Cerium Pyrazolates Grafted onto Mesoporous Silica SBA-15: Reversible CO Uptake and Catalytic Cycloaddition of Epoxides and Carbon Dioxide.

The activation and catalytic conversion of CO is a current topic relating to molecular chemistry and materials science alike. As a transdisciplinary field of research, surface organometallic chemistry (SOMC) might be applicable to perform synergistically, thus striking a new path in sustainable chemistry. Both ceric and cerous rare-earth-metal pyrazolates, which were recently shown to reversibly insert CO and to promote the catalytic cycloaddition of epoxides and carbon dioxide, were grafted onto large-pore mesoporous silica SBA-15, thermally pretreated at 500 °C. The obtained hybrid materials [Ce(Mepz)]@SBA-15, Ce(Mepz)(thf)@SBA-15, Ce(Mepz)@SBA-15, and [Ce(Mepz)(thf)]@SBA-15 (Mepz = 3,5-dimethylpyrazolato) were characterized by DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy), solid-state H/C NMR spectroscopy, elemental analysis, ICP/OES, and N physisorption. The lanthanum(III)-based material [La(Mepz)(thf)]@SBA-15 was synthesized for better assessment of the cerous materials being highly sensitive to oxidation. To mimic ceric surface species, Ce[OSi(OBu)]Cl was treated with 1 equiv of K(Mepz), generating the mixed pyrazolyl/siloxy complex KCe[OSi(OBu)](Mepz) featuring a cerium(IV)-bonded terminal pyrazolato ligand. All hybrid materials show efficient and reversible carbon dioxide uptake of maximum 20 wt % in the solid state. When combined with tetra--butylammonium bromide (TBAB), the hybrid materials catalyze the cycloaddition of CO and epoxides, displaying good conversion of various epoxides and reusability.