Halide Size-Selective Binding by Cucurbit[5]uril–Alkali Cation Complexes in the Gas Phase

We report data that suggest complexes with alkali cations capping the portals of cucurbit[5]­uril (CB[5]) bind halide anions size-selectively as observed in the gas phase: Cl^– binds inside the CB[5] cavity, Br^– is observed both inside and outside, and I^– binds weakly outside. This is reflected in sustained off-resonance irradiation collision-induced dissociation (SORI-CID) experiments: all detected Cl^– complexes dissociate at higher energies, and Br^– complexes exhibit unusual bimodal dissociation behavior, with part of the ion population dissociating at very low energies and the remainder dissociating at significantly higher energies comparable to those observed for Cl^–. Decoherence cross sections measured in SF₆ using cross-sectional areas by Fourier transform ion cyclotron resonance techniques for [CB[5] + M₂X]⁺ (M = Na, X = Cl or Br) are comparable to or less than that of [CB[5] + Na]⁺ over a wide energy range, suggesting that Cl^– or Br^– in these complexes are bound inside the CB[5] cavity. In contrast, [CB[5] + K₂Br]⁺ has a cross section measured about 20% larger than that of [CB[5] + Na]⁺, suggesting external binding that may correspond with the weakly bound component seen in SORI. While I^– complexes with alkali cation caps were not observed, alkaline earth iodides with CB[5] yielded complexes with cross sections 5–10% larger than that of [CB[5] + Na]⁺, suggesting externally bound iodide. Geometry optimization at the M06-2X/6-31+G* level of ab initio theory suggests that internal anion binding is energetically favored by approximately 50–200 kJ mol^–1 over external binding; thus, the externally bound complexes observed experimentally must be due to large energetic barriers hindering the passing of large anions through the CB[5] portal, preventing access to the interior. Calculation of the barriers to anion egress using MMFF//M06-2X/6-31+G* theory supports this idea and suggests that the size-selective binding we observe is due to anion size-dependent differences in the barriers.