Strong and Selective Anion Binding within the Central Cavity of Molecular Knots and Links.

A molecular pentafoil knot and doubly and triply entwined [2]catenanes based on circular Fe(II) double helicate scaffolds bind halide anions in their central cavities through electrostatic and CH···X(-) hydrogen-bonding interactions. The binding is up to (3.6 ± 0.2) × 10(10) M(-1) in acetonitrile (for pentafoil knot [2·Cl](PF6)9), making these topologically complex host molecules some of the strongest synthetic noncovalent binders of halide anions measured to date, comparable in chloride ion affinity to silver salts.