Toward a general method for CCC N-heterocyclic carbene pincer synthesis: Metallation and transmetallation strategies for concurrent activation of three C–H bonds

A general solution to the preparation of pincer complexes that require the formation of two N-heterocyclic carbenes (NHC’s) and the activation of an aryl C–H bond is reported. The reaction of a phenylene-bridged bis(imidazolium) salt with Zr(NMe2)4 generated the requisite CCC–NHC Zr pincer complex, which has been characterized by X-ray crystallography. Subsequent manipulation of the Zr coordination sphere by reaction with MeI demonstrated the robustness of the ligand geometry at Zr and led to the isolation and structural characterization of a CCC–NHC Zr triiodide pincer complex. A variation of the methodology has been applied to a saturated NHC analog to produce the corresponding CCC–NHC Zr pincer complex. Importantly, it has been found that CCC–NHC Zr pincer complexes can be generated in situ and transmetallated with an appropriate Rh source to generate CCC–NHC Rh pincer structures. These two methodologies, metallation of CCC–NHC precursors with transition metal amido complexes combined with transmetallation, hold great promise for opening general synthetic pathways to a wide variety of transition metal CCC–NHC pincer complexes.

Graphical abstractA zirconium amido reagent has been successfully applied to deprotonate two imidazolium salts and activate and aryl C–H bond concurrently generating a CCC–NHC Zr pincer complex, which has been shown to efficiently transmetallate to Rh.Figure optionsDownload full-size imageDownload as PowerPoint slide