Reactions of the polynuclear chain complex [Cu(L)(H 2O)] n (H 2L is 5-[(2′-carboxyphenyl)azo]-1,3-dimethylbarbituric acid) with imidazole. Crystal structures of the dinuclear and mononuclear derivatives and EPR study of the dimer-monomer equilibrium in solution

Abstract

On treating the polynuclear chain complex [Cu(L)(H 2O)] n (H 2L is 5-[(2′-carboxyphenyl)azo]-1, 3-dimethylbarbituric acid) with imidazole the complexes [Cu(L)(Him)] 2 ( 1) and [Cu(L)(Him) 2] ( 2) are obtained. The structures of 1 and 2 have been determined by X-ray crystallographic methods. The structure of 1 consists of dinuclear units in which the copper ions are bridged by the barbituric ring. It crystallizes in the monoclinic space group P2 1/ c with cell constants a = 8.654(2), b = 22.64(2), c = 8.8048(8) Å, β = 103.76(1)°, Z = 2, R = 0.051 and R w = 0.041. The structure of 2 consists of discrete [Cu(L)(Him) 2] molecules held together by intermolecular hydrogen bonds to form infinite chains. It crystallizes in the triclinic space group P 1 with cell constants a 8.268(2), b = 9.412(3), c = 13.798(5) Å, α = 99.07(3), β = 91.20(3), γ = 108.64(2)°, Z = 2, R = 0.041 and R w = 0.035. Comparison of the crystal structure of 1 with analogous complexes containing nitrogen heterocyclic ligands reveals that the replacement of imidazole by pyrazole or pyridine leads to very different crystal structures, which has been attributed to the different nature of the nitrogen heterocyclic ligand used in their syntheses. In dmso solution, EPR spectra of both complexes clearly indicate the existence of a monomer-dimer equilibrium.