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 (Ea) than OA pathways, with cationic complexes generally
displaying higher barriers. Complexes containing NHCs with strongly
electron-withdrawing substituents consistently showed lower Ea across all pathways, while those bearing electron-donating
groups resulted in higher Ea. Electron-donating
abilities of NHCs were quantified with electrostatic potential (ESP)
parameters viz., molecular electrostatic potential minimum (Vmin), and potential at the carbene carbon nucleus
(VC). Quantitative correlations were found
between Ea and both Vmin and VC. Our results demonstrate
that Vmin and VC 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.