Picking on Carbonate: Kinetic Selectivity in the Encapsulation of Anions

Supramolecular hosts bind to inorganic anions at a fast rate and select them in proportion with thermodynamic stability of the corresponding [anion⊂host] complexes, forming in a reversible manner. In this study, we describe the action of hexapodal capsule 1 and its remarkable ability to select anions based on a large span of rates by which they enter this host. The thermodynamic affinity of 1 toward eighteen anions extends over eight orders of magnitude (0a <10⁸  M^-1 ; ¹ H NMR spectroscopy). The capsule would retain CO₃^2- (K_a =10⁷  M^-1 ) for hours in the presence of eleven competing anions, including stronger binding SO₄^2- , HAsO₄^2- and HPO₄^2- (K_a =10⁷ -10⁸  M^-1 ). The observed selection resulted from 1 possessing narrow apertures (ca. 3×6 Å) comparable in size to anions (d=3.5-7.1 Å) slowing down the encapsulation to last from seconds to days. The unorthodox mode of action of 1 sets the stage for creating hosts that pick anions by their ability to access the host.