas an aurophilic interaction. Thus, again, despite the differencein the geometry the presence of the longer spacer group does notfavour formation of aurophilic interactions between molecules inthe solid state. The situation in the crystal structure of 5 is slightlycomplicated by the presence of a disordered dichloromethanemolecule in the lattice but this does not form any short contactswith the central backbone of the molecule.

From the structural data obtained from the structures of 1–5 it is apparent that the formation of intermolecular aurophilicinteractions in the solid state is not favoured by the presenceof longer spacer groups in complexes with the general formula[(R3P)Au(C≡C)X(C≡C)Au(PR3)] regardless of whether the back-bone geometry of the molecules is linear or kinked. However,other factors such as the bulk of the auxiliary phosphine ligandsand the electronic properties of the central spacer group and thephosphines must also play a part in the final packing arrangementof the molecules.

Electronic structure of dinuclear gold(I) r-acetylides

Then, in order to obtain more information regarding the elec-tronic properties of these [(R3P)Au(C≡C)X(C≡C)Au(PR3)] (X =pyridine or bipyridine) complexes, at least within the individualmolecular units, density functional theory (DFT) calculationswere undertaken. Geometry optimisation was performed on [H3P–Au–C≡C–X–C≡C–Au–PH3] (X = 2,5-pyridine (6); X = 2,5′-bipyridine (7); X = 2,6′-bipyridine (8)) using the solid statemolecular structure of 1, 4, and 5 as a starting point. The processfollows that described in the Experimental section and selectedbond lengths and angles for 6–8 are summarised in Table 2, whichcan be compared to the data relating to the crystallographicallydetermined structures (Table 1). While quantitative agreementbetween the observed and calculated data is not expected due thenature of the calculations and structural approximations involved,the optimised geometry of 6–8 is in good general agreement withthe experimentally observed structural trends which allows for ahigh degree of confidence in the accuracy of the computations,and the conclusions drawn from them.

Table 2 Selected bond lengths (A) and bond angles (◦) of 6–8

Au–C(≡C) Au–P C≡C (C≡)C–Au–P C≡C–Au

6 1.988 2.351 1.224 179.6 178.57 1.989 2.351 1.225 179.9 179.98 1.988 2.351 1.224 179.8 178.9

The geometry optimised structures of 6–8 presented the samegross structural features as the experimentally determined struc-tures, having rod-like molecular backbones with each Au(I) centreassuming a linear two-coordinated geometry. The Au–C bondis 1.988–1.989 A in length (c.a. 1.984(15)–2.11(5) A for thecrystallographically determined structures). The Au–P bond is2.351 A, while the C≡C is 1.224–1.225 A, both bonds beingslightly longer that those found in experimentally determinedstructures of phosphine gold(I) r-acetylides. In the geometryoptimised structure of each of 6–8 there is less than 1◦ deviationfrom linearity in the two C–Au–P and C≡C–Au bond angles,which is one of the primary differences in structure compared tothe solid state structures of 1–5. This difference is not surprisingsince the larger deviation from linearity in 1–5 is attributed tocrystal packing forces and intermolecular interactions, especiallythe presence of Au · · · Au interactions which is not taken in accountin the calculations on 6–8.

An analysis of the orbital structure of 6–8 reveals the HOMO(highest occupied molecular orbital) is calculated to be essentiallydelocalised over the central portion of the molecular frameworkcomprising the acetylene units and spacer group with negligibleinteraction with the Au(I) centres. For each system the HOMO isbonding in character between the carbon atoms of the acetyleniccarbon atoms and anti-bonding with respect to the C(sp)–C(sp2) ethynyl–aromatic bonds, while the converse is true of theLUMO. These interactions are illustrated for 6 in Fig. 7.

Fig. 7 (a) HOMO and (b) LUMO orbitals of 6.

Electronic and emission spectroscopy of dinuclear gold(I)r-acetylides

The absorption and emission spectra of the dinuclear gold(I) r-acetylides 1–5 were measured at room temperature, from 10−5 Msamples in CH2Cl2, and the results are summarised in Table 3together with equivalent data for the desilylated terminal di-ynes.

In general, the profiles of the absorption spectra of the dinucleargold(I) r-acetylides are similar to that of the equivalent acetylideligands with the kmax position shifting to longer wavelengths. Thestrong dependence of the absorption spectra profile of the gold

1640 | Dalton Trans., 2008, 1635–1646 This journal is © The Royal Society of Chemistry 2008

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online

Table 3 Absorption and emission spectra of dinuclear gold(I) r-acetylides 1–5

Absorption Emissiona

k/nm (e/104 dm3 mol−1 cm−1) k/nm (relative intensityb)

2,5-Diethynyl pyridine 261(∼2.4), 281(∼1.1), 289(∼1.2), 298(∼1.0) —1 268(1.6), 276(1.9), 299(3.2), 313(4.5), 332(8.2) 344(1), 382(sh, 0.35), 493(0.09), 528(0.06)2 236(2.4), 299(3.0), 313(3.6), 331(6.4) 349(1), 383(sh, 0.38), 494(0.17), 528(0.11)2,6-Diethynyl pyridine 283(sh, ∼1.4), 291(∼1.6), 300(∼1.5) —3 261(4.8), 285(sh, 2.1), 307(sh, 2.1), 317(2.6), 324(2.5) 339(1), 355(sh, 0.82), 436(0.07), 464(0.05)2,5′-Diethynylbipyridine

268(∼2.3), 301(sh, ∼3.2), 313(∼4.2), 327(∼3.3) —

4 341(7.9), 359(7.4) 368(0.75), 388(1)2,6′-Diethynylbipyridine

247(∼2.2), 297(∼1.8), 309(∼1.6) —

5 280(4.9), 312(3.0), 324(sh, 2.5) 342(1), 448(0.05)

a Excited at kmax. b Intensity of the strongest peak is assigned as 1.

Table 4 Absorption and emission data of 2

k/nm (CH2Cl2) k/nm (toluene)

Abs. 299, 312, 331 300, 313, 333Em.a 349, 383(sh), 494(w), 528(w) 348, 385(sh), 494(vw)

a Excited at kmax.

compounds on their respective ligands hints at the ligand-centred(LC) nature of these transitions. Although the insensitivity of lowenergy absorption bands towards the auxiliary ligand of Au(I) (1vs. 2) precludes the Au(I)-centred origin of the transitions, the red-shift on binding to an Au centre is consistent with the extensionof p-conjugation onto the Au(I) centres through the acetylenicbridges. Therefore, transitions in these absorption spectra areligand-dominated p–p*, but may mix with a little r(Au–C) inthe HOMO and possibly some Au 6pp character in the LUMO.23

Although the possibility of a MLCT origin for these absorptionbands cannot be totally ruled out, it is probably of minorimportance, since the representative examples show no significantchange in their absorption and emission behaviour in the less polartoluene with respect to the more polar dichloromethane (Table 4).This is confirmed by the electronic structure of 6, which gives rise toa HOMO that originates solely from ligand orbital contributions(Fig. 7).

There is little electronic interaction between the two Au moietiesin dinuclear gold(I) compounds through a two-ring bridge (4 and5). However, such electronic interaction is detectable between thetwo platinum centres in the related two-ring di-yne complexes,[(tBu3P)2Pt(C≡C)(bipyridine)(C≡C)Pt(PtBu3)2], because of thebetter orbital matching between the platinum centres and theacetylide ligand than in gold(I) r-acetylides.23 It is likely thatthe electronic communication between the two gold centres inthree-ring di-yne complexes is even more remote, since this kindof electronic communication decreases drastically as the distancebetween two metal centres increases.64

There is a sequential increase in the absorption maxima as thenumber of bridging aromatic rings increases (1, 2 and 4). However,for cases where the spacer unit is a 2,6′-pryridine group (3 and 5)there was no obvious red shift observed on proceeding from 2,6-pyridine to 6,6′-disubstituted 2,2′-bipyridine (Table 3). Here, the6,6′-linkage obviously disrupts the direct p-delocalisation to the

acetylenic unit, which emphasises the importance of the lineargeometry of a spacer for effective conjugation. This is supportedby the electronic structure calculation for the HOMO of 8 asdepicted in Fig. 8.

Fig. 8 (a) HOMO and (b) LUMO orbitals of 8.

The emission behaviour of the dinuclear gold(I) complexes aresimilar to their absorptions in terms of their emission maximaas a function of the bridging heterocyclic rings. Each individualexcitation spectrum has a similar profile to its absorption,suggesting the emission arises from the lowest-energy absorptionband. The variation in the excitation wavelength has little influenceon the emission spectrum, indicating a single emissive state ormultiple states that are in equilibrium. There is always someoverlap between the low-energy onset of an absorption spectrumand the high-energy end of the corresponding emission spectrum.The Stokes shifts of these dinuclear complexes are small to modest.Such a closeness of the emission spectrum to the absorption spec-trum suggests the origin of the lowest emission state is a singlet.Several dinuclear gold(I) complexes are found to have emissivestates with a lifetime of the order of nanoseconds,12,14 whichsomewhat strengthens the singlet origin argument. The emissionof gold(I) acetylides is strongly dependent on the acetylide ligands,yet nearly independent of the ancillary ligands around Au(I) (1 vs.2). Based on these observations, the lowest emission state in thesegold complexes can be tentatively assigned as ligand-dominated1(p–p*).

This journal is © The Royal Society of Chemistry 2008 Dalton Trans., 2008, 1635–1646 | 1641

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online

Table 5 Absorption and emission data of 4 and 5 in CH3OH–CH2Cl2 (1 : 1) and in the presence of Zn(BF4)2 and CF3COOH at room temperature

4 k/nm 5 k/nm

ccomplex (∼10−5 M) in CH3OH–CH2Cl2 (1 : 1)a Abs. 341(7.4), 357(6.7) 276(5.2), 313(br, 3.2)Em. 370(0.69), 390(1) 350(1)

Zn(BF4)2 (∼10−3 M) Abs. 303(3.9), 311(4.0), 361(8.3), 378(8.7) 269(5.6), 358(2.4), 373(2.7)Em.a 398(1), 413(0.96) 387(1), 403(sh, 0.88)

CF3COOH (∼10−3 M) Abs. 317(2.9), 362(4.2), 378(4.4) 268(4.3), 360(2.1), 375(sh, 1.9)Em.a 439(br) 422(br)

a Excited at kmax.

Reactivity studies on [(R3P)Au(C≡C)X(C≡C)Au(PR3)] (R = Ph,X = 2,5′-bipyridine (4); R = Ph, X = 2,6′-bipyridine (5))

Complexes 4 and 5 each have a bipyridine spacer group that isnot coordinated to a metal and is thus available for binding tometal ions or alternatively protons through the nitrogen donoratoms. Upon exposure of 4 and 5 to Zn(BF4)2 the maxima of theabsorption spectra red-shifts towards longer wavelengths (Table 5)and exposure to CF3COOH induces very similar changes (Fig. 9).Given that excess CF3COOH is used, it is likely that both nitrogenatoms of the bipyridine group are protonated. The bipyridine canadopt a cisoid conformation when it chelates to Zn2+ via the twoimine nitrogen atoms. However, this is unlikely to be the adoptedconformation for a doubly protonated bipyridine because of stericfactors. The doubly protonated bipyridine is more likely to adopt atransoid conformation. The apparent resemblance in the absorp-tion profiles in the presence of Zn(BF4)2 and CF3COOH stronglysuggests the predominant role of electrostatic perturbation, ratherthan the forced coplanarity by zinc(II) chelation, since protonatedbipyridine does not necessarily enforce coplanarity between thetwo pyridine rings. Even though there is overall similarity betweenthe Zn2+-coordinated species and the protonated one, the spectrumassociated with the protonated species is broader and the vibronicstructure in the bands is less distinct, indicating the presence of asmall amount of charge transfer.

Fig. 9 Absorption spectrum of complex 4 in CH2Cl2–CH3OH (I) and inthe presence of CF3COOH (II).

The ortho-linkage of gold(I) ethynyl groups with respect to thenitrogen atoms in 5, where a larger induction effect on bindingto Zn2+/H+ is expected, gives rise to a larger bathochromic shiftthan its 5,5′-analogue 4. The presence of gold(I) ethynyl groupsattached in a meta position experience less of an inductioneffect on binding to Zn2+/H+, such that the Zn2+-coordinatedor protonated complex 5 exhibits similar absorption energies to

the analogous species of 4, despite the shorter kmax of 5 in itsZn2+/H+ unbound form. In the studies reported here there wasno discernable decomposition of the starting gold complexes ashas been observed in protonation studies on other alkynylgoldcomplexes that contain pyridine units.65 The 31P NMR spectra donot indicate the presence of additional phosphorus environmentsupon reaction with Zn2+ or H+.

The interaction of bipyridine with metal ions or protons willmodify the electronic and optical properties of a conjugated systemwith built-in bipyridine units in its backbone. This has beenexploited to tune the performances of a number of bipyridine-containing conjugated polymers.66–69 The ions Zn2+ and H+ bothexert a similar influence on the absorption of 4 and 5, yet eachhas a very different effect on the emission in a CH2Cl2–MeOHmixed solvent (Fig. 10). The causes may be traced to the higherpolarisability of H+ and possible hydrogen bonding betweenprotonated 4 or 5 and methanol. It is clear that much moresignificant geometric relaxations from the H+ polarisation andhydrogen bonding occur in the excited state before it deactivatesto the lowest emission state, as manifested by the longer kem of 4and 5 on binding to H+ than on binding to Zn2+. The broadnessand lack of structure in the emission profile for both protonatedspecies also indicates increased charge transfer character of itsexcited state, when compared to the comparatively sharper andvibronically structured emission of the Zn2+-coordinated species.Although the coordination of Zn2+ or protonation only results indifferences in lower energy absorption bands between 4 and 5, thedifferences in their emission are evident. The Zn2+/H+-bound 4emits at lower energies than the analogous species of 5, suggestingthat the expanded p-electron cloud of 4 in its excited state ismore susceptible to the polarised Zn2+/H+ than more confinedp-electron cloud of 5 in its excited state. The emission of Zn2+/H+-bound 4 is also significantly enhanced, whereas no appreciable

Fig. 10 Emission spectra of complex 4 in CH2Cl2–CH3OH (I) and in thepresence of CF3COOH (II).

1642 | Dalton Trans., 2008, 1635–1646 This journal is © The Royal Society of Chemistry 2008

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online

change in emission intensity is observed for 5 on binding toZn2+/H+.

Conclusion

A study of a series of digold ethynyl complexes with pyridineor bipyridine spacer groups that have the general formula[(R3P)Au(C≡C)X(C≡C)Au(PR3)] (R = Ph, X = 2,5-pyridine (1);R = Cy (cyclohexane), X = 2,5-pyridine (2); R = Ph, X = 2,6-pyridine (3); R = Ph, X = 2,5′-bipyridine (4); R = Ph, X =2,6′-bipyridine (5)), shows that the formation of an aurophilicAu · · · Au interaction only occurs in complex 1, in the solidstate, when the spacer group, X, is relatively short and when theauxiliary phosphine ligand is not excessively bulky. This obser-vation supports conclusions drawn previously from the analysisof the structures of a series of digold ethynyl complexes withvarious length spacer groups.21 However, the length and geometricconfiguration of the spacer group and the bulk of the phosphineligand represent only the steric factors that contribute to the solidstate packing of these complexes and electronic factors have notbeen taken into consideration. Solution absorption and emissionspectroscopy, supported by DFT calculations, show that thereis little contribution from the gold centres to the delocalisationwithin the complexes and the p-orbitals extend only over thecentral spacer group and the two acetylene bonds. In solution,the bipyridine complexes 4 and 5 are capable of coordinating Zn2+

ions or protons and this coordination is characterised from bothabsorption and emission studies.

Experimental

General

All reactions were carried out under an atmosphere of drynitrogen using standard Schlenk techniques. Solvents were freshlydistilled, dried and degassed by standard procedures before use.70

Infrared spectra were recorded using a NaCl cell on a PERKINELMER PARAGON 1000 FT-IR spectrometer. UV/vis spectrawere recorded on Perkin-Elmer Lamda-12 spectrometer andCary 100 Bio UV-visible spectrometer. 1H, 13C and 31P NMRspectra were recorded on Bruker28 DRX-400/500 spectrome-ters. Chemical shifts in ppm are relative to the residue solventresonance (1H and 13C) and external 85% H3PO4 (31P). Massspectra were recorded on KRATOS CONCEPT/MSI CONCEPTIH/MICROMASS PLATFORM-LC mass spectrometers. Ele-mentary analyses were performed at Department of Chemistry,University of Cambridge. Solution emission spectra were recordedat 293 K on AMINCO Bowman Series 2 Luminescence Spec-trometer as 10−5 mol dm−3 solutions in CH2Cl2. [AuPPh3Cl],71

[Au(PCy3)Cl],72 2,5-bis(trimethylsilylethynyl)pyridine73 and 2,6-bis(trimethylsilylethynyl)pyridine73 were prepared by literaturemethods.

Ligand syntheses

5,5′-Bis(trimethylsilylethynyl)-2,2′-bipyridine. To a solution of5,5′-dibromo-2,2′-bipyridine (1.2 g, 3.8 mmol) in iPr2NH–THF(70 cm3, 1 : 1 v/v) under nitrogen was added a catalytic mixture ofCuI (20 mg), Pd(OAc)2 (20 mg) and PPh3 (60 mg). The solution wasstirred for 20 min at 50 ◦C and then trimethylsilylethyne (1.64 g,

16.7 mmol) was added. The reaction mixture was left with stirringfor 20 h at 75 ◦C. The completion of the reaction was determinedby silica TLC and IR spectroscopy. The solution was allowed tocool down to room temperature, filtered and the solvent mixtureremoved under reduced pressure. The residue was subjected tosilica column chromatography using dichloromethane–hexane (1 :1) to afford the product as a white solid in 68% yield (0.9 g). IR(CH2Cl2): m(C≡C) 2159 cm−1. 1H NMR (d, 400 MHz, CDCl3): 0.27(s, 18H, H of TMS), 7.84 (dd, 3JHH = 8.2, 4JHH = 2.1, 2H, H4 and 4′ ),8.34 (dd, 3JHH = 8.2, 4JHH = 0.7, 2H, H3 and 3′ ), 8.71 (dd, 4JHH = 2.1,4JHH = 0.7, 2H, H6 and 6′ ). 13C NMR (d, 400 MHz, CDCl3): −0.20(C of TMS); 99.44, 101.74 (–C≡C–); 120.33 (ipso-C), 120.46,139.75, 152.05, 154.19 (ipso-C) (aromatic C of pyridines). ESI(m/z): 349.12 (Calc. Mr = 348.598). Anal. Calc. for C20H24N2Si2:C, 68.91; H, 6.94; N, 8.04. Found: C, 68.83; H, 6.94; N, 7.98%.

6,6′-Bis(trimethylsilylethynyl)-2,2′-bipyridine. This compoundwas synthesised by the same method as for 5,5′-bis(trimethyl-silylethynyl)-2,2′-bipyridine but using 0.7 g (2.2 mmol) of 6,6′-dibromo-2,2′-bipyridine. The product was purified by silica col-umn chromatography with dichloromethane–hexane (2 : 3) aseluent. The product was obtained as a white solid in a 74% yield(0.57 g). IR (CH2Cl2): m(C≡C) 2160 cm−1. 1H NMR (d, 400 MHz,CDCl3): 0.28 (s, 18H, H of TMS), 7.47 (dd, 3JHH = 7.8 Hz, 4JHH =1.0 Hz, 2H, H5 and 5′ ), 7.75 (t, 3JHH = 7.8 Hz, 2H, H4 and 4′ ), 8.41(dd, 3JHH = 7.8 Hz, 4JHH = 1.0 Hz, 2H, H3 and 3′ ). 13C NMR (d,400 MHz, CDCl3): −0.25 (C of TMS); 94.52, 103.94 (–C≡C–);121.10, 127.77, 136.94, 142.35 (ipso-C), 155.71 (ipso-C) (aromaticC of pyridines). ESI (m/z): 349.12 (Calc. Mr = 348.598). Anal.Calc. for C20H24N2Si2: C, 68.91; H, 6.94; N, 8.04. Found: C, 67.78;H, 6.98; N, 7.90%.

Complex synthesis

General synthetic procedure for preparing the Au(I) complexes.To a freshly prepared terminal alkyne (ca. 0.2 mmol, from thereaction of trimethylsilyl-protected alkyne with KOH–MeOH) inCH2Cl2 (30 mL) was added gold(I) phosphine chloride (stoichio-metric amount), followed by MeOH–NaOMe (20 mL, containingca. 2–30 mg Na). The mixture was stirred under N2 at roomtemperature overnight and then filtered through cellulose. Thefiltrate was evaporated to dryness under reduced pressure. CH2Cl2

was added to the residue and the resulting suspension was stirredfor 15 min and filtered. The filtrate was reduced in volume, loadedon to a short alumina column, and then eluted with mixed solventsof THF (or ethyl acetate)–hexane. The solvents were removedin vacuo to yield pale yellow powders. Pure products were obtainedeither by layering concentrated CH2Cl2 solution with hexane, or bydiethyl ether vapour diffusion into concentrated CH2Cl2 solutions.

[(Ph3PAu)(C≡C)(2-(C5H3N)-5)(C≡C)(AuPPh3)] (1). Synthe-sis was by the general procedure using a freshly prepared alkynefrom 35 mg (0.13 mmol) 2,5-bis(trimethylsilylethynyl)pyridine and130 mg (0.26 mmol) [Au(PPh3)Cl]. Yield: 92 mg (67%) yellowcrystalline solid. IR (CH2Cl2): m(C≡C) 2121 cm−1. 1H NMR (d,500 MHz, CDCl3): 7.29 (dd, 3JHH = 8.1 Hz, 4JHH = 0.8 Hz, 1H, H3),7.30–7.56 (m, 30H, H of PPh3), 7.61 (dd, 3JHH = 8.1 Hz, 4JHH =2.1 Hz, 1H, H4), 8.64 (dd, 4JHH = 2.1 Hz, 4JHH = 0.8 Hz, 1H, H6).13C NMR (d, 500 MHz, CDCl3): 100.96, 101.17 (–C≡C–); 119.42(ipso-C), 125.94, 138.48, 141.57 (ipso-C), 153.09 (aromatic C of

This journal is © The Royal Society of Chemistry 2008 Dalton Trans., 2008, 1635–1646 | 1643

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online

pyridine); 129.07, 129.16, 129.40 (d, ipso-C), 129.84 (d, ipso-C),131.52, 134.22, 134.33 (C of PPh3). 31P{1H} NMR (d, 400 MHz,CDCl3): 42.69, 42.80. LSIMS (m/z): 1044.2 (Calc. Mr = 1043.648).Anal. Calc. for Au2C44H32NP2: C, 51.78; H, 3.19; N, 1.34. Found:C, 51.69; H, 3.19; N. 1.37%.

[(Cy3PAu)(C≡C)(2-(C5H3N)-5)(C≡C)(AuPCy3)] (2). Synthe-sised by the general procedure using a freshly prepared alkynefrom 40 mg (0.15 mmol) 2,5-bis(trimethylsilylethynyl)pyridine and150 mg (0.30 mmol) [Au(PCy3)Cl]. Yield: 92 mg (58%) yellowcrystalline solid. IR (CH2Cl2): m(C≡C) 2117cm−1. 31P{1H} NMR(d, 400 MHz, CDCl3): 57.22. LSIMS (m/z): 1080 (Calc. Mr =1079.688). Anal. Calc. for Au2C45H69NP2·1.25CH2Cl2: C, 46.83;H, 6.08; N, 1.18. Found: C, 46.80; H, 6.04; N. 1.10%.

[(PPh3Au)(C≡C)(2-(C5H3N)-6)(C≡C)AuPPh3)] (3). Synthe-sised by the general procedure using a freshly prepared alkynefrom 35 mg (0.13 mmol) 2,6-bis(trimethylsilylethynyl)pyridine and130 mg (0.26 mmol) [Au(PPh3)Cl]. Yield: 100 mg (75%) yellowcrystalline solid. IR (CH2Cl2): m(C≡C) 2115 cm−1. 1H NMR (d,500 MHz, CDCl3): 7.24 (s, 1H, H4), 7.39–7.56 (m, 32H, H3 and 5

and H of PPh3). 13C NMR (d, 500 MHz, CDCl3): 103.02 (br, –C≡C–); 125.08, 135.26, 144.24 (ipso-C) (aromatic C of pyridine);129.02, 129.11, 129.66 (d, ipso-C), 130.10 (ipso-C), 131.41, 134.24,134.35 (C of PPh3). 31P{1H} NMR (d, 400 MHz, CDCl3): 42.72.LSIMS (m/z): 1044.2 (Calc. Mr = 1043.648). Anal. Calc. forAu2C45H33NP2: C, 51.78; H, 3.19; N, 1.34. Found: C, 51.66; H,3.28; N. 1.24%.

[(PPh3Au)(C≡C)(5-(C5H3N)(C5H3N)-5′)(C≡C)(AuPPh3)] (4).Synthesised by the general procedure using a freshly preparedalkyne from 40 mg (0.115 mmol) 5,5′-bis(trimethylsilylethynyl)-2,2′-bipyridine and 115 mg (0.23 mmol) [Au(PPh3)Cl]. Yield: 52 mg(40%) pale yellow powder. IR (CH2Cl2): m(C≡C) 2117 cm−1. 1H

NMR (d, 500 MHz, CDCl3): 7.43–7.75 (m, 30H, H of PPh3), 7.85(dd, 3JHH = 8.3 Hz, 4JHH = 2.1 Hz, 2H, H4 and 4′ ), 8.26 (dd, 3JHH =8.3 Hz, 4JHH = 0.8 Hz, 2H, H3 and 3′ ), 8.76 (dd, 4JHH = 2.1 Hz, 4JHH =0.8 Hz, 2H, H6 and 6′ ). 13C NMR (d, 500 MHz, CDCl3): 100.89,101.10 (–C≡C–); 120.19, 121.71 (ipso-C), 139.86, 152.50, 153.08(ipso-C) (aromatic C of pyridines); 129.11, 129.20, 129.34 (ipso-C), 129.79 (ipso-C), 131.58, 134.21, 134.32 (C of PPh3). 31P{1H}NMR (d, 400 MHz, CDCl3): 42.78. LSIMS (m/z): 1121 (Calc.Mr = 1120.734). Anal. Calc. for Au2C50H36N2P2: C, 53.59; H,3.24; N, 2.50. Found: C, 53.34; H, 3.37; N. 2.31%.

[(PPh3Au)(C≡C)(6-(C5H3N)(C5H3N)-6′)(C≡C)(AuPPh3)] (5).Synthesised by the general procedure using a freshly preparedalkyne from 40 mg (0.115 mmol) 6,6′-bis(trimethylsilylethynyl)-2,2′-bipyridine and 115 mg (0.23 mmol) [Au(PPh3)Cl]. Yield: 47 mg(37%) light yellow solid. IR (CH2Cl2): m(C≡C) 2116cm−1. 1H NMR(d, 500 MHz, CDCl3): 7.41–7.57 (m, 32H, H5 and 5′ and H of PPh3),7.63 (t, 3JHH = 7.8 Hz, 2H, H4 and 4′ ), 8.42 (dd, 3JHH = 7.8 Hz, 4JHH =1.1 Hz, 2H, H3 and 3′ ). 13C NMR (d, 500 MHz, CDCl3): 103.42,103.63 (–C≡C–); 119.61, 127.16, 136.22, 143.40 (ipso-C), 155.75(ipso-C) (aromatic C of pyridines); 129.06, 129.15, 129.47 (ipso-C), 129.91 (ipso-C), 131.53, 134.24, 134.35 (C of PPh3). 31P{1H}NMR (d, 400 MHz, CDCl3): 42.73. LSIMS (m/z): 1121 (Calc.Mr = 1120.734). Anal. Calc. for Au2C50H36N2P2: C, 53.59; H,3.24; N, 2.50. Found: C, 53.14; H, 3.45; N. 2.46%.

X-Ray crystallography

For crystal data, see Table 6.

Data collection and reduction. The crystals of 1–5 weremounted in inert oil on glass fibres. Data were measured usingMo-Ka radiation (k = 0.71069 A) with a Bruker Kappa CCDdiffractometer (2–5) and on a Bruker AXS SMART CCD

Table 6 Details of the crystal data, data collection parameters and structure refinement for complexes 1–5

1 2 3 4 5

Empirical formula C44H32Au2N2P2 C46H71Au2Cl2NP2 C45H33Au2NP2 C50H36Au2N2P2 C52H40Au2Cl4N2P2

Formula weight 1044.59 1164.81 1043.60 1120.68 1290.53Temperature/K 180(2) 180(2) 180(2) 180(2) 180(2)Wavelength (k)/A 0.6887 0.71073 0.71069 0.71069 0.71069Crystal system Triclinic Triclinic Monoclinic Monoclinic TriclinicSpace group P1 (No. 2) P1 (No. 2) C2/c (No. 15) P21/c (No. 14) P1 (No. 2)a/A 8.881(2) 9.1956(3) 26.524(4) 7.0693(1) 9.4980(12)b/A 13.638(3) 10.3609(3) 8.705(2) 13.7695(4) 10.9270(8)c/A 16.551(3) 12.8658(4) 18.646(3) 21.3934(6) 12.4680(16)a/◦ 74.65(3) 103.998(2) 90 90 72.057(6)b/◦ 83.64(3) 100.256(2) 120.30(3) 96.947(2) 81.173(5)c /◦ 71.06(3) 96.941(2) 90 90 79.945(6)Volume/A3 1827.8(6) 1153.00(6) 3717.1(16) 2067.16(9) 1205.3(2)Z 2 1 4 2 1Dc/Mg m−3 1.898 1.678 1.865 1.800 1.778l/mm−1 8.139 6.571 8.004 7.204 6.405Crystal size/mm 0.12 × 0.08 × 0.08 0.30 × 0.18 × 0.05 0.03 × 0.03 × 0.01 0.30 × 0.07 × 0.05 0.12 × 0.08 × 0.05h range/◦ 1.75–20.00 3.62–27.48 1.78–25.08 3.53–27.49 2.93–25.02Reflections collected 9084 12208 5889 18945 10292Independent reflections 3749 5234 3264 4705 4218Rint 0.0949 0.1206 0.0904 0.0699 0.0348Observed reflections [I>2r(I)] 2053 4596 1856 3943 3668Restraints/parameters 12/195 38/246 0/227 0/253 419/363Goodness of fit 1.045 1.048 1.032 1.040 1.041R1 [I>2r(I)] 0.0817 0.0579 0.0529 0.0309 0.0314wR2 (all data) 0.2231 0.1603 0.2116 0.0764 0.0655

1644 | Dalton Trans., 2008, 1635–1646 This journal is © The Royal Society of Chemistry 2008

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online

diffractometer on Station 9.8 of the STFC Daresbury Laboratory(1) using silicon monochromated radiation of wavelength k =0.6887 A; both instruments were fitted with an Oxford Cryostreamlow-temperature attachment.

Structure solution and refinement. Structures were solved bydirect methods (SHELXS-8674) and subjected to full-matrix least-squares refinement on F 2 (program SHELXL-9775 for 1, 2, 4 and 5and SHELXL-93 for 376). Despite the use of synchrotron radiationcrystals of 1 gave very weak diffraction patterns and in therefinement to compensate for this only the gold and phosphorusatoms were assigned anisotropic displacement parameters andhydrogen atoms were not included in the model. One of the phenylrings was clearly disordered over two sites and the two rings wererefined as rigid groups with partial occupancies that summed tounity. The pyridine nitrogen atom was treated in the refinementas being disordered over two symmetry related positions. In thecrystal structure of 2, in the central arene ring the nitrogen atomwas treated as being disordered over four positions in the ring andwas refined with 25% occupancy on each site. The four C–H groupsin the ring were then refined with 75% occupancy. The crystalstructure also included a molecule of dichloromethane solvent thatwas disordered over two sites and the two orientations were refinedso that the occupancy summed to unity. In the crystal structure of5 one of the phenyl rings was disordered over two positions andthe two orientations were refined with partial occupancies thatsummed to unity. There was also a disordered dichloromethanemolecule in the lattice, disordered over two orientations. Thiswas again modelled and refined in two orientations with theatomic occupancies summed to unity. Excluding the exceptionsmentioned above non-hydrogen atoms were generally refined withanisotropic displacement parameters and hydrogen atoms wereincluded using a riding model.

DFT calculations

The crystal structures of 1, 4, and 5 were used as startinggeometries for the geometry optimisations of [H3P–Au–C≡C–X–C≡C–Au–PH3] (X = 2,5-pyridine (6); X = 2,5′-bipyridine(7); X = 2,6′-bipyridine (8)) using Gaussian03.77 The geometryoptimisation was performed at a B3LYP level78 using as a basis set3-21G** for Au and 6-31G** for all other atoms.

Acknowledgements

We are grateful to the EPSRC for funding for a post doctoralfellowship (to O. F. K.), an EPSRC Senior Fellowship (toP. R. R.) and for a grant to purchase the Bruker Nonius KappaCCD diffractometer. We acknowledge a grant from the EPSRCNational Service for Computational Software to undertake DFTcalculations and the STFC for a beamtime allocation. We arealso grateful for financial support from the European Commis-sion project SANEME (under the framework of the 5th ISTprogramme, contract number IST-1999-10323). P. L. thanks theCambridge Overseas Trust and the Overseas Research Schemefor financial support. The award of a DAAD grant (GemeinsamesHochschulsonderprogramm III von Bund und Landern) (to B. A.)is gratefully acknowledged.

References

1 P. Nguyen, P. Gomez-Elipe and I. Manners, Chem. Rev., 1999, 99,1515; N. J. Long and C. K. Williams, Angew. Chem., Int. Ed., 2003, 42,2586.

2 D. Beljonne, H. F. Wittmann, A. Kohler, S. Graham, M. Younus, J.Lewis, P. R. Raithby, M. S. Khan, R. H. Friend and J. L. Bredas,J. Chem. Phys., 1996, 105, 3868.

3 J. H. Burroughs, D. D. C. Bradley, A. R. Brown, R. N. Marks, K.Mackay, R. H. Friend, P. L. Burn and A. B. Holmes, Nature, 1990,347, 539.

4 N. Tissler, G. J. Denton and R. H. Friend, Nature, 1996, 82, 695.5 J. M. Halls, C. A. Walsh, N. C. Greenham, E. A. Marseglia, R. H.

Friend, S. C. Moratti and A. B. Holmes, Nature, 1995, 376, 498.6 G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science,

1995, 270, 1789.7 F. Garnier, R. Hajlaoui, A. Yasser and P. Svwastra, Science, 1994, 265,

1684.8 H. Sirringhaus, N. Tissler and R. H. Friend, Science, 1998, 280, 1741.9 J. B. Torrance, Low Dimensional Conductors and Superconductors,

NATO ASI Ser., Ser. B, 1987, 155, 113.10 T. B. Marder, G. Lesley, Z. Yuan, H. B. Fyfe, P. Chow, G. Stringer,

I. R. Jobe, N. J. Taylor, I. D. Williams and S. K. Kurtz, Materials forNonnlinear Optics: Chemical Perspectives, ACS Symp. Ser., 1991, 455,605.

11 H. B. Fyfe, M. Mlekuz, G. Stringer, N. J. Taylor and T. B. Marder,Inorganic and Organometallic Polymers with Special Properties, NATOASI Ser., Ser. E, 1992, 206, 331.

12 V. W.-W. Yam and K. M.-C. Wong, Top. Curr. Chem., 2005, 257, 1.13 R. J. Puddephatt, Chem. Commun., 1998, 1055.14 V. W. W. Yam and K. K.-W. Lie, Chem. Soc. Rev., 1999, 28.15 V. W. W. Yam, K. K.-W. Lie and K. M.-C. Wang, J. Organomet. Chem.,

1999, 578, 3.16 A. S. Batsanov, M. A. Fox, J. A. K. Howard, J. A. H. MacBridge and

K. Wade, J. Organomet. Chem., 2000, 610, 20.17 A. S. Batsanov, M. A. Fox, T. G. Hibbert, J. A. K. Howard, R. Kivekas,

A. Laromaine, R. Sillanpaa, C. Vinas and K. Wade, Dalton Trans.,2004, 3822.

18 I. R. Whittall, A. M. McDonagh, M. G. Humphrey and M. Samoc,Adv. Organomet. Chem., 1999, 43, 349.

19 M.-C. Brandys, M. C. Jennings and R. J. Puddephatt, J. Chem. Soc.,Dalton Trans., 2000, 4601.

20 P. Li, B. Ahrens, K.-H. Choi, M. S. Khan, P. R. Raithby, P. J. Wilsonand W.-Y. Wong, CrystEngComm, 2002, 4, 405.

21 P. Li, B. Ahrens, N. Feeder, P. R. Raithby, S. J. Teat and M. S. Khan,Dalton Trans., 2005, 874.

22 J. Vicente, M. T. Chicote, M. M. Alvarez-Falcon, M.-D. Abrisqueta,F. J. Hernandez and P. G. Jones, Inorg. Chim. Acta, 2003, 347, 67.

23 M. J. Irwin, J. J. Vittal and R. J. Puddephatt, Organometallics, 1997, 16,3541.

24 V. W. W. Yam and S. W. K. Choi, J. Chem. Soc., Dalton Trans., 1996,4227.

25 V. W. W. Yam, S. W. K. Choi and K. K. Cheung, Organometallics, 1996,15, 1734.

26 J. Vicente, M.-T. Chicote, M. M. Alvarez-Falcon and D. Bautista,Organometallics, 2004, 23, 5707.

27 J. Vicente, M. T. Chicote, M. M. Alvarez-Falcon and P. G. Jones,Organometallics, 2005, 24, 2764.

28 T. Yamamoto, K. Sugiyama, T. Kushida, T. Inoue and T. Kanbara,J. Am. Chem. Soc., 1996, 118, 3930.

29 M. S. Khan, M. K. Al-Suti, M. R. A. Al-Mandhary, B. Ahrens, J. K.Bjernemose, M. F. Mahon, L. Male, P. R. Raithby, R. H. Friend, A.Kohler and J. S. Wilson, Dalton Trans., 2003, 65; M. S. Khan, M. R. A.Al-Mandhary, M. K. Al-Suti, B. Ahrens, M. F. Mahon, L. Male, P. R.Raithby, C. E. Boothby and A. Kohler, Dalton Trans., 2003, 74; M. S.Khan, M. R. A. Al-Mandhary, M. K. Al-Suti, F. R. Al-Battashi, S.Al-Saadi, B. Ahrens, J. K. Bjernemose, M. F. Mahon, P. R. Raithby,M. Younus, N. Chawdhury, A. Kohler, E. A. Marseglia, E. Tedesco, N.Feeder and S. J. Teat, Dalton Trans., 2004, 2377.

30 M. S. Khan, M. R. A. Al-Mandhary, M. K. Al-Suti, A. K. Hisham,P. R. Raithby, B. Ahrens, M. F. Mahon, L. Male, E. A. Marseglia, E.Tedesco, R. H. Friend, A. Kohler, N. Feeder and S. J. Teat, J. Chem.Soc., Dalton Trans., 2002, 1358.

31 I. R. Whittall, M. G. Humphrey and D. C. R. Hockless, Aust. J. Chem.,1997, 50, 991.

This journal is © The Royal Society of Chemistry 2008 Dalton Trans., 2008, 1635–1646 | 1645

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online

32 G. Jia, R. J. Puddephatt, J. D. Scott and J. J. Vittal, Organometallics,1993, 12, 3565.

33 M. A. MacDonald, R. J. Puddephatt and G. P. A. Yap, Organometallics,2000, 19, 2194.

34 W. J. Hunks, M. A. MacDonald, M. C. Jennings and R. J. Puddephatt,Organometallics, 2000, 19, 5063.

35 N. C. Payne, R. Ramachandran and R. J. Puddephatt, Can. J. Chem.,1995, 73, 6.

36 D. Li, X. Hong, C. M. Che, W. C. Lo and S. M. Peng, J. Chem. Soc.,Dalton Trans., 1993, 2929.

37 S. J. Shieh, X. Hong, S. M. Peng and C. M. Che, J. Chem. Soc., DaltonTrans., 1994, 3067.

38 B. C. Tzeng, W. C. Lo, C. M. Che and S. M. Peng, Chem. Commun.,1996, 181.

39 T. E. Muller, S. W. K. Choi, D. M. P. Mingos, D. Murphy, D. J. Williamsand V. W. W. Yam, J. Organomet. Chem., 1994, 484, 209.

40 C. M. Che, H. K. Yip, W. C. Lo and S. M. Peng, Polyhedron, 1994, 13,887.

41 M. I. Bruce and D. N. Duffy, Aust. J. Chem., 1986, 39, 1697.42 M. I. Bruce, K. R. Grundy, M. J. Liddell, M. R. Snow and E. R. T.

Tiekink, J. Organomet. Chem., 1988, 344, C49.43 G. A. Carriedo, V. Riera, X. Solans and J. Solans, Acta Crystallogr.,

Sect. C, 1988, 44, 978.44 R. H. Naulty, M. P. Cifuentes, M. G. Humphrey, S. Houbrechts, C.

Boutton, A. Persoons, G. A. Heath, D. C. R. Hockless, B. Luther-Davies and M. Samoc, J. Chem. Soc., Dalton Trans., 1997, 4167.

45 I. R. Whittall, M. G. Humphrey, M. Samoc, B. Luther-Davies andD. C. R. Hockless, J. Organomet. Chem., 1997, 544, 189.

46 I. R. Whittall, M. G. Humphrey, S. Houbrechts, J. Maes, A. Persoons,S. Schmid and D. C. R. Hockless, J. Organomet. Chem., 1997, 544, 277.

47 L. Lu, S.-Y. Poon and W.-Y. Wong, J. Organomet. Chem., 2005, 690,5036.

48 O. Schuster, R.-Y. Liau, A. Schier and H. Schmidbaur, Inorg. Chim.Acta, 2005, 358, 1429.

49 G. Hogarth and M. M. Alvarez-Falcon, Inorg. Chim. Acta, 2005, 358,1386.

50 W. M. Khairul, L. Porres, D. Albesa-Jove, M. S. Senn, M. Jones, D. P.Lydon, J. A. K. Howard, A. Beeby, T. B. Marder and P. J. Low, J. ClusterSci., 2006, 17, 65.

51 G. Ferguson, J. F. Gallagher, J. D. Kennedy, A.-M. Kelleher and T. R.Spalding, Dalton Trans., 2006, 2133.

52 J. Vicente, M.-T. Chicote, M. M. Alvarez-Falcon and P. G. Jones,Organometallics, 2005, 24, 5956.

53 M. I. Bruce, M. Jevric, B. W. Skelton, M. E. Smith, A. H. White andN. N. Zaitseva, J. Organomet. Chem., 2006, 691, 361.

54 D. Millar, L. N. Zakharov, A. L. Rheingold and L. H. Doerrer, Acta.Crystallogr., Sect. C, 2005, 61, m90.

55 O. Schuster, U. Monkowius, H. Schmidbaur, R. S. Ray, S. Kruger andN. Rosch, Organometallics, 2006, 25, 1004.

56 H.-Y. Chao, W. Lu, Y. Li, M. C. W. Han, C.-M. Che, K.-K. Chung andN. Zhu, J. Am. Chem. Soc., 2002, 124, 14696.

57 N. C. Baenziger, W. E. Bennett and D. M. Soboroff, Acta Crystallogr.,Sect. B, 1976, 32, 962.

58 R. C. Bott, G. A. Bowmaker, R. W. Bucklery, P. C. Healy and M. C. S.Perera, Aust. J. Chem., 1999, 52, 271.

59 P. Pyykko, J. Li and N. Runeberg, Chem. Phys. Lett., 1994, 218, 133.60 H. Schmidbaur, Gold: Progress in Chemistry, Biochemistry and Tech-

nology, J. Wiley & Sons, Chichester, 1999.61 H. Schmidbaur, W. Graf and G. Muller, Angew. Chem., Int. Ed. Engl.,

1988, 27, 417.62 A. Bondi, J. Phys. Chem., 1964, 68, 441.63 C. A. Tolman, Chem. Rev., 1977, 77, 313.64 O. F. Koentjoro, R. Rousseau and P. J. Low, Organometallics, 2001, 20,

4502.65 N. C. Habermehl, F. Mohr, D. J. Eisler and R. J. Puddephatt,

Can. J. Chem., 2006, 84, 111.66 Y. Eichen, G. Nakhmanovich, V. Gorelik, O. Epshtein, J. M. Poplawski

and E. Ehrenfreund, J. Am. Chem. Soc., 1998, 120, 10463.67 G. Zotti, S. Zecchin, G. Schiavon, A. Berlin and M. Penso, Chem.

Mater., 1999, 11, 3342.68 L. Trouillet, A. De Nicola and S. Guillerez, Chem. Mater., 2000, 12,

1611.69 M. Bouachrine, J.-P. Lere-Porte, J. J. E. Moreau, F. Serein-Spirau and

C. Torreilles, J. Mater. Chem., 2000, 10, 263.70 W. F. L. Armarengo and D. D. Perrin, Purification of Laboratory

Chemicals, 4th edn, Butterworth Heinemann, Bath, UK, 1996.71 M. I. Bruce, B. K. Nicholson and O. Bin Shawkataly, Inorg. Synth.,

1989, 26, 325.72 R. Uson, A. Laguna and M. Laguna, Inorg. Synth., 1989, 26, 86.73 S. Takahashi, Y. Kuroyama, K. Sonogashira and N. Hagihara, Synthe-

sis, 1980, 627.74 G. M. Sheldrick, SHELXS-86, Program for Crystal Structure Solution,

University of Gottingen, Germany, 1986.75 G. M. Sheldrick, SHELXL-97, Program for refinement of crystal

structures, University of Gottingen, Germany, 1997.76 G. M. Sheldrick, SHELXL-93, Program for Crystal Structure Refine-

ment, University of Gottingen, Germany, 1993.77 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.

Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N.Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone,B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H.Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X.Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo,J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,R. Cammi, C. Pomelli, J. Ochterski, P. Y. Ayala, K. Morokuma, G. A.Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul,S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P.Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,B. G. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople,GAUSSIAN 03 (Revision C.01), Gaussian, Inc., Wallingford, CT,2004.

78 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.

1646 | Dalton Trans., 2008, 1635–1646 This journal is © The Royal Society of Chemistry 2008

Dow

nloa

ded

by S

yrac

use

Uni

vers

ity o

n 11

/04/

2013

17:

24:2

7.

Publ

ishe

d on

24

Janu

ary

2008

on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/B

7166

64H

View Article Online