Computational Study of Methane C-H Activation by Diiminopyridine Nitride/Nitridyl Complexes of 3d Transition Metals and Main-Group Elements.

The C-H bond activation of methane using Ph,MePDI-M≡N [Ph,MePDI = 2,6-(PhN═CMe)2C5H3N] (M = V, Mn, Fe, Co, Ni, Al, or P) has been studied via three reaction pathways: [2σ + 2π] addition, hydrogen atom abstraction (HAA), and direct insertion. The activating ligand is a nitride/nitridyl (N), with diiminopyridine (PDI) as the supporting ligand. Calculations show reasonable C-H activation barriers for Co, Ni, Al, and P Ph,MePDI nitrides, complexes that favor an HAA pathway. Electrophilic Ph,MePDI nitride complexes of the earlier metals with a nucleophilic actor ligand-V, Mn, Fe-follow a [2σ + 2π] addition pathway for methane activation. Free energy barriers for methyl migration, Ph,MePDI-M(CH3)═NH → Ph,MePDI-M-N(H)CH3, are also interesting in the context of alkane functionalization; discriminating factors in this mechanistic step include the strengths of the σ-bond and metal-actor ligand π-bond that are broken and the electrophilicity of the actor ligand to which methyl migrates.