Displacement of pyridine-2-methanol from dichloro(pyridine-2-methanolato)gold(III) in acidic solution. Ring opening at oxygen

Abstract

The kinetics of the displacement of pyridine-2-methanol (N–OH) from dichloro(pyridine-2-methanolato)gold(III), [Au(N–O)Cl₂], have been studied in 5% aqueous methanol at 25 °C. In the presence of LiCl and perchloric acid the reaction consists of a pre-equilibrium protonation of the oxygen followed, first, by ring opening at oxygen accompanied by the entry of chloride or solvent, and then by displacement of the N-bonded pyridine-2-methanol to give [AuCl₄]^–. This final stage is very similar to the displacement of 2-(methoxymethyl)pyridine (N–OMe) from [Au(N–OMe)Cl₃]. The ligand is not displaced in the absence of chloride. The reaction between [AuCl₄]^– and excess pyridine-2-methanol in 5% aqueous methanol was also studied at 25 °C. The initial attachment of the ligand by nitrogen is followed by rapid ring closing and a second molecule of ligand enters more slowly. The final product is [Au(N–O)₂]⁺(isomeric form unknown), stabilised by the basic conditions generated by the excess ligand. Monodentate pyridine-2-methanol behaves as if it has a pK_a of 5.6 with a steric hindrance comparable to that of 2-methylpyridine.