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) 2 MX and TpM(L)X (M = Ir, Ru, Os, and Rh; acac = acetylacetonate, Tp = tris(pyrazolyl)borate; X = CH 3, OH, OMe, NH 2, 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−NR 2 systems versus M−CH 3 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.