Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH_2/OH Motifs Relevant to H_2O Activation by the Oxygen Evolving Complex in Photosystem II
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/II)_3Mn^(II)–OH_2 clusters leads to internal reorganization resulting in formal oxidation at Mn to generate Fe^(III/II)_3Mn^(III)–OH. ^(57)Fe Mössbauer spectroscopy reveals that oxidation state changes (three for Fe^(III/II)_3Mn–OH_2 and four for Fe^(III/II)_3Mn–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 pK_a (17.4) of the [Fe^(III)_2Fe^(II)Mn^(II)–OH_2] cluster and the reduction potentials of the [Fe_3Mn–OH_2] and [Fe_3Mn–OH] clusters were used to analyze the O–H bond dissociation enthalpies (BDE_(O–H)) for multiple cluster oxidation states. BDE_(O–H) increases from 69 to 78 and 85 kcal/mol for the [Fe^(III)Fe^(II)_2Mn^(II)–OH_2], [Fe^(III)_2Fe^(II)Mn^(II)–OH_2], and [Fe^(III)_3Mn^(II)–OH_2] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe_3Mn–OH_x] system was obtained by constructing a potential–pK_a diagram; the shift in reduction potentials of the [Fe_3Mn–OH_x] clusters in the presence of different bases supports the BDE_(O–H) values reported for the [Fe_3Mn–OH_2] clusters. A lower limit of the pK_a for the hydroxide ligand of the [Fe_3Mn–OH] clusters was estimated for two oxidation states. These data suggest BDE_(O–H) values for the [Fe^(III)_2Fe^(II)Mn^(III)–OH] and [Fe^(III)_3Mn^(III)–OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe_3Mn═O] in this and related multinuclear systems
