Engineering hierarchical porosity in MOFs for host-guest chemistry with large organometallic complexes

The microporous nature of metal-organic frameworks (MOFs) often limits their capacity to incorporate large molecular guests, such as organometallic catalysts. In this work, we demonstrate a defect-engineering strategy for the Zr-based MOF UiO-66 to generate hierarchical pore structures capable of hosting the bulky Lehn-type complex [Re(bpy-4-COOH)(CO)₃Cl]. By introducing missing-linker and missing-cluster defects-both during synthesis and through a selective ligand removal (SeLiRe) process-we modulate the framework's pore structure and volume. Using a post-synthetic modification approach, 2,2'-bipyridine-4-carboxylic acid (bpy-4-COOH) is anchored into the MOF structure via solvent-assisted ligand incorporation, followed by complexation with [Re(CO)₅Cl]. A comprehensive suite of characterization techniques including TGA, Ar-physisorption, STEM-EDX, solid-state NMR, XAS and other spectroscopic methods confirmed the formation and uniform distribution of the Re-complex within the MOF porosity. Our results show that the introduced defects and the associated creation of mesoporosity are essential for successful incorporation of the large Re-complex, while nearly defect-free UiO-66 cannot be modified with the ligand post-synthetically. The use of the SeLiRe process enables us to gain reasonable control over the amount of the Re-complex inside the MOF and leads to a homogeneous distribution throughout the particles. Photocatalytic CO₂RR experiments show CO as the main product with high selectivity when using TEOA as a sacrificial agent. This work demonstrates the potential of engineering hierarchical porosity in MOFs for immobilizing large, catalytically active molecular species in a stable and well-defined environment.