Spacer Length-Independent Shuttling of the Pillar[5]arene Ring in Neutral [2]Rotaxanes

Tomoki Ogoshi; Daisuke Kotera; Shungo Nishida; Takahiro Kakuta; Tada-Aki Yamagishi; Albert M Brouwer

doi:10.1002/chem.201800104

Spacer Length-Independent Shuttling of the Pillar[5]arene Ring in Neutral [2]Rotaxanes

Chemistry. 2018 Apr 25;24(24):6325-6329. doi: 10.1002/chem.201800104. Epub 2018 Mar 8.

Authors

Tomoki Ogoshi^{1

2}, Daisuke Kotera¹, Shungo Nishida¹, Takahiro Kakuta¹, Tada-Aki Yamagishi¹, Albert M Brouwer³

Affiliations

¹ Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan.
² WPI Nano Life Science Institute, Kanazawa University, Kakuma-machi, Kanazawa, 920-1192, Japan.
³ van't Hoff Institute for Molecular Sciences, University of Amsterdam, P.O. Box 94157, 1090 GD, Amsterdam, The Netherlands.

Abstract

For a series of neutral [2]rotaxanes consisting of a pillar[5]arene ring and axles possessing two stations separated by flexible spacers of different lengths, the free energies of activation for the ring shuttling between the stations were found to be independent of the spacer length. The constitution of the spacer affects the activation energies: replacement of CH₂ groups by repulsive oxygen atoms in the axle increases the barrier. The explanation for the observed length-independence lies in the presence of a barrier for re-forming the stable co-conformation, which makes the ring travel back and forth along the thread in an intermediate state.

Keywords: NMR spectroscopy; macrocyclic compounds; molecular dynamics; pillar[5]arenes; rotaxanes.