Crystal structures of complexes between alkali-metal salts and cyclic polyethers. Part II. Complex formed from sodium bromide and 2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacyclo-octadeca-2,11-diene (‘di-benzo-18-crown-6’)

Abstract

Sodium bromide reacts in methanol with the cyclic ether dibenzo-18-crown-6(L) to yield crystals of formula NaBrL,2H₂O. Three-dimensional X-ray crystal structure analysis has shown that there are two kinds of sodium complex in the crystal, (A) a complexed cation, and (B) a complexed ion-pair. In both, the sodium ions have a hexagonal bipyramidal environment, with six oxygen atoms from L in the equatorial plane; the apices are occupied by water molecules at 2·31 and 2·27 Å in (A), while one apex is occupied by a water molecule at 2·35 Å, and the other by a bromide ion at 2·82 Å in (B). Hydrogen bonding involving the unco-ordinated molecule of water and the unco-ordinated bromide ion, as well as the co-ordinated water molecules and bromide ion, results in a chain parallel to the b axis of the crystal with the two complexes alternately along it. Both molecules of cyclic ether have normal bond lengths and angles and the same conformation as found previously in the (Rb_0·55, Na_0·45)NCS complex. Sodium–oxygen (ether) distances average 2·705 Å(2·64–2·84 Å) in (A), and 2·72 Å(2·54–2·89 Å) in (B).

Two formula units, C₂₀H₂₄O₆, NaBr, 2H₂O, comprise the asymmetric unit in a monoclinic unit cell with a= 10·558, b= 14·036, c= 30·677 Å, β= 91·3°, and space group P2₁/c. Intensities were measured with a four-circle diffractometer. Non-hydrogen atoms were located from Patterson and Fourier syntheses and their parameters were refined by full-matrix least-squares methods to a final R of 0·083 for 2880 observations.