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Making helical structures by spontaneous processes

Helical structures are inherently chiral; they have a right- or left-handed helicity. If we can obtain helical structures in a simple one-step procedure from a linear organic molecule, the conversion would be useful to regulate molecular functions that are associated with the helical structures. We have been investigating such a spontaneous helix formation process by taking advantage of the labile character of metal ions. We designed a series of linear chain-like molecules that can form helical structures upon metal complexation.

The design of oligo(salen)-type ligands for spontaneous helix formation

The molecules have a structure in which several salen-type coordination sites are linked together. When d-block metal ions are added, formation of four coordination bonds at each salen-type coordination site makes the entire molecule adopt a helical structure. Since the resultant helical metal complexes have a lot of phenoxo oxygen atoms directing inside, they are expected to strongly bind to additional metal ions.

The schematic animation of spontaneous helix formation from an oligo(salen) ligand by metal complexation.
[Hover to animate]
A helical metal complex　that is expected to be obtained from an acyclic oligo(salen) ligand

We found that the salen-oligomer ligands (n = 2,3,4) formed a helical structure upon complexation with zinc(II) acetate when a hard metal ion (Ca2+, lanthanide, etc.) is present. The hard metal ions are strongly bound in the helical cavity surrounded by oxygen atoms. The helical complexes existed as an equilibrated mixture of right- and left-handed forms, and the complex having longer oligomer ligands underwent slower helix inversion. The helix inversion rates also depended on the central metal ions. For example, the [LZn3La]3+ underwent much slower helix inversion than [LZn3Ba]2+. Such a dynamic feature of the helical metal complexes would be useful to design a system in which the structural conversion can dynamically change various kinds of chiral functions.

Helical metal complexes obtained by the multiple complexation of oligo(salen)-type ligands (n=2,3,4). Ca2+, Lanthanide(III), etc. were used as the guest.
The crystal structure of helical Zn2Eu complex obtained by complexation of bis(salen) ligand. [Hover to animate]
The crystal structure of helical Zn3La complex obtained by complexation of tris(salen) ligand. [Hover to animate]



[References]
“Novel Synthetic Approach to Trinuclear 3d-4f Complexes: Specific Exchange of the Central Metal of a Trinuclear Zinc(II) Complex of a Tetraoxime Ligand with a Lanthanide(III) Ion” Akine, S.; Taniguchi, T.; Nabeshima, T. Angew. Chem. Int. Ed. 2002, 41, 4670-4673.
doi:10.1002/anie.200290011
“Ca2+- and Ba2+-Selective Receptors Based on Site-Selective Transmetalation of Multinuclear Polyoxime-Zinc(II) Complexes” Akine, S.; Taniguchi, T.; Saiki, T.; Nabeshima, T. J. Am. Chem. Soc. 2005, 127, 540-541.
doi:10.1021/ja046790k
“Guest-dependent Inversion Rate of a Tetranuclear Single Metallohelicate” Akine, S.; Taniguchi, T.; Matsumoto, T.; Nabeshima, T. Chem. Commun. 2006, 4961-4963.
doi:10.1039/b610641b
“Helical Metallohost-Guest Complexes via Site-Selective Transmetalation of Homotrinuclear Complexes” Akine, S.; Taniguchi, T.; Nabeshima, T. J. Am. Chem. Soc. 2006, 128, 15765-15774.
doi:10.1021/ja0646702
“Synthesis of Acyclic Tetrakis- and Pentakis(N2O2) Ligands for Single-helical Heterometallic Complexes with a Greater Number of Winding Turns” Akine, S.; Matsumoto, T.; Sairenji, S.; Nabeshima, T. Supramol. Chem. 2011, 23, 106-112.
doi:10.1080/10610278.2010.514906