Comparison of the Electrochemical Reduction of Pentadentate Binucleated Nickel(II) and Copper(II) Compounds in Dimethylformamide

A series of binuclear complexes of nickel(II) (Ni₂L) and copper(II) (Cu₂L) (L consists of a deprotonated pentadentate ligand derived from the condensation of acetylacetone or salicylaldehyde with l,3-diaminopropan-2-ol or l,5-diaminopentan-3-ol and an exogenous two-atom bridge between the two metal centers consisting of deprotonated pyrazole and related species) have been prepared and the redox processes in dimethylformamide examined at the mercury and platinum electrodes. For the Ni₂L series, two nearly reversible electrode processes are observed at very negative potentials. Evidence for structurally different forms of the complexes is provided. At low temperatures, in the absence of light and oxygen, controlled-potential electrolysis experiments can generate a nickel(I) species (as indicated by ESR) by a one-electron-reduction step, although more than one species is formed in this experiment. E° values for the two reduction processes, described in eq 1 and 2, are well-separated. By contrast, the Cu₂L complexes are kinetically labile and both [Cu₂L]_- and [Cu₂L]_2- are very unstable. Two one-electron-reduction steps are observed at considerably less negative potentials than for Ni₂L. The data obtained do not fit the model for simple electron transfer and rapid structural rearrangement; the redox cross-reaction.