We are currently seeking models for the various oxidation levels (S, states; n = -1 through 4) of the photosynthetic water oxidation center (WOC), to assist in the elucidation of its nature and mode of action.1'l The WOC requires four Mn atoms for activity, and these are 0x0-bridged and ligated by 0and perhaps N-based groups, primarily of aspartate and glutamate carboxylates in the protein matrix; recent reports suggest the nitrogen ligation to be minimal, at the level of 1 +_ 1 per four Mn centers. The WOC also requires CI' for activity, and it has been suggested, for example, that the CIo is a ligand of the Mn atoms, or that it is involved in acid/base chemistry at the polypeptide^.['-^' The CIQ is an essential cofactor for the WOC; in the absence of Cl0, an abnormal S, state is formed and advancement to S, is b l ~c k e d . ~~' . ~~ Whatever its precise role, CIQ thus seems to be important at the higher S, states.
We recently described the preparation of various Mn complexes that were proposed as potential models of certain S, states.[5. The complexes [Mn,O,(O,CR),(bpy),J and [Mn,O,(O,CR),(bpy),]o~ @ (bpy = 2,2'-bipyridine), whose [Mn,O,Jz@ core contains a Mn, unit that is either planar or nonplanar ("butterfly" structure), were proposed as models for So and Sl.t5371 In addition, the complexes [Mn4O,C1,(OAc),(HIm)J~~, (HIm = imidazole) and [Mn40,C1,(OAc),(py),], which contain three Mn"' and one Mn'" in a distorted Mn,O,CI cubane core, were proposed as