Electrochemistry of the Copper-Nickel Series of Heteropolymetallic Complexes
[(m₄-O){NC₅H₄[C(O)NEt₂]-3}₄Cu_4-x{Ni(H₂O)}_xCl₆ with x=0 to 4

Bizuneh Workie, Christopher E. Dubé ¹, Samuel P. Kounaves*, Levent Aksu ² and Albert Robbat, Jr.
Department of Chemistry, Tufts University, Medford, Massachusetts 02155

Geoffrey Davies
Department of Chemistry, Northeastern University, Boston, Massachusetts 02115

* Corresponding Author
1 Present Address:  Department of Chemistry, Boston College, Boston MA 02167
2 Permanent Address:  Department of Chemistry, Sutcu Imam University, K. Maras (Turkey)

The electrochemistry of the tetranuclear copper-nickel heteropolymetallic complexes (m₄-O)L₄Cu_4-x(Ni(H₂O))_xCl₆ , where x = 0 to 4 and L=N,N-diethylnicotinamide (denc), were studied at a platinum electrode in dimethylsulfoxide 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 voltammograph. The CVs of all the Cu containing complexes show a quasi-reversible redox couple in the potential range 0.250 V to - 0.450 V vs. Ag/0.01 M AgHFP/CH₃CN. 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 CV results indicate that electron transfer initially occurs only to the Cu(II) centers 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 Cu₄, Cu₃Ni, Cu₂Ni₂, and CuNi₃ complexes were 3.1, 2.1, 1.8, and 0.57, respectively. The diffusion coefficient for all the complexes was 2.2(± 0.2)´10^-6 cm²s^-1. The electronic spectrum of the Cu₄ complex taken after exhaustive electrolysis shows that 1/4 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. We also propose a model based on statistically determined electron transfer to Cu(II) in particular faces.