Stereochemical and structural characteristics of single- and double-site Pd(II)–N-heterocyclic carbene complexes: Promising catalysts in organic syntheses ranging from CC coupling to olefin polymerizations

This review discusses the principles underlying the design, synthesis, and catalytic application of mono- and binuclear Pd(II)–N-heterocyclic carbene (NHC) complexes. The main points of focus are the key proceedings made over the past one and a half decades in the design and development of Pd–NHC complexes and their application as catalysts in various organic syntheses. The catalytic behaviors of single- and double-site chelates of Pd-bearing NHC ligands originating from imidazole, triazole, tetrazole, pyridine, benzothiazole, and benzimidazole moieties were reviewed. A vast number of Pd–NHC complexes have been used as catalysts in various organic syntheses on account of their stability in air and moisture, as well as their low-to-moderate cost and the availability of Pd in stable and variable oxidation states. The catalytic performances of these chelates in reactions ranging from CC coupling to olefin polymerizations are mainly due to the stereochemically diverse topologies of the catalysts. The extent of activity depends on both the steric and electronic properties of the substituents at the heteroatom of a core moiety. Polymer- or silica-supported mononuclear Pd–NHC catalysts are distinguished by their high catalytic activity compared with their unsupported mononuclear counterparts. Emphasis is made on the stereochemical aspects of the Pd complexes, which determine the appropriate catalytic property as well as the consequences of product formation.

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► The ligand design for the formation of mono- and binuclear Pd complexes with N-heterocyclic carbene (NHC) scaffolds was explored.
► Particular attention was paid to Pd–NHC complexes exhibit isomerism in different solvents.
► Catalytic ability of the different classes of Pd–NHC complexes was evaluated and compared.
► Effect of N-substituents, ancillary ligands and type of NHC backbone on catalytic performances was investigated.
► Tuning the catalytic properties of Pd–NHC complexes by changing the system into a heterogeneous catalyst.