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)-amin oimidazole-5(4)-carboxamide (AImC), 2,2'-biimidazole (BiIm) (I. Torok et al ., J. Inorg. Biochem. 71 (1998) 7-14), and bis(imidazol-2-yl)methane (BiImM ) (K. Varnagy et al., J. Chem. Sec., Dalton Trans. (1994) 2939-2945) have b een evaluated by using the recently published stability constants and by ap plying the recently established log K-ML(M), versus pK(HL)(H), straight-lin e plots (L.E. Kapinos et al., Inorg. Chim. Acta 280 (1998) 50-56) which hol d for simple imidazole-type ligands. The indicated analysis regarding the i ntramolecular equilibrium between a monodentatally imidazole-nitrogen-coord inated (open) species and a chelated isomer provides various helpful insigh ts, 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 t he series Zn(ImA)(+) (87%) <Ni(ImA)(+) ( 96%) <Cu(ImA)(+) (99.5%). A carbon yl oxygen, if sterically favorably positioned as in the M(AImC)(2+) complex es, may also participate well in chelate formation. In this way a carbonyl group can certainly be activated via metal ion coordination and become read y for further reactions. For Ni2+, Cu2+ and Zn2+ the formation degree of th e chelated M(AImC)(2+) isomers varies between about 30 and 75%. In all of t he so-called 'open' species the metal ion is solely coordinated to a pyridi ne-like nitrogen of the imidazole residue. Some further observations of bio logical interest are indicated. (C) 2000 Elsevier Science Inc. AU rights re served.