Development of Molecular Catalysts for CO2 Reduction and Nanomaterial Catalysts for Oxidation Reactions

The thesis presented here is focused on two aspects of transition metal mediated catalysis research- one is designing homogeneous rhenium complex with suitable ligand framework for electrochemical reduction of carbon dioxide to address dual issues regarding green house gas removal and hydrocarbon fuel production; another being fabrication of heterogeneous mesoporous manganese oxide to catalyze organic fine chemical synthesis under aerobic atmospheric condition. The quest for conquering fossil fuel energy dependence leads us to develop methodologies for using CO2 as a renewable resource. Electrochemical reduction of CO2 has been considered as a promising procedure for this purpose. Coordination complexes of rhenium and α-diimine ligands are known to resolve the bottleneck of activating the thermally stable and kinetically inert CO2 molecule. In the beginning of the thesis, I have described the development of a new family of α-diimine ligand coordinated rhenium complexes that exhibits remarkable catalytic activity compared to the existing list of analogous systems. The reported compounds in this work consist of three different ligand systems- quinoline, naphthyridine and benzonaphthyridine bound to pyridine or thiazole moieties in the coordination sphere. It has been shown that rate of CO2 reduction and turn over frequency depends significantly on the nature of the ligands. Overall, complexes that have pyridine outperform those having thiazole, with the benzonaphthyridine complexes showing superior activity with rate constant and catalytic turnover of 103 orders. The second part of my thesis describes the design and application of thermally stable and tunable mesoporous manganese oxide to catalyze coupling and aromatization reactions. From the viewpoint of green chemistry, synthesizing valuable organic molecules by using no or minimum additive and maintaining mild reaction condition is of utter importance. We demonstrate synthesis of aromatic nitrogen containing heterocyclic molecules from their non aromatic counterparts by employing robust and inexpensive manganese oxide catalyst in the presence of no other oxidant/additive but air. The same manganese oxide material when fabricated to act as a support for copper oxide catalyst exhibited excellent efficiency for C-O, C-N and C-S bond formation in Ullmann type reaction. Not only the synthetic methodology but the underlying mechanism and role of lattice oxygen present in the structure of catalyst are explained in details following experimental and theoretical studies. The catalytic protocols discussed here have several advantages over the already reported procedures in terms of product separation, reusability of the catalyst, absence of additive, water as by product and air as the terminal oxidant