1,2-Dimethyldiborane is an
organoboron compound with the formula
CH3)BH2sub>2. Structurally, it is related to
diborane, but with methyl groups replacing terminal hydrides on each boron. It is the dimer of methylborane, CH
3BH
2, the simplest alkylborane.
1,2-Dimethyldiborane can exist in a cis- and a trans arrangement. 1,2-Dimethyldiborane is an easily condensed, colorless gas that ignites spontaneously in air.
An isomer of 1,2-dimethyldiborane is
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. It is a colorless gas that ignites in air.
Formation
The methylboranes were first prepared by H. I. Schl ...
, known as unsymmetrical dimethyldiborane, which has two methyl groups on one boron atom. Other methylated versions of diborane including
methyldiborane,
trimethyldiborane
Trimethyldiborane, (CH3)3B2H3 is a molecule containing boron carbon and hydrogen. It is an alkylborane, consisting of three methyl group substituted for a hydrogen in diborane. It can be considered a mixed dimer: (CH3)2BH2BH(CH3) or dimethylborane ...
,
tetramethyldiborane.
Trimethylborane exists as a monomer.
Preparation
Methylboranes were first prepared by
H. I. Schlesinger and A. O. Walker in the 1930s.
[ ]
In a more modern synthesis, 1,2-dimethyldiborane is produced by treating lithium methylborohydride with hydrogen chloride:
[
:2 LiCH3BH3 + 2 HCl → (CH3BH2)2 + 2 H2 + 2 LiCl
Instead of hydrogen chloride, ]methyl iodide
Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one h ...
or trimethylsilyl chloride can be used.[
Lithium methylborohydride can be made by treating methylboronic esters with ]lithium aluminium hydride
Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li Al H4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic ...
.
Miscellaneous routes
Methylboranes arise the reaction of diborane and trimethylborane. This reaction produces 1- methyldiborane, 1,1-dimethyldiborane, 1,1,2-trimethyldiborane
Trimethyldiborane, (CH3)3B2H3 is a molecule containing boron carbon and hydrogen. It is an alkylborane, consisting of three methyl group substituted for a hydrogen in diborane. It can be considered a mixed dimer: (CH3)2BH2BH(CH3) or dimethylborane ...
, 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.[ dimethylchloroborane and methyldichloroborane are also produced as gaseous products.][
When Cp2Zr(CH3)2 reacts with diborane, a borohydro group inserts into the zirconium-carbon bond, and methyl diboranes are produced.
In ether dimethylcalcium reacts with diborane to produce dimethyldiborane and calcium borohydride:
:Ca(CH3)2 + 2 B2H6 → Ca(BH4)2 + B2H4(CH3)2
1,2-Dimethyldiborane is produced by the room temperature disproportionation of ]trimethyldiborane
Trimethyldiborane, (CH3)3B2H3 is a molecule containing boron carbon and hydrogen. It is an alkylborane, consisting of three methyl group substituted for a hydrogen in diborane. It can be considered a mixed dimer: (CH3)2BH2BH(CH3) or dimethylborane ...
.
Physical and spectroscopic properties
''cis''-1,2-Dimethyldiborane melts at −132.5 °C; ''trans''-1,2-dimethyldiborane melts at −102 °C.[ The vapour pressure for the symmetrical isomer is given by Log P = 7.523−(1290/T).][
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.
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.
]
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.[
1,2-Dimethyldiborane slowly converts to 1,1-dimethyldiborane.][
:(MeBH2)2 + 4 H2O → CH3B(OH)2 + 4 H2
Symmetrical dimethyldiborane reacts with trimethylamine to yield a solid adduct trimethylamine-methylborane (CH3)3N·BH2CH3.][
When dimethyldiborane is combined with ammonia and heated, B-methyl ]borazole
Borazine, also known as borazole, is a non-polar inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benzene. For t ...
s are produced. These borazoles can have one, two or three methyl groups substituted on the boron atoms.
Related species
*Lithium trihydromethylborate H3BH3sup>−.[
*Isomers of diethyldiborane can be produced by analogous methods.
*1,2- 2,2- and 2,4-dimethyltetraborane,]
References
Extra reading
*
*
*
*
* mass spectroscopy
* charge distribution and atom location calculations
{{Boron compounds
Alkylboranes