Structural and electronic effects on the polarographic half-wave potentials of copper (II) chelate complexes

Polarographic measurements in dimethylformamide solution have been performed on 37 bis-chelate Cu(II) complexes containing primarily salicylaldimine, geta-ketoamine, beta-iminoamine, and pyrrole-2-aldimine ligand systems. The complexes were selected in order to reveal the effect of stereochemical and donor atom variations on half-wave potentials. Mono- and binuclear complexes undergo one-and two-electron reductions, respectively, which were established for representative cases by controlled potential coulometry. The response of Cu(II)/Cu(I) potentials to these variations reveals that nonplanar bis-chelate complexes are easier to reduce than their planar analogs, rigid planar tetradentate or related planar bis-chelate complexes, and complexes differing only in donor atoms are more readily reduced in the order N4 less than N2O2 less than N2S2. Binuclear complexes are reduced in one two-electron or two resolvable one-electron steps depending on ligand structure. Potentials of these and other complexes previously examined are considered in relation to the markedly positive potentials of "blue" copper proteins and ligand structural features which might afford Cu(II)/Cu(I) potentials in the protein range are noted.