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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry