Electrochemistry of the copper–nickel series of heteropolymetallic complexes [(µ4-O){NC5H4[C(O)NEt2 ]-3}4Cu4-x {Ni(H2O)}xCl6 ] (x = 0–4)

Abstract

The electrochemistry of the tetranuclear copper–nickel heteropolymetallic complexes [(µ ₄ -O)L ₄ Cu _4-x { Ni(H ₂ O)} _x Cl ₆ ] [x = 0–4, L = N,N-diethylnicotinamide (denc)] were studied at a platinum electrode in dimethyl sulfoxide with 0.20 M tetrabutylammonium hexafluorophosphate as supporting electrolyte. At potentials more cathodic than -1.0 V the complexes are electrodeposited as Cu–Ni alloy and metal oxide films and display a complicated set of cyclic voltammographs. The voltammographs of all the Cu-containing complexes show a quasi-reversible redox couple in the potential range 0.250 to -0.450 V vs. Ag–AgPF ₆ (0.01 M)–CH ₃ CN. As the number of Cu atoms decreases in the complex, the peak currents i _pa and i _pc decrease proportionally and the peak potential shifts anodically. The cyclovoltammetric (CV) results indicate that electron transfer initially occurs only to the Cu ^II centres and that the electron-transfer reaction appears to be quasi-reversible. Using steady-state voltammetry at an ultramicroelectrode in combination with chronoamperometry at a microelectrode and exhaustive electrolysis at a Hg-pool electrode, the number of electrons (n) transferred for this initial reduction of the Cu ₄ , Cu ₃ Ni, Cu ₂ Ni ₂ and CuNi ₃ complexes were 3.1, 2.1, 1.8 and 0.57, respectively. The diffusion coefficient for all the complexes was 2.2(±0.1) × 10 ^- cm ² s ^-1 . The electronic spectrum of the Cu ₄ complex taken after exhaustive electrolysis shows that one quarter of the Cu atoms remain in the Cu ^II form and that the Cu ^I complex remains stable. Since only a single CV peak results for all of the complexes, the electron transfer is most likely consecutive with very closely spaced E° potentials. A model based on statistically determined electron transfer to Cu ^II in particular faces is also proposed.