Determination of the π‑Accepting Properties of Borate‑, Aluminate‑, and Gallate-Functionalized N‑Heterocyclic Carbenes by 77 Se NMR Spectroscopy

Anionic N-heterocyclic carbenes with weakly coordinating borate, aluminate, and gallate moieties of the type [(F 5 C 6 ) 3 E-NHC] − (E = B, Al, Ga) were isolated as lithium salts by the lithiation of 1,3-bis­(2,4,6-trimethyl­phenyl)­imidazolin-2-ylidene (IMes) or 1,3-bis­(2,6-diiso­propyl­phenyl)­imidazolin-2-ylidene (IDipp) followed by the addition of E­(C 6 F 5 ) 3 (E = B, Al, Ga). Treatment with elemental selenium afforded the lithium salts of the corresponding anionic selenourea derivatives [{(F 5 C 6 ) 3 E-NHC}­Se] − (NHC = IMes, E = B; NHC = IDipp, E = B, Al, Ga), which were examined, among other things, by means of 77 Se­{ 1 H} NMR spectroscopy to assess the π-accepting properties of the WCA-NHC ligands in comparison with their neutral NHC congeners.