STRUCTURE AND MIXED-VALENCE STATES OF THE TRINUCLEAR IRON(II,III,III) PIVALATE COMPLEX-PIVALIC ACID ADDUCT, [FE3O(O(2)CCME(3))(6)(ME(3)CCO(2)H)(3)](ME=-CH3)

The structure of the mixed-valence trinuclear iron pivalate complex-pivalic acid adduct, [Fe3O(O(2)CCMe(3))(6)( Me(3)CCO(2)H)(3)] (Me = -CH3), 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) Angstrom, V = 6456(1) Angstrom(3) and Z = 4, without solvate molecules of crystallization. The adduct is first characterized to be a mixed-valence Fe3O compound with carboxylic acid coordinating to the iron atom. The acid is coordinated through the carbonyl O atom trans to the mu(3)-oxo atom, disordered for crystallographic mirror symmetry. Although there are two crystallographically non-equivalent iron sites, no significant difference is seen between their coordination environments at room temperature. The complex shows a valence-trapped state below similar to 100 K and a valence-detrapped state above similar to 230 K up to room temperature on the Fe-57 Mossbauer time-scale (similar to 10(-7)-10(-8) s), whereas it shows an evidence of valence trapping on the IR time-scale (similar to 10(-12)-10(-13) s) at room temperature. The temperature-dependent valence transformation is found to culminate with a first-order phase transition at similar to 230 K.