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 betwee ...

Al₂( CH₃)₆ (abbreviated as Al₂Me₆ 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 Al₂R₆ and

diborane Diborane(6), generally known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Diborane is a key boron compound with a variety of applications. It has attracte ...

are analogous (R = alkyl). In Al₂Me₆, 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 AlMe₃.

Synthesis

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

Applications

Catalysis

Starting with the invention of Ziegler-Natta catalysis,

organoaluminium compound Organoaluminium chemistry is the study of compounds containing bonds between carbon and aluminium. It is one of the major themes within organometallic chemistry. Illustrative organoaluminium compounds are the dimer trimethylaluminium, the monomer ...

s 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 bo ...

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 and ...

Methylaluminoxane Methylaluminoxane, commonly called MAO, is a mixture of organoaluminium compounds with the approximate formula (Al(CH3)O)''n''. It is usually encountered as a solution in (aromatic) solvents, commonly toluene but also xylene, cumene, or mesitylene ...

, 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 mate ...

s such as Al₂O₃ 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 (substra ...

or atomic layer deposition. 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 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. ...

) of aluminium-containing

compound semiconductor Semiconductor materials are nominally small band gap insulators. The defining property of a semiconductor material is that it can be compromised by doping it with impurities that alter its electronic properties in a controllable way. Because of ...

s, 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 potent ...

, AlP,

AlSb Aluminium antimonide (AlSb) 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. Its ...

, AlGaAs, AlInGaAs, AlInGaP,

AlGaN Aluminium gallium nitride (AlGaN) is a semiconductor material. It is any alloy of aluminium nitride and gallium nitride. The bandgap of AlxGa1−xN can be tailored from 3.4eV (xAl=0) to 6.2eV (xAl=1). AlGaN is used to manufacture light-emitting di ...

AlInGaN ''Alingan'' is a Hindi film starring Tushar Purwar, Anil Rastogi ,Vivek Anand Mishra and Prashant Rai in lead roles. Film has been shot at Varanasi , Prayagraj and Bhadrawah in Jammu. It was released on 25 March 2022. Plot Alingan is a story of ...

, 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 Al₂O₃ via the processes of

or atomic layer deposition. The Al₂O₃ provides excellent surface passivation of p-doped silicon surfaces. The Al₂O₃ 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: :Al₂Me₆ + 3 H₂O → Al₂O₃ + 6 CH₄ Under controlled conditions, the reaction can be stopped to give

methylaluminoxane Methylaluminoxane, commonly called MAO, is a mixture of organoaluminium compounds with the approximate formula (Al(CH3)O)''n''. It is usually encountered as a solution in (aromatic) solvents, commonly toluene but also xylene, cumene, or mesitylene ...

: :AlMe₃ + H₂O → 1/n lMeOsub>n + 2 CH₄ 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 aroun ...

gives the dialuminium diamide dimer: :2 AlMe₃ + 2 HNMe₂ → lMe₂NMe₂sub>2 + 2 CH₄

Reactions with metal chlorides

TMA reacts with many metal halides to install alkyl groups. When combined with gallium trichloride, it gives

trimethylgallium Trimethylgallium, often abbreviated to TMG or TMGa, is the organogallium compound with the formula Ga(CH3)3. It is a colorless, pyrophoric liquid. Unlike trimethylaluminium, TMG adopts a monomeric structure. When examined in detail, the monomeric ...

. Al₂Me₆ 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 conta ...

to give (AlMe₂Cl)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 Tebbe's reagent is the organometallic compound with the formula (C5H5)2TiCH2ClAl(CH3)2. It is used in the methylenation of carbonyl compounds, that is it converts organic compounds containing the R2C=O group into the related R2C=CH2 derivative. It ...

, 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 ar ...

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 bo ...

s, is prepared from TMA and

titanocene dichloride Titanocene dichloride is the organotitanium compound with the formula ( ''η''5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowl ...

. In combination with 20 to 100 mol % Cp₂ZrCl₂ (

zirconocene dichloride Zirconocene dichloride is an organozirconium compound composed of a zirconium central atom, with two cyclopentadienyl and two chloro ligands. It is a colourless diamagnetic solid that is somewhat stable in air. Preparation and structure Zirconoce ...

), the (CH₃)₂Al-CH₃ adds "across" alkynes to give vinyl aluminum species that are useful in

in a reaction known as carboalumination.

Adducts

As for other "electron-deficient" compounds, trimethylaluminium gives

adduct An adduct (from the Latin ''adductus'', "drawn toward" alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all co ...

s R₃N.AlMe₃. The Lewis acid properties of AlMe₃ have been quantified. The enthalpy data show that AlMe₃ is a

hard Hard may refer to: * Hardness, resistance of physical materials to deformation or fracture * Hard water, water with high mineral content Arts and entertainment * ''Hard'' (TV series), a French TV series * Hard (band), a Hungarian hard rock supe ...

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 In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...

DABCO DABCO (1,4-diazabicyclo .2.2ctane), also known as triethylenediamine or TEDA, is a bicyclic organic compound with the formula N2(C2H4)3. This colorless solid is a highly nucleophilic tertiary amine base, which is used as a catalyst and reagent i ...

, 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, succeeding t ...

ATREX mission (

Anomalous Transport Rocket Experiment The Anomalous Transport Rocket Experiment (ATREX) is a heliophysics experiment that was conducted by NASA's Goddard Space Flight Center out of the Wallops Flight Facility located on the Eastern Shore of Virginia. The mission entailed the use of ...

) 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 Methyllithium is the simplest organolithium reagent with the empirical formula CH3Li. This s-block organometallic compound adopts an oligomeric structure both in solution and in the solid state. This highly reactive compound, invariably used in s ...

, 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