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 CuIII or a CuIV complex. Kinetic analysis of pulse radiolysis measurements coupled with DFT calculations point out that CuIII (CO3 )n3-2n complexes are the active intermediates in the electrolysis of CuII (CO3 )n2-2n solution. The results enable the evaluation of E°[(CuIII/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 CuIII (CO3 )n3-2n to yield hydroxyl radicals. Significant charge transfer from the coordinated carbonate to CuIII results in the formation of C2 O62- by means of a second-order reaction of CuIII (CO3 )n3-2n . 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.