Biomimetic Oxygen Reduction by Cofacial Porphyrins at a Liquid–Liquid Interface

Oxygen reduction catalyzed by cofacial metalloporphyrins at the 1,2-dichlorobenzene-water interface was studied with two lipophilic electron donors of similar driving force, 1,1'-dimethylferrocene (DMFc) and tetrathiafulvalene (TTF). The reaction produces mainly water and some hydrogen peroxide, but the mediator has a significant effect on the selectivity, as DMFc and the porphyrins themselves catalyze the decomposition and the further reduction of hydrogen peroxide. Density functional theory calculations indicate that the biscobaltporphyrin, 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene, Co(2)(DPX), actually catalyzes oxygen reduction to hydrogen peroxide when oxygen is bound on the "exo" side ("dock-on") of the catalyst, while four-electron reduction takes place with oxygen bound on the "endo" side ("dock-in") of the molecule. These results can be explained by a "dock-on/dock-in" mechanism. The next step for improving bioinspired oxygen reduction catalysts would be blocking the "dock-on" path to achieve selective four-electron reduction of molecular oxygen.