Two Manganese Metalloporphyrin Frameworks Constructed from a Custom-Designed Porphyrin Ligand Exhibiting Selective Uptake of CO2over CH4and Catalytic Activity for CO2Fixation

Abstract

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 [Mn8.65(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.