N-Heterocyclic Carbene Ligand Effects on Pd-Catalyzed C–H Activation: A Computational Analysis

C–H activation presents a sustainable strategy for direct inert C–H bond functionalization. This density functional theory (DFT) study examines methane activation by neutral and cationic Pd(II)-methoxy complexes via oxidative addition (OA) and σ-bond metathesis (SBM). We elucidate the trans effect and trans influence of 21 electronically and structurally diverse N-heterocyclic carbene (NHC) ligands in these processes. SBM pathways exhibited significantly lower activation energies (E_a) than OA pathways, with cationic complexes generally displaying higher barriers. Complexes containing NHCs with strongly electron-withdrawing substituents consistently showed lower E_a across all pathways, while those bearing electron-donating groups resulted in higher E_a. Electron-donating abilities of NHCs were quantified with electrostatic potential (ESP) parameters viz., molecular electrostatic potential minimum (V_min), and potential at the carbene carbon nucleus (V_C). Quantitative correlations were found between E_a and both V_min and V_C. Our results demonstrate that V_min and V_C serve as useful descriptors for analyzing the trans effect and trans influence of NHC ligands. This holds particularly true in systems lacking prominent secondary interactions and steric hindrance. These correlations highlight the critical role of NHC ligand design and offer a predictive framework to guide future catalyst optimization for alkane functionalization.