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.