SYNTHESIS, SPECTRA AND ELECTROCHEMICAL-BEHAVIOR OF BIOMIMETIC COPPER(II) COMPLEXES WITH CUN5 AND CUN6 CHROMOPHORES

The Cu(ClO4)2 complexes of the two pentadentate ligands, N-(2-hydroxyethyl)-N,N',N'-tris(benzimidazol-2'-ylmethyl)-1,2-ethanediamine and its 6'-methyl derivative and four sexadentate ligands, N,N,N',N'-tetrakis(benzimidazol-2'-ylmethyl)-1,2-ethanediamine, N,N,N',N'-tetrakis(benzimidazol-2'-ylmethyl)-1,2-cyclohexanediamine and their 6'-methyl derivatives have been isolated and studied. They are of the type CuL(ClO4)2.nH2O where n = 0, 1. All the complexes exhibit two ligand field bands and the nuBAR(max) and epsilon values are lower and higher, respectively, than the range known for CuN4 chromophores. Compared to the latter they also have high g(parallel), low A(parallel) and high g(parallel)/A(parallel) values. These spectral data are consistent with an appreciable displacement and/or tetrahedral twist of copper from the N4 square plane of the complexes. The anchoring of a bulky benzimidazole moiety to N,N'-bis(benzimidazol-2-ylmethyl)-1,2-ethanediamine to incorporate a N-donor axial to the planar CuN4 chromophore decreases CFSE and enhances the Cu(II)-Cu(I) redox potential. However, addition of two benzimidazole nitrogens to the N4 ligand to obtain a CuN6 chromophore, depresses the redox potential. Further, the fusion of a cyclohexyl ring into the above CuN6 chromophore dramatically leads to elevation in E1/2 with increase in reversibility. The introduction of a methyl group in the aromatic ring, far away from the redox centre, depresses E1/2.