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

Complexes [(Fe2MOL3)-M-III-O-II] (M = Fe, Co, Ni, Cu; 2, 3, 4, 5) have been synthesised in which L2- is a pentadentate ligand designed to coordinate a ll three metal atoms in the central cluster and to inhibit dissociation and solvent exchange processes. Crystal structures for 2, 4 and 5 show threefo ld symmetry, attributed to rotational disorder. Magnetisation data for 2 in dicate strong superexchange between basis oxidation states Fe(3+, 3+, 2+). Comparisons of IR spectra across the series of complexes con rm the non-thr eefold 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 voltamme try and spectroelectrochemical measurements show that the mixed-valence iro n complex 2 can be oxidised reversibly to give the tri-iron(III) complex [F e3OL3](+) and reduced reversibly and quasireversibly to give respectively [ Fe3OL3](-) and tri-iron(II) [Fe3OL3](2-), E-0 = 85, -635, -1230 mV (versus Fc(+/0)) in dichloromethane (T = 298 K, 0.1 M [n-Bu4N][PF6]). Mossbauer spe ctra of 2 indicate significant valence delocalisation even at low temperatu re (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 M ossbauer absorptions are consistent with increasing proportions of trimer m olecules with greater delocalisation, i.e. Fe(2.75+, 2.75+, 2.5+). In froze n solution (THF) the spectra indicate increasing proportions of molecules f ully valence-delocalised on the Mossbauer time scale. The data are accounte d for with a model which places the complex at the Robin-Day class III/II b orderline. It combines strong superexchange with significant double exchang e even at the lowest temperatures, while at higher temperatures in solution complete valence delocalisation occurs through intramolecular electron tra nsfer at rates intermediate between the IR and NMR time scales.