Synthesis of Metal-Organic Frameworks with Various N-Heterocyclic Carbene Transition Metal Complexes and Their Applications to Catalysis

Abstract

Metal−organic frameworks (MOFs) are crystalline hybrid materials consisting of metal ions and organic linkers. Among the organic linkers, N-heterocyclic carbenes (NHCs) are thought to be a good candidate for the construction of functional MOFs. Because NHCs exhibit strong σ-electron donating property, NHCs can easily stabilize the various metal complexes, and facilitates their use in organometallic chemistry. Due to their unique properties, immobilization of NHC ligands having metal complexes into MOFs as organic linkers may lead to the preparation of MOFs exhibiting distinctive catalytic activity in organic reactions. By utilization of easy tunability of MOFs and the unique properties of metal complexes embedded MOFs, MOFs having NHC metal complexes can be prepared and studied in the organic reactions for catalytic applications. In chapter 2, before the preparation of NHC metal complex embedded MOFs, imidazolium which was known as NHC precursor, was reacted with zinc nitrate for the construction of imidazolium based MOF. Depending on the types of counteranions of the imidazolium linkers, two distinct MOFs were synthesized. The presence of the bicarbonate as a counteranion of the imidazolium, led to the generation of 2D sheet structure and inhibited further three dimensional growth. However, by the adoption of chloride anion of the imidazolium ligands, 3D MOF was formed and evolved 4-fold interpenetrated structure. The 3D MOF exhibited high porosity in spite of folded structure with BET surface area of 1400 m2/g. Interestingly, unlike the usual trend of lower stability of MOFs having zinc cluster towards water, the 3D MOF exhibited unique tolerance toward organic solvent and even water for 6 months without the collapse of the framework due to the presence of multiple π- π interactions. Moreover, due to the presence of imidazolium groups in 3D MOF as organic linkers, this MOF showed cationic function with strong adsorption toward anionic guests in aqueous medium. In chapter 3, in order to prepare NHC having meal complex embedded MOFs, the organic linker from imidazolium based MOF we previously reported was utilized and incorporated the metal complexes to the imidazolium ligand and reacted with zinc nitrate to produce MOFs bearing the NHC metal complexes (CuCl, CuBr, CuI and AuCl). Interestingly, these MOFs have metal complexes coordinated by NHC ligands inside the pores of MOFs and 4-fold interpenetrated cube structure. Moreover, these MOFs exhibit distinctive catalytic activity in various organic reactions, such as copper catalyzed click reaction, multi-component reaction and gold catalyzed hydroamination reaction and can be reused 4 times without decomposition of the framework and significant decrease of the conversion rate in the organic reactions. Because of the incorporation of the various metal complexes into MOFs, they can be utilized as a universal template for the preparation of functional MOFs for practical applications.