Samarium- and Ytterbium-Grafted Periodic Mesoporous Silica for Carbon Dioxide Capture and Conversion

Immobilized coordination compounds of Lewis acidic metals are powerful catalytic components of systems for the cycloaddition of CO₂ to epoxides that do not require sophisticated coordination frameworks to harness the metal center and modulate its activity. Surface organometallic chemistry (SOMC) is a valuable methodology to prepare well-defined and site-isolated surface complexes and coordination compounds on metal oxides, with ligand environments easily adjustable to a targeted catalytic reaction. In this work, the SOMC methodology is applied to prepare Sm^II, Yb^II, and Sm^III alkoxide surface complexes on periodic mesoporous (organo)silica of distinct pore symmetry/size for application in the CO₂ cycloaddition reaction. The surface complexes are readily accessible by the grafting of the bis(trimethylsilyl)amide precursors Ln^II[N(SiMe₃)₂]₂(THF)₂ (Ln = Sm, Yb) and Sm^III[N(SiMe₃)₂]₃, followed by ligand exchange with alcohols (ethanol and neopentanol). The use of periodic mesoporous supports led to hybrid materials with relatively high surface areas and pore sizes, affording good performance in CO₂ capture and in the cycloaddition of CO₂ to epoxides under mild conditions (60-80 °C, 1-10 bar). In terms of catalytic performance, recyclability, and low amount of added nucleophile TBAX (X = Br, I), the most active materials prepared in this work compare well to a variety of previously reported SOMC-derived surface complexes and to other heterogeneous Lewis acids displaying more elaborate ligand environments.