Global and local reactivity of N-heterocyclic carbenes with boron and phosphorus atoms: An analysis based on spin polarized density functional framework

A global and local analysis on some N-heterocyclic carbenes containing boron and phosphorus atoms has been performed within the spin polarized density functional theory (SP-DFT) framework. The philicities for spin polarization have been found linearly related to the triplet-singlet energy gap along this series. In agreement with the experimental evidence, these compounds exhibit very low values of the electrophilicity index. In order to gain further insights into the bonding nature and charge and spin reorganization to spin polarization processes associated to these compounds; local considerations were made using simple model approximations to SP-DFT Fukui descriptors. A higher depletion of density of charge is observed in the boron and phosphorus carbenes as compared with imidazol-2-ylidene taken as the reference compound, whereas the spin density is shown to be accumulated on the carbenic carbon atom in all compounds. The SP-DFT indices used here allow us to assess that the aluminum analogue compounds have a slightly higher electrophilic character than the boron compounds, in agreement with previous experimental results.