Allosteric Binding of K+ to Crown Ether Macrocycles Appended to a Lanthanum Double Decker System

A benzocrown-appended lanthanum(III) porphyrin double decker has been synthesized which is capable of binding simple K+ in a cooperative, allosteric fashion to form a 1 : 4 complex. Analysis of this system by means of the Hill equation gave an overall binding constant (Kb) of 1.0 × 1014 (mol dm−3)−4 and a Hill coefficient of 4.0. This same system, however, binds Na+ in a conventional, linear fashion. These results indicate that the novel allosteric binding occurs as the result of 1 : 2 sandwich complexes between K+ and appended crown ether moieties of the two joined porphyrins. The suitability of lanthanum(III) over cerium(IV) for preparing this highly crowded double decker porphyrin is also noted.