Abstract
DFT calculations were performed to elucidate the oxidative addition mechanism of the dimeric palladium(II) abnormal N-heterocyclic carbene complex 2 in the presence of phenyl chloride and NaOMe base under the framework of a Suzuki-Miyaura cross-coupling reaction. Pre-catalyst 2 undergoes facile, NaOMe-assisted dissociation, which led to monomeric palladium(II) species 5, 6, and 7, each of them independently capable of initiating oxidative addition reactions with PhCl. Thereafter, three different mechanistic routes, path a, path b, and path c, which originate from the catalytic species 5, 7, and 6, were calculated at M06-L-D3(SMD)/LANL2TZ(f)(Pd)/6-311++G**//M06-L/LANL2DZ(Pd)/6-31+G* level of theory. All studied routes suggested the rather uncommon Pd-II/Pd-IV oxidative addition mechanism to be favourable under the ambient reaction conditions. Although the Pd-0/Pd-II routes are generally facile, the final reductive elimination step from the catalytic complexes were energetically formidable. The Pd-II/Pd-IV activation barriers were calculated to be 11.3, 9.0, 26.7kcalmol(-1) (G(L)(S-D3)) more favourable than the Pd-II/Pd-0 reductive elimination routes for path a, path b, and path c, respectively. Out of all the studied pathways, path a was the most feasible as it comprised of a Pd-II/Pd-IV activation barrier of 24.5kcalmol(-1) (G(L)(S-D3)). To further elucidate the origin of transition-state barriers, EDA calculations were performed for some key saddle points populating the energy profiles.