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

Abstract

This work demonstrates a new nonconventional ligand design, imidazole/pyridine-based nonsymmetrical ditopic ligands (1 and 1S ), to construct a dynamic open coordination cage from nonsymmetrical building blocks. Upon complex formation with Pd2+ at a 1:4 molar ratio, 1 and 1S initially form mononuclear PdL4 complexes (Pd2+ (1)4 and Pd2+ (1S )4 ) without formation of a cage. The PdL4 complexes undergo a stoichiometrically controlled structural transition to Pd2 L4 open cages ((Pd2+ )2 (1)4 and (Pd2+ )2 (1S )4 ) capable of anion binding, leading to turn-on anion binding. The structural transitions between the Pd2 L4 open cage and the PdL4 complex are reversible. Thus, stoichiometric addition (2 equiv) of free 1S to the (Pd2+ )2 (1S )4 open cage holding a guest anion ((Pd2+ )2 (1S )4 ⋅G- ) enables the structural transition to the Pd2+ (1S )4 complex, which does not have a cage and thus causes the release of the guest anion (Pd2+ (1S )4 +G- ).