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

Abstract

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 [Mn_3MO_n] complexes indicating a link between Lewis acidity and the tuning of redox potentials, a series of triiron complexes was prepd. incorporating a range of redox-inactive metals. Crystallog. characterization revealed a tetrametallic Fe_3M (M = Ca, Sr, Zn, La, Sc) bridged by a μ_4-oxo and a μ_2-hydroxo ligands. The complexes were isolated as the Fe^(III)3 or the Fe^(III)_3Fe^(II) species as established crystallog. and via Mossbauer spectroscopy. In the reduced clusters one of the two Fe centers not bound to the hydroxo moiety displayed an elongated Fe-(μ_4-O) distance, indicating valence localization. Electrochem. studies revealed that the redox-inactive centers modulate the clusters' redn. potentials (E_(1/2)), which span a window of 0.5 V. The complexes displayed a linear dependence between E_(1/2) 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 [Fe_3MO(OH)] and previous ones of [Mn_3MO_n] (n = 2, 4) moieties support the generality of the above relationship between the redn. potentials of heterometallic oxido clusters and the Lewis acidity of incorporated cations, as applied to clusters of different redox-active metals. Furthermore, the aforementioned sepn. between the site of cluster redn. 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.