Nanocage containing metal-organic framework constructed from a newly designed low symmetry tetra-pyrazole ligand

Abstract

1368-Tetra(1H-pyrazol-4-yl)-9H-carbazole (H4CTP), a tetra-pyrazole ligand with Cs symmetry, has been synthesized based on a carbazole core. A solvothermal reaction of this ligand with NiCl2·6H2O gave a three-dimensional (3-D) metal-organic framework (MOF), [Ni(H4CTP)Cl2]·nS (BUT-41), which crystallized in the cubic space group Pm-3 in spite of H4CPT with a central carbazole core and four peripheral pyrazole rings has low symmetry. The framework of BUT-41 can be regarded as a four-connected 3-D net with the rhr topology when both the organic ligand and the metal center are considered as four-connected nodes. Nanocages with internal diameter of 2 nm are present in the framework of BUT-41, which are formed by interconnecting 12 H4CTP ligands and 20 Ni(II) ions. Each nanocage connects with six adjacent cages through sharing hexagonal windows with diameter over 7 Å, resulting in 3-D intersecting channels of the MOF. Although the tetra-pyrazole ligand is not deprotonated after coordination with the metal ions, powder X-ray diffraction and N2 adsorption experiments reveal that the framework of BUT-41 is rigid and permanently porous with the Brunauer-Emmett-Teller surface area up to 1551 m2 g−1. Furthermore, gas adsorption experiments show that this MOF selectively adsorbs CO2 over N2 and CH4.