Valence Trapping of Mixed-Valence [Fe₃O(O₂CCH₃)₆(py)3]•S (S = Solvent) Complexes at High Pressure

The transformation from valence detrapped to valence trapped for two oxo-centered trinuclear iron acetate complexes is studied at pressures up to 95 kbar with the use of a diamond anvil cell. Variable-pressure ⁵⁷Fe Mössbauer spectra are presented for ⁵⁷Fe-enriched [Fe₃O(O₂CCH₃)₆(py)₃]•py (1) and [Fe₃O(O₂CCH₃)₆(py)₃]•CHCl₃ (2), where py is pyridine. At 298 K and applied pressures less than ~20 kbar, each of the complexes gives a spectrum with a single quadrupole-split doublet, which indicates that complexes 1 and 2 are interconverting faster than the Mössbauer time scale under these conditions. Application of pressure in excess of ~80 kbar leads to both complexes becoming valence trapped, as indicated by two doublets in the Mössbauer spectrum with an area ratio of ~2:1 (Fe^III:Fe^II). At intermediate pressures, each of these complexes gives a Mössbauer spectrum that can be fit as a superposition of a valence detrapped doublet and a valence trapped four-line pattern. The nature of the pressure-induced transformations observed in complexes 1 and 2 is discussed with reference to the phase diagram derived from a spin-Hamiltonian theoretical approach that parametrizes intermolecular interactions in terms of the molecular field approximation.