Stepwise Halide-Triggered Double and Triple Catenation of Self-Assembled Coordination Cages

A simple self-assembled [Pd2L4] coordination cage consisting of four carbazole-based ligands was found to dimerize into the interpenetrated double cage [3X@Pd4L8] upon the addition of 1.5equivalents of halide anions (X=Cl-, Br-). The halide anions serve as templates, as they are sandwiched by four Pd-II cations and occupy the three pockets of the entangled cage structure. The subsequent addition of larger amounts of the same halide triggers another structural conversion, now yielding a triply catenated link structure in which each Pd-II node is trans-coordinated by two pyridine donors and two halide ligands. This simple system demonstrates how molecular complexity can increase upon a gradual change of the relative concentrations of reaction partners that are able to serve different structural roles.