Crystal structures and Mossbauer spectra of mixed-valence dinuclear iron(II,III) complexes: detrapped valence states

Mixed-valence mu-phenolate bis(mu-carboxylate) diiron(II,III) complexes [(F eFeIII)-Fe-II(bpmp)(L)(2)][BF4](2) have been prepared, where Hbpmp represen ts 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol and HL is benzoi c acid (Hba) 1, phenylacetic acid (Hpaa) 2, 3-phenylpropionic acid (Hppa) 3 , 4-phenylbutyric acid (Hpba) 4, 5-phenylvaleric acid (Hpva) 5, 6-phenylhex anoic acid (Hpha) 6 or p-methoxybenzoic acid (Hmba) 7. The Mossbauer spectr a were measured for the complexes and the crystal structures of 4 and 7 det ermined. Complexes 1, 2 and 4 show trapped Mossbauer spectra due to iron(II ) and iron(III), 5-7 show detrapped Mossbauer spectra at 293 K and trapped ones at 78 K and 3 shows a similar temperature dependence to that of 1, but differs in some points. The quadrupole splittings and isomer shifts of 3 i ncrease on rising temperature. The spectra are explained in terms of intram olecular electron exchange between two energetically inequivalent vibronic states FeA2+FeB3+ and FeA3+FeB2+: the apparent valence states of the iron a toms an 2.2 and 2.8 on the Mossbauer timescale at 293 K. At both 293 and 13 0 K the cations of complex 7 sit on a center of symmetry, and the two irons in this cation are crystallographically equivalent. The mean Fe-O length i s intermediate between Fe-II-O and Fe-III-O values, which indicates that bo th iron atoms are in an averaged valence state at 293 and 130 K when consid ered in terms of a static description. However, Mossbauer spectra of the co mplex show that the valence states of the irons are localized to iron-(II) and -(III) at 130 K and are delocalized at 293 K on the characteristic Moss bauer timescale. Complex 4 is composed of iron-(II) and -(III) moieties, in accordance with the observation that the valence states of the irons are l ocalized on the Mossbauer timescale (10(-7) s) at 293 K.