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

A new beta-cyclodextrin dimer, 2,2'-bipyridyl-4,4'-dimethyl-bridged-bis(6-monodeoxy-6-ammonio-beta-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 pK(a1) 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. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.