N-heterocyclic carbenes and parent cations: acidity, nucleophilicity, stability, and hydrogen bonding-Electrochemical Study and ab initio calculations

N-Heterocyclic carbenes (NHCs) are widely used as organocatalysts.Their reactivity (and instability) is related to their basicityand nucleophilicity, which, in turn, are linked to their scaffold.NHCs can be generated by chemical deprotonation or electrochemicalreduction of the parent azolium cations, NHCH+s.Cyclic voltammetry enabled the reduction potential of theNHCH+s to be determined; the reduction potential is relatedto the acidity of the NHCH+s and the oxidation potential ofthe NHCs, which is related to the nucleophilicity of the NHCs.It was thus possible to order different NHCH+s and NHCs bytheir acidity and nucleophilicity, respectively. A study on thestability of NHCs was also performed in the absence and in thepresence of acetic acid to assess the possibility of the coexistenceof NHC and an acid in the same solution, opening thepossibility of co-catalysis. Finally, ab initio calculations confirmedthe presence, in DMF, of hydrogen-bonded NHCH+–NHC adducts, which could influence catalyst activity.