The `super acid' BF3H2O stabilized by 1,4-dioxane: new preparative aspects and the crystal structure of BF3H2O·C4H8O2

Abstract

Highly Brønsted-acidic boron trifluoride monohydrate, a widely used `super acid-catalyst', is a colourless fuming liquid that releases BF3 at room temperature. Compared to the liquid components, i.e. boron trifluoride monohydrate and 1,4-dioxane, their 1:1 adduct, BF3H2O·C4H8O2, is a solid with pronounced thermal stability (m.p. 401–403 K). The crystal structure of the long-time-stable easy-to-handle and weighable compound is reported along with new preparative aspects and the results of 1H, 11B, 13C and 19F spectroscopic investigations, particularly documenting its high Brønsted acidity in acetonitrile solution. The remarkable stability of solid BF3H2O·C4H8O2 is attributed to the chain structure established by O—H...O hydrogen bonds of exceptional strength {O2...H1—O1 [O...O = 2.534 (3) Å] and O1—H1...O3i [2.539 (3) Å] in the concatenating unit >O2...H1—O1—H2...O3i<}, taking into account the molecular (non-ionic) character of the structural moieties. Indirectly, this structural feature documents the outstanding acidification of the H2O molecule bound to BF3 and reflects the super acid nature of BF3H2O. In detail, the C 2 2(7) zigzag chain system of hydrogen bonding in the title structure is characterized by the double hydrogen-bond donor and double (κO,κO′) hydrogen-bond acceptor functionality of the aqua ligand and dioxane molecule, respectively, the almost equal strength of both hydrogen bonds, the approximatety linear arrangement of the dioxane O atoms and the two neighbouring water O atoms. Furthermore, the approximately planar arrangement of B, F and O atoms in sheets perpendicular to the c axis of the orthorhombic unit cell is a characteristic structural feature.