Increased valence theory and the four electron reduction of O2 to H2O

The four electron reduction of oxygen to water, without the intermediate formation of hydrogen peroxide has been reported on dicofacial dicobalt porphyrins and planar dicobaltchelates, containing two CoN4 units in the plane of the molecules. These two classes of compounds display "reversed selectivity" with respect to the solution pH. Dicofacial dicobalt porphyrins directly reduce O2 to H2O in acid solution, and to hydrogen peroxide in alkaline solution. Planar dicobaltchelates show four electron reduction Of O2 in alkaline solution, but yield exclusively hydrogen peroxide at low pH. By using the increased valence theory, an attempt is made to provide a valence bond representation for the electronic reorganization that could be associated with each of these phenomena. For the results obtained in acid solution, this theory indeed offers an excellent description. In alkaline solution, however, application of the increased valence theory alone does not fully account for the experimental observations, unless the following condition is also satisfied. Besides bridge adsorption of oxygen, appropriate values for the redox potentials of the Co centres also seem to be a prerequisite for the occurrence of direct reduction of oxygen to water