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

Abstract

Mixed-valence µ-phenolate bis(µ-carboxylate) diiron(II,III) complexes [FeIIFeIII(bpmp)(L)2][BF4]2 have been prepared, where Hbpmp represents 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol and HL is benzoic acid (Hba) 1, phenylacetic acid (Hpaa) 2, 3-phenylpropionic acid (Hppa) 3, 4-phenylbutyric acid (Hpba) 4, 5-phenylvaleric acid (Hpva) 5, 6-phenylhexanoic acid (Hpha) 6 or p-methoxybenzoic acid (Hmba) 7. The Mossbauer spectra were measured for the complexes and the crystal structures of 4 and 7 determined. 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 increase on rising temperature. The spectra are explained in terms of intramolecular electron exchange between two energetically inequivalent vibronic states FeA2+FeB3+ and FeA3+Fe B2+: the apparent valence states of the iron atoms are 2.2 and 2.8 on the Mossbauer timescale at 293 K. At both 293 and 130 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 is intermediate between FeII–O and FeIII–O values, which indicates that both iron atoms are in an averaged valence state at 293 and 130 K when considered in terms of a static description. However, Mossbauer spectra of the complex show that the valence states of the irons are localized to iron-(II) and -(III) at 130 K and are delocalized at 293K on the characteristic Mossbauer timescale. Complex 4 is composed of iron-(II) and -(III) moieties, in accordance with the observation that the valence states of the irons are localized on the Mossbauer timescale (10–7 s) at 293 K.