1,2-Dimethyldiborane
# 1,2-Dimethyldiborane

1,2-Dimethyldiborane is an organoboron compound with the formula $[(CH_3)BH_2]_2.$ 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, 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.

In a more modern synthesis, 1,2-dimethyldiborane is produced by treating lithium methylborohydride with hydrogen chloride:

    2 $LiCH_3BH_3$ + 2 HCl → $(CH_3BH_2)_2$ + 2 $H_2$ + 2 LiCl

Instead of hydrogen chloride, methyl iodide or trimethylsilyl chloride can be used.

Lithium methylborohydride can be made by treating methylboronic esters with lithium aluminium hydride.

### 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 $(CH_3)_2O.BH_3$ leaving methylborane which rapidly dimerises to 1,2-dimethyldiborane. 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.

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. dimethylchloroborane and methyldichloroborane are also produced as gaseous products.

When $Cp_2Zr(CH_3)_2$ 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:

    $Ca(CH_3)_2$ + 2 $B_2H_6$ → $Ca(BH_4)_2$ + $B_2H_4(CH_3)_2$

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 $C_s.$ A trans-1,2-dimethyldiborane molecule has point group $C_2.$ 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 alkenes 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:

    $(MeBH_2)_2$ + 4 $H_2O$ → $CH_3B(OH)_2$ + 4 $H_2$

Symmetrical dimethyldiborane reacts with trimethylamine to yield a solid adduct trimethylamine-methylborane $(CH_3)_3N·BH_2CH_3.$

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 $[CH_3BH_3]^−.$
* 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.
