Selective Anion Recognition by a Dynamic Quadruple Helicate.

An M2 L4 quadruple helicate, formed by wrapping four molecules of 1,4-bis(3-pyridyloxy)benzene (L1 ) about two palladium(II) centers, is shown to bind anions within its internal cavity. 1 H NMR 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 [Pd2 (L1 )4 ]4+ helicates containing ClO4 , BF4 - and I- reveal that the cavity size can dynamically change in response to the size of the guest.