1,2-Dimethyldiborane is an
organoboron compound
Organoborane or organoboron compounds are chemical compounds of boron and carbon that are organic derivatives of BH3, for example trialkyl boranes. Organoboron chemistry or organoborane chemistry is the chemistry of these compounds.
Organoboron ...
with the formula
3)BH2">CH3)BH2sub>2. Structurally, it is related to
diborane
Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...
, 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. Sch ...
, known as unsymmetrical dimethyldiborane, which has two methyl groups on one boron atom. Other methylated versions of diborane including
methyldiborane,
trimethyldiborane,
tetramethyldiborane
Dimethylborane, (CH3)2BH is the simplest dialkylborane, consisting of a methyl group substituted for a hydrogen in borane. As for other boranes it normally exists in the form of a dimer called tetramethyldiborane or tetramethylbisborane or TMDB (( ...
.
Trimethylborane
Trimethylborane (TMB) is a toxic, pyrophoric gas with the formula B(CH3)3 (which can also be written as Me3B, with Me representing methyl).
Properties
As a liquid it is colourless. The strongest line in the infrared spectrum is at 1330 cmâ ...
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 or ]trimethylsilyl chloride
Trimethylsilyl chloride, also known as chlorotrimethylsilane is an organosilicon compound (silyl halide), with the formula (CH3)3SiCl, often abbreviated Me3SiCl or TMSCl. It is a colourless volatile liquid that is stable in the absence of water. I ...
can be used.[
Lithium methylborohydride can be made by treating methylboronic esters with lithium aluminium hydride.]
Miscellaneous routes
Methylboranes arise the reaction of diborane
Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...
and trimethylborane
Trimethylborane (TMB) is a toxic, pyrophoric gas with the formula B(CH3)3 (which can also be written as Me3B, with Me representing methyl).
Properties
As a liquid it is colourless. The strongest line in the infrared spectrum is at 1330 cmâ ...
. This reaction produces 1- methyldiborane, 1,1-dimethyldiborane, 1,1,2- trimethyldiborane, and 1,1,2,2-tetramethyldiborane
Dimethylborane, (CH3)2BH is the simplest dialkylborane, consisting of a methyl group substituted for a hydrogen in borane. As for other boranes it normally exists in the form of a dimer called tetramethyldiborane or tetramethylbisborane or TMDB (( ...
. By treating monomethyldiborane with ether, dimethyl ether borane (CH3)2O.BH3 leaving methylborane which rapidly dimerises to 1,2-dimethyldiborane. The reaction is complex.
Tetramethyl lead
Tetramethyllead, also called tetra methyllead and lead tetramethyl, is a chemical compound used as an antiknock additive for gasoline. Its use is being phased out for environmental considerations.
The National Institute for Occupational Safety a ...
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.
Other methods to form methyldiboranes include treating hydrogen with trimethylborane between 80 and 200 °C under pressure, or treating a metal borohydride
Borohydride refers to the anion , which is also called tetrahydroborate, and its salts. Borohydride or hydroborate is also the term used for compounds containing , where ''n'' is an integer from 0 to 3, for example cyanoborohydride or cyanotrihyd ...
with trimethylborane in the presence of hydrogen chloride
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...
, aluminium chloride
Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms hexahydrate with the formula , containing six water molecules of hydration. Both are colourless crystals, but samples are often contam ...
or boron trichloride
Boron trichloride is the inorganic compound with the formula BCl3. This colorless gas is a reagent in organic synthesis. It is highly reactive toward water.
Production and structure
Boron reacts with halogens to give the corresponding trihalides. ...
. If the borohydride is sodium borohydride
Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula Na BH4. This white solid, usually encountered as an aqueous basic solution, is a reducing agent that finds applica ...
, then methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...
is a side product. If the metal is lithium, then no methane is produced.[ 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.]
Physical and spectroscopic properties
''cis''-1,2-Dimethyldiborane melts at −132.5 °C; ''trans''-1,2-dimethyldiborane melts at −102 °C. 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. Vapour pressure is approximated by Log P = 7.363−(1212/T).[ 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 alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s to produce mono and dialkylmethylboranes. More methylated boranes are less stable.[
1,2-Dimethyldiborane slowly converts to 1,1-dimethyldiborane.]
Methylborane hydrolyzes to methylboronic acid:[
:(MeBH2)2 + 4 H2O → CH3B(OH)2 + 4 H2
Symmetrical dimethyldiborane reacts with ]trimethylamine
Trimethylamine (TMA) is an organic compound with the formula N(CH3)3. It is a colorless, hygroscopic, and flammable tertiary amine. It is a gas at room temperature but is usually sold as a 40% solution in water. (It is also sold in pressurized ...
to yield a solid adduct trimethylamine-methylborane (CH3)3N·BH2CH3.[
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.]
Under normal conditions dimethyldiborane does not react with hydrogen.
Related species
*Lithium trihydromethylborate 3BH3">H3BH3sup>−.[
*Isomers of diethyldiborane can be produced by analogous methods.
*1,2- 2,2- and 2,4-dimethyltetraborane,] 1,2-dimethylpentaborane 2,3-dimethylpentaborane, 4,5-dimethylhexaborane, and 5,6- 6,8- 6,9-dimethyldecaborane.
References
Extra reading
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* mass spectroscopy
* charge distribution and atom location calculations
{{Boron compounds
Alkylboranes