Boron monofluoride or fluoroborylene is a chemical compound with the formula BF, one atom of boron and one of fluorine. It is an unstable gas, but it is a stable ligand on transition metals, in the same way as carbon monoxide. It is a subhalide, containing fewer than the normal number of fluorine atoms, compared with boron trifluoride. It can also be called a borylene, as it contains boron with two unshared electrons. BF is isoelectronic with carbon monoxide and dinitrogen; each molecule has 14 electrons.[1]
Structure
The experimental B–F bond length is 1.26267 Å.[2][3][4] Despite being isoelectronic to the triple-bonded species CO and N2, computational studies generally agree that the true bond order is much lower than 3. One reported computed bond order for the molecule is 1.4, compared with 2.6 for CO and 3.0 for N2.[5]
Lewis dot diagram structures show three formal alternatives for describing bonding in boron monofluoride.
BF is unusual in that the dipole moment is inverted with fluorine having a positive charge even though it is the more electronegative element. This is explained by the 2sp orbitals of boron being reoriented and having a higher electron density. Backbonding, or the transfer of π orbital electrons for the fluorine atom, is not required to explain the polarization.[6]
Preparation
Boron monofluoride can be prepared by passing boron trifluoride gas at 2000 °C over a boron rod. It can be condensed at liquid nitrogen temperatures (−196 °C).[7]
Properties
Boron monofluoride molecules have a dissociation energy of 7.8 eV or heat of formation −27.5±3 kcal/mole[1][8] or 757±14 kJ/mol.[2] The first ionization potential is 11.115 eV.[2] The spectroscopic constants vibrational frequency ωe of BF+ (X 2Σ+) is 1765 cm−1 and for neutral BF (X 1Σ+) it is 1402.1 cm−1.[2][9] The anharmonicity of BF is 11.84 cm−1.[9]
Reactions
BF can react with itself to form polymers of boron containing fluorine with between 10 and 14 boron atoms. BF reacts with BF3 to form B2F4. BF and B2F4 further combine to form B3F5. B3F5 is unstable above −50 °C and forms B8F12. This substance is a yellow oil.[7]
BF reacts with acetylenes to make the 1,4-diboracyclohexadiene ring system. BF can condense with 2-butyne forming 1,4-difluoro-2,3,5,6-tetramethyl-1,4-diboracyclohexadiene. Also, it reacts with acetylene to make 1,4-difluoro-1,4-diboracyclohexadiene.[7] Propene reacts to make a mix of cyclic and non-cyclic molecules which may contain BF or BF2.[2]
BF reacts with oxygen: BF + O2 → OBF + O; with chlorine: BF + Cl2 → ClBF + Cl; and with nitrogen dioxide BF + NO2 → OBF + NO.[10]
Ligand
A naïve analysis would suggest that BF is isoelectronic with carbon monoxide (CO) and so could form similar compounds to metal carbonyls. As discussed above (see § Structure), BF has a much lower bond order, so that the valence shell around boron is unfilled. Consequently, BF as a ligand is much more Lewis acidic; it tends to form higher-order bonds to metal centers, and can also bridge between two or three metal atoms (μ2 and μ3).[11]
Working with BF as a ligand is difficult due to its instability in the free state.[12] Instead, most routes tend to use derivatives of BF3 that decompose once coordinated.
In a 1968 conference report, Kämpfer et al claimed to produce Fe(BF)(CO)4 via reaction of B2F4 with Fe(CO)5, but modern chemists have not reproduced the synthesis, and the original compound has no crystallographic characterization.[13][14] The first modern demonstration of BF coordinated to a transition element is due to Vidovic and Aldrige, who produced [(C5H5)Ru(CO)2]2(μ2-BF) (with BF bridging both ruthenium atoms) in 2009.[15] To make the compound, Vidovic and Aldridge reacted NaRu(CO)2(C5H5) with (Et2O)·BF3; the boron monofluoride ligand then formed in-place.[14]
Vidovic and Aldridge also developed a substance with the formula (PF3)4FeBF by reacting iron vapour with B2F4 and PF3.[2] Hafnium, thorium, titanium, and zirconium can form a difluoride with a BF ligand at the low temperature of 6K. These come about by reacting the atomic metal with BF3.[2]
The first fully characterized molecule featuring BF as a terminal ligand was synthesized by Drance and Figueroa in 2019, by sterically hindering the formation of a dimer. In the molecule, boron is double-bonded to iron.[16]
References
^ abHildenbrand, Donald L.; Murad, Edmond (1965). "Dissociation Energy of Boron Monofluoride from Mass-Spectrometric Studies". The Journal of Chemical Physics. 43 (4): 1400. Bibcode:1965JChPh..43.1400H. doi:10.1063/1.1696932.
^ abcdefghiVidovic, Dragoslav; Aldridge, Simon (2011). "Coordination chemistry of group 13 monohalides". Chemical Science. 2 (4): 601. doi:10.1039/C0SC00508H.
^Cazzoli, G.; Cludi, L.; Degli Esposti, C.; Dore, L. (1989). "The millimeter and submillimeter-wave spectrum of boron monofluoride: Equilibrium structure". Journal of Molecular Spectroscopy. 134 (1): 159–167. Bibcode:1989JMoSp.134..159C. doi:10.1016/0022-2852(89)90138-0. ISSN0022-2852.
^Martinie, R. J.; Bultema, J. J.; van der Wal, M. N.; Burkhart, B. J.; van der Griend, D. A. & de Kock, R. L. (2011). "Bond Order and Chemical Properties of BF, CO, and N2". Journal of Chemical Education. 88 (8): 1094–1097. Bibcode:2011JChEd..88.1094M. doi:10.1021/ed100758t.
^Fantuzzi, Felipe; Cardozo, Thiago Messias; Nascimento, Marco Antonio Chaer (28 May 2015). "Nature of the Chemical Bond and Origin of the Inverted Dipole Moment in Boron Fluoride: A Generalized Valence Bond Approach". The Journal of Physical Chemistry A. 119 (21): 5335–5343. Bibcode:2015JPCA..119.5335F. doi:10.1021/jp510085r. PMID25531385.
^ abDyke, John M.; Kirby, Colin; Morris, Alan (1983). "Study of the ionization process BF+ (X 2Σ+ ) ← BF(X 1Σ+ ) by high-temperature photoelectron spectroscopy". J. Chem. Soc., Faraday Trans. 2. 79 (3): 483–490. doi:10.1039/F29837900483.
^Xu, Liancai; Li, Qian-shu; Xie, Yaoming; King, R. Bruce; Schaefer, Henry F. (15 March 2010). "Major Difference between the Isoelectronic Fluoroborylene and Carbonyl Ligands: Triply Bridging Fluoroborylene Ligands in Fe3(BF)3(CO)9 Isoelectronic with Fe3(CO)12". Inorganic Chemistry. 49 (6): 2996–3001. doi:10.1021/ic902511m. PMID20143841.
^Xu, Liancai; Li, Qian-shu; King, R. Bruce (May 2012). "Fluoroborylene ligands in binuclear ruthenium carbonyls: Comparison with their iron analogues". Polyhedron. 38 (1): 44–49. doi:10.1016/j.poly.2012.02.003.
^Drance et al. 2019: "Previously, Vidovic and Aldridge reported that two equivalents of the ruthenium-based nucleophile Na[CpRu(CO)2] (Cp, cyclopentadienyl; [C5H5]−) reacts with boron trifluoride diethyl etherate (BF· 3Et 2O) with the formal loss of two equivalents of sodium fluoride (NaF) to produce the bridging BF complex ((μ2-BF)[CpRu(CO)2]2) (20). The latter is the only crystallographically characterized compound in which BF functions as a ligand to a metal center."
^ abXu, L.; Li, Q.-S.; Xie, Y.; King, R. B.; Schaefer, H. F. III (2010). "Binuclear fluoroborylene manganese carbonyls". Inorganica Chimica Acta. 363 (13): 3538–3549. doi:10.1016/j.ica.2010.07.013.
