Mixed-ligand complexes of cuII-1, 10-o-phenanthroline and its analogues characterized by computer-aided electron spin resonance spectroscopy

Abstract

An extensive reinvestigation based on computer-aided ESR spectroscopy, for Cu-1,10-o-phenanthroline (CuOP) and its analogue Cu-2,9-dimethyl-1,10-o-phenanthroline (CuDMP) has been performed in the liquid and solid phases under a large range of pH, temperature and mole ratio conditions. Equilibria involved in solution and coordination behaviour as a function of pH have been characterized by a careful analysis of magnetic ESR parameters. Selective isotopic enrichment and computer-aided ESR spectroscopy have been used for precise determination of spin-Hamiltonian parameters and correlation times. CuOP and its analogue CuDMP have been reported to posses very different chemical and biological characteristics despite being conformationally homologous. The 2,9-DMP form possesses a marked specificity and stereochemistry that confers upon the bidentate cupric chelate potent oxidative properties. This could be attributed to the steric hindrance to normal planar configuration caused by the presence of methyl groups close to the metal coordination site, a feature not shared by 1,10-phenanthroline. A structure–activity relationship is proposed on the basis of difference in magnetic parameters and consequent evaluation of molecular orbital coefficients K and α² under the assumption of an effective D_4h tetragonally distorted octahedral symmetry. Our results are in favour of two distinct structures for OP and 2,9-DMP copper(II) complexes in solution: a tetragonal structure probably with rhombic distortion for OP and a flat tetrahedral structure for DMP. The presence of one or two ligands coordinating the metal ion does not seem to play a determining role in the structural arrangement. On the contrary the presence of substituent methyl groups in positions 2- and 9- is fundamental for explaining the striking differences shown by the analogue compounds and can reconcile the manifestation of unexpected properties.