Theoretical Studies On Dinitrogen Activation And Functionalization By Binuclear/Multinuclear Metal Hydride Complexes

The activation and transformation of dinitrogen under mild conditions has attracted considerable attention,but the research is quite challenging due to the inertness of dinitrogen.Multinuclear transition-metal hydride complexes could active dinitrogen without extra reductants,which could satisfy the requirement of sustainable development.Therefore,deep understanding of related reaction mechanism is of great importance.Nevertheless,it is difficult to conduct mechanistic studies at the molecular and electronic levels though current experimental approaches.In this thesis,a series of dinitrogen activation and functionalization by binuclear or multinuclear transition-metal hydrides have been computationally studied based on density functional theory,and the main results are summarized as follows:（1）Dinitrogen activation by trichromium tetrahydride complex has been computationally studied.The reaction mechanism involves sequential dinitrogen coordination,dihydrogen elimination,N-N bond cleavage,and N-H formation.The hydride rearrangement prior to H₂elimination is the rate-limiting step,while N-H formation is relatively facile.The oxidation states of three Cr centers are changing among Cr（II）,Cr（III）,and Cr（IV）during the reaction process.This indicates that the flexible oxidation state of Cr center played an important role in the cleavage and hydrogenation of N₂.（2）A two-state reactivity scenario has been computationally disclosed for dinitrogen activation by a diniobium tetrahydride.This reaction is initiated with N₂ coordination and H₂elimination via a triplet pathway,followed by a singlet route going through N-H formation,N-N cleavage,and the second H₂ release.These results suggest that the oxidation state and spin state of metal complexes could account for its ability to coordinate to N₂.Remarkably,it is revealed that alkali metal cations play a crucial role which is capable of decreasing the activation barriers of N-H formation and subsequent H₂ elimination by electrostatic effect and steric effect,respectively.（3）The mechanism for dinitrogen cleavage by a dititanium polyhydride has been computationally investigated.Remarkably,the H₂ elimination prior to N-N cleavage is the rate-limiting step of N₂ activation,which is accomplished by coupling of two terminal hydrides,and planar PNP-pincer ligand could stabilize the corresponding transition state.Besides,tetrahydrofuran（THF）solvent could also promote the H₂ elimination due to the similar polarity of the corresponding intermediates or transition states to THF molecule.（4）The mechanism of dinitrogen cleavage with CO₂ by dititanium hydride complex has been computationally investigated.The initial step is[2+2]cycloaddition between CO₂ and Ti₂complex,since more electron could transfer from the highest occupied orbital of metal complex to the antibonding orbital of CO₂,enhancing the interaction between metal complex and CO₂.In addition,electrophilic addition to Ti-N₂ complex of CO₂ could promote N-N cleavage.Although different product yielded under different experimental conditions,all the pathways for transformation of CO₂ with N₂ involves sequential[2+2]cycloaddition,H₂ elimination,N-N bond cleavage,and double decomposition between CO₂ and Ti=N bond.