DFT Studies Of Thermal Activation Of Methane By [M(H)(OH)]~+ (M=Rh,Pd)

The methane catalyzed by transition metal complexes as catalysts has allowed the controlled transfer of carbene units into organic substrates. The activation of C-H and C-C bonds of alkanes by gas-phase atomic transition metal ions has been studied intensely over the past decade because of its immense scientific and industrial importance. A number of experimental and theoretical studies of atomic transition-metal ions with small alkanes have provided a wealth of insight concerning the intrinsic interactions of metal ions with bonds composed of carbon and hydrogen atoms. The studies demonstrate that methane can be spontaneously activated by the third-row transition metal ions Os+, Ir+, Pt+, etc., yielding the metallic carbene cations and H2. First-row and second-row transition-metal cations are found to be much less reactive toward methane than their third row counterparts, and activation of methane is rarely observed. The addition of a single ligand to the metal center can dramatically alter the reactivity, and the kind of reaction have been studied in theoretical investigation at the DFT level, but there is little theoretical report about the mechanism of methane catalyzed by 4d latter transition metal.In this paper, we chose several typical reactions that have been carefully studied using quantum methods, obtained some interesting results. On the basis of the molecular orbital theory, the tradition transition state theory as well as quantum chemistry theory, the systems chose have been investigated using Density Functional Theory (DFT), the Moller-Plesset correlation energy correction MPn and the polarized continuum model (PCM). The structures of the reagents, the reaction products and transition states along the reaction paths have been obtained, then obtained the reaction surfaces, the spectrum datum, thermodynamic datum as well as the information of orbitals. The reaction mechanism has been argued deeply using these data.The whole paper consists of four chapters. Chapter 1 mainly reviews the evolution of methane catalyzed by ligated transition metal. The second chapter summarizes the theory of quantum chemistry and calculation methods of this paper. The contents of two chapters were the basis and background of our studies and offer us with userful and reliable quantum methods.In chapter 3, the dehydrogenation reaction of [Rh(H)(OH)]~+and CH4 has been investigated theoretically using density functional theory (DFT) calculation done. With regard to mechanistic aspects, theσ-complex assisted metathesis (σ-CAM) and the oxidative addition/reductive elimination (OA/RE) reaction paths have been found. After extensive sampling of the potential-energy surfaces, the finding shows that theσ-CAM and OA/RE reactions favor energetically low-spin singlet state PES in a spin-conserving manner, respectively. To study the effects of covalent ligands, comparing with the reactions for the RhH~+ and [Rh(H)(OH)]~+ toward CH4 are discussed. The DFT calculations suggest that the different systems have the more similar and no barriers in excess of the endothermic for the H2 elimination reaction on the low-spin state PES. For example, the low barrier of transition state for theσ-CAM mechanism has few s-electrons on the low spin singlet state PES. Furthermore, our work provides a explanation for why the [Rh(H)(OH)]~+ and CH4 has the large increase in barrier height compared with the [RhH]~+ system. However, From the ground state to excited state needs 20.7 kcal/mol energies, [Rh(H)(OH)]~+ may not be a good methane activation substance compared with the RhH~+/CH4 reaction system. After extensive sampling of the potential-energy surfaces (PESs) the finding shows that theσ-complex assisted metathesis (σ-CAM) process favor energetically low-spin singlet state in a spin-conserving manner.Chapter 4, in this paper we have carried out a theoretical investigation at the DFT (B3LYP) level of the mechanism of methane catalyzed by ligated transition metal [Pd(H)(OH)]~+, the dehydrogenation reaction of [Pd(H)(OH)]~+ toward methane has been investigated theoretically. Both high- and low-spin states potential-energy surfaces for the reaction were built up by using density functional theory (DFT). As compared with the [PdH]~+ system, there is having many features in common. Our calculations indicate that the ground-states species have low electron spin and low s population in the metal-center for the transition states. After extensive sampling of the potential-energy surfaces (PESs), the finding shows that theσ-complex assisted metathesis (σ-CAM) process favor energetically low-spin singlet state in a spin-conserving manner.