Mechanistic Studies on the Role of [Cu^II(CO₃)_n]²⁻²ⁿ as a Water Oxidation Catalyst: Carbonate as a Non-Innocent Ligand

Recently it was reported that copper bicarbonate/carbonate complexes are good electro-catalysts for water oxidation. However, the results did not enable a decision whether the active oxidant is a Cu^III or a Cu^IV complex. Kinetic analysis of pulse radiolysis measurements coupled with DFT calculations point out that Cu^III(CO₃)_n ³⁻²ⁿ complexes are the active intermediates in the electrolysis of Cu^II(CO₃)_n ²⁻²ⁿ solution. The results enable the evaluation of E°[(Cu^III/II(CO₃)_n)_aq]≈1.42 V versus NHE at pH 8.4. This redox potential is in accord with the electrochemical report. As opposed to literature suggestions for water oxidation, the present results rule out single-electron transfer from Cu^III(CO₃)_n ³⁻²ⁿ to yield hydroxyl radicals. Significant charge transfer from the coordinated carbonate to Cu^III results in the formation of C₂O₆ ²⁻ by means of a second-order reaction of Cu^III(CO₃)_n ³⁻²ⁿ. The results point out that carbonate stabilizes transition-metal cations at high oxidation states, not only as a good sigma donor, but also as a non-innocent ligand.