Mechanistic Studies on the Role of [Cu^II(CO₃)_<i>n</i>]^{2−2<i>n</i>} as a Water Oxidation Catalyst: Carbonate as a Non‐Innocent Ligand

Abstract

Abstract 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 (CO 3 ) n 3−2 n complexes are the active intermediates in the electrolysis of Cu II (CO 3 ) n 2−2 n solution. The results enable the evaluation of E °[(Cu III/II (CO 3 ) 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 (CO 3 ) n 3−2 n to yield hydroxyl radicals. Significant charge transfer from the coordinated carbonate to Cu III results in the formation of C 2 O 6 2− by means of a second‐order reaction of Cu III (CO 3 ) n 3−2 n . 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.