Effect of cyclodextrin dimers with bipyridyl and biphenyl linking groups on carboxyl ester hydrolysis catalyzed by zinc complex

Two new β-cyclodextrin dimers, biphenyl-4,4′-bis(6-monodeoxy-6-ammoniomethyl-β-cyclodextrin) (Host 1, H1) and 2,2′-bipyridyl-5,5′-bis(6-monodeoxy-6-ammoniomethyl-β-cyclodextrin) (Host 2, H2), were synthesized and further assembled with a zinc complex containing a hydrophobic group to construct two host–guest systems, ZnL(H1) and ZnL(H2) (L = 4-(4′-tert-butylbenzyl)diethylenetriamine), acting as supramolecular metallohydrolase models. In such system, the chemical equilibrium constants were determined by pH potentiometric titration at 298 ± 0.1 K. Two deprotonation constants of the Zn(II)-coordinated water, 8.68 ± 0.03 (pKa1) and 10.50 ± 0.04 (pKa2), were obtained respectively for the ZnL(H1) system whereas only one pKa1 value was obtained for ZnL(H2) (9.38 ± 0.02). The kinetics of p-nitrophenyl acetate (pNA) hydrolysis catalyzed by ZnL(H1) and ZnL(H2) were examined respectively from pH 7.56 to 10.56 at 298 ± 0.1 K. The pH profile of rate constant of pNA hydrolysis catalyzed by ZnL(H1) exhibits an exponential increase with second-order rate constants of 0.98 and 8.41 M−1 s−1 for mono- and di-hydroxyl active species, indicating a potent catalytic activity relative to the reported mononuclear and polynuclear Zn(II) species. However, the pH profile of rate constant of hydrolysis catalyzed by ZnL(H2) only shows a saturated kinetic behavior with the second-order rate constant of 1.26 M−1 s−1, suggesting a kinetic process controlled only by an acid–base equilibrium. All kinetic results are in good agreement with thermodynamic data for hydroxyl activated species in ZnL(H1) and ZnL(H2) systems.

Graphical abstractTwo new cyclodextrin dimers were synthesized and applied as supramolecular catalyst precursors, in which one hydrophobic cavity is assembled with zinc complex and another hydrophobic cavity is used to capture p-nitrophenyl acetate. Contrast to bipyridyl-linking system, biphenyl-linking cyclodextrin system exhibits higher catalytic activity and the second-order rate constant presents exponential growth with the increasing value of pH.Figure optionsDownload full-size imageDownload as PowerPoint slide