Electronic and Steric Effects in Gold(I) Phosphine Thiolate Complexes

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 degrees 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 degrees 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 degrees 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.