Mixed-Valence States of [Fe₃O(CH₂XCO₂)₆(H₂O)₃]·nH₂O (X = H, Cl, and Br) Characterized by X-ray Crystallography and ⁵⁷Fe-Mössbauer Spectroscopy

Abstract

Mixed-valence states of μ-oxo-centered trinuclear iron carboxylate complexes, [Fe3O(CH2BrCO2)6(H2O)3] (1), [Fe3O(CH3CO2)6(H2O)3]·2H2O (2), and [Fe3O(CH2ClCO2)6(H2O)3]·3H2O (3), were characterized by X-ray crystallography and 57Fe-Mössbauer spectroscopy. Compound 1 has a trapped-valence-type structure, i.e., one Fe(II)-type atom and two Fe(III)-type atoms. The central Fe3O atoms construct nearly an isosceles triangle array with Fe−O bond distances of 1.86(1), 1.83(1), and 2.00(1) Å at 293 K. In the case of compound 2, the X-ray structure shows good equivalence in geometry around the three iron atoms. Results of X-ray crystallography of 1 and 2 are consistent with the results of Mössbauer spectroscopy. The Mössbauer spectra of 1 show a trapped-valence state below 296 K. The Mössbauer spectra of compound 2 are simulated using a three-site relaxation model where an increased rate of intramolecular electron transfer is assumed among the three iron atoms with an increase in temperature. Black crystals of 3 were converted to red powders of [Fe3O(CH2ClCO2)6(H2O)3] (4) due to loss of all three crystal water molecules when kept in air at ambient temperature. Mössbauer spectra of the dehydrated compound 4 show a temperature-independent trapped-valence state. On the other hand, there are three quadrupole doublets in the Mössbauer spectrum of the hydrated compound 3 measured at 86 K, one of which was ascribed to an FeII state and the remaining two to an FeIII (FeIIIA and FeIIIB) state. It is found that two of the three doublets (FeII and FeIIIA) converge without appreciable line broadening with an increase in temperature, while the other doublet (FeIIIB) remains unchanged, indicating valence delocalization over the two iron atoms. The X-ray structure of compound 3 also indicates that valence delocalization takes place only between two iron atoms. At 112 K the central Fe3O atoms array with the Fe−O bond distances of 2.003(2) (Fe1), 1.853(2) (Fe2), and 1.849(2) Å (Fe3). A temperature increase to 293 K changes the bond distances to 1.957(3) (Fe1) and to 1.895(3) Å (Fe2), while the bond distance of Fe3−O is almost unchanged (1.846(3) Å). There are intra- and intermolecular hydrogen bonds formed by the crystal water and coordinated water molecules in compound 3. The results suggest that hydrogen-bonding interaction affects the local environment of iron atoms and induces valence delocalization between the two iron atoms.