Efficient PEPPSI-Themed Palladium N‑Heterocyclic Carbene Precatalysts for the Mizoroki–Heck Reaction

Two
sets of six PEPPSI (pyridine enhanced precatalyst preparation,
stabilization, and initiation) themed palladium complexes of amide-functionalized
N-heterocyclic carbenes [PdBr2(NHC)­(Py)], where NHC = 1-acetamido-3-R-imidazolin-2-ylidene,
1-acetamido-3-R-benzimidazolin-2-ylidene and Py = pyridine, were prepared.
Solid-state structures of all these complexes were determined by single-crystal
X-ray diffraction methods. All complexes under scrutiny were found
to adopt a trans arrangement with square-planar geometry.
These complexes showed excellent catalytic activity toward the Mizoroki–Heck
cross-coupling reaction of aryl chlorides and styrene. Benzimidazole-derived
complexes exhibited better catalytic activity than imidazole-based
complexes. Formation of palladium nanoparticles in the reaction mixture
was evidenced by dynamic light scattering and transmission electron
microscopy studies and a mercury poisoning experiment, suggesting
the possible involvement of palladium nanoparticles in the catalytic
reactions.