Carboxylic ester hydrolysis catalyzed by a host–guest system constructed by cyclodextrin dimer and zinc complex

Abstract

A new β-cyclodextrin dimer, 2,2′-bipyridyl-4,4′-dimethyl-bridged-bis(6-monodeoxy-6-ammonio-β-cyclodextrin) (BCD, Host) was synthesized and further assembled with a zinc complex containing a hydrophobic group as an inclusion complex, ZnL(BCD) (ZnL, L = 4-(4′-tertbutylbenzyl) diethylenetriamine, Guest). The complex was characterized by elemental analysis and NMR spectra and applied as a new supramolecular system of catalyzing carboxylic ester hydrolysis. In such host–guest system, the two deprotonation constants of the Zn II-coordinated water molecules, 8.38 ± 0.01 and 10.42 ± 0.02 for p K a1 and p K a2 respectively, were obtained by pH potentiometric titration at 298 ± 0.1 K. The kinetics of p-nitrophenyl acetate (pNA) hydrolysis catalyzed by ZnL(BCD) was carried out at pH 7.54–10.56 and 298 ± 0.1 K. The pH profile of observed hydrolysis rate constants catalyzed by ZnL(BCD) exhibits an exponential growth, indicating that the kinetic process is controlled by two acid–base equilibriums. Obtained kinetic results are in good agreement with thermodynamic data for hydroxyl active species in ZnL(BCD) system. Second-order rate constants of 0.15 and 2.64 M −1 s −1 were calculated for mono- and di-hydroxyl active species in ZnL(BCD) system, respectively. Compared with an unassembled host or guest analogue, Zn(dien) (dien = diethylenetriamine), the supramolecular system exhibits much higher catalytic activity.