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

The O-2-evolving complex (OEC) of photosystem II (PSII) carries out the fou r-electron oxidation of water to dioxygen. We had previously reported react ions between manganese complexes containing the ligands dipicolinate (dpa) and 2,2':6,2"-terpyridine (terpy) and oxygen atom-transfer reagents potassi um peroxomonosulfate (oxone), sodium hypochlorite and dimethyldioxirane tha t led to homogeneous catalytic O-2 evolution. The species responsible for c atalysis are the complexes of formula [(H2O)LMn(O)(2)MnL(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 s ource of oxygen atoms in the molecular oxygen evolved, so this complex is a functional model for photosynthetic water oxidation. Here, we present a ki netic analysis of the reaction between [Mn(dpa)(2)](-) and oxone discussing both the mechanism of O-2 evolution and of the side reaction that produces permanganate. The O-2-evolving reaction is first-order in manganese and se cond-order in oxone, with a k(obs) of 37 700 +/- 260 M-2 h(-1). MnO4- forma tion is first order in both manganese and oxone, with a k(obs) of 570 +/- 1 4 M-1 h(-1). All measurements were taken at a pH of 4.5; under this conditi on the rate of MnO4- formation is approximately equal to the rate of O-2 ev olution, preventing the reaction from being truly catalytic at this pH. The kinetics are interpreted in terms of the reaction between a Mn=O-containin g intermediate and oxone as being the key step in O-O bond formation. (C) 2 000 Elsevier Science S.A. All rights reserved.