Ligand lone-pair influence on hydrocarbon C-H activation: A computational perspective

Mid to late transition metal complexes that break hydrocarbon C-H bonds by transferring the hydrogen to a heteroatom ligand while forming a metal-alkyl bond offer a promising strategy for C-H activation. Here we report a density functional (B3LYP, M06, and X3LYP) analysis of cis-(acac)₂MX and TpM(L)X (M = Ir, Ru, Os, and Rh; acac = acetylacetonate, Tp = tris(pyrazolyl)borate; X = CH₃, OH, OMe, NH₂, and NMe ₂) systems for methane C-H bond activation reaction kinetics and thermodynamics. We address the importance of whether a ligand lone pair provides an intrinsic kinetic advantage through possible electronic d _π-p_π repulsions for M-OR and M-NR₂ systems versus M-CH₃ systems. This involves understanding the energetic impact of the X ligand group on ligand loss, C-H bond coordination, and C-H bond cleavage steps as well as understanding how the nucleophilicity of the ligand X group, the electrophilicity of the transition metal center, and cis-ligand stabilization effect influence each of these steps. We also explore how spectator ligands and second- versus third-row transition metal centers impact the energetics of each of these C-H activation steps.