Speciation and transformation of Co(II)/Ni(II)–citrate–imidazole ternary system – synthesis, spectroscopic and structural studies

Abstract

The cobalt and nickel imidazole citrate complexes [Co(Im) 6][Co Δ(Im) 3(Hcit)] [Co Λ(Im) 3(Hcit)] · 4H 2O ( 1) and [Ni(Im) 6][Ni Δ(Im) 3(Hcit)] [Ni Λ(Im) 3(Hcit)] · 4H 2O ( 2) (Im=imidazole, H 4cit=citric acid) were synthesized in aqueous solutions, which were isolated in pure crystalline forms and characterized spectroscopically. The X-ray structural analyses revealed that both compounds are isomorphous and consist of two types of metal ions in different coordination modes. One is octahedrally bound by six imidazole ligands, and the other is quasi-octahedrally coordinated by three imidazole and one citrate ligands, in which the citrate ion coordinates to the metal ions tridentately through α-hydroxyl, α-carboxyl and one of the β-carboxyl groups, leaving the other deprotonated β-carboxyl group free, which form intramolecular strong hydrogen bond with α-hydroxyl group. The co-existences of [M(Im) 6] 2+ and [M(Im) 3(Hcit)] − in the double salts ( 1) and ( 2) (M=Co 2+, Ni 2+) show an equilibrium between imidazole and imidazole–citrate species, which is further supported by the conversions of imidazole or citrate complexes to ( 1) and ( 2). The formations of the mixed ligand complexes present a case that the Co 2+ and Ni 2+ might interact competitively with monodentate imidazole and tridentate citrate ligands.