Electronic Profiling of N‑Phosphine Oxide-Substituted Imidazolin-2-ylidenes (PoxIms) and Imidazolidin-2-ylidenes (SPoxIms)

A detailed electronic study of the N-phosphine oxide functionalized imidazolin-2-ylidenes (PoxIms) and imidazolidin-2-ylidenes (SPoxIms) has been performed experimentally using IR, 13C, and 77Se NMR spectroscopies. While the net donor/acceptor properties of the (S)­PoxIms could not be differentiated via IR spectroscopy (TEP), NMR spectroscopic methods (HEP, Se) reveal that the (S)­PoxIms are slightly weaker σ-donors but stronger π-acceptors compared to common NHCs. Moreover, backbone and substituent-effects could also be resolved by the latter, allowing for a ranking of their electronic properties. Finally, the donicities of these well-designed NHC ligands in their bidentate κ2-C,O modes were evaluated using HEP2 and compared to those of classical chelators.