Boron monofluoride monoxide

Boron oxyfluoride

Names
IUPAC name Fluoro(oxo)borane
Other names boron monofluoride monoxide
Identifiers
CAS Number	23361-56-0
3D model (JSmol)	Interactive image
ChemSpider	124393
PubChem CID	141025
CompTox Dashboard (EPA)	DTXSID20177920
InChI InChI=1/BFO/c2-1-3 Key: FHYICEHKTRQYRP-UHFFFAOYSA-N
SMILES B(=O)F
Properties
Chemical formula	BFO
Molar mass	45.81 g·mol^−1
Appearance	Gas
Thermochemistry
Std enthalpy of formation (ΔfH^⦵298)	48.0 ± 3.0 kcal/mol[1]
Related compounds
Related oxyhalides	boron oxychloride
Related compounds	boron monofluoride boron monoxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Boron monofluoride monoxide or oxoboryl fluoride^[2] or fluoroxoborane is an unstable inorganic molecular substance with formula FBO. It is also called boron fluoride oxide, fluoro(oxo)borane or fluoro-oxoborane. The molecule is stable at high temperatures, but below 1000 °C condenses to a trimer (BOF)₃ called trifluoroboroxin. FBO can be isolated as a triatomic non-metallic molecule in an inert gas matrix, and has been condensed in solid neon and argon.^[3] When an attempt is made to condense the gas to a solid in bulk, a polymeric glass is formed, which is deficient in fluoride, and when heated forms a glassy froth like popcorn.^[4][5] Boron fluoride oxide has been studied because of its production in high energy rocket fuels that contain boron and fluorine, and in the form of an oxyfluoride glass. BOF glass is unusual in that it can condense directly from gas.^[6]

Properties

Monomer

The FBO molecule is linear with structure F-B=O. The F-B bond length is 1.283 Å, and B-O bond is 1.207 Å.^[7]

The infrared spectrum of BFO has vibrational bands at 1900, 1050, and 500 cm⁻¹.^[8] Spectroscopic constants of the ¹⁰BFO molecule are B=9349.2711 MHz D=3.5335 kHz and for ¹¹BFO molecule they are B=9347.3843 MHz D=3.5273 kHz^[9] The monomer is stable either at low pressures, or temperatures over 1000 °C. Below this temperature, the monomers associate to form a trimer^[10] called trifluoroboroxole.^[11]

Heat of formation Δ_fH^⦵
_{298 K} is predicted to be -146.1 kcal/mol. Proton affinity 149.6 kcal/mol.^[12]

Trimer

If a hot BFO gas is cooled slowly it dismutates back into B₂O₃ and BF₃.^[13] At room temperature this dismutation completes in an hour.^[13]

Boron fluoride oxide forms a trimer with a ring composed of alternating oxygen and boron atoms, with fluorine bonded to the boron. (BFO)₃. The ring structure puts it in the class of boroxols.^[14] This is also called trifluoroboroxin. The trimer is the predominant form in gas at 1000K.^[13] When heated to 1200K it mostly converts to the monomer BFO.^[13] Boron oxyfluoride can be condensed from vapour to a fluorine deficient glass at temperatures below 190° by very rapid cooling. When heated this deposit has a temperature at which it loses more BF₃ to form a frothy or porous glass that resembles popcorn. The glass deposited at lower temperatures has a higher proportion of fluorine. Deposits at -40 °C are predicted to have a 1:1 ratio of fluorine to oxygen.^[5] Below -135° (BFO)₃ is stable.^[15]

The heat of formation of the trimer from the monomer (BFO)₃ → 3BFO is 131 kcal/mol.^[16]

Glass

Boron oxyfluoride glass is transparent and colourless. It is stable in dry air, but it is hygroscopic and in normal air becomes white and opaque. When heated the glass will encounter a glass transition temperature (T_g) at which it ceases to be a glass, and produces BF₃ gas and a boron oxyfluoride with less fluorine is left behind. This glass transition temperature is determined from where the pressure of BF₃ produced exceeds the strength of the glass. The hypothetical structure of BOF glass, is of long chains of B-O-B-O with fluorine attached to each boron. These can be considered as BO₂F triangles linked in a chain by O atoms. These chains are tangled up like spaghetti in the glass. When the substance becomes fluorine deficient, crosslinks with oxygen form between the chains, and it becomes more two dimensional in structure.^[17] BF₃ is produced when the terminals of two linear −(BF)O− chains join with each other.^[6] These ends contain -O-BF₂, and when two meet, BF₃ can be eliminated and the chain extended with oxygen.^[18]

Occurrence

BFO is expected to form in supernovae II output in gas between 1,000 and 2,000 °C and pressures around 10⁻⁷ bar.^[19]

Preparation

Otto Ruff noticed that a mixture of BF₃ and SiF₄ passing over molten B₂O₃ produced some SiO₂ and redistributed B₂O₃ into cold parts of the reaction tube. He speculated that there must be some heat stable intermediate that converted back into the original components on cooling.^[20][21] Several years later, Paul Baumgarten and Werner Bruns made the boron oxyfluoride trimer by passing BF₃ over solid B₂O₃ at 450 °C.^[20][22]

BFO is an intermediate in the hydrolysis of BF₃ along with BF(OH)₂, BF₂OH and boric acid.

• BF₃ + H₂O → BFO + 2HF;
• BF₂OH → BFO + HF;
• BF(OH)₂ → BFO + H₂O^[23]

Another way in which BFO can be made is to vapourise B₂O₃ with BF₃.^[5]

When BF₃ is heated with air, BFO gas predominates from 2800° to 4000 °C, being a maximum at 3200°. Above 4000 °C BO dominates.^[8]

Hot BF₃ passed over some oxides such as SiO₂ forms BFO.^[24] Other oxides that can yield boron oxyfluoride are magnesium oxide, titanium dioxide, carbonates or alumina.^[25]

In the plasma phase HF reacts with BO₂H⁺
₂, B₂OH⁺, B₃O⁺
₄, B₂O⁺
₄, B₂O⁺
₂, B₂OH⁺ to make FBO, and other products including FBOH and FBO⁺.^[26]

Related

The B-O-F molecule theoretically exists but it releases energy when it rearranges to F-B-O.^[27][28] A related molecule is BOF₂.^[29] Molecules related to the trimer include B₃O₃ClF₂, B₃O₃Cl₂F, and (BOCl)₃.^[30]

FBO is predicted to be able to insert noble gas atoms between the fluorine and boron atom yielding FArBO, FKrBO and FXeBO. The molecules are predicted to be linear.^[31]

Uses

Boron oxyfluoride could be used in boriding steel. By using a gas, sticking solids onto the steel is avoided. Also this method allows control of the boron concentration, and mostly forms Fe₂B instead of the more brittle FeB.^[25] Burning boron releases much energy, so its use in explosives or fuel is being researched. To maximise energy output, both fluorine and oxygen are used to react, and thus FBO and related molecules are formed and may be in the exhaust.^[26]

References

1. Larson, J. W.; McMahon, T. B. (1987). "Boron monofluoride monoxide". 26 (24). NIST: 4018. doi:10.1021/ic00271a011. Retrieved 20 May 2015. {{cite journal}}: Cite journal requires |journal= (help)
2. Kuchitsu, Kozo (2013-03-09). Structure of Free Polyatomic Molecules: Basic Data. Springer Science & Business Media. p. 28. ISBN 9783642457487.
3. Jacox, Marilyn E. (December 1994). "The vibrational energy levels of small transient molecules isolated in neon and argon matrices". Chemical Physics. 189 (2): 149–170. Bibcode:1994CP....189..149J. doi:10.1016/0301-0104(94)00143-X.
4. Boussard-Plédel, Catherine; Le Floch, Marie; Fonteneau, Gilles; Lucas, Jacques (July 1997). "Vapor Phase Deposition of Foaming Boron Oxyfluoride Vitreous Materials". Materials Research Bulletin. 32 (7): 805–811. doi:10.1016/S0025-5408(97)00050-0.
5. Boussard-Plédel, Catherine; Le Floch, Marie; Fonteneau, Gilles; Lucas, Jacques; Sinbandhit, Sourisak; Shao, J.; Angell, C.A.; Emery, Joël; Buzaré, J.Y. (February 1997). "The structure of a boron oxyfluoride glass, an inorganic cross-linked chain polymer". Journal of Non-Crystalline Solids. 209 (3): 247–256. Bibcode:1997JNCS..209..247B. doi:10.1016/S0022-3093(96)00548-0.
6. Polishchuk, S. A.; Ignat’eva, L. N.; Marchenko, Yu. V.; Bouznik, V. M. (5 March 2011). "Oxyfluoride glasses (A review)". Glass Physics and Chemistry. 37 (1): 1–20 (14). doi:10.1134/S108765961101010X. S2CID 97609959.
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8. Yoder, M.John (December 1974). "High temperature arc studies of infrared radiation from boron and tungsten oxides". Journal of Quantitative Spectroscopy and Radiative Transfer. 14 (12): 1317–1328. Bibcode:1974JQSRT..14.1317Y. doi:10.1016/0022-4073(74)90100-9.
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12. Nguyen, Minh Tho; Vanquickenborne, L. G.; Sana, Michel; Leroy, Georges (May 1993). "Heats of formation and proton affinities of some oxoborons (R-B≡O) and sulfidoborons (R-B≡S) with R = hydrogen, fluorine, chlorine, and methyl group". The Journal of Physical Chemistry. 97 (20): 5224–5227. doi:10.1021/j100122a010.
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14. Goubeau, J.; Keller, H. (December 1952). "Über Boroxol-Verbindungen Darstellung, physikalische und chemische Eigenschaften". Zeitschrift für anorganische und allgemeine Chemie (in German). 267 (1–3): 1–26. doi:10.1002/zaac.19522670102.
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16. Hildenbrand, Donald L.; Theard, Lowell P.; Saul, Albert M. (1963). "Transpiration and Mass Spectrometric Studies of Equilibria Involving BOF(g) and (BOF)₃(g)". The Journal of Chemical Physics. 39 (8): 1973. Bibcode:1963JChPh..39.1973H. doi:10.1063/1.1734569.
17. Boussard-Plédel, Catherine; Fonteneau, Gilles; Lucas, Jacques (July 1995). "Boron oxyfluoride glasses in the B-O-F system: new polymeric spaghetti-type glasses". Journal of Non-Crystalline Solids. 188 (1–2): 147–152. doi:10.1016/0022-3093(95)00183-2.
18. Lucas, Jacques (May 1995). "Non-conventional spaghetti-type glasses". Journal of Non-Crystalline Solids. 184: 21–24. Bibcode:1995JNCS..184...21L. doi:10.1016/0022-3093(95)00087-9.
19. Hoppe, P.; Lodders, K.; Strebel, R.; Amari, S.; Lewis, R. S. (10 April 2001). "Boron in Presolar Silicon Carbide Grains from Supernovae". The Astrophysical Journal. 551 (1): 478–485. Bibcode:2001ApJ...551..478H. doi:10.1086/320075.
20. Baumgarten, Paul; Bruns, Werner (6 September 1939). "Über die Umsetzung von Borfluorid mit Bortrioxyd, Boraten, Carbonaten und Nitraten und zur Kenntnis eines mutmaßlichen Boroxyfluorides (BOF)₃". Berichte der Deutschen Chemischen Gesellschaft (A and B Series) (in German). 72 (9): 1753–1762. doi:10.1002/cber.19390720921.
21. Ruff, Otto; Braida, A.; Bretschneider, O.; Menzel, W.; Plaut, H. (18 May 1932). "Die Darstellung, Dampfdrucke und Dichten des BF₃, AsF₅ und BrF₃". Zeitschrift für anorganische und allgemeine Chemie (in German). 206 (1): 59–64. doi:10.1002/zaac.19322060108.
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23. Zhang, Lei; Zhang, Weijiang; Xu, Jiao; Ren, Xin (7 December 2014). "Synthesis of enriched ¹⁰B boric acid of nuclear grade". Transactions of Tianjin University. 20 (6): 458–462. Bibcode:2014TrTU...20..458Z. doi:10.1007/s12209-014-2303-x. S2CID 91517483.
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28. Soto, Maribel R. (April 1995). "Rate Constant Determinations for HBO + F Channels from ab Initio Reaction Path Calculations". The Journal of Physical Chemistry. 99 (17): 6540–6547. doi:10.1021/j100017a039.
29. Mathews, C.Weldon (January 1966). "The 4465-Å emission spectrum of the BOF₂ molecule". Journal of Molecular Spectroscopy. 19 (1–4): 203–223. Bibcode:1966JMoSp..19..203M. doi:10.1016/0022-2852(66)90242-6.
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31. Lin, Tsun-Yi; Hsu, Jeng-Bin; Hu, Wei-Ping (February 2005). "Theoretical prediction of new noble-gas molecules OBNgF (Ng=Ar, Kr, and Xe)". Chemical Physics Letters. 402 (4–6): 514–518. Bibcode:2005CPL...402..514L. doi:10.1016/j.cplett.2004.12.090.