Modulation of Rotational Dynamics in Halogen-Bonded Cocrystalline Solids

Abstract

Dynamic processes are responsible for the functionality of a range of materials, biomolecules, and catalysts. We report a detailed systematic study of the modulation of methyl rotational dynamics via the direct and the indirect influence of noncovalent halogen bonds. For this purpose, a novel series of cocrystalline architectures featuring halogen bonds (XB) to tetramethylpyrazine (TMP) is designed and prepared using gas-phase, solution, and solid-state mechanochemical methods. Single-crystal X-ray diffraction reveals the capacity of molecular bromine as well as weak chloro-XB donors to act as robust directional structure-directing elements. Methyl rotational barriers (E a) measured using variable-temperature deuterium solid-state NMR range from 3.75 ± 0.04 kJ mol–1 in 1,3,5-trichloro-2,4,6-trifluorobenzene·TMP to 7.08 ± 0.15 kJ mol–1 in 1,4-dichlorotetrafluorobenzene·TMP. E a data for a larger series of TMP cocrystals featuring chloro-, bromo-, and iodo-XB donors are shown to be governed by a combination of steric and electronic factors. The average number of carbon–carbon close contacts to the methyl group is found to be a key steric metric capable of rationalizing the observed trends within each of the Cl, Br, and I series. Differences between each series are accounted for by considering the strength of the σ-hole on the XB donor. One possible route to modulating dynamics is therefore via designer cocrystals of variable stoichiometry, maintaining the core chemical features of interest between a given donor and acceptor while simultaneously modifying the number of carbon close contacts affecting dynamics. These principles may provide design opportunities to modulate more complex geared or cascade dynamics involving larger functional groups.

σ-Hole Effect-Induced Electroluminescence of Halogen Cocrystals for Determination of Iodide in Seawater

“…Halogen bonds (XBs) are noncovalent interactions formed between the electron-deficient halogen atoms and Lewis bases. Compared with conventional hydrogen bonds, XBs exhibit more intense noncovalent interactions and excellent directionality, making them a powerful complementary tool in the cocrystal engineering self-assembly toolbox . The large positively electric region at the tip of the halogen atoms is often referred to as σ-hole. , The σ-hole effect has emerged as a powerful tool for constructing novel structures with new reactivity. , In their interrelationship, halogen bonds mediate the σ-hole-based charge transfer .…”

Triple Activation Facilitates Modular and Stoichiometric Control over Cocrystals of Phenazine

“…While cocrystallizations have provided several productive avenues for the rational design of new solid forms, − they have also proved to be an efficient way of designing and exploring complex landscapes of competing noncovalent interactions (NCIs). − A deeper understanding of these delicate and reversible NCIs and their associated molecular recognition principles dictates the scope of accessible complex supramolecular architectures. − …”