Abstract
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.
Highlights
Brønsted-acidic boron trifluoride monohydrate, a widely used ‘super acid-catalyst’, is a colourless fuming liquid that releases BF3 at room temperature
The remarkable stability of solid BF3H2OÁC4H8O2 is attributed to the chain structure established by O—HÁ Á ÁO hydrogen bonds of exceptional strength {O2Á Á ÁH1—O1 [OÁ Á ÁO = 2.534 (3) A ] and O1—H1Á Á ÁO3i [2.539 (3) A ] in the concatenating unit >O2Á Á ÁH1—O1—H2Á Á ÁO3i
This structural feature documents the outstanding acidification of the H2O molecule bound to BF3 and reflects the super acid nature of BF3H2O
Summary
Solutions of boron trifluoride in water have been under investigation for more than 200 years (Gay-Lussac & Thenard, 1809; Davy, 1812; Berzelius, 1824). Meerwein (1933) was able to isolate the BF3 dihydrate and, after addition of one further equivalent of BF3 at low temperature, the BF3 monohydrate . Meerwein (1933) was able to isolate the BF3 dihydrate and, after addition of one further equivalent of BF3 at low temperature, the BF3 monohydrate Both hydrates were examined in detail (Klinkenberg & Ketelaar, 1935; McGrath et al, 1944; Greenwood & Martin, 1951; Wamser, 1951; Pawlenko, 1959) and while the dihydrate was shown to be distillable without decomposition under reduced pressure, boron trifluoride monohydrate releases BF3 above its melting point of 279.2 K. At room temperature, it is a colourless fuming liquid with a density of 1.8 g mlÀ1. Stabilization of the mono- and dihydrate with 18-crown-6 (Bott et al, 1991; Feinberg et al, 1993; Simonov et al, 1995) or of BF3H2O with dicyclohexane-18-crown-6 (Fonar et al, 1997) led to three further crystal structures containing the BF3H2O moiety and, as the most recent example, stabilization with triphenylphosphane oxide (Chekhlov, 2005) gave a crystalline 1:2 adduct of BF3H2O and (C6H5)3PO
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