Density Functional Study On The Activation Of Low Alkanes By Selected Transition Metal And Oxide Ions

The C-H and C-C bond activation of low alkanes has been extensively studied over the past decades due to its importance in energy conversion and organic synthesis. The successful development of practical C-H and C-C activation methods, which could require reagents that are sufficiently reactive to cleave the strong C-H and C-C bonds but still able to be selective and controllable, would revolutionize the strategies available for the synthesis of natural products, pharmaceuticals, and other industrially relevant materials.As the simplest hydrocarbons, methane and ethane are easily accessible. A number of experimental and theoretical studies on the reactivity of bare transition-metal and oxide ions with methane and ethane have provided a wealth of insights into the C-H bond activation process. UB3LYP method is emploied to investigate the detailed mechanisms of the reaction of transition metal/ions/oxides with alkanes here.Firstly, the potential energy surfaces (PESs) and the detailed mechanisms of the reaction of platinum cation with ethane have been investigated by the UB3LYP method. Calculations show that the overall reactions are exothermic by34.0and38.9kcal/mol for1,1-and1,2-elimination of single dihydrogen, respectively, and3.3kcal/mol for twofold dihydrogen elimination. Predicted substantial differences in exothermicity and barriers for the rate-determining steps between single and twofold dehydrogenations could justify the conclusions from FTICR-MS experimental studies on the dehydrogenation of C2H6by Pt+.Secondly, the following conclusions can be drawn from our density functional investigation on the potential energy surface relevant for the interaction of Rh+with ethane: The minimum energy reaction pathways for the most of the C-H and C-C bond activation reactions of ethane by the rhodium ion are found not to be one of two PESs of a certain spin state. The spin crossings of the potential surfaces of different spin multiplicities are likely involved. Among these reactions, the minimum energy pathway of1,2-dehydrogenation can be described as Rh+(3F)+C2H6â†'RhC2H6+(31)â†'1TS1/2â†'HMC2H5+(12)â†'1T2/3â†'(H)2Rh(C2H4)+(13)â†'(H2)Rh(C2H4)+(34)â†'Rh(C2H4)+(35)+H2. While the minimum energy pathway of C-C bond activation for the elimination of methane can be described as Rh+(3F)+C2H6â†'MC2H6+(31)â†'3TS1/9â†'1TS9/10â†' CH3-RhH+-CH2(110)â†'1TS10/11â†'(CH4)RhCH2+(111)â†'1RhCH2+CH4.Thirdly,from our density functional investigation on the potential energy surface relevant for the interaction of ReOn+with methane we can draw that the minimum energy pathway of ReO2+CH4â†'ReO2CH2++H2can be described as3ReO2+CH4â†'ReO2+(CH4)+(31)â†'1TS1/2â†'HReO2CH3+(12)â†'1TS2/3â†'(H2)ReO2(CH2)+(13)â†'ReO2(CH2)+(14)+H2.The pathway of ReO3++CH4â†'ReO3CH2++H2can be described as1ReO3++CH4â†'ReO3+(CH4)+(15)â†'1TS5/6â†'HReO3CH3+(16)â†'1TS6/7â†'(H2)ReO3(CH2)+(17)â†'ReO3(CH2)+(18)+H2.An addition-elimination pathway can be found to interpret the mechanism for the reaction Re04++CH4â†'ReO4H++CH3.The differences in exothermicity and barriers for the rate-determining steps could justify the conclusions from experimental studies.