Theoretical Studies On Mechanisms Of N₂ Activation And Alkene Hydrogenation Catalyzed By Transition-Metal Complexes

N2 is abundant in the atmosphere and N2 fixation is a very important process on earth. Compared with the original energy-intensive Haber-Bosch process, which converts atmospheric nitrogen into ammonia under high temperatures and pressures, the functionalization of dinitrogen activated by soluble transition-metal (TM) complexes is an energy-efficient and atom economic process under mild conditions. In addition, the catalytic hydrogenation of unsaturated compounds is the trend of cost savings and catalytic effciency. Based on the goals above, this paper is trying to perform a systemic theoretical study on the activation and functionalization of dinitrogen activated by binuclear transition-metal complexes (BTMs) and alkene hydrogenation catalyzed by base metal complexes using theoretical method with integrating experiment. The following contents are included:(a) A DFT study on the carboxylation of hafnocene and ansa-zirconocene dinitrogen complexes with CO2 indicates that the most favorable initial CO2 insertion into M-N (M=Hf/Zr) adopts a stepwise way rather than a concerted [2+2] way. The calculated results provides the regioselectivity of the N-C formation in experiments, which explains the difference in the major products formation for hafnocene and ansa-zirconocene system. And the reaction mechanisms of CO assisted N2 cleavage and functionalization activated by dinuclear hafnium complex are studied using DFT method. Several key intermediates (Ia, Ib, Ic and Id) with axial/equatorial N=C=O coordination structures are found to be of importance along reaction pathways of CO assisted N2 functionalization, which could provide a profound theoretical insight in the C-N bond formation and N-N bond cleavage, as well as the originality of different isomers in a mechanistic aspect.In addition, a comparative analysis in ring tension and charge distribution unveils the different activity of N-N bond cleavage in the CO and CO2 involved direct N-C bond formation reactions,(b) The mechanism of the dinitrogen hydrogenation triggered by bimetallic complexes, [L2M]2(μ-η2:η2-N2) (M=Ta and Zr, L=Sita-type or Chirik-type ligand), is investigated by density functional theory. The calculated results show that side-on ditantalum dinitrogen complex owning Sita-type ligands favors the pathway of homolytic dihydrogen splitting while hydrogenation products are generated. However, the dihydrogen splitting switches to the heterolytic pathway as the dominant mechanism when Zr is the metal center. Ditantalum dinitrogen complex is much easier to undergo hydrogenation from the side-on coordination mode than the side-on-end-on mode with Sita-type or Chirik-type ligands. With these findings from the computational study, this work identifies that different metal centers and coligands (Sita-type or Chirik-type) in different binding modes (side-on-end-on or side-on bridged) dictate the pathway of dihydrogen cleavage triggerred by the bimetallic complexes.(c) This part studied bent and planar structures of M2N2 cores of a series of dinuclear early transition-metal complexes (M= Zr, Hf, Nb, Ta, Mo and W) containing a side-on bridging dinitrogen ligand using DFT method. The calculated results propose three key factors favoring bent structured:(1) the availability of single electron in metal centers which leads to the bonding interaction between two metal atoms, (2) no remarkable steric effect around metal centers, and (3) the cis conformation of the ligands in dinitrogen dinuclear complexes. In addition, bent and planar structures of M2N2 could be transformed into each other if steric hindrance is slight.(d) The mechanism of directed alkene hydrogenation catalyzed by base metal complexes has been studied by means of DFT calculations. The calculations suggest that the reaction involves a CoII catalytic cycle facilitated by coordination of a second H2. The catalytic cycle begins with a T-shaped (diphosphine)CoH2 that coordinates and inserts the alkene, followed by the coordination of a second equivalent of H2. The resulting CoII intermediate undergoes a rate-determining σ bond metathesis step. Furthermore, 1,2-insertion of alkene into the Co-H bond is preferred over the 2,1-insertion. Different substrates are also applied in the proposed favorable mechanism, and the theoretical calculation results are in agreement with the experimental results. The effect of transition-metal modulation on the activity of olefin hydrogenation catalyst is clarified. The different valence and effect of metal centers in the olefin hydrogenation reaction mechanism are compared. The law of stereoselectivity of olefin hydrogenated products is put forward. Combined with the experimental results, functional grouops in olefins substrate impacting on the yeild is specified.