Tricyanoborane-Functionalized Anionic N-Heterocyclic Carbenes: Adjustment of Charge and Stereo-Electronic Properties

The 1-methyl-3-(tricyanoborane)imidazolin-2-ylidenate anion (2) was obtained in high yield by deprotonation of the B(CN)(3)-methylimidazole adduct 1. Regarding charge and stereo-electronic properties, anion 2 closes the gap between well-known neutral NHCs and the ditopic dianionic NHC, the 1,3-bis(tricyanoborane)imidazolin-2-ylidenate dianion (IIb). The influence of the number of N-bonded tricyanoborane moieties on the sigma-donating and pi-accepting properties of NHCs was assessed by quantum chemical calculations and verified by experimental data on 2, IIb, and 1,3-dimethylimidazolin-2-ylidene (IMe, IIa). Therefore NHC 2, which acts as a ditopic ligand via the carbene center and the cyano groups, was reacted with alkyl iodides, selenium, and [Ni(CO)(4)] yielding alkylated imidazoles 3 and 4, the anionic selenium adduct 5, and the anionic nickel tricarbonyl complex 8, respectively. The results of this study prove that charge, number of coordination sites, buried volume (%V-bur) and sigma-donor and pi-acceptor abilities of NHCs can be effectively fine-tuned via the number of tricyanoborane substituents.