Light Metal Pyrazolates Grafted onto Periodic Mesoporous Silica for Carbon Dioxide Capture and Transformation

Recent advances have shown that light metal pyrazolate complexes not only achieve high CO₂ uptake but are also able to convert epoxides and CO₂ to cyclic carbonates catalytically. Surface organometallic chemistry (SOMC) combines reactive metal complexes with the durability and robustness of a support material to form environmentally even more benign materials for CO₂ capture and conversion. In this study, light metal pyrazolates with a variety of oxidation states and ionic radii were grafted onto mesoporous silica SBA-15₅₀₀ affording the hybrid materials [Mg(pz^<i>t</i>Bu2)₂]₂@SBA-15₅₀₀, Al(pz^<i>t</i>Bu2)₃@SBA-15₅₀₀, Ti^+IV(pz^Me2)₄@SBA-15₅₀₀, and Ti^+III(pz^<i>t</i>Bu2)₃@SBA-15₅₀₀. The hybrid materials were characterized via N₂ physisorption, elemental analysis, ICP/OES, DRIFTS, and solid-state NMR spectroscopy, suggesting successful grafting with monometallic surface species and revealing a CO₂ uptake of up to 11 wt%. In addition, Ti^+IV(pz^Me2)[OSi(O<i>t</i>Bu)₃]₃ was synthesized as a model complex for surface species likely present for Ti^+IV(pz^Me2)₄@SBA-15₅₀₀. Complex Ti^+IV(pz^Me2)[OSi(O<i>t</i>Bu)₃]₃ is also able to insert CO₂ under the formation of the carbamate complex Ti^+IV(CO₂·pz^Me2)[OSi(O<i>t</i>Bu)₃]₃, emulating material CO₂@Ti^+IV(pz^Me2)₄@SBA-15₅₀₀. All hybrid materials under study are active catalysts in the cycloaddition of epoxides with CO₂ to form cyclic carbonates. The magnesium hybrid material [Mg(pz^<i>t</i>Bu2)₂]₂@SBA-15₅₀₀ exceeds its homogeneous congener, featuring high conversion even for bulkier epoxides along with a desirable reusability.