SYNTHESIS, STRUCTURE, AND ELECTRONIC SPECTROSCOPY OF NEUTRAL, DINUCLEAR GOLD(I) COMPLEXES - GOLD(I)-GOLD(I) INTERACTIONS IN SOLUTION AND IN THE SOLID-STATE

A series of neutral, dinuclear gold(I) complexes containing phosphine and thiolate ligands have been synthesized and characterized by elemental analysis and by H-1 and P-31 NMR and UV-visible spectroscopy. Crystal structures of two of the complexes are reported. [Au2(p-tc)2(dppb)] (4) crystallizes in the triclinic space group P1BAR (Z = 2) with unit cell dimensions a = 10.757(2) angstrom, b = 13.177(2) angstrom, c = 14.630(3) angstrom, alpha = 82.23(1)degrees, beta = 83.16(1)degrees, and gamma = 75.42(1)degrees; R = 0.0286. [Au2(p-tc)2(dpppn)] (5) crystallizes in the monoclinic space group P2(1)/n (Z = 4) with unit cell dimensions a = 12.007(1) angstrom, b = 25.292(5) angstrom, c = 13.421(2) angstrom, and beta = 94.92(1)degrees; R = 0.0432. The structures of 4 and 5 are similar; each has linear, two-coordinate gold atoms connected via a bridging bis(phosphine) to form an open-chain dinuclear complex. The dinuclear units are then connected via short intermolecular gold(I)-gold(I) interactions to form polymeric chains. The intermolecular Au-Au distances for 4 and 5 are 3.094(l) and 3.200(1) angstrom, respectively. Other similar complexes include [Au2(p-tc)2(dppm)] (1), [Au2(p-tc)2(dppe)] (2), and [Au2(p-tc)2(dppp)]( 3). Cyclic dinuclear gold(I) complexes with bridging bis(phosphine) and dithiolate ligands are also reported: [Au(dppe)(pdt)Au] (6), [Au(dppp)(pdt)Au] (7), [Au(dppb)(pdt)Au] (8), [Au(dpppn)(pdt)Au] (9), and [Au(dppe)(tdt)Au] (10). The following abbreviations are used: dppm = bis(diphenylphosphino)methane; dppe = 1,2-bis(diphenylphosphino)ethane; dppp = 1,3-bis(diphenylphosphino)propane; dppb = 1,4-bis(diphenylphosphino) butane; dpppn = 1,5-bis(diphenylphosphino)pentane; PMe3 = trimethylphosphine; PPh3 = triphenylphosphine; p-tc = p-thiocresol; pdt = 1,3-propanedithiol; tdt = 3,4-toluenedithiol. Spectral features of the dinuclear gold complexes are compared to those of the mononuclear complexes Au(PPh3)(p-tC) (11) and Au(PMe3)(p-tc) (12). Analysis of the electronic absorption spectra from Gaussian band spectral fitting, comparison to free ligand spectra, studies of solvatochromism, and alterations of the phosphine and thiolate ligands indicate that the two lowest energy transitions in 1-9, 11, and 12 are LMCT (S --> Au) transitions. The spectra for the open-chain complexes, 2-5 in which the length of the bis(phosphine) backbone successively increases by 1, are very similar and show no absorption bands below 3.0 X 10(4) cm-1. However, the lowest energy transition for the smallest member of this series, 1 (which is yellow), is significantly red shifted, occurring at 2.8 X 10(4) cm-1. A similar trend is observed for the two smallest members of the cyclic series, 6 and 7 compared to 8 and 9. Variable-temperature NMR experiments reveal the presence of dynamic processes for 1, 6, and 9. The activation energy, determined by line shape analysis, is approximately 10 kcal/mol for 1 and 9 and less than 10 kcal/mol for 6. The observed red shift in the LMCT transitions and the variable-temperature behavior are consistent with the presence of intramolecular gold(I)-gold(I) interactions in solution for 1 and 6-9. The implications of these results for biological systems are considered.