Nonclassical carbenes as noninnocent ligands

Abstract

The introduction of monoheteroatom stabilized cyclic carbenes, less than a decade after the discovery of stable acyclic and N-heterocyclic carbenes (NHCs), soon led to the development of complex ligands fulfilling more than a simple spectator role. Particularly the imidazol-4-ylidenes (Im4), cyclic (alkyl)(amino)carbenes (CAACs) and 1,2,3-triazol-5-ylidenes (trz) have emerged as ligands for transition metal complexes that demonstrate a noninnocent role. The advancements of these nonclassical N-heterocyclic carbenes as noninnocent ligands are reviewed in this contribution. A short overview of the unique (stereo)electronic features of these classes of carbenes is provided. The focus of this study is however on the impact of Im4, CAACs and trz on the metal complex reactivity in catalytic applications where the ligands exhibit a chemically or a redox noninnocent role in cooperative, bifunctional or redox-switchable catalysis. The introduction of ligand-based chirality and the role played in chiral induction are included, as well as the effect of multinuclear cooperativity in polymetallic complexes of these nonclassical carbenes. The extension of these ligands to macromolecular systems, including surface functionalization for catalytic or material applications, are showcased. Finally, the potential for the nonclassical carbenes as noninnocent ligands in non-catalytic applications, such as light emitting and biological applications, are highlighted.