Comparison of carbene and imidazole bonding to a copper(I) center

The bonding of (1,4,7-trithiacyclononane)copper(I) to N-methylimidazole and to N,N′-dimethylimidazol-2-ylidene (IMe) as its pseudo-tautomer has been compared by crystallography, spectroscopy, and electrochemical analyses. In the solid state, a shorter Cu–L bond is observed for L = carbene than for L = imidazole, which indicates that electronic effects override potential steric hindrance in this pair of complexes. The oxidation potential difference reveals that the imidazole is a substantially stronger donor than the carbene ligand, and it allowed for estimating the Lever electronic parameter for the carbene ligand, EIMe = +0.30 V.

Comparison of N-methylimidazole with its pseudo-tautomer N,N′-dimethylimidazol-2-ylidene (IMe) in a copper(I) complex revealed a stronger bonding of the carbene ligand yet a stronger electron donor ability of the imidazole; the Lever electronic parameter for the carbene ligand EIME was determined.Figure optionsDownload as PowerPoint slideHighlights
► Shorter copper-ligand distance for sterically more crowded NHC (X-ray)
► Stronger electron donation for imidazole than for NHC (e-chem)
► Lever electronic parameter for the carbene, ENHC is +0.30 V.