Kinetic analysis of the O2-forming reaction between [Mn(III)(dpa)2]− (dpa=dipicolinate) and potassium peroxomonosulfate

The O2-evolving complex (OEC) of photosystem II (PSII) carries out the four-electron oxidation of water to dioxygen. We had previously reported reactions between manganese complexes containing the ligands dipicolinate (dpa) and 2,2′:6,2′′-terpyridine (terpy) and oxygen atom-transfer reagents potassium peroxomonosulfate (oxone), sodium hypochlorite and dimethyldioxirane that led to homogeneous catalytic O2 evolution. The species responsible for catalysis are the complexes of formula [(H2O)LMn(O)2MnL(OH2)]n±, where L=dpa (n=−1) or L=terpy (n=+3). In the case of the reaction of the terpy complex and hypochlorite, isotope studies showed that water is the source of oxygen atoms in the molecular oxygen evolved, so this complex is a functional model for photosynthetic water oxidation. Here, we present a kinetic analysis of the reaction between [Mn(dpa)2]− and oxone discussing both the mechanism of O2 evolution and of the side reaction that produces permanganate. The O2-evolving reaction is first-order in manganese and second-order in oxone, with a kobs of 37 700±260 M−2 h−1. MnO4 − formation is first order in both manganese and oxone, with a kobs of 570±14 M−1 h−1. All measurements were taken at a pH of 4.5; under this condition the rate of MnO4 − formation is approximately equal to the rate of O2 evolution, preventing the reaction from being truly catalytic at this pH. The kinetics are interpreted in terms of the reaction between a MnO-containing intermediate and oxone as being the key step in OO bond formation.