Tetramethyldiborane
Chemical compound
From Wikipedia, the free encyclopedia
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 ((CH3)2BH)2.[2] Other combinations of methylation occur on diborane, including monomethyldiborane, trimethyldiborane, 1,2-dimethylborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms.[3] The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.[4][5]
| |
| Names | |
|---|---|
| IUPAC name
Tetramethyldiborane(6) | |
| Other names
Dimethylborane dimer | |
| Identifiers | |
3D model (JSmol) |
|
| |
| |
| Properties | |
| (CH 3) 2BH 2B(CH 3) 2 | |
| Molar mass | 83.777 |
| Appearance | Colorless liquid |
| Odor | Pungent; |
| Melting point | −72.5 °C (−98.5 °F; 200.7 K) |
| Boiling point | 68.6 °C (155.5 °F; 341.8 K) |
| Hazards | |
| NFPA 704 (fire diamond) | |
| Related compounds | |
| trimethylborane dimethyldiborane diethylborane | |
Related compounds |
Borane tetramethyl aluminium hydride tetramethyl gallium hydride methylalane |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
| |
Properties
Tetramethyldiborane has two boron atoms linked by a two hydrogen atom bridge, and each boron is linked to two methyl groups. A tetramethyldiborane molecule belongs to the D2h point group. Its infrared spectrum shows a strong absorption band at 1602 cm−1 due to bridging hydrogen, a weak band at 1968 cm−1 and lines due to methyl between 900 and 1400 cm−1.[6] In the molecule the boron to hydrogen distance is 1.36 Å, the boron to boron distance is 1.84 Å; the boron to carbon distance is 1.590 Å; the angle of boron-boron to carbon is 120.0°; the boron-carbon-hydrogen angle is 112.0°.[7] The NMR J coupling between two boron-11 nuclei in tetramethyldiborane is 55 Hz.[8]
Tetramethyldiborane melts at −72.5 °C and boils at 68.6 °C.[9] Vapour pressure is approximated by Log P = 7.687−(1643/T).[9] Tetramethyldiborane has a vapour pressure of 48 mm Hg at 0 °C.[10] Heat of vapourisation was measured at 7.3 kcal/mol.[11] The predicted heat of formation for the liquid is ΔH0f=−65 kcal/mol, and for the gas −57 kcal/mol.[11]
A gas chromatograph can be used to determine the amounts of the methyl boranes in a mixture. The order they pass through are diborane, monomethyldiborane, trimethylborane, 1,1-dimethyldiborane, 1,2-dimethyldiborane, trimethyldiborane, and lastly tetramethyldiborane.[12]
The nuclear resonance shift for the bridge hydrogen is 8.90 ppm, compared to 10.49 for diborane.[13]
Preparation
Dimethylborane is formed when lithium dimethylborohydride Li(CH3)2BH2 reacts with an acid.[14] The lithium dimethylborohydride can be made from a dimethylborinic ester and lithium monoethoxy aluminium hydride.[14]
Methylboranes are also formed by the reaction of diborane and trimethylborane. This reaction produces four different substitutions of methyl with hydrogen on diborane. Produced is 1-methyldiborane, 1,1-dimethyldborane, 1,1,2-trimethyldiborane and 1,1,2,2-tetramethyldiborane.[3] The latter is maximised when trimethylborane is six times the concentration of diborane.[10]
Other methods to form methyldiboranes include reacting hydrogen with trimethylborane between 80 and 200 °C under pressure, or reacting 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.[4] dimethylchloroborane and methyldichloroborane are also produced as gaseous products.[4]
Atomic hydrogen converts trimethylborane on a graphene monolayer surface to dimethylborane which dimerises to tetramethyldiborane.[15]
Reactions
Dimethylborane reacts with alkenes with the highest yield in ether to produce a dimethylalkylborane.[14] The dimethylalkylboranes can then be converted to a tertiary alcohol by oxidative carbonylation. This requires heating to 150° with carbon monoxide under 50 bars of pressure, and then oxidation with hydrogen peroxide.[16]
Methylboranes such as tetramethyldiborane disproportionate in the gas phase to trimethylborane and diborane at room temperature.[3] The time period is on the order of a few hours, and disproportionation is faster the higher the temperature.[4] At 0 °C disproportionation takes about a day.[4] At −78.5 °C methyldiborane disproportionates slowly first to diborane and 1,1-dimethyldiborane.[17] In solution methylborane is more stable against disproportionation than dimethylborane.[14]
- 4(CH3)3B2H3 ⇌ (CH3)4B2H2 + B2H6 K=0.0067.[18]
- 3B2H2Me4 ⇌ 2 B2H3Me3 + 2 BMe3
Dimethylborane is hydrolyzed in water to Dimethylborinic acid (CH3)2BOH.[3]
Dimethyldiborane spontaneously inflames when exposed to air.[17]
Ammonia and tetramethyldiborane combine to form a white solid at −78 °C. The solid decomposes above 10 °C.[19] The structure of the solid is ionic [(CH3)2B(NH3)2]+ [(CH3)2BH2]−.[19][20] A simple adduct BHMe3.NH3 is formed from tetramethyldiborane and ammonia in ether. This also forms during the thermal decomposition of the diammoniate.[21]
Acetonitrile reacts slowly with tetramethyldiborane at room temperature to form dimeric ethylideneaminodimethylborane (CH3CH=NB(CH3)2)2. This has a cis and a trans isomer, one melting at 76 °C and another at −5 °C.[22]
Tetramethyldiborane reacts with sodium in liquid ammonia to make a salt with formula Na2HB(CH3)2 called sodium dimethylboryl. The salt is white and stable to 90 °C.[21] With potassium K2HB(CH3)2 potassium dimethylboryl is formed.[23] Calcium metal react with tetramethyldiborane to make CaHB(CH3)2.NH3.[21]
Tetramethyldiborane combines with dimethylphosphine to yield an adduct of dimethylborane.[21]
Tetramethyldiborane reacts with organic borates to form methylboronic esters.
- 2 (CH3)4B2H2 + 4 B(OR)3 ⇌ 6 CH3(OR)2 + (CH3)2B2H4.[24]
Tetramethyldiborane acts as a catalyst to enable the same results from trimethylborane:
- (CH3)3B + 2 B(OR)3 → 3 CH3(OR)2[24]
Related
The tetramethylborate anion (CH3)4B− only has one boron atom.[25]