Methyldiborane Explained

Methyldiborane, CH3B2H5, or monomethyldiborane is the simplest of alkyldiboranes, consisting of a methyl group substituted for a hydrogen in diborane. As with other boranes it exists in the form of a dimer with a twin hydrogen bridge that uses three-center two-electron bonding between the two boron atoms, and can be imagined as methyl borane (CH3BH2) bound to borane (BH3).[1] Other combinations of methylation occur on diborane, including 1,1-dimethylborane, 1,2-dimethyldiborane, trimethyldiborane, tetramethyldiborane, and trimethylborane (which is not a dimer). At room temperature the substance is at equilibrium between these molecules.

The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.[2] [3]

Formation

Methylboranes are 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. The reaction is complex. At 0 °C when diborane is in excess, monomethyldiborane is initially produced, coming to a steady but low level, and 1,1-dimethyldiborane level increases over a long time, until all trimethylborane is consumed. Monomethyldiborane ends up at equilibrium with a mixture of diborane and dimethyldiborane. At 0° the equilibrium constant for 2B2H5Me ←→ B2H6 + (BH2Me)2 is around 0.07, so monomethyldiborane will typically be the majority of the mixture, but there will still be a significant amount of diborane and dimethyldiborane present.[4] Monomethyldiborane yield is best with a ratio of 4 of diborane to 1 of trimethylborane.[5] The yield of trimethyldiborane is maximised with ratio of 1 of diborane to 3 of trimethylborane.[5]

When methyllithium reacts with diborane, monomethyldiborane is produced in about a 20% yield.[6]

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.[7]

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.[2] dimethylchloroborane and methyldichloroborane are also produced as gaseous products.[2]

When Cp2Zr(CH3)2 reacts with borane dissolved in tetrahydrofuran, a borohydro group inserts into the zirconium carbon bond, and methyl diboranes are produced.[8]

When trimethylgallium reacts with diborane at -45°, methyldiborane is produced along with dimethylgallium borohydride.

2(CH3)3Ga + B2H6 → (CH3)2GaBH4 + CH3B2H5.[9] At room temperature trimethylgallium reacts with diborane to make a volatile substance that decomposes to gallium metal along with methyldiborane.

(CH3)3Ga + 3B2H6 → Ga + 3CH3B2H5 +1.5H2.[9]

Properties

The compound boils at −43 °C.[10] Methyldiborane liquid has a density of 0.546 g/ml at −126° At −78.5 the vapour pressure is 55 torr.

Methyldiborane HCH3BH2BH2 has one methyl group and a hydrogen on a boron atom. The other boron atom is only bound to hydrogen atoms. A bridge of two hydrogen atoms links the boron atoms together. The methyldiborane molecule has the following measurements: B1 is the boron atom not attached to the methyl group and B2 is the boron atom that has methyl group attached, and Hμ is one of the bridge hydrogen atoms between the boron atoms. The distance between boron atoms is 1.82 Å, Distance between boron atoms and bridging hydrogen atoms is 1.34 Å. Distances to non bridging hydrogens from B1 are 1.195 and 1.187 Å. B2 distance to non bridging hydrogen is 1.2 Å. Distance between two bridging hydrogen atoms is 1.96 Å. The carbon to boron bond is 1.49 Å long. The angle subtended from the bridging hydrogens to the boron to boron axis is 47°. The angle of carbon to the boron-boron axis is 120°. The dipole moment of methyldiborane is 0.56 D.[11] It has a vapour pressure of 61 mm Hg at −77.2 °C.[5] The predicted heat of formation for the liquid is ΔH0f=−14 kcal/mol, and for the gas −9 kcal/mol.[12]

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.[13]

The nuclear resonance shift for the bridge hydrogen is 10.09 ppm, compared to 10.49 for diborane.[14]

Reactions

At -78.5 °C methyldiborane disproportionates slowly first to diborane and 1,1-dimethyldiborane.[15] In solution methylborane is more stable against disproportionation than dimethylborane.[16]

2MeB2H5 1,1-Me2B2H4 + B2H6 K=2.8 Me=CH3.[17]

By reacting methyldiborane with ether, dimethylether borine is formed (CH3)2O.BH3 leaving methylborane which rapidly dimerises to 1,2-dimethyldiborane.[18] [19]

Methyldiborane is hydrolyzed in water to methylboronic acid CH3B(OH)2.[18] Methyldiborane reacts with trimethylamine to yield solid derivatives trimethylamine-methylborane (CH3)3N—BHCH3 and trimethylamine-borane (CH3)3N—BH3.[18]

Methyldiborane is pyrophoric, spontaneously inflaming when exposed to air.[15]

When methyldiborane is oxidised around 150 °C a substance 2-methyl-1,3,4-trioxadiboralane is produced. This is a ring of three oxygen atoms and two boron atoms, with methyl attached to one boron atom. At the same time dimethyltrioxadiboralane and trimethylboroxine are also formed, and also hydrocarbons, diborane, hydrogen, and dimethoxyborane (dimethyl methylboronic ester).[20]

When methyldiborane, or dimethyldiborane is combined with ammonia, aminodimethylborine (NH2BMe2) is formed and on heating around 180 °C B-methyl borazoles are produced. These borazoles can have zero, one, two or three methyl groups substituted on the boron atoms (B3N3H6, MeB3N3H5, Me2B3N3H4 or Me3B3N3H3).[21] [22]

A specific way to make 1,2-dimethyldiborane is to react methyldiborane with a sufficient amount of a Lewis base. This will strip off borane to combine with the Lewis base, and let two methyl borane molecules dimerise.[23]

Methyldiborane can methylate tetraborane.

CH3B2H5 + B4H10 → 2-CH3B4H9 + B2H6[24]

Related

Bis(trimethylphosphine) methyldiborane is an adduct of methyldiborane formed when methylpentaborane (1-CH3B5H8 or 2-CH3B5H8) is reacted with trimethylphosphine.[25]

Extra reading

Notes and References

  1. Srebnik. Morris. Cole. Thomas E.. Brown. Herbert C.. Methylborane - a remarkable unhindered monoalkylborane which achieves the controlled sequential hydroboration of representative alkenes. Tetrahedron Letters. January 1987. 28. 33. 3771–3774. 10.1016/s0040-4039(00)96380-9.
  2. Long. L. H.. Wallbridge. M. G. H.. 646. The chemistry of boron. Part VI. New preparative methods and decomposition studies relating to methyldiboranes. Journal of the Chemical Society. 1965. 3513–3520. 10.1039/JR9650003513.
  3. Schlesinger. H. I.. Walker. A. O.. Hydrides of Boron. IV. The Methyl Derivatives of Diborane. Journal of the American Chemical Society. April 1935. 57. 4. 621–625. 10.1021/ja01307a009.
  4. Book: van Aalten. Lloyd. Seely. G. R.. Oliver. Juhn. Ritter. D. M.. Kinetics and Equilibria in the Alkylation of Diborane Preliminary Report. 32. 1 June 1961. 107–114. 10.1021/ba-1961-0032.ch012. American Chemical Society. Advances in Chemistry. 978-0-8412-0033-3.
  5. Carpenter. J.H.. Jones. W.J.. Jotham. R.W.. Long. L.H.. The Raman spectra of the methyldiboranes—II Monomethyldiborane and trimethyldiborane, and characteristic frequencies of the methyldiboranes. Spectrochimica Acta Part A: Molecular Spectroscopy. September 1971. 27. 9. 1721–1734. 10.1016/0584-8539(71)80227-1. 1971AcSpA..27.1721C. free.
  6. Massey. A. G.. Chapter 2. The typical elements. Part II: Group III. Annual Reports on the Progress of Chemistry, Section A. 1979. 76. 13–15. 10.1039/IC9797600013.
  7. Holliday. A.K.. N. Jessop. G.. The reaction of tetramethyllead with diborane. Journal of Organometallic Chemistry. November 1967. 10. 2. 291–293. 10.1016/s0022-328x(00)93089-4.
  8. Marsella. John A.. Caulton. Kenneth G.. Dealkylation of zirconium(IV) by borane: the intimate mechanism of an alkyl transfer reaction. Journal of the American Chemical Society. May 1982. 104. 9. 2361–2365. 10.1021/ja00373a005.
  9. Schlesinger. H. I.. Brown. Herbert C. The Borohydrides of Gallium. Journal of the American Chemical Society. September 1943. 65. 9. 1787. 10.1021/ja01249a512. 17 August 2015.
  10. Web site: Thermal reaction of diborane with trimethylborane. 4 September 1958. Lamneck, John H Jr . Kaye, Samuel. National Advisory Committee for Aeronautics.
  11. Chiu. C. W.. Burg. A. B.. Beaudet. R. A.. Molecular structure determination of methyldiborane. Inorganic Chemistry. March 1982. 21. 3. 1204–1208. 10.1021/ic00133a064.
  12. Web site: Altschuller. Aubrey P.. Calculated Heats of Formation and Combustion of Boron Compounds (Boron, Hydrogen, Carbon, Silicon). NACA Research Memorandum. National Advisory Committee for Aeronautics. 14 August 2015. Cleveland, Ohio. 22. 4 October 1955.
  13. Seely. G. R.. Oliver. J. P.. Ritter. D. M.. Gas-Liquid Chromatographic Analysis of Mixtures Containing Methyldiboranes. Analytical Chemistry. December 1959. 31. 12. 1993–1995. 10.1021/ac60156a032.
  14. Leach. John B.. Ungermann. Charles B.. Onak. Thomas P.. Proton magnetic resonance studies on methyl and chloro substituted diboranes. Journal of Magnetic Resonance. January 1972. 6. 1. 74–83. 10.1016/0022-2364(72)90088-1. 1972JMagR...6...74L.
  15. Book: Duward F. Shriver. Bunting. Roger K.. Inorganic Syntheses, Volume 19. 22 Sep 2009. John Wiley and Sons. 237–238. https://books.google.com/books?id=Tk3HUFj9XnkC&pg=PA237. 55 1-Methyldiborane. 978-0-471-04542-7.
  16. Brown. Herbert C.. Cole. Thomas E.. Srebnik. Morris. Kim. Kee Won. Hydroboration. 79. Preparation and properties of methylborane and dimethylborane and their characteristics as hydroborating agents. Synthesis of tertiary alcohols containing methyl groups via hydroboration. The Journal of Organic Chemistry. December 1986. 51. 25. 4925–4930. 10.1021/jo00375a031.
  17. Book: Onak. Thomas. Stone. F. G. A.. West. Robert. Advances in Organometallic Chemistry. 1 January 1966. Academic Press. New York, London. 284. https://books.google.com/books?id=AHEPoIx22iwC&pg=PA266. 19 August 2015. Carboranes and Organo-Substituted Boron Hydrides. 978-0-08-058004-3.
  18. Bell. R. P.. Emeléus. H. J.. The boron hydrides and related compounds. Quarterly Reviews, Chemical Society. 1948. 2. 2. 132. 10.1039/QR9480200132.
  19. Book: Adams. Roy M.. Metal-Organic Compounds. Organoboron Compounds. September 1959. 23. Metal-Organic Compounds. 92. 10.1021/ba-1959-0023.ch010. 17 August 2015. https://www.thevespiary.org/rhodium/Rhodium/Vespiary/talk/files/4262-AIC-023-METAL-ORGANIC-COMPOUNDS-13db1.pdf?topic=2297.0. Advances in Chemistry. 978-0-8412-0024-1.
  20. Barton. Lawrence. Crump. John M.. Wheatley. Jeffrey B.. Trioxadiborolanes from the oxidation of methyldiborane. Journal of Organometallic Chemistry. June 1974. 72. 1. C1–C3. 10.1016/s0022-328x(00)82027-6.
  21. Sheldon. J. C.. Smith. B. C.. The borazoles. Quarterly Reviews, Chemical Society. 1960. 14. 2. 202. 10.1039/QR9601400200.
  22. Schlesinger. H. I.. Horvitz. Leo. Burg. A. B.. Hydrides of Boron. VI. The Action of Ammonia on the Methyl Diboranes. Journal of the American Chemical Society. March 1936. 58. 3. 409–414. 10.1021/ja01294a008.
  23. Book: Wade. Kenneth. Electron Deficient Compounds. 1971. Thomas Nelson. 978-1-4684-6056-8. 91–92. https://books.google.com/books?id=7oXgBwAAQBAJ&pg=PA92. 17 August 2015. The General Chemistry of the Boron Hydrides.
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  25. Kameda. Mitsuaki. Driscoll. Jerry A.. Kodama. Goji. Formation and reactions of bis(trimethylphosphine)-methyldiborane(4). Inorganic Chemistry. September 1990. 29. 19. 3791–3795. 10.1021/ic00344a028.

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