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

Abstract

The electrochemical behaviour of a family of monomeric copper(II) complexes of the related tetraanionic chelating ligands N,N ′-o-phenylenebis(oxamate) (L¹) and its methylamide (L²) and bis(methylamide) (L³) has been investigated by cyclic voltammetry in acetonitrile at 25 °C and 0.1 mol dm^-3 NEt₄ClO₄ 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)–L¹ 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 d⁹ (Cu^II, square planar) to the low-spin d⁸ (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 [PPh₄][CuL³]·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 Å) typical of trivalent copper.

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