Heterometallic Triiron-Oxo/Hydroxo Clusters: Effect of Redox-Inactive Metals

Abstract

A series of tetranuclear oxo/hydroxo clusters comprised of three Fe centers and a redox-inactive metal (M) of various charge is reported. Crystallographic studies show an unprecedented Fe3M(μ4-O)(μ2-OH) core that remains intact upon changing M or the oxidation state of iron. Electrochemical studies reveal that the reduction potentials (E1/2) span a window of 500 mV and depend upon the Lewis acidity of M. Using the pKa of the redox-inactive metal aqua complex as a measure of Lewis acidity, these compounds display a linear dependence between E1/2 and 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.

Trivalent Lewis Acidic Cations Govern the Electronic Properties and Stability of Heterobimetallic Complexes of Nickel

“…While the electronic structure of the metal complexes is most strongly perturbed by the trivalent cations, the effects engendered by these metals are in line with those observed for divalento r monovalenti ons. This is in agreement with work examining the effect of redox-inactive cations on the reduction potentials of manganese [23] or iron [24] oxo clusters. The chemical shift for the signal corresponding to the imine proton in the 1 HNMR spectra for the bimetallic complexes provides au seful readout of the effects of incorporation of Lewis acids.…”

“…As in the spectroscopic data (vide supra), the trivalent and highly Lewis acidic cations resulted in the most pronounceds hift of E p,c .A lthough the Lewis acidity of yttrium is greater than that of neodymium (pK a = 8.3 vs. 9.0), the reduction peak potential measured for L-Ni-Nd is more positive than that of L-Ni-Y.This can be ascribed to uncertainties in the electrochemical data, as interpretations regarding peak potentials for irreversible processes can be complicated by secondary effects. Similar uncertainty was encountered by Agapie and co-workers, [23,24] and we have quantified our error here as 70 AE 12 mV/pK a which correspondst oAE 17 %e rror.T hus, there is ac lear and uniformt rend of modulated reductionp otential upon coordination of Lewis acidic metalst oL-Ni,e ven for the unusualtrivalent analogues that are the focus of this study.…”

“…In these studies we elected to use the pK a of the redox-inactive metal-aquac omplex as am easure of Lewis acidity, [38] an approach that has successfully been employed in severall iterature reports. [23,24] Specifically,w ec hose sodium (pK a = 14.7), calcium (12.7),n eodymium (9.0), and yttrium (8.3) as redox-inactivem etals for the presentw ork. Yttrium and neodymium are especially attractive, as they are relevant to enhancements in oxidative reactivity with iron compounds (vide supra).…”

“…Similar resultsh ave been demonstrated for iron analogues of these compounds, with am easured dependence on the Lewis acidity of the redox-active metal center of 70 mV per pK a unit. [24] The attenuated coupling in these two cases can be ascribed to differences between iron and manganese,a lthough exact comparisons are difficult.…”

“…Notably,t he Lewis acidities of Y 3 + and other trivalent ions are significantly greatert han other ions such as Ca 2 + or Sr 2 + ;c onsequently, there is ap rivileged position for Y 3 + and Sc 3 + in modulating the reactivityo ft he iron complexes described above. [19] However,d espite the relevance of these heterometallic effects to possible applicationsi nc atalysis [2] and energy science, [22] chemical preparation of compounds that contain trivalent Lewis-acidic ions with at ransition metal has received less attentiont han it deserves.T sui et al and Herbert et al have quantified Lewis acidity effects in multimetallic manganese [23] and iron [24] oxo-bridged clusters, respectively.T hey find ad istinctive shift in reduction potential of their clusters as af unction of the Lewis acidity of ab ound redox-inactivem etal,i ncluding the trivalent rare-earth cations Y 3 + and Nd 3 + . One possible strategy for preparation of heterobimetallic complexes involves synthesis of ditopic frameworks with binding motifs that display distinct ligand properties and that can therefore facilitate orthogonal metallations.…”