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

Abstract

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 unknown^3-5. Aluminium compounds in the +1 oxidation state are known, but are thermodynamically unstable with respect to disproportionation. Compounds of this type are typically oligomeric^6-8, although monomeric systems that possess a metal-centred lone pair, such as Al(Nacnac)^Dipp (where (Nacnac)^Dipp = (NDippCR)₂CH and R =  ^t Bu, Me; Dipp = 2,6- ⁱ Pr₂C₆H₃), have also been reported^9,10. Coordination of these species, and also of (η⁵-C₅Me₅)Al, to a range of Lewis acids has been observed^11-13, but their primary mode of reactivity involves facile oxidative addition to generate Al(III) species^6-8,14-16. 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.