Forum Oxygen Evolution Catalysis by a Dimanganese Complex and Its Relation to Photosynthetic Water Oxidation Ranitendranath Tagore, Robert H. Crabtree,* and Gary W. Brudvig* Department of Chemistry, Yale UniVersity, P.O. Box 208107, New HaVen, Connecticut 06520-8107 Received November 21, 2006 [Mn 2 III/IV (μ-O) 2 (terpy) 2 (OH 2 ) 2 ](NO 3 ) 3 (1, where terpy ) 2,2′:6′2′′-terpyridine) acts as a water-oxidation catalyst with HSO 5 - 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 [Mn 2 III/IV (μ-O) 2 ] 3+ complex is converted to the[Mn 2 IV/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 HSO 5 - . With excess HSO 5 - , the rate of absorbance change was found to be first-order in [1] and nearly zero-order in [HSO 5 - ]. At relatively low [HSO 5 - ], 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 HSO 5 - , 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 [HSO 5 - ], we propose a difference in reactivity of the [Mn 2 III/IV (μ-O) 2 ] 3+ complex upon binding of HSO 5 - to the Mn III site as compared to the reactivity upon binding HSO 5 - to the Mn IV site. The kinetic distinctness of the Mn III and Mn IV 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. Introduction The oxygen-evolving complex (OEC) of photosystem II consists of manganese ions connected by μ-oxo linkages 1 and catalyzes the oxidation of water to molecular oxygen. Synthetic manganese compounds have been studied exten- sively as models of the OEC, with an abundance of structurally relevant complexes. 2 However, functional anal- ogy to the OEC has only been rarely achieved with manganese. Instead, ruthenium has provided the first ex- amples of homogeneous water-oxidation catalysts. The complexes [(bpy) 2 (H 2 O)Ru III ORu III (H 2 O)(bpy) 2 ] 4+ (bpy ) 2,2′-bipyridine) and [(phen) 2 (H 2 O)Ru III ORu III (H 2 O)(phen) 2 ] 4+ (phen ) 1,10-phenanthroline) were reported in 1982 by Meyer et al. to catalyze oxygen evolution via water oxidation in the presence of Ce 4+ . 3,4 These complexes were shown to retain their oxo-bridged structures in the II/II, III/III, III/IV, IV/IV, IV/V, and V/V oxidation states, leading to the existence of extensive redox chemistry. 4–7 Mechanistic studies 5–12 have implicated the V/V oxidation state, formu- lated as [Ru V tO] 2 O, as the oxygen-evolving species. At- * To whom correspondence should be addressed. E-mail: gary.brudvig@ yale.edu (G.W.B.), robert.crabtree@yale.edu (R.H.C.). (1) Kirby, J. A.; Robertson, A. S.; Smith, J. P.; Thompson, A. C.; Cooper, S. R.; Klein, M. P. J. Am. Chem. Soc. 1981, 103, 5529–5537. (2) Mukhopadhyay, S.; Mandal, S. K.; Bhaduri, S.; Armstrong, W. H. Chem. ReV. 2004, 104, 3981–4026. (3) Gersten, S. W.; Samuels, G. J.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 4029–4030. (4) Gilbert, J. A.; Eggleston, D. S.; Murphy, W. R.; Geselowitz, D. A.; Gersten, S. W.; Hodgson, D. J.; Meyer, T. J. J. Am. Chem. Soc. 1985, 107, 3855–3864. (5) Lei, Y.; Hurst, J. K. Inorg. Chim. Acta 1994, 226, 179–185. (6) Lei, Y.; Hurst, J. K. Inorg. Chem. 1994, 33, 4460–4467. (7) Yamada, H.; Hurst, J. K. J. Am. Chem. Soc. 2000, 122, 5303–5311. (8) Binstead, R. A.; Chronister, C. W.; Ni, J.; Hartshorn, C. M.; Meyer, T. J. J. Am. Chem. Soc. 2000, 122, 8464–8473. Inorg. Chem. 2008, 47, 1815-1823 10.1021/ic062218d CCC: $40.75  2008 American Chemical Society Inorganic Chemistry, Vol. 47, No. 6, 2008 1815 Published on Web 03/10/2008