Mixed-Valence States of [Fe₃O(CH₂<i>X</i>CO₂)₆(H₂O)₃]·<i>n</i>H₂O (<i>X</i> = 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, [Fe 3 O(CH 2 BrCO 2 ) 6 (H 2 O) 3 ] ( 1 ), [Fe 3 O(CH 3 CO 2 ) 6 (H 2 O) 3 ]·2H 2 O ( 2 ), and [Fe 3 O(CH 2 ClCO 2 ) 6 (H 2 O) 3 ]·3H 2 O ( 3 ), were characterized by X-ray crystallography and 57 Fe-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 Fe 3 O 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 [Fe 3 O(CH 2 ClCO 2 ) 6 (H 2 O) 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 Fe II state and the remaining two to an Fe III (Fe IIIA and Fe IIIB ) state. It is found that two of the three doublets (Fe II and Fe IIIA ) converge without appreciable line broadening with an increase in temperature, while the other doublet (Fe IIIB ) 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 Fe 3 O 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.