Selective Anion Recognition by a Dynamic Quadruple Helicate

An M2L4 quadruple helicate, formed by wrapping four molecules of 1,4-bis(3-pyridyloxy)benzene (L-1) about two palladium(II) centers, is shown to bind anions within its internal cavity. (HNMR)-H-1 exchange experiments provide a quantitative measure of anion selectivity and reveal a preference for ClO4- over the other tetrahedral anions BF4- and ReO4- and the octahedral anion PF6-. X-ray crystal structures of [Pd-2(L-1)(4)](4+) helicates containing ClO4, BF4- and I- reveal that the cavity size can dynamically change in response to the size of the guest.