Metal nanoparticles have been recognized and widely explored as unique catalysts for carbon−carbon coupling reactions. However, due to their extreme tendency to agglomeration, the generation and stabilization of metal nanoparticles in a porous matrix is an important research field. Herein, novel mesoporous phenolic resin-supported palladium nanoparticles (Pd@NH 2 -MPRNs) were prepared via direct anionic exchange followed by gentle reduction by using primary amine-functionalized ordered mesoporous phenolic resin as the support. The obtained Pd@NH 2 -MPRN material still possessed large surface area and ordered two-dimensional hexagonal mesoporous structure. Meanwhile, uniform and well-dispersed palladium nanoparticles were formed in the mesoporous channels, which could be attributed to an efficient complexation and stabilization effect derived from the primary amine groups. As a result, it can promote Suzuki coupling of less activated aromatic bromides to various biaryls in water with high conversion and selectivity. This excellent performance was attributed to small particle sizes, ordered mesopores, and a hydrophobic pore surface, which resulted in the decreased diffusion limitation and the increased active site accessibility. It is noted that it is competitive with the best palladium catalysts known for water-medium Suzuki coupling reaction, and it can be reused at least seven times without significant reduction in the catalytic efficiency, showing a good recyclability. Therefore, this work provides a new potential platform for designing and fabricating robust ordered mesoporouspolymer-supported metal nanoparticles for various catalytic applications.
Conformational dynamics of active sites in enzymes enable great control over the catalytic process. Herein, we constructed a metal–organic framework with conformationally dynamic active sites (Rh2-ZIF-8). The active sites in Rh2-ZIF-8 were composed of the imidazolate-bridged bimetallic center with a catalytic dirhodium moiety and structural zinc site. Even though the coordination sphere of the dirhodium species was saturated with two circularly arranged esp groups and two axial 2-MeIm ligands, it could still effectively catalyze the direct synthesis of N–H aziridines from olefins with high activity. We found that such a self-adaptive catalytic process was based on the dynamic breakage and reformation of the rhodium–zinc imidazolate bridges. Interestingly, the in situ generated dirhodium site with a unique Rh2(esp)2(2-MeIm)1 configuration was able to exhibit obviously enhanced selectivity compared to homogeneous catalyst Rh2(esp)2. Furthermore, the surrounding zinc imidazolate groups could effectively protect the dirhodium moieties from harsh environments, and this ultimately endowed it with high stability.
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