Development of 7-membered N-heterocyclic carbene ligands for transition metals

We recently reported the first example of a seven-membered N-heterocyclic carbene (NHC) ligand for transition metals. These ligands are attractive because the heterocyclic framework, derived from 2,2′-diaminobiphenyl, exhibits a torsional twist that results in a chiral, C2-symmetric structure. The present report outlines the synthetic efforts that led to the development of these ligands together with the synthesis and structural characterization of metal complexes bearing seven-membered NHCs as ancillary ligands. The identity of nitrogen substituent, neopentyl versus 2-adamantyl, influences the synthetic accessibility and stability of the seven-membered amidinium salts and the NHC–metal complexes obtained via in situ deprotonation/metallation. Computational analysis of the seven-membered ring structures reveals the Hückel antiaromatic 8π electron system achieves significant Möbius aromatic stabilization upon undergoing torsional distortion of the heterocyclic ring.

Graphical abstractThe syntheses of metal complexes bearing seven-membered N-heterocyclic carbenes are described, including X-ray characterization of one amidinium salt carbene precursor, one silver(I)–carbene complex, and two palladium(II)–carbene complexes. Computational analyses reveal a Möbius aromatic stabilization of the formally antiaromatic seven-membered framework.Figure optionsDownload full-size imageDownload as PowerPoint slide