Crystallographic evidence for different modes of interaction of BF₃/BF₄^- species with water molecules and 18-crown-6

We report the crystal and molecular structures of the complex of 18-C-6 with H₃O⁺BF₄^- (I) and the complex of 18-C-6 with BF₃OH₂·H₂O (II). The different modes of appearance of the "BF₃" species as BF₃, BF₃OH₂, BF₃OH₂·H₂O and BF₄^-, as well as their structurally significant intermolecular and intramolecular interactions, are discussed. In complex I the oxonium ion is bound at the centre of the 18-C-6 macrocycle. The oxonium oxygen is located practically equidistant (2.68-2.73 Å) from the six macrocyclic ethereal oxygens. The BF₄^- counter-ion is positioned 7.3 Å away from the oxonium ion in the same general plane of the crown ether. This anion is not involved in any direct intermolecular contacts, a fact that may explain why it is spherically disordered. In complex II there is no guest molecule (or ion) present in the "cavity" of the macrocycle, but there are two hydrogen-bonded systems of BF₃OH₂·H₂O that are interacting with the crown ether on either side of the general macrocyclic plane. Complex II features three types of hydrogen bonds-the O(water)-H⋯O(crown) bonds (2.83 and 2.85 Å), the O(water)⋯H-O(BF₃) bond (2.49 Å) and the O(BF₃)-H⋯O(crown) bond (2.65 Å). The strong intermolecular O(crown)⋯O(water)⋯O(BF₃) and O(crown)⋯O(BF₃) interactions stabilize the normally unstable BF₃OH₂·H₂O species.