Electronic and Steric Effects in Gold(I) Phosphine ThiolateComplexes

Abstract

The unusual yellow color of Au 2 (dppm)(SR) 2 (R = 4‐tolyl; dppm = diphenylphosphinomethane) is attributed to a red‐shift in the S→Au charge transfer caused by destabilization of the sulfur highest occupied molecular orbital (HOMO). Variable temperature experiments show two broad bands at ‐80°C in the 31 P{ 1 H} NMR spectrum of Au 2 (dppm)(SR) 2 and the activation energy for interconversion is 10 kcal/mol. Only one sharp band is observed down to ‐80°C in the spectrum of the white complex, Au 2 (dppe)(SR) 2 (dppe = diphenylphosphinoethane). Molecular mechanics calculations on Au 2 (dppm)(SR) 2 and Au 2 (dppe)(SR) 2 reveal that, for Au 2 (dppe)(SR) 2 , a series of maxima and minima, separated by 2.5 kcal/mol, occur every 120° which is consistent with rotation around an unhindered carbon‐phosphorus single bond. The Au atoms are not within bonding distance in any conformation. Computational results for Au 2 (dppm)(SR) 2 indicate one minimum energy structure in which the Au‐P bonds are anti. There is a high energy conformation (9 kcal/mol above the global minimum) where overlap between golds is maximized. The implications of gold‐gold bonding in this complex are discussed. The steric influence of the thiolate ligand has been examined by synthesizing a series of dinuclear gold(I) complexes in which the steric properties of the thiolate are varied: Au 2 (dppm)(SR) 2 (R = 2,6‐dichlorophenyl; 2,6‐dimethylphenyl; 3,5‐dimethylphenyl). The 2,6‐disubstituted complexes are white, while the 3,5‐dimethyl complex is yellow. These results, along with VT‐NMR experiments, are consistent with the conclusion that the more sterically‐bulky thiolates hinder the close approach of the golds in the dinuclear complexes.