Formation of a Strong Heterogeneous Aluminum Lewis Acid on Silica

Al(OC(CF3)3)(PhF) reacts with silanols present on partially dehydroxylated silica to form well‐defined ≡SiOAl(OC(CF3)3)2(O(Si≡)2) (1). 27Al NMR and DFT calculations with a small cluster model to approximate the silica surface show that the aluminum in 1 adopts a distorted trigonal bipyramidal coordination geometry by coordinating to a nearby siloxane bridge and a fluorine from the alkoxide. Fluoride ion affinity (FIA) calculations follow experimental trends and show that 1 is a stronger Lewis acid than B(C6F5)3 and Al(OC(CF3)3)(PhF) but is weaker than Al(OC(CF3)3) and iPr3Si+. Cp2Zr(CH3)2 reacts with 1 to form [Cp2ZrCH3][≡SiOAl(OC(CF3)3)2(CH3)] (3) by methide ion. This reactivity pattern is similar to reactions of organometallics with the proposed strong Lewis acid sites present on Al2O3. Al(OC(CF3)3)3(PhF) reacts with partially dehydroxylated silica to form very strong Lewis acid sites. The well‐defined aluminum Lewis acids are significantly stronger than common molecular Lewis acids and engage in alkyl ion reactions that are uncommon using well‐defined supported Lewis acids.