Lewis acidic redox-inactive metal centers have long been known to affect the redox reactivity of transition metal complexes. To further our understanding of the relationship between a cation's Lewis acidity and its effects on redox processes, a systematic study of structurally analogous heteromultimetallic complexes of redox inactive metals (M) displaying a broad range of Lewis acidities is necessary. Inspired by recent reports on [Mn3MOn] complexes indicating a link between Lewis acidity and the tuning of redox potentials, a series of triiron complexes was prepared incorporating a range of redox-inactive metals. Crystallographic characterization revealed a tetrametallic Fe3M (M = Ca, Sr, Zn, La, Sc) bridged by a �4-oxo and a �2-hydroxo ligands. The complexes were isolated as the FeIII3 or the FeIII3FeII species as established crystallographically and via M�ssbauer spectroscopy. In the reduced clusters one of the two Fe centers not bound to the hydroxo moiety displayed an elongated Fe�(�4-O) distance, indicating valence localization. Electrochemical studies revealed that the redox-inactive centers modulate the clusters' reduction potentials (E1/2), which span a window of 0.5 V. The complexes displayed a linear dependence between E1/2 and the pKa of the M-aqua ion, a measure of Lewis acidity, with a slope of ca. 70 mV per pKa unit. The current study of [Fe3MO(OH)] and previous ones of [Mn3MOn] (n = 2, 4) moieties support the generality of the above relationship between the reduction potentials of heterometallic oxido clusters and the Lewis acidity of incorporated cations, as applied to clusters of different redox-active metals. Furthermore, the aforementioned separation between the site of cluster reduction and the (OH) ligand (a possible acid/base functionality) makes these clusters potential targets for the study of the relationship between proton and electron transfers in metal-oxide clusters.

Modulation of redox potential of Fe-<sub>3</sub> oxo/hydroxo clusters by Lewis acids

248th National Meeting (2014)

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Modulation of redox potential of Fe-3 oxo/hydroxo clusters by Lewis acids

Modulation of redox potential of Fe-₃ oxo/hydroxo clusters by Lewis acids