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6.1 Conclusions

The work presented in this thesis has primarily focussed on the synthetic modification of trialdehyde and diamine precursors towards the preparation of novel POCs, with an emphasis on tuning their gas sorption and separation properties.

CC14 was isolated after purification of a cage product mixture generated upon scrambling the cage precursors TFB, Me3TFB and R,R-CHDA. Packing in a

window-to-window and isoreticular fashion to its parent cage CC3, this asymmetric cage possessed a constricted interior cavity and PLE as a result of the incorporated methyl groups occupying accessible space within the pore structure. Although this resulted in lower gas uptakes, kinetic measurements demonstrated the ability of

CC14 to hinder the diffusion of Xe to a greater extent than that observed in CC3, making it a potential candidate for the separation of Xe from Kr gas during the reprocessing of UNF.

Enantiomerically-pure derivatives of CHDA were also prepared and successfully reacted with TFB in the preparation of new POC molecules. CC16 was isolated as a phase-pure [4+6] cage, with the introduction of twelve peripheral methyl groups frustrating the molecular packing, generating additional extrinsic porosity and improving the gas sorption properties in comparison to CC3, including an apparent BET surface area of 1023 m2 g-1. Alternatively, the introduction of twelve peripheral hydroxyl groups made cage formation difficult, with the poor solubility of the resulting cage CC17 making its preparation, purification and crystallisation a challenge. Adopting a protection-deprotection strategy via the TBDMS- functionalised cage CC18, CC17 could be isolated, although the gas sorption properties of this cage in the amorphous state did not exhibit any selectivity towards a particular gas. Utilising a commercially-sourced diamine which possessed bulky ethanoanthracene functionality, CC19 was successfully synthesised and characterised. Isolated as an [8+12] cage, it is the largest imine bond-based cage prepared to date that is derived from the TFB precursor. However, the inherent flexibility of the bonds resulted in structural collapse upon desolvation.

In a separate investigation, CC1β and CC3 were demonstrated to be suitable candidates for the capture and separation of Xe and Kr under conditions mimicking those experienced in the reprocessing of UNF, with the measured Xe uptake and

144 Xe/Kr selectivity for both cages outperforming the previous-best porous material. In the case of CC3, the unprecedented performance was a direct consequence of the dimensions of its interconnected 3-D diamondoid pore network in relation to the Xe guest, with the findings validating complementary MD simulations.

6.2 Future Outlook

Despite the field of POCs still being relatively young, developments have been rapid, with cages of varying composition, stoichiometry, size and porosity being prepared in the intervening years. With respect to the cages prepared and presented in this thesis, employing derivatives of traditional precursors has been shown to successfully alter the properties in comparison to their parent cage. Although more sterically-demanding TFB derivatives could not be incorporated into the cage structure, synthetically modifying the diamine precursor was demonstrated to be an effective derivatisation strategy. Introducing other functional groups1-3 onto cyclised diamine precursors will help build on the results presented herein and may lead to materials with superior properties. In the case of CC17, a more detailed investigation would be beneficial in finding a suitable route to its isolation, whether that be through elucidating conditions using the original diol diamine, or finding a protection-deprotection strategy that takes advantage of improved stabilities inferred upon post-synthetic “tying”.4 CC17 is interesting due to the peripheral hydroxyl functionalities, which could possibly enable the cage to be reacted with metal salts in the preparation of cage-MOF materials,5 allow chemical attachment to a surface in the preparation of chiral chromatography columns6 or permit other PSM strategies to tune the gas sorption properties.

Future imine bond-based cages may benefit from a move away from the traditional trialdehyde-diamine approach, with the majority of precursors now being assessed. For example, the use of aldehydes based on the structures of certain extended tetratopic MOF linkers may lead to cages of alternate shapes and packing arrangements. Other approaches could involve the construction of robust architectures from carbon-carbon bonds, following the example of Avellaneda et al.7

or employing alkyne metathesis, which would allow thermodynamic control of the reaction products.8 Alternatively, utilising bonds which have been successfully employed in the preparation of COFs, including nitroso,9 hydrazone10 and boroxine11

145 linkages, could be implemented in the formation of more structurally or chemically robust POCs.

With respect to Xe/Kr separation, future materials prepared specifically for this application need to adhere to the criteria outlined by Banerjee et al.,12 with narrow pores of uniform width, which are large enough to accommodate a single Xe atom, being a critical prerequisite. Due to the difficulty of predicting how a POC molecule will self-assemble and subsequently pack in the solid state, synthesising new cages specifically for this purpose is not trivial. Therefore, the use of simulation studies to guide towards and identify promising candidates, which has been successfully implemented for MOFs, will prove crucial.

6.3 References

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3. Y. N. Belokon, J. Fuentes, M. North and J. W. Steed, Tetrahedron, 2004, 60, 3191-3204.

4. M. Liu, M. A. Little, K. E. Jelfs, J. T. A. Jones, M. Schmidtmann, S. Y. Chong, T. Hasell and A. I. Cooper, J. Am. Chem. Soc., 2014, 136, 7583-7586. 5. R. A. Smaldone, R. S. Forgan, H. Furukawa, J. J. Gassensmith, A. M. Z. Slawin, O. M. Yaghi and J. F. Stoddart, Angew. Chem. Int. Ed., 2010, 49, 8630-8634.

6. A. Kewley, A. Stephenson, L. Chen, M. E. Briggs, T. Hasell and A. I. Cooper, Chem. Mater., 2015, 27, 3207-3210.

7. A. Avellaneda, P. Valente, A. Burgun, J. D. Evans, A. W. Markwell-Heys, D. Rankine, D. J. Nielsen, M. R. Hill, C. J. Sumby and C. J. Doonan, Angew. Chem., 2013, 52, 3746-3749.

8. Q. Wang, C. Zhang, B. C. Noll, H. Long, Y. Jin and W. Zhang, Angew. Chem. Int. Ed., 2014, 53, 10663-10667.

9. D. Beaudoin, T. Maris and J. D. Wuest, Nat. Chem., 2013, 5, 830-834.

10. F. J. Uribe-Romo, C. J. Doonan, H. Furukawa, K. Oisaki and O. M. Yaghi,

J. Am. Chem. Soc., 2011, 133, 11478-11481.

11. A. P. Côté, A. I. Benin, N. W. Ockwig, M. O'Keeffe, A. J. Matzger and O. M. Yaghi, Science, 2005, 310, 1166-1170.

12. D. Banerjee, A. J. Cairns, J. Liu, R. K. Motkuri, S. K. Nune, C. A. Fernandez, R. Krishna, D. M. Strachan and P. K. Thallapally, Acc. Chem. Res., 2015, 48, 211-219.