Crystal structures of complexes between alkali-metal salts and cyclic polyethers. Part I. Complex formed between rubidium sodium isothiocyanate and 2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxocyclo-octadeca-2,11-diene (‘dibenzo-18-crown-6’)

Abstract

A three-dimensional X-ray crystal structure analysis has been carried out on a complex formed from 2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxocyclo-octadeca-2,11-diene (known as ‘dibenzo-18-crown-6’) and rubidium sodium isothiocyanate and the chemical formula [(Rb_0·55, Na_0·45)NCS(C₂₀H₂₄O₆)] established. The crystal also contains uncomplexed molecules of dibenzo-18-crown-6 which have the same bond lengths and angles as, but a different conformation from, the complexed molecules. There are four molecules of complex and two centro-symmetrical molecules of uncomplexed ether in a monoclinic unit cell having a= 19·322, b= 12·948, c= 13·001 Å, β= 105·78°, and space group P2₁/a. The structure was solved by direct methods, followed by Fourier and difference Fourier syntheses. All the atoms including hydrogen have been located; full-matrix refinement converged with R 0·116.

The six oxygen atoms of the complexed ether are coplanar and equidistant from the metal atom, with Rb–O(mean) 2·91 and Na–O(mean) 2·80 Å. A hexagonal pyramid about the rubidium is completed by the nitrogen of the isothiocyanate (Rb-N 2·91 Å) but the sodium is not in contact with the anion (Na-N 3·32 Å).