Tuning the Electronic and Ligand Properties of Remote Carbenes: A Theoretical Study

The effect of annulation and carbonylation on the electronic and ligating properties of remote N-heterocyclic carbenes (rNHCs) has been studied quantum-chemically. The thermodynamic stability of these complexes has been assessed on the basis of their hydrogenation and stabilization energies, while HOMO-LUMO gaps are used to measure the kinetic stabilities. Annulated/carbonylated rNHCs are found to be weaker s donors but better p acceptors compared with the parent rNHCs. The reactivity of these rNHCs has been studied by evaluating their nucleophilicity and electrophilicity indices. The nucleophilicity values are in good agreement with the sigma basicities of all of the rNHCs. The P-31 NMR chemical shifts of the corresponding rNHC-phosphinidene adducts have been calculated and found to correlate well with the p acidities of these rNHCs.