The influence of ligating atom identity on complex cation stability and anionic speciation in hydrogen-bonding solvate ionic liquids

Solvate ionic liquids (SILs) are a rapidly growing family of materials with potential applications in energy storage, metal deposition and extraction, reaction media for synthetic chemistry, and as pharmaceutical delivery agents. As with traditional ionic liquids, the broad synthetic versatility available in designing SILs provide an excellent framework for meeting the diverse needs of these technological niches. This work explores how structural variations to the molecular solvent impact SIL structure and properties. The solvent molecules investigated have a core structural motif similar to triethylene glycol and tetraethylene glycol with variations in ligating atom identity (O, N, and S atoms). All of the SILs are created from equimolar mixtures of a molecular solvent and lithium bis(trifluoromethanesulfonyl)imide, LiNTf 2 . Ionic interactions are monitored with IR and Raman spectroscopy and supplemented by DFT analyses of the various complex cations. In general, ligating atom identity and location within a molecular solvent has a major impact on complex cation stability. This, in turn, affects the ability of the anions to form ionically-associated species with the Li + ions. For example, replacing select oxygen atoms of triethylene glycol with sulfur atoms destabilizes the complex cation and increases ionic association. In contrast, replacing the oxygen atoms with primary or secondary amines tends to produce the opposite effects. Lengthening the solvent molecule from four to five ligating atoms makes the solvent molecule more competitive in binding the Li + cation. SILs based on tetraethylene glycol and tetraethylenepentamine produce more stable complex cations and have higher abundances of unassociated, “free” NTf 2 – ions compared to shorter tetradentate analogs.