Stimuli-responsive chirality inversion of metallohelices and related dynamic metal complexes

Abstract

Metal-coordination structures, which contain coordination bonds with a dynamic nature, are generally useful as a platform for stimuli-responsive molecules and materials. This feature was successfully utilized to obtain various kinds of dynamic chiral metal-containing structures that undergo a chirality inversion. This review illustrates the metal-containing helices and related chiral structures that can undergo stimuli-responsive chirality inversion. They are categorized into several structural types that allow different kinds of chirality inversions, e.g., the Δ/Λ chirality inversion of octahedral complexes and their self-assembled analogues, P/M chirality inversion of helical complexes, inversion of the P/M twist chirality of acyclic, macrocyclic, and cage-like complexes, etc. In principle, the chirality inversion described in this review is not a perfect conversion between enantiomers but diastereomers. The conversion basically involves the reversal of the dynamic chirality based on metal coordination while keeping the invariant carbon-centered chirality in the organic ligands. The chirality inversion is driven by the equilibrium shift caused by external stimuli, e.g., environmental changes such as solvent exchange, dissolution/crystallization, etc., or additives such as a proton, cation, organic molecule, etc., or chemical transformation such as redox reactions. In addition, strategies to control the response speeds and time profiles of the chirality inversion have been developed for some chiral metal-containing systems.