Solvate Molecule Effects and Unusual57Fe Mössbauer Line Broadening in the Valence Detrapping of Mixed-Valence [Fe3O(O2CCH3)6(3-Et-py)3]·S

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Abstract
A new series of mixed-valence μ3-oxo-bridged Fe3O complexes with the composition [Fe3O(O2CCH3)6(3-Et-py)3]·S, where 3-Et-py is 3-ethylpyridine and the solvate molecule S is either 0.5C6H5CH3 (1), 0.5C6H6 (2), CH3CN (3), or CH3CCl3 (4), is reported. The complex [Fe3O(O2CCH3)6(3-Et-py)3]·0.5C6H5CH3 (1) crystallizes in the orthorhombic space group Fdd2 which at 298 K has a unit cell with a = 22.726(8) Å, b = 35.643(14) Å, c = 20.816(6) Å, and Z = 16. Refinement with 5720 observed [F > 5σ(Fo)] reflections gave R = 0.0337 and Rw = 0.0390. An analysis of the bond lengths in complex 1 shows that it is the most valence-trapped Fe3O complex reported at room temperature. The complex [Fe3O(O2CCH3)6(3-Et-py)3]·CH3CCl3 (4) crystallizes in the triclinic space group P1̄ which at 238 K has a unit cell with a = 12.764(2) Å, b = 13.1472(2) Å, c = 15.896(3) Å, α = 78.01(2)°, β = 89.38(2)°, γ = 61.38(1)°, and Z = 2. Refinement with 6264 observed [F > 5σ(Fo)] reflections gave R = 0.0435 and Rw = 0.0583. In this μ3-oxo-bridged complex all three iron ions are inequivalent. Powder X-ray diffraction patterns taken at room temperature show that complexes 1 and 2 are isostructural and that complexes 3 and 4 are isostructural. Variable-temperature 57Fe Mössbauer spectra were collected for all four complexes. The data for complexes 1 and 2 clearly indicate that these two complexes are totally valence trapped. On the other hand, Mössbauer spectra (43−293 K) for complexes 3 and 4 show that these two complexes become valence detrapped at temperatures near room temperature. Two doublets are seen at low temperature and they move together to become a single doublet at ∼293 K. Examination of the line width versus temperature for each of the two components of the two doublets points to a curiosity. The two components of the “FeIII” doublet and the lower-velocity component of the “FeII” doublet do not exhibit any line broadening, whereas the higher velocity “FeII” component shows a surge in line width in the ∼70−150 K range. Possible explanations for these unusual line width responses are discussed.

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