C-H-pi interactions in 1-n-butyl-3-methylimidazolium tetraphenylborate molten salt: solid and solution structures

Area of Science:

• Ionic Liquids
• Supramolecular Chemistry
• Materials Science

Background:

• Understanding the structure-property relationships of ionic liquids is crucial for their application.
• 1-n-butyl-3-methylimidazolium tetraphenylborate is a representative ionic liquid whose solution behavior requires detailed investigation.

Purpose of the Study:

• To elucidate the inter-ionic interactions and solution structures of 1-n-butyl-3-methylimidazolium tetraphenylborate in different solvents.
• To investigate the influence of solvent polarity on ion pairing and aggregation phenomena.

Main Methods:

• Crystal structure analysis to identify initial inter-ionic interactions.
• Nuclear Magnetic Resonance (NMR) spectroscopy, including 1H NOESY, to probe solution-phase ion contacts.
• 13C-NMR relaxation time measurements to determine apparent ionic volumes and aggregation behavior.

Main Results:

• Crystal structure reveals significant C-H-pi interactions between the imidazolium cation and tetraphenylborate anion.
• In [D6]DMSO, the ionic liquid exists as solvent-separated ion pairs, with minimal cation-anion interaction observed via NMR.
• In CDCl3, concentration-dependent NMR spectra and NOESY data indicate the formation of contact ion pairs and larger aggregates due to C-H-pi interactions and hydrogen bonding.

Conclusions:

• The solvent plays a critical role in determining the ion pairing state and aggregation behavior of 1-n-butyl-3-methylimidazolium tetraphenylborate.
• C-H-pi interactions are prevalent in both solid and solution states, influencing the observed structures.
• The formation of aggregates in less polar solvents like CDCl3 is driven by these specific intermolecular interactions.