Density Functional Theory Studies On Iron-based Complexes For The Mechanism Of Water Oxidation Reaction

Directly using sunlight to split water into hydrogen and oxygen through an artificial photosynthesis system is an ideal way to convert solar energy into green and clean chemical energy.The potential application of this system in daily life is anticipated to solve the energy crisis and environmental pollution problems at the same time.In the artificial photosynthesis system,the half-reaction of water oxidation is the source of electrons and protons throughout the water splitting process.This water oxidation reaction has also been identified as one of the key factors hindering the wider use of solar energy.Therefore,it is necessary to explore an efficient and stable molecular water oxidation catalyst.Among the many water oxidation catalysts,the Fe-based water oxidation catalyst has received extensive attentions because it is the most abundant metal element on the earth,and it has many advantages such as abundance,cheapness,non-toxicity and harmlessness.Thanks to the rapid development of quantum chemistry methods and highperformance computers,first-principle calculations under the framework of density functional theory(DFT)are playing an increasingly important role in the interpretation of reaction mechanisms,prediction of molecular properties,and design of highly active catalysts.In this dissertation,we have systematically studied the structure and electronic properties of two dinuclear Fe-based water oxidation catalysts and their corresponding catalytic water oxidation mechanisms through DFT calculations.This dissertation consists of five chapters:The first chapter introduces the background and the review of relevant literatures.Inspired by the natural photosynthesis system,the artificial photosynthesis system is getting more and more attentions.It starts with a brief introduction to the related knowledge of the photosystem ? and the OEC in the photosynthesis system of nature.Then it focuses on the water oxidation reaction of the artificial photosynthesis system,together with detailed introductions to some molecular water oxidation catalysts that have caused popularities in the literatures,especially the Fe-based catalysts.At the end of this chapter,the development of the DFT method used in this dissertation is briefly described.In the second chapter,we study the mechanism of dinuclear iron complex[(H2O)-Fe?-(ppq)-O-(ppq)-Fe?Cl]3+(Fe?(ppq),ppq=2-(pyrid-2'-yl)-8-(1",10"phenanthrolin-2"-yl)-quinoline)and mononuclear iron complex[Cl-Fe?(dpa)Cl]+(Fe?(dpa),dpa=N,N-di(1,10-phenanthrolin-2-yl)-N-isopentylamine)catalyzing the oxygen release reaction(OER).A decomposition-and-reaction mechanism is proposed for the OER with the dinuclear Fe?(ppq)complex as the initial state of the catalytic agent.In this mechanism,the high-valent dinuclear iron complex first dissociates into two mononuclear moieties,and the oxidized mononuclear iron complexes directly catalyze the formation of an O-O bond through a nitrate attack pathway with nitrate functioning as a cocatalyst.DFT calculations reveal that it is the electron-deficient microenvironment around the iron center that gives rise to the remarkable catalytic activity observed experimentally.Therefore,the outstanding performance of the Fe?(ppq)catalyst can be ascribed to the high reactivity of its mononuclear moieties in a high oxidation state,which is concomitant with the structural stability of the lowvalent dinuclear complex.The theoretical insights provided by this study could be useful for the optimization and design of novel ironbased water oxidation catalysts.In the third chapter,the mechanism of the water oxidation catalyzed by the dinuclear Fe-based complex[Fe2(?-O)(OH2)2(TPA)2]4+ is systematically studied.It is proposed that there are two possible ways for the Fe?=O moieties in the oxidized complex to change from the anti to syn position.One is that Fe?=O moieties rotate around the Fe-O-Fe aixs,and the other is that one of the Fe?=O moieties rotates around the Fe atom in the direction perpendicular to the O-Fe-O-Fe plane.These two ways of rotation will form into two different complexes.The O-O bond of these two complexes is formed by the combination of two Fe?=O moieties(I2M).Therefore,the high catalytic activity of the complex[Fe2(?-O)(OH2)2(TPA)2]4+ can be attributed to its relatively flexible ligand structure,making the Fe?=O moieties in the oxidized complex easily converted to a syn position.The theoretical insights provided by this research provide an important reference for studying the catalytic mechanism of the complex with the trans ligand,and also provide a new strategy for the design of novel metal-centered catalysts.In the fourth chapter,we study the water oxidation mechanism in photosystem?.Two mechanisms for the formation of O-O bond are studied:one is the water nucleophilic attack mechanism,and the other is the oxyl-oxo mechanism.Density functional theory calculations reveal that the O-O bond for the water oxidation reaction in photosystem ? is formed through the oxyl-oxo mechanism rather than the water nucleophilic attack mechanism.In the fifth chapter,we summarize this dissertation,and also look forward to the further developments of molecular catalysts.Some brief introductions are also given to the works we will carry out in the future.