Development of metal-organic frameworks based on N-heterocyclic ligands

Metal-organic frameworks (MOFs) have firmly established in the field of porous materials since their early discovery in the 1990s. With a plethora of applications, these materials offer endless possibilities owing to their tunable, modular nature. The vast amount of synthesized MOFs indicates an utmost interest in this topic among researchers in academia and industry. However, bulk scale uses of MOFs for gas storage, catalysis, water purification and complex separations require such MOFs to have a definite, large pore size in the nanometer range. As MOFs are modular, the production of such large pore materials requires a separate synthesis of particular building blocks called ligands. Most pathways to obtain large ligands rely on the use of palladium-catalyzed coupling chemistry, thus incurring enormous costs and low scalability of the corresponding MOFs. In this thesis, we explore metal-organic frameworks based on N-heterocyclic ligands. We show how the use of these heterocycles has allowed us to easily construct ligands and new MOFs with them, and we showcase various applications of these materials. In Chapter 1, we briefly overview the existing state of the art in ligand design for metal-organic frameworks and define thesis aims. In Chapter 2, we show how a new ligand forms a Cu-Sp5 MOF, which has one of the highest CO2/N2 adsorption selectivities in flue gas conditions reported to date. We investigate the origins of such remarkable performance via multiple characterization techniques. Chapter 3 reports a new MOF Cu-Sp5-BF4. We for the first time access N-heterocyclic carbene (NHC) adduct chemistry in a MOF via ligand post-synthetic modification. We then reveal how this process alters the dimensionality of the material and how it improves the accessibility of NHC sites for a second-step iridium grafting. We finally shed light on iridium speciation in the final material using a combination of EXAFS and XPS methods, and how it changes upon a catalytic reaction of stilbene hydrogenation. Chapter 4 illustrates a new approach for ligand synthesis for large pore MOFs. We show the development of a new concept in ligand design that we call LigX. Using the LigX approach, we simplify large pore MOF ligand synthesis by avoiding time- and cost-intensive Pd-based chemistry. We show the synthesis of structurally diverse MOFs using this methodology.