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Molecular shape sorting using molecular organic cages

Abstract

The energy-efficient separation of chemical feedstocks is a major sustainability challenge. Porous extended frameworks such as zeolites or metal–organic frameworks are one potential solution to this problem. Here, we show that organic molecules, rather than frameworks, can separate other organic molecules by size and shape. A molecular organic cage is shown to separate a common aromatic feedstock (mesitylene) from its structural isomer (4-ethyltoluene) with an unprecedented perfect specificity for the latter. This specificity stems from the structure of the intrinsically porous cage molecule, which is itself synthesized from a derivative of mesitylene. In other words, crystalline organic molecules are used to separate other organic molecules. The specificity is defined by the cage structure alone, so this solid-state ‘shape sorting’ is, uniquely, mirrored for cage molecules in solution. The behaviour can be understood from a combination of atomistic simulations for individual cage molecules and solid-state molecular dynamics simulations.

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Figure 1: Molecules to separate molecules.
Figure 2: Synthesis of organic cages.
Figure 3: Solid-state shape selectivity can be understood in terms of the structure and dynamics of discrete, isolated cage molecules.
Figure 4: The diamondoid pore network for crystalline CC3 carries the steric signature of the discrete cage building units.
Figure 5: Solid-state shape selectivity and chromatographic separations using crystalline porous organic cages.
Figure 6: Solid-state molecular simulations reveal diffusion mechanisms for the organic guests.

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Acknowledgements

The authors acknowledge funding from the EPSRC (EP/H000925/1) and the Leverhulme Trust (F/00025/Al). A.C. is a Royal Society Wolfson award holder. The authors thank J.T.A. Jones for assistance with NMR measurements.

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Authors

Contributions

A.C. conceived the project. T.M., D.A. and A.C. designed the experiments. T.M. prepared the cages and carried out the sorption experiments. K.J. conceived the modelling strategy and performed the molecular simulations. M.S. solved the single-crystal structures and S.C. refined the powder X-ray diffraction data. T.M. and A.A. performed the breakthrough experiments.

Corresponding author

Correspondence to Andrew I. Cooper.

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The authors declare no competing financial interests.

Supplementary information

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Supplementary information (PDF 2052 kb)

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Supplementary movie 1 (WMV 4433 kb)

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Supplementary movie 2 (MOV 12923 kb)

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Supplementary movie 3 (MOV 24790 kb)

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Supplementary movie 4 (MOV 6705 kb)

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Crystallographic data for the complex between mesitylene and compound CC3 (CIF 56 kb)

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Crystallographic data for the complex between meta-xylene and compound CC3 (CIF 88 kb)

Supplementary information

Crystallographic data for the complex between para-xylene and compound CC3 at 100K (CIF 42 kb)

Supplementary information

Crystallographic data for the complex between para-xylene and compound CC3 at 295K (CIF 41 kb)

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Mitra, T., Jelfs, K., Schmidtmann, M. et al. Molecular shape sorting using molecular organic cages. Nature Chem 5, 276–281 (2013). https://doi.org/10.1038/nchem.1550

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