Immobilization of Pd(II) on MOFs as a highly active heterogeneous catalyst for Suzuki–Miyaura and Ullmann-type coupling reactions

Abstract

Palladium chloride was successfully immobilized on MOF-253, a MOF material with un-coordinated 2,2′-bipyridine moieties, through post-synthetic modification. The coordination interaction between the 2,2′-bipyridine moieties and Pd cations in the MOF-253·PdCl2 material was demonstrated by X-ray photoelectron spectroscopy (XPS) analysis. The prepared MOF-253·0.05PdCl2 was shown to be highly active, selective, and recyclable for the Suzuki–Miyaura cross-coupling and Ullmann homocoupling of a wide range of aryl halides including electron-rich and electron-poor aryl iodides/bromides, heteroaryl iodides, and even aryl chlorides, affording the corresponding biaryl compounds in good to excellent yields under mild reaction conditions. It is suggested that the 2,2′-bipyridine moieties in MOF-253 provide electron-rich environment for Pd(II) cations, and thus facilitate the initial adsorption and insertion of aryl halides molecules to Pd active sites during the carbon–carbon coupling reactions. Moreover, MOF-253·PdCl2 showed remarkably higher activity than the homogeneous Pd(bpy)Cl2 and PdCl2(CH3CN)2, which might be due to the presence of charge transfers between adjacent ligands and metals in the MOF catalyst. The superior catalytic performances of metal ions immobilized on MOFs to their homogenous counterparts might bring new opportunities in the development of highly efficient metal catalysts combining advantages of both homogeneous and heterogeneous catalysis.