Effect of Ionic Size on Solvate Stability of Glyme-Based Solvate Ionic Liquids

A series
of binary mixtures composed of glymes (triglyme, G3; tetraglyme,
G4; pentaglyme, G5) and alkali-metal bis­(trifluoromethanesulfonyl)­amide
salts (M­[TFSA]; M = Li, Na, and K) were prepared, and the correlation
between the composition and solvate stability was systematically investigated.
Their phase diagrams and Raman spectra suggested complexation of the
glymes with M­[TFSA] in 1:1 and/or 2:1 molar ratio(s). From isothermal
stability measurements, it was found that the formation of structurally
stable complexes in the solid state did not necessarily ensure their
thermal stability in the liquid state, especially in the case of 2:1
complexes, where uncoordinating or highly exchangeable glyme ligands
existed in the molten complexes. The phase-state-dependent Raman spectra
also supported the presence of free glymes in certain liquid complexes.
The effect of the electric field induced by the alkali-metal cations
on the oxidative stability of certain glyme complexes was examined
by linear sweep voltammetry and quantum chemical calculations. Although
the actual oxidative stability of complexes did not necessarily reflect
the calculated HOMO energy levels of the glymes, the strong electric
field induced by the smaller M+ cations and proper coordination
structures impart high stability to the glyme complexes. The results
of thermogravimetry of complexes with different M+ cations
revealed that a balance of competitive interactions of the M+ ions with the glymes and [TFSA]− anions predominates
the thermal stability.