Synthesis, spectra and electrochemical behaviour of biomimetic copper(II) complexes with CuN₅ and CuN₆ chromophores

Abstract

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 max and e 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 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 E½ with increase in reversibility. The introduction of a methyl group in the aromatic ring, far away from the redox centre, depresses E½ .