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). T he different modes of appearance of the ''BF3'' species as BF3, BF3OH2 , BF3OH2.H2O and,BF4-, as well as their structurally significant inter molecular 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 Angstrom) from the six macrocyclic ethereal oxygens. The BF4- counter-ion is po sitioned 7.3 Angstrom away from the oxonium ion in the same general pl ane of the crown ether. This anion is not involved in any direct inter molecular contacts, a fact that may explain why it is spherically diso rdered. In complex II there is no guest molecule (or ion) present in t he ''cavity'' of the macrocycle, but there are two hydrogen-bonded sys tems of BF3OH2.H2O that are interacting with the crown ether on either side of the general macrocyclic plane. Complex II features three type s of hydrogen bonds-the O(water)-H...O(crown) bonds (2.83 and 2.85 Ang strom), the O(water)...H-O(BF3) bond (2.49 Angstrom) and the O(BF3)-H O(crown) bond (2.65 Angstrom. The strong intermolecular O(crown>...O(w ater)...O(BF3) and O(crown)...O(BF3) interactions stabilize the normal ly unstable BF3OH2.H2O species.