The synthesis and crystal structure of [K(18-crown-6)]+[NCS·SO2]–(18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane)

Abstract

Reaction of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) with potassium thiocyanate in liquid sulfur dioxide gives, on recrystallisation from diethyl ether–sulfur dioxide, yellow crystals of the complex [K(18-crown-6)]⁺[NCS·SO₂]^– in 98% yield. Vibrational spectroscopy points to the formation of a weak donor–acceptor complex anion [NCS·SO₂]^–, and this was confirmed by X-ray crystallography: triclinic, P(no. 2), a= 7.635(2), b= 9.595(2), c= 8.171(1)Å, α= 102.95(2), β= 95.68(2), γ= 111.91(2)°, Z= 1. The centrosymmetric [K(18-crown-6)]⁺ cations are centred on special positions on the corners of the unit cell, and the complex anion is disordered about the inversion centre at (½, ½, ½). The nature of the disorder makes it difficult to locate the positions of some light atoms in the [NCS·SO₂]^– anion, although the S–S bond length, 2.736(9)Å, is consistent with the lability of the complex. Dissociation pressure measurements at different temperatures were used to determine a binding energy for gaseous [NCS·SO₂]^– of 66 kJ mol^–1, a value which is compared with those of related sulfite complexes in relation to Pearson's concept of ‘absolute hardness’.