Electrocatalytic Water Oxidation by a Trinuclear Copper(II) Complex

We
report a trinuclear copper­(II) complex, [(DAM)­Cu3(μ3-O)]­[Cl]4 (1, DAM = dodecaaza
macrotetracycle), as a homogeneous electrocatalyst for water oxidation
to dioxygen in phosphate-buffered solutions at pH 7.0, 8.1, and 11.5.
Electrocatalytic water oxidation at pH 7 occurs at an overpotential
of 550 mV with a turnover frequency of ∼19 s–1 at 1.5 V vs NHE. Controlled potential electrolysis (CPE) experiments
at pH 11.5 over 3 h at 1.2 V and at pH 8.1 for 40 min at 1.37 V vs
NHE confirm the evolution of dioxygen with Faradaic efficiencies of
81% and 45%, respectively. Rinse tests conducted after CPE studies
provide evidence for the homogeneous nature of the catalysis. The
linear dependence of the current density on the catalyst concentration
indicates a likely first-order dependence on the Cu precatalyst 1, while kinetic isotope studies (H2O versus D2O) point to involvement of a proton in or preceding the rate-determining
step. Rotating ring-disk electrode measurements at pH 8.1 and 11.2
show no evidence of H2O2 formation and support
selectivity to form dioxygen. Freeze-quench electron paramagnetic
resonance studies during electrolysis provide evidence for the formation
of a molecular copper intermediate. Experimental and computational
studies support a key role of the phosphate as an acceptor base. Moreover,
density functional theory calculations highlight the importance of
second-sphere interactions and the role of the nitrogen-based ligands
to facilitate proton transfer processes.