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.

The 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 as PowerPoint slide