In situ preparation of palladium nanoclusters in cerium metal-organic frameworks Ce-MOF-808, Ce-UiO-66 and Ce-BTC as nanoreactors for room temperature Suzuki cross-coupling reaction

Ultrasmall thiolated palladium nanoclusters (Pd-n(l-Cys)(m)) were immobilized inside the cavities of three kinds of cerium-organic frameworks (Ce-MOFs), with different structures and coordinate environments Ce-MOF-808, Ce-UiO-66 and Ce-BTC as sacrificial templates, donated as core-shell catalysts (Pd-n@Ce-MOFs). The Pd-n(l-Cys)(m) clusters have an ultrasmall size with narrow distribution (1 +/- 0.2 nm), amounting to a cluster size of ca. 40 Pd atoms. For comparison, the palladium nanoclusters were loaded over the three Ce-MOFs with the well-known impregnation method, donated as supported material catalysts (Pd-n/Ce-MOFs). Pd-n@Ce-MOFs and Pd-n/Ce-MOFs catalysts were utilized for the Suzuki cross-coupling reaction in an environmentally friendly solvent system (ethanol/water) under mild reaction conditions. Pd-n@Ce-MOFs catalysts showed a remarkably high catalytic activity, due to the unique atomic packing structure and electronic properties of the palladium nanoclusters and the porosity and high specific surface area of the Ce-MOFs that have a great influence on the Suzuki cross- coupling reaction. Pd-n@Ce-MOF-808 showed the highest turnover frequency (132,530.1 h(-1)) observed in the microwave-assisted Suzuki cross-coupling reaction with a very low Pdn loading (0.007 mol%, 0.0224 mu mol Pd). Moreover, the Pd-n@Ce-MOF-808 catalyst was readily recovered and recycled in at least five cycles without significant leaching or loss of catalytic activity. The particle size of the palladium nanoclusters inside the Ce-MOFs cavities is still around 1 +/- 0.2 nm as confirmed by high resolution-transmission electron microscopy (HR-TEM) analysis. The crystallinity of the Pd-n@Ce-MOFs and Pd-n/Ce-MOFs catalysts was characterized by X-ray diffraction (XRD). The charge of unloaded palladium clusters (Pd-n(l-Cys)(m)) and loaded palladium clusters in Ce-MOFs (Pd-n@Ce-MOFs) and on Ce-MOFs (Pd-n/Ce-MOFs) was investigated by X-ray photoelectron spectroscopy (XPS). Liquid nitrogen sorption isotherms at -196 degrees C for the prepared catalysts were measured to investigate their texture properties.

Automated summary

Ultra-small thiolated palladium nanoclusters were immobilized inside cerium-organic frameworks, resulting in core-shell catalysts with high catalytic activity in Suzuki cross-coupling reactions. Among them, Pd-n@Ce-MOF-808 exhibited the highest turnover frequency and could be recycled multiple times without significant loss of activity. This study highlights the potential of palladium nanoclusters supported in MOFs for efficient catalysis in environmentally friendly conditions.