Trimethylaluminium
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| Trimethylaluminium | |
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| Chemical name | trimethylaluminium |
| Alternative names | trimethyl aluminium aluminium trimethyl |
| Chemical formula | C6H18Al2 |
| Empirical formula | C3H9Al |
| Molecular mass | 144.18 g/mol |
| Melting point | 15.0 °C |
| Boiling point | 125.0 °C |
| Density | 0.752 g/cm3 |
| CAS number | 75-24-1 |
| SMILES | |
| 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 when the vapor is released in air. Monomeric trimethylaluminium is unstable and virtually unknown.
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[edit] Structure and bonding
The rare species AlMe3 features an aluminium atom bonded to three methyl groups.[citation needed] It has idealized threefold symmetry.
As suggested by the octet rule, TMA tends to dimerize via the Al centers sharing a pair of methyl groups. The dimer, Al2Me6, is analogous to diborane with respect to bonding. As with diborane, the constituents of the dimer are held together by a 3-center-2-electron bond: the shared methyl groups bridge between the two aluminium atoms, with each bridging methyl group contributing two electrons to the bridge bond. The Al-C(terminal) and Al-C(bridging) distances are 1.97 and 2.14 Å, respectively.[1]
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. This electron-deficient situation causes TMA to be very reactive toward any substance towards Lewis bases, i.e. any substance that provide basic electron-pairs. For example upon treatment with AlCl3, TMA forms (AlMe2Cl)2.
[edit] 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.
TMA is also used in semiconductor fabrication to grow thin film, high-k dielectrics such as Al2O3 via the processes of Chemical Vapor Deposition or Atomic Layer Deposition.
TMA forms a complex with the tertiary amine DABCO [1], which is safer to handle than TMA itself.[2]
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.[3]
[edit] 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.
[edit] References
- ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
- ^ Biswas, K.; Prieto, O.; Goldsmith, P. J.Woodward, S. Remarkably Stable (Me3Al)2DABCO and Stereoselective Nickel-Catalyzed AlR3 (R = Me, Et) Additions to Aldehydes Angewandte Chemie International Edition Volume 44, Pages 2232 - 2234 (2005).
- ^ * Negishi, E.; Matsushita, H. Palladium-Catalyzed Synthesis of 1,4-Dienes by Allylation of Alkenyalane: α-Farnesene [1,3,6,10-Dodecatetraene, 3,7,11-trimethyl-]" Organic Syntheses, Collected Volume 7, p.245; Vol. 62, p.31.
