Oxygen atom transfer and oxidative water incorporation in cuboidal Mn3MO(n) complexes based on synthetic, isotopic labeling, and computational studies.

The oxygen-evolving complex (OEC) of photosystem II contains a Mn(4)CaO(n) catalytic site, in which reactivity of bridging oxidos is fundamental to OEC function. We synthesized structurally relevant cuboidal Mn(3)MO(n) complexes (M = Mn, Ca, Sc; n = 3,4) to enable mechanistic studies of reactivity and incorporation of μ(3)-oxido moieties. We found that Mn(IV)(3)CaO(4) and Mn(IV)(3)ScO(4) were unreactive toward trimethylphosphine (PMe(3)). In contrast, our Mn(III)(2)Mn(IV)(2)O(4) cubane reacts with this phosphine within minutes to generate a novel Mn(III)(4)O(3) partial cubane plus Me(3)PO. We used quantum mechanics to investigate the reaction paths for oxygen atom transfer to phosphine from Mn(III)(2)Mn(IV)(2)O(4) and Mn(IV)(3)CaO(4). We found that the most favorable reaction path leads to partial detachment of the CH(3)COO(-) ligand, which is energetically feasible only when Mn(III) is present. Experimentally, the lability of metal-bound acetates is greatest for Mn(III)(2)Mn(IV)(2)O(4). These results indicate that even with a strong oxygen atom acceptor, such as PMe(3), the oxygen atom transfer chemistry from Mn(3)MO(4) cubanes is controlled by ligand lability, with the Mn(IV)(3)CaO(4) OEC model being unreactive. The oxidative oxide incorporation into the partial cubane, Mn(III)(4)O(3), was observed experimentally upon treatment with water, base, and oxidizing equivalents. (18)O-labeling experiments provided mechanistic insight into the position of incorporation in the partial cubane structure, consistent with mechanisms involving migration of oxide moieties within the cluster but not consistent with selective incorporation at the site available in the starting species. These results support recent proposals for the mechanism of the OEC, involving oxido migration between distinct positions within the cluster.