One-pot preparation of magnetic N-heterocyclic carbene-functionalized silica nanoparticles for the Suzuki–Miyaura coupling of aryl chlorides: improved activity and facile catalyst recovery

Abstract

Based on a reverse micelle strategy, we successfully synthesized new magnetic silica nanoparticles functionalized with a bulky N-heterocyclic carbene (N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, denoted as IPr) precursor through the co-condensation of IPr-bridged organosilane and tetraalkoxysilane in a one-pot reaction. TEM and SEM investigations revealed that the particle sizes of the synthesized materials were uniformly distributed in the range 15–30 nm, and could be tuned by varying the amount of siliceous precursors. FT-IR and XPS characterizations showed that the IPr ligand was successfully incorporated onto magnetic silica nanoparticles. Such materials show good coordination capability toward Pd(acac)2 (acac = acetylacetonate), leading to a higher loading in comparison with magnetic silica nanoparticles (without functionalization). This Pd-loaded material is active toward the Suzuki–Miyaura couplings of challenging aryl chlorides under relatively mild conditions (at 80 °C). An 81% yield for biphenyl was achieved in the presence of 0.32 mol% of Pd within 8 h, using iso-propyl alcohol as a solvent and KOtBu as a base. The activity of the functionalized nanoparticles is much higher than that of mesoporous silica-based catalysts, as well as a commercial Pd/C catalyst. This catalyst can be easily isolated by using a magnetic field and directly used in the next reaction cycle without significant loss of its activity.