Formation of a Strong Heterogeneous Aluminum Lewis Acid on Silica

Abstract

Al(OC(CF 3 ) 3 )(PhF) reacts with silanols present on partially dehydroxylated silica to form welldefined �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 .

Effects of surface acidity on the structure of organometallics supported on oxide surfaces

“…71 We investigated the reaction of Al(OC(CF 3 ) 3 )(PhF) 72 with SiO 2-700 . 73 The choice of this particular combination was driven by two complementary rationales. First, Al(OC(CF 3 ) 3 )(PhF) is a significantly stronger Lewis acid than B(C 6 F 5 ) 3 based on fluoride ion affinity calculations.…”

“…In PhF with mild heating RSiOAl(OC(CF 3 ) 3 ) 2 (O(SiR) 2 ) forms as discussed above. 73 In perfluorohexanes at 25 1C Al(OC(CF 3 ) 3 )(PhF) reacts with SiO 2-700 to form the bridging silanol RSi-OHÁ Á ÁAl(OR F ) 3 , Fig. 12(b).…”

An Amorphous Teflate Doped Aluminium Chlorofluoride: A Solid Lewis‐Superacid for the Dehydrofluorination of Fluoroalkanes

“…In all of these materials Lewis‐acidity and amorphicity are mainly due to the distortion of the bulk structure by chloride or isopropoxide groups. Note also that recently a heterogeneous Al‐based Lewis Acid was generated by treatment of partially dehydroxylated silica with Al(OC(CF 3 ) 3 ) 3 (PhF) [8] …”

“…Note also that recently a heterogeneous Al-based Lewis Acid was generated by treatment of partially dehydroxylated silica with Al(OC(CF 3 ) 3 ) 3 (PhF). [8] The pentafluoroorthotellurate group ([OTeF 5 ] À , teflate group) is an interesting substitute for fluoride, because the teflate group mirrors the electron withdrawing properties of fluoride, but it is considerably bulkier. [9] It can be considered as chemical inert towards electrophiles.…”

Tailored Lewis Acid Sites for High-Temperature Supported Single-Molecule Magnetism

“…During grafting, one anionic ligand present in the precursor would be transferred to a Lewis acidic site on the surface to form an ion pair, constituted by a negatively charged support and a cationic metal complex. [28][29][30] To avoid the formation of fast relaxing neutral species via protonation of organic ligands, the use of a support with Lewis acid and ideally no Brønsted acid sites is necessary. [27,31] Low-coordinate Al(III) centers are known to act as strong Lewis acids and can be generated on various supports by different methods such as dehydroxylation of γ-alumina, [28,29] incorporation of aluminum atoms in a rigid framework (zeolites) [32,33] or grafting of alkylaluminum species on surfaces.…”