First pseudorotaxane-like [3]complexes based on cryptands and paraquat: self-assembly and crystal structures

Area of Science:

• Supramolecular Chemistry
• Organic Synthesis
• Host-Guest Chemistry

Background:

• Cryptands are macrocyclic compounds known for their ability to bind guests.
• Understanding host-guest stoichiometry is crucial for designing molecular recognition systems.

Purpose of the Study:

• To synthesize a new cryptand, bis(1,3,5-phenylene)tri(1,4,7,10-tetraoxadecyl) (3a).
• To investigate the host-guest stoichiometry of the new cryptand with paraquat.
• To explore the factors influencing complex stoichiometry in cryptand systems.

Main Methods:

• Synthesis of cryptand 3a using pseudo-high dilution conditions.
• Complexation studies in solution using association constant measurements.
• Mass spectrometry for gas-phase stoichiometry determination.
• X-ray crystallography for solid-state structure analysis.

Main Results:

• Cryptand 3a was synthesized in good yield.
• A 1:1 host-guest complex stoichiometry was observed in acetone solution and the gas phase.
• Solid-state X-ray crystallography revealed a 2:1 stoichiometry for complexes of 3a and a related cryptand (3b) with paraquat.
• The guest molecule was found to partially occupy the cavities of two cryptand molecules in the solid state.

Conclusions:

• This study demonstrates for the first time that different stoichiometries can occur for the same host-guest pair in cryptand-based complexes.
• The observed variability in stoichiometry is dependent on the phase (solution, gas, or solid state).
• These findings have implications for the design of supramolecular assemblies with tunable properties.