Equilibrium studies of binary and mixed-ligand complexes of zinc(II) involving imidazole, histamine, and L-histidine as ligands

Abstract

Binary and mixed-ligand complexes formed by zinc(II) with imidazole, histamine, and L-histidine ligands (A) have been investigated by potentiometry in aqueous solution at 37 °C and I= 0.15 mol dm^–3(Na[ClO₄]) and the relevant stability constants are evaluated using advanced computer techniques. The results suggest that in the zinc(II)–imidazole, –histamine, and –L-histidine binary systems, the complex formation process is favoured by the π-electron system of the imidazole group in both the 1 :1 and 1 :2 complexes. It appears that the extra proton in the ZnAH histamine complex and ZnA₂H₂ and ZnA₂H L-histidine complexes resides on the primary amino-group of the respective ligands. The mixed-ligand systems, (i) zinc(II)–imidazole (A)–histamine (B), (ii) zinc(II)–imidazole (A)–L-histidine (B), and (iii) zinc(II)–histamine (A)–L-histidine (B) respectively showed the presence of the mixed species, (i) ZnAB and ZnA₂B, (ii) ZnAB, and (iii) ZnAB, ZnABH, and ZnABH₂. Formation of ZnA₂B in system (i) was found to be less favoured. All other mixed species do have marked stabilities. The probable site of protonation in ZnABH and ZnABH₂ species in system (iii) is discussed in terms of their stability constant data.