Co₇-Cluster-Based Metal–Organic Frameworks with Mixed Carboxylate and Pyrazolate Ligands: Construction and CO₂ Adsorption and Fixation

Abstract

Cluster-based metal–organic frameworks (MOFs) supported by mixed ligands usually have unique properties due to the combined features from different ligands. In this regard, it is rather rare for carboxylate and pyrazolate ligands as "struts" to be present in one MOF. Herein, four isostructural heptanuclear Co(II)-based MOFs based on mixed carboxylate and pyrazolate ligands, [Co7(μ3-OH)4(BDP)2(IPA-X)5] (BUT-127-X; X = H, OH, Br, and NH2, respectively; BDP2– = 1,4-benzenedipyrozolate and IPA2– = isophthalate), were synthesized through carefully tuning the solvothermal reaction conditions. Structurally, in these MOFs, alternately connecting Co7(μ3-OH)4 clusters with carboxylate ligands (IPA2–-X) generate two-dimensional (2D) layers, which are further supported by the pyrazolate ligands (BDP2–), giving rise to the final pillared-layer frameworks with well adjustable pore surface environments. They have permanent porosity with high Brunauer–Emmett–Teller surfaces from 1030 to 1314 m2 g–1, as well as good chemical stability. Interestingly, four Co(II) atoms in the Co7(μ3-OH)4 cluster exhibit a five-coordinated configuration that affords open metal sites. The feature allows these MOFs to present good CO2 adsorption performance and catalytic activity for the CO2 fixation, where BUT-127-NH2 gave the best catalytic performance for the cycloaddition of CO2 with epoxides into cyclic carbonates under mild conditions (1 bar, 25 and 50 °C).