Crystallographic evidence for different modes of interaction of BF 3/BF 4− species with water molecules and 18-crown-6

Abstract

We report the crystal and molecular structures of the complex of 18-C-6 with H 3O +BF 4 − ( I) and the complex of 18-C-6 with BF 3OH 2·H 2O ( II). The different modes of appearance of the “BF 3” species as BF 3, BF 3OH 2, BF 3OH 2·H 2O and BF 4 −, 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 BF 4 − 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 BF 3OH 2·H 2O 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(BF 3) bond (2.49 Å) and the O(BF 3)-H⋯O(crown) bond (2.65 Å). The strong intermolecular O(crown)⋯O(water)⋯O(BF 3) and O(crown)⋯O(BF 3) interactions stabilize the normally unstable BF 3OH 2·H 2O species.