Sulfonatocalixarene Counterion Exchange Binding Model in Action: Metal-Ion Catalysis Through Host-Guest Complexation

Abstract

p-Sulfonatocalixarene water soluble macrocyclic host receptors are known to form cooperative ternary complexes with complementary organic guest and metal cations. This property may be explored to enhance the interaction of weak nitrogen ligands with metal cations in a confined space showing some resemblance to metal-containing enzymes. However, the best of our knowledge, catalytic potential of this property remains unexplored. In this work the Ni2+ catalyzed hydrolysis of a picolinate ester (2,4-dinitrophenyl picolinate, 1) was used as a model reaction to evaluate the effect of sulfonatocalixarene macrocycles in the kinetics of this reaction. The results show that the host molecules promote the reaction through simultaneous complexation of the metal cation and the substrate and, in the case of the larger calixarenes containing more basic phenol groups, substantially higher rate enhancements are observed owing to additional assistance provided by base/nucleophilic catalysis. However, due the ionic nature of these receptors auto-inhibition of the reaction is observed at higher concentrations due counterion (Na+) binding that competes with the catalytically active Ni2+ -complexes.