Unusual centrosymmetric structure of [M(18-crown-6)]+(M= Rb, Cs and NH4) complexes stabilized in an environment of hexachloridoantimonate(V) anions

Abstract

In (1,4,7,10,13,16-hexaoxacyclooctadecane)rubidium hexachloridoantimonate(V), [Rb(C12H24O6)][SbCl6], (1), and its isomorphous caesium {(1,4,7,10,13,16-hexaoxacyclooctadecane)caesium hexachloridoantimonate(V), [Cs(C12H24O6)][SbCl6]}, (2), and ammonium {ammonium hexachloridoantimonate(V)–1,4,7,10,13,16-hexaoxacyclooctadecane (1/1), (NH4)[SbCl6]·C12H24O6}, (3), analogues, the hexachloridoantimonate(V) anions and 18-crown-6 molecules reside across \overline{3} axes passing through the Sb atoms and the centroids of the 18-crown-6 groups, both of which coincide with centres of inversion. The Rb+[in (1)], Cs+[in (2)] and NH4+[in (3)] cations are situated inside the cavity of the 18-crown-6 ring; they are situated on \overline{3} axes and are equally disordered about centres of inversion, deviating from the centroid of the 18-crown-6 molecule by 0.4808 (13), 0.9344 (7) and 0.515 (8) Å, respectively. Interaction of the ammonium cation and the 18-crown-6 group is supported by three equivalent hydrogen bonds [N...O = 2.928 (3) Å and N—H...O = 162°]. The centrosymmetric structure of [Cs(18-crown-6)]+, with the large Cs+cation approaching the centre of the ligand cavity, is unprecedented and accompanied by unusually short Cs—O bonds [2.939 (2) and 3.091 (2) Å]. For all three compounds, the [M(18-crown-6)]+cations and [SbCl6]−anions afford linear stacks along thecaxis, with the cationic complexes embedded between pairs of inversion-related anions.