Cyclopropenimine as pincer ligand and strong electron donor in proton sponges

Abstract

The basicity of cyclopropenimines (CPIs) and cyclopropenimino‐based proton sponges is investigated by means of density functional theory (DFT) calculations and for the first time directly compared with the basicity of phosphazenes. It is found that CPIs are more basic than the corresponding phosphazenes in the gas phase. Proton sponges based on CPI as pincer ligands also possess higher or at least the same gas‐phase proton affinity as proton sponges based on phosphazenes. However, in comparison with phosphazenes, CPI‐based pincer ligands possess greater conformational flexibility, which enables almost complete avoidance of nitrogen lone pairs. This leads to the substantially smaller destabilization of neutral base in CPI proton sponges. Utilizing homodesmotic reactions, we have shown that significant contribution to the proton affinity of CPI proton sponges is an electron‐donating effect of the second CPI substituent, whereas only a smaller portion of the stabilization energy should be attributed to intramolecular hydrogen bond. Further, it was shown that tetrasubstituted CPI naphthalenes possess very low ionization potential, which qualifies them as very strong electron donors. Finally, utilizing cyclopropenimino substituents, it was found that theoretical gas basicity limit of 370 kcal mol−1 recently calculated by Leito and coworkers could be extended beyond 370 kcal mol−1 in a case of cyclopropenimino phosphorus carbenes and cylopropenimino phosphorus ylides. Copyright © 2016 John Wiley & Sons, Ltd.