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Highly Brønsted-acidic boron trifluoride monohydrate, a widely used 'super acid-catalyst', is a colourless fuming liquid that releases BF 3 at room temperature. Com-pared to the liquid com-ponents, i.e. boron trifluoride monohydrate and 1,4-dioxane, their 1:1 adduct, BF 3 H 2 O·C 4 H 8 O 2 , 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 1 H, 11 B, 13 C and 19 F spectroscopic investigations, particularly documenting its high Brønsted acidity in aceto-nitrile solution. The remarkable stability of solid BF 3 H 2 O·C 4 H 8 O 2 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⋯O3 i [2.539 (3) Å] in the concatenating unit &gt;O2⋯H1-O1-H2⋯O3 i &lt;}, taking into account the mol-ecular (non-ionic) character of the structural moieties. Indirectly, this structural feature documents the outstanding acidification of the H 2 O mol-ecule bound to BF 3 and reflects the super acid nature of BF 3 H 2 O. 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 (κ O ,κ O ') 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.

acta crystallographica. section e, crystallographic communications

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