Tuning carbon dioxide adsorption affinity of zinc(II) MOFs by mixing bis(pyrazolate) ligands with N-containing tags

The four zinc(II) mixed-ligand metal–organic frameworks (MIXMOFs) Zn(BPZ)x(BPZNO2)1–x, Zn(BPZ)x(BPZNH2)1–x, Zn(BPZNO2)x(BPZNH2)1–x, and Zn(BPZ)x(BPZNO2)y(BPZNH2)1–x−y (H2BPZ = 4,4′-bipyrazole; H2BPZNO2 = 3-nitro-4,4′-bipyrazole; H2BPZNH2 = 3-amino-4,4′-bipyrazole) were prepared through solvothermal routes and fully investigated in the solid state. Isoreticular to the end members Zn(BPZ) and Zn(BPZX) (X = NO2, NH2), they are the first examples ever reported of (pyr)azolate MIXMOFs. Their crystal structure is characterized by a three-dimensional open framework with one-dimensional square or rhombic channels decorated by the functional groups. Accurate information about ligand stoichiometric ratio was determined (for the first time on MIXMOFs) through integration of selected ligands skeleton resonances from 13C cross polarized magic angle spinning solid-state NMR spectra collected on the as-synthesized materials. Like other poly(pyrazolate) MOFs, the four MIXMOFs are thermally stable, with decomposition temp...