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, 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 Huckel antiaromatic 8 pi electron system achieves significant Mobius aromatic stabilization upon undergoing torsional distortion of the heterocyclic ring. (c) 2005 Elsevier B.V. All rights reserved.