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.
cis-1,2-Dimethyldiborane | |
trans-1,2-Dimethyldiborane | |
Names | |
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Preferred IUPAC name
1,2-Dimethyldiborane(6) | |
Other names
Symmetrical dimethyldiborane | |
Identifiers | |
3D model (JSmol) |
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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).
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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
References
Extra reading
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