Density Functional Theory Studies On Iron Based Molecular Catalyst For Water Oxidation

Inspired by natural photosynthesis,developing the artificial simulation of photosynthesis with using the solar energy for hydrogen production,is generally recognized as one of the most feasible methods.Artificial photosynthesis is a multi,step cooperative system,in which water oxidation is also a multi-electron,proton transfer process,thus becoming the key step of solar energy conversion.Therefore,a highly efficient and stable water oxidation catalyst is the best choice for solving the water oxidation reaction.At present,iron-based catalysts are increasingly favored by scientists due to advantages of high mineral content,environmental friendliness and cheap.Because of the limitations of experimental detection methods,the complex inter*mediates involved in the process of water oxidation are difficult to be characterized one by one.With the development of quantum chemistry methods and the gradual improvement of computer computing power,Density Functional Theory?DFT?has become extremely mature for computing the molecular dynamics,thermodynamics,reaction mechanism and intermediate prediction.Based on the experimental observations and density functional theory,the three-dimensional space,electronic structure and reaction mechanism of iron-based catalysts have been investigated systematically and comprehensively in this paper.Accordingly,we understand the role of electron-withdrawing groups in increasing the catalytic efficiency of molecular catalysts.At the same time,the cycle process catalyzed by the dual-core catalyst is clearly understood.Combining these explorations provides some ideas for humans to design more efficient and stable catalysts.This dissertation consists of four chapters.The first chapter presents the background knowledge and a comprehensive review of relevant literature.Taking the photosystem ? and oxygen evolving complex?OEC?of plant photosynthesis as the starting point,the core components of the artificial photosynthesis system were described in detailed,including photosensitizer,proton reduction catalyst and water oxidation reaction.Subsequently,the water oxidation reaction was introduced,and the water oxidation catalysts of different systems were summarized and the reaction mechanism of the molecular water oxidation catalyst was analyzed.Next,the currently popular molecular water oxidation catalysts are introduced.At the end of this chapter,the development of density functional theory and Gaussian software is described briefly.In the second chapter,we studed the complex,[Fe?OTf?₂?Pytacn?]?OTf=CF3SO3-,Pytacn = 1-?2'-pyridylmethyl?-4,7-dimethyl-1,4,7-triazacyclononane?,and several derivatives,such as[Fe?OTf?2?E,HPytacn?]?E=-Cl,-CO2Et and-NO2?and[Fe?OTf?2?E RPytacn?]?R=-F and R=-Me?,as well as electron-withdrawing group effect on catalytic activity are made.In this work,we propose an oxygen radical mechanism based on density functional theory?DFT?calculations for the six complexes.The crucial O-O bond-formation step is elucidated.Furthermore,we propose a simple charge-pair interaction model to characterize the effect of electron-withdrawing groups on the catalytic efficiency.It is clearly demonstrated that an electron-withdrawing group with a higher electronegativity is associated with a lower Gibbs free energy barrier for the O-O bond formation,which then leads to a more active catalyst.The theoretical insights provided in this work could be useful for the design of highly efficient Fe-based water oxidation catalysts.In the third chapter,we studied the complex,[?TPA?₂Fe₂??-O?Cl₂]²⁺,and explained the reaction mechanism of dual-core iron-based catalysts for water oxidation by oxygen-oxygen bond formation pathway.At the same time,our theoretical calculations fit well with the observed phenomena in the experiment.Meanwhile,our theoretical calculations are in good agreement with the observed phenomena in the experiment,firstly,the structure of the double-bridge??-O???-OAc?diiron center was an active intermediate in the process of releasing oxygen;secondly,UV-vis spectra showed that high-valent iron oxo?Fe?=O?intermediate could promote the O-O bond formation.Finally,the 18O-labeling experiments showed that the O atoms in O2 were derived from both water and oxone.In the fourth chapter,we look forward to the future of molecular water oxidation catalysts,indicating that developing the efficient,stable,long-lasting and environmentally friendly catalysts will have a long way to go,and it is urgent to cooperate with more experimental and theoretical researchers.In summary,starting from the microscopic reaction mechanism of mononuclear iron-based water oxidation catalyst,combined with density functional theory,the positive effects of electron-withdrawing groups on the properties of the catalyst are elaborated.Through the theoretical analysis of the mechanism of the dual-core iron-based catalyst,the catalytic cycle of the dual-core iron-based catalyst is clearly understood.Whether it is a single-core or dual-core catalyst,the theoretical understanding will be fed back to the research and development of the experiment,and the catalyst for artificial photosynthesis will move forward.