Stereochemical activity of lone pairs. The crystal and molecular structures of the salts of chloro(1,4,7,10,13,16-hexaoxacyclo-octadecane)tin(II). Calculation of macrocyclic cavity size by force field methods

Abstract

The crystal structures of the trichlorostannate(II), (1), and perchlorate, (2), salts of chloro (1,4,7,10,13,16-hexaoxacyclo-octadecane)tin(II), [Sn(18-crown-6)Cl]⁺, have been determined from X-ray diffractometer data by Patterson and Fourier methods. Crystals of both complexes are triclinic, space group P, with Z= 2 and 4, in unit cells of dimensions a= 12.049(8), b= 7.733(8), c= 12.717(9)Å, α= 107.0(1), β= 82.1(1), γ= 89.0(1)° and a= 8.053(8), b= 23.964(15), c= 11.747(8)Å, α= 92.6(1), β= 118.8(1), γ= 88.3(1)°, respectively. The structures were refined to R= 0.073 and 0.088 for 2 991 and 4 032 observed reflections respectively. Both complexes contain [Sn(18-crown-6)Cl]⁺ in which the tin occupies a hexagonal pyramidal site being bonded to all six crown oxygens [Sn–O 2.592(6)–2.883(7)Å] and a chlorine atom [Sn–Cl 2.428(2)Å] in an axial position. Structure (2) contains two independent sets of these complex cations. The tin lone pair exhibits much reduced stereochemical activity and this behaviour is discussed on the basis of the structures and the Mössbauer parameters. Molecular mechanics calculations of hole size show that the best fit for a metal within 18-crown-6 in the D_3d conformation is provided by M–O bond lengths of 2.85 Å.