Organometallic Ionic Plastic Crystals Based on a Half-Sandwich-Type [Ru(C ₅ H ₅ )(PMe ₃ ) ₃ ] ⁺ Cation

Ionic plastic crystals (IPCs) are soft crystalline phases that have attracted increasing attention in recent years because of their unique solid-state properties. Although a number of organometallic sandwich complexes form IPC phases, examples based on half-sandwich complexes remain scarce, and their molecular design has not yet been explored in detail. In this study, we synthesized salts composed of the half-sandwich cation [Ru(C5H5)(PMe3)3]+ and various anions, and examined their thermal properties. Among them, the salts with Tf3C–, CPFSA–, Ad-BF3–, B(CN)4–, and PF6– anions underwent transitions to IPC phases at 352, 300, 454, 392, and 406 K, respectively. The transition temperatures to the IPC phase (Tc) generally decreased with increasing anion volume, although deviations from this trend were considerable, suggesting that Tc in this system is also influenced by molecular shape and local molecular arrangements. In contrast, the salts with CB11H12– and FSA– anions did not exhibit IPC phases; the latter displayed five solid-state phase transitions. All salts exhibiting an IPC phase showed alternating cation–anion arrangements in the low-temperature phase and substantially increased disorder in the high-temperature phases. Compared with previously reported half-sandwich cations bearing less symmetric ligand sets, the present cation appears to favor IPC formation, although the transition temperatures remain relatively high.