Evaluation of intramolecular equilibria in complexes formed between substituted imidazole ligands and nickel (II), copper (II) or zinc (II)

The metal ion-binding properties of imidazole-4-acetate (ImA-), 4(5)-aminoimidazole-5(4)-carboxamide (AImC), 2,2'biimidazole(BiIm) (I. Török et al., J. Inorg. Biochem. 71 (1998) 7-14), and bis (imidazol-2-yl)methane(BiImM) (K. Várnagy et al., J. Chem. Soc., Dalton Trans. (1994) 2939-2945) have been evaluated by using the recently published stability constants and by applying the recently established log K(ML)M versus pK(HL)H straight-line plots (L. E. Kapinos et al., Inorg. Chim. Acta 280 (1998) 50-56) which hold for simple imidazole-type ligands. The indicated analysis regarding the intramolecular equilibrium between a monodentatally imidazole-nitrogen-coordinated (open) species and a chelated isomer provides helpful insights, e.g., the formation degree of chelates is more favored if six-membered rings can be formed, as in the case with M(BiImM)2+ compared to M(BiIm)2+, though in both instances the formation degree of the chelates is large. The formation degree of chelates in the M(ImA)+ complexes increases in the series Zn(ImA)+ (87%)