1,2-Dimethyldiborane

1,2-Dimethyldiborane is an organoboron compound with the formula [(CH3)BH2]2. Structurally, it is related to diborane, but with methyl groups replacing terminal hydrides on each boron. It is the dimer of methylborane, CH3BH2, the simplest alkylborane.[1] 1,2-Dimethyldiborane can exist in a cis- and a trans arrangement.[2] 1,2-Dimethyldiborane is an easily condensed, colorless gas that ignites spontaneously in air.

1,2-Dimethyldiborane

cis-1,2-Dimethyldiborane

trans-1,2-Dimethyldiborane
Names
Preferred IUPAC name
1,2-Dimethyldiborane(6)
Other names
Symmetrical dimethyldiborane
Identifiers
3D model (JSmol)
  • InChI=1S/C2H10B2/c1-3-5-4(2)6-3/h3-4H,1-2H3
    Key: CMZSWMSJMMTTMF-UHFFFAOYSA-N
  • C[BH]1[H][BH](C)[H]1
Properties
(CH3BH2)2
Molar mass 55.72 g mol−1
Appearance Colorless gas
Melting point −124.9 °C (−192.8 °F; 148.2 K)
Boiling point 4 °C (39 °F; 277 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N (what is checkY☒N ?)
Infobox references

An isomer of 1,2-dimethyldiborane is 1,1-dimethyldiborane, known as unsymmetrical dimethyldiborane, which has two methyl groups on one boron atom. Other methylated versions of diborane including methyldiborane, trimethyldiborane, tetramethyldiborane. Trimethylborane exists as a monomer.

Preparation

Methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.[3][4]

In a more modern synthesis, 1,2-dimethyldiborane is produced by treating lithium methylborohydride with hydrogen chloride:[1]

2 LiCH3BH3 + 2 HCl → (CH3BH2)2 + 2 H2 + 2 LiCl

Instead of hydrogen chloride, methyl iodide or trimethylsilyl chloride can be used.[5]

Lithium methylborohydride can be made by treating methylboronic esters with lithium aluminium hydride.[5]

Miscellaneous routes

Methylboranes arise the reaction of diborane and trimethylborane. This reaction produces 1-methyldiborane, 1,1-dimethyldiborane, 1,1,2-trimethyldiborane, and 1,1,2,2-tetramethyldiborane. By treating monomethyldiborane with ether, dimethyl ether borane (CH3)2O.BH3 leaving methylborane which rapidly dimerises to 1,2-dimethyldiborane.[6] The reaction is complex.

Tetramethyl lead reacts with diborane to give a range of methyl-substituted diboranes, ending up at trimethylborane, but including 1,1-dimethyldiborane, and trimethyldiborane. Other products are hydrogen gas and lead metal.[7]

Other methods to form methyldiboranes include treating hydrogen with trimethylborane between 80 and 200 °C under pressure, or treating 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.[3] dimethylchloroborane and methyldichloroborane are also produced as gaseous products.[3]

When Cp2Zr(CH3)2 reacts with diborane, 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 is produced by the room temperature disproportionation of trimethyldiborane.[10]

Physical and spectroscopic properties

cis-1,2-Dimethyldiborane melts at −132.5 °C; trans-1,2-dimethyldiborane melts at −102 °C.[11] The cis-1,2-dimethyldiborane molecule has point group Cs. A trans-1,2-dimethyldiborane molecule has point group C2. Unsymmetrical dimethyldiborane melts at −150.2 °C.[12] Vapour pressure is approximated by Log P = 7.363−(1212/T).[12] The vapour pressure for the symmetrical isomer is given by Log P = 7.523−(1290/T).[12]

Gas chromatography can be used to determine the amounts of the methyl boranes in a mixture. The order of elution are: diborane, monomethyldiborane, trimethylborane, 1,1-dimethyldiborane, 1,2-dimethyldiborane, trimethyldiborane, and last tetramethyldiborane.[13]

The nuclear resonance shift for the bridge hydrogen is 9.55 ppm for the unsymmetrical isomer and 9.73 ppm for the symmetrical isomers, compared to 10.49 for diborane.[14]

Reactions

Methylborane shows little tendency to disproportionate (redistribute) at room temperature. It reacts stepwise with alkenes to produce mono and dialkylmethylboranes. More methylated boranes are less stable.[5]

1,2-Dimethyldiborane slowly converts to 1,1-dimethyldiborane.[15]

Methylborane hydrolyzes to methylboronic acid:[6]

(MeBH2)2 + 4 H2O → CH3B(OH)2 + 4 H2

Symmetrical dimethyldiborane reacts with trimethylamine to yield a solid adduct trimethylamine-methylborane (CH3)3N·BH2CH3.[6]

When dimethyldiborane is combined with ammonia and heated, B-methyl borazoles are produced. These borazoles can have one, two or three methyl groups substituted on the boron atoms.[16][17]

Under normal conditions dimethyldiborane does not react with hydrogen.[18]

  • Lithium trihydromethylborate [CH3BH3]−.[5]
  • Isomers of diethyldiborane can be produced by analogous methods.[19]
  • 1,2- 2,2- and 2,4-dimethyltetraborane,[20] 1,2-dimethylpentaborane[21] 2,3-dimethylpentaborane,[22] 4,5-dimethylhexaborane,[23] and 5,6- 6,8- 6,9-dimethyldecaborane.[24]

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. 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): 3513–3520. doi:10.1039/JR9650003513. (subscription required)
  4. 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.
  5. 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 Sgents. Synthesis of Tertiary Alcohols Containing Methyl Groups via Hydroboration". The Journal of Organic Chemistry. 51 (25): 4925–4930. doi:10.1021/jo00375a031.
  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. (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.
  11. Hedberg, Lise; Hedberg, Kenneth; Kohler, David A.; Ritter, David M.; Schomaker, Verner (May 1980). "Electron-diffraction investigations of the molecular structures of cis- and trans-1,2-dimethyldiborane". Journal of the American Chemical Society. 102 (10): 3430–3434. doi:10.1021/ja00530a021.
  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. 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.
  14. 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.
  15. 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.
  16. Sheldon, J. C.; Smith, B. C. (1960). "The borazoles". Quarterly Reviews, Chemical Society. 14 (2): 202. doi:10.1039/QR9601400200.
  17. 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.
  18. Adams, Roy M. (September 1959). "Organoboron Compounds" (PDF). Metal-Organic Compounds. Advances in Chemistry. Vol. 23. p. 92. doi:10.1021/ba-1959-0023.ch010. ISBN 0-8412-0024-6. Retrieved 17 August 2015.
  19. Mikhailov, B. M. (April 1962). "The Chemistry Of Diborane". Russian Chemical Reviews. 31 (4): 209. Bibcode:1962RuCRv..31..207M. doi:10.1070/RC1962v031n04ABEH001281. S2CID 250909492.
  20. Deever, William R.; Ritter, David M. (November 1969). "Methyltetraboranes. I. 2-Methyl and 1,2-, 2,2-, and 2,4-dimethyl derivatives". Inorganic Chemistry. 8 (11): 2461–2467. doi:10.1021/ic50081a043.
  21. Addison, C. C.; Davidson, G. (1973). "Elements of Group III". Inorganic Chemistry of the Main-Group Elements. Vol. 1. p. 68. doi:10.1039/9781847556370-00053. ISBN 978-0-85186-752-6. (subscription required)
  22. Onak, Thomas; Friedman, Lawrence B.; Hartsuck, Jean A.; Lipscomb, William N. (July 1966). "Rearrangement of 1,2- to 2,3-Dimethylpentaborane(9)". Journal of the American Chemical Society. 88 (14): 3439–3440. doi:10.1021/ja00966a051.
  23. Shore, S. G. (1975). "Nido and Arachno Boron Hydrides". In Muetterties, Earl L (ed.). Boron Hydride Chemistry. Academic Press. p. 150. ISBN 0-12-509650-X.
  24. Dunstan, I.; Williams, R. L.; Blay, N. J. (1960). "970. Boron hydride derivatives. Part V. Nucleophilic substitution in decaborane". Journal of the Chemical Society (IV): 5012–5015. doi:10.1039/JR9600005012. Retrieved 19 August 2015.

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