Ligand Properties of Boron-Substituted Five‑, Six‑, and Seven-Membered Heterocyclic Carbenes: A Theoretical Study

The electronic properties of boron-substituted five-, six-, and seven-membered heterocyclic carbenes have been studied using quantum chemical methods. The stability of carbenes has been examined from the values of their respective singlet–triplet and HOMO–LUMO gaps. Both the singlet–triplet and the HOMO–LUMO gaps indicate higher stability for six- and seven-membered P-heterocyclic carbenes (PHCs) containing boron atoms at the α position with respect to phosphorus atoms. While PHCs are better π acceptors, the π acidities of NHCs can be tuned by substituting a boron atom in the α position with respect to nitrogen. This is revealed by the energies of a π-symmetric unoccupied orbital centered at the central carbon atom. Reactivity of these carbenes has been discussed in terms of nucleophilicity and electrophilicity index. The calculated relative redox potential values and ¹³C NMR parameters are found to correlate well with the π acidities of the respective carbenes

Electronic and Ligand Properties of Annelated Normal and Abnormal (Mesoionic) <i>N</i>‑Heterocyclic Carbenes: A Theoretical Study

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 ³¹P NMR chemical shifts of corresponding carbene-phosphinidene adducts have been found to correlate well with the π acidity of the NHCs