Effect of Monoelectronic Oxidation of an Unsymmetrical Phenoxido-Hydroxido Bridged Dicopper(II) Complex

A (μ-hydroxido, μ-phenoxido)Cu^IICu^II complex 1 has been synthesized using an unsymmetrical ligand bearing an N, N-bis(2-pyridyl)methylamine (BPA) moiety coordinating one copper and a dianionic bis-amide moiety coordinating the other copper(II) ion. Electrochemical mono-oxidation of the complex in DMF occurs reversibly at 213 K at E_1/2 = 0.12 V vs Fc⁺/Fc through a metal-centered process. The resulting species (complex 1⁺) is only stable at low temperature and has been spectroscopically characterized by UV-vis-NIR cryo-spectroelectrochemical and EPR methods. DFT and TD-DFT calculations, consistent with experimental data, support the formation of a Cu^IICu^III phenoxido-hydroxido complex. Low-temperature chemical oxidation of 1 by NOSbF₆ yields a tetranuclear complex 2(SbF₆)(NO₂) which displays two binuclear Cu^IICu^II subunits. The X-ray crystal structure of 2(SbF₆)(NO₂) evidences that the nitrogen of the terminal amide group is protonated and the coordination of the amide occurs via the O atom. The bis-amide moiety appears to be a non-innocent proton acceptor along the redox process. Alternatively, protonation of complex 1 leads to the complex 2(ClO₄)₂, as evidenced by X-ray crystallography, cyclic voltammetry, and ¹H NMR.