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Synthesis of contra-helical trefoil knots with mechanically tuneable spin-crossover properties


Molecular knots attract attention on account of their topological intricacies and potential application. Tying molecular knots with different topologies, on larger length scales, remains challenging, not to mention the difficulties with harnessing their topological characteristics in order to modulate their properties. Here, we report a general approach to construct torus knots from two coaxially nested multistranded contra-helices. As a proof of concept, a series of two iron(II)-templated contra-helical trefoil knots have been synthesized near-quantitatively in one step. Among these, one features a long trefoil knot—a 111-atom closed loop that is ~11 nm long. The thermally induced spin crossover of the two iron(II) centres in each knot can be modulated in opposing directions by changing the intramolecular mechanical strain. The synthesis of molecular knots with mechanically tuneable properties enables the unleashing of their stimuli-responsive multifunctionalities. One of these molecular knots exhibits, during crystallization, narcissistic self-sorting, which allows the manual separation of enantiomers. A purely organic trefoil knot, obtained by reductive demetallation of its precursor, is also characterized in the solid state by X-ray crystallography.

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Fig. 1: Assembling a torus knot in a contra-helical tubular manner by nesting coaxially a small multistranded helix inside a larger reverse helix.
Fig. 2: Syntheses of the trefoil knots TK3, TK4, TK6 and TK7 as well as reduction of TK4 to produce a demetallated fully organic trefoil knot TK4D.
Fig. 3: Single-crystal X-ray (super)structures of the trefoil knots TK3, TK4, TK6 and TK7 as well as CD spectra for TK4 and TK6.
Fig. 4: Single-crystal X-ray structures of the wholly organic trefoil knot TK4D.
Fig. 5: SCO investigation of TK3, TK4 and TK6 by VT 1H NMR spectroscopy, VT magnetic susceptibility measurement and VT X-ray crystallography.

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Data availability

Crystallographic data for the structures reported in this Article are available from the Cambridge Crystallographic Data Centre with the following codes: TK3-100K (CCDC 2143936), TK3-273K (CCDC 2143937), TK4-100K (CCDC 2143938), TK4-273K (CCDC 2143939), Λ-(+)-TK6-100K (CCDC 2143940), Λ-(+)-TK6-273K (CCDC 2143941), Δ-(−)-TK6-100K (CCDC 2143942), Δ-(−)-TK6-273K (CCDC 2143943), TK4D-100K (CCDC 2143944) and TK7-100K (CCDC 2151636). Other data that support the findings of this study are available in the paper and Supplementary Information.


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We are grateful for financial support from the National Natural Science Foundation of China (grant nos. 22171232 and 21971211), the Natural Science Foundation of Zhejiang Province (grant no. 2022XHSJJ007), the Qiantang River Talent Foundation (grant no. QJD1902029) and Westlake University. We thank X. Lu and X. Shi, X. Miao, Z. Chen and C. Zhang for their help in recording NMR spectra, X-ray data collection of diffraction dots, CD spectroscopy and magnetic measurement, respectively. We thanks X. Lin and C. Wu for their very helpful discussion on magnetism. This research was supported by both the Instrumentation and Service Center for Molecular Science and the Instrumentation and Service Center for Physical Science, as well as by Westlake University HPC Center. We also thank the staff of the BL17B beamline of National Facility for Protein Science in Shanghai at Shanghai Synchrotron Radiation Facility for assistance during data collection.

Author information

Authors and Affiliations



Z.L. and L.W. conceived the idea, designed the research and produced the manuscript. L.W. and Z.L. carried out experiments and analysed the data. L.W. contributed to NMR spectroscopic analysis. L.W., M.T. and L.J. contributed to X-ray crystallographic analyses. Y.C. and J.L. contributed to mass spectrometric analyses. Z.L. is the principal investigator of the Laboratory for Supramolecular Organic Functional Assemblies and supervised the research. L.B., S.W. and Y.L. discussed and commented on the manuscript.

Corresponding author

Correspondence to Zhichang Liu.

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Competing interests

Z.L. and L.W. are inventors on a Chinese patent application (Application No. CN202210972052.X). The remaining authors declare no competing interests.

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Peer review information

Nature Synthesis thanks Paul Kruger and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Alison Stoddart, in collaboration with the Nature Synthesis team.

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Supplementary information

Supplementary Information

Experimental procedures and characterization data. Supplementary Discussion, Figs. 1–79, Tables 1–19 and refs. 1–18

Supplementary Video 1

A video of the X-ray crystal structure of TK3.

Supplementary Video 2

A video of the X-ray crystal structure of TK4.

Supplementary Video 3

A video of the X-ray crystal structure of TK4D.

Supplementary Video 4

A video of the X-ray crystal structure of TK6.

Supplementary Video 5

A video of the X-ray crystal structure of TK7.

Supplementary Data 1

Crystallographic data for TK3-100K (CCDC 2143936).

Supplementary Data 2

Crystallographic data for TK3-273K (CCDC 2143937).

Supplementary Data 3

Crystallographic data for TK4-100K (CCDC 2143938).

Supplementary Data 4

Crystallographic data for TK4-273K (CCDC 2143939).

Supplementary Data 5

Crystallographic data for TK4D (CCDC 2143944).

Supplementary Data 6

Crystallographic data for Λ-(+)-TK6-100K (CCDC 2143940).

Supplementary Data 7

Crystallographic data for Λ-(+)-TK6-273K (CCDC 2143941).

Supplementary Data 8

Crystallographic data for Δ-(−)-TK6-100K (CCDC 2143942).

Supplementary Data 9

Crystallographic data for Δ-(−)-TK6-273K (CCDC 2143943).

Supplementary Data 10

Crystallographic data for TK7 (CCDC 2151636).

Source data

Source Data Fig. 3

Source data for CD spectra.

Source Data Fig. 5

Source data for VT NMR spectra and VT magnetism.

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Wu, L., Tang, M., Jiang, L. et al. Synthesis of contra-helical trefoil knots with mechanically tuneable spin-crossover properties. Nat. Synth 2, 17–25 (2023).

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