Hydrogen-Bonded Complexes of Carboxylate Anions and Dextrins in an Aprotic Polar Solvent

Abstract

Complexation of p-methylbenzoate (p-CH3C6H4CO2−) and alkanoate anions (CnCO2−) with cyclodextrins (CDs) and acyclic dextrins (Gns) through hydrogen bonding in an aprotic polar solvent, [D6]DMSO, has been studied by means of 1H NMR spectroscopy. Although undissociated p-methylbenzoic acid, p-CH3C6H4CO2H, does not interact with dextrins, p-CH3C6H4CO2− binds through hydrogen-bonding interactions with fairly large binding constants (K) both to native CDs such as α-, β-, and γ-CDs and to Gns. Similar complexation was observed with CnCO2− anions. The K values are related to the basicity of the carboxylate anions. 1H NMR spectroscopy shows that the CO2− group of the guest interacts with the secondary OH groups at the vicinal 2- and 3-positions of the dextrins, while the primary OH groups do not participate at all. Formation of the hydrogen-bonded complex of β-CD and p-CH3C6H4CO2− is an entropically favorable process. Addition of a small amount of D2O suppresses the formation of the hydrogen-bonded complexes, suggesting that hydrogen-bonding interactions between simple hosts possessing dense OH groups as hydrogen-bond donors and guests with CO2− groups as hydrogen-bond acceptors hardly occur in aqueous solution.