Stabilization of copper(III) complexes by disubstituted oxamides and related ligands

The electrochemical behaviour of a family of monomeric copper(II) complexes of the related tetraanionic chelating ligands N,N'-o-phenylenebis(oxamate) (L(1)) and its methylamide (L(2)) and bis(methylamide) (LS) has been investigated by cyclic voltammetry in acetonitrile at 25 degrees C and 0.1 mol dm(-3) NEt(4)ClO(4) as supporting electrolyte. The copper(III)-copper(II) reduction potentials have been found to span a potential range from +0.41 to -0.02 V (vs. saturated calomel electrode), being reversible for all cases except the copper(II)-L(1) complex. The trend in formal potentials along this series is explained in terms of the stronger donor properties of the deprotonated-amido nitrogen atoms than those of the carboxylate oxygen ones. Hence, the stabilization of the trivalent oxidation state of copper is attributed to the increasing number of deprotonated-amido donor groups. A perfect correlation has been observed within this family between the Cu-III-Cu-II potentials and the visible absorption maxima of the copper(II) complexes. The relative gain in crystal-field stabilization energy for the change from the d(9) (Cu-II, square planar) to the low-spin d(8) (Cu-III, square-planar) electronic configuration is the main factor in the overall thermodynamic stability of the copper((III)) complexes. The molecular structure of the stable copper(III) complex [PPh(4)l[CuL(3)]. MeCN has been determined by single-crystal X-ray analysis. The metal is in a nearly square-planar environment formed by the four amido nitrogen atoms of the chelating ligand, with short Cu-N bond distances (1.84-1.88 Angstrom) typical of trivalent copper.