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

C−H bond activations of benzene and methane by M( η 2 -O 2 CH) 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 ( E a ) of 16.1 and 21.2 kcal/mol and reaction energies (Δ E ) of −16.5 and −25.8 kcal/mol for M = Pd and Pt, respectively, to afford M( η 2 -O 2 CH)(C 6 H 5 )( η 1 -HCOOH), where MP4(SDQ) values are given hereafter and a negative Δ E value represents that the reaction is exothermic. The C−H bond activation of methane proceeds with E a values of 21.5 and 17.3 kcal/mol and Δ E values of −8.3 and −13.3 kcal/mol for M = Pd and Pt, respectively, to afford M( η 2 -O 2 CH)(CH 3 )( η 1 -HCOOH). However, C−H bond activations of benzene and methane by Pd(PH 3 ) 2 need a large E a value, and these reactions are significantly endothermic: E a = 26.5 kcal/mol and Δ E = 22.1 kcal/mol for benzene and E a = 34.7 kcal/mol and Δ E = 31.5 kcal/mol for methane. Also, the C−H bond activation of methane by Pt(PH 3 ) 2 needs a large E a value (28.1 kcal/mol) with moderate endothermicity (Δ E = 7.0 kcal/mol), while the C−H bond activation of benzene by Pt(PH 3 ) 2 occurs with a moderate E a value (17.3 kcal/mol) and a negative Δ E value (−3.9 kcal/mol). From these results, the following conclusions are presented: (1) Pd( η 2 -O 2 CH) 2 can perform easily the C−H bond activations of benzene and methane but Pd(PH 3 ) 2 cannot. This is because the formate ligand assists the C−H bond activation through formation of a strong O−H bond. (2) Pt( η 2 -O 2 CH) 2 more easily performs the C−H bond activation of methane but much less easily the C−H bond activation of benzene than Pd( η 2 -O 2 CH) 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−C 6 H 5 and M−CH 3 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( η 2 -O 2 CH) 2 is characterized to be heterolytic C−H bond fission, while the C−H bond activation by M(PH 3 ) 2 is characterized to be homolytic C−H bond fission.