Theoretical Studies Of The Reaction Mechanisms Of Proton And Carbon Dioxide Reduction Catalyzed By Mononuclear Transition Metal Complexes

The artificial photosynthesis system includes three components:photosensitizer excitation,electrons transfer and the redox reactions.Those reduction reactions can produce fuels or useful chemicals.A key issue is to develop highly efficient and stable catalysts for proton reduction and carbon dioxide reduction.Selecting out the most promising structure through computational before experimental synthesis is an efficient way to develop the catalyst we need.In this dissertation,the mechanism and selectivity of several mononuclear transition metal complexes electrocatalyzing proton reduction and electrocatalyzing/photocatalyzing carbon dioxide reduction reactions are investigated using density functional theory.The calculation results provide several reasonable suggestions for future catalyst design.(1)Density functional theory(DFT)calculation reveals that the proton reduction of mononuclear cobalt and iron complex catalysts containing tetradentate phosphorus ligands(MP₄N₂,M=Fe,Co)is a heterolytic mechanism.The phosphate from the buffer was found to play a crucial role in the reaction.For the more efficient cobalt catalyst,the starting species is a six-coordinated Co^? complex,via two sequential proton-coupled electron transfer reductions lead to the formation of a Co^?-H intermediate with a dihydrogen phosphate ligand.Subsequently,the H-H bond formation takes place via coupling of the Co^?-H and the proton from the dihydrogen phosphate ligand.When the phosphate ligand is displaced by a water molecule,the total barrier for the dihydrogen formation increases by 11.0 kcal mol^-1.For the iron catalyst,the overall mechanism is essentially the same;however,the first reduction is likely the rate-limiting step.The calculated results well explained the experimental results.(2)The iron-porphyrin complex with four positively-charged N,N,N-trimethyl-4-ammoniumphenyl substituents is an efficient electrocatalyst for the selective reduction of CO₂ to CO in aqueous solution.Density functional calculations have elucidated the porphyrin was a redox non-innocent ligand and can accept two electrons and one proton,while the ferrous ion keeps its oxidation state as+2 during the reduction.The Fe^?-porphyrin diradical intermediate then performs a nucleophilic attack on CO₂,coupled with two electron transfer from the porphyrin ligand to the CO₂ moiety.This is followed by an intramolecular proton transfer from the porphyrin nitrogen to the carboxylate oxygen,affording a Fe^?-COOH intermediate.Finally,a proton transfered from the carbonic acid in the aqueous solution to the hydroxyl moiety,results in the cleavage of the C-O bond and the production of a CO molecule.The formation of Fe^?-hydride species,a critical intermediate for the production of H₂ and formic acid,was found to be kinetically very unfavorable,even though it is thermodynamically more favorable.The prevention of this metal-hydride formation pathway explains why this catalyst is highly selective for the reduction of CO₂ in aqueous solution.(3)DFT calculations for the photocatalyzed reduction of CO₂ by the mononuclear cobalt complexes containing N5 ligands were performed to elucidate the catalytic pathways of the formation of CO,formate,and H₂ in this system.The results showed that the first reduction process of the catalyst requires light exciteation,but the second step not.Co^?-COOH and Co^?-H complexes are extremely important intermediates in this reaction.The protonation of Co^?-COOH can lead to the cleavage of the C-O bond and generation of CO,while Co^?-H can fix CO₂ to form formic acid.The results revealed that the oxygen atom at the cis-coordination site in the catalyst plays an important role in stabilizing the transition state during the transformation of CO₂ at the cobalt center.The calculated results explain the experimental products distribution well.(4)The density functional method was used to calculate a highly efficient ruthenium complex for the reduction of carbon dioxide.This Ru complex can act as both photosensitizer and catalyst.The calculation results showed that Ru^?-S is excited from the ground state to the excited triplet state through a metal-to-ligand charge tranfer(MLCT)process under light,then ³Ru^?-S was quenched by the electron donor BIH(1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole(BIH))and reduced to ²Ru^I-S.After that,CH₃CN molecular dissociate to form Ru^I,and then Ru^I is reduced to produce Ru⁰ BI·.Ru⁰can nucleophilically attack carbon dioxide to form Ru^?-CO₂^2-intermediate and then undergo two protonation processes to generate Ru^?-CO.Ru⁰ can also be protonated to form Ru^?-H.Ru^?-H is more favour in thermodynamical than Ru^?-CO₂,but is unfavourable in kinetical as the energy barrier is higher than the formation of Ru^?-CO₂^2-.The calculation results well explained the selectivity of this Ru complex.