Intramolecular C-H•••F hydrogen bonding-induced 1,2,3-triazole-based foldamers

This paper reports the first series of artificial secondary structures that are induced by intermolecular C-H center dot center dot center dot F hydrogen bonding. Oligomers that contain two four, six, and eight 1,2,3-triazole units have been designed and prepared by connecting the neighbouring 1,2,3-triazole units with 4,6-difluoro-m-phenylene linker(s). Two triphenylmethyl groups are appended at the ends of the backbones to suppress the stacking of the backbones and provide solubility. X-Ray analysis and H-1-NMR and two-dimensional heteronuclear Overhauser enhancement spectroscopic (HOFSY) experiments support that the 1 2,3-triazote backbones adopt folded or helical conformations due to the formation of continuous three-centred C-H center dot center dot center dot F hydrogen bonding. Quantum chemical calculations reveal that the longest 8-mer foldamer can form a one-turn helical cavity with a diameter of ca. 1.7 nm. Halide anion competition experiments show that the intramolecular C H F hydrogen bonding is more stable than the well-established intermolecular C-H center dot center dot center dot X- (X = Cl and I) hydrogen bonding.