Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II

摘要

We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation of Fe^III/II3Mn^II–OH2 clusters leads to internal reorganization resulting in formal oxidation at Mn to generate Fe^III/II3Mn^III–OH. ⁵⁷Fe Mӧssbauer spectroscopy reveals that oxidation state changes (three for Fe^III/II3Mn–OH2 and four for Fe^III/II3Mn–OH clusters) occur exclusively at the Fe centers; the Mn center is formally Mn^II when water is bound and Mn^III when hydroxide is bound. Experimentally determined pKa (17.4) of the [Fe^III2Fe^IIMn^II–OH2] cluster and the reduction potentials of the [Fe3Mn–OH2] and [Fe3Mn–OH] clusters were used to analyze the O–H bond dissociation enthalpies (BDEO–H) for multiple cluster oxidation states. BDEO–H increases from 69, to 78, and 85 kcal/mol for the [Fe^IIIFe^II2Mn^II-OH2], [Fe^III₂Fe^IIMn^II-OH₂], and [Fe^III₃Mn^II-OH₂] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe₃Mn–OH_x] system was obtained by constructing a potential–pK_a diagram; the shift in reduction potentials of the [Fe₃Mn–OH_x] clusters in the presence of different bases supports the BDE_O–H values reported for the [Fe₃Mn–OH₂] clusters. A lower limit of the pK_a for the hydroxide ligand of the [Fe₃Mn–OH] clusters was estimated for two oxidation states. These data suggest BDE_O–H values for the [Fe^III₂Fe^IIMn^III–OH] and [Fe^III₃Mn^III–OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe₃Mn=O] in this and related multinuclear systems.