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

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 degrees 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 (KOBu)-Bu-t 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.