First Examples of Structurally Imposing Eight-Membered-Ring (Diazocanylidene) N-Heterocyclic Carbenes: Salts, Free Carbenes, and Metal Complexes

Eight-membered-ring (diazocanylidene) N-heterocyclic carbenes (8-NHCs) are reported for the first time. The precursor salts, readily prepared by the amidine route, are converted to the free carbene by treatment with KHMDS; in general, the free NHCs can be isolated, and in one example the molecular structure was obtained. Silver complexes were prepared by direct reaction of Ag2O with the diazocanylidinium salts, and Rh complexes were formed by treatment of in situ formed free carbene with the appropriate Rh precursor complex. The salts and complexes were fully characterized using spectroscopic methods, including in some cases by X-ray crystallography. Key features of these new ring-expanded heterocyclic carbenes (RE-NHCs) are the extreme steric strain they impose on the metal center (they are the most sterically imposing class of NHC reported to date) and their high donor capacity, being some of the most basic NHCs currently available.