![]() cis-1,2-Dimethyldiborane
| |
![]() trans-1,2-Dimethyldiborane
| |
Names | |
---|---|
Preferred IUPAC name
1,2-Dimethyldiborane(6) | |
Other names
Symmetrical dimethyldiborane
| |
Identifiers | |
3D model (
JSmol)
|
|
| |
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).
|
1,2-Dimethyldiborane is an organoboron compound with the formula [(CH3)BH22. 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.
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]
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]
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]
1,2-Dimethyldiborane is produced by the room temperature disproportionation of trimethyldiborane. [10]
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]
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]
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]
![]() cis-1,2-Dimethyldiborane
| |
![]() trans-1,2-Dimethyldiborane
| |
Names | |
---|---|
Preferred IUPAC name
1,2-Dimethyldiborane(6) | |
Other names
Symmetrical dimethyldiborane
| |
Identifiers | |
3D model (
JSmol)
|
|
| |
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).
|
1,2-Dimethyldiborane is an organoboron compound with the formula [(CH3)BH22. 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.
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]
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]
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]
1,2-Dimethyldiborane is produced by the room temperature disproportionation of trimethyldiborane. [10]
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]
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]
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]