Vibrational and theoretical study of the complexation of LiPF6 and LiClO4 by di(ethylene glycol) dimethyl ether

Solid [(diglyme)2Li+]PF6− and [(diglyme)2Li+]ClO4− salts melting at 132 and 66 °C, respectively, have been isolated. They involve two diglyme molecules wrapping the six-coordinated cation and a “spectroscopically free” anion in a similar local structure of complexation as in the P(EO)6,LiPF6 and P(EO)6,LiClO4 systems. The structures and vibrational assignments of the [(diglyme)2Li+]X− salts have been established by comparison with the corresponding IR and Raman spectra of liquid and solid diglyme and by using ab initio calculations. In particular, the atomic motions involved in the so-called “rattling”, D-LAM or “breathing” modes have been described in terms of a distorted octahedral cage of oxygen coordinating the lithium. Various (diglyme)n,LiClO4 samples (n=12–2) could be melted without decomposition and the Raman profile of the symmetric stretching vibration ν1 of the anion has been studied as a function of the temperature. An equilibrium is evidenced between two kinds of 2/1 complexes: in the first one, the anions are “spectroscopically free” (ν1=933 cm−1) and the cation is coordinated to six ether oxygen as in the solid state, while in the second, less stable by ∼14 kJ mol−1, the cation is coordinated to five ether oxygen and one perchlorate oxygen to form contact ion-pairs (ν1=938 cm−1).