Molecular design of a ''molecular syringe'' mimic for metal cations using a 1,3-alternate calix[4]arene cavity

A 1,3-alternate calix[4]arene bearing a nitrogen-containing crown cap at one side and a bis(ethoxyethoxy) group at another side has been synthesized. H-1 NMR spectroscopic studies showed that Ag+ is bound to the crown-capped side (log K-ass. = 9.78: CD2Cl2:CD3OD = 4:1 v/v,30 degrees C), and the dissociation of Ag+ from this cavity is very slow. When the nitrogen atom in the crown ring is protonated with trifluoroacetic acid, Ag+ is pushed out to the bis(ethoxyethoxy) side through a pi-basic tube of the 1,3-alternate calix[4]arene. The dissociation of the complex from the bis(ethoxyethoxy) side occurs relatively fast. On the other hand, when the nitrogen . H+ in the crown ring is deprotonated with Li2CO3 and diazabicycloundecene, Ag+ is sucked back to the crown-capped side through the pi-basic tube. These chemically-switchable actions well imitate the function of a ''syringe'', using the pi-basic tube as a pipette and the crown ring as a rubber cap. We believe that this prototype of a ''molecular syringe'' is a novel molecular architecture for the action of metal cations.