Formula C6H18Al2 Density 752 kg/m³ Melting point 15 °C | Molar mass 144.18 g/mol Boiling point 125 °C Appearance Colorless liquid | |
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Trimethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al2(CH3)6 (abbreviated as Al2Me6 or TMA) as it exists as a dimer. This colorless liquid is an industrially important compound but must be handled with care due to its pyrophoricity; it evolves white smoke (aluminium oxides) when the vapor is released into the air.
Contents
Structure and bonding
Al2Me6 exists mostly as a dimer at room temperature and pressure, analogous in structure and bonding to diborane. As with diborane, the molecules are connected by 2 3-center-2-electron bonds: 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 an utterance of "electron-deficient" molecules and tend to undergo reactions with Lewis bases that would give products consisting of 2-centered-2-electron bonds. For example, upon treatment with amines gives adducts R3N-AlMe3. Another reaction that gives products that follow the octet rule is the reaction of Al2Me6 with aluminium trichloride to give (AlMe2Cl)2.
The monomer species AlMe3, which has an aluminium atom bonded to three methyl groups, occurs at high temperature and low pressure. VSEPR Theory predicts and electron diffraction confirms that it has a trigonal planar (threefold) symmetry, as observed in BMe3.
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 NaClTMA 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 deposit 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, which is safer to handle than TMA itself.
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 carboalumination.
The NASA ATREX mission (Anomalous Transport Rocket Experiment) employed the white smoke that TMA forms on air contact to study the high altitude jet stream.
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 impurities, (b) oxygenated and organic impurities.