Multidentate NHC complexes of group IX metals featuring carbon-based tethers: Synthesis and applications

Abstract Incorporation of N-tethers to N-heterocyclic carbene ligands to form a powerful class of multidentate, responsive ligand frameworks that have proven to be more than just ancillary ligands with reported success and usefulness in a multitude of applications. NHC complexes of group IX transition metals (Co, Rh, Ir) have enjoyed considerable research interest over the last two decades, owing to their vast range of accessible oxidation states, favourable stabilities and reactivities, as well as multi-functional use. The inclusion of carbon as a secondary binding atom has been extremely useful to help enable the resulting metal complex for complex C–H activation, small molecule fixation and activation, and unusual migratory insertion reactions. This comprehensive review communicates results that demonstrate the versatility of this carbon-based multidentate ligand series through inclusion of synthetic routes available to access the metal-NHC complexes, their associated stability and reactivity pathways, as well as advances made in several different applications, most notably homogeneous catalysis.