Direct X-ray observation of trapped CO₂ in a predesigned porphyrinic metal-organic framework

Metal-organic frameworks (MOFs) are emerging microporous materials that are promising for capture and sequestration of CO₂ due to their tailorable binding properties. However, it remains a grand challenge to pre-design a MOF with a precise, multivalent binding environment at the molecular level to enhance CO₂ capture. Here, we report the design, synthesis, and direct X-ray crystallographic observation of a porphyrinic MOF, UNLPF-2, that contains CO₂-specific single molecular traps. Assembled from an octatopic porphyrin ligand with [Co₂(COO)₄] paddlewheel clusters, UNLPF-2 provides an appropriate distance between the coordinatively unsaturated metal centers, which serve as the ideal binding sites for in situ generated CO₂. The coordination of Co^II in the porphyrin macrocycle is crucial and responsible for the formation of the required topology to trap CO₂. By repeatedly releasing and recapturing CO₂, UNLPL-2 also exhibits recyclability. CO ₂-Specific traps are generated in a rationally designed porphyrinic MOF, UNLPF-2 (see figure). With a precise binding environment, CO₂ trapping is directly observed by X-ray crystallography. More remarkably, UNLPF-2 can repeatedly release and recapture CO₂, exhibiting a good recyclability. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.