Equilibrium and solution structural study of the interaction of tri- and tetra-dentate polyimidazole ligands with transition metal ions

Abstract

Copper(II) and zinc(II) complexes of two polyimidazole derivatives, 4-(imidazol-4-ylmethyl)-2-(imidazol-2-ylmethyl)imidazole (TRIM) and bis[4-(imidazol-4-ylmethyl)-imidazol-2-yl]methane (TIM), containing three and four methylene-linked imidazole rings as donor groups, have been studied by potentiometry, UV–VIS, EPR and NMR spectroscopic methods. The data revealed that both ligands form extremely stable and varied complexes with zinc(II) and copper(II). In equimolar solutions of the metal ions and TRIM, two and three imidazole co-ordinated MAH and MA species were formed. The complex Zn(TRIM) probably has tetrahedral geometry. The formation of bis-complexes has also been detected with the ligand in excess. The data revealed 6N and 5N co-ordinated central ions in ZnA₂ and CuA₂, respectively. In MA complexes of TIM, the ligand is co-ordinated to the metal ions via all the four imidazole units. Formation of bis-complexes has only been found in the zinc(II) containing system. Ternary systems of zinc(II)–TRIM and –TIM have also been studied with L-cysteine as a second ligand. The potentiometric and NMR results established the formation of ternary complexes with different protonation states in relatively high amount, in spite of the high stability of the parent complexes.