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 M-aqua complex as a measure of Lewis acidity, these compounds display a linear dependence between E1/2 and acidity, with a slope of ∼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.

Figure 1

Truncated solid-state structures of (a) 1-Ca, (b) 1-Sr, (c) 2-Sc, (d) 1-Zn, (e) 2-Ca and (f) 2-La. Portions of the ligand (L), hydrogen atoms and outer-sphere anions are omitted for clarity. Thicker lines emphasize the [MFe₃O₂] moiety.

Figure 2

Zero-field ⁵⁷Fe Mössbauer spectra for 1-Ca and 2-Ca: (80 K, data: black dots, spectral fit: green line, deconvolution: red and blue lines; residual: grey dots.

Figure 3

Cyclic voltammograms corresponding to the [MFe^III₃O(OH)]/[MFe^III₂Fe^IIO(OH)] redox couple (M = Sc³⁺, La³⁺, Zn²⁺, Ca²⁺, and Sr²⁺) in 0.1 M NBu₄PF₆ in CH₂Cl₂/1,2-DME (9:1). Scan rate of 200 mV/s. Potentials are referenced to Fc/Fc⁺.

Figure 4

Reduction potentials of MFe₃O(OH) complexes (green circles), MMn₃O₂ complexes (blue diamonds), and MMn₃O₄ complexes (red squares) vs. pK_a of the corresponding M(aqua)ⁿ⁺ ion as a measure of Lewis acidity. Potentials were referenced to Fc/Fc⁺.

Scheme 1

Synthesis of complexes 1-M (M = Ca, Sr, Zn), 2-M (M = Ca, Sc, La).