Hydration, Ion Pairing, and Sandwich Motifs in Ammonium Nitrate Complexes of Crown Ethers

Ammonium nitrate coordinates to 18-crown-6 through a near-ideal ‘tripod’ arrangement of hydrogen bonds to alternate crown ether oxygen atoms. Ion pairing interactions are apparent, with the strength of this ion pairing interaction appearing to be reduced by hydration, representing a potentially significant role of water in cation–molecule complexes. The bis(benzo-15-crown-5) complex of ammonium nitrate is of sandwich type structure, with the ammonium ion complexed between two roughly parallel crown ether rings through an unusual but efficient set of linear, bifurcated, and off-axis hydrogen bonding interactions.