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- Aluminium(I) nucleophiles are a group of inorganic and organometallic nucleophilic compounds containing at least one aluminium metal center in the +1 oxidation state with a lone pair of electrons strongly localized on the aluminium(I) center. Prevalent aluminium(III) compounds such as aluminium trihalides (AlCl3, AlBr3, AlI3) are regularly employed in organic synthesis as electrophiles or Lewis acids. However, upon reducing of the metal center, aluminium(I) compounds may gain a lone pair which confers them nucleophilic character. While many aluminium(I) compounds are thermodynamically unstable due to their low oxidation state and act as good reducing agents, recent synthetic developments allowed for the isolation of stable aluminium(I) compounds. The first example of an isolable aluminium(I) compound was the tetrameric (AlCp*)4 (Cp* = pentamethylcyclopentadienyl) reported by Schnöckel and coworkers in 1991, while the first monomeric aluminium(I) compound was isolated on a β-diketiminate NacNac-type ligand by Roesky and coworkers in 2000. This initial monomeric aluminium(I) neutral compound and other closely related β-diketiminate supported aluminium(I) compounds were predicted to display ambiphilic behavior: electrophilic character due to the formally vacant aluminium p-orbital, as well as nucleophilic character due to the presence of a lone pair. However, in practice, their nucleophilic character was not observed through coordination to electrophiles or nucleophilic substitution; instead, their main mode of reactivity involves oxidative addition pathways, due to the low oxidation state of the aluminium center. Nevertheless, these seminal findings along with more recent advances in the synthesis, isolation, and characterization of aluminium(I) compounds allowed for the discovery of a novel type of reactivity at aluminium centers: nucleophilicity at aluminium(I) anions, referred to as aluminyl anions. The first isolation of a nucleophilic aluminium center was achieved in 2018 by Aldridge, Goicoechea and coworkers when they were able to synthesize the first aluminyl anion, following the discoveries of gallium and indium analogues, heavier group 13 analogues which are more stable than aluminium in the lower +1 oxidation state. Since then, several other nucleophilic aluminyl anions have been synthesized and characterized. (en)
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