Effect of Quenched Disorder on Phase Transitions in the Mixed-Valence Solid Solution [Fe^III₂Fe^II_1-xCO^II_xO(O₂CCH₃)₆(py)₃](py)

The effect of adding permanently distorted [Fe^II_I2Co^IIO(O₂CCH₃)₆(py)₃)(py) (2) complexes to the lattice of the isostructural mixed-valence Fe^III₂Fe^IIO complex 1 to form solid solutions of [Fe^Ili2Fe^,I_1-xCo^II_iO(O₂CCH3)₆(py)3](py) is examined. Single-crystal 298 K X-ray structures are presented for [Fe₃O(O₂CCH₃)₆(py)₃](py) (1) and Fe^III₂Co^IIO complex 2. Both complexes crystallize in the rhombohedral space group R32. Complex 1 has a unit cell with a = 17.558 (7) Å and c = 11.074 (4) Å, where Z = 3. Complex 2 crystallizes with a unit cell of a = 17.583 (6) Å and c = 11.074 (3) Å, with Z = 3. Complex 2 is statically disordered, whereas complex 1 is dynamically disordered at 298 K. In both cases Fe₂MO (M = Fe, Co) complexes and pyridine solvate molecules are alternatively stacked along the crystallographic C₃ axis. Each Fe₂MO and pyridine solvate molecule is at a 32 symmetry site, when the effects of the disorder are considered. Powder XRD data for 1, 2, and the x = 0.5 solid solution (complex 3) show that complex 3 is indeed a solid solution and not just a mixture of crystals of 1 and 2. Adiabatic calorimetry was used to measure the heat capacity at constant pressure, C_P, in the 12-300 K range for a 17.4981-g sample of complex 2 and a 13.6562-g sample of complex 3. For the x = 0.5 solid solution complex 3 three C_P peaks were seen at 84.5, 148.0, and 168.8 K. A single broad C_P peak was seen at 148.5 K for Fe^,III₂Co^II complex 2. This one broad peak gave a total entropy gain for the phase transition of 10.32 J K^-1 mol^-1, which is close to a value of ΔS = R In 3 = 9.13 J K^-1 mol^-1. Solid-state ²H NMR results for randomly oriented polycrystalline and magnetically oriented microcrystalline samples of 2 confirmed that the 148.5 K phase transition for Fe^III₂Co^II complex 2 is due only to a cooperative onset of motion of the pyridine solvate molecules. The total entropy gain for the x = 0.5 solid solution was calculated to be ΔS = 19.74 J K^-1 mol^-1. This AS value is rationalized in terms of the onset of valence detrapping in the Fe₂O complexes and pyridine solvate molecules in the solid-solution complex 3. In agreement with the heat capacity results, ⁵⁷Fe Mössbauer data for [Fe^III₂Fe^II_1-xCo^II_xO(O₂CCH₃)₆(py)₃](py) show that this solid solution converts from a ferrodistortive phase to a antiferrodistortive phase at ~85 K, finally to be completely valence detrapped and dynamic in a paradistortive phase at ~ 175 K. The Fe^III₂Co^IIO complexes in the solid-solution complex 3 behave as an unusual type of quenched disorder. They do not serve simply as lattice sites missing a mixed-valence Fe₃O complex. Each Fe^III₂Co^IIO complex is permanently distorted, and due to py-py contacts with neighboring trinuclear complexes it acts as an unusual element of disorder that appears to prohibit the appearance of an ordered state at a concentration of Fe^III₂Co^IIO complex below that predicted by percolation theory.