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(OCnH2n+1)3 and 1,3,5-C6H3(OCnH2n+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 degrees C and -18-62 degrees 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 degrees 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 CCpH...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.