Hairpin helicates: a missing link between double-helicates and trefoil knots.

We have prepared a novel ligand in which two qpy metal-binding domains are linked by a polyoxyethylene spacer. The new ligand reacts with copper(ii) salts to form dinuclear double helicates with a hairpin structure. The hairpin complex and a model complex have been structurally characterised. The hairpin structure is formed regioselectively as the head-to-tail (HT) stereoisomer. Detailed electrochemical and spectroelectrochemical studies of the complexes are reported.