Physico-chemical properties of metal ion sites in cuproproteins: an investigation of selected copper(II) complexes

Abstract

A systematic investigation of the electronic absorption spectra, the e.s.r. spectra, and the half-wave potentials of a series of copper(II) Schiff base complexes with salicylideneimine and o-aminobenzylideneimine ligands has been completed. Assignments have been made for the electronic spectra of salicylideneimine complexes of flattened tetrahedral coordination geometry and for a number of o-aminobenzylideneimine complexes. Intense absorption in the visible region of the spectrum of the former complexes has been attributed to x→dxy charge transfer. Reduction of tetrabenzo[b,f,j,n][1,5,9,13]tetraazacyclo-hexadecinecopper(II) nitrate with thiol or ascorbic acid produces an intensely blue species exhibiting charge transfer absorption at 14,400 cm-1 (ε ca. 8,000 M-1 cm-1). The e.s.r. parameters were found to be sensitive to changes in the ligand field strength, the covalency of the metal-ligand bonding, and the symmetry of the coordination site. Polarograms were recorded for pyridine solutions of the complexes. The low half-wave potentials are consistent with extensive stabilization of the cupric state relative the consistent with extensive stabilization of the cupric state relative to the cuprous by the Schiff base ligands. The various factors influencing the half-wave potentials are discussed. Three-dimensional X-ray analysis of NM’-1,2-propylenebis (o-aminobenzylideneiminato)copper(II) showed the molecule to adopt an umbrella conformation. Small tetrahedral distortions of the donor atoms are necessary to accommodate the gauche 1,2-propylenediimine ring, the atoms of which appear to be disordered. The origins of the deviations from overall molecular planarity are discussed. The results of these investigations are compared with the physicochemical properties of the metal ion sites in cuproproteins and the various models for the ‘blue’ copper ion are discussed.