Trimethylaluminium

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.

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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.^[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. 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.

Another more complicated reaction that gives products that follow the octet rule, Al₂Me₆ reacts with aluminium trichloride to give AlMe₂Cl)₂.

The species AlMe₃, which features an aluminium atom bonded to three methyl groups is unknown. VSEPR Theory predicts that such a molecule would have idealized 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 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, 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]

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.

References

• Informative commercial link to Trimethylaluminum and other metalorganics.

• Interactive Vapor Pressure Chart for metalorganics.