Synthesis, structure and reaction chemistry of a nucleophilic aluminyl anion.

The reactivity of aluminium compounds is dominated by their electron deficiency and consequent electrophilicity; these compounds are archetypal Lewis acids (electron-pair acceptors). The main industrial roles of aluminium, and classical methods of synthesizing aluminium-element bonds (for example, hydroalumination and metathesis), draw on the electron deficiency of species of the type AlR3 and AlCl3 1,2 . Whereas aluminates, [AlR4 ]- , are well known, the idea of reversing polarity and using an aluminium reagent as the nucleophilic partner in bond-forming substitution reactions is unprecedented, owing to the fact that low-valent aluminium anions analogous to nitrogen-, carbon- and boron-centred reagents of the types [NX2 ]- , [CX3 ]- and [BX2 ]- are unknown3-5 . Aluminium compounds in the +1 oxidation state are known, but are thermodynamically unstable with respect to disproportionation. Compounds of this type are typically oligomeric6-8 , although monomeric systems that possess a metal-centred lone pair, such as Al(Nacnac)Dipp (where (Nacnac)Dipp  = (NDippCR)2 CH and R =  t Bu, Me; Dipp = 2,6- i Pr2 C6 H3 ), have also been reported9,10 . Coordination of these species, and also of (η5 -C5 Me5 )Al, to a range of Lewis acids has been observed11-13 , but their primary mode of reactivity involves facile oxidative addition to generate Al(III) species6-8,14-16 . Here we report the synthesis, structure and reaction chemistry of an anionic aluminium(I) nucleophile, the dimethylxanthene-stabilized potassium aluminyl [K{Al(NON)}]2 (NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene). This species displays unprecedented reactivity in the formation of aluminium-element covalent bonds and in the C-H oxidative addition of benzene, suggesting that it could find further use in both metal-carbon and metal-metal bond-forming reactions.