A Combining Theoretical And Experimental Study On The Activation Of CO₂

CO₂ is both the greenhouse gases responsible for global warming and also is the most abundant reserves of carbon source on earth.Along with the CO₂ emissions rapidly increased,to developing the efficient methods for CO₂ recovery from the atmosphere becoming more and more important.CO₂ capture and utilization is one of the important means of its resources.The main works of this paper are theoretically studying the reaction mechanism of the recently reported utilize CO₂ by DFT/B3YLP.The reaction mechanism for the reduction of CO₂ activated by（tBuArN）3M≡N（M=Nb,V,Ta）was studied by the means of DFT calculations.We found hat（tBuArN）3Ta≡N held the best activity among the three（tBuArN）3M≡N complexes studied.Our results also indicated that the reaction of（tBuArN）3M≡N with CO₂ occurred under orbital control involving the HOMO-3 orbital of（tBuArN）3M≡N,which could give higher overlapping with the LUMO of the CO₂ molecule.The substitutions on the amino donor ligands studied here took larger effect on the HOMO structure of the（tBBuArN）3M≡N molecules.The electronic structure of the（tBuArN）3M≡N complexes also showed their ability for activating CO₂ molecules,in the order of M=V<Nb<Ta.And then,we theoretically study the mechanism of the both isomers of terminal tantalaziridine-hydride（TaH）react with CO₂ to give bimetallic methlene diolates.The results reveal showed that the reduction of CO₂ with the terminal tantalum hydrides was carried out by two-step reaction mechanisms,via formate intermediate.And mechanism of the tautomerism between（Ar[tBuCH2]N）2（η2-tBu）（H）CNAr)TaH and（Ar[tBuCH2]N）2（k2-CH2C（Me）2CH2Ar）TaH had aslo been investigated theoretically by DFT.The results reveal that this reaction is accomplished by two steps via tridentateligand intermediate.This kind of intermediate can be considered as the activating reagent to activate the C-H bond.Then,we theoretical study the mechanisms of the reduction reaction between CO₂ and catecholborane/PCP-pincer nickel hydride by DFT method The results reveal that the reaction between CO₂/formaldehyd and catecholborane is very difficult to be carried out at room temperature because of the high activation energy barrier.However,the reduction reaction between CO₂/formaldehyd and PCP-pincer nickel hydride holds a low activation energy barrier.It indicates that the energy barrier between the CO₂/formnaldehyd and borane is able to largely decrease as the PCP-pincer nickel hydride to be an efficient catalyst.And the main function of catalyst（PCP-pincer nickel hydride）is to activate the C≡O bonds of CO₂ and HCHO in this reactian.At last,we study the CO₂ splitting reaction mechanism with NHC as catalyst and aromatic aldehyde as oxygen acceptor.We designed two distinct channels:Ⅰ and Ⅱ.In the charmel I,the first step of this reaction is direct activation of CO₂ by the NHC to fonnm an imidazolium carboxylate.However in channel Ⅱ,the first step of this reaction was addition of the catalyst（NHC）to the aldehyde to form the nucleophilic Breslow intermediate.It is difficult to precede the reactions path at room temperature because of the high activation energy barrier.And under H2O assistance,most of the high activation energy barriers involving in proton transfer are able to largely decrease respectively.From our calculation,a trace amount of water could also act as a co-catalyst in reaction system,and plays a necessary role.These DFT studies strongly support a novel water-assisted proton-relay mechanism in proton transfer step.Then we will use the combining method theoretical simulation and experimental to design and synthesis of the novel NHC.Furthermore,we well experimental study the novel NHC as a catalyst,NHC as catalyst and aromatic aldehyde as oxygen acceptor to reduce CO₂.