Cooperative Loading and Release Behavior of a Metal–Organic Receptor

In order to design artificial chemical
systems that are capable
of achieving complex functions, it is useful to design synthetic receptors
that mimic their biological counterparts. Biological functions are
underpinned by properties that include specific binding with high
affinity and selectivity, cooperativity, and release triggered by
external stimuli. Here we show that a metal–organic receptor
constructed through subcomponent self-assembly can selectively and
cooperatively load and release oxocarbon anions. The flexible coordination
spheres of its cadmium­(II) centers allow the receptor to dynamically
adjust its structure upon exchanging four triflate or triflimide counterions
for two oxocarbon anions, resulting in strong cooperativity and very
tight binding, with an apparent association constant for C5O52– of 5 × 1010 M–1. Substituting the cadmium­(II) ions for copper­(I)
by switching solvent prompted a structural reorganization and release
of the oxocarbon anions. Its cooperative behavior allows the receptor
to carry a greater payload than would be possible in a noncooperative
analogue.