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

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. Novel chelating NHC palladium complexes were synthesized, characterized by NMR, HR‐MS and single crystal X‐ray diffraction, and then reacted with amine‐functionalized magnetic nanoparticles to produce the magnetically supported NHC palladium complexes. The immobilized version showed highly activity, stability, and recyclability in Suzuki–Miyaura reaction and the reduction of 4‐nitrophenol to 4‐aminophenol.