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

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(-)).