Cooperativity and Complexity in the Binding of Anions and Cations to a Tetratopic Ion-Pair Host

Cooperative
interactions play a very important role in both natural
and synthetic supramolecular systems. We report here on the cooperative
binding properties of a tetratopic ion-pair host 1. This
host combines two isophthalamide anion recognition sites with two
unusual “half-crown/two carbonyl” cation recognition
sites as revealed by the combination of single-crystal X-ray analysis
of the free host and the 1:2 host:calcium cation complex, together
with two-dimensional NMR and computational studies. By systematically
comparing all of the binding data to several possible binding models
and focusing on four different variants of the 1:2 binding model,
it was in most cases possible to quantify these complex cooperative
interactions. The data showed strong negative cooperativity (α
= 0.01–0.05) of 1 toward chloride and acetate
anions, while for cations the results were more variable. Interestingly,
in the competitive (CDCl3/CD3OD (9:1, v/v))
solvent, the addition of calcium cations to the tetratopic ion-pair
host 1 allosterically switched “on” chloride
binding that is otherwise not present in this solvent system. The
insight into the complexity of cooperative interactions revealed in
this study of the tetratopic ion-pair host 1 can be used
to design better cooperative supramolecular systems for information
transfer and catalysis.