Oxygen Evolution Catalysis by a Dimanganese Complex and Its Relation to Photosynthetic Water Oxidation

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Abstract
[Mn2III/IV(μ-O)2(terpy)2(OH2)2](NO3)3 (1, where terpy = 2,2′:6′2′′-terpyridine) acts as a water-oxidation catalyst with HSO5− as the primary oxidant in aqueous solution and, thus, provides a model system for the oxygen-evolving complex of photosystem II (Limburg, J.; et al. J. Am. Chem. Soc. 2001, 123, 423–430). The majority of the starting [Mn2III/IV(μ-O)2]3+ complex is converted to the[Mn2IV/IV(μ-O)2]4+ form (2) during this reaction (Chen, H.; et al. Inorg. Chem. 2007, 46, 34–43). Here, we have used stopped-flow UV–visible spectroscopy to monitor UV–visible absorbance changes accompanying the conversion of 1 to 2 by HSO5−. With excess HSO5−, the rate of absorbance change was found to be first-order in [1] and nearly zero-order in [HSO5−]. At relatively low [HSO5−], the change of absorbance with time is distinctly biphasic. The observed concentration dependences are interpreted in terms of a model involving the two-electron oxidation of 1 by HSO5−, followed by the rapid reaction of the two-electron-oxidized intermediate with another molecule of 1 to give two molecules of 2. In order to rationalize biphasic behavior at low [HSO5−], we propose a difference in reactivity of the [Mn2III/IV(μ-O)2]3+ complex upon binding of HSO5− to the MnIII site as compared to the reactivity upon binding HSO5− to the MnIV site. The kinetic distinctness of the MnIII and MnIV sites allows us to estimate upper limits for the rates of intramolecular electron transfer and terminal ligand exchange between these sites. The proposed mechanism leads to insights on the optimization of 1 as a water-oxidation catalyst. The rates of terminal ligand exchange and electron transfer between oxo-bridged Mn atoms in the oxygen-evolving complex of photosystem II are discussed in light of these results.