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2020-09-07 - QuinoneHel - ChemRxiv.pdf (4.66 MB)
MicroED Elucidation of Diverse Solid-State Packing in a Family of Electron-Deficient Expanded Helicenes
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revised on 10.09.2020 and posted on 10.09.2020by Adrian Samkian, Gavin R. Kiel, Christopher G. Jones, Harrison Bergman, Julia Oktawiec, Hosea Nelson, T. Don Tilley
Solid-state packing plays a defining role in the properties
of a molecular organic material, but it is difficult to elucidate in the
absence of single crystals that are suitable for X-ray diffraction. Here, we
demonstrate the coupling of divergent synthesis with microcrystal electron
diffraction (MicroED) for rapid assessment of solid-state packing motifs, using
a class of chiral nanocarbons – expanded helicenes – as a proof of concept. Two
highly selective oxidative dearomatizations of a readily-accessible helicene
provided a divergent route to four electron-deficient analogues containing
quinone or quinoxaline units. Crystallization efforts consistently yielded microcrystals that were unsuitable for single
crystal X-ray diffraction, but ideal for MicroED. This technique facilitated
the elucidation of solid-state structures of all five compounds with
<1.1 Å resolution. The otherwise-inaccessible data revealed a range of notable
packing behavior, including four
different space groups, homochirality in a crystal for a helicene with an
extremely low enantiomerization barrier, and nanometer scale cavities. The
results of this study suggest that MicroED will soon become an indispensable tool for high-throughput investigations in
pursuit of next-generation organic materials.