Colorless organometallic ionic liquids from cationic ruthenium sandwich complexes: thermal properties, liquid properties, and crystal structures of [Ru(η5-C5H5)(η6-C6H5R)][X] (X = N(SO2CF3)2, N(SO2F)2, PF6)

Abstract

A series of ionic liquids containing cationic ruthenium complexes ([Ru(C₅H₅)(C₆H₅R)]⁺) were prepared, and their thermal properties were investigated (R = C₄H₉ (1a), C₈H₁₇ (1b), OCH₂OCH₃ (2a), O(CH₂CH₂O)₂CH₃ (2b), O(CH₂)₃CN (3a), O(CH₂)₆CN (3b), CO(CH₂)₂CH₃ (4a), CO(CH₂)₆CH₃ (4b)). Bis(trifluoromethanesulfonyl)amide (TFSA) and bis(fluorosulfonyl)amide (FSA) were used as counter anions. These ionic liquids were colorless and stable toward air and light. These salts exhibited glass transitions upon cooling from the melt (T_g = −82 °C to −55 °C), and the glass transition temperatures of the salts increased as the polarity of the substituents increased (alkyl < ether < cyano < carbonyl). The decomposition temperatures decreased in the order of alkyl > cyano > carbonyl > ether. The viscosities, solvent polarities, and refractive indices of the salts of 1a and 2a were also evaluated. Hexafluorophosphate (PF₆) salts were also prepared, which were solids with high melting points (T_m = 65–127 °C). X-ray crystal structure analyses of these salts revealed the importance of intermolecular contacts involving the ring hydrogens. The PF₆ salt of 2a exhibited an order–disorder phase transition.