Trimethylaluminium

Trimethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al₂(CH₃)₆ (abbreviated as Al₂Me₆ or TMA), as it exists as a dimer. This colorless liquid is pyrophoric. It is an industrially important compound, closely related to triethylaluminium.^[3][4]

Trimethylaluminium

Names
IUPAC name Trimethylalumane
Other names Trimethylaluminum; aluminium trimethyl; aluminum trimethyl
Identifiers
CAS Number	75-24-1 Y
3D model (JSmol)	dimer: Interactive image monomer: Interactive image
ChemSpider	10606585 Y
ECHA InfoCard	100.000.776
PubChem CID	16682925
UNII	AV210LG46J Y
CompTox Dashboard (EPA)	DTXSID9058787
InChI InChI=1S/3CH3.Al/h3*1H3; Y Key: JLTRXTDYQLMHGR-UHFFFAOYSA-N Y InChI=1/3CH3.Al/h3*1H3;/rC3H9Al/c1-4(2)3/h1-3H3 Key: JLTRXTDYQLMHGR-MZZUXTGEAJ
SMILES dimer: C[Al-](C)([CH3+]1)[CH3+][Al-]1(C)C monomer: C[Al](C)C
Properties
Chemical formula	C6H18Al2
Molar mass	144.17 g/mol 72.09 g/mol (C3H9Al)
Appearance	Colorless liquid
Density	0.752 g/cm3
Melting point	15 °C (59 °F; 288 K)
Boiling point	125–130 °C (257–266 °F; 398–403 K)[1][2]
Solubility in water	Reacts
Vapor pressure	1.2 kPa (20 °C) 9.24 kPa (60 °C)[1]
Viscosity	1.12 cP (20 °C) 0.9 cP (30 °C)
Thermochemistry
Heat capacity (C)	155.6 J/mol·K[2]
Std molar entropy (S⦵298)	209.4 J/mol·K[2]
Std enthalpy of formation (ΔfH⦵298)	−136.4 kJ/mol[2]
Gibbs free energy (ΔfG⦵)	−9.9 kJ/mol[2]
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	Pyrophoric
GHS labelling:
Pictograms	[1]
Signal word	Danger
Hazard statements	H250, H260, H314[1]
Precautionary statements	P222, P223, P231+P232, P280, P370+P378, P422[1]
NFPA 704 (fire diamond)	3 4 3 W
Flash point	−17.0 °C (1.4 °F; 256.1 K)[1]
Related compounds
Related compounds	Triethylaluminium
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Structure and bonding

The structure and bonding in Al₂R₆ and diborane 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.^[5] 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.^[6]

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 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 Al₂O₃ 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 Al₂O₃ via the processes of chemical vapor deposition 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:

AlMe₃ + H₂O → 1/n [AlMeO]_n + 2 CH₄

Alcoholysis and aminolysis reactions proceed comparably. For example, dimethylamine gives the dialuminium diamide dimer:^[7]

2 AlMe₃ + 2 HNMe₂ → [AlMe₂NMe₂]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.^[8] Al₂Me₆ reacts with aluminium trichloride to give (AlMe₂Cl)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.^[9] In combination with 20 to 100 mol % Cp₂ZrCl₂ (zirconocene dichloride), the (CH₃)₂Al-CH₃ adds "across" alkynes to give vinyl aluminium species that are useful in organic synthesis in a reaction known as carboalumination.^[10]

Adducts

As for other "electron-deficient" compounds, trimethylaluminium gives adducts R₃N.AlMe₃. The Lewis acid properties of AlMe₃ have been quantified.^[11] The enthalpy data show that AlMe₃ 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.^[12]

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.

References

1. Sigma-Aldrich Co., Trimethylaluminum. Retrieved on 2014-05-05.
2. "Trimethyl aluminum".
3. Krause, Michael J.; Orlandi, Frank; Saurage, Alfred T.; Zietz, Joseph R. (2000). "Aluminum Compounds, Organic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a01_543. ISBN 978-3-527-30673-2.
4. C. Elschenbroich (2006). Organometallics. VCH. ISBN 978-3-527-29390-2.
5. Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5.
6. Vass, Gábor; Tarczay, György; Magyarfalvi, Gábor; Bödi, András; Szepes, László (2002). "HeI Photoelectron Spectroscopy of Trialkylaluminium and Dialkylaluminium Hydride Compounds and Their Oligomers". Organometallics. 21 (13): 2751–2757. doi:10.1021/om010994h.
7. Lipton, Michael F.; Basha, Anwer; Weinreb, Steven M. (1979). "Conversion of Esters to Amides with Dimethylaluminium Amides: N,N-Dimethylcyclohexanecarboxamide". Organic Syntheses. 59: 49. doi:10.15227/orgsyn.059.0049.
8. Gaines, D. F.; Borlin, Jorjan; Fody, E. P. (1974). "Trimethylgallium". Inorganic Syntheses. Vol. 15. pp. 203–207. doi:10.1002/9780470132463.ch45. ISBN 978-0-470-13246-3.
9. Pine, S. H.; Kim, V.; Lee, V. (1990). "Enol ethers by methylenation of esters: 1-Phenoxy-1-phenylethene and 3,4-dihydro-2-methylene-2H-1-benzopyran". Org. Synth. 69: 72. doi:10.15227/orgsyn.069.0072.
10. Negishi, E.; Matsushita, H. (1984). "Palladium-Catalyzed Synthesis of 1,4-Dienes by Allylation of Alkenyalane: α-Farnesene [1,3,6,10-Dodecatetraene, 3,7,11-trimethyl-]". Organic Syntheses. 62: 31. doi:10.15227/orgsyn.062.0031.
11. Henrickson, C. H.; Duffy, D.; Eyman, D. P. (1968). "Lewis acidity of Alanes. Interactions of Trimethylalane with Amines, Ethers, and Phosphines". Inorganic Chemistry. 7 (6): 1047–1051. doi:10.1021/ic50064a001.
12. Vinogradov, Andrej; Woodward, S. (2010). "Palladium-Catalyzed Cross-Coupling Using an Air-Stable Trimethylaluminium Source. Preparation of Ethyl 4-Methylbenzoate". Organic Syntheses. 87: 104. doi:10.15227/orgsyn.087.0104.

External links

• NASA ATREX mission article.