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

Abstract

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 pK 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, the latter system breaks the gas-phase hyperbasicity threshold of 300 kcal mol, while its pK = 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.

On the Significance of Lone Pair/Lone Pair and Lone Pair/Bond Pair Repulsions in the Cation Affinity and Lewis Acid/Lewis Base Interactions

“…For example, a recent study on the simple pnictogen oxides shows that the PA of the oxygen atom in these compounds increases as As–O > N–O > P–O. 33…”

Substituted ketenes offer exceptional carbon bases in gas phase: Computational study by density functional theory method

“…The expansion of π‐system in ketene 9 using the cyclohexadiene and cyclopropene rings causes its PA to increase drastically close to the hyperbasicity limit (hyperbases are called compounds with PA ≥ 1,255.2). [ 43 ] The increments in HOMA values of three‐ and six‐membered rings in the conjugate acid of ( 9+H ) + compared with the neutral molecule 9 to the amounts of 0.46 and 1.05, respectively, point out to the positive charge delocalization in these rings and cation stability. The increasing NICS values associated with the protonated form of ( 9+H ) + for both rings, also testifies to this.…”

Design of Robust Organosuperbases and Anion Receptors by Combination of Azine Heterocycle Skeleton and Phosphazene Motif

“…Diverse potential applications of the superbases in different fields such as the catalytic applications has always been an impetus in chemistry. During the recent years, a lot of experimental and theoretical studies have been reported on design of the strong organosuperbases and new proton sponges . This is not surprising due to two reasons including: 1) acid‐base chemistry is one of the main research fields in chemistry, 2) the advantages of the organosuperbases are based on their better solubility in the organic solvents, moderate conditions of reaction and their excellent stability .…”