Synthesis, NMR spectroscopy, and crystal structure of the 1:2 host: guest complex of 18-crown-6 with lithium phenoxide

Abstract

The title complex crystallizes in the space group Pcab with a = 16.019(7), b = 19.62(1), c = 15.847(5) Å, and Z = 8. The structure was solved by direct methods and refined by full matrix least-quares calculations to a residual, R, of 0.083. The monomeric structure lacks a true centre of symmetry although the two lithiums have similar chemical environments. Each lithium cation is coordinated to three different oxygens of the hexaether and to both phenoxide oxygens. The hexaether is slightly elongated allowing both Li(1) and Li(2) to lie in the cavity formed by the crown with values of −0.077 and 0.074 Å, respectively, out of the plane approximated by the six oxygens. Both cations are strongly associated with the phenol moieties. The distances between the lithium and the phenoxide oxygens range from 1.859–1.893 Å, while distances of 1.940–2.420 Å are found with the ether oxygens. The Li(1)—Li(2) distance is found to be relatively short at 2.35 (2) Å.Evidence for motion in the 18-crown-6 portion of the solid complex has been obtained from 13C CPMAS NMR spectroscopy. At 298 K, a dipolar washout phenomenon occurs since the motional correlation times are of the order of the inverse of the 1H decoupling field amplitude. At lower temperatures, the motion becomes slow enough to permit resolution of the chemical shift differences of unique carbon sites of the crown ether. Consistent with the X-ray analysis, only five resonances are evident for the phenoxide carbons, indicating that the two aromatic rings are nearly equivalent in their crystallographic environment. The solution 13C spectrum shows only five resonances in total down to 188 K indicative of rapid conformational averaging of all the 18-crown-6 carbon resonances and rapid rotation about the O-phenyl bond. Keywords: crown ethers, NMR, stereochemistry, dynamics, X-ray.