Table of Contents
Metal-Based Drugs
Volume 1 (1994), Issue 5-6, Pages 405-417
http://dx.doi.org/10.1155/MBD.1994.405

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

Department of Chemistry, University of Maine, Orono 04469-5706, ME, USA

Copyright © 1994 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The unusual yellow color of Au2(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 P31{H1} NMR spectrum of Au2(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, Au2(dppe)(SR)2 (dppe = diphenylphosphinoethane). Molecular mechanics calculations on Au2(dppm)(SR)2 and Au2(dppe)(SR)2 reveal that, for Au2(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 Au2(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: Au2(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.