Theoretical Study On The Reaction Mechanism Of VB Group Transition Metallocene Hydrides With Substituted Alkynes RC?CR(R=COOMe,COOH,Me)

Hydride complexes of transition metals play important role as intermediates in a number of catalytic hydrogenation processes because of their high chemical reactivity.While hydride complexes of VB group transition metallocene represent one of the most important class of them,especially Cp₂MH₃ and Cp₂M?L?H?Cp=C₅H₅,C₅Me₅,C₅H₄Si Me₃,C₅H₃?Si Me₃?₂;M=Nb,Ta;L=CO,isocyanides,phosphines or phosphites?family.The reactivity of these compounds has been extensively studied because they are especially appropriate for the investigation of microscopic reversibility,beta-elimination,which is a very common step in industrial catalytic reactions.Furthermore,insertion processes of unsaturated molecules,such as carbon disulfide,carbon dioxide,olefins,alkynes,heterocumulenes etc.,into the metal hydride bond,constitute fundamental chemical steps in several catalytic cycles.In particular,the insertion of an alkyne into metal–hydride bond is a key elementary step in catalytic polymerization and hydrogenation processes,which has received considerable attention.Experimental results showed that the reaction products,mechanisms and stereoselectivity depend on several factors,such as the nature of the metal center,ancillary ligands and substituents on the alkynes.Therefore,it is of great significant to study the reaction between hydride metallocene complexes containing different metals and different ligands and alkynes containing different substituents,which can provide a reasonable explanation to the experimental phenomenon,reveal the influence factors of the reaction,understand the nature of the reaction,and provide predictions and theoretical guidance for new chemical reactions in this field.In this paper,the reaction mechanisms of VB group transition metallocene hydrides with substituted alkynes RC?CR?R=COOMe,COOH,Me?have been investigated and compared based on density functional theory?DFT?calculations.The changes of chemical bonds and the kinetic characteristics of reactions are also discussed.Geometry optimizations,frequency calculations,single point energy corrections of reactants,transition states,intermediates and products as well as intrinsic reaction coordinates?IRC?from transition states were all performed using Gaussian 09 package.The changes of chemical bonds in the reactions were probed based on quantum theory of?atoms in molecules??QTAIM?,using AIMALL,Multiwfn,and Topmod programs.The kinetic characteristics are discussed based on transition state theory?TST?.This thesis mainly includes the following four parts:1.The insertion of an alkyne into transition metal–hydride bonds is a key elementary step in catalytic polymerization and hydrogenation processes.It was found that a Z-or E-type alkenyl complex can be formed through trans/cis stereospecific processes.In the first part,the reaction mechanism of Cp₂M?L?H[Cp=?⁵-C₅H₅;M=Nb,V;L=CO,P?OMe?₃]with dimethylacetylene dicarboxylate?DMAD?,and the factors of influencing the stereoselectivity have been investigated based on density functional theory calculations.The calculated results show that for L=CO,the Z-isomer product forms first even at low temperatures because of the low Gibbs free energy barrier??G^#?,then the Z-pro converts to E-pro.While for L=P?OMe?₃,the exclusive product is the E-isomer.For different metal centers,the mechanisms of Cp₂M?CO?H+DMAD?M=Nb and V?reactions are similar,while their products are different at room temperature.For M=Nb,because the energy barrier of the isomerization from Z-pro to E-pro is low and the relative free energies of Z-pro and E-pro are almost equal,both Z-pro and E-pro can be obtained.While for the Cp₂V?CO?H+DMAD reaction,only the Z-pro can be obtained under mild conditions,E-pro can be obtained only at high temperatures.For the reactions of Cp₂M?CO?H+DMAD?M=V and Nb?,the formation of E-isomer products proceeds via two five-membered ring transition states.The calculated results provide a reasonable explanation to the experimental observations and predict a new insertion reaction.2.In the second part,the reaction mechanism of Cp₂Nb?CO?H?Cp=?⁵-C₅H₅?with acetylenedicarboxylic?ADCA?has been investigated based on density functional theory calculations.Kinetic calculation and topological analyses of electron density have been conducted.The calculated results show that the reaction can proceed easily even at very low temperature.Z-pro Cp₂Nb?CO???¹-?Z?-HOOCC=CHCOOH?and the isomer E-pro can be got via two parallel steps.The yield rate of E-pro is larger than that of Z-pro because of its lower energy barrier.In addition,the carboxylato-containing niobocene compound Cp₂Nb?CO??k¹-OOCCH=CCOOH?can also be got as the most stable product through a proton transfer process.Along the reaction pathway,there is a?²-?C,C?-Nb agostic structure.The results provide useful information and theoretical predictions on much more such insertion reactions.3.Experimental results showed that the different substituents on alkynes influence the reaction products and mechanism as they react with Cp MH₃?M=Nb and Ta?.Alkynes featured electron-withdrawing substituents afford the fumaric ester complexes;in contrast,those featured electron-donating groups lead to the displacement ofH₂,with the formation of metallocene alkyne hydride complexes.In the third part,the reaction mechanisms of Cp₂NbH₃?Cp=?⁵-C₅H₅?with substituted alkynes RC?CR?R=COOMe?1?and Me?2??have been investigated based on density functional theory?DFT?calculations.The calculated results show that the reaction mechanisms and products of title reactions can be regulated by the nature of substituent of alkyne.As alkynes featured electron-withdrawing substituent COOMe,the corresponding fumaric ester complex Cp₂Nb H?trans-Me O₂CCH=CHCO₂Me?can be obtained at ambient temperature through an insertion process.As alkynes featured electron-donating substituent Me,the products are hydride niobocene Cp₂Nb H?Me C?CMe?andH₂ by the elimination of hydrogen molecule,and they only can be gotten with the irradiation of UV light.Our studies give reasonable explanations to the experimental facts and predict a kind of new chemical reaction on this domain.4.In the fourth part,the reaction mechanism of Cp₂VH₃?Cp=?⁵-C₅H₅?with dimethylacetylene dicarboxylate Me O₂CC?CCO₂Me?DMAD?has been investigated based on DFT calculations.The changes of chemical bonds in reaction paths have been conducted using AIM theory.The results show that the DMAD inserts into the V-H bond of Cp₂VH₃ in trans-bends manner first in the reaction.The two hydrogen atoms in Cp₂VH₃ add to the acetylenic carbons of DMAD respectively by two steps to get the trans-addition product??⁵-C₅H₅?₂VH?trans-Me O₂CCH=CHCO₂Me?.The reaction can proceed at room temperature.Compared to??⁵-C₅H₅?TaH₃and??⁵-C₅H₄Si Me₃?₂NbH₃,the reaction containing Cp₂VH₃proceeds more smoothly.The study in this part suggests that cheap metallocene hydrides Cp₂VH₃can replace expensive Cp₂MH₃?M=Nb,Ta?compounds,applying to the functionalization of alkynes.Based on the discussions in above four parts,it can be seen that:the metals and ligands in metalocene hydrides,and the properties of substituents in alkynes have obvious influences on the mechanism,products and selectivity of such reactions.The period of the metal element has little influence on the reaction mechanism and products,but the conditions under which the reaction takes place are different.The ligands in hydrides mainly affect the stereoselectivity of the reaction.The substituents in alkynes directly influence the reaction mechanisms and products.Therefore,the reaction mechanism and stereoselectivity of products can be experimentally regulated by selecting metal center,ligand of metallocene hydrides,and substituent of alkynes.The innovations of this paper are as follows.1.The impact of metal center and ligand L in VB group metallocene hydrides Cp₂M?L?H?M=Nb,V?on the stereoselectivity of the insertion reactions of substituted-alkyne into the metal-hydride bond are clarified.Due to the little influence of different VB group metals on the reaction mechanism,the metallocene hydrides containg cheap metal V can be used to functionalize alkynes instead of noble metal Nb and Ta.2.The process of proton transfer in the reaction of Cp₂Nb?CO?H with alkynes is discovered by theoretical calculation.An agostic structure?²-?C,C?-Nb is found in the reaction between Cp₂Nb?CO?H and acetylenedicarboxylic?ADCA?.3.The regulating effects of alkyne substituents on the reactions between Cp₂Nb?CO?H and alkynes are determined.The mechanisms and products are predicted for the reactions between Cp₂NbH₃and alkynes with different substituents.