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(pztBu2)22@SBA-15500, Al(pztBu2)3@SBA-15500, Ti+IV(pzMe2)4@SBA-15500, and Ti+III(pztBu2)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(pzMe2)OSi(OtBu)33 was synthesized as a model complex for surface species likely present for Ti+IV(pzMe2)4@SBA-15500. Complex Ti+IV(pzMe2)OSi(OtBu)33 is also able to insert CO2 under the formation of the carbamate complex Ti+IV(CO2·pzMe2)OSi(OtBu)33, emulating material CO2@Ti+IV(pzMe2)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(pztBu2)22@SBA-15500 exceeds its homogeneous congener, featuring high conversion even for bulkier epoxides along with a desirable reusability.
Kracht et al. (Tue,) studied this question.