Stabilization of copper(III) complexes by substituted oxamate ligands

Abstract

A new series of monomeric copper(II) complexes of the related substituted oxamate ligands N,N′-naphthalene-1,8-diylbis(oxamate) (L²) and N,N′-trimethylenebis(oxamate) (L³) have been synthesized. The molecular structures of [NBu₄]₂[CuL²] and [PPh₄]₂[CuL³]·2H₂O have been determined by single-crystal X-ray analysis. The structure of the previously reported complex [PPh₄]₂[CuL¹], where L¹ is the parent o-phenylenebis(oxamate), has been also determined. These are mononuclear four-co-ordinate copper(II) complexes with the metal center in a more or less distorted square-planar environment formed by the two amido nitrogen and two carboxylate oxygen atoms from the two oxamato groups of each tetradentate chelating ligand. The bond lengths at the metal atom are similar for all three complexes, the Cu–N bond distances (1.89–1.93 Å) being shorter than the Cu–O ones (1.93–1.97 Å). The bond angles around the metal are different from one complex to the other. They are closer to 90°, corresponding to the ideal square-planar geometry, for the copper(II)–L² and –L³ complexes as a result of the alternating 5-6-5-membered chelate ring system afforded by L² and L³, respectively. The values of the Cu^III–Cu^II redox potential in acetonitrile for this family of complexes range from 0.41 to 0.27 V (vs. saturated calomel electrode, 25 °C and 0.1 mol dm^–3 NEt₄ClO₄ as supporting electrolyte), the redox process being only reversible for the copper(II)–L³ species. The stabilization of the trivalent oxidation state of copper in this complex is attributed to the stronger basicity of the aliphatic amido nitrogens with respect to that of the aromatic amido ones. The trend in formal potential along this series is mainly controlled by the size of the chelate rings around the metal ion.