Displacement by chloride of pyridine-2-carboxylate from dichloro(pyridine-2-carboxylato)gold(III) in acidic solution; the position of ring opening

Abstract

The kinetics of the reaction [AuCl2(N–O)]+ 2Cl– [graphic omitted] [AuCl4]–++HN–OH (where N–O = pyridine-2-carboxylate) have been studied at 25 °C in methanol–water (95:5, v/v). In the presence of excess of chloride, the reaction involves a fast and reversible ring opening with Au–N bond breaking (K1= 3.16 dm3 mol–1) and a reversible protonation of the free nitrogen (K2= 640 dm3 mol–1, corresponding to a pKa of 2.80) followed by a typical associatively activated displacement of the O-bonded ligand by chloride, with the usual rate law, kobs.=k1+k2[Cl–] where k1(the solvolytic rate constant)= 1.9 × 10–4 s–1 and k2= 4.54 × 10–2 dm3 mol–1 s–1. In the absence of chloride a solvolytic ring opening can be observed but the reaction stops before the monodentate ligand is displaced.