Calixarene-Assisted Pd Nanoparticles in Organic Transformations: Synthesis, Characterization, and Catalytic Applications in Water for C–C Coupling and for the Reduction of Nitroaromatics and Organic Dyes

A new type of ternary hybrid, Pd@MCM-Calixox, based on mesoporous silica, calixarene conjugate, and Pd(0) nanoparticles (NPs) was synthesized by sacrificial oxidation of allylic calixarene conjugate eventually functionalized with mesoporous silica without using any external reducing agent. The role of the calix conjugate in the formation of Pd@MCM-Calixox has been established. The hybrid, Pd@MCM-Calixox, was characterized by different techniques to support the formation of well-dispersed Pd(0) NPs of 12 ± 2 nm size. The catalyst, Pd@MCM-Calixox, has been proven to be a resourceful one in water in three different types of reactions, namely, Suzuki C-C cross coupling, reduction of both hydrophilic and hydrophobic nitroaromatic compounds, and reduction and degradation of cationic, anionic, and neutral organic dyes. The catalyst exhibited higher turnover frequencies for all these transformations even when a very low concentration of Pd-based catalyst was used. The Pd@MCM-Calixox hybrid catalyst can be recycled several times without experiencing any significant loss in the activity. Also, the regenerated catalyst showed retention of well-spread Pd(0) species even after several catalytic cycles. The tetraallyl calixarene, allylCalix, conjugate acts as a reducing agent, also controls the size, and yields the well-dispersed Pd(0) NPs. The calix conjugate further provides a hydrophobic core in assisting the diffusion of hydrophobic substrates toward catalytic sites.