N-Heterocyclic Carbene Based Nickel and Palladium Complexes: A DFT Comparison of the Mizoroki-Heck Catalytic Cycles

A theoretical investigation has been performed on nickel- and palladium-N-heterocyclic carbene (NHC) complexes in the Mizoroki-Heck (MH) reaction. A density functional theory (DFT) comparison of reaction pathways in cationic and neutral catalytic cycles for both metals was made using the M06 functional. The oxidative addition barrier is much lower for NHC-Ni than that for NHC-Pd for both mono- and bis-ligand pathways; the bis-ligand pathway (i.e., where both NHCs remain coordinated) is energetically more favorable for both metals. The migratory insertion steps have lower free energy barriers for NHC-Ni than for NHC-Pd complexes for both neutral and cationic pathways. beta-Hydride elimination is computed as thermodynamically difficult for NHC-Ni complexes, indicating a possible reluctance in the elimination of the Ni-based hydride intermediate and formation of the Heck adduct at room temperature. The base-assisted reductive elimination is easier for NHC-Pd than for NHC-Ni complexes in the neutral and cationic pathways, in agreement with reported experimental observations.