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

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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 AuIII 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 pKa 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.

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