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

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 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 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 the c axis, with the cationic complexes embedded between pairs of inversion‐related anions.