Elucidation of Diverse Solid‐State Packing in a Family of Electron‐Deficient Expanded Helicenes via Microcrystal Electron Diffraction (MicroED)**
Adrian E. Samkian, Gavin R. Kiel, Christopher G. Jones, Harrison M. Bergman, Julia Oktawiec, Hosea M. Nelson, T. Don Tilley
Abstract
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. Herein, 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 behaviors, including four different space groups, homochirality in a crystal for a helicene with an extremely low enantiomerization barrier, and nanometer scale cavities.