C–H hydrogen bond and halogen bond directed self-assembly of ethereal podands and C–X⋯F−/HF2− halogen bonding in solution

Abstract

A series of halogenated podands (P1–P8) have been synthesized from 1,3,5-tris(bromomethyl)mesitylene and halogen-substituted phenols to study the structure-directing roles of C–H hydrogen bonds and halogen bonds in the formation of self-assembled supramolecular frameworks. The ethereal podands are suitable for the study of C–H hydrogen bonds in the crystalline state due to the presence of three different types of C–H donors, such as methyl, methylene, and aromatic –CH groups, and three different types of acceptors such as ethereal oxygen, halogens, and aromatic rings. Single crystal X-ray structures of three halophenyl-functionalized podands (P2, P3 and P4) and a cyanophenyl-functionalized podand (P9) were determined at room temperature (298 K), and detailed Hirshfeld surface analysis of the crystal structures was performed to quantify the close contact contributions (in %) from different types of non-covalent interactions involved in the self-assembly of podands. The crystal structures of the halophenyl-functionalized podands (P2, P3, and P4) showed self-assembly primarily via intermolecular C–H⋯O, C–H⋯π, and halogen bonding interactions. Hirshfeld surface analysis of the crystal structures revealed significantly higher contributions from H⋯C and H⋯X (X = halogen) close contacts in comparison to H⋯O and X⋯C close contacts in 2D-fingerprint plots. The self-assembly of the cyanophenyl-functionalized podand (P9) was largely governed by intermolecular C–H⋯O and C–H⋯N interactions. The podand crystals showed relatively high thermal stability in thermogravimetry analysis (250–290 °C), which can be attributed to the hydrogen and halogen bond assisted formation of 3D supramolecular frameworks. ¹⁹F-NMR spectra of tetraethylammonium fluoride in the presence of an equivalent amount of tris(2-halophenoxymethyl)mesitylene podand (P5 or P6) showed a downfield shift of the fluoride signal (Δδ 3.2 ppm) indicating C–X⋯F⁻ halogen bonding in solution. Further, ¹⁹F-NMR spectra of hydrogen bifluoride (HF₂⁻) in the presence of P5 or P6 showed a large upfield shift of ≈21–22 ppm suggesting C–X⋯F⁻⋯H–F halogen bonding in solution.