Recent advances in PEPPSI–themed palladium NHC complexes: Applications ranging from homogeneous catalysis to electrocatalysis

Abstract

N–heterocyclic carbenes (NHCs) have garnered significant interest particularly among theorists and organometallic chemists due to their structural diversity and ability to bind with almost every transition metal atom. However, their remarkable potential as catalysts and nucleophilic reagents is less widely recognized. Palladium catalysed reactions enable diverse organic transformations under mild reaction conditions with quantitative yields of the desired products, making them highly valuable. Traditionally, phosphorus-based ligands and catalysts have influenced the catalysis domain; however, their exorbitant prices, and susceptibility to degradation towards air and moisture exploits their practical application. These challenges have driven the exploration for robust and cost-effective alternatives with NHCs in combination with palladium gaining prominence for their remarkable stability and σ–donating properties. These Pd–NHC complexes have emerged as novel and versatile nucleophilic entities, representing as one of the most effective classes in organometallic chemistry. Within this category Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation, (PEPPSI) type Pd complexes have attracted significant attention as efficient substitutes for phosphine-based Pd counterparts, their inherent air– and moisture– stability, combined with exceptional catalytic activity, mainly in cross–coupling and other key organic transformations, has further solidified their prominence in modern catalysis. Pd-PEPPSI complexes possess a pyridine molecule as a labile ligand (through-away entity), which facilitates substrates to bind with the metal centre and thereby renders the transformation reaction. The strong σ–donor facets of NHCs have led to their greater use as supporting ligands in organometallic chemistry predominantly in the industry sector relevant catalytic processes. Beyond their widespread application in catalysis, Pd–NHC complexes have also garnered increasing attention in materials science, explicitly for their potential applications as Pd–NHC complexes, with a focus on recent literature detailing their synthesis, structural diversity, and catalytic applications in both organic transformations as well as in electrocatalytic water splitting reactions.