Implementation of the Hard‐Soft Acid‐Base Principle for the Direct Synthesis of Bimetallic Zirconium‐Nickel and Hafnium‐Nickel Metal‐Organic Frameworks with a Polar Pore Environment

An important direction in MOFs is the development of materials with well-defined and ordered heterogeneity toward novel functionalities and control of important properties at the atomic scale. A great approach is the designed synthesis of mixed-metal MOFs (M'MOFs) made of different building units based on distinct metal cations. However, their in-situ formation and co-assembly into an extended crystalline framework present great challenges due to the different kinetic and thermodynamic parameters. We report the successful synthesis of two isostructural, bimetallic Zr-Ni and Hf-Ni M'MOFs following the hard and soft acid and base (HSAB) principle. Single-crystal X-ray diffraction analysis revealed the formation of a 3D structure that features an in-situ formed [Ni(HPyC)₄Cl₂]^4- 4-c nonplanar metallo-ligand that bridges 8-c Zr(Hf)₆-clusters. The resulting (4, 8)-c scu-type framework shows polygonal 1D channels, with a largest pore diameter of approximately 9 Å, decorated by Cl^- anions. This polar pore environment was found fully accessible, and both materials were investigated for Xe/Kr and CO₂/CH₄ separation. The results are very promising, showing that the IAST selectivity calculated for a 20:80 Xe/Kr molar mixture is 8.2 and 7.1 at 298 K for the Zr_Ni-scu-MOF-1 and Hf_Ni-scu-MOF-1, respectively, which is comparable to some of the best-performing materials.