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

<p>1368-Tetra(1<i>H</i>-pyrazol-4-yl)-9<i>H</i>-carbazole (H<sub>4</sub>CTP), a tetra-pyrazole ligand with <i>C</i><sub>s</sub> symmetry, has been synthesized based on a carbazole core. A solvothermal reaction of this ligand with NiCl<sub>2</sub>·6H<sub>2</sub>O gave a three-dimensional (3-D) metal-organic framework (MOF), [Ni(H<sub>4</sub>CTP)Cl<sub>2</sub>]·nS (<b>BUT-41</b>), which crystallized in the cubic space group <i>Pm</i>-3 in spite of H<sub>4</sub>CPT with a central carbazole core and four peripheral pyrazole rings has low symmetry. The framework of <b>BUT-41</b> can be regarded as a four-connected 3-D net with the <b><i>rhr</i></b> 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 <b>BUT-41</b>, which are formed by interconnecting 12 H<sub>4</sub>CTP 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 N<sub>2</sub> adsorption experiments reveal that the framework of <b>BUT-41</b> is rigid and permanently porous with the Brunauer-Emmett-Teller surface area up to 1551 m<sup>2</sup> g<sup>−1</sup>. Furthermore, gas adsorption experiments show that this MOF selectively adsorbs CO<sub>2</sub> over N<sub>2</sub> and CH<sub>4</sub>.</p>