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 [Fe^IIFe^III(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 Mössbauer spectra were measured for the complexes and the crystal structures of 4 and 7 determined. Complexes 1, 2 and 4 show trapped Mössbauer spectra due to iron(II) and iron(III), 5–7 show detrapped Mössbauer 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 Fe_A²⁺Fe_B³⁺ and Fe_A³⁺Fe _B²⁺: the apparent valence states of the iron atoms are 2.2 and 2.8 on the Mössbauer 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 Fe^II–O and Fe^III–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, Mössbauer 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 Mössbauer 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 Mössbauer timescale (10^–7 s) at 293 K.