Lithium-7 NMR studies of macrobicyclic complexes in ambient temperature molten salts

Abstract

Complexation of the Li + ion by macrobicyclicdiazapolyoxa ligands (cryptands) was studied in a 45:45 mol% aluminum(III) chloride-N-( n-butyl)pyridinium chloride mixture at 40°C. Lithium-7 NMR measurements at this temperature and a field strength of 42.28 kG, showed that the exchange of the Li + ion between the “free” and complexed sites is slow, and separate resonance signals were observed for the free and complexed Li + ion. Conditional formation constants were obtained from the integrated areas of the resonance lines; the complex stabilities were found to decrease in the order C221·Li + > C222·Li + > C222B·Li +. In the case of the C222·Li + complex, a decomplexation rate constant of ∼ 5 × 10 2 s −1 was obtained at the coalescence temperature of 40°C. Solid state 7Li NMR spectra were obtained for the C211·LiAlCl 4 and C222B·LiAlCl 4 complexes, in the static and magic angle spinning modes. These measurements indicate that the electric field gradient at the 7Li nucleus in these complexes is small, yielding narrow resonance lines in the MAS mode.