C−H Bond Activation of Benzene and Methane by M(η2-O2CH)2(M = Pd or Pt). A Theoretical Study

Abstract

C−H bond activations of benzene and methane by M(η2-O2CH)2 (M = Pd or Pt) are theoretically investigated with density functional theory (DFT), MP2-MP4(SDQ), and CCSD(T) methods. The C−H bond activation of benzene takes place with activation energies (Ea) of 16.1 and 21.2 kcal/mol and reaction energies (ΔE) of −16.5 and −25.8 kcal/mol for M = Pd and Pt, respectively, to afford M(η2-O2CH)(C6H5)(η1-HCOOH), where MP4(SDQ) values are given hereafter and a negative ΔE value represents that the reaction is exothermic. The C−H bond activation of methane proceeds with Ea values of 21.5 and 17.3 kcal/mol and ΔE values of −8.3 and −13.3 kcal/mol for M = Pd and Pt, respectively, to afford M(η2-O2CH)(CH3)(η1-HCOOH). However, C−H bond activations of benzene and methane by Pd(PH3)2 need a large Ea value, and these reactions are significantly endothermic: Ea = 26.5 kcal/mol and ΔE = 22.1 kcal/mol for benzene and Ea = 34.7 kcal/mol and ΔE = 31.5 kcal/mol for methane. Also, the C−H bond activation of methane by Pt(PH3)2 needs a large Ea value (28.1 kcal/mol) with moderate endothermicity (ΔE = 7.0 kcal/mol), while the C−H bond activation of benzene by Pt(PH3)2 occurs with a moderate Ea value (17.3 kcal/mol) and a negative ΔE value (−3.9 kcal/mol). From these results, the following conclusions are presented: (1) Pd(η2-O2CH)2 can perform easily the C−H bond activations of benzene and methane but Pd(PH3)2 cannot. This is because the formate ligand assists the C−H bond activation through formation of a strong O−H bond. (2) Pt(η2-O2CH)2 more easily performs the C−H bond activation of methane but much less easily the C−H bond activation of benzene than Pd(η2-O2CH)2, because the intermediate, Pt(II)−benzene complex, is too stable. (3) Benzene more easily undergoes C−H bond activation than does methane. The higher reactivity of benzene is interpreted in terms of M−C6H5 and M−CH3 bond energies and the bonding interaction of benzene π and π* orbitals with M d orbitals. Analysis of electron distribution implicitly indicates that the C−H bond activation by M(η2-O2CH)2 is characterized to be heterolytic C−H bond fission, while the C−H bond activation by M(PH3)2 is characterized to be homolytic C−H bond fission.