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 ΔS = 17.18 ± 1.42 J K^-1 mol^-1, which is close to ΔS = R In 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 ~208 K with ΔS = 28.10 ± 0.44 J K^-1 mol^-1, which is close to ΔS = R In 32 (=28.82 J K^-1 mol^-1). The valence-detrapping phase transition in both cases involves each Fe₃0 complex converting from being statically trapped in one vibronic state to dynamically interconverting between four vibronic states. For both 1 and 2, this contributes ΔS = R In 4. Even though it has been reported that the CHC1₃ solvate molecules in 1 cooperatively convert from being static in one lattice position to jumping between eight positions in the phase transition, the CHC1₃ 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 CHC1₃ solvate molecules only contribute ΔS = R In 2. The nature of the motion of the chloroform solvate molecule is examined in detail by employing solid-state ²H NMR spectroscopy on magnetically oriented crystalline samples of [Fe₃0(0₂CCH₃)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 ~ 18° off the C₃ axis. A decrease in the temperature reduces this angle to 10° at 145 K and presumably 0° below the ~94 K phase transition. Thus, for complex 2 at the ~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 CHC1₃ molecules in 2 at ~94 K involve only a head-to-tail motion with the C-H vector on the c axis.