Pd Nanoparticles Supported On C₃N₄ As An Efficient And Reusable Catalyst For Heck Reaction Under Ligand-free Condition

Palladium-catalyzed Heck coupling reactions which are a powerful and practical way for C-C bond formation are widely applied as an effective organic synthesis methodology in medicine, dyes, pesticides, household chemicals, luminescent materials and other industries. Now, Heck reactions have received continuous attentions for the extensive applications. Among all of these, research on catalysts for Heck reactions has always been an issue. Palladium salts combined with special ligands are generally used in conventional methods for good activity and selectivity. Commercialized Pd(OAc)2, Pd2(dba)3, Pd(PPh3)4 are commom catalyst. However, the preparation of these ligands are difficult and costly, and these ligands depend on harsh reaction conditions due to their instability, moreover, both the homogeneous catalysts and ligands cannot be recycled, so it remains important to explore new ligand-free and heterogeneous catalytic system for efficient Heck reactions. Considering that the activation mechanism of homogeneous Pd-containing catalysts depends on the introduction of special ligands, we are interested in improving the activity of heterogeneous Pd NPs-based catalysts by increasing their electron density via support effects. In other words, the choice of an ideal support is significant which can act as ligand at the same time.Graphitic C3N4, as a kind of ancient polymeric semiconductor, not only have the excellent properties of graphene such as large surface area, mechanical strength, chemical stability and light weight, but also have enhanced the functional properties because of the incorporation of nitrogen atoms.In this paper, Pd nanoparticles supported on g-C3N4 were synthesized through a facile chemical reduction method with PdCl2 as the precursor. X-ray diffraction (XRD), X-ray photoelectron (XPS) and transmission electron microscopy (TEM) were performed to characterize the structures and testified that dispersed tiny Pd nanoclusters were successfully loaded on the g-C3N4 support. The resultant complex could be used as an efficient catalyst for Heck cross-coupling reactions under ligand-free condition and the experimental results showed that the catalyst could not only catalyze aryl iodides in excellent yields but also promote the reaction of aryl chlorides up to 27% yield. In addition, the Pd/g-C3N4 composite could be recycled up to six runs without significant loss of activity. Therefore, the catalyst system displayed in this paper will have wide application prospect no matter from the environmental protection or industrial application.