From MOF-74-Zn to Triazolate-Directed Nonsymmetric Assembly of Chiral Zn₆ @Zn₆ Clusters

The creation of new cluster building blocks, as well as new ligand coordination modes, are among the most effective ways to develop new framework materials. Yet, large and chiral clusters are both difficult to create and relatively few. Here, by studying the competing coordination of different azolates against carboxylate and combined carboxylate/phenolate, it is shown that the impact of azolates in the MOF-74 synthesis system differs dramatically, leading to the synthesis of MOF-74, UTSA-74, and CPM-72 for 2-methylimidazole, 1,2,4-triazole, and 1,2,3-triazole, respectively. The new CPM-72 contains a novel chiral Zn₁₂ triazolate cluster, which features a trigonal-prismatic Zn₆ core inside an octahedral Zn₆ shell. In contrast with MOF-74 with fully deprotonated and symmetrically bonded 2,5-dihydroxyterephthalic acid (H₄ DOBDC), H₄ DOBDC adopts an unusual nonsymmetric bonding mode in CPM-72 (carboxylate only at one end and carboxylate/phenolate at the other), resulting in a highly porous and intrinsically chiral 3D framework. The nonsymmetric bonding mode by H₄ DOBDC, apparently dictated by the chiral Zn₁₂ cluster, can be replicated with 2-hydroxyterephthalic acid (H₃ OBDC), leading to the synthesis of porous isoreticular CPM-73.