Model complexes for the carboxylate-histidine-metal triad systems in metalloenzymes. Synthesis, crystal structures and spectroscopic properties of [M(Him)(2)(O(2)CMe)(2)] (M=Zn-II or Co-II, Him equals imidazole)

Two monomeric complexes [M(Him)(2)(O(2)CMe)(2)] (M = Zn-II 1 or Co-II 2, 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) Angstrom, and M-N 1.996(2)-2.005(2) and 2.020(4)-2.030(5) Angstrom 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 analogous to those found for metalloenzymes, The IR, Raman and C-13 NMR spectra of the complexes have been recorded and discussed in relation to the crystal structures.