Heat Capacity Study of the Abrupt Valence-detrapping Phase Transition of Mixed-valence [Mn₃O(O₂CCH₃)₆(py)₃]⋅ Py

The heat capacity of the mixed-valence complex [Mn₃O(O₂CCH₃)₆(py)₃].py, where py is pyridine, has been measured with an adiabatic calorimeter between 13 and 300 K. A phase transition with a sharp heat capacity peak has been found at 184.65 K. The enthalpy and entropy of the phase transition are ΔH = 6460 J mol^-1and ΔS = 35.77 J K^_1mol^-1. From a comparison of the present calorimetric results with the results of single-crystal X-ray diffraction, magnetic susceptibility, and solid-state²H NMR studies, it is concluded that the phase transition is associated with the onset of rapid intramolecular electron transfer in the mixed-valence Mn₃0 complexes and the orientational disordering of the pyridine solvate molecules. The former contribution to ΔS is R In 4, while the latter is R In 18, where R is the gas constant. The total entropy gain from these two contributions, R In 72 (=35.56 J K^_1mol'¹), agrees well with the observed ΔS. In the high-temperature valence-detrapped phase each Mn₃0 complex is dynamically interconverting between four configurations, where in three cases the “extra” electron resides on one manganese ion and the Mn₃0 unit is distorted as an isosceles triangle and in the fourth configuration the Mn₃0 unit is a symmetric equilateral triangle that is electronically delocalized. The contribution of R In 18 results from the disordering of the pyridine solvate molecule about a pseudo-C₆axis perpendicular to the plane of the pyridine and 3-fold reorientations about the crystallographic C₃axis where the plane of the pyridine solvate molecule in each of its positions is tipped ~15° off the C₃axis. Together with the isostructural mixed-valence complex [Fe₃O(O₂CCH₃)₆(py)₃](CHCl₃), the present complex [Mn₃O(O₂CCH₃)₆(py)₃]⋅py is one of two R32 symmetry trinuclear mixed-valence complexes in which the static electron delocalization state has clearly been found to occur as a constituent in the valence-detrapped phase.