13C, 31P and 15N NMR studies of the ligand exchange reactions of auranofin and chloro(triethylphosphine)gold (I) with thiourea

The interaction of thiourea (Tu) with auranofin (Et(3)PAuSATg) and its analogue, Et3PAuCl has been studied using C-13, P-31 and N-15 NMR spectroscopy. It is observed that Tu is able to replace both the ligands, Et3P and SATg(-) simultaneously from gold(I) in auranofin, forming [Et3P-Au-Tu](+) and Tu-Au-SATg complexes. However, no separate resonances for these species were observed either due to their rapid exchange with auranofin and thus giving only the average resonances or because the chemical shifts of either two species are same so that they cannot be resolved. The displaced SATg(-) is oxidized to its disulfide, (SATg)(2). However, some of the displaced Et3P is oxidized to Et3PO while the remaining reacts with Tu to form Et3P-Tu species, characterized by delta P-31 of 1.0 ppm, assigned after an independent reaction between Et3P and Tu. In an experiment using a 0.05 M solution of auranofin, the Et3PO resonance appeared in auranofin spectrum after 4 days of addition of 1.0 equivalent of Tu, showing that the reaction is slow. A resonance for free Et3P is also detected in P-31 NMR on the addition of CN-. It is also observed that Tu reacts with Et3PAuCl to form [Et3P-Au-Tu](+) via displacement of Cl-, consistent with an upfield shift of 6.2 ppm in >C=S resonance of Tu in C-13 NMR. In N-15 NMR, a smaller downfield, instead of an upfield shift, in NH2 resonance of Tu on its addition to auranofin and Et3PAuCl indicates that it is not binding to gold(I) through nitrogen. (C) 2002 Elsevier Science B.V. All rights reserved.