Selective Inclusion of Extra-Framework Ligands in Nanoporous Vanadium-Based Frameworks Using an Optimized Synthesis Approach for Potential Applications in Catalysis and Separation

Metal–organic frameworks (MOFs) are nanoporous, crystalline materials with highly tunable structures. Here, we investigate the concept of extra-framework ligands as a strategy for accessing additional tunability in MOFs. Specifically, we used a combination of initial synthetic exploration and subsequent Latin hypercube sampling (LHS) and Bayesian optimization campaigns to synthetically incorporate or exclude extra-framework ligands in the NU-2001 system, a vanadium-based metal–organic framework constructed from the bulky, three-dimensional (3D) linker bicyclo[2.2.2]octane-1,4-dicarboxylic acid (BODCA). This synthetic approach enabled efficient screening of a design space consisting of around 2,700 points through only 55 total reactions. Analysis of the local coordination environment and oxidation state of the vanadium centers suggests that the extra-framework ligands are charge-balancing when incorporated. Comprehensive characterization of both the guest and guest-free phases using X-ray diffraction, adsorption measurements, and X-ray absorption spectroscopy reveals that incorporation of extra-framework ligands alters pore geometry, while also influencing framework stability and flexibility. The combination of thermal stability, redox-active vanadium centers, and additional tunability via selective incorporation of extra-framework ligands in the NU-2001 system warrants further investigation of these materials for important catalysis and separation applications.