Theoretical Researches On The Formation And Catalytic Machanisms Of Lindqvist [W₆O₁₉]^2- And Keggin [PW₁₂O₄₀]^3- Polyoxometalates

Polyoxometalate （POM） chemistry has been in existence for more than a centuryand still attracts much attention due to their potential applications in catalysis,medicine, magnetism, optics, conductivity, and so forth. The formation mechanism isvery challenging for POM chemistry. Because it is difficult to reveal the formationmechanism from experimental work alone, theoretical study will help us tounderstand the structural changes and the interaction between building blocks in thePOM polymerization processes from isolated anions to nanoclusters. Therefore, thedetermination of the formation mechanism is not only able to explain theexperimental facts at the atomic and molecular scale, but also to provide usefulguidance on the design and synthesis of POMs. It would lead to more efficientsynthesis of novel POM-based functional materials and improve the properties ofexisting materials.On the other hand, polyoxometalate catalysis is one of the most active area of itsapplications so far, consisting of acid catalysis, redox catalysis, or both kinds.Compared with the synthesis of novel polyoxometalate catalysts, however, it isindispensable to understand their catalytic mechanisms. Thus, quantum chemicalmethods show some progressiveness in predicting the reactivity and provide greathelp in revealing the catalytic mechanism.In this thesis, DFT calculations have been performed to investigate the formationmechanism of Lindqvist [W₆O₁₉]^2-and α-Keggin-type [PW₁₂O₄₀]^3-, which willprovide a better understanding of the formation process of POMs and offer sometheoretical guidances for the synthesis of new structures. Then, DFT approaches wereapplied to explore the mechanism of water splitting catalyzed bysingle-Ru-substituted polyoxometalates and discuss the effect of the hetero atoms onthe catalytic activity. The present work has been focused on the following threeaspects:（1） The formation mechanism is always a fundamental and confused issue forpolyoxometalate chemistry. Two formation mechanisms （M1and M2） of theLindqvist anion [W₆O₁₉]^2-have been adopted to investigate it’s self-assembly reaction pathways at density functional theory （DFT） level. Potential energy surfaces revealthat both the two mechanisms are thermodynamically favorable and overall barrierlessat room temperature, but M2is slightly dominant than M1. The formation ofpentanuclear species [W₅O₁₆]^2-and [W₅O₁₅（OH）]^-is recognized as therate-determining steps in the whole assembly polymerization processes. These twosteps involve the highest energy barriers with30.48kcal/mol and28.90kcal/mol,respectively for M1and M2.[W₄O₁₃]^2-and [W₄O₁₂（OH）]^-are proved to be the moststable building blocks. In addition, DFT results reveal that formation of [W₃O₁₀]^2-experiences a lower barriers along the chain channel.（2） Based on the previous work, the structural characterization and thethermodynamic behavior of various possible intermediates in the formation ofα-Keggin-type [PW₁₂O₄₀]^3-anion in aqueous solution were analyzed by densityfunctional method. Based on two proposed mechanisms （order of reaction withheteroatom）, thermodynamic analysis indicates that [HPO₄]^2-,[WO₃（OH）]^-,[W2O7]^2-,[W₃O₁₀]^2-,[W₄O₁₃]^2-,[W₅O₁₆]^2-, and [PW₂O₉]^-will be mainly involved in thepolymerization processes of [PW₁₂O₄₀]^3-anion. The starting reaction from isodimerhas been determined through transition state search for the initial steps. Heteroatomhas been introduced in the heterotrimer [PW₂O₉]^-at the second step whichexperiences almost the same barrier with isotrimer [W₃O₁₀]^2-. From geometrictopology and thermodynamics, the defect structures are more suitable for buildingblocks.（3） Water oxidation is a key half reaction in energy conversion scheme. Thereaction mechanism for the oxidation of H2O to O₂catalyzed by single-Ru-substitutedpolyoxometalates [Ru^III（H₂O）XW₁₁O₃₉]^5–（X=Si, Ge, P） was investigated by themeans of DFT method. The electronic structure results of the pre-activationintermediates indicate that the aqua ligand is prone to accommodate proton coupledelectron transfer （PCET） process to achieve the active group [Ru^V＝O], and the highvalent oxo-ruthenium （V） species are responsible for the O-O forming event. Threepossible proton acceptors are designed for the rate-determining step （Ob, Oa, and H2O）,the calculation results support that the bridge Obatom of the polytungstate ligand willact as the most plausible proton acceptor in the formation of O-O bond, with anenergy barrier of28.43kcal/mol. A detailed information of the peroxidicintermediates in the oxidation process have been characterized by DFT calculations,both the peroxo-species [Ru^IV（OO）SiW₁₁O₃₉]^6–and [Ru^V（OO）SiW₁₁O₃₉]^5–show thesix-coordinate isomer with an open, terminal geometry is more favorable than the close seven-coordinate ones. In addition, the nature of the hetero atoms with XO₄^n–(X=Si^IV, Ge^IV, P^V) was crucial in controlling the catalytic activity, in the order of X=Ge>Si> P.