Formation of a Strong Heterogeneous Aluminum Lewis Acid on Silica.

Al(OC(CF 3 ) 3 )(PhF) reacts with silanols present on partially dehydroxylated silica to form well-defined ≡SiOAl(OC(CF 3 ) 3 ) 2 (O(Si≡) 2 ) (1). 27 Al 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(C 6 F 5 ) 3 and Al(OC(CF 3 ) 3 )(PhF) but is weaker than Al(OC(CF 3 ) 3 ) and i Pr 3 Si + . Cp 2 Zr(CH 3 ) 2 reacts with 1 to form [Cp 2 ZrCH 3 ][≡SiOAl(OC(CF 3 ) 3 ) 2 (CH 3 )] (3) by methide abstraction. This reactivity pattern is similar to reactions of organometallics with the proposed strong Lewis acid sites present on Al 2 O 3 .