Organoaluminium chemistry is the study of compounds containing bonds between carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 and aluminium

Aluminium

Aluminium 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....

 bond. It is one of the major themes within organometallic chemistry

Organometallic chemistry

Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...

. Illustrative organoaluminium compounds are the dimer trimethylaluminium

Trimethylaluminium

Trimethylaluminium is the chemical compound with the formula Al26, abbreviated as Al2Me6, 2 or the abbreviation TMA. This pyrophoric, colorless liquid is an industrially important organoaluminium compound...

, the monomer triisobutylaluminium

Triisobutylaluminium

Triisobutylaluminium is an organoaluminium compound with the formula Al3. This a colorless pyrophoric liquid is mainly used to make linear primary alcohols and α-olefins.-Structure:...

, and the titanium-aluminium compound called Tebbe's reagent

Tebbe's reagent

The 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...

. The behavior of organoaluminium compounds can be understood in terms of the polarity of the C−Al bond and the high Lewis acidity of the three-coordinated species. Industrially, these compounds are mainly used for the production of polyolefin

Polyolefin

A polyolefin is a polymer produced from a simple olefin as a monomer. For example, polyethylene is the polyolefin produced by polymerizing the olefin ethylene. An equivalent term is polyalkene; this is a more modern term, although polyolefin is still used in the petrochemical industry...

s.

History

The first organoaluminium compound (C₂H₅)₃Al₂I₃ was discovered in 1859. Organoaluminium compounds were however little known until the 1950s when Karl Ziegler

Karl Ziegler

Karl Waldemar Ziegler was a German chemist who won the Nobel Prize in Chemistry in 1963, with Giulio Natta, for work on polymers. The Nobel Committee recognized his "excellent work on organometallic compounds [which]...led to new polymerization reactions and ... paved the way for new and highly...

 and colleagues discovered the direct synthesis of trialkylaluminium compounds and applied these compounds to catalytic olefin polymerization. This line of research ultimately resulted in the Nobel Prize to Ziegler.

Aluminium(III) compounds

Organoaluminium compounds generally feature three- and four-coordinate Al centers, although higher coordination number

Coordination number

In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....

s are observed with inorganic ligands such as fluoride

Fluoride

Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...

. In accord with the usual trends, four-coordinate Al prefers to be tetrahedral. In contrast to boron, aluminium is a larger atom and easily accommodates four carbon ligands. The triorganoaluminium compounds are thus usually dimeric with a pair of bridging

Bridging ligand

A bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually restricted to small ligands such as pseudohalides or to ligands that are...

 alkyl ligands, e.g., Al₂(C₂H₅)₄(μ-C₂H₅)₂. Thus, despite its common name of triethylaluminium, this compound contains two aluminium centres, and six ethyl group

Ethyl group

In chemistry, an ethyl group is an alkyl substituent derived from ethane . It has the formula -C2H5 and is very often abbreviated -Et.Ethylation is the formation of a compound by introduction of the ethyl functional group, C2H5....

s. When the organoaluminium compound contain hydride

Hydride

In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...

 or halide

Halide

A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...

, these smaller ligands tend to occupy the bridging sites. Three coordination occurs when the R groups

Side chain

In organic chemistry and biochemistry, a side chain is a chemical group that is attached to a core part of the molecule called "main chain" or backbone. The placeholder R is often used as a generic placeholder for alkyl group side chains in chemical structure diagrams. To indicate other non-carbon...

 is bulky, e.g. Al(Mes)₃ (Mes = 2,4,6-Me₃C₆H₂ or mesityl) or isobutyl.

Ligand exchange in trialkylaluminium compounds

The trialkylaluminium dimers often participate in dynamic equilibria, resulting in the interchange of bridging and terminal ligands as well as ligand exchange between dimers. Even in noncoordinating solvent

Solvent

A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

s, Al-Me exchange is fast, as confirmed by proton NMR

Proton NMR

Proton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen is used, practically all of the hydrogen consists of the...

 spectroscopy. For example, at −25 °C the ¹H NMR spectrum of Me₆Al₂ comprises two signals in 1:2 ratio, as expected from the solid state structure. At 20 °C, only one signal is observed because exchange of terminal and bridging methyl groups is too fast to be resolved by NMR. The high Lewis acidity of the monomeric species is related to the size of the Al(III) center and its tendency to achieve an octet configuration

Octet rule

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 (The octet rule is a chemical rule of thumb that states that atoms of low (...

.

Low valent organoaluminium compounds

The first organoaluminium compound with a Al-Al bond was reported in 1988 as (((Me₃Si)₂CH)₂Al)₂ (a dialane). They are typically prepared reduction of the dialkylaluminium chlorides by metallic potassium:₂ + 2 K → R₂Al-AlR₂ + 2 KCl

Another notable group of alanes are tetraalanes containing for Al(I) centres. These compounds adopt a tetrahedrane

Tetrahedrane

Tetrahedrane is a platonic hydrocarbon with chemical formula 44 and a tetrahedral structure. Extreme angle strain prevents this molecule from forming naturally....

 core, as illustrated by (Cp*

Pentamethylcyclopentadiene

1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diolefin with the formula C5Me5H . 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted as Cp*...

Al)₄ and ((Me₃Si₃C)Al)₄. The cluster [Al₁₂(i-Bu)₁₂]^2- was obtained from related investigations on the reduction of organoaluminium compounds. This dianion adopts an icosahedral structure reminiscent of dodecaborate ([B₁₂H₁₂]^2-). Its formal oxidation state is less than one.

From alkyl halides and aluminium

Industrially, simple aluminium alkyls of the type Al₂R₆ (R = Me, Et) are prepared in a two-step process beginning with the alkylation

Alkylation

Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion or a carbene . Alkylating agents are widely used in chemistry because the alkyl group is probably the most common group encountered in...

 of aluminium powder:

2 Al + 3 CH₃CH₂Cl → (CH₃CH₂)₃Al₂Cl₃

The reaction resembles the synthesis Grignard reagents. The product, (CH₃CH₂)₃Al₂Cl₃, is called ethylaluminium sesquichloride

Ethylaluminium sesquichloride

Ethylaluminium sesquichloride, also called EASC, is an industrially important organoaluminium compound used primarily as a precursor to triethylaluminium and as a catalyst component in Ziegler-Natta type systems for olefin and diene polymerizations...

. The term sesquichloride refers to the fact that, on average, the Cl:Al ratio is 1.5. These sesquichlorides can be converted to the triorganoaluminium derivatives by reduction:

2 (CH₃CH₂)₃Al₂Cl₃ + 6 Na → (CH₃CH₂)₆Al₂ + 2 Al + 6 NaCl

This method is used for production of trimethylaluminium and triethylaluminium

Triethylaluminium

Triethylaluminium is an organoaluminium compound. This volatile, colorless liquid is highly pyrophoric, igniting immediately upon exposure to air. It is normally stored in stainless steel containers either as a pure liquid or as a solution in hydrocarbon solvents such as hexane, heptane, or ...

.

Hydroalumination

Aluminium powder reacts directly with certain terminal alkenes in the presence of hydrogen. The process entails two steps, the first producing dialkylaluminium hydrides. Such reactions are typically conducted at elevated temperatures and require activation by trialkylaluminium reagents:

3 Al + 3/2 H₂ + 6 CH₂=CHR → [HAl(CH₂CHR)₂]₃

For nonbulky R groups, the organoaluminium hydrides are typically trimeric. In a subsequent step, these hydrides are treated with more alkene to effect hydroalumiunation:

2 [HAl(CH₂CHR)₂]₃ + 3 CH₂=CHR → 3 [Al₂(CH₂CHR)₃

Diisobutylaluminium hydride

Diisobutylaluminium hydride

Diisobutylaluminium hydride, DIBAL, DIBAL-H or DIBAH, is a reducing agent with the formula 2, where i-Bu represents isobutyl...

, which is dimeric, is prepared by hydride elimination from triisobutylaluminium:

2 i-Bu₃Al → (i-Bu₂AlH)₂ + 2 (CH₃)₂C=CH₂

carboalumination

Organoaluminium compounds can react with alkenes and alkynes in carboalumination. Monoaddition is only possible when the alkene is substituted. The reaction is regioselective for 1-alkenes. The so-called ZACA reaction

ZACA reaction

The Zirkonium-catalyzed Asymmetric Carbo-Alumination reaction was developed by Nobel laureate Ei-ichi Negishi. It facilitates the chiral functionalization of alkenes using organoaluminium compounds under the influence of chiral bis-indenylzirkonium catalysts...

 is an example of an asymmetric carboalumination.

Laboratory preparations

Although the simple members are commercially available at low cost, many methods have been developed for their synthesis in the laboratory, including metathesis or transmetalation

Transmetalation

Transmetalation is a general chemical reaction type in organometallic chemistry describing the exchange of ligands between two metal centers....

. Metathesis of aluminium trichloride with RLi or RMgX gives the trialkyl:

AlCl₃ + 3 BuLi → Bu₃Al + 3 LiCl

• Transmetalation

2 Al + 3 HgPh₂ → 2 AlPh₃ + 3 Hg

Lewis acidity

Organoaluminium compounds are hard acids and readily form adducts with bases such as pyridine

Pyridine

Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...

, THF

Tetrahydrofuran

Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...

 and tertiary amines. These adducts are tetrahedral at Al.

Electrophiles

The Al-C bond is polarized such that the carbon is highly basic. Acids react to give alkanes. For example, alcohols give alkoxide

Alkoxide

An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They can be written as RO−, where R is the organic substituent. Alkoxides are strong bases and, when R is not bulky, good nucleophiles and good ligands...

s:

AlR'₃ + ROH → (R'₂Al−OR)_n + R'H

A wide variety of acids can be employed beyond the simple mineral acids. Amines give amido derivatives. With carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, trialkylaluminium compounds give the dialkylaluminium carboxylate:

AlR₃ + CO₂ → R₂AlO₂CR

The conversion is reminiscent of the carbonation of Grignard reagents.

With oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 one obtains the corresponding alkoxides, which can be hydrolysed to the alcohols:

AlR₃ + 3/2 O₂ → Al(OR)₃

Alkene polymerization

Industrially, organoaluminium compounds can be used as catalysts for alkene polymerization to polyolefins, for example the catalyst methylaluminoxane

Methylaluminoxane

Methylaluminoxane, 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...

.