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

Abstract

The interaction of thiourea (Tu) with auranofin (Et 3PAuSATg) and its analogue, Et 3PAuCl has been studied using 13C, 31P and 15N NMR spectroscopy. It is observed that Tu is able to replace both the ligands, Et 3P and SATg − simultaneously from gold(I) in auranofin, forming [Et 3P-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 Et 3P is oxidized to Et 3PO while the remaining reacts with Tu to form Et 3P-Tu species, characterized by δ 31P of 1.0 ppm, assigned after an independent reaction between Et 3P and Tu. In an experiment using a 0.05 M solution of auranofin, the Et 3PO 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 Et 3P is also detected in 31P NMR on the addition of CN −. It is also observed that Tu reacts with Et 3PAuCl to form [Et 3P-Au-Tu] + via displacement of Cl −, consistent with an upfield shift of 6.2 ppm in >CS resonance of Tu in 13C NMR. In 15N NMR, a smaller downfield, instead of an upfield shift, in NH 2 resonance of Tu on its addition to auranofin and Et 3PAuCl indicates that it is not binding to gold(I) through nitrogen.