The Reaction of Boron Fluoride with Water. I. Preparation and Some Properties of Pure Boron Fluoride Dihydrate1

Hydroxytricyanoborate Anion: Synthetic Aspects and Structural, Chemical, and Spectroscopic Properties

In recent years, salts of the hydridotricyanoborate anion [BH(CN)₃]^- (MHB) have become readily available. In spite of the unusually high stability of the MHB anion, it can be used as a valuable starting material for the preparation of selected tricyanoborates, for example, the boron-centered nucleophile B(CN)₃^2-. A further unprecedented example is the hydroxytricyanoborate anion [B(OH)(CN)₃]^- that is accessible by oxidation of (H₃O)MHB with elemental bromine in water. The Brønsted acid (H₃O)[B(OH)(CN)₃] was isolated as a crystalline solid. It serves as a versatile starting material for the synthesis of coordination compounds, metal salts, and ionic liquids. The [B(OH)(CN)₃]^- anion shows a rich coordination chemistry and a high tendency to form hydrogen-bonded motifs as demonstrated by a series of salts with different types of cations. Furthermore, the [B(OH)(CN)₃]^- anion itself serves as starting material for new tricyanoborates such as the unusual trianion [B{OB(CN)₃}₃]^3- and the silylated anions [B(OSiR₃)(CN)₃]^- (R = Me, Et, Ph). Some of these follow-up products have been characterized by single-crystal X-ray diffraction, e.g., [nBu₄N]₃[B{OB(CN)₃}₃] and [nBu₄N][B(OSiPh₃)(CN)₃].

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.