1,1-Dimethyldiborane

1,1-Dimethyldiborane is the organoboron compound with the formula (CH3)2B(μ-H)2BH2. A pair of related 1,2-dimethyldiboranes are also known.[3] It is a colorless gas that ignites in air.

1,1-Dimethyldiborane
Names
IUPAC name
1,1-Dimethyldiborane
Other names
Unsymmetrical dimethyldiborane
Identifiers
3D model (JSmol)
Properties
(CH
3)
2B
2H
4
Molar mass 55.72 g mol−1
Appearance Colorless gas
Melting point −150.2 °C (−238.4 °F; 123.0 K)
Boiling point −4 °C (25 °F; 269 K)
Solubility in organic solvents Soluble in ether, pentane, tetrahydrofuran
Thermochemistry
-25 kcal/mol
Hazards
NFPA 704 (fire diamond)
Related compounds
Related alkyl boranes
 Methyldiborane
Trimethyldiborane
Tetramethyldiborane
Trimethylborane
Diethyldiborane
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

The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.[4][5] Methylboranes are formed by the reaction of diborane and trimethylborane. This reaction produces four different substitution of methyl with hydrogen on diborane. Produced are 1-methyldiborane, 1,1-dimethyldborane, 1,1,2-trimethyldiborane, and 1,1,2,2-tetramethyldiborane.[6]

Tetramethyl lead reacts 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-dimethyldiborane, and trimethyldiborane. The other outputs of the reaction are hydrogen gas and lead metal.[7]

Other methods to form methyldiboranes include heating trimethylborane with hydrogen. Alternatively trimethylborane reacts with borohydride salts with 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.[8]

In ether dimethylcalcium reacts with diborane to produce dimethyldiborane and calcium borohydride:[9]

Ca(CH3)2 + 2 B2H6 → Ca(BH4)2 + B2H4(CH3)2

1,2-dimethyldiborane slowly converts on standing to 1,1-dimethyldiborane.[10]

Gas chromatography can be used to determine the amounts of the methyl boranes in a mixture. The order they elute are diborane, monomethyldiborane, trimethylborane, 1,1-dimethyldiborane, 1,2-dimethyldiborane, trimethyldiborane, and finally tetramethyldiborane.[11]

Selected properties

1,1-Dimethyldiborane has a dipole moment of 0.87 d.[12] The predicted heat of formation for the liquid is ΔH0f=-31 kcal/mol, and for the gas -25 kcal/mol. Heat of vapourisation was measured at 5.5 kcal/mol.[13]

Reactions

At −78.5 °C, methyldiborane disproportionates slowly, first to diborane and 1,1-dimethyldiborane.[14] In solution methylborane is more stable against disproportionation than dimethylborane.[15][16]

2 MeB2H5 → 1,1-Me2B2H4 + B2H6, K = 2.8, Me = CH3
3 [1,1-Me2B2H4] → 2 Me3B2H3 + B2H6, K = 0.00027

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.[17]

Gentler oxidation of 1,1-dimethyldiborane at 80 °C yields 2,5-dimethyl-1,3,4-trioxadiboralane, a volatile liquid that contains a ring of two boron and three oxygen atoms.[18] An intermediate in this reaction is two molecules of dimethylborylhydroperoxide (CH3)2BOOH. (CAS 41557-62-5)[19] When methyldiborane is oxidised around 150 °C a similar substance methyltrioxadiboralane is produced. At the same time dimethyltrioxadiboralane and trimethylboroxine are also formed, and also hydrocarbons, diborane, hydrogen, and dimethoxyborane (dimethyl methylboronic ester).[18]

References
  1. Jane E. Macintyre, ed. (1994-11-10). Dictionary of Organometallic Compounds. p. 468. ISBN 9780412430602.
  2. Baker, Charles J. (2001). The Fire Fighter's Handbook of Hazardous Materials. Jones & Bartlett Learning. p. 152. ISBN 9780962705212.
  3. 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.
  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. Bell, R. P.; Emeléus, H. J. (1948). "The boron hydrides and related compounds". Quarterly Reviews, Chemical Society. 2 (2): 132. doi:10.1039/QR9480200132.
  7. 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.
  8. 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.
  9. James, B. D.; Wallbridge, M. G. H. (1970). "Metal Tetrahydroborates". In Lippard, Stephen J. (ed.). Progress in Inorganic Chemistry, Volume 11. Wiley. p. 185. ISBN 0471-54081-1.
  10. Lehmann, Walter J.; Wilson, Charles O.; Shapiro, I. (1960). "Infrared Spectra of Alkyldiboranes. III. 1,2-Dimethyl- and 1,2-Diethyldiboranes". The Journal of Chemical Physics. 33 (2): 590. Bibcode:1960JChPh..33..590L. doi:10.1063/1.1731190.
  11. 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.
  12. Chiu, C. W.; Burg, A. B.; Beaudet, R. A. (15 March 1983). "Microwave spectrum, dipole moment, barrier to internal rotation of 1,1‐dimethyldiborane". The Journal of Chemical Physics. 78 (6): 3562–3566. Bibcode:1983JChPh..78.3562C. doi:10.1063/1.445182.
  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. Bunting, Roger K. (22 Sep 2009). "55 1-Methyldiborane". In Duward F. Shriver (ed.). Inorganic Syntheses, Volume 19. John Wiley and Sons. pp. 237–238. ISBN 978-0471045427.
  15. Brown, Herbert C.; Cole, Thomas E.; Srebnik, Morris; Kim, Kee Won (December 1986). "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. 51 (25): 4925–4930. doi:10.1021/jo00375a031.
  16. 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.
  17. 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.
  18. Barton, Lawrence; Crump, John M.; Wheatley, Jeffrey B. (June 1974). "Trioxadiborolanes from the oxidation of methyldiborane". Journal of Organometallic Chemistry. 72 (1): C1–C3. doi:10.1016/s0022-328x(00)82027-6.
  19. Barton, Lawrence; Crump, John M. (November 1973). "Oxidation of 1,1-dimethyldiborane. Gas-phase peroxide intermediates". Inorganic Chemistry. 12 (11): 2506–2510. doi:10.1021/ic50129a003.
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