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A molecular endless (74) knot

Abstract

Current strategies for the synthesis of molecular knots focus on twisting, folding and/or threading molecular building blocks. Here we report that Zn(ii) or Fe(ii) ions can be used to weave ligand strands to form a woven 3 × 3 molecular grid. We found that the process requires tetrafluoroborate anions to template the assembly of the interwoven grid by binding within the square cavities formed between the metal-coordinated criss-crossed ligands. The strand ends of the grid can subsequently be joined through within-grid alkene metathesis reactions to form a topologically trivial macrocycle (unknot), a doubly interlocked [2]catenane (Solomon link) and a knot with seven crossings in a 258-atom-long closed loop. This 74 knot topology corresponds to that of an endless knot, which is a basic motif of Celtic interlace, the smallest Chinese knot and one of the eight auspicious symbols of Buddhism and Hinduism. The weaving of molecular strands within a discrete layer by anion-template metal–ion coordination opens the way for the synthesis of other molecular knot topologies and to woven polymer materials.

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Fig. 1: Synthesis of interwoven 3 × 3 grids [Zn916](BF4)18 and [Fe916](BF4)18, macrocycle 3, Solomon link 4 and 74 knot 2.
Fig. 2: 1H NMR spectra (600 MHz (except (f)), 298 K (except (f))) of building block 1, metal-coordinated intermediate [Zn916](BF4)18 and ring-closed products 2–4.
Fig. 3: Structure of 74 knot coordination complex [Fe92](BF4)18, based on the X-ray crystal structure of [Fe916](BF4)18 for the 3 × 3 grid region with cyclized end groups modelled using Merck molecular force field.
Fig. 4: Mechanism of the formation of unknot macrocycle 3, Solomon link 4 and 74 knot 2 by connecting strand ends within a 3 × 3 interwoven grid, [M916](BF4)18 (M = Zn(ii), Fe(ii)).

Data availability

Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre (http://www.ccdc.cam.ac.uk/) under CCDC number 2022144. These data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. All other data supporting the findings of this study are available within the paper and its Supplementary Information, or are available from the Mendeley data repository at https://doi.org/10.17632/g5b854f68w.1.

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Acknowledgements

We thank J. E. Beves and J.-F. Ayme (now University of New South Wales and BASF SE, respectively) for early ligand designs for this target topology; the Engineering and Physical Sciences Research Council (EPSRC; EP/P027067/1), the European Research Council (ERC; Advanced Grant no. 786630), and East China Normal University for funding; the EPSRC National Mass Spectrometry Service Centre for high-resolution mass spectrometry; the Diamond Light Source for synchrotron beam time on I19 (XR029); networking contributions from the COST Action CA17139, EUTOPIA and Alberto Valero for the video of the rotating 74 knot structure. D.A.L. is a Royal Society Research Professor.

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Contributions

J.J.D., S.D.P.F., J.-F.L. and S.L.W. carried out the synthesis and characterization studies. G.F.S.W. solved the crystal structure. D.A.L. directed the research. All the authors contributed to the analysis of the results and the writing of the manuscript.

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Correspondence to David A. Leigh.

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Supplementary information

Supplementary Information

Experimental procedures and characterization data. Supplementary discussion, Figs. 1–40, Tables 1–3, spectra 1–21 and references 1–21.

Supplementary Video 1

Movie of endless 74 knot based on the X-ray crystal structure of the 3 × 3 grid with Merck Mechanics Force Field (MMFF)-modelled cyclized end groups.

Supplementary Data

3 × 3 grid crystallographic data.

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Leigh, D.A., Danon, J.J., Fielden, S.D.P. et al. A molecular endless (74) knot. Nat. Chem. 13, 117–122 (2021). https://doi.org/10.1038/s41557-020-00594-x

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