Synthesis and Characterization of N‑Trifluoromethyl N‑Heterocyclic Carbene Ligands and Their Complexes

Starting from N-trifluoromethyl benzimidazole (1), a series of N-trifluoromethyl benzimidazolium salts 2a–f·HA have been prepared and fully characterized. These were engaged in the formation of [Ir­(CO)2(NHC)­Cl], [Rh­(COD)­(NHC)­Cl], [Se­(NHC)], and [Au­(NHC)­Cl] derivatives. IR analysis of [Ir­(CO)2(NHC)­Cl] complexes revealed that the trifluoromethyl substituent on nitrogen significantly decreases the σ-donating ability of the carbene carbon. On the other hand, the π-acceptor property of these novel ligands is enhanced. Examination of the 77Se NMR resonance of [Se­(NHC)] adducts and the redox potentials of [Rh­(NHC)­(COD)­Cl] complexes further supports this assumption. In addition, the efficiency of these new N-trifluoromethyl NHC ligands was investigated in π-acidic Au­(I)-catalyzed hydroalkoxylation of cyclohexene. The gold complexes bearing NHCs 2a and 2c–e compete with [Au­(PPh3)­Cl] in terms of catalytic activity.