Modulation of Rotational Dynamics in Halogen-Bonded Cocrystalline Solids

Dynamicprocesses are responsible for the functionality of a rangeof materials, biomolecules, and catalysts. We report a detailed systematicstudy of the modulation of methyl rotational dynamics via the directand the indirect influence of noncovalent halogen bonds. For thispurpose, a novel series of cocrystalline architectures featuring halogenbonds (XB) to tetramethylpyrazine (TMP) is designed and prepared usinggas-phase, solution, and solid-state mechanochemical methods. Single-crystalX-ray diffraction reveals the capacity of molecular bromine as wellas weak chloro-XB donors to act as robust directional structure-directingelements. Methyl rotational barriers (E (a)) measured using variable-temperature deuterium solid-state NMR rangefrom 3.75 & PLUSMN; 0.04 kJ mol(-1) in 1,3,5-trichloro-2,4,6-trifluorobenzene & BULL;TMPto 7.08 & PLUSMN; 0.15 kJ mol(-1) in 1,4-dichlorotetrafluorobenzene & BULL;TMP. E (a) data for a larger series of TMP cocrystalsfeaturing chloro-, bromo-, and iodo-XB donors are shown to be governedby a combination of steric and electronic factors. The average numberof carbon-carbon close contacts to the methyl group is foundto be a key steric metric capable of rationalizing the observed trendswithin each of the Cl, Br, and I series. Differences between eachseries are accounted for by considering the strength of the & sigma;-holeon the XB donor. One possible route to modulating dynamics is thereforevia designer cocrystals of variable stoichiometry, maintaining thecore chemical features of interest between a given donor and acceptorwhile simultaneously modifying the number of carbon close contactsaffecting dynamics. These principles may provide design opportunitiesto modulate more complex geared or cascade dynamics involving largerfunctional groups.