Phase Diagrams and Solvate Structures of Binary Mixtures of Glymes and Na Salts

We prepared a series of binary mixtures
composed of selected Na
salts and glymes (tetraglyme, G4, and pentaglyme, G5) with different
salt concentrations and anionic species ([X]−: [N­(SO2CF3)2]− = [TFSA]−, [N­(SO2F)2]− = [FSA]–, ClO4–,
PF6–) and studied the effects of concentration,
anionic structure, and glyme chain length on their phase diagrams
and solvate structures. The phase diagrams clearly illustrate that
all the mixtures form 1:1 complexes, [Na­(G4 or G5)1]­[X].
The thermal stability of the equimolar mixtures was drastically improved
in comparison with those of diluted systems, indicating that all the
glyme molecules coordinate to Na+ cations to form equimolar
complexes. Single-crystal X-ray crystallography revealed that [Na­(G5)1]­[X] forms characteristic solvate structures in the crystalline
state irrespective of the paired anion species. A comparison of the
solvate structures of the glyme–Na complexes with those of
the glyme–Li complexes suggests that the ionic radii of the
coordinated alkali-metal cations have substantial effects on the resulting
solvate structures. The Raman bands of the complex cations were assigned
by quantum chemical calculations. Concentration dependencies of cationic
and anionic Raman spectra show good agreement with the corresponding
phase diagrams. In addition, the Raman spectra of the 1:1 complexes
strongly suggest that the glymes coordinate to Na+ cation
in the same way in both liquid and crystalline states. However, the
aggregated structure in the crystalline state is broken by melting,
which is accompanied by a change in the anion coordination.