# Trimethylaluminium

Trimethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al$_2$(CH$_3$)$_6$ (abbreviated as Al$_2$Me$_6$ or TMA), as it exists as a dimer. This colorless liquid is pyrophoric. It is an industrially important compound, closely related to triethylaluminium.

## Structure and bonding

The structure and bonding in Al$_2$R$_6$ and diborane are analogous (R = alkyl). In Al$_2$Me$_6$, 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 CH$_3$Cl + 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 polyolefins, such as polyethylene and polypropylene. 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 dielectrics such as Al$_2$O$_3$ via the processes of chemical vapor deposition or atomic layer deposition. TMA is the preferred precursor 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.

### 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$_2$O$_3$ via the processes of chemical vapor deposition or atomic layer deposition. The Al$_2$O$_3$ provides excellent surface passivation of p-doped silicon surfaces. The Al$_2$O$_3$ layer is typically the bottom layer with multiple silicon nitride (Si$_x$N$_y$) layers for capping.

## Reactions
### Hydrolysis and related protonolysis reactions

Trimethylaluminium is hydrolyzed readily, even dangerously:
    Al$_2$Me$_6$ + 3 H$_2$O → Al$_2$O$_3$ + 6 CH$_4$
Under controlled conditions, the reaction can be stopped to give methylaluminoxane:
    AlMe$_3$ + H$_2$O → 1/n [AlMeO]$_n$ + 2 CH$_4$
Alcoholysis and aminolysis reactions proceed comparably. For example, dimethylamine gives the dialuminium diamide dimer:
    2 AlMe$_3$ + 2 HNMe$_2$ → [AlMe$_2$NMe$_2$]$_2$ + 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$_2$Me$_6$ reacts with aluminium trichloride to give (AlMe$_2$Cl)$_2$.
TMA/metal halide reactions have emerged as reagents in organic synthesis. Tebbe's reagent, which is used for the methylenation of esters and ketones, is prepared from TMA and titanocene dichloride. In combination with 20 to 100 mol % Cp$_2$ZrCl$_2$ (zirconocene dichloride), the (CH$_3$)$_2$Al-CH$_3$ adds "across" alkynes to give vinyl aluminium species that are useful in organic synthesis in a reaction known as carboalumination.

### Adducts

As for other "electron-deficient" compounds, trimethylaluminium gives adducts R$_3$N.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 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.