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