Remarkable enhancement of Ca2+ affinity using a redox-switchable coordinating group

Abstract

Redox-switchable coordinating groups hold significant promise for metal purification, detection, and the design of stimuli-responsive materials. However, existing designs often involve complex structures where a redox-active metal interferes with the target metal interaction. In this study, we demonstrate the use of a simple organic motif, namely 2-nitropyridine, as a redox-switchable coordinating group. This group was conjugated with a diaza-crown ether to yield a ligand for Ca⁺² coordination. Under normal conditions, this ligand demonstrates weak metal interactions attributed to the electronic properties of the nitro groups. However, upon reduction, it transitions to a radical anion state with a strong affinity for Ca⁺². Notably, the required redox potential shifts depending on the metal ion present in the solution, dictated by the charge density of the complexed ion. This behavior facilitates the recognition of various metal ions in a solution, opening possibilities for applications in biological or industrial sensing.