Bonding properties and electronic structures of glycylglycinato copper(II) complexes. Molecular structure of acetylhistaminegly-cylglycinatocopper(II) dihydrate

Abstract

Mixed ligand diglycinatocopper(II) complexes of the Cu(glygly)L·nH2O type, where glygly stands for [NH2-CH2 CONCH2CO2]2− and L for imidazole (n = 1.5), N-methylimidazole (n = 1), 2-methylimidazole (n = 2), 4-methylimidazole (n = 2), 4-phenylimidazole (n = 2), N-acetylhistamine (n = 2) and NH3 (n = 2), were prepared and characterized by elemental analyses, i.r., vis. and e.p.r. spectroscopic measurements. The molecular structure of [Cu(glygly)(achmH)]·2H2O (achmH = acetylhistamine) was determined using three dimensional XRD data. The structure consists of distorted square planar [Cu(glygly)-(achmH)] units interconnected via the peptide oxygen at the apex to complete a square pyramidal structure, Cu—O-(peptide) 2.477(2) A. The H2O molecules, not binding directly to the copper ion, involve in intermolecular hydrogen bonding with the copper units. The dianionic glygly ligand and the imidazole ring bind strongly to the central copper ion with Cu—N(amino) 2.045(6) A, Cu—N-(peptide) 1.891(5) A, Cu—O(carboxylate) 2.001(4) A and Cu—N(imidazole) 1.956(5) A. The dihedral angle between the imidazole nucleus and the CuN3O xy plane is 6.0°. Similar structures with a CuN3O coordination plane are proposed for the imidazole complexes, based on spectroscopic data. The bonding properties of the glygly ligand and the unidentate imidazole ligands are elucidated and discussed with reference to the electronic structures of the complexes deduced from Gaussian analyses.