Synthesis

AlCl is synthesized by reaction of liquid aluminium with gaseous HCl at 1200 K and 0.2 mbar to yield gaseous AlCl and hydrogen gas.^[1] At 77 K, AlCl is a dark red solid which turns black upon disproportionation at temperatures higher than 180 K. At temperatures under 77 K and dissolved in a matrix of polar and non-polar solvents, it exists as a metastable solution whose reactivity can be studied. AlBr, a red oil, is prepared similarly from liquid aluminium metal and gaseous HBr.^[4]

${\displaystyle {\ce {Al (l) + HCl (g) -> AlCl (g) + 1/2 H2 (g)}}}$

Due to the nature of HF, which possesses a bond much stronger than that of its congeners,^[8] AlF is synthesized instead by the comproportionation of Al and AlF₃ which are pressed and mixed into pellets.^[9] The pellets are then loaded into a graphite furnace and heated to 1050 K.^[9]

${\displaystyle {\ce {2 Al (s) + AlF3 (s) -> 3 AlF (g)}}}$

Stability increases with mass: while AlCl decomposes at 77 K or above, AlBr remains stable up to 253 K.^[1][4] Remarkably, it has been discovered that at any given temperature, the vapor pressure of AlF is orders of magnitude lower than that of other aluminium monohalides.^[9]

Disproportionation

At room temperature, AlX compounds tend to disproportionate to Al and AlX₃. When dark red, solid AlCl is allowed to warm up, it turns black to yield aluminium metal and the more stable aluminium (III) chloride salt.^[1]

${\displaystyle {\ce {2 AlCl (s)-> Al (s) +AlCl3 (s)}}}$

Dohmier et al. documented that the exception is AlBr. AlBr is stable enough at temperatures under -30 C that it comproportionates to AlBr₂ in the presence of AlBr₃.^[2]

${\displaystyle {\ce {AlBr (s) + AlBr3 (s) -> 2 AlBr2 (s)}}}$

Oligomerization

In Lewis basic solutions, AlX compounds have a tendency to oligomerize.^[2]

Molecular structure of the adduct of NEt₃ with AlBr, [Al₄Br₄(NEt₃)₄)], in the solid state.^[2]

β-Diketiminato systems

β-Diketiminato ligands, an example of a NacNac ligand. It is anionic and bidentate. Shown are tautomers of a substituted HNacNac ligand precursor, and an idealized complex (right) of the conjugate base (M = metal, L = other ligand)

In 2018, Liu et al. reviewed the chemistry of aluminium (I) with β-diketiminato ligands,^[10] widely used ligands with immense versatility in electronic and steric properties. These aluminium (I) complexes have immense potential for small molecule activation.^[10]

Synthesis

β-diketiminato aluminium alkyls and aluminium halides are synthesized by adding a trialkyl aluminium compound to the initial β-diketiminate ligands, adding iodine, and the reducing with potassium.^[10]

A synthetic route to aluminum (I) complexes supported by β-diketiminate ligands.^[11]

[1+2] Cycloadditions

Al(I) compounds exhibit behavior analogous to that of singlet carbenes.^[10] Like carbenes, they undergo [1+2] cycloadditions with alkynes and azides to afford three membered ring derivatives such as dialuminacyclohexadiene.^[1][10]

Similarly to the nucleophilic carbon center in the carbene, the lone pair on the aluminium center binds to the first azide equivalent. Nitrogen gas is liberated. With the second equivalent of azide, a five-member ring is formed.

Aluminium (I) systems undergo reactions with azides in the same fashion as carbenes.^[10]

Reactions with other small molecules

Such aluminium (I) complexes can activate water as well as elemental phosphorus, oxygen, and sulfur to yield bridged dimers. This occurs via partial reduction of the elemental small molecule.^[10]

Activation of elemental phosphorus (P₄) by an aluminium (I) complex

AlCp* ligand.

AlCp*

AlCp*, consisting of aluminium (I) bonded with the pentamethylcyclopentadiene anion ((CCH3)₅⁻), was first synthesized in 1991 by Dohmier et al.^[12] (AlCp*)_4, a yellow crystalline solid, is first produced from the combination of AlCl and MgCp*_2.^[6] When vaporized, the long Al-Al bonds (276.9 pm)^[12] split, and monomeric molecules of [AlCp*] are created.

As revealed through Schnockel's work, [AlCp*] reacts by inserting itself into other bonds. Reaction with Al₂I₆ yields subvalent halide species; reaction with As₄tBu₄ yields As-Al bonds.^[6] When reacted with transition metal-cyclopentadienyl complexes such as NiCp₂, it offers a straightforward pathway to compounds containing aluminium-transition metal bonds, which has great potential for important catalytic reactions.^[2]

NiCp₂ and AlCp* react to form a butterfly structure.

The LUMO orbitals of CO are shown here. These orbitals are antibonding π orbitals.

As with other AlR ligands, [AlCp*] can be regarded as a CO analogue, as it possesses 2 empty π orbitals and engages in similar coordination modes (terminal and bridging).^[6] This similarity implies the possibility of pi backbonding interactions between AlCp*and metals it complexes to.