|Other names||Trimethylaluminium; aluminium trimethyl|
|Molar mass||144.18 g/mol|
|Except where noted otherwise, data are given for|
materials in their standard state
(at 25 °C, 100 kPa)
Infobox disclaimer and references
Trimethylaluminium is the chemical compound with the formula Al2(CH3)6, abbreviated as Al2Me6, (AlMe3)2 or the misnomer TMA. This pyrophoric, colorless liquid is an industrially important organoaluminium compound. It evolves white smoke (aluminium oxides) when the vapor is released into the air.
Structure and bonding
Al2Me6 exists as a dimer, analogous in structure and bonding to diborane. As with diborane, the metalloids are connected by a 3-center-2-electron bond: the shared methyl groups bridge between the two aluminium atoms. The Al-C(terminal) and Al-C(bridging) distances are 1.97 and 2.14 Å, respectively. The carbon atoms of the bridging methyl groups are each surrounded by five neighbors: three hydrogen atoms and two aluminium atoms. The methyl groups interchange readily intramolecularly and intermolecularly. 3-Centered-2-electron bonds are a hallmark of "electron-deficient" molecules, which tend to be reactive toward Lewis bases that would give products consisting of 2-centered-2-electron bonds. For example upon treatment with amines gives adducts R3N-AlMe3.
Synthesis and applications
TMA is prepared via a two-step process that can be summarized as follows:
- 2 Al + 6 CH3Cl + 6 Na → Al2(CH3)6 + 6 NaCl
TMA is mainly used for the production of methylaluminoxane, an activator for Ziegler-Natta catalysts for olefin polymerisation. TMA is also employed as a methylation agent. Tebbe's reagent, which is used for the methylenation of esters and ketones, is prepared from TMA. TMA is often released from sounding rockets as a tracer in studies of upper atmospheric wind patterns.
In combination with Cp2ZrCl2 (zirconocene dichloride), the (CH3)2Al-CH3 adds "across" alkynes to give vinyl aluminum species that are useful in organic synthesis in a reaction known as carbozirconation.
Semiconductor grade TMA
TMA is the preferred metalorganic source for metalorganic vapour phase epitaxy (MOVPE) of aluminium-containing compound semiconductors, such as AlAs, AlN, AlP, AlSb, AlGaAs, AlInGaAs, AlInGaP, AlGaN, AlInGaN, AlInGaNP etc. Criteria for TMA quality focus on (a) elemental impurites, (b) oxygenated and organic impurities.
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