Gas Sorption Properties of Isostructural Co-MOFs Containing Dipyridylporphyrin Linkers with Different Substituents at the 10,20-meso-Positions

Two new isostructural 3D dipyridylporphyrin-based MOFs, [Co(DpyDClP)]₆·18H₂O (I) and [Co(DpyDCNP)]₆·18H₂O (II), were prepared and structurally characterized by X-ray crystallography [DpyDClP: 5,15-di(4-pyridyl)-10,20-di(4-chlorophenyl)porphyrin; DpyDCNP: 5,15-di(4-pyridyl)-10,20-di(4-cyanophenyl)porphyrin]. They display exactly the same framework structures, notwithstanding different substituents at the 10,20-meso-positions of the dipyridylporphyrin backbones. Both substituents, -Cl for I and -CN for II, are completely exposed to the open space of solvent-free I and II. Therefore, the two MOFs have the same framework structures with distinct pore functionalities. Solvent-free I and II have solvent-accessible void volumes of 17.3 and 14.9%, respectively. The thermal properties of I and II are very similar, but the gas sorption properties strongly depend on the pore dimensions and functionalities. Compound I has a higher uptake of CO₂ compared with that of II due to its larger void volume. Particularly, CO₂ sorption isotherms at 196 K indicate dramatically different patterns depending on the meso-substituents. Whereas I shows S-shape isotherms for CO₂ adsorption/desorption at 196 K, II does not. The sorption of N₂, H₂, and CH₄ by the two MOFs was also investigated.