Theoretical Study On The Activation Of Methane By Transition Metal Complexes MX⁺(M=Pt,Au,Ag,Cu;X=CO,F,Cl,Br,I)

As a raw material of fuel and chemical products,methane plays an important role in industrial production and life.However,it is very difficult to break the thermodynamically stable and dynamically inert C-H bond in methane.For more than a century,transforming methane from fossil or biological resources into more valuable raw materials has been one of the central challenges to solve the global energy problem.The activation and selective functionalization of methane is still a huge challenge.In this paper,the activation of methane by transition metal ions connected with ligands is studied theoretically.Density functional theory(DFT)at B3LYP method level was used to optimize the structure of each stagnation point in the reaction process,construct a reliable reaction potential energy surface for the reaction with methane;The intermediate and transition states were obtained by geometric configuration optimization and frequency calculation,and verified by intrinsic reaction coordinate(IRC)calculation.Natural bond orbital(NBO),charge analysis(NPA)and interaction analysis are carried out to better understand the mechanism of methane activation from the micro point of view,and the theoretical data are used to provide a theoretical basis for the experiment.The whole thesis consists of three parts.In the first part,the reaction of transition metal complex Pt(CO)⁺activated methane connected with closed-shell ligand was studied theoretically.The results show that the metal complex connected with the closed-shell ligand CO reduces the activation ability of methane,and its activation ability is lower than that of the bare metal cation Pt⁺.In the second part,the reaction of Pt X⁺(X=F,Cl,Br,I)with methane was studied theoretically.These reactions take place more easily along the low-spin potential energy surface.In the elimination of HX(X=F,Cl,Br,I),the formal oxidation state of the metal ion appears to be conserved,and the importance of this reaction channel decreases in going as the sequence:X=F,Cl,Br,I.A reversed trend is observed in the losses of small closed-shell molecule H₂ for X=F,Cl,Br,while it is not favorable for Pt I⁺in the loss of either HI or H₂.In the eliminations of HX,the mechanisms are different.The transfer form of H is from proton to atom,last to hydride.While for the loss of H₂,the transfer of H is in the form of hydride for all the X ligands.The reason is mainly due to the electronegativity of halogens.The calculated results are consistent with the experimental.In the third part,the reaction of noble metal complex MF⁺(M=Au,Ag,Cu)with methane was studied theoretically.In the study,it is found that the reaction tends to be on the surface of the double state with low spin,and the energy does not cross.The reaction path of elimination HF or H₂ was found in Au F⁺/CH₄ system.The binding ability between Ag⁺and ligand F was weak in Ag F⁺/CH₄system.New activation channels were found in Cu F⁺/CH₄ system,and the products were Cu H⁺and CH₃F.The reaction pathways of the three metal ion complexes all involve the cleavage of M-F(M=Au,Ag,Cu)bond and dissociate HF.The energy of each stagnation point on this channel is lower than that of the ground state reactant,and it is an exothermic reaction,which is favorable for kinetics and thermodynamics.Compared with the direct activation of methane by bare metal cations,open-shell ligand F increases the formal oxidation state of metals and enhances the activation ability of metal ions to methane.