Pressure versus temperature effects on intramolecular electron transfer in mixed-valence complexes

Mixed-valence trinuclear carboxylates, [M₃O(O ₂CR)₆L₃] (M=metal, L=terminal ligand), have small differences in potential energy between the configurations M ^IIM^IIIM^III⇔ M^IIIM ^IIM^III⇔M^IIIM^IIIM^II, which means that small external changes can have large structural effects, owing to the differences in coordination geometry between M²⁺ and M ³⁺ sites (e.g., about 0.2 Å for Fe-O bond lengths). It is well-established that the electron transfer (ET) between the metal sites in these mixed-valence molecules is strongly dependent on temperature and on the specific crystal environment; however, herein, for the first time, we examine the effect of pressure on the electron transfer. Based on single-crystal X-ray diffraction data that were measured at 15, 90, 100, 110, 130, 160, and 298 K on three different crystals, we first unexpectedly found that our batch of Fe ₃O (O₂CC(CH₃)₃)₆(C ₅H₅N)₃ (1) exhibited a different temperature dependence of the ET process than previous studies of compound 1 have shown. We observed a phase transition at around 130 K that was related to complete valence trapping and Hirshfeld surface analysis revealed that this phase transition was governed by a subtle competition between C-H⋯pi; and π⋯pi; intermolecular interactions. Subsequent high-pressure single-crystal X-ray diffraction at pressures of 0.15, 0.35, 0.45, 0.74, and 0.96 GPa revealed that it was not possible to trigger the phase transition (i.e., valence trapping) by a reduction of the unit-cell volume, owing to this external pressure. We conclude that modulation of the ET process requires anisotropic changes in the intermolecular interactions, which occur when various directional chemical bonds are affected differently by changes in temperature, but not by the application of pressure.