Oxygen atom transfer and oxidative water incorporation in cuboidal Mn ₃MO_n complexes based on synthetic, isotopic labeling, and computational studies

The oxygen-evolving complex (OEC) of photosystem II contains a Mn ₄CaO_n catalytic site, in which reactivity of bridging oxidos is fundamental to OEC function. We synthesized structurally relevant cuboidal Mn₃MO_n complexes (M = Mn, Ca, Sc; n = 3,4) to enable mechanistic studies of reactivity and incorporation of μ₃-oxido moieties. We found that Mn^IV ₃CaO₄ and Mn^IV₃ScO₄ were unreactive toward trimethylphosphine (PMe₃). In contrast, our Mn ^III₂Mn^IV₂O₄ cubane reacts with this phosphine within minutes to generate a novel Mn^III ₄O₃ partial cubane plus Me₃PO. We used quantum mechanics to investigate the reaction paths for oxygen atom transfer to phosphine from Mn^III₂Mn^IV₂O ₄ and Mn^IV₃CaO₄. We found that the most favorable reaction path leads to partial detachment of the CH ₃COO^- ligand, which is energetically feasible only when Mn(III) is present. Experimentally, the lability of metal-bound acetates is greatest for Mn^III₂Mn^IV₂O ₄. These results indicate that even with a strong oxygen atom acceptor, such as PMe₃, the oxygen atom transfer chemistry from Mn₃MO₄ cubanes is controlled by ligand lability, with the Mn^IV₃CaO₄ OEC model being unreactive. The oxidative oxide incorporation into the partial cubane, Mn^III ₄O₃, was observed experimentally upon treatment with water, base, and oxidizing equivalents. ¹⁸O-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.