Mechanistic Studies on the Role of [CuII(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 degrees[(Cu-III/II-(CO3)(n))(aq)] approximate to 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 C2O62- 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.