Heck reaction using palladium-benzimidazole catalyst: synthesis, characterisation and catalytic activity

Abstract

Palladium is used as a catalyst for carbon-carbon couplings such as in the Heck, Stille, and Suzuki reactions. This research reports on two palladium catalysts for the Heck reaction, namely bi-1-3-bis(phenylmethyl) benzimidazolium bromo palladium(II) (Pd-NHC-H) and bi-1,3-bis[(4-chlorophenyl)methyl]-1-benzimidazolium bromo palladium(II) (Pd-NHC-Cl). The catalyst preparation involved two steps of synthesis. First was the synthesis of N-heterocyclic carbene (NHC) ligands by reacting benzimidazole with benzyl bromide and 4-chlorobenzylbromide. It was then followed by the reacting of both synthesized ligands with palladium chloride. The synthesized NHC ligands and catalysts were characterized using Fourier Transform Infrared (FT-IR), 1H and 13C Nuclear Magnetic Resonance (NMR), and UV-Vis spectroscopy. All results gave evidence of the successfulness of the synthesized compounds. A preliminary complexation study between the Pd2+ cation and N-heterocyclic carbene (NHC) ligand showed stoichiometric of 1 Pd2+ cation to 2 NHC ligand. The catalytic activity of the Pd-NHC-H and Pd-NHC-Cl was studied on a Heck carbon-carbon coupling reaction between 1-bromo-4-nitrobenzene and styrene at different catalyst loadings and reaction times. The catalytic performance was analysed using GC-FID. Results showed that excellent catalytic activity was achieved using Pd-NHC-Cl with a catalyst loading of 0.5 mmol% and 60 minutes of reaction time.