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.

Automated summary

Dynamic processes of materials, biomolecules, and catalysts can be modulated by noncovalent halogen bonds. A study on modulating methyl rotational dynamics using direct and indirect influences of halogen bonds is reported. The study includes the design and preparation of cocrystalline architectures featuring halogen bonds to tetramethylpyrazine (TMP). The results show that the behavior of chloride, bromide, and iodide halogen bond donors can be explained by steric and electronic factors.