Dynamic Open Coordination Cage from Nonsymmetrical Imidazole–Pyridine Ditopic Ligands for Turn-On/Off Anion Binding

Article abstract of DOI:10.1002/anie.201911097

This work demonstrates a new nonconventional ligand design, imidazole/pyridine-based nonsymmetrical ditopic ligands (1 and 1^S), to construct a dynamic open coordination cage from nonsymmetrical building blocks. Upon complex formation with Pd²⁺ at a 1:4 molar ratio, 1 and 1^S initially form mononuclear PdL₄ complexes (Pd²⁺(1)₄ and Pd²⁺(1^S)₄) without formation of a cage. The PdL₄ complexes undergo a stoichiometrically controlled structural transition to Pd₂L₄ open cages ((Pd²⁺)₂(1)₄ and (Pd²⁺)₂(1^S)₄) capable of anion binding, leading to turn-on anion binding. The structural transitions between the Pd₂L₄ open cage and the PdL₄ complex are reversible. Thus, stoichiometric addition (2 equiv) of free 1^S to the (Pd²⁺)₂(1^S)₄ open cage holding a guest anion ((Pd²⁺)₂(1^S)₄?G^?) enables the structural transition to the Pd²⁺(1^S)₄ complex, which does not have a cage and thus causes the release of the guest anion (Pd²⁺(1^S)₄+G^?).