Electron transfer in a trinuclear oxo-centred mixed-valence iron complex, in solid and solution states

Complexes [Fe^III₂M^IIOL₃] (M = Fe, Co, Ni, Cu; 2, 3, 4, 5) have been synthesised in which L^2- is a pentadentate ligand designed to coordinate all three metal atoms in the central cluster and to inhibit dissociation and solvent exchange processes. Crystal structures for 2, 4 and 5 show threefold symmetry, attributed to rotational disorder. Magnetisation data for 2 indicate strong superexchange between basis oxidation states Fe(3+, 3+, 2+). Comparisons of IR spectra across the series of complexes confirm the non-threefold symmetry of the mixed-valence cluster on the vibrational time scale, both in the solid state and in solution. Proton NMR spectra in solution at room temperature do not distinguish the three iron sites, suggesting that pseudo-rotation by thermal electron transfer also operates. Cyclic voltammetry and spectroelectrochemical measurements show that the mixed-valence iron complex 2 can be oxidised reversibly to give the tri-iron(III) complex [Fe₃OL₃]⁺ and reduced reversibly and quasireversibly to give respectively [Fe₃OL₃]^- and tri-iron(II) [Fe₃OL₃]^2-, E⁰ = 85,-635, -1230 mV (versus Fc^+/0) in dichloromethane (T = 298 K, 0.1 M [n-Bu₄N][PF₆]). Mössbauer spectra of 2 indicate significant valence delocalisation even at low temperature (4.2 K) with estimated valences Fe(2.9+, 2.9+, 2.2+) in the solid state. At higher temperatures no lifetime broadening is observed but additional Mössbauer absorptions are consistent with increasing proportions of trimer molecules with greater delocalisation, i.e. Fe(2.75+, 2.75+, 2.5+). In frozen solution (THF) the spectra indicate increasing proportions of molecules fully valence-delocalised on the Mössbauer time scale. The data are accounted for with a model which places the complex at the Robin-Day class III/II borderline. It combines strong superexchange with significant double exchange even at the lowest temperatures, while at higher temperatures in solution complete valence delocalisation occurs through intramolecular electron transfer at rates intermediate between the IR and NMR time scales.