Boosting the performance by the water solvation shell with hydrogen bonds on protonic ionic liquids: insights into the acid catalysis of the glycosidic bond

Abstract

Hydrogen-bonding (HB) induced by the water solvation shell is vital in chemical and biological systems. Herein, HBs related to the binding behavior of protonic ionic liquids (PILs) with water molecules were found to dominate in the acid catalysis of the glycosidic bond in hydrolysis. Spectroscopic techniques, including temperature-related NMR and solvatochromic UV/vis, were employed to investigate the distinct HB interactions of the PILs in the continuous solvent phase. The results revealed that both the sulfonic groups and the N-containing groups synergistically govern the deprotonation of the PILs and consequently the reaction efficiency of the PILs via HBs. Furthermore, the intermolecular HBs between the PIL and the solvent clusters were found to be responsible for the superior catalytic activity of the PILs over the conventional sulfuric acid. A linear correlation between the hydrolysis rate (k–k₀) and catalyst concentration was established in the PILs and sulfuric acid. Their linearization indicates a secondary proton release from the anion of PILs, which was not observed in the case of sulfuric acid. Computational studies showed that the PIL-catalysed mechanism obeyed the dynamic “one-step” protonation of the glycosidic bond by the surrounding explicit solvent water molecules. A lower free energy barrier was obtained by the PILs in comparison with sulfuric acid. The reasons behind this were concluded to be the easy secondary proton release from the PILs by the consecutive nucleophilic activation of water molecules via HBs, offering a more acidic solution for reactant protonation, and the other was the background positive charge of solvent water molecules induced by HBs, stabilizing the transition state in the process.