Advancements in Pd-Based Supported Porous Nanocatalysts for the C-C Cross-Coupling Reactions

Palladium (Pd) is a 4d transition metal with electronic configuration [Kr] 4d10 5s0, and it is one of the most widely studied metals in the periodic table due to its versatile catalytic role in organic synthesis. The choice of ligands that can coordinate with Pd sites plays a crucial role in the progress of the reaction. Due to the coexistence of multiple oxidation states (Pd(0)/Pd(II)), the active Pd sites of the catalysts can participate in various stages of the coupling reaction. The Pd-catalyzed C-C coupling reactions proceed through four steps: (1) oxidative addition of the reactant to the catalytic site, (2) transmetallation, (3) rearrangements of ligand centers and (4) reductive elimination to the coupling products. For the heterogeneous Pd nanocatalysts, active Pd sites are often strongly bound (chelated) with the solid catalyst surfaces. In this review, we have highlighted the advancements made in the heterogeneous Pd nanocatalysts with an emphasis on the types of different classes of porous solids, which could ligate with the Pd centers via strong covalent bonds. The high specific surface areas and small Pd sites of these nanocatalysts provide a larger number of catalytic sites and thus facilitate the reaction. Mechanistic aspects of the C-C cross-coupling reactions are discussed in the context of the structure–reactivity relationship.