SOMC@Periodic Mesoporous Silica Nanoparticles: Meerwein–Ponndorf–Verley Reduction Promoted by Immobilized Rare-Earth-Metal Alkoxides

Abstract

The Meerwein–Ponndorf–Verley (MPV) reduction is a reaction that offers a mild reduction of aldehydes and ketones to the corresponding alcohols. Although described as a catalytic reaction, its real-life applicability suffers from the necessity of using the standard catalyst [Al(OiPr)3] in stoichiometric amounts or even in excess. Rare-earth-metal-based catalysts are capable of performing in these reactions in a truly catalytic fashion. The ceric alkoxide [Ce(OiPr)4]3 has been synthesized via silylamine elimination from Ce[N(SiHMe2)2]4 with isopropyl alcohol, its trimetallic solid-state structure has been determined by X-ray diffraction, and its performance in the MPV reduction of 4-tBu-cyclohexanone has been examined and compared to that of cerous [Ce(OCH2tBu)3]4. Spherical mesoporous silica nanoparticles with an MCM-41-type honeycomb pore symmetry, termed MSN-MCM-41 (particle size, ca. 250 nm diameter; pore size, 2.6 nm diameter), are employed for grafting the molecular precursors Ce[N(SiHMe2)2]4, [Ce(OiPr)4]3, Ce[N(SiMe3)2]3, and La[N(SiMe3)2]3 according to the methods of surface organometallic chemistry (SOMC). The MPV reductions carried out with the homogeneous and heterogeneous catalysts reveal (a) a better performance of Ce(III) in comparison to Ce(IV), (b) better performance of La[N(SiMe3)2]3@MSN-MCM-41 in comparison to Ce[N(SiMe3)2]3@MSN-MCM-41 (high sensitivity of Ce(III)-grafted materials), and (c) reusability of the grafted catalyst systems. All hybrid materials were characterized by PXRD, N2 physisorption, and 1H/13C/29Si MAS NMR and FTIR spectroscopies as well as elemental analysis.