Triplet Oxygen Evolution Catalyzed by a Biomimetic Oxomanganese Complex: Functional Role of the Carboxylate Buffer

Abstract

Photosynthetic oxygen evolution involves water splitting into triplet oxygen, protons, and electrons, as follows: 2H2O → 3O2 + 4e–+ 4H+. The reaction is catalyzed by the oxomanganese complex of photosystem II, embedded in the thylakoid membrane of green plant chloroplasts and internal membranes of cyanobacteria. Biomimetic synthetic complexes have been developed over the years, although the reactivity of most of these complexes remains to be established. Here, we report a computational study of water splitting catalyzed by the mixed-valent oxomanganese dimer [H2O(terpy)MnIII(μ-oxo)2MnIV(terpy)OH2]3+ (terpy = 2,2′:6′,2″-terpyridine) in acetate buffer, with emphasis on the origin of triplet oxygen and the noninnocent role of carboxylate ligands in the underlying reaction mechanism. Our calculations suggest triplet oxygen evolution from an end-on (η1) Mn(III)-superoxo species, which forms from a hydroperoxo intermediate generated by nucleophilic attack of substrate water onto an oxyl radical Mn(IV)–O•. Carboxylate groups of acetate facilitate formation of the oxyl radical by shifting the redox potential of the Mn complex upon exchange with water ligands, and catalyze the O–O bond formation by deprotonating the nucleophilic water molecule. These findings provide valuable insights on the origin of triplet oxygen and on the regulatory role of the environment surrounding the inorganic core of oxomanganese complexes during catalytic oxygen evolution.