Electrochemical study of a trinuclear copper(II) system: the Cu^II₂Cu^II-Cu^II₃ couple and its inhibition by proton

Abstract

An electrochemical study of a group of spinwiloublet (S=1/2) trinuclear copper(II) complexes. (Cu₃Op(OH)_1-p (ligand)₃](CIO₄)_2-p(0≤p≤1; ligand = RNC(R')C(R')NO-; R = Et, n-Pr, n-Bu, Ph; R'=Me. Ph), is reported. In general, the complexes as isolated contain both Cu3O and Cu₃OH cores held by three peripheral oximato bridges of the ligand Cyclic voltammetry (both dc and ac) and constant potential coulometry establish that only the Cua core displays the novel one-electron-transfer process Cu^IIICu^II₂+e-⇌Cu^II₃ with E°₂₉₈ in the range 03-0.4 V vs. SCE in several solvents at a platinum working electrode. The rare Cu(III)-Cu(II) mixed-valence species thus discovered have an intense electronic absorption band at ∼830 nm. The heterogeneous rate constants of electron transfer are reported. Base (NEt₃) converts Cu₃OH to a Cu₃O species in an equilibrium deprotonation reaction. On the other hand an acid (HClO₄) brings about the reverse conversion, Cu₃O→Cu₃OH. The influence of these reactions on the electrochemical response is analyzed. The identification of a second couple Cu^II₃+e-⇌Cu^II₂Cu^I (E°₂₉₈'∼-0.4 V vs. SCE) at a mercury working electrode in which only the Cu₃OH core can participate is briefly reported. The symbiotic relationship between electron transfer and proton transfer is noted.