Biomimetic Oxygen Reduction
by Cofacial Porphyrins
at a Liquid–Liquid Interface
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Abstract
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<sub>2</sub>(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