Electronic and Steric Parameters of 76 N-Heterocyclic Carbenes in Ni(CO)₃(NHC)

Abstract

We report electron-donor and steric properties of a diverse group of representative N-heterocyclic carbene (NHC) ligands quantified with the help of DFT calculations. This study afforded the conventional TEP data (Tolman electronic parameter = νCO (A1) of Ni(CO)3(NHC)), which allowed ranking 76 NHC ligands in order of increasing donor power. The TEP data reveal several general trends concerning the influence of NHC ring size, substitution, and annulation. The calculations also provided reaction enthalpies for CO elimination from the Ni(CO)3(NHC) complexes and formation of the 16-electron Ni(CO)2(NHC) species. This reaction is largely under steric control, which allowed defining a new steric descriptor for NHC ligands, r (“repulsiveness”) = 10 × (7.568 − 0.003172TEP − 0.0446ΔH), ranging from 0.0 for the smallest (ImNH2) to 8.0 for the most repulsive carbene (ImNAd2) of this work. Ni(CO)3L and Os(H2)Cl2(CO)L2 complexes eliminate CO and H2, respectively, more readily with L = NHC vs PR3 ligands. Apparently, even relatively small NHC ligands are very sterically demanding, and this property of N-heterocyclic carbenes may play a major role in coordination chemistry and catalysis.