Crystal structures of complexes between alkali-metal salts and cyclic polyethers. Part VII. Complex formed between dibenzo-24-crown-8 (6,7,9,10,12,13,20,21,23,24,26,27-dodecahydrodibenzo[b,n]-1,4,7,10,13,16,19,22-octaoxacyclotetracosin) and two molecules of potassium isothiocyanate

Abstract

Potassium thiocyanate reacts with dibenzo-24-crown-8 giving crystals of formula C₂₄H₃₂O₈,2KNCS. The molecular structure has been determined by three-dimensional X-ray analysis from diffractometer data. Crystals are monoclinic, space group P2₁/c, with Z= 2 in a unit cell of dimensions: a= 9·902(8), b= 18·55(4), c= 8·572(8)Å, β= 106·9(2)°. All atoms, including hydrogen, were located. Full-matrix least-squares refinement led to a final R factor of 0·079 for 1184 independent reflections.

The ligand oxygen atoms are almost coplanar, with potassium atoms 0·66 Å on each side of this plane. Each K⁺atom is within bonding distance of five oxygen atoms, three bonding independently (K–O 2·732–2·825 Å) and the fourth and fifth unequally shared between the two potassium atoms (K–O 2·898, 2·979 Å). One thiocyanate ion is above and one below the ligand ring plane, each bridging the two potassium ions through nitrogen (K–N 2·87, 2·88 Å). A benzene ring from another ligand is in a position for its π-electrons to form an electrostatic attraction to the potassium ion, so there are eight sources of electron density round the cation and the structure has three-dimensional electrostatic bonding throughout.