Using Hydrogen-Bonds and Herringbone Packing to Design Interfaces of 4,4‘-Disubstituted meso-Hydrobenzoin Crystals. The Importance of Recognizing Unfavorable Packing Motifs

Five pure 4,4‘-disubstituted meso -hydrobenzoins and one solid solution have been investigated crystallographically. The aim was to establish whether a favorable hydrogen-bonding network and aromatic herringbone packing motif might make the lamellar structure shared by meso -hydrobenzoin (mHB) and meso -hydroanisoin sufficiently robust to serve as a reliable template on which to construct isomorphous surfaces containing various functional groups. Complete structures of the 4,4‘-difluoro-, 4,4‘-dichloro-, and 4,4‘-dibromo- meso -hydrobenzoins and an average structure of the disordered 4,4‘-dimethyl derivative were determined by X-ray diffration. Partial structures of twinned or disordered crystals of the 4,4‘-diethyl derivative and of the 1:1 mixed crystal of the 4,4‘-dimethyl and 4,4‘-dibromo derivatives were determined by morphological and optical studies supplemented by diffraction. While the fluoro and ethyl compounds preserve the complete layer structure of mHB, and the bromo and chloro compounds preserve most of its features, the methyl compound packs in a completely different structure. Unlike either of its pure components, the methyl/bromo solid solution packs in the mHB layer structure. Hydrogen bonding and herringbone packing do not overwhelmingly favor a constant layer motif. Competing influences due to size, charge, and O−H···X hydrogen bonding (X = Cl, Br) of substituents are discussed. For crystal design purposes it is important to identify unfavorable packing motifs, such as the side-by-side arrangement of parallel C−Cl or C−Br groups.