Cerium Pyrazolates Grafted onto Mesoporous Silica SBA-15: Reversible CO2 Uptake and Catalytic Cycloaddition of Epoxides and Carbon Dioxide

The activation and catalytic conversion of CO2 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 CO2 and to promote the catalytic cycloaddition of epoxides and carbon dioxide, were grafted onto large-pore mesoporous silica SBA-15(500), thermally pretreated at 500 degrees C. The obtained hybrid materials [Ce(Me(2)pz)(4)](2)@SBA-15(500), Ce(Me(2)pz)(4)( thf)@SBA-15(500), Ce-4(Me(2)pz) (12)@SBA-15(500), and [Ce(Me(2)pz)(3)(thf)](2)@SBA-15(500) (Me(2)pz = 3,5-dimethylpyrazolato) were characterized by DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy), solid-state H-1/C-13 NMR spectroscopy, elemental analysis, ICP/OES, and N-2 physisorption. The lanthanum(III)-based material [La(Me(2)pz)(3)(thf)](2)@SBA-15(500) was synthesized for better assessment of the cerous materials being highly sensitive to oxidation. To mimic ceric surface species, Ce[OSi(OtBu)(3)](3)Cl was treated with 1 equiv of K(Me(2)pz), generating the mixed pyrazolyl/siloxy complex KCe[OSi(OtBu)(3)](4)(Me(2)pz) 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-n-butylammonium bromide (TBAB), the hybrid materials catalyze the cycloaddition of CO2 and epoxides, displaying good conversion of various epoxides and reusability.