B. Workie et al., ELECTROCHEMISTRY OF THE COPPER-NICKEL SERIES OF HETEROPOLYMETALLIC COMPLEXES (4)-O)(NC5H4[C(O)NET2]-3)(4)CU4-X(NI(H2O))(X)CL-6] (X=0-4), Journal of the Chemical Society. Dalton transactions, (10), 1997, pp. 1739-1745
The electrochemistry of the tetranuclear copper-nickel heteropolymetal
lic complexes [(mu(4)-O)L4Cu4-x{Ni-(H2O)}xCl(6)][x = 0-4, L = N,N-diet
hylnicotinamide (denc)] were studied at a platinum electrode in dimeth
yl sulfoxide with 0.20 M tetrabutylammonium hexafluorophosphate as sup
porting electrolyte. At potentials more cathodic than -1.0 V the compl
exes are electrodeposited as Cu-Ni alloy and metal oxide films and dis
play 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-AgPF6 (0.01 M)-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 cyclovoltammetric (CV) results indicate that electron
transfer initially occurs only to the Cu-II centres and that the elect
ron-transfer reaction appears to be quasi-reversible. Using steady-sta
te voltammetry at an ultramicroelectrode in combination with chronoamp
erometry at a microelectrode and exhaustive electrolysis at a Hg-pool
electrode, the number of electrons (n) transferred for this initial re
duction of the Cu-4, Cu3Ni, Cu2Ni2 and CuNi3 complexes were 3.1, 2.1,
1.8 and 0.57, respectively. The diffusion coefficient for all the comp
lexes was 2.2(+/-0.1) x 10(-6) cm(2) s(-1). The electronic spectrum of
the Cu-4 complex taken after exhaustive electrolysis shows that one q
uarter of the Cu atoms remain in the Cu-II form and that the Cu-I comp
lex 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 degrees potentials. A model based on statistically d
etermined electron transfer to Cu-II in particular faces is also propo
sed.