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

Highly Brønsted-acidic boron trifluoride monohydrate, a widely used 'super acid-catalyst', is a colourless fuming liquid that releases BF₃ at room temperature. Com-pared to the liquid com-ponents, 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 com-pound 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 aceto-nitrile 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 mol-ecular (non-ionic) character of the structural moieties. Indirectly, this structural feature documents the outstanding acidification of the H₂O mol-ecule 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 ortho-rhom-bic unit cell is a characteristic structural feature.