Electronic and ligand properties of annelated normal and abnormal (mesoionic) N-heterocyclic carbenes: A theoretical study

摘要

The effect of annelation and carbonylation on the electronic and ligating properties of N-heterocyclic carbenes (NHCs) has been studied quantum chemically. The thermodynamic and kinetic stability of these NHCs have been assessed on the basis of their singlet-triplet and HOMO-LUMO gaps respectively. Both annelation and carbonylation have been found to decrease the stability of NHCs. Compared to nonannelated carbenes, annelated and carbonylated carbenes are found to be weaker σ donors but better π acceptors. However, the effect of carbonylation is more pronounced than annelation toward increasing the π acidity of the NHCs. The reactivity of these carbenes has been discussed in terms of nucleophilicity and electrophilicity indices. The calculated values of the relative redox potential and ~(31)P NMR chemical shifts of corresponding carbene-phosphinidene adducts have been found to correlate well with the π acidity of the NHCs.