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Distal conformational locks on ferrocene mechanophores guide reaction pathways for increased mechanochemical reactivity

Abstract

Mechanophores can be used to produce strain-dependent covalent chemical responses in polymeric materials, including stress strengthening, stress sensing and network remodelling. In general, it is desirable for mechanophores to be inert in the absence of force but highly reactive under applied tension. Metallocenes possess potentially useful combinations of force-free stability and force-coupled reactivity, but the mechanistic basis of this reactivity remains largely unexplored. Here, we have used single-molecule force spectroscopy to show that the mechanical reactivities of a series of ferrocenophanes are not correlated with ring strain in the reactants, but with the extent of rotational alignment of their two cyclopentadienyl ligands. Distal attachments can be used to restrict the mechanism of ferrocene dissociation to proceed through ligand ‘peeling’, as opposed to the more conventional ’shearing’ mechanism of the parent ferrocene, leading the dissociation rate constant to increase by several orders of magnitude at forces of ~1 nN. It also leads to improved macroscopic, multi-responsive behaviour, including mechanochromism and force-induced cross-linking in ferrocenophane-containing polymers.

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Fig. 1: Schematic illustration of the proposed mechanochemical dissociation mechanisms and their relationship to metallocene structure.
Fig. 2: FCP polymers used in this study and their respective behaviour in SMFS experiments.
Fig. 3: The relationship between FCP structure and the structural changes that accompany ligand dissociation.
Fig. 4: Correlation between side-chain angle at ligand dissociation and the SMFS plateau force.
Fig. 5: Multifunctional mechanochemical responses of cis-3FCP.

Data availability

All the data generated and/or analysed during the current study are available as Supplementary Information, and the datasets supporting Fig. 2b,c through the Duke Research Data Repository (https://doi.org/10.7924/r4gq6z428).

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Acknowledgements

The polymer synthesis, SMFS studies and mechanistic analysis formed a part of work supported by the National Science Foundation under grant no. CHE-1904016 to C.T. and S.L.C. The bulk mechanochromism and cross-linking studies formed a part of work supported by the US Army Research Laboratory and the Army Research Office under grant W911NF-15-0143 to S.L.C. In addition, C.T. acknowledges partial support from the National Science Foundation EPSCoR Program under grant no. OIA-1655740. The authors thank P. Zhang for help with the DFT calculations.

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Y.Z. and S.L.C. conceived and designed the experiments. Y.Z., Z.W., Y.S. and Y.L. performed the synthesis. Z.W. and T.B.K. collected the AFM data. Y.Z., Z.W., T.B.K., Y.S. and C.T. analysed the data. L.S. and M.F. performed the mechanical testing. Y.S. and E.X. performed the DFT calculations. Y.Z., Y.S., C.T. and S.L.C. wrote the manuscript. All authors discussed the results and commented on the manuscript.

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Correspondence to Stephen L. Craig.

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Supplementary Information

Supplementary Discussion, Figs. 1–47 and Tables 1–18.

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Zhang, Y., Wang, Z., Kouznetsova, T.B. et al. Distal conformational locks on ferrocene mechanophores guide reaction pathways for increased mechanochemical reactivity. Nat. Chem. 13, 56–62 (2021). https://doi.org/10.1038/s41557-020-00600-2

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