Highly Cooperative Valence Detrapping of Mixed-Valence [Mn₃O(O₂CCH₃)₆(py)₃](py) in the Solid State

Heat capacity at constant pressure versus temperature data for [Mn₃O(O₂CCH₃)₆(py)₃](py) show that there is a first-order phase transition at 184.65 K. The phase transition is characterized by total enthalpy and entropy gains of ΔH = 6460 J mol^-1 and ΔS = 35.77 J K^-1 mol^-1, respectively. The experimental ΔS is approximately equal to R ln 72 and is accounted for by two contributions: the Mn₃O complexes converting from valence trapped to valence detrapped (R ln 4) and the pyridine solvate molecules converting from being static to dynamically gaining access to 18 different orientations (R In 18). The 18 different solvate molecule orientations result from the plane of each pyridine solvate jumping between three positions about the C₃ crystal axis, together with a rotation in each position about a local pseudo C₆ axis. To account for the ²H NMR results the average orientation of the plane of the pyridine is not along the C₃ axis, but tipped off of it by ~15°. Variable-temperature ²H NMR for a single crystal of [Mn₃O(O₂CCD₃)₆(py)₃](py) oriented with the magnetic field parallel to the c axis establishes that there is an abrupt valence detrapping at the phase transition. Detailed ²H NMR experimentation employing a new selective irradiation technique on a single crystal of [Mn₃O(O₂CCH₃)₆(C₅D₅N)₃](C₅D₅N) verifies the onset and mechanism of motion of the pyridine solvate molecule at the phase transition. An explanation is given as to why the present Mn₃O complex exhibits valence detrapping abruptly in a first-order phase transition, whereas the isostructural Fe₃O complex valence detraps gradually in a higher order phase transition.