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

Abstract

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.