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

Area of Science:

• Supramolecular Chemistry
• Host-Guest Chemistry
• Physical Chemistry

Background:

• Cucurbit[5]uril (CB[5]) are macrocyclic hosts with a unique portal structure.
• Alkali and alkaline earth cations are known to cap the portals of CB[5].
• Understanding anion binding within these capped systems is crucial for molecular recognition.

Purpose of the Study:

• To investigate the size-selective binding of halide anions (Cl-, Br-, I-) to alkali cation-capped CB[5] complexes.
• To elucidate the factors governing internal versus external anion binding.
• To explore the role of kinetic barriers in host-guest complexation.

Main Methods:

• Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) experiments to probe complex stability.
• Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry to measure decoherence cross sections.
• Computational chemistry: *Ab initio* and molecular mechanics calculations (M06-2X/6-31+G*, MMFF) for geometry optimization and barrier calculations.

Main Results:

• Chloride anions bind internally within the CB[5] cavity, while iodide binds externally.
• Bromide exhibits bimodal dissociation, indicating both internal and external binding populations.
• Experimental and computational data suggest kinetic barriers, not thermodynamic favorability, dictate external binding for larger anions.

Conclusions:

• CB[5] complexes with alkali cation caps exhibit size-selective halide anion binding.
• Anion size-dependent kinetic barriers at the CB[5] portal govern the observed binding modes.
• This work provides insights into the dynamic processes controlling host-guest interactions in supramolecular systems.