Quantum Chemical Studies Of The Reaction Mechanism Of Water Oxidation Catalyzed By Mn-and Fe-based Complexes

Inspired by nature,a promising scheme to use solar energy is artificial photosynthesis(AP).Water oxidation is at the heart of this system and is both thermodynamically and kinetically very unfavorable.Even though significant progress has been made during the last few decades in catalytic water oxidation,the mechanism remains unclear due to the experiment limitations,such as the difficulty of capturing the short-lived intermediates and transition state structures.Nowadays,it is possible to model chemical reactions at the atomic level via computational chemistry.The redox potential,p K_as,and reaction pathways can be calculated.In this thesis,density functional calculations were employed to study the mechanism of water oxidation driven by homogeneous Mn-or Fe-based water oxidation complexes,which would help design more efficient and stable water oxidation catalysts.The thesis mainly includes the following aspects:(1)Density functional calculations were performed to elucidate the mechanism of water oxidation catalyzed by a mononuclear Mn complex,([LMn^?(H₂O)₂]²⁺,(L=Py₂N(t Bu)₂)),which can catalyze water oxidation electrochemically in an aqueous solution.The intramolecular oxo-oxo coupling and water nucleophilic attack(WNA)pathways were proposed to prompt the O-O bond formation.The calculations showed that the first process is one-electron oxidation coupled with the release of two protons.Then sequential two proton-coupled electron-transfer processes generate[LMn^?(O)(O)]⁺.It triggers the O-O bond formation via direct coupling of two oxo ligands or WNA.The rate-determining step is the O-O bond formation,with barriers around 17 kcal/mol.The further oxidized Mn^? intermediate for the O-O bond formation was suggested to be unfavorable.Furthermore,the C-H activation on the tertiary butyl of the ligand has a similar barrier compared with the O-O bond formation,which explains the low turnover number of this catalyst.(2)The mechanism of water oxidation catalyzed by a mononuclear iron-based complex([Cl-Fe^?-(dpa)-Cl]⁺)was investigated by density functional calculations.We found that the nitrate could function as a co-catalyst to prompt the O-O bond formation.The ligand exchange between one chloride and a water molecule generates the catalytically active species[Cl-Fe^?-(dpa)-OH₂]²⁺,the starting species of the catalytic cycle.Firstly,two oxidation processes lead to the generation of[Cl-Fe^?-(dpa)=O]²⁺,which triggers the critical O-O bond formation via nitrate nucleophilic attack(NNA)mechanism with a calculated barrier of 11.0 kcal/mol.The barrier of WNA was found to be 5.5 kcal/mol higher.Also,DFT-based molecular dynamic simulations on the explicitly solvated system showed that the barrier increases about 12 kcal/mol for NNA pathway.In contrast,the barriers for WNA are very similar for these two different approaches.Therefore,it is important to consider explicit solvent when investigating pathways involving anions.Furthermore,the C-H oxidative activation of the methyl group on the dpa ligand may be the reason for the low catalytic efficiency.(3)The heterotrinuclear complex(A₀,{[Ru ^?(trpy)]₂(?-[Mn ^?(bpp)₂])(OAc)₂}²⁺)was found to be capable of catalyzing water oxidation both electrochemically and photochemically.DFT calculations were used to study the mechanism of this catalytic reaction.Two heteronuclear metal centers(Mn and Ru)was proposed to facilitate the O-O bond formation.Firstly,two acetates were displaced by four water molecules to generate the catalytically active species A.In A,every Ru^?center has one water ligand,and two water molecules coordinate with the Mn^? center.The calculations showed that a series of oxidation and deprotonation events take place from A,leading to the formation of the complex RM-1(Ru^?Mn^?Ru^?),which is the starting point of the catalytic cycle.Then,three oxidation processes from RM-1 lead to the generation of the catalytically competing species RM-4([Ru^?-O?]₂(?-[Mn ^?-O?])),which triggers the O-O bond formation with a calculated barrier of 7.2 kcal/mol.The direct coupling of the two adjacent oxo ligands bound to the Ru^? and Mn^? leads to the production of a superoxide intermediate.Subsequent O₂release turns out to be quite facile.Other possible pathways were found to be less favorable,including water nucleophilic attack,the coupling of an oxo and a hydroxide.