Molecular aggregation of annular dinuclear gold(I) compounds containing bridging diphosphine and dithiolate ligands

A series of annular dinuclear Au(I) complexes containing diphosphine (R2P(CH2)(n)PR2; R = Me, n = 1, dmpm; R = Me, n = 2, dmpe; R = Ph, n = 1, dppm; R = Ph, n = 2, dppe) and dithiolate (dtc S2CNEt2-, i-mnt = S2C2(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 = PF6, 2; P-P = dppe, X = PF6, 3; P-P = PPh3, X = PF6, 4) and [Au-2(P-P)(S-S)I (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) Angstrom b = 48.220(11) Angstrom, and c = 15.273(2) Angstrom, R = 0.0542; [Au-2(dppe)(i-mnt)] (8), space group P2(1)/n, with a = 11.793(2) Angstrom, b = 19.607(2) Angstrom, and c = 15.349(2) Angstrom, 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 Delta H values of similar to 15 kcal/mol and Delta S values of similar to-35 to similar to-46 cal/(K mol). Concentration dependence of emission spectra of annular compounds in acetonitrile also supports association in solution. Emissions at similar to-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 ((LMCT)-L-3) transitions give emission bands at similar to 550 nm. Compound 4 has a S --> Au (LMCT)-L-3 transition at 500 nm. P-31 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.