Solvent Induced Luminescence in Supramolecular Heterobimetallic Gold(I)- Copper(I) Complexes with a Bidentate Nitrile Ligand

Heterobimetallic complexes [Cu(N C-Cy-C N)2][Au(C6F5)2]·0.5 Toluene (1) and [Cu(N C-CyC N)2][Au(C6F5)2]·CH2Cl2 (2) and [AuCu(C6F5)2(N C-Cy-C N)2]·CH2Cl2 (3) (with the same stoichiometry but different structure) have been synthesized and the crystal structure of complex 2·CH2Cl2 has been characterized through X-ray diffraction studies. The structure shows a cationic [Cu(N C-Cy-C N)2] + polymer that runs parallel to the crystallographic y axis, which is formed by the copper centers and the nitrile bridging ligands, and [Au(C6F5)2] anions that link the polymeric chain via non-classical C-H···Au hydrogen bonds. Complexes 1, 2 and 3 only differ in the solvents and the time used for their synthesis but this greatly affects their photophysical properties. Complexes 1, 2 and 3 are brightly luminescent in solid state at room temperature with lifetimes in the microseconds range. Complexes 1 and 2 display emissions arising from IL transitions while for complex 3 an emission arising from a MLCT is proposed. Heterometallic Au(I)-M(I) complexes (M = Ag(I), Tl(I) and Cu(I)) built up through the acid-base reaction between [Au(C6X5)2] and M salts are an important class of compounds from experimental, theoretical [1, 2] and photophysical [3] viewpoints. In most cases this interest arises from the presence of metallophilic interactions which are usually responsible for the arrangements found in the solid state such as dimers, polymeric linear chains, 2D-sheets or 3Dnetworks. What is found in these cases is that the presence of Au(I)···M closed-shell interactions governs the structural dispositions found in the solid state. Also, many of these complexes display fascinating luminescent properties that are closely related to the presence of metallophilic interactions. Taking into account the heterometals, the ligands, the intermetallic distances and the number of metal-metal interactions it is possible to obtain complexes in which the luminescent properties vary from blue to red emissions and from fluorescent to phosphorescent emitters. This trends have been recently revised [3] and just as an example, while heterometallic Au-Ag and Au-Cu polymeric complexes built up from [Au2M2(C6F5)4(NCCH3)] (M = Ag or Cu) [4] tetranuclear units display fluorescent yellow-green or yellow emissions, discrete molecules as {[Tl( toluene)][Au(C6Cl5)2]} [5] is a blue emitter with a lifetime in the microseconds range. In both examples the synthetic strategy is common but the heterometals, the ligands and the intermetallic interactions differ and are responsible for a different photoluminescent behaviour. In another example, a *Address correspondence to these authors at the Departamento de Química, Universidad de La Rioja, UA-CSIC. Complejo Científico-Tecnológico, 26004-Logroño, Spain; Tel: +34 941 299 642; Fax +34 941 299 621; E-mail: [email protected] fluorescent red emission is obtained from {[AuTl(C6Cl5)2 (bipy)]n·0.5toluene}n (bipy = 4,4’-bipyridine) complex [6]. As regards Au(I)-Cu(I) complexes built up through this acid-base strategy, we have recently reported the synthesis and structural characterization of complexes [AuCu(C6F5)2 (N CCH3)(C4H4N2)]n [7] and [Au2Cu2(C6F5)4(N CCH3)2]n [4] in which the copper(I) centers display a distorted tetrahedral environment. In the first case, the tetrahedral environment is completed by the coordination of two pyrimidine ligands, one acetonitrile ligand and a strong Au(I)···Cu(I) metallophilic interaction. In the second example the copper center completes its coordination sphere by the coordination to only one acetonitrile molecule, two Au(I)···Cu(I) contacts and one Cu(I)···C contact. What it seems likely is that when the copper centers are ligand-unsaturated the basic aurate fragment is able to stabilize the copper atom through metallophilic interactions.(see Scheme 1) These structural situations in which metallophilic interactions take place lead to compounds that display very interesting photoluminescent properties. Another interesting structural feature is the key role that solvent molecules would play in the formation of different structural arrangements found in the solid state and their influence in the photophysical properties [8]. At this point we wondered whether the presence of larger amounts of a nitrile ligand would compete or not with the metallophilic interactions leading to well-separated ionic counterparts [Au(C6F5)2] and [CuL2] + (L = bidentate nitrile ligand) (see Scheme 1c). This strategy would give rise to a new family of complexes in which the acid-base stacking would be prevented by complete saturation of the Cu(I) centers with nitrile ligands and the photoluminescent properties, if present, would have a different origin. 74 The Open Inorganic Chemistry Journal, 2008, Volume 2 Fernández et al. Hence, herein we report the synthesis and photophysical properties of a new type of Au-Cu complexes [Cu(N C-CyC N)2][Au(C6F5)2]·0.5 Toluene (1), [Cu(N C-CyC N)2][Au(C6F5)2]·CH2Cl2 (2) and [AuCu(C6F5)2(N C-CyC N)2]·CH2Cl2 (3) using the bidentate N-donor ligand 1,4cyclohexanedicarbonitrile bonded to Cu(I) in different solvents as toluene and dichloromethane. The use of different solvents for the crystallization of the complexes 1, 2 and 3 leads to new structural arrangements in the solid state. We also study the relationship between the luminescent properties and the possible structural changes upon addition of dichloromethane to complex 1 in a fast reaction or in a slow diffusion process of n-hexane into dichloromethane. 2. RESULTS AND DISCUSSION The ligand 1,4-cyclohexanedicarbonitrile reacts with the gold-copper starting material [AuCu(C6F5)2(N C-Me)2] in a 2:1 molar ratio in toluene as solvent, leading to a compound of stoichiometry [Cu(N C-Cy-C N)2][Au(C6F5)2]·0.5 Toluene (1), by displacement of the acetonitrile ligand. Addition of CH2Cl2 to complex 1 and slow diffusion of n-hexane leads to complex [Cu(N C-Cy-C N)2][Au(C6F5)2]·CH2Cl2 (2). By contrast, addition of CH2Cl2 to complex 1 and further evaporation of the solvent under vacuum leads to complex [AuCu(C6F5)2(N C-Cy-C N)2]·CH2Cl2 (3). All physical and spectroscopic properties are in accordance with the proposed stoichiometries. It is worth mentioning that complexes 2 and 3 display the same stoichiometry but a different arrangement in their structures. Taking into account their photophysical properties (see below) a Au(I)···Cu(I) intermetallic interaction is proposed for complex 3. Complex 1 is soluble in CH2Cl2 and partially soluble in toluene while complexes 2 and 3 are soluble in CH2Cl2 and insoluble in toluene. It is worth mentioning that upon addition of CH2Cl2 to complex 1 it converts into complex 3 but addition of toluene to complex 3 does not recover complex 1. Complex 3 is also soluble and decomposes in CH3CN and MeOH. Complexes 1, 2 and 3 show similar F NMR profiles, displaying the signals corresponding to the C6F5 groups bonded to Au(I) in the free [Au(C6F5)2] units at approximately -114.9 (Fo), -161.7 (Fp), -162.9 (Fm) ppm. The H NMR spectra for complexes 1, 2 and 3 show the signals corresponding to the cis/trans mixture (ca 70:30) of the ligand 1,4-cyclohexanedicarbonitrile that, in solution, seems to be detached from the copper(I) centers since the chemical shifts are similar to the ones of the free ligand. In the case of complex 2 the crystal structure displays a cis conformation for the 1,4-cyclohexanedicarbonitrile ligand. Single crystals of the pentafluorophenyl complex 2·CH2Cl2 suitable for x-ray diffraction studies were obtained by slow diffusion of n-hexane into a solution of the complex in dichloromethane. Its crystal structure consists of a cationic polymer that runs parallel to the crystallographic y axis, which is formed by the coppercenters and the nitrile bridging ligands, and [Au(C6F5)2] anions that link the polymeric chain via non-classical C-H···Au hydrogen bonds (Fig. 1). The gold(I) atoms are linearly coordinated to two pentafluorophenyl rings with typical Au-C bond distances (2.046(5) and 2.051(5) Å), while the copper(I) centers display a tetrahedral environment by coordination to four nitrogen atoms of different nitrile molecules, which act as bridges between two copper centers (see Fig. 2). The Cu-N bond lengths of 1.982(4), 1.996(4), 1.998(5) and 2.025(5) Å are longer than in the related acetonitrile complex [Au2Cu2(C6F5)4(N CCH3)2]n [4] (1.903(4) Å), but are in general shorter than the Cu-N distance to acetonitrile in [AuCu(C6F5)2(N CCH3)(C4H4N2)]n (C4H4N2 = Pyrimidine) [7] (2.048(4) Å). Interestingly, although Au···Cu bonding interactions have been observed in the Au/Cu nitrile derivatives [Au2Cu2(C6F5)4(N CCH3)2]n [4] and [AuCu(C6F5)2 Au R R