Trimethyldiborane
Chemical compound
From Wikipedia, the free encyclopedia
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 methylborane.[1] called 1,2-dimethyldiborane.[2] Other combinations of methylation occur on diborane, including monomethyldiborane, 1,2-dimethyldiborane, tetramethyldiborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms, so it is difficult to keep it pure.[3] The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.[4][5]
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| Names | |
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| IUPAC name
1,1,2-Trimethyldiborane | |
| Other names
Trimethyldiborane(6) | |
| Identifiers | |
3D model (JSmol) |
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PubChem CID |
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CompTox Dashboard (EPA) |
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| Properties | |
| (CH 3) 3B 2H 3 | |
| Molar mass | 69.75 g mol−1 |
| Appearance | Colorless pyrophoric liquid |
| Melting point | −122.9 °C (−189.2 °F; 150.2 K) |
| Boiling point | 45.5 °C (113.9 °F; 318.6 K) |
| Thermochemistry | |
Std enthalpy of formation (ΔfH⦵298) |
48 kcal/mol |
| Related compounds | |
| trimethylborane tetramethyldiborane dimethyldiborane methyldiborane | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Formation
Trimethylborane is formed by the reaction of diborane and trimethylborane. This reaction produces four different substitution of methyl with hydrogen on diborane. Produced is 1-methyldiborane, 1,1-dimethyldborane, 1,1,2-trimethyldiborane and 1,1,2,2-tetramethyldiborane. By reacting monomethyldiborane with ether, dimethylether borine is formed [(CH3)2O].BH3 leaving methylborane which rapidly dimerises to 1,2-dimethyldiborane.[3] The reaction is complex.[6] The yield of trimethyldiborane is maximised with ratio of 1 of diborane to 3 of trimethylborane.[7]
Tetramethyl lead can react with diborane in a 1,2-dimethoxyethane solvent at room temperature to make a range of methyl substituted diboranes, ending up at trimethylborane, but including 1,1-di, tridiborane. The other outputs of the reaction are hydrogen gas and lead metal.[8]
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]
When Cp2Zr(CH3)2 reacts with borane dissolved in tetrahydrofuran, a borohydro group inserts into the zirconium carbon bond, and methyl diboranes are produced.[9]
Properties
Trimethyldiborane has two methyl groups on one boron atom, and one methyl and a hydrogen on the second boron atom. A bridge of two hydrogen atoms links the boron atoms together. The molecule is expected to have a Cs point group due to rapid rotation of the methyls. The infrared spectrum of trimethyldiborane has a strong absorption band at 2509 cm−1 due to the non-bridge boron-hydrogen bond.[10] It has a vapour pressure of 51 mm Hg at -22.8 °C; 61 mm Hg at -18.4 °C and[7] 83 mm Hg at 0 °C.[11] Vapour pressure can be approximated by Log P = 7.673 - (1527/T).[12] The boiling point is 45.5 °C, and the melting point is -122.9.[12]
The predicted heat of formation for liquid trimethyldiborane is ΔH0f=-48 kcal/mol, and for the gas -41 kcal/mol. Heat of vapourisation ΔHvap was measured at 7.0 kcal/mol.[13]
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.[14]
The nuclear resonance shift for the bridge hydrogen is 9.27 ppm, compared to 10.49 for diborane.[15]
Reactions
Trimethyldiborane partially disproportionates over a period of hours at room temperature to yield tetramethyldiborane and 1,2-dimethyldiborane. Over a period of weeks 1,1-dimethyldiborane appears as well.[16]
Trimethyldiborane is hydrolyzed in water to methylboronic acid CH3B(OH)2 and dimethylborinic acid (CH3)2B(OH).[3]
Trimethyldiborane spontaneously inflames when exposed to air.[18]
Trimethyldiborane reacts with liquid ammonia initially forming methylborohydride anions and (CH3)2B(N3)2+ cations.[19][20][21]
Related
Trimethylborane (CH3)3B has a similar-sounding name, and many similar properties, but only has one boron atom.[4] Trimethylhydroborate (CH3)3BH− is an anion with one boron atom. It can form a lithium salt.[22]