Stabilization of Copper(III) Ions with Deprotonated Hydroxyiminoamide Ligands: Syntheses, Structures, and Electronic Properties of Copper(II) and Copper(III) Complexes

Copper(II) and copper(III) complexes were prepared with two novel ligands, N,Nʹ-bis(2-(1-hydroxyimino-2-methyl-1phenyl)propyl)dimethylmalondiamide (H4mal55) and N,Nʹ bis(-(1-hydroxyiminoethyl)phenyl)dimethylmalondiamide (H4mal66), both of which contain two amide and two oxime functionalities as potential ligand donor groups. The two copper (II) complexes (NEt4)[Cu(Hmal55)] (1) and (NEt4)[Cu(Hmal66)] (2) can be reversibly oxidized in acetonitrile at a redox potential of -0.120 and -0.075 V vs. the Fc/Fc+ redox couple, respectively. While the quantitative electrolysis of 1 results in the preparation of the oxidized complex [Cu(Hmal55)] (3), which is sufficiently stable to be isolated, isolation of the oxidation product of 2 was not attempted because of its long-term instability. The properties of the complexes were investigated by means of various spectroscopic methods (UV-vis, ESR, NMR, and IR spectroscopy) and by X-ray structure analysis. The structure determinations and the spectroscopic investigations of the complexes reveal a square-planar CuN4 coordination environment for each complex in the solid state and in acetonitrile solution. In both the oxidized and reduced oxidation states of the complexes, the coordinated ligands remain triply deprotonated with a hydrogen atom bridging both oxime oxygen atoms. The ligands can therefore be regarded as pseudo-macrocyclic. The characterization of the oxidation products clearly identifies the electron-transfer reaction as being metal-centered. For the first time, the structure of a copper(III) complex with a ligand containing oximes as donor groups was determined. The redox potentials of the copper complexes are compared to related CuIII/CuII redox couples.