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

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 ZnII-coordinated water molecules, 8.38 ± 0.01 and 10.42 ± 0.02 for pKa1 and pKa2 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.

Graphical abstractA new β-cyclodextrin dimer was synthesized and further assembled with a zinc complex containing a hydrophobic group as a new supramolecular system of catalyzing ester hydrolysis. Thermodynamic properties of the supramolecular complex in solution and kinetics of catalyzing p-nitrophenyl acetate hydrolysis were investigated and obtained very good consistent results in hydroxyl active species. Compared with an unassembled host or guest analogue, the supramolecular complex exhibits much higher catalytic activity.Figure optionsDownload full-size imageDownload as PowerPoint slide