Structure and Bonding in Neutral and Cationic 14-Electron Gold Alkyne pi Complexes

Cyclododecyne (5) as a prototype unstrained alkyne was coordinated to either the neutral [AuCl] fragment or to two different cationic [Au(NHC)](+) entities (NHC=N-heterocyclic carbene), and the resulting complexes 6, 8, and 10 were characterized by X-ray crystallography and NMR spectroscopy. Since the structure of cyclododecyne in the solid state could also be obtained after in situ crystallization, a comparison was possible that provides insights into structural changes imposed on the alkyne by the different gold fragments. These data are interpreted on the basis of a DFT analysis of the bonding situation in the individual compounds, which provides insights into the very first elementary step common to many gold-catalyzed transformations.