Reactivity of heterocyclic nitrogen donors in systems containing the tetrachloroaurate(III) anion

Abstract

A series of gold(III) complexes of the type [AuCl₃(L)] has been prepared and characterised (L = oxazole, benzoxazole, thiazole, their benzo and methyl-substituted derivatives, or 2-methylbenzoselenazole). The five-membered N,O- N,S- and N,Se-heterocyclic bases are all bound to Au^III through nitrogen. The kinetics of the displacement of L by chloride to give [AuCl₄]^– has been studied in methanol–water (95:5, v/v) at 25.0°C and I= 0.20 mol dm^–3(LiClO₄). The equilibrium constants for the reversible processes have also been determined. The reactions of the corresponding pyridine, 4-chloro-, 4-cyano- and 2,6-bis(chloromethyl)-pyridine complexes have also been reexamined under the same conditions. The equilibrium constants, K₂, depend upon the basicity of the nitrogen in the ligands and points for all ligands, irrespective of ring size and composition, lie roughly on the same log K₂versus pK_a curve. There is no significant systematic steric effect on the equilibrium constants of the sort found for the more basic methyl pyridines. The complexes of the five-membered heterocyclic ligands are approximately ten times less reactive than those of the six-membered N-heterocycles of comparable basicity and exhibit steric retardation from ortho-methyl substituents. The nucleophilicities of these ligands have been calculated and five-membered N,O- and N,S-heterocycles are considerably less reactive than six-membered N-heterocycles of similar basicity.