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

aInstitut für Anorganische Chemie und Strukturchemie, Lehrstuhl II: Material- und Strukturforschung, Heinrich-Heine-Universität Düsseldorf, Universitätsstrasse 1, D-40225 Düsseldorf, Germany
*Correspondence e-mail: peter.barthen@hhu.de, wfrank@hhu.de

Edited by E. V. Boldyreva, Russian Academy of Sciences, Russia (Received 26 June 2019; accepted 21 October 2019; online 31 October 2019)

Highly Brønsted-acidic boron trifluoride monohydrate, a widely used `super acid-catalyst', is a colourless fuming liquid that releases BF3 at room temperature. Com­pared to the liquid com­ponents, 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 com­pound 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 aceto­nitrile 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 mol­ecular (non-ionic) character of the structural moieties. Indirectly, this structural feature documents the outstanding acidification of the H2O mol­ecule bound to BF3 and reflects the super acid nature of BF3H2O. In detail, the C22(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 ortho­rhom­bic unit cell is a characteristic structural feature.

1. Chemical context

Solutions of boron trifluoride in water have been under investigation for more than 200 years (Gay-Lussac & Thenard, 1809[Gay-Lussac, J. L. & Thenard, L. J. (1809). Ann. Phys. 32, 1-15.]; Davy, 1812[Davy, J. (1812). Phil. Trans. R. Soc. 102, 352-363.]; Berzelius, 1824[Berzelius, J. J. (1824). Liebigs Ann. Chem. 46, 48-58.]). Meerwein (1933[Meerwein, H. (1933). Ber. Dtsch Chem. Ges. A/B, 66, 411-414.]) was able to isolate the BF3 dihydrate and, after addition of one further equivalent of BF3 at low temperature, the BF3 monohydrate also. Both hydrates were examined in detail (Klinkenberg & Ketelaar, 1935[Klinkenberg, L. J. & Ketelaar, J. A. A. (1935). Recl Trav. Chim. Pays Bas, 54, 959-961.]; McGrath et al., 1944[McGrath, J. S., Stack, G. G. & McCusker, P. A. (1944). J. Am. Chem. Soc. 66, 1263-1264.]; Greenwood & Martin, 1951[Greenwood, N. N. & Martin, R. L. (1951). J. Chem. Soc. pp. 1915-1921.]; Wamser, 1951[Wamser, C. A. (1951). J. Am. Chem. Soc. 73, 409-416.]; Pawlenko, 1959[Pawlenko, S. (1959). Z. Anorg. Allg. Chem. 300, 152-158.]) 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 ml−1. To examine the acidity of the monohydrate, reactions with ethers, alcohols and carb­oxy­lic acids etc. were performed by Meerwein & Pannwitz (1934[Meerwein, H. & Pannwitz, W. (1934). J. Prakt. Chem. 141, 123-148.]). They obtained BF3H2O·C4H8O2, which they called the dioxane salt of boron trifluoride monohydrate, by adding BF3H2O to a solution of 1,4-dioxane in petroleum naphta. BF3H2O·C4H8O2 (1) precipitates as needle-shaped crystals which melt at 401–403 K with decom­position (Meerwein & Pannwitz, 1934[Meerwein, H. & Pannwitz, W. (1934). J. Prakt. Chem. 141, 123-148.]). Unexpectedly, the experiment described in §6[link] resulted in the same product. The primordial idea of this experiment was to prepare an anhydrous solution of HBF4 from HBF4/H2O (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 `HBF4 solution' impressively illustrates how efficently BF3 is stabilized by water and dioxane. The reactions and equilibria of HBF4-, BF3-, H2O- and HF-containing systems have been examined in detail (Pawlenko, 1968[Pawlenko, S. (1968). Chem. Ing. Tech. 40, 52-55.]; Gascard & Mascherpa, 1973[Gascard, C. & Mascherpa, G. (1973). J. Chim. Phys. 70, 1040-1047.]; Christe et al., 1975[Christe, K. O., Schack, C. J. & Wilson, R. D. (1975). Inorg. Chem. 14, 2224-2230.]; Mootz & Steffen, 1981a[Mootz, D. & Steffen, M. (1981a). Z. Anorg. Allg. Chem. 482, 193-200.]; Yeo & Ford, 2006[Yeo, G. A. & Ford, T. A. (2006). S. Afr. J. Chem. 59, 129-134.]; Dubey et al., 2007[Dubey, A., Saha, H. P., Pink, R. H., Badu, S. R., Mahato, D. N., Scheicher, R. H., Mahanti, K. M., Chow, L. & Das, T. P. (2007). Hyperfine Interact. 176, 45-50.]) and it remains amazing that BF3H2O, unlike the other boron trihalide/water mixtures, releases the strong Lewis-acid (BF3) unhydrolysed. Investigations by Greenwood & Martin (1951[Greenwood, N. N. & Martin, R. L. (1951). J. Chem. Soc. pp. 1915-1921.]) showed that BF3H2O is highly ionized in the liquid state and that the Hammett acidity of H[BF3OH] is H0 = −11.4. By NMR spectroscopic determination of the thermodynamic acidity function from 13C chemical-shift changes of the signals of unsaturated ketones at infinite dilution in the acid under investigation, Farcasui & Ghenciu (1992[Farcasui, D. & Ghenciu, A. (1992). J. Catal. 134, 126-133.]) found boron trifluoride monohydrate to be super acidic, with H0 < −14. The applications of this super acid are numerous, e.g. as a highly effective catalyst for several Friedel–Craft reactions (Yoneda et al., 1969[Yoneda, N., Hasegawa, E., Aomura, K. & Ohtsuka, H. (1969). Bull. Jpn Petrol. Inst. 11, 54-58.]; Oyama et al., 1978[Oyama, T., Hamano, T., Nagumo, K. & Nakane, R. (1978). Bull. Chem. Soc. Jpn, 51, 1441-1443.]; Liu et al., 2003[Liu, L., Wang, X. & Li, C. (2003). Org. Lett. 5, 361-363.]; Prakash et al., 2016[Prakash, G. K. S., Gurung, L., Marinez, E. R., Mathew, T. & Olah, G. A. (2016). Tetrahedron Lett. 57, 288-291.], and references therein). The long-time-stable and easy-to-handle solid 1 provides the `super acid BF3H2O' in a safe and efficient way.

[Scheme 1]

Although Meerwein & Pannwitz (1934[Meerwein, H. & Pannwitz, W. (1934). J. Prakt. Chem. 141, 123-148.]) isolated com­pound 1 (m.p. 401–403 K) and a solid, in which BF3H2O 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 BF3H2O moieties bound to O-donor mol­ecules. The crystal structure of boron trifluoride monohydrate itself has been reported by Mootz & Steffen (1981b[Mootz, D. & Steffen, M. (1981b). Z. Anorg. Allg. Chem. 483, 171-180.]), after redetermination of the crystal structure of the dihydrate in the same year (Mootz & Steffen, 1981c[Mootz, D. & Steffen, M. (1981c). Acta Cryst. B37, 1110-1112.]; Bang & Carpenter, 1964[Bang, W. B. & Carpenter, G. B. (1964). Acta Cryst. 17, 742-745.]). Stabilization of the mono- and dihydrate with 18-crown-6 (Bott et al., 1991[Bott, S. B., Alvanipour, A. & Atwood, J. L. (1991). J. Incl. Phenom. Macrocycl. Chem. 10, 153-158.]; Feinberg et al., 1993[Feinberg, H., Columbus, I., Cohen, S., Rabinovitz, M., Selig, H. & Shoham, G. (1993). Polyhedron, 12, 2913-2919.]; Simonov et al., 1995[Simonov, Y. A., Gelmboldt, V. O., Ganin, E. V., Dvorkin, A. A., Fonar, M. S., Ostapchuk, L. V. & Lipkovsky, Y. (1995). Russ. J. Coord. Chem. 21, 724-729.]) or of BF3H2O with di­cyclo­hexane-18-crown-6 (Fonar et al., 1997[Fonar, M. S., Simonov, Y. A., Mazus, M. D., Ganin, E. V. & Gelmboldt, V. O. (1997). Crystallogr. Rep. 42, 790-794.]) led to three further crystal structures containing the BF3H2O moiety and, as the most recent example, stabilization with tri­phenyl­phosphane oxide (Chekhlov, 2005[Chekhlov, A. N. (2005). Russ. J. Coord. Chem. 31, 9-13.]) gave a crystalline 1:2 adduct of BF3H2O and (C6H5)3PO.

2. 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[link] 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[Buschmann, J., Müller, E. & Luger, P. (1986). Acta Cryst. C42, 873-876.]). Compared to the mean equivalent isotropic displacement parameter (Ueq) of the C and O atoms in the 1,4-dioxane moiety [= 0.0427 (6) Å2], the mean Ueq value of B1, O1 and F1 to F3 in the aqua­tri­fluorido­boron moiety [0.0867 (8) Å2] is dramatically higher and correction for libration is needed prior to com­parison with the geometries of BF3H2O moieties in related com­pounds. In Table 1[link], the uncorrected and corrected (Schomaker & Trueblood, 1968[Schomaker, V. & Trueblood, K. N. (1968). Acta Cryst. B24, 63-76.]; RG = 0.0241) B—O and B—F bond lengths of 1 are given in com­parison to the bond lengths of BF3H2O (Mootz & Steffen, 1981c[Mootz, D. & Steffen, M. (1981c). Acta Cryst. B37, 1110-1112.]) and BF3H2O·H2O (Mootz & Steffen, 1981b[Mootz, D. & Steffen, M. (1981b). Z. Anorg. Allg. Chem. 483, 171-180.]). After correction, the values of 1 agree well with those of the hydrates and those in almost undistorted BF4 as found in Li[BF4] at 200 K [1.387 (3)–1.391 (3) Å; Matsumoto et al., 2006[Matsumoto, K., Hagiwara, R., Mazej, Z., Goreshnik, E. & Žemva, B. (2006). J. Phys. Chem. B, 110, 2138-2141.]] or in H5O2[BF4] [1.381 (2)–1.399 (2) Å; Mootz & Steffen, 1981a[Mootz, D. & Steffen, M. (1981a). Z. Anorg. Allg. Chem. 482, 193-200.]]. The bond-valence sum of B1 is as expected taking into account the `uncorrected' nature of the r0 values used (Brown & Altermatt, 1985[Brown, I. D. & Altermatt, D. (1985). Acta Cryst. B41, 244-247.]). Inter­estingly, for all com­pounds mentioned in Table 1[link], the B—F bond perpendicular to the plane of the aqua ligand (1: B1—F3; BF3H2O: B1—F2; BF3H2O·H2O: B1—F3) is slightly but significantly longer than the other two B—F bonds, probably attributable to a small destabilizing inter­action with the oxygen lone pair. The F—B—O angles in all three com­pounds [1: 105.6 (3)–109.8 (3)°; BF3H2O: 105.9 (4)–108.1 (4)°; BF3H2O·H2O: 106.3 (1)–109.8 (1)°] are smaller than the F—B—F angles [1: 109.9 (3)–112.1 (3)°; BF3H2O: 111.2 (4)–113.0 (4)°; BF3H2O·H2O: 109.8 (1)–114.0 (1)°]. This fits to the observation (Table 1[link]) that the B—O bond in the BF3H2O moiety is relatively weaker than the B—F bonds and the planar geometry of BF3 is preserved in the aqua com­plex to some extent. Furthermore, for all three com­pounds, the O—B—F angle including the F atom that is approximately in plane with the aqua ligand [1: O1—B1—F1 = 105.6 (3)°; BF3H2O: O—B—F3 = 105.9 (4)°; BF3H2O·H2O: O1—B—F2 = 106.3 (1)°] is significantly smaller than the other O—B—F angles. This observation may be attributed to an attractive F⋯H inter­action within the moiety.

Table 1
Selected bond lengths (Å)

Values for BF3H2O·C4H8O2, BF3H2O (Mootz & Steffen, 1981b[Mootz, D. & Steffen, M. (1981b). Z. Anorg. Allg. Chem. 483, 171-180.]) and BF3H2O·H2O (Mootz & Steffen, 1981c[Mootz, D. & Steffen, M. (1981c). Acta Cryst. B37, 1110-1112.]) 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 {r0[B—O(F)] = 1.371 (1.281), B = 0.37; Brown & Altermatt, 1985[Brown, I. D. & Altermatt, D. (1985). Acta Cryst. B41, 244-247.]}; in braces are the values corrected for libration (Schomaker & Trueblood, 1968[Schomaker, V. & Trueblood, K. N. (1968). Acta Cryst. B24, 63-76.]).

B1—O1 1.473 (4) [0.76] {1.528 (4) [0.65]} 1.532 (6) [0.64] 1.512 (2) [0.68]
B1—F1 1.361 (4) [0.81] {1.409 (4) [0.71]} 1.383 (5) [0.76] 1.377 (2) [0.77]
B1—F2 1.332 (4) [0.87] {1.396 (4) [0.73]} 1.399 (5) [0.73] 1.382 (2) [0.76]
B1—F3 1.333 (4) [0.87] {1.410 (4) [0.71]} 1.382 (5) [0.76] 1.390 (2) [0.74]
       
Σs(B–O,F) [3.31] {[2.80]} [2.89] [2.96]
[Figure 1]
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. The direction of hydrogen bonding is given by dashed lines.

Although both BF3H2O and 1,4-dioxane are liquids at room temperature, adduct 1 is a solid with a remarkably high melting point (401–403 K), mainly resulting from the concatenation of the mol­ecular com­ponents via O—H⋯O hydrogen bonding, as shown in Fig. 2[link]. The high stability might be correlated to the exceptional strength of both O2⋯H1—O1 [O⋯O = 2.534 (3) Å] and O1—H1⋯O3i [2.539 (3) Å] in the concatenating >O2⋯H1—O1—H2⋯O3i< unit. Indirectly, this structural feature documents the outstanding acidification of the H2O mol­ecule bound to BF3 and reflects the super acid nature of BF3H2O. Further details of the hydrogen bonding are given in Table 2[link]. To the best of our knowledge, there is no example of a water ligand bonded to a nonmetal or a metal with the ligand engaged in a hydrogen bond of similar strength (O⋯O < 2.60 Å) to an O atom of a dioxane mol­ecule. In the adduct 18-crown-6·BF3H2O (m.p. 345 K), mentioned in §1[link], the aqua ligand is hydrogen bonded to two O-donor atoms and the O⋯O distances are 2.76 and 2.80 Å (Feinberg et al., 1993[Feinberg, H., Columbus, I., Cohen, S., Rabinovitz, M., Selig, H. & Shoham, G. (1993). Polyhedron, 12, 2913-2919.]). In the structure of BF3H2O·H2O, the nonligating water mol­ecule plays a similar role as bridging species as the dioxane mol­ecule in 1. The O⋯O distances in the characteristic ⋯H—O—H⋯O(H2)⋯H—O—H⋯ unit are 2.631 and 2.643 Å (Mootz & Steffen, 1981c[Mootz, D. & Steffen, M. (1981c). Acta Cryst. B37, 1110-1112.]), i.e. as com­pared to the very strong Brønsted acids fluoro­sulfuric acid [O⋯O = 2.643 (1) Å] or tri­fluoro­methane­sulfonic acid [O⋯O = 2.640 (4) Å] (Bartmann & Mootz, 1990[Bartmann, K. & Mootz, D. (1990). Acta Cryst. C46, 319-320.]), for example, the hydrogen bonding is of the same strength in the dihydrate and much stronger in the adduct 1.

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A D—H H⋯A DA D—H⋯A
O1—H1⋯O2 0.82 (5) 1.72 (5) 2.534 (3) 175 (5)
O1—H2⋯O3i 0.82 (5) 1.72 (5) 2.539 (3) 170 (5)
Symmetry code: (i) [-x+{\script{3\over 2}}, y-{\script{1\over 2}}, z].
[Figure 2]
Figure 2
The zigzag chain of hydrogen-bonded moieties in the crystal of 1 [view direction [001]; 30% probability ellipsoids; symmetry codes: (A) −x + [{3\over 2}], y − [{1\over 2}], z; (B) x, y + 1, z]. Features indicative for the mode of concatenation of the characteristic building blocks by hydrogen bonding are: (i) double hydrogen-bond donor and double (κO,κO′) hydrogen-bond acceptor functionality of the aqua ligand and dioxane moiety, respectively; (ii) almost equal strength of both hydrogen bonds; (iii) an approximatety linear arrangement of the dioxane O atoms and the two neighbouring water O atoms (e.g. O1, O3A, O2A and O1A); (iv) an approximately planar arrangement of B1, F1, O1, O2 and O3.

3. Supra­molecular features

As mentioned before, in the solid of 1 the aqua ligand of the BF3H2O moiety acts as a hydrogen-bond donor in two directions, establishing a C22(7) graph set (Etter, 1990[Etter, M. C. (1990). Acc. Chem. Res. 23, 120-126.]) (Fig. 2[link]). 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[link] shows the arrangement of the chains in the solid due to van der Waals inter­actions.

[Figure 3]
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.

4. Database survey

A search of the Cambridge Structural Database (CSD; Version 5.40, November 2018 update; Groom et al., 2016[Groom, C. R., Bruno, I. J., Lightfoot, M. P. & Ward, S. C. (2016). Acta Cryst. B72, 171-179.]) for the BF3H2O moiety yielded six structures: the crown ether adducts 18-crown-6 mono­aqua­tri­fluorido­boron toluene semisolvate (CSD refcode SIXFOU; Bott et al. 1991[Bott, S. B., Alvanipour, A. & Atwood, J. L. (1991). J. Incl. Phenom. Macrocycl. Chem. 10, 153-158.]), 18-crown-6 bis­(mono­aqua­tri­fluorido­boron) dihydrate (LEKYIJ; Feinberg et al. 1993[Feinberg, H., Columbus, I., Cohen, S., Rabinovitz, M., Selig, H. & Shoham, G. (1993). Polyhedron, 12, 2913-2919.], Simonov et al., 1995[Simonov, Y. A., Gelmboldt, V. O., Ganin, E. V., Dvorkin, A. A., Fonar, M. S., Ostapchuk, L. V. & Lipkovsky, Y. (1995). Russ. J. Coord. Chem. 21, 724-729.]) and di­cyclo­hexano-18-crown-6 bis­(mono­aqua­tri­fluorido­boron) (NIYGAD; Fonar et al., 1997[Fonar, M. S., Simonov, Y. A., Mazus, M. D., Ganin, E. V. & Gelmboldt, V. O. (1997). Crystallogr. Rep. 42, 790-794.]); the phosphane oxide adduct mono­aqua­tri­fluorido­boron bis­(tri­phenyl­phosphane oxide) (XATWAR; Chekhlov, 2005[Chekhlov, A. N. (2005). Russ. J. Coord. Chem. 31, 9-13.]); two transition-metal coordination com­pounds [CIGVUJ10 (Van Rijn et al., 1987[Van Rijn, J., Reedijk, J., Dartmann, M. & Krebs, B. (1987). J. Chem. Soc. Dalton Trans. pp. 2579-2593.]) and UKAJIA (Orain et al., 2010[Orain, P.-Y., Capon, J.-F., Gloaguen, F., Pétillon, F. Y., Schollhammer, P., Talarmin, J., Zampella, G., De Gioia, L. & Roisnel, T. (2010). Inorg. Chem. 49, 5003-5008.])], 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[Mootz, D. & Steffen, M. (1981b). Z. Anorg. Allg. Chem. 483, 171-180.]) on the inorganic parent com­pound BF3H2O and there are two reports on the dihydrate BF3H2O·H2O (Mootz & Steffen, 1981c[Mootz, D. & Steffen, M. (1981c). Acta Cryst. B37, 1110-1112.]; Bang & Carpenter, 1964[Bang, W. B. & Carpenter, G. B. (1964). Acta Cryst. 17, 742-745.]).

5. NMR spectroscopy

NMR studies of BF3H2O·C4H8O2 have not been published so far. Ford & Richards (1956[Ford, P. T. & Richards, R. E. (1956). J. Chem. Soc. pp. 3870-3874.]) have shown by low-temperature NMR investigations that, in the solid state, BF3H2O and BF3H2O·H2O are not ionized. Diehl (1958[Diehl, P. (1958). Helv. Phys. Acta, 31, 685-712.]) reported the 19F NMR spectra of BF3H2O in aqueous solution. He observed separate broad resonances which he attributed to HBF3OH, HBF4, HBF2(OH)2 and HBF(OH)3 in concentrated solutions at 243 K with coalescence of the peaks at higher temperatures. Gillespie & Hartman (1967[Gillespie, R. J. & Hartman, J. S. (1967). Can. J. Chem. 45, 859-863.]) have shown by low-temperature (193 K) 1H and 19F NMR spetroscopy that BF3H2O is formed in dilute solutions in acetone containing both water and BF3. They found two major peaks in the 19F NMR spectrum and assigned the low-field peak (−146.05 ppm) to the 1:1 com­plex of BF3 with acetone and the high-field peak (−146.59 ppm) to BF3H2O in acetone. The corresponding 1H NMR signals were detected by Gillespie & Hartmann at 12.42 ppm as multipletts. In our experiments, in the presence of CD3CN and 1,4-dioxane and at a significantly higher temperature (297 K), the protons were detected as a broad singlet at 9.41 ppm. Gottlieb et al. (1997[Gottlieb, H. E., Kotlyar, V. & Nudelman, A. (1997). J. Org. Chem. 62, 7512-7515.]) 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 1H nuclei in the title com­pound is depicted by a shift of more than 7 ppm to higher frequencies (H2O in CD3CN: s, 2.13 ppm; Fulmer et al. 2010[Fulmer, G. R., Miller, A. J. M., Sherden, N. H., Gottlieb, H. E., Nudelman, A., Stoltz, B. M., Bercaw, J. E. & Goldberg, K. I. (2010). Organometallics, 29, 2176-2179.]). The chemical shifts of the NMR signals belonging to 1,4-dioxane are close to those of the uncom­plexed com­pound (C4H8O2 in CD3CN: 1H: s, 3.60 ppm; 13C: 68.5 ppm; Fulmer et al., 2010[Fulmer, G. R., Miller, A. J. M., Sherden, N. H., Gottlieb, H. E., Nudelman, A., Stoltz, B. M., Bercaw, J. E. & Goldberg, K. I. (2010). Organometallics, 29, 2176-2179.]). Due to the com­parable donor numbers (Gutmann, 1976[Gutmann, V. (1976). Coord. Chem. Rev. 18, 225-255.]) of aceto­nitrile (NMR solvent) and 1,4-dioxane, it can be concluded that the acidity of BF3H2O is not critically reduced by 1,4-dioxane with respect to its application as a super acid-catalyst.

The NMR sample was investigated in a 5 mm precision glass NMR tube (Wilmad 507) at 297 K in the deuterium-locked mode on a Bruker Avance III 400 MHz spectrometer operating at 400.17, 376.54, 128.23 or 100.62 MHz for 1H, 19F, 11B and 13C nuclei, respectively. The 1H NMR and 13C chemical shifts were referenced with respect to tetra­methyl­silane yielding the chemical shift for CD3CN (contains CD2HCN) as 1.96 ppm and CD3CN as 118.7 ppm. The 19F chemical shifts were referenced with respect to CFCl3 (0 ppm) as external standard. The 11B chemical shifts were referenced with respect to BF3·(C2H5)2O (0 ppm) as external standard. 68 mg of ground crystals were dissolved in 0.5 ml CD3CN to prepare the NMR sample: 1H NMR: 3.71 (s, 8H, C4H8O2), 9.41 (s, 2H, H2O). 19F NMR: −148.10 (s, 11BF3), −148.04 (s, 10BF3). 11B NMR: −0.1 (s, 11BF3). 13C NMR: 68.0 [t, 1J(C,H) = 189 Hz, C4H8O2].

6. 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 [BF3OH] by 19F 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 HBF4/H2O, 4.53 g (0.11 mol) HF/H2O 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. Analysis calculated (%) for C4H10BF3O3: 27.62 C, 5.80 H; found: 27.84 C, 5.87 H.

7. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3[link]. The positions of all H atoms were identified via subsequent ΔF 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 Uiso values were set at 1.2Ueq(C) for methyl­ene H atoms. For the H atoms of the aqua ligand, positional parameters and Uiso values were refined.

Table 3
Experimental details

Crystal data
Chemical formula H2BF3O·C4H8O2
Mr 173.93
Crystal system, space group Orthorhombic, Pbca
Temperature (K) 223
a, b, c (Å) 7.6835 (5), 12.929 (1), 15.2326 (13)
V3) 1513.2 (2)
Z 8
Radiation type Mo Kα
μ (mm−1) 0.16
Crystal size (mm) 0.69 × 0.48 × 0.42
 
Data collection
Diffractometer Stoe IPDS
Absorption correction Multi-scan (Blessing, 1989[Blessing, R. H. (1989). J. Appl. Cryst. 22, 396-397.])
Tmin, Tmax 0.673, 0.920
No. of measured, independent and observed [I > 2σ(I)] reflections 19913, 1481, 932
Rint 0.085
(sin θ/λ)max−1) 0.617
 
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.067, 0.138, 1.38
No. of reflections 1481
No. of parameters 108
H-atom treatment H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3) 0.60, −0.42
Computer programs: X-AREA (Stoe & Cie, 2009[Stoe & Cie (2009). X-AREA. Stoe & Cie GmbH, Darmstadt, Germany.]), SHELXS (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]), SHELXL2014 (Sheldrick, 2015[Sheldrick, G. M. (2015). Acta Cryst. C71, 3-8.]), SHELXTL (Sheldrick, 2008[Sheldrick, G. M. (2008). Acta Cryst. A64, 112-122.]) and publCIF (Westrip, 2010[Westrip, S. P. (2010). J. Appl. Cryst. 43, 920-925.]).

Supporting information


Computing details top

Data collection: X-AREA (Stoe & Cie, 2009); cell refinement: X-AREA (Stoe & Cie, 2009); data reduction: X-AREA (Stoe & Cie, 2009); program(s) used to solve structure: SHELXS (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).

Aquatrifluoridoboron–1,4-dioxane (1/1) top
Crystal data top
H2BF3O·C4H8O2Dx = 1.527 Mg m3
Mr = 173.93Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, PbcaCell parameters from 7579 reflections
a = 7.6835 (5) Åθ = 2.7–25.9°
b = 12.929 (1) ŵ = 0.16 mm1
c = 15.2326 (13) ÅT = 223 K
V = 1513.2 (2) Å3Prisms, colourless
Z = 80.69 × 0.48 × 0.42 mm
F(000) = 720
Data collection top
Stoe IPDS
diffractometer
932 reflections with I > 2σ(I)
Radiation source: sealed tubeRint = 0.085
φ–scanθmax = 26.0°, θmin = 2.7°
Absorption correction: multi-scan
(Blessing, 1989)
h = 99
Tmin = 0.673, Tmax = 0.920k = 1515
19913 measured reflectionsl = 1818
1481 independent reflections
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.067Hydrogen site location: difference Fourier map
wR(F2) = 0.138H atoms treated by a mixture of independent and constrained refinement
S = 1.38 w = 1/[σ2(Fo2) + 1.3744P]
where P = (Fo2 + 2Fc2)/3
1481 reflections(Δ/σ)max < 0.001
108 parametersΔρmax = 0.60 e Å3
0 restraintsΔρmin = 0.42 e Å3
Special details top

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
F10.2228 (2)0.10315 (15)0.11881 (16)0.0760 (7)
F20.2173 (4)0.0515 (2)0.05362 (17)0.1020 (9)
F30.2125 (4)0.0419 (2)0.19661 (18)0.1177 (11)
O10.4628 (3)0.0027 (2)0.1262 (3)0.0968 (14)
H10.516 (6)0.057 (4)0.133 (3)0.108 (17)*
H20.515 (6)0.053 (4)0.128 (3)0.109 (17)*
O20.6367 (2)0.16850 (14)0.13995 (14)0.0435 (5)
O30.8588 (2)0.33805 (14)0.11459 (14)0.0425 (5)
C10.8229 (3)0.1569 (2)0.1462 (2)0.0409 (7)
H110.87050.13720.08890.049*
H120.85110.10210.18830.049*
C20.9021 (4)0.2567 (2)0.1756 (2)0.0435 (8)
H210.85850.27450.23410.052*
H221.02880.24930.17920.052*
C30.6730 (3)0.3498 (2)0.1084 (2)0.0410 (7)
H310.64490.40460.06630.049*
H320.62560.36960.16570.049*
C40.5934 (4)0.2498 (2)0.0791 (2)0.0448 (8)
H410.46660.25730.07560.054*
H420.63660.23190.02050.054*
B10.2712 (4)0.0021 (3)0.1232 (3)0.0412 (8)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
F10.0439 (11)0.0559 (12)0.128 (2)0.0169 (9)0.0004 (13)0.0007 (12)
F20.0930 (18)0.113 (2)0.1005 (19)0.0035 (16)0.0243 (16)0.0500 (16)
F30.118 (2)0.141 (2)0.0942 (19)0.035 (2)0.0120 (17)0.0528 (18)
O10.0264 (12)0.0294 (13)0.234 (4)0.0014 (11)0.008 (2)0.0130 (18)
O20.0295 (10)0.0357 (10)0.0652 (14)0.0044 (8)0.0025 (9)0.0052 (10)
O30.0292 (10)0.0355 (10)0.0629 (14)0.0033 (8)0.0010 (10)0.0012 (10)
C10.0308 (16)0.0374 (15)0.0544 (18)0.0027 (12)0.0045 (13)0.0001 (14)
C20.0314 (14)0.0430 (17)0.056 (2)0.0027 (13)0.0078 (14)0.0035 (14)
C30.0318 (15)0.0367 (15)0.0544 (19)0.0014 (12)0.0022 (13)0.0054 (14)
C40.0348 (15)0.0442 (17)0.055 (2)0.0003 (13)0.0100 (15)0.0025 (14)
B10.0299 (16)0.0422 (18)0.052 (2)0.0037 (15)0.0039 (18)0.0006 (15)
Geometric parameters (Å, º) top
F1—B11.361 (4)C1—C21.495 (4)
F2—B11.332 (4)C1—H110.9800
F3—B11.333 (4)C1—H120.9800
O1—B11.473 (4)C2—H210.9800
O1—H10.82 (5)C2—H220.9800
O1—H20.82 (5)C3—C41.499 (4)
O2—C41.440 (3)C3—H310.9800
O2—C11.442 (3)C3—H320.9800
O3—C31.439 (3)C4—H410.9800
O3—C21.442 (3)C4—H420.9800
B1—O1—H1121 (3)O3—C3—H31109.8
B1—O1—H2119 (3)C4—C3—H31109.8
H1—O1—H2120 (4)O3—C3—H32109.8
C4—O2—C1110.4 (2)C4—C3—H32109.8
C3—O3—C2110.4 (2)H31—C3—H32108.2
O2—C1—C2109.5 (2)O2—C4—C3110.1 (2)
O2—C1—H11109.8O2—C4—H41109.6
C2—C1—H11109.8C3—C4—H41109.6
O2—C1—H12109.8O2—C4—H42109.6
C2—C1—H12109.8C3—C4—H42109.6
H11—C1—H12108.2H41—C4—H42108.2
O3—C2—C1110.1 (2)F3—B1—F2109.9 (3)
O3—C2—H21109.6F3—B1—F1111.0 (3)
C1—C2—H21109.6F2—B1—F1112.1 (3)
O3—C2—H22109.6F3—B1—O1108.3 (3)
C1—C2—H22109.6F2—B1—O1109.8 (3)
H21—C2—H22108.2F1—B1—O1105.6 (3)
O3—C3—C4109.5 (2)
C4—O2—C1—C258.7 (3)C2—O3—C3—C458.7 (3)
C3—O3—C2—C159.2 (3)C1—O2—C4—C358.9 (3)
O2—C1—C2—O358.6 (3)O3—C3—C4—O258.5 (3)
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O1—H1···O20.82 (5)1.72 (5)2.534 (3)175 (5)
O1—H2···O3i0.82 (5)1.72 (5)2.539 (3)170 (5)
Symmetry code: (i) x+3/2, y1/2, z.
Selected bond lengths (Å) in BF3H2O·C4H8O2, BF3H2O (Mootz &amp; Steffen, 1981b) and BF3H2O·H2O (Mootz &amp; Steffen, 1981c) in the left, middle and right columns, respectively; in square brackets are the corresponding bond valences and valence sums calculated using the Brown formalism [r0(B—O(F) = 1.371(1.281), B = 0.37; Brown &amp; Altermatt, 1985]; in braces are the values corrected for libration (Schomaker &amp; Trueblood, 1968). top
B1—O11.473 (4) [0.76] {1.528 (4) [0.65]}1.532 (6) [0.64]1.512 (2) [0.68]
B1—F11.361 (4) [0.81] {1.409 (4) [0.71]}1.383 (5) [0.76]1.377 (2) [0.77]
B1—F21.332 (4) [0.87] {1.396 (4) [0.73]}1.399 (5) [0.73]1.382 (2) [0.76]
B1—F31.333 (4) [0.87] {1.410 (4) [0.71]}1.382 (5) [0.76]1.390 (2) [0.74]
Σs(B–O,F)[3.31] {[2.80]}[2.89][2.96]
 

Acknowledgements

We thank E. Hammes and P. Roloff for technical support, and C. Siemes for the preparation of the structural formula scheme.

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