Adaptive Halide Binding with Anion Size: Triggering Chiral D3 to Trigonal Prism Changes in a Molecular Cage

We report the IR spectroscopy and structural analysis of the halide ions F-, Cl-, and Br- bound inside a bicyclic amide-based cryptand cage (1) under cryo-cooled conditions in the gas phase. We record infrared spectra of the cryo-cooled gas-phase ions in the hydride stretch and fingerprint regions using IR-UV double resonance methods. The IR spectra in the NH and CH stretch regions are remarkably simple, reflecting highly symmetric structures for all three 1·X- complexes. We compare the experimental spectra with calculated candidate structures using first-principles predictions of the infrared spectra in the NH, CH, and fingerprint regions. We assign the 1·F- complex to a 9-coordinate, D3 symmetry structure that contains six NH···F- and three aromatic CH···F- H-bonds, much as it does in the crystalline solid, with a trigonal twist angle α = 35°. While the gas-phase and X-ray structures of 1·F- are similar, the crystal structure is compressed by its surroundings, with the distance between the two bridgehead tertiary amines that is 0.58 Å shorter than our assigned gas-phase structure. In contrast, the 1·Cl- and 1·Br- complexes are uniquely assigned based on their infrared spectra to C3h symmetry structures in which the six amide N's form a 6-coordinate trigonal prism (α = 0°) binding pocket, while the two tertiary N's form caps on the prism's triangular faces. To accommodate these larger halide ions, the three aromatic rings rotate so that their closest CH groups are nearly tangential to the spherical halide ion at the cage's center.