Double-Helical Chain Directed by Sulfate and Phosphate Tetramers

The assembly of helical structures with artificial molecules has been one of the focuses of supramolecular chemistry, while anion-based helical structures, especially infinite helical chains, have rarely been reported. Herein we present a strategy to assemble double-strand helical chains based on the anion coordination (hydrogen bonding in nature) of a C₂-symmetric bis-monourea ligand L¹, with a sulfate anion (2) or dihydrogen phosphate tetramers (3). Both of the assemblies were elucidated by single-crystal structures. Moreover, a control single-crystal structure, 1, which was formed by one ligand L¹ and two chloride ions, was also obtained for comparison, highlighting the necessity of the four coordination sites of the sulfate anion and dihydrogen phosphate tetramers for assembling the double-strand helical chains. The sulfate-based double-strand helix 2 was featured with absolute chirality as a result of the self-resolving crystallization. Within the chain-like structure of 2, the V-shaped configuration of L¹ led to the formation of cavities that are suitable for encapsulating TEA⁺ cations through C–H···π interactions, thus making helix 2 appear as an infinite molecular train. Similarly, the anion-cluster dihydrogen phosphate tetramer, as an enlarged analogy of the sulfate anion, also providing four oxygen-based binding sites, induced another infinite molecular train 3 with the bigger TPA⁺ cations loaded in the cavities. In contrast with the chiral structure of 2, the structure of 3 is mesomeric, showing the sensitivity of the anionic helix toward the anion node.