Seven-Membered Cyclic Diamidoalumanyls of Heavier Alkali Metals: Structures and C-H Activation of Arenes

Like the previously reported potassium-based system, rubidium and cesium reduction of [{SiN ^Dipp}AlI] ({SiN ^Dipp} = {CH ₂SiMe ₂NDipp} ₂) with the heavier alkali metals [M = Rb and Cs] provides dimeric group 1 alumanyl derivatives, [{SiN ^Dipp}AlM] ₂. In contrast, similar treatment with sodium results in over-reduction and incorporation of a formal equivalent of [{SiN ^Dipp}Na ₂] into the resultant sodium alumanyl species. The dimeric K, Rb, and Cs compounds display a variable efficacy toward the C-H oxidative addition of arene C-H bonds at elevated temperatures (Cs > Rb > K, 110 °C) to yield (hydrido)(organo)aluminate species. Consistent with the synthetic experimental observations, computational (DFT) assessment of the benzene C-H activation indicates that rate-determining attack of the Al(I) nucleophile within the dimeric species is facilitated by π-engagement of the arene with the electrophilic M ⁺ cation, which becomes increasingly favorable as group 1 is descended.