Dynamic Helicity Control of Oligo(salamo)-Based Metal Helicates

Abstract

Much attention has recently focused on helical structures that can change their helicity in response to external stimuli. The requirements for the invertible helical structures are a dynamic feature and well-defined structures. In this context, helical metal complexes with a labile coordination sphere have a great advantage. There are several types of dynamic helicity controls, including the responsive helicity inversion. In this review article, dynamic helical structures based on oligo(salamo) metal complexes are described as one of the possible designs. The introduction of chiral carboxylate ions into Zn 3 La tetranuclear structures as an additive is effective to control the P/M ratio of the helix. The dynamic helicity inversion can be achieved by chemical modification, such as protonation/deprotonation or desilylation with fluoride ion. When (S)-2-hydroxypropyl groups are introduced into the oligo(salamo) ligand, the helicity of the resultant complexes is sensitively influenced by the metal ions. The replacement of the metal ions based on the affinity trend resulted in a sequential multistep helicity inversion. Chiral salen derivatives are also effective to bias the helicity; by incorporating the gauche/anti transformation of a 1,2-disubstituted ethylene unit, a fully predictable helicity inversion system was achieved, in which the helicity can be controlled by the molecular lengths of the diammonium guests.

Self-assembled stereomutation with supramolecular chirality inversion

“…The exchange of metal ions and/ or anions can modulate the chirality of supramolecular polymers and their corresponding aggregates by metal coordination and electrostatic interactions. 7,78 In 1969, Tachibana and Kamara found that metal cations can induce handedness inversion of nanoaggregates in the selfassociation of D/L-12-hydroxystearic acid and its alkali soaps in various solvents. 79 The D-and L-acids assembled into M-and P-type twisted aggregates, respectively, while the Li soap aggregates into the oppositely handed twists to those of the corresponding acid.…”

“…The exchange of metal ions and/or anions can modulate the chirality of supramolecular polymers and their corresponding aggregates by metal coordination and electrostatic interactions. 7,78…”

Evaluation of the stereoselectivity for titanium(IV)-based coordination entities induced by the enantiopure diphenylethene-1,2-diamine ligand

“…Another way to achieve the stereocontrol of metallo-supramolecular structures implies the coordination of chiral organic additives to coordination sites that do not participate in the construction of the helical framework of the molecule [9]. However, this strategy is much more confidential in the case of helicates [10], as most of the reported helicates contain metal ions where all the coordination sites are occupied by ligating atoms belonging to the organic strands conferring the helical structure of the complex.…”

Self-Healing Metallacycle-Cored Supramolecular Polymers Based on a Metal–Salen Complex Constructed by Orthogonal Metal Coordination and Host–Guest Interaction with Amino Acid Sensing

“…Metal complexes of salen-type ligands derived from Schiff bases have aroused long-term concern due to their ubiquitous use in selective adsorption, enantioselective separation, and catalytic chemical transformations, such as epoxidation of olefins, aziridination of styrene, cyclopropanation of olefins, and so on. − Simultaneously with the development of synthetic approaches to this kind of compounds, an interest emerged in adjusting their properties through various physical or chemical modifications. Particularly, incorporation of metal–salen complexes into supramolecular assemblies is expected to afford the corresponding materials with more different characteristics, such as photophysical, magnetic, electronic, and catalytic properties …”