Synthesis, spectra and electrochemical behaviour 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 ν max and ϵ values are lower and higher, respectively, than the range known for CuN4 chromophores. Compared to the latter they also have high g∥, low A∥ and high g∥/A∥ 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 CuIICuI 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 E 1 2 with increase in reversibility. The introduction of a methyl group in the aromatic ring, far away from the redox centre, depresses E 1 2 .