1,2-Dimethyldiborane

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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[1]
    Key: CMZSWMSJMMTTMF-UHFFFAOYSA-N[2]
  • 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 ?)

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.

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]

Related species

  • 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

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    doi:10.1016/s0040-4039(00)96380-9
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  2. doi:10.1063/1.1669454
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  3. ^
    doi:10.1039/JR9650003513
    . (subscription required)
  4. doi:10.1021/ja01307a009
    .
  5. ^
    doi:10.1021/jo00375a031
    .
  6. ^
    doi:10.1039/QR9480200132
    .
  7. doi:10.1016/s0022-328x(00)93089-4
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  8. doi:10.1021/ja00373a005
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  9. ISBN 0471-54081-1
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  10. doi:10.1063/1.1731675
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  11. doi:10.1021/ja00530a021
    .
  12. ^
    ISBN 9780080580043
    . Retrieved 14 August 2015.
  13. doi:10.1021/ac60156a032
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  14. doi:10.1016/0022-2364(72)90088-1
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  15. doi:10.1063/1.1731190
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  16. doi:10.1039/QR9601400200
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  17. doi:10.1021/ja01294a008
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  18. ISBN 0-8412-0024-6
    . Retrieved 17 August 2015.
  19. S2CID 250909492
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  20. doi:10.1021/ic50081a043
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  21. ISBN 978-0-85186-752-6
    . (subscription required)
  22. doi:10.1021/ja00966a051
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  23. ISBN 0-12-509650-X
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  24. doi:10.1039/JR9600005012
    . Retrieved 19 August 2015.

Extra reading
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