Molecular Aggregation of Annular Dinuclear Gold(I) Compounds Containing Bridging Diphosphine and Dithiolate Ligands

A series of annular dinuclear Au(I) complexes containing diphosphine (R(2)P(CH(2))(n)()PR(2); R = Me, n = 1, dmpm; R = Me, n = 2, dmpe; R = Ph, n = 1, dppm; R = Ph, n = 2, dppe) and dithiolate (dtc = S(2)CNEt(2)(-), i-mnt = S(2)C(2)(CN)(2)(2)(-)) ligands were synthesized: [Au(2)(P-P)(S-S)]X (S-S = dtc: P-P = dmpm, X = Cl, 1; P-P = dppm, X = PF(6), 2; P-P = dppe, X = PF(6), 3; P-P = PPh(3), X = PF(6), 4) and [Au(2)(P-P)(S-S)] (S-S = i-mnt: P-P = dmpm,5; P-P = dppm, 6; P-P = dmpe, 7; P-P = dppe, 8). Crystal structures of two complexes are reported. Pertinent crystallographic data: [Au(2)(dmpm)(i-mnt)] (5), space group Fdd2, with a = 19.574(3) Å, b = 48.220(11) Å, and c = 15.273(2) Å, R = 0.0542; [Au(2)(dppe)(i-mnt)] (8), space group P2(1)/n, with a = 11.793(2) Å, b = 19.607(2) Å, and c = 15.349(2) Å, R = 0.0448. Each molecule has two gold atoms bridged by a dithiolate ligand on one side and a diphosphine ligand on the other side, thus forming an eight- or nine-membered ring digold complex. The tendency of the digold(I) compounds to aggregate through an intermolecular Au-Au interaction depends on the ligands. Among the structures determined, complex 5 forms a polymeric chain and compound 8 is monomeric. Molecular aggregation also occurs in solution. Concentration-dependent absorption spectra of Au-dtc compounds suggest that an equilibrium between the monomer and dimer exists. Equilibrium constants corresponding to the intermolecular Au-Au interaction range from 38 to 137 M(-)(1) with DeltaH values of approximately 15 kcal/mol and DeltaS values of approximately -35 to approximately -46 cal/(K mol). Concentration dependence of emission spectra of annular compounds in acetonitrile also supports association in solution. Emissions at approximately 400-440 nm assignable to spin-allowed metal-centered transitions from monomeric Au(2) at lower concentrations and dimeric Au(4) at higher concentrations are observed. Compounds 1-3 and 5-8 in the glass state having spin-forbidden dithiolate ligand to gold charge transfer ((3)LMCT) transitions give emission bands at approximately 550 nm. Compound 4 has a S --> Au (3)LMCT transition at 500 nm. (31)P variable-temperature NMR experiments were performed for 1-3. The dynamic process is attributed to molecular aggregation through an intermolecular Au-Au interaction in solution. The activation energies are approximately 9 kcal/mol for 1 and 2.