Two Manganese Metalloporphyrin Frameworks Constructed from a Custom-Designed Porphyrin Ligand Exhibiting Selective Uptake of CO2 over CH4 and Catalytic Activity for CO2 Fixation

In this work, two new metal-metalloporphyrin frameworks (MMPFs), namely, MMPF-12 and MMPF-13, having unique structural formulas as [Mn24O78(Mn-dcdbp)(12)] and [Mn-8.65(dcdbp)(8)](DMF)(14)(H2O)(8), respectively, were synthesized via the reaction of the custom-designed porphyrin ligand 5,15-bis(3,5-dicarboxyphenyl)-10,20-bis(2,6-dibromophenyl) porphyrin (dcdbp) and hydrated manganese nitrate under solvothermal conditions. Single-crystal X-ray diffraction analysis reveals three-dimensional porous structures with MMPF-12 exhibiting nearly complete metalation of the ligand, whereas MMPF-13, unusually, contains only a small fraction of metalated porphyrin. Gas sorption studies attest to a permanent porosity together with selective adsorption of CO2 over CH4. Additionally, both MMPFs catalyzed CO2 cycloaddition of electronically and sterically substituted epoxide substrates, thereby providing access to important products under mild conditions. Results also showed that the ligand-metalated MMPF-12, containing trimer Mn clusters, has a higher catalytic activity than its congener MMPF-13, which has only dimer clusters. Thus, Mn clusters likely play an important role in both the structure of the MMPFs and their activity toward the transformation of CO2, with the enhanced catalytic activity being ascribed to the conserved porous structure.

Automated summary

This study successfully synthesized two new metal-metalloporphyrin frameworks (MMPF-12 and MMPF-13) via solvothermal conditions, showing unique structures with either fully or partially metalated ligands, as well as selective adsorption of CO2 and catalytic activity towards CO2 cycloaddition. The enhanced catalytic activity of ligand-metalated MMPF-12 compared to MMPF-13 is likely attributed to the presence of trimer Mn clusters in its structure.