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 pK_a of 2.80) followed by a typical associatively activated displacement of the O-bonded ligand by chloride, with the usual rate law, k_obs.=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.