Tuning the Electronic Properties of N-Heterocyclic Carbenes

The electron‐donating properties of N‐heterocyclic carbenes ([N,N′‐bis(2,6‐dimethylphenyl)imidazol]‐2‐ylidene and the respective dihydro ligands) with 4,4′‐R‐substituted aryl rings (4,4′‐R=NEt2, OC12H25, Me, H, Br, S(4‐tolyl), SO(4‐tolyl), SO2(4‐tolyl)) were studied. Twelve new N‐heterocyclic carbene (NHC) ligands were synthesized as well as the respective iridium complexes [IrCl(cod)(NHC)] and [IrCl(CO)2(NHC)]. Cyclic voltammetry (ΔE1/2) and IR ($\tilde \nu $(CO)) can be used to measure the electron‐donating properties of the carbene ligands. Modifying the 4‐positions with electron‐withdrawing substituents (4‐R=−SO2Ar, ΔE1/2=+0.92 V) results in NHC ligands with virtually the same electron‐donating capacity as a trialkylphosphine in [IrCl(cod)(PCy3)] (ΔE1/2 =+0.95 V), while [IrCl(cod)(NHC)] complexes with 4‐R=NEt2 (ΔE1/2= +0.59 V) show drastically more cathodic redox potentials and significantly enhanced donating properties. Closing the gap between phosphines and NHC ligands: N‐heterocyclic carbenes (NHCs) substituted with electron‐deficient groups (see scheme) have the same donor strength as electron‐rich trialkylphosphines.