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) was anchored into the MOF structure via solventassisted 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 defectsand the associated creation of mesoporosity-is essential for successful incorporation of the large Re-complex, while nearly defect-free UiO-66 cannot be modified with the ligand postsynthetically. The use of the SeLiRe process enabled us to gain reasonable control over the amount of the Re-complex inside the MOF and led to a homogeneous distribution throughout the particles. Preliminary, photocatalytic CO 2 RR 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.