Pyrazolate-Based Porphyrinic Metal–Organic Framework with Extraordinary Base-Resistance

Abstract

Guided by a top-down topological analysis, a metal-organic framework (MOF) constructed by pyrazolate-based porphyrinic ligand, namely, PCN-601, has been rationally designed and synthesized, and it exhibits excellent stability in alkali solutions. It is, to the best of our knowledge, the first identified MOF that can retain its crystallinity and porosity in saturated sodium hydroxide solution (∼ 20 mol/L) at room temperature and 100 °C. This almost pushes base-resistance of porphyrinic MOFs (even if MOFs) to the limit in aqueous media and greatly extends the range of their potential applications. In this work, we also tried to interpret the stability of PCN-601 from both thermodynamic and kinetic perspectives.

Design and Construction of Chemically Stable Metal-Organic Frameworks

“…15 The idea is exemplified in the case of PCN-601. The connectivity of both the Ni 8 cluster (a 12-connected node) and the pyrazolate-based porphyrinic ligand TPP 4− (a 4-connected linker) is very high among the reported inorganic clusters and organic ligands.…”

“…In the work of PCN-601, Wang et al rationalized that the length of the ligand is related to the activation energy of ligand dissociation and thus affects the decomposition of MOFs. 15 In Scheme 1, two isoreticular frameworks are presented, which are constructed by a long and a short ligand, respectively. Regardless of the mechanism of substitution (dissociative or associative), ligands coordinated to metal nodes are sure to be bent in transition states.…”

“…PCN-333 is an elongated version of MIL-100, in which each face of the supertetrahedron is a 4,4′,4″-s-triazine-2,4,6-triyl-tribenzoate ligand (TATB) instead of BTC in MIL-100 (Figure 3) be modeled as stepwise substitutions of bridging ligands with attacking species, such as H 2 O. 15 They often undergo a dissociative mechanism due to the saturated coordination environment around the metal ions. If the dissociative pathway is represented in a reaction coordinate diagram, the initial release of a ligand would be the rate-determining step and have a much larger activation energy than the other steps in the reaction.…”

“…Because the strength of Zr-oxyanion bond is very high and the length of BDC ligand is short, the rate of ligand dissociation (reverse reaction of crystal growth) is very low. 15 This leads to a limited rate of structural reparation in the crystal growth of UiO-66 and makes UiO-66 often come with a high ratio of deficiency. However, structural defects have little impact on the chemical stability of UiO-66.…”

“…30 (Figure 14). 15 PCN-601 is demonstrated to be extremely stable, especially in alkali aqueous solutions. It can maintain sound crystallinity and porosity even after being boiled in saturated NaOH aqueous solution (20 mol/L) at 100 °C for 24 h. The intactness of PCN-601 is also examined by UV-Vis adsorption spectrum by submerging the treated MOF into DMF for 24 h and comparing its spectrum with the standard solution of H 4 TPP in DMF.…”