Trimethylaluminium is one of the simplest examples of an
organoaluminium compound. Despite its name it has the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...
Al2(
CH3)
6 (abbreviated as Al
2Me
6 or TMA), as it exists as a
dimer
Dimer may refer to:
* Dimer (chemistry), a chemical structure formed from two similar sub-units
** Protein dimer, a protein quaternary structure
** d-dimer
* Dimer model, an item in statistical mechanics, based on ''domino tiling''
* Julius Dimer ( ...
. This colorless liquid is
pyrophoric
A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...
. It is an industrially important compound, closely related to
triethylaluminium
Triethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al2( C2H5)6 (abbreviated as Al2Et6 or TEA), as it exists as a dimer. This colorless liquid is pyrophoric. It is an industrially ...
.
Structure and bonding
The structure and bonding in Al
2R
6 and
diborane are analogous (R = alkyl). In Al
2Me
6, the Al-C(terminal) and Al-C(bridging) distances are 1.97 and 2.14 Å, respectively. The Al center is tetrahedral. 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. At higher temperatures, the dimer
cracks into monomeric AlMe
3.
Synthesis
TMA is prepared via a two-step process that can be summarized as follows:
:2 Al + 6
CH3Cl + 6 Na → Al
2(CH
3)
6 + 6 NaCl
Applications
Catalysis
Starting with the invention of
Ziegler-Natta catalysis,
organoaluminium compounds have a prominent role in the production of
polyolefin
A polyolefin is a type of polymer with the general formula (CH2CHR)n where R is an alkyl group. They are usually derived from a small set of simple olefins (alkenes). Dominant in a commercial sense are polyethylene and polypropylene. More speciali ...
s, such as
polyethylene
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including b ...
and
polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer propylene.
Polypropylene
belongs to the group of polyolefins a ...
.
Methylaluminoxane, which is produced from TMA, is an activator for many transition metal catalysts.
Semiconductor applications
TMA is also used in semiconductor fabrication to deposit thin film, high-k
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the ma ...
s such as Al
2O
3 via the processes of
chemical vapor deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (subst ...
or
atomic layer deposition
Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas-phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors (a ...
. TMA is the preferred precursor for
metalorganic vapour phase epitaxy
Metalorganic vapour-phase epitaxy (MOVPE), also known as organometallic vapour-phase epitaxy (OMVPE) or metalorganic chemical vapour deposition (MOCVD), is a chemical vapour deposition method used to produce single- or polycrystalline thin films. ...
(
MOVPE) of aluminium-containing
compound semiconductors, such as
AlAs,
AlN
Aluminium nitride ( Al N) is a solid nitride of aluminium. It has a high thermal conductivity of up to 321 W/(m·K) and is an electrical insulator. Its wurtzite phase (w-AlN) has a band gap of ~6 eV at room temperature and has a potenti ...
,
AlP,
AlSb,
AlGaAs
Aluminium gallium arsenide (also gallium aluminium arsenide) ( Alx Ga1−x As) 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 ...
,
AlInGaAs,
AlInGaP,
AlGaN,
AlInGaN,
AlInGaNP, etc. Criteria for TMA quality focus on (a) elemental impurities, (b) oxygenated and organic impurities.
Photovoltaic applications
In deposition processes very similar to semiconductor processing, TMA is used to deposit thin film, low-k (non-absorbing) dielectric layer stacks with Al
2O
3 via the processes of
chemical vapor deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (subst ...
or
atomic layer deposition
Atomic layer deposition (ALD) is a thin-film deposition technique based on the sequential use of a gas-phase chemical process; it is a subclass of chemical vapour deposition. The majority of ALD reactions use two chemicals called precursors (a ...
. The Al
2O
3 provides excellent surface passivation of p-doped silicon surfaces. The Al
2O
3 layer is typically the bottom layer with multiple silicon nitride (Si
xN
y) layers for capping.
Reactions
Hydrolysis and related protonolysis reactions
Trimethylaluminium is hydrolyzed readily, even dangerously:
:Al
2Me
6 + 3 H
2O → Al
2O
3 + 6 CH
4
Under controlled conditions, the reaction can be stopped to give
methylaluminoxane:
:AlMe
3 + H
2O → 1/n
lMeOsub>n + 2 CH
4
Alcoholysis and aminolysis reactions proceed comparably. For example,
dimethylamine
Dimethylamine is an organic compound with the formula (CH3)2NH. This secondary amine is a colorless, flammable gas with an ammonia-like odor. Dimethylamine is commonly encountered commercially as a solution in water at concentrations up to arou ...
gives the dialuminium diamide dimer:
:2 AlMe
3 + 2 HNMe
2 →
lMe2NMe2sub>2 + 2 CH
4
Reactions with metal chlorides
TMA reacts with many metal halides to install alkyl groups. When combined with gallium trichloride, it gives
trimethylgallium. Al
2Me
6 reacts with
aluminium trichloride
Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms hexahydrate with the formula , containing six water molecules of hydration. Both are colourless crystals, but samples are often contam ...
to give (AlMe
2Cl)
2.
TMA/metal halide reactions have emerged as reagents in
organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
.
Tebbe's reagent, which is used for the methylenation of
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...
s and
ketone
In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...
s, is prepared from TMA and
titanocene dichloride. In combination with 20 to 100 mol % Cp
2ZrCl
2 (
zirconocene dichloride), the (CH
3)
2Al-CH
3 adds "across" alkynes to give vinyl aluminum species that are useful in
organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
in a reaction known as carboalumination.
Adducts
As for other "electron-deficient" compounds, trimethylaluminium gives
adducts R
3N
.AlMe
3. The Lewis acid properties of AlMe
3 have been quantified. The enthalpy data show that AlMe
3 is a
hard acid and its acid parameters in the
ECW model are E
A =8.66 and C
A = 3.68.
These adducts, e.g. the complex with the
tertiary amine DABCO, are safer to handle than TMA itself.
The
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeedin ...
ATREX mission (
Anomalous Transport Rocket Experiment) employed the white smoke that TMA forms on air contact to study the high altitude jet stream.
Synthetic reagent
TMA is a source of methyl nucleophiles, akin to
methyl lithium, but less reactive. It reacts with ketones to give, after a hydrolytic workup, tertiary alcohols.
Safety
Trimethylaluminium is pyrophoric, reacting violently with air and water.
References
External links
NASA ATREX mission article
{{Authority control
Organoaluminium compounds
Coordination complexes
Methyl complexes