Cage-To-Cage Transformations in Self-Assembled Coordination Cages Using "Acid/Base" or "Guest Binding-Induced Strain" as Stimuli

Abstract

Controlling supramolecular systems between different functional forms by utilizing acids/bases as stimuli is a formidable challenge, especially where labile coordination bonds are involved. A pair of acid/base responsive, interconvertible 1,5-enedione/pyrylium based Pd₂L₄-type cages are prepared that exhibit differential guest binding abilities towards disulfonates of varied sizes. A three-state switch has been achieved, where (i) a weakly coordinating base induced cage-to-cage transformation in the first step, (ii) a strongly coordinating base triggered cage disassembly as the second step, and (iii) the third step shows acid(strong) promoted generation of initial cage, thereby completing the cycle. To our surprise, binding of a specific disulfonate guest facilitated cage-to-cage transformations by inducing strain on the cage assembly thereby opening the labile pyrylium rings of the cage. Through a competitive guest binding study, we demonstrated the superior guest binding capability of the octacationic pyrylium-based cage over a similar-sized tetracationic cage. These results provide a reliable approach to reversibly modulate the guest binding properties of acid/base-responsive self-assembled coordination cages.

Keywords: Coordination cages; Host-Guest Chemistry; Self-assembly; Supramolecular chemistry; pH switch.