Oxygen Atom Transfer and Oxidative Water Incorporation in Cuboidal Mn_(3)MO_n Complexes Based on Synthetic, Isotopic Labeling, and Computational Studies

Abstract

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.

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