Model studies of benzimidazole complexes of Cu, Zn, and Cd. Crystal structures of 1,5-bis(benzimidazol-2-yl)-3-thiapentane complexes of CuBr2 and Cu(NO3)2 and nuclear magnetic resonance studies of exchange of 1,6-bis(benzimidazol-2-yl)-2,5-dithiahexane on Cd

Abstract

Some complexes of bridged bis(benzimidazole) ligands of various bridge types with Cu, Zn, and Cd have been examined as potential models of structure and mobility of biological metal binding sites. Two distinct (distorted trigonal bipyramidal and square-based pyramidal verging on distorted octahedral) environments have been found in crystal structures for-copper bromide and nitrate complexes of one ligand, 1,5-bis(benzimidazol-2-yl)-3-thiapentane. N.m.r. studies of the exchange of 1,6-bis(benzimidazol-2-yl)-2,5-dithiahexane [or its ring-methylated analogue 1,6-bis(5′,6′-dimethylbenzimidazolyl)-2,5-dithiahexane] in cadmium chloride mixtures show that it occurs via a rate-determining dissociative step, where there is cadmium salt in excess of a 1 : 1 ratio of M : L, but that bimolecular ligand exchange is rate-determining when the ratio M : L is less than unity. A hypothesis as to the minimum rearrangements necessary to explain the n.m.r. spectra is proposed. The crystal structure of 1,6-bis(1′-octylbenzimidazolyl)-2,5-dithiahexane was obtained. This ligand, lacking prototropy, served to test the role of protons in the hypothesis.