Trimethylaluminium is the
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with the formula
AlAluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
2(CH
3)
6, abbreviated as Al
2Me
6, (AlMe
3)
2 or the abbreviation TMA. This pyrophoric, colorless liquid is an industrially important
organoaluminiumOrganoaluminium chemistry is the study of compounds containing bonds between carbon and aluminium bond. It is one of the major themes within organometallic chemistry. Illustrative organoaluminium compounds are the dimer trimethylaluminium, the monomer triisobutylaluminium, and the...
compound. It evolves white smoke (aluminium oxides) when the vapor is released into the air.
Structure and bonding
Al
2Me
6 exists as a dimer, analogous in structure and bonding to
diboraneDiborane is the chemical compound consisting of boron and hydrogen with the formula B2H6. It is a colorless gas at room temperature with a repulsively sweet odor. Diborane mixes well with air, easily forming explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature...
. 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 an utterance of "electron-deficient" molecules and tend to reactions with Lewis bases that would give products consisting of 2-centered-2-electron bonds. For example upon treatment with amines gives
adductAn adduct is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is considered a distinct molecular species...
s R
3N-AlMe
3. Another reaction that gives products that follow the
octet ruleThe octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low (The octet rule is a chemical rule of thumb that states that atoms of low (...
is the reaction of Al
2Me
6 with aluminium trichloride to give (AlMe
2Cl)
2.
The species AlMe
3, which would feature an
aluminiumAluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
atomThe atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
bonded to three methyl groups is unknown.
VSEPR TheoryValence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...
predicts that such a molecule would have idealized threefold
symmetryThe symmetry group of an object is the group of all isometries under which it is invariant with composition as the operation...
, as observed in BMe
3.
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
methylaluminoxaneMethylaluminoxane, commonly called MAO, is a white solid with the general formula n.-Physical properties:MAO is pyrophoric, and is violently reactive with any chemical bearing an acidic proton. However, MAO is generally used as a solution in an hydrocarbon due to its relatively high solubility in...
, an activator for
Ziegler-Natta catalystA Ziegler–Natta catalyst is a catalyst used in the synthesis of polymers of 1-alkenes . Three types of Ziegler–Natta catalysts are currently employed:* Solid and supported catalysts based on titanium compounds...
s for olefin polymerisation. TMA is also employed as a methylation agent.
Tebbe's reagentThe Tebbe reagent is the organometallic compound with the formula 2TiCH2ClAl2. It used in the methylenation of carbonyl compounds, that is it converts organic compounds containing the R2C=O group into the related R2C=CH2 derivative...
, which is used for the methylenation of
esterEsters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s and
ketoneIn organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
s, 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
dielectricA dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
s such as Al
2O
3 via the processes of
Chemical Vapor DepositionChemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
or
Atomic Layer DepositionAtomic layer deposition is a thin film deposition technique that is based on the sequential use of a gas phase chemical process. The majority of ALD reactions use two chemicals, typically called precursors. These precursors react with a surface one-at-a-time in a sequential manner...
.
TMA forms a complex with the tertiary amine
DABCODABCO or 1,4-diazabicyclo[2.2.2]octane is a chemical compound. It is a polyurethane and Baylis-Hillman reaction catalyst, complexing ligand and Lewis base. It is used to regulate the reaction rate in Flexplay time-limited DVDs by adjusting pH. Antioxidants, like DABCO, are used to improve the...
, which is safer to handle than TMA itself.
In combination with Cp
2ZrCl
2 (zirconocene dichloride), the (CH
3)
2Al-CH
3 adds "across" alkynes to give vinyl aluminium species that are useful in
organic synthesisOrganic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
in a reaction known as carboalumination.
Semiconductor grade TMA
TMA is the preferred metalorganic source for
metalorganic vapour phase epitaxyMetalorganic vapour phase epitaxy , also known as organometallic vapour phase epitaxy or metalorganic chemical vapour deposition , is a chemical vapour deposition method of epitaxial growth of materials, especially compound semiconductors, from the surface reaction of organic compounds or...
(MOVPE) of aluminium-containing
compound semiconductorA compound semiconductor is a semiconductor compound composed of elements from two or more different groups of the periodic table . These semiconductors typically form in groups 13-16 ,...
s, such as
AlAsAluminium arsenide or aluminum arsenide is a semiconductor material with almost the same lattice constant as gallium arsenide and aluminium gallium arsenide and wider band gap than gallium arsenide.-Properties:...
,
AlNAluminium nitride is a nitride of aluminium. Its wurtzite phase is a wide band gap semiconductor material, giving it potential application for deep ultraviolet optoelectronics.-History:...
,
AlPAluminium phosphide is an inorganic compound used as a wide band gap semiconductor and a fumigant. This colourless solid is generally sold as a grey-green-yellow powder due to the presence of impurities arising from hydrolysis and oxidation.-Properties:...
,
AlSbAluminium antimonide is a semiconductor of the group III-V family containing aluminium and antimony. The lattice constant is 0.61 nm. The indirect bandgap is approximately 1.6 eV at 300 K, whereas the direct band gap is 2.22 eV....
,
AlGaAsAluminium gallium arsenide is a semiconductor material with very nearly the same lattice constant as GaAs, but a larger bandgap. The x in the formula above is a number between 0 and 1 - this indicates an arbitrary alloy between GaAs and AlAs.The bandgap varies between 1.42 eV and 2.16 eV...
, AlInGaAs, AlInGaP,
AlGaNAluminium gallium nitride is a semiconductor material. It is an alloy of aluminium nitride and gallium nitride.AlGaN is used to manufacture light-emitting diodes operating in blue to ultraviolet region, where wavelengths down to 250 nm were achieved. It is also used in blue semiconductor...
, AlInGaN, AlInGaNP etc. Criteria for TMA quality focus on (a) elemental impurites, (b) oxygenated and organic impurities.
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