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

• Published in: Journal of organometallic chemistry Vol. 690; no. 24; pp. 6143 - 6155
• Main Authors: Scarborough, Christopher C., Popp, Brian V., Guzei, Ilia A., Stahl, Shannon S.
• Format: Journal Article
• Language: English
• Published: LAUSANNE Elsevier B.V 01.12.2005; Elsevier
• Subjects: Chemistry; Chemistry, Inorganic & Nuclear; Chemistry, Organic; N-heterocyclic carbene; Palladium; Physical Sciences; Science & Technology; X-ray structure; N-heterocyclic carbene; X-ray structure; Palladium; RHODIUM; STABLE CARBENES; STRUCTURAL-CHARACTERIZATION; OLEFIN METATHESIS; BIS(N-HETEROCYCLIC CARBENE); GOLD(I) COMPLEXES; PALLADIUM(II) COMPLEXES; AROMATICITY; MODULAR APPROACH; palladium; CATALYZED AEROBIC OXIDATION
• ISSN: 0022-328X; 1872-8561
• DOI: 10.1016/j.jorganchem.2005.08.022

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. 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, C 2-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.