Investigation of Factors Stabilizing Formally Zerovalent Copper Macrocyclic Complexes

Electrochemical reduction of [Cu(Me₄[14]1,4,8,11-tetraeneN₄)(N-Me-imidazole)](PF₆)₂, [Cu(N₄)MeIm]²⁺, has been studied in detail to ascertain the factors favoring stabilization of formally zerovalent copper complexes. At platinum, gold, glassy-carbon, and mercury electrodes, two chemically reversible one-electron-reduction processes are observed in dichloromethane or acetone. In dichloromethane, the reversible half-wave potential for the first reduction process is dependent on the concentration of MeIm, implying that this process corresponds to the reaction [Cu(N₄)MeIm]²⁺+ e^-⇌ [Cu(N₄)]⁺+ MeIm. The second one-electron-reduction step is independent of MeIm concentration and is assumed to correspond to the process [Cu(N₄)]⁺+ e^-⇌ [Cu(N₄)]°. The oxygen-sensitive copper(I) complex [Cu(N₄)]⁺can be identified as the product of either a one-electron controlled-potential reductive electrolysis in dichloromethane or acetone or chemical reduction with cobaltocene. The extremely air- and moisture-sensitive formally copper(0) complex [Cu(N₄)]° can be identified after a two-electron controlled-potential reductive electrolysis of [Cu(N₄)MeIm]²⁺or a one-electron reduction of [Cu(N₄)]⁺at -70 °C in acetone. The rate of decomposition of the zerovalent complex to elemental copper is enhanced by acetonitrile, platinum electrodes, and water. Reaction with oxygen is very rapid, but does not regenerate the starting material. The stabilization of a formally zerovalent copper complex is considered to be favored by factors such as appropriate charge, geometry, nature of the macrocyclic ligand, and correct choice of solvent and electrodes. A reversible one-electron-oxidation process observed at mercury but not at other electrodes is attributed to interaction of MeIm with mercury following dissociation of the macrocyclic complex: 2[Cu(N₄)MeIm]²⁺⇌ 2[Cu(N₄)]²⁺+ 2MeIm; 2MeIm + Hg ⇌ [Hg(MeIm)₂]²⁺+ 2e^-. At platinum,gold, and glassy-carbon electrodes a chemically irreversible oxidation process is observed at different potentials to the oxidation process at mercury electrodes. However, unlike copper macrocyclic systems such as [Cu[14]aneN₄]²⁺([Cu(cyclam)]²⁺), no stable copper(III) complex could be detected under conditions of cyclic voltammetry even with scan rates of 500 mV s^-1. That is, factors that stabilize formally zerovalent complexes appear to destabilize copper(III) complexes.