Use of molecular electrostatic potential at the carbene carbon as a simple and efficient electronic parameter of N-heterocyclic carbenes

摘要

Topographical analysis of the molecular electrostatic potential (MESP) has been carried out for a variety of N-heterocyclic carbenes at the B3LYP, BP86, and M05 levels of density functional theory (DFT) using a 6-311++G **basis set. The electron rich character of the carbene carbon is assessed in terms of the absolute minimum of the MESP at the carbene lone pair region (V_(min)), as well as the MESP at the carbene nucleus (V _C). A linear relationship is established between the V_C and Tolman electronic parameter (TEP) which suggested the use of the former as a simple and efficient descriptor for the electron donating power of N-heterocyclic carbene ligands toward metal coordination. The V_(min) of the carbene also showed good correlation with TEP. However, the deviation from linearity was higher than V_C-TEP correlation, and the reason for this was attributed to the steric effect of N-substituents at the lone pair region. The greater coordinating power of N-heterocyclic carbenes over phosphines is explained on the basis of deeper V_(min) values obtained for the former, and in fact even the V_(min) of the least electron rich N-heterocyclic carbene is comparable to the highly electron rich phosphine ligands. Thus the MESP topographical approach presented herein offers quantification of the inherent electron donating power of a free N-heterocyclic carbene ligand.