Abstract
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)(3)Mn(II)-OH(2) clusters leads to internal reorganization resulting in formal oxidation at Mn to generate Fe(III/II)(3)Mn(III)-OH. (57)Fe Mössbauer spectroscopy reveals that oxidation state changes (three for Fe(III/II)(3)Mn-OH(2) and four for Fe(III/II)(3)Mn-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 p K(a) (17.4) of the [Fe(III)(2)Fe(II)Mn(II)-OH(2)] cluster and the reduction potentials of the [Fe(3)Mn-OH(2)] and [Fe(3)Mn-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)(2)Mn(II)-OH(2)], [Fe(III)(2)Fe(II)Mn(II)-OH(2)], and [Fe(III)(3)Mn(II)-OH(2)] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe(3)Mn-OH (x)] system was obtained by constructing a potential-p K(a) diagram; the shift in reduction potentials of the [Fe(3)Mn-OH (x)] clusters in the presence of different bases supports the BDE(O-H) values reported for the [Fe(3)Mn-OH(2)] clusters. A lower limit of the p K(a) for the hydroxide ligand of the [Fe(3)Mn-OH] clusters was estimated for two oxidation states. These data suggest BDE(O-H) values for the [Fe(III)(2)Fe(II)Mn(III)-OH] and [Fe(III)(3)Mn(III)-OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe(3)Mn═O] in this and related multinuclear systems.