Model complexes for the carboxylate–histidine–metal triad systems in metalloenzymes. Synthesis, crystal structures and spectroscopic properties of [M(Him)2(O2CMe)2](M = ZnII or CoII, Him = imidazole)

Abstract

Two monomeric complexes [M(Him)₂(O₂CMe)₂](M = Zn^II1 or Co^II2, Him = imidazole) have been synthesized and structurally characterised by X-ray analysis. The complexes are isostructural. In each structure the metal atom is co-ordinated by a pair of acetate groups and a pair of Him ligands in a distorted-tetrahedral N₂O₂ environment with M–O 1.965(3)–1.991(2) and 1.972(4)–2.013(4)Å, and M–N 1.996(2)–2.005(2) and 2.020(4)–2.030(5)Å for 1 and 2, respectively. The solid-state structures of both complexes involve intermolecular N–H ⋯ O hydrogen bonds between the non-co-ordinated Him nitrogen atoms and the acetate oxygen atoms, with the acetate groups acting in both syn and anti modes, resulting in two types of carboxylate–imidazole–metal systems analogoues to those found for metalloenzymes. The IR, Raman and ¹³C NMR spectra of the complexes have been recorded and discussed in relation to the crystal structures.