Structure and mixed-valence states of the trinuclear iron(II,III,III) pivalate complex-pivalic acid adduct, [Fe 3O(O 2CCMe 3) 6(Me 3CCO 2H) 3] (Me = −CH 3)

Abstract

The structure of the mixed-valence trinuclear iron pivalate complex-pivalic acid adduct, [Fe 3O(O 2CCMe 3) 6(Me 3CCO 2H 3 (Me = −CH 3), was characterized by single crystal X-ray diffraction. The adduct crystallizes in the orthorhombic system, space group Cmcm, with a = 18.437(1), b = 20.339(2), c = 17.171(1) A ̊ , V = 6456(1) A ̊ 3 and Z = 4 , without solvate molecules of crystallization. The adduct is first characterized to be a mixed-valence Fe 3O compound with carboxylic acid coordinating to the iron atom. The acid is coordinated through the carbonyl O atom trans to the μ 3-oxo atom, disordered for crystallographic mirror symmetry. Although there are two crystallography non-equivalent iron sites, no significant difference is seen between their coordination environments at room temperature. The complex shows a valence-trapped state below ∼ 100 K and a valence-detrapped state above ∼ 230 K up to room temperature on the 57Fe Mössbauer time-scale (∼10 −7 − 10 −8 s), whereas it shows an evidence of valence trapping on the IR time-scale (∼ 10 −12 − 10 −13 s) at room temperature. The temperature-dependent valence transformation is found to culminate with a first-order phase transition at ∼ 230 K.