Immobilized Pd on a NHC functionalized metal-organic framework MIL-101(Cr): an efficient heterogeneous catalyst in Suzuki-Miyaura coupling reaction in water

A novel Pd-NHC functionalized metal-organic framework (MOF) based on MIL-101(Cr) was synthesized and used as an efficient heterogeneous catalyst in the C-C bond formation reactions. Using this heterogeneous Pd catalyst system, the Suzuki-Miyaura coupling reaction was accomplished well in water, and coupling products were obtained in good to excellent yields in short reaction time. The Pd-NHC-MIL-101(Cr) was characterized using some different techniques, including Fourier transform-infrared, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy, inductively coupled plasma and elemental analysis. The microscopic techniques showed the discrete octahedron structure of MIL-101(Cr), which is also stable after chemical modification process to prepare the catalyst system. The TEM images of the catalyst showed the existence of palladium nanoparticles immobilized in the structure of the catalyst, while no reducing agent was used. It seems that the NHC groups and imidazolium moieties in the structure of the MOF can reduce Pd (II) to Pd (0) species. This modified MOF substrate can also prevent aggregation of Pd nanoparticles, resulting in high stability of them in organic transformation. The Pd-NHC-MIL-101(Cr) catalyst system could be simply extracted from the reaction mixture, providing an efficient synthetic method for the synthesis of biaryls derivatives using the aforementioned coupling reaction. The Pd-NHC-MIL-101(Cr) catalyst could be recycled in this organic reaction with almost consistent catalytic efficiency.