American Chemical Society
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pseudo[1]Catenane-Type Pillar[5]thiacrown Whose Planar Chiral Inversion is Triggered by Metal Cation and Controlled by Anion

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posted on 2018-07-05, 00:00 authored by Eunji Lee, Huiyeong Ju, In-Hyeok Park, Jong Hwa Jung, Mari Ikeda, Shunsuke Kuwahara, Yoichi Habata, Shim Sung Lee
Chiral inversion of single molecules has been a challenging task because chirality information controls structures and functions of various molecules, artificial nanostructures, DNA, and proteins. Herein we present a pseudo[1]­catenane-type molecule whose planar chiral inversion is driven by a metal ion under the control of anions for the first time. Considering an in–out equilibrium of a fused thiacrown and the soft metal binding, pillar[5]­thiacrown (rac-L) was synthesized. Two planar-chiral enantiomers of rac-L (in-pS-L and in-pR-L) were isolated and the absolute configuration was determined by circular dichroism and single crystal X-ray analysis. The in-pS-L recognizes Hg2+ to trigger the chiral inversion to out-pR-L, to our surprise; it takes place only in the presence of ClO4 or NO3 among the anions used. In the mercury­(II) perchlorate complex solution, anion-exchange from ClO4 to I or removal of Hg2+ by addition of S2– makes the system reversible. The crystallographic approach reveals that the anions act as coordination mode-directing species (endo- or exo-coordination) which play a decisive role on the chiral inversion. For instance, the week coordinating ClO4 allows Hg2+ to locate inside the thiacrown (endo-coordination) which causes the chiral inversion from in-pS-L to out-pR-L due to the expansion of the thiacrown unit upon endo-mode complexation. Oppositely, the strong coordinating I takes Hg2+ out of the thiacrown (exo-coordination) without large conformational changes of the thiacrown, resulting in no chiral inversion. A series of experimental works was also accomplished with the other enantiomer in-pR-L, which afforded identical results. Consequently, the chiral inversion is governed by steric factors that arise from the coordination modes depending on the coordinating ability of anions. This work demonstrates the first chiral inversion induced by combination of metal ion and anion and presents a new perspective on the supramolecular coordination chemistry of pillar­[n]­arenes.