Metal-Ligand Cooperative Catalysis of β-Protic Complexes

Metal-ligand cooperation is a powerful strategy to realize efficient and selective transformation and energy conversion. This review aims to provide an overview of the catalysis of pyrazole and protic N-heterocyclic carbene (pNHC) complexes as a new class of metal-ligand bifunctional catalysts. These ligands feature an acidic NH group at the β-position relative to the metal center. Such “β-protic” complexes often exhibit metal-ligand cooperative reactivity owing to the proton transfer at the NH group in the second coordination sphere, coupled with metal-centered events. In this review, some representative metal-ligand cooperative catalysts other than protic pyrazole and NHC complexes are first introduced. The following sections describe the hydrogenation and transfer hydrogenation as well as “borrowing hydrogen”-type carbon-carbon and carbon-nitrogen bond-forming reactions catalyzed by these β-protic complexes. Catalytic transformations independent of hydrogen-transfer processes are also discussed, including cyclization and annulation reactions.