Trimethyldiborane

Trimethyldiborane, (CH3)3B2H3 is a molecule containing boron carbon and hydrogen. It is an alkylborane, consisting of three methyl group substituted for a hydrogen in diborane. It can be considered a mixed dimer: (CH3)2BH2BH(CH3) or dimethylborane and methylborane.[1] called 1,2-dimethyldiborane.[2] Other combinations of methylation occur on diborane, including monomethyldiborane, 1,2-dimethyldiborane, tetramethyldiborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms, so it is difficult to keep it pure.[3] The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.[4][5]

Trimethyldiborane
Names
IUPAC name
1,1,2-Trimethyldiborane
Other names
Trimethyldiborane(6)
Identifiers
3D model (JSmol)
Properties
(CH
3)
3B
2H
3
Molar mass 69.75 g mol−1
Appearance Colorless pyrophoric liquid
Melting point −122.9 °C (−189.2 °F; 150.2 K)
Boiling point 45.5 °C (113.9 °F; 318.6 K)
Thermochemistry
48 kcal/mol
Related compounds
Related alkyl boranes
 trimethylborane
tetramethyldiborane
dimethyldiborane
methyldiborane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N (what is YN ?)
Infobox references

Formation

Trimethylborane is formed by the reaction of diborane and trimethylborane. This reaction produces four different substitution of methyl with hydrogen on diborane. Produced is 1-methyldiborane, 1,1-dimethyldborane, 1,1,2-trimethyldiborane and 1,1,2,2-tetramethyldiborane. By reacting monomethyldiborane with ether, dimethylether borine is formed [(CH3)2O].BH3 leaving methylborane which rapidly dimerises to 1,2-dimethyldiborane.[3] The reaction is complex.[6] The yield of trimethyldiborane is maximised with ratio of 1 of diborane to 3 of trimethylborane.[7]

Tetramethyl lead can react with diborane in a 1,2-dimethoxyethane solvent at room temperature to make a range of methyl substituted diboranes, ending up at trimethylborane, but including 1,1-di, tridiborane. The other outputs of the reaction are hydrogen gas and lead metal.[8]

Other methods to form methyldiboranes include reacting hydrogen with trimethylborane between 80 and 200 °C under pressure, or reacting a metal borohydride with trimethylborane in the presence of hydrogen chloride, aluminium chloride or boron trichloride. If the borohydride is sodium borohydride, then methane is a side product. If the metal is lithium then no methane is produced.[4] Dimethylchloroborane and methyldichloroborane are also produced as gaseous products.[4]

When Cp2Zr(CH3)2 reacts with borane dissolved in tetrahydrofuran, a borohydro group inserts into the zirconium carbon bond, and methyl diboranes are produced.[9]

Properties

Trimethyldiborane has two methyl groups on one boron atom, and one methyl and a hydrogen on the second boron atom. A bridge of two hydrogen atoms links the boron atoms together. The molecule is expected to have a Cs point group due to rapid rotation of the methyls. The infrared spectrum of trimethyldiborane has a strong absorption band at 2509 cm−1 due to the non-bridge boron-hydrogen bond.[10] It has a vapour pressure of 51 mm Hg at -22.8 °C; 61 mm Hg at -18.4 °C and[7] 83 mm Hg at 0 °C.[11] Vapour pressure can be approximated by Log P = 7.673 - (1527/T).[12] The boiling point is 45.5 °C, and the melting point is -122.9.[12]

The predicted heat of formation for liquid trimethyldiborane is ΔH0f=-48 kcal/mol, and for the gas -41 kcal/mol. Heat of vapourisation ΔHvap was measured at 7.0 kcal/mol.[13]

A gas chromatograph can be used to determine the amounts of the methyl boranes in a mixture. The order they pass through are diborane, monomethyldiborane, trimethylborane, 1,1-dimethyldiborane, 1,2-dimethyldiborane, trimethyldiborane, and last tetramethyldiborane.[14]

The nuclear resonance shift for the bridge hydrogen is 9.27 ppm, compared to 10.49 for diborane.[15]

Reactions

Trimethyldiborane partially disproportionates over a period of hours at room temperature to yield tetramethyldiborane and 1,2-dimethyldiborane. Over a period of weeks 1,1-dimethyldiborane appears as well.[16]

3[1,1-(CH3)3B2H4] ⇌ 2 (CH3)3B2H3 + B2H6 K=0.00027[17]
4(CH3)3B2H3 ⇌ (CH3)4B2H2 + B2H6 K=0.0067[17]

Trimethyldiborane is hydrolyzed in water to methylboronic acid CH3B(OH)2 and dimethylborinic acid (CH3)2B(OH).[3]

Trimethyldiborane spontaneously inflames when exposed to air.[18]

Trimethyldiborane reacts with liquid ammonia initially forming methylborohydride anions and (CH3)2B(N3)2+ cations.[19][20][21]

Trimethylborane (CH3)3B has a similar-sounding name, and many similar properties, but only has one boron atom.[4] Trimethylhydroborate (CH3)3BH− is an anion with one boron atom. It can form a lithium salt.[22]

References
  1. Srebnik, Morris; Cole, Thomas E.; Brown, Herbert C. (January 1987). "Methylborane - a remarkable unhindered monoalkylborane which achieves the controlled sequential hydroboration of representative alkenes". Tetrahedron Letters. 28 (33): 3771–3774. doi:10.1016/s0040-4039(00)96380-9.
  2. Low, M. J. D. (1968). "Characteristic Infrared Frequencies of Methyldiboranes". The Journal of Chemical Physics. 48 (5): 2386–2387. Bibcode:1968JChPh..48.2386L. doi:10.1063/1.1669454.
  3. Bell, R. P.; Emeléus, H. J. (1948). "The boron hydrides and related compounds". Quarterly Reviews, Chemical Society. 2 (2): 132. doi:10.1039/QR9480200132.
  4. Long, L. H.; Wallbridge, M. G. H. (1965). "646. The chemistry of boron. Part VI. New preparative methods and decomposition studies relating to methyldiboranes". Journal of the Chemical Society (Resumed): 3513. doi:10.1039/JR9650003513. (subscription required)
  5. Schlesinger, H. I.; Walker, A. O. (April 1935). "Hydrides of Boron. IV. The Methyl Derivatives of Diborane". Journal of the American Chemical Society. 57 (4): 621–625. doi:10.1021/ja01307a009.
  6. van Aalten, Lloyd; Seely, G. R.; Oliver, Juhn; Ritter, D. M. (1 June 1961). Kinetics and Equilibria in the Alkylation of Diborane Preliminary Report (PDF). Advances in Chemistry. 32. American Chemical Society. pp. 107–114. doi:10.1021/ba-1961-0032.ch012. ISBN 0-8412-0033-5.
  7. Carpenter, J.H.; Jones, W.J.; Jotham, R.W.; Long, L.H. (September 1971). "The Raman spectra of the methyldiboranes—II Monomethyldiborane and trimethyldiborane, and characteristic frequencies of the methyldiboranes". Spectrochimica Acta Part A: Molecular Spectroscopy. 27 (9): 1721–1734. Bibcode:1971AcSpA..27.1721C. doi:10.1016/0584-8539(71)80227-1.
  8. Holliday, A.K.; N. Jessop, G. (November 1967). "The reaction of tetramethyllead with diborane". Journal of Organometallic Chemistry. 10 (2): 291–293. doi:10.1016/s0022-328x(00)93089-4.
  9. Marsella, John A.; Caulton, Kenneth G. (May 1982). "Dealkylation of zirconium(IV) by borane: the intimate mechanism of an alkyl transfer reaction". Journal of the American Chemical Society. 104 (9): 2361–2365. doi:10.1021/ja00373a005.
  10. Cowan, R. D. (1949). "The Infra-Red Spectra of Borine Carbonyl and Tetramethyldiborane". The Journal of Chemical Physics. 17 (2): 218. Bibcode:1949JChPh..17..218C. doi:10.1063/1.1747225.
  11. Lamneck, John H , Jr & Kaye, Samuel (4 September 1958). "Thermal reaction of diborane with trimethylborane". National Advisory Committee for Aeronautics.CS1 maint: uses authors parameter (link)
  12. Onak, Thomas (1 January 1966). Stone, F. G. A.; West, Robert (eds.). Advances in Organometallic Chemistry. New York, London: Academic Press. p. 284. ISBN 9780080580043. Retrieved 14 August 2015.
  13. Altschuller, Aubrey P. (4 October 1955). "Calculated Heats of Formation and Combustion of Boron Compounds (Boron, Hydrogen, Carbon, Silicon)" (PDF). NACA Research Memorandum. Cleveland, Ohio: National Advisory Committee for Aeronautics. p. 22. Retrieved 14 August 2015.
  14. Seely, G. R.; Oliver, J. P.; Ritter, D. M. (December 1959). "Gas-Liquid Chromatographic Analysis of Mixtures Containing Methyldiboranes". Analytical Chemistry. 31 (12): 1993–1995. doi:10.1021/ac60156a032.
  15. Leach, John B.; Ungermann, Charles B.; Onak, Thomas P. (January 1972). "Proton magnetic resonance studies on methyl and chloro substituted diboranes". Journal of Magnetic Resonance. 6 (1): 74–83. Bibcode:1972JMagR...6...74L. doi:10.1016/0022-2364(72)90088-1.
  16. Lehmann, Walter J.; Wilson, Charles O.; Shapiro, I. (1961). "Infrared Spectra of Alkyldiboranes. V. Tri- and Tetramethyl- and Ethyldiboranes". The Journal of Chemical Physics. 34 (3): 783. Bibcode:1961JChPh..34..783L. doi:10.1063/1.1731675.
  17. Onak, Thomas (1 January 1966). "Carboranes and Organo-Substituted Boron Hydrides". In Stone, F. G. A.; West, Robert (eds.). Advances in Organometallic Chemistry. New York, London: Academic Press. p. 284. ISBN 9780080580043. Retrieved 19 August 2015.
  18. Urben, Peter; Pitt, M. J., eds. (2007). Bretherick's Handbook of Reactive Chemical Hazards (7th ed.). Amsterdam: Elsevier. p. 527. ISBN 978-0-12-373945-2.
  19. Jungfleisch, Francis (1973). "Reactions of Methyl Substituted Diboranes and 2,2-Dimethyltetraborane with Amine Bases". Ohio State University. p. 64. Retrieved 15 August 2015.
  20. Sheldon, J. C.; Smith, B. C. (1960). "The borazoles". Quarterly Reviews, Chemical Society. 14 (2): 202. doi:10.1039/QR9601400200.
  21. Schlesinger, H. I.; Horvitz, Leo; Burg, A. B. (March 1936). "Hydrides of Boron. VI. The Action of Ammonia on the Methyl Diboranes". Journal of the American Chemical Society. 58 (3): 409–414. doi:10.1021/ja01294a008.
  22. "inorganic materials research division - The Berkeley Lab" (PDF).

Extra reading
  • Carpenter, J. H.; Jones, W. J.; Jotham, R. W.; Long, L. H. (1968). "Laser-source Raman spectroscopy and the Raman spectra of the methyldiboranes". Chemical Communications (London) (15): 881. doi:10.1039/C19680000881.
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