SUBTLE EFFECTS OF SOLVATE MOLECULES ON THE RATE OF INTRAMOLECULAR ELECTRON-TRANSFER OF MIXED-VALENCE COMPLEXES IN THE SOLID-STATE

The microscopic nature of the valence-detrapping phase transition of [Fe3O(O2CCH3)6(4-Me-py)3](CHCl3) (2) is studied. The constant-pressure heat capacity of a 16.3703-g sample of 2 in the 14-300 K range is presented. An apparently first-order phase transition with two C(p) peaks at 93.48 and 95.04 K was observed. After correction for the normal heat capacity, the total entropy gain for the phase transition was found to be DELTA-S = 17.18 +/- 1.42 J K-1 mol-1, which is close to DELTA-S = R ln 8 (= 17.29 J K-1 mol-1). Isostructural [Fe3O(O2CCH3)6(py)3](CHCl3) (1) has been reported to have a "double-peaked" phase transition at approximately 208 K with DELTA-S = 28.10 +/- 0.44 J K-1 mol-1, which is close to DELTA-S = R ln 32 (= 28.82 J K-1 mol-1). The valence-detrapping phase transition in both cases involves each Fe3O complex converting from being statically trapped in one vibronic state to dynamically interconverting between four vibronic states. For both 1 and 2, this contributes DELTA-S = R ln 4. Even though it has been reported that the CHCl3 solvate molecules in 1 cooperatively convert from being static in one lattice position to jumping between eight positions in the phase transition, the CHCl3 molecules in 2 only start jumping between two positions where the C-H vector is pointing either up or down along the c axis (C3 axis). For 2, the CHCl3 solvate molecules only contribute DELTA-S = R ln 2. The nature of the motion of the chloroform solvate molecule is examined in detail by employing solid-state H-2 NMR spectroscopy on magnetically oriented crystalline samples of [Fe3O(O2CCH3)6(4-Me-py)3](CDCl3). In these crystals, the C-D vector is jumping between positions on a cone axially symmetric about the C3 axis. At 290 K, this cone is approximately 18-degrees off the C3 axis. A decrease in the temperature reduces this angle to 10-degrees at 145 K and presumably 0-degrees below the approximately 94 K phase transition. Thus, for complex 2 at the approximately 94 K phase transition the c axis of the crystal is too small to permit the chloroform C-H vector to precess about the c axis (C3 axis). The cooperative motion of the CHCl3 molecules in 2 at approximately 94 K involve only a head-to-tail motion with the C-H vector on the c axis.