Density Functional Theory Studies On Dimanganese Based Molecular Catalyst For Biomimetic Water Oxidation

Making direct use of sunlight to split water into hydrogen and oxygen through artificial photosynthetic systems is an ideal way to convert solar energy into green clean chemical energy. Potential applications of this system in daily life would solve both problems of energy crisis and the environmental pollution simultaneously. In the system of artificial photosynthetic, the water oxidation half-reaction is the source for both electrons and protons during the overall water splitting process. This water oxidation reaction has also been determined as one of the key factors that hinder the wider utilization of solar energy. Therefore, it has become necessary to explore for an efficient and stable catalysts for molecular water oxidation reaction. Among numerous water oxidation catalysts, manganese-based complexes are specifically interesting, as it is the metal chosen by nature to guide the water oxidation reaction in photosynthesis.Benefited from the rapid development of the quantum chemistry methods and the high performance computers, first-principles calculation in the framework of density functional theory (DFT) plays a more and more vital role in the field of theoretical explanation and material property prediction. In this dissertation, we have systemically investigated the structure and the electronic properties of a biomimetic dimanganese water oxidation catalyst, and its corresponding catalytic mechanism through DFT calculations. Based on this, a rational rearrangement of the dimanganese catalyst is then designed by means of ligand transformation for a more efficient water oxidation catalyst.This dissertation consists of four chapters. The first chapter presents the background knowledge and a comprehensive review of relevant literature. It starts with a brief introduction to the composition of photosystem II found in the photosynthesis system of a plant. Then it focuses on the artificial photosynthesis system which realizes the proton reduction reaction and the critical water oxidation reaction. The reaction for water oxidation is explained in great detail. Then, brief comparisons toward some of the widely used molecular water oxidation catalysts in the literatures are made. Finally, the theoretical method used in this dissertation, particularly the basics of density functional theory is explained. As the reaction of the biomimetic dimanganese complex1is carried out in the presence of water solvent, the solvation effect is also discussed.In the second chapter, the research object of this dissertation-a biomimetic dimanganese water oxidation catalyst:[H2O(terpy)MnⅢ(μ-O)2MnⅣ(terpy)OH2]3+(tpy=2,2’,6’,2’-terpyridine, denoted as complex1) is introduced. The developments in both the experimental research and the theoretical investigations are summarized. It is followed by the characterization and the study of the structural properties possesses by complex1, calculation details are also presented.In the third chapter, Theoretical investigations aimed at the elucidation of the asymmetric features in the geometric and electronic structures of complex1, as well as their influences on the chemical functions of the two manganese centers are made. With the insight gained from the first-principles calculations, the asymmetry of complex1is studied in the acetate buffer solution. Both thermodynamic and kinetic aspects are explored in detail, with both the structural and chemical asymmetries of the two manganese centers fully considered. It is concluded that the larger steric repulsion associated with the Mn(Ⅳ) center plays a decisive role, as it leads to the predominant acetate coordination at the Mn(Ⅲ) ion. This conclusion has resolved the controversy regarding the preferential for acetate binding to complex1.In the fourth chapter, based on the gained knowledge on complex1, theoretical investigations are carried out for a structural rearrangement of [H2O(2-bpNP)MnⅢ(μ-O)2MnⅣ(2-bpNP)OH2]3+(2-bpNP=2-([2,2’-bipyridin]-6-yl)-1,8-naphthyridine, termed as complex2). The complex2acts as a structural model for biomimetic manganese complex in the oxygen evolving center of photosystem II. Previous calculations by density functional theory had shown that the previously proposed manganese water splitting complex1could be redesigned rationally to include an efficient proton acceptor ligand. By the replacement of catalyst in the water oxidation reaction from complex1to complex2, the barrier corresponded to the O-O bond formation step decreased greatly, and the reaction becomes feasible at room temperature. It has also been observed that there is no significant impact on the effect of steric repulsion during the course of reaction. This study shines an important light into the design of a more efficient water splitting catalyst and provides insight onto the O2evolution mechanism in the photosystem Ⅱ.To summarize, we have studied the micro reaction mechanism of manganese-based water oxidation catalyst by using the density functional theory calculations. According to an in-depth analysis of the relationships between the structures and the properties of the catalyst molecules, the catalytic performance is effectively improved through a rational ligand design. Above results are valuable to guide the experiment of the applicable artificial photosynthesis systems.