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

The crystal structure of BF3H2O·C4H8O2 – the dioxane adduct of the ’super acid’ BF3H2O – is reported along with new preparative aspects and results of 1H, 11B and 13C and 19F spectroscopic investigations. The pronounced thermal stability of the solid adduct (m.p. 128–130 °C) in comparison to the liquid components is attributed to the chain structure established by O—H⋯O hydrogen bonds of exceptional strength taking into account the molecular (non-ionic) character of the structural moieties.

Highly Brønsted-acidic boron trifluoride monohydrate, a widely used 'super acid-catalyst', is a colourless fuming liquid that releases BF 3 at room temperature. Compared to the liquid components, i.e. boron trifluoride monohydrate and 1,4-dioxane, their 1:1 adduct, BF 3 H 2 OÁC 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 compound 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 acetonitrile solution. The remarkable stability of solid BF 3 H 2 OÁC 4 H 8 O 2 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) Å ] and O1-H1Á Á ÁO3 i [2.539 (3) Å ] in the concatenating unit >O2Á Á ÁH1-O1-H2Á Á ÁO3 i <}, taking into account the molecular (non-ionic) character of the structural moieties. Indirectly, this structural feature documents the outstanding acidification of the H 2 O molecule 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 (O,O 0 ) 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.

Chemical context
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 BF 3 dihydrate and, after addition of one further equivalent of BF 3 at low temperature, the BF 3 monohydrate also. 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 BF 3 above its melting point of 279.2 K. At room temperature, it is a colourless fuming liquid with a density of 1.8 g ml À1 . To examine the acidity of the monohydrate, reactions with ethers, alcohols and carboxylic acids etc. were performed by Meerwein & Pannwitz (1934 (Meerwein & Pannwitz, 1934). Unexpectedly, the experiment described in x6 resulted in the same product. The primordial idea of this experiment was to prepare an anhydrous solution of HBF 4 from HBF 4 /H 2 O (1:1 w:w) by distilling off water as the 1,4-dioxane/water azeotrope with coincident replacement of water by an excess of 1,4-dioxane. The dioxane adduct 1 starts to precipitate after a short period of time if a small amount of water remains in the resulting liquid. The formation of 1 in a 'HBF 4 solution' impressively illustrates how efficently BF 3 is stabilized by water and dioxane. The reactions and equilibria of HBF 4 -, BF 3 -, H 2 O-and HFcontaining systems have been examined in detail (Pawlenko, 1968;Gascard & Mascherpa, 1973;Christe et al., 1975;Mootz & Steffen, 1981a;Yeo & Ford, 2006;Dubey et al., 2007) and it remains amazing that BF 3 H 2 O, unlike the other boron trihalide/water mixtures, releases the strong Lewis-acid (BF 3 ) unhydrolysed. Investigations by Greenwood & Martin (1951) showed that BF 3 H 2 O is highly ionized in the liquid state and that the Hammett acidity of H[BF 3 OH] is H 0 = À11.4. By NMR spectroscopic determination of the thermodynamic acidity function from 13 C chemical-shift changes of the signals of unsaturated ketones at infinite dilution in the acid under investigation, Farcasui & Ghenciu (1992) found boron trifluoride monohydrate to be super acidic, with H 0 < À14. The applications of this super acid are numerous, e.g. as a highly effective catalyst for several Friedel-Craft reactions (Yoneda et al., 1969;Oyama et al., 1978;Liu et al., 2003;Prakash et al., 2016, and references therein). The long-time-stable and easy-to-handle solid 1 provides the 'super acid BF 3 H 2 O' in a safe and efficient way.
Although Meerwein & Pannwitz (1934) isolated compound 1 (m.p. 401-403 K) and a solid, in which BF 3 H 2 O is stabilized by 1,8-cineole (m.p. 344-346 K) more than 80 years ago, the crystal structures of these compounds are still unknown and the reasons for the unexpected high thermal stability, especially of the dioxane adduct, are still unknown. Generally, there are very rare examples of crystal structures with BF 3 H 2 O moieties bound to O-donor molecules. The crystal structure of boron trifluoride monohydrate itself has been reported by Mootz & Steffen (1981b), after redetermination of the crystal structure of the dihydrate in the same year (Mootz & Steffen, 1981c;Bang & Carpenter, 1964). Stabilization of the mono-and dihydrate with 18-crown-6 (Bott et al., 1991;Feinberg et al., 1993;Simonov et al., 1995) or of BF 3 H 2 O with dicyclohexane-18-crown-6 (Fonar et al., 1997) led to three further crystal structures containing the BF 3 H 2 O moiety and, as the most recent example, stabilization with triphenylphosphane oxide (Chekhlov, 2005) gave a crystalline 1:2 adduct of BF 3 H 2 O and (C 6 H 5 ) 3 PO.

Structural commentary
Compound 1 was found to crystallize in the orthorhombic space group Pbca with eight formula units in the unit cell and all components in general positions. Fig. 1 shows the asymmetric unit of the crystal structure, which contains aquatrifluoridoboron and 1,4-dioxane molecular moieties. The dioxane moiety is free of any kind of conformational disorder often recognized in the case of saturated six-membered ring species. Bond lengths, angles and torsion angles defining the chair conformation are in excellent agreement with the expectations for a 'fully ordered' dioxane molecule, e.g. found in the structure of uncomplexed 1,4-dioxane at 153 K (Buschmann et al., 1986). Compared to the mean equivalent isotropic displacement parameter (U eq ) of the C and O atoms in the 1,4-dioxane moiety [= 0.0427 (6) Å 2 ], the mean U eq value of B1, O1 and F1 to F3 in the aquatrifluoridoboron moiety [0.0867 (8) Å 2 ] is dramatically higher and correction for libration is needed prior to comparison with the geometries of BF 3 H 2 O moieties in related compounds. In Table 1, the uncorrected and corrected (Schomaker & Trueblood, 1968 (Mootz & Steffen, 1981b) and BF 3 H 2 OÁH 2 O (Mootz & Steffen, 1981c) in the left, middle and right columns, respectively; in square brackets are the corresponding bond valences and the valence sums calculated using the Brown formalism {r 0 [B-O(F)] = 1.371 (1.281), B = 0.37; Brown & Altermatt, 1985}; in braces are the values corrected for libration (Schomaker & Trueblood, 1968

Figure 1
Diagram of the asymmetric unit of the crystal structure of compound 1, displaying the atom-labelling scheme. Anisotropic displacement ellipsoids are drawn at the 40% probability level and the radii of H atoms are chosen arbitrarily.  Mootz & Steffen, 1981a]. The bond-valence sum of B1 is as expected taking into account the 'uncorrected' nature of the r 0 values used (Brown & Altermatt, 1985). Interestingly, for all compounds mentioned in Table 1 (3) 170 (5) Symmetry code: (i) Àx þ 3 2 ; y À 1 2 ; z.

Figure 3
Packing diagram of 1 (view direction [010]) documenting the arrangement of the zigzag chains to flat sheets perpendicular to the c axis. Inspection of the intermolecular distances gives no evidence for interactions stronger than van der Waals forces between the chains.

Supramolecular features
As mentioned before, in the solid of 1 the aqua ligand of the BF 3 H 2 O moiety acts as a hydrogen-bond donor in two directions, establishing a C 2 2 (7) graph set (Etter, 1990) (Fig. 2). The propagation vector of the zigzag chain is parallel to the b axis of the unit cell. Note the almost equal strength of both hydrogen bonds. Fig. 3 shows the arrangement of the chains in the solid due to van der Waals interactions.  Feinberg et al. 1993, Simonov et al., 1995 and dicyclohexano-18-crown-6 bis(monoaquatrifluoridoboron) (NIYGAD; Fonar et al., 1997); the phosphane oxide adduct monoaquatrifluoridoboron bis(triphenylphosphane oxide) (XATWAR; Chekhlov, 2005); two transition-metal coordination compounds [CIGVUJ10 (Van Rijn et al., 1987) and UKAJIA (Orain et al., 2010)], containing cocrystallized monoaquatrifluoridoboron moieties. As mentioned above, in addition to these reports on compounds having organic components, there is the report of Mootz & Steffen (1981b) Diehl (1958) reported the 19 F NMR spectra of BF 3 H 2 O in aqueous solution. He observed separate broad resonances which he attributed to HBF 3 OH, HBF 4 , HBF 2 (OH) 2 and HBF(OH) 3 in concentrated solutions at 243 K with coalescence of the peaks at higher temperatures. Gillespie & Hartman (1967) have shown by low-temperature (193 K) 1 H and 19 F NMR spetroscopy that BF 3 H 2 O is formed in dilute solutions in acetone containing both water and BF 3 . They found two major peaks in the 19 F NMR spectrum and assigned the low-field peak (À146.05 ppm) to the 1:1 complex of BF 3 with acetone and the high-field peak (À146.59 ppm) to BF 3 H 2 O in acetone. The corresponding 1 H NMR signals were detected by Gillespie & Hartmann at 12.42 ppm as multipletts.

Database survey
In our experiments, in the presence of CD 3 CN and 1,4dioxane and at a significantly higher temperature (297 K), the protons were detected as a broad singlet at 9.41 ppm. Gottlieb et al. (1997) indicated that the influence of temperature on the NMR shift overcompensates the influence of the solvent if the basicity of the solvents is similar. Apart from this effect, the high acidity of the oxygen-bonded 1 H nuclei in the title compound is depicted by a shift of more than 7 ppm to higher frequencies (H 2 O in CD 3 CN: s, 2.13 ppm; Fulmer et al. 2010). The chemical shifts of the NMR signals belonging to 1,4dioxane are close to those of the uncomplexed compound (C 4 H 8 O 2 in CD 3 CN: 1 H: s, 3.60 ppm; 13 C: 68.5 ppm; Fulmer et al., 2010). Due to the comparable donor numbers (Gutmann, 1976) of acetonitrile (NMR solvent) and 1,4-dioxane, it can be concluded that the acidity of BF 3 H 2 O is not critically reduced by 1,4-dioxane with respect to its application as a super acidcatalyst.

Synthesis and crystallization
All preparations and sample manipulations were carried out in tetrafluoroethylene hexafluoropropylene block copolymer (FEP) vessels. Tetrafluoroboric acid solution (50 wt% in water; Fluka Chemicals) was probed for its content of [BF 3 OH] À by 19 F NMR spectroscopy. Depending on the quantity of these anions, hydrofluoric acid (48 wt% in water, Sigma-Aldrich) was added. In a typical experiment, to 131.4 g (1.24 mol) of HBF 4 /H 2 O, 4.53 g (0.11 mol) HF/H 2 O was added at 273 K. The mixture was stirred for 15 min, before 430 g of 1,4-dioxane was added at the same temperature. Subsequently, the reaction mixture was heated and the 1,4-dioxane-water azeotrope was distilled off under normal pressure until the boiling point (361 K) began to change. 368 g of azeotrope was removed by the distillation and the residue was a pale-brown solution. This solution was stored in a sealed FEP flask under an atmosphere of dry argon (Argon 5.0). After 1 h, the formation of colourless crystals of 1 started and was allowed to continue for 9 d. The crystals were isolated under an argon atmosphere and washed with hexane/1,4-dioxane (10:1 v/v) three times using Schlenk techniques. 40.7 g (0.23 mol) were collected after drying the almost hexagonal colourless crystals in an argon stream (40 min). Compound 1 is stable at room temperature and shows a poor solubility in 1,4-dioxane, but a good solubility in acetonitrile.
An elemental analysis was performed with a HEKATECH EA 3000 elemental analyser using Callidus 2E3 software. 1.7 mg of freshly ground crystals were used and a modifier was added to suppress the influence of the high fluorine content.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The positions of all H atoms were identified via subsequent ÁF syntheses. In the refinement, a riding model was applied, using idealized C-H bond lengths, as well as H-C-H and C-C-H angles. The U iso values were set at 1.2U eq (C) for methylene H atoms. For the H atoms of the aqua ligand, positional parameters and U iso values were refined.  (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015), SHELXTL (Sheldrick, 2008) and publCIF (Westrip, 2010 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: publCIF (Westrip, 2010). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.