Design of Exceptionally Strong Organic Superbases Based on Aromatic Pnictogen Oxides: Computational DFT Analysis of the Oxygen Basicity in the Gas Phase and Acetonitrile Solution.

DFT B3LYP calculations convincingly showed that aromatic pnictogen oxides offer scaffolds suitable for tailoring powerful organic superbases exhibiting exceptional oxygen basicity in both the gas phase and polar aprotic acetonitrile solution. With their protonation enthalpies and pKa values, they surpass the basicity of classical proton sponges and related nitrogen bases. The most potent system is provided with two arsenic oxide moieties on the phenanthrene framework assisted by the two phosphazeno groups in the para-position to both basic centers. With its proton affinity PA = 300.5 kcal mol-1, the latter system breaks the gas-phase hyperbasicity threshold of 300 kcal mol-1, while its pKa = 54.8 promotes it as an unprecedented superbase in acetonitrile. The origin of such a dramatic basicity enhancement is traced to a fine interplay between (a) steric repulsions of the two negatively charged oxygens destabilizing a neutral base, (b) favorable intramolecular [O-H···O]- hydrogen bonding in conjugate acids, and (c) efficient cationic resonance upon protonation supported by the electron-donating substituents. Given the growing interest in highly basic compounds together with related basic catalysts and metal complexing agents, we hope that the results presented here will open a new avenue of research in these fields and direct attention toward utilizing aromatic pnictogen oxides in designing improved organic materials.