Thermal Properties and Crystal Structures of Ionic Liquids from Ruthenium Sandwich Complexes with Trialkoxybenzene Ligands: Effects of Substituent Positions and Alkyl Chain Lengths

A series of ionic liquids containing [Ru­(C₅H₅)­(arene)]⁺, where the arene ligands are 1,2,3-C₆H₃(OC_nH_2n+1)₃ and 1,3,5-C₆H₃(OC_nH_2n+1)₃ (n = 6, 12, and 18), were synthesized. The counteranions were PF₆ and FSA (bis­(fluorosulfonyl)­amide). The melting points of the PF₆ and FSA salts were 33–93 °C and −18–62 °C, respectively. In the PF₆ salts, the melting points of the 1,3,5-derivatives were lower than those of the 1,2,3-derivatives by 28–55 °C, whereas no such tendency was observed in the FSA salts. X-ray crystallography showed the formation of lamellar structures in the PF₆ salts of the 1,2,3-derivatives in the solid state. In these crystals, cation–anion pairs via C_CpH···F H-bond-like interactions were commonly observed. Several 1,3,5-derivatives showed metastable phases with lower melting points than the stable phases. The salts of the 1,2,3-derivative (n = 2) were also synthesized and characterized.