Abnormal C5-bound N-heterocyclic carbenes: Extremely strong electron donor ligands and their iridium(I) and iridium(III) complexes

Imidazolium salts are found to bind abnormally via C5 to iridium(I) and iridium(III) to give air-stable monodentate N-heterocyclic carbene complexes. Abnormal ligand binding was verified by X-ray diffraction in both Ir(I) and Ir(III) complexes. In the case of Ir(l), it is necessary to block the C2 and C4 positions to form a stable sterically protected C5-bound complex. Infrared spectroscopy on carbonyl derivatives indicates that abnormally bound N-heterocyclic carbenes are much stronger electron donors than their ubiquitous C2-bound counterparts. The Tolman electronic parameter for 1-isopropyl-2,4-diphenyl-3-methylimidazolin-5-ylidene is 2039 cm(-1), compared to ca. 2050 cm(-1) for typical NHCs.