New chelating N-heterocyclic carbene palladium complexes immobilized on magnetic nanoparticles: Synthesis, characterization, and catalytic properties

Abstract

Two chelating N-heterocyclic carbene (NHC) palladium complexes 3a-b containing N-(3-bromopropyl)-N′-thioether difunctionalized imidazol-2-ylidene were synthesized by direct metalation of the precursor imidazolium salts and characterized by NMR and HR-MS. The molecular structure of 3a has been further characterized unambiguously by X-ray single crystal analysis. The subsequent substitution reaction with amine-functionalized magnetic nanoparticles (MNPs-NH₂) afforded two highly active, stable, and recyclable magnetic nanoparticle supported NHC palladium complexes, MNPs-NHC-Pd I-II, which were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), inductively coupled plasma optical emission spectrometry (ICP-OES), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The MNPs-NHC-Pd complexes exhibited high catalytic activity in the Suzuki–Miyaura cross-coupling reaction of aryl bromides with arylboronic acids, as well as in the reduction of toxic 4-nitrophenol (4-NP) under mild conditions. The recycling experiment of MNPs-NHC-Pd I in the Suzuki–Miyaura cross-coupling reaction between 4-bromoanisole and phenylboronic acid showed that the yield of 4-methoxybiphenyl remained high at 88% within 0.5 h at 60 °C, indicating that the catalytic activity was well maintained for at least 10 cycles.