Multi-temperature X-ray diffraction, Mössbauer spectroscopy and magnetic susceptibility studies of a solvated mixed-valence trinuclear iron formate, [Fe3O(HCO2)6(NC5H4CH3)3]·1.3(NC5H4CH3)

Abstract

Structural analysis of the solvated mixed-valence (MV) trinuclear iron carboxylate, [Fe₃O(HCO₂)₆(γ-pic)₃]·x(γ-pic), 1, (γ-pic = 4-methylpyridine; x ≈ 1.3), has been performed from X-ray diffraction data at four temperatures. Analysis of the X-ray diffraction and variable temperature Mössbauer data for 1 clearly shows the presence of localised valence states for iron atoms at and below 100 K. Both physical methods agree that compound 1 exhibits a continuously growing degree of electron transfer (ET) between the three iron sites with increasing temperature; however, the extent of ET is significantly larger for one of the iron(III) sites. All three terminal γ-pic groups remain ordered at and below room temperature, whereas the γ-pic solvent molecules have a temperature dependent disorder over a number of positions. The distribution of the solvent over disorder positions is suggested to influence the degree of ET between the metal sites. From the molecular geometry in the Fe₃O-core, an estimate of 260 cm⁻¹ in energy difference between the two vibronic states corresponding to electronic localisation on either of the two iron atoms involved in ET is obtained. This value is close to an estimate for the energy difference assessed by using a Blume–Emery–Griffith Hamiltonian to describe the localised–delocalised transition in mixed valence molecular compounds.