pH-Responsive chelating N-heterocyclic dicarbene palladium(ii) complexes: recoverable precatalysts for Suzuki–Miyaura reaction in pure water

Abstract

A hydrophobic palladium complex of the chelating N-heterocyclic dicarbene (NHDC) (3) was synthesized through in situ deprotonation of a bisimidazolium salt (2) by Pd(OAc)₂ in DMSO, subsequent basic hydrolysis and acidification resulted in the formation of four NHDC palladium complexes bearing carboxylic functional groups (4–7). Complexes 4–7 are air-stable, pH-responsive and water-soluble under basic conditions due to deprotonation of the carboxylic groups. The catalytic activities of the hydrophilic catalytic systems from 4–7 were preliminarily evaluated using the Suzuki–Miyaura cross-coupling reaction in pure water, high catalytic activity and chemical selectivity were observed in comparison with their hydrophobic analogue from 3. Either the catalytic aqueous solution or the pH-responsive catalysts were easily separated and could be recycled at least four times. TEM analysis revealed that Pd(0) nanoparticles were formed after the catalytic reaction.