Theoretical Study On The Reaction Mechanism Of PdX~+ And PtX~+(X=F,Cl,Br) Activated Small Molecule Alkanes

The C-H and C-C bonds of small molecule alkanes activated by transition metal ions have been widely used in organic synthesis and catalysis.Currently,a large number of experimental and theoretical studied have reported that transition metal ions can effectively activated C-H and C-C bond of alkanes.However,some transition metal ions could not effectively activated alkanes,such as the ground state Cr⁺does not active to any alkanes.Therefore,chemists have tried to add ligands to transition metals to improve the catalytic activity of metals.The results showed that adding appropriate ligands to transition metal ions could effectively improve the reactivity of metals to alkanes.In this thesis,density functional theory at B3LYP method level was used.The mechanism of PdX⁺and PtX⁺?X=F,Cl and Br?activated small alkanes were studied in detail.Through theoretical calculations for each research system,we obtain the geometric structures and corresponding thermodynamic parameters of the stable points involved in reaction,and draw the potential energy surface maps of different spin states,which provided favorable reference for further experimental research.The whole thesis consists of five chapters:In chapter 1,we introduced the development and application of quantum chemistry,and then discussed the research status of small alkanes activated by mononuclear transition metal and transition metal ligand ions.Finally,the research work in this paper was summarized.In chapter 2,the related theories,basis set and methods involved in this article were summarizedIn Chapter 3,the reaction mechanisms of the singlet and triplet PdX⁺?X=F,Cl and Br?to activate the C-H and C-C bonds of methane,ethane,propane and n-butane were studied.The results show that the ground state was the triplet state,the reaction started from the ground state and the spin inversion occurred before the transition state TS1/2,then the reaction proceeded along the low spin potential energy surface,which was a typical“two-state reaction”.In the process of PdX⁺activated the first C-H bond of the alkanes,the hydrogen atom was transferred to the halogen in two ways:oxidative addition/reductive elimination?OA/RE?or?-complex-assisted metathesis??-CAM?.In the reaction of PdX⁺activated methane,only one C-H bond was activated,and the hydrogen atom was transferred to halogen in OA/RE way,the final product was HX.For the PdX⁺/C₂H₆ system,the two C-H bonds of ethane were activated.Base on the origin of the hydrogen atom in the liberated H₂,it can be divided into 1,2-and 1,1-elimination,the final product was a mixture of HX and H₂.The first C-H bond activation in the reaction system of propane and butane,there was competition between the C1-H and C2-H bond,and the activation of C2-H bond was easier;In addition,except for similar to 1,2-and 1,1-elimination of ethane,there was also the activation of third C-H bond?for C₃H₈?and C-C bond(for C₄H₁₀).The main product of PdX⁺activated alkanes was HX.In Chapter 4,the reaction mechanisms of singlet and triplet PtX⁺?X=F,Cl and Br?to activate the C-H and C-C bonds of propane and n-butane were studied.The resulted show that the ground state was singlet for PtF⁺,the whole reaction proceeded on the singlet potential energy surfaces in spin-allowed manner in the reaction of PtF⁺with propane and butane.For Cl and Br,the triplet state was the ground state,spin inversion occurs between the two spin states at the entrance channels of the initial complex IM1,then the reaction proceeded along the low-spin potential energy surface,which was the mechanism of“two-state reaction”.There were no ion-molecular complexes formed on the singlet PES,¹PtX⁺activated the first C-H bond of propane and butane to cleavage directly to form hydride complex.In the first C-H bond activation process,there was competition between C1-H and C2-H bonds,and the activation of C2-H bonds was easier.Similar to PdX⁺activated propane and butane,except for the 1,2-elimination of H₂?the 1,1-elimination reaction of hydrogen molecules was not observed?,there was third C-H bond?for C₃H₈?and C-C bond(for C₄H₁₀)activation.In the system of PtX⁺with alkanes,the final product was mainly HX.The energy at each stable point of the singlet PESs was lower than that of the ground state reactants,indicated that propane and butane could be effectively activated by PtX⁺.In Chapter 5,the reaction mechanisms of NiX⁺,PdX⁺and PtX⁺?X=F,Cl and Br?activated small molecules were compared.The resulted showed that the reaction mechanisms of NiX⁺,PdX⁺and PtX⁺to activate small molecule alkanes were very similar,but there were also differences in some aspects.For NiX⁺,PdX⁺,and PtX⁺?except PtF⁺?,the ground states were triplet states,there was spin flip between the triplet states and the singlet state,which was a typical“two-state reaction”?TSR?model.For PtF⁺,the singlet state was the ground state,the whole reaction was performed in a spin-allowed manner on the low-spin potential energy surface.NiX⁺,PdX⁺and PtX⁺?except ¹PtX⁺?interacted with alkanes to form a relatively loose ion-molecular complex.After the first C-H bond was activated,the hydrogen atoms were transferred in two ways:OA/RE and?-CAM.In the ¹PtX⁺activated alkanes system,the first C-H bond directly activated cleavage to form a hydride complex.The reactivity order of NiX⁺,PdX⁺and PtX⁺for small molecule alkanes was NiX⁺<PdX⁺<PtX⁺.