Low-temperature enantiotropic k2 phase transition in the ionic 222-cryptand complex with LiClO4

Area of Science:

• Supramolecular Chemistry
• Crystal Engineering
• Computational Chemistry

Background:

• The macrobicyclic polyether 222-cryptand is known for its ability to encapsulate cations.
• Understanding the structural dynamics of cryptand-cation complexes is crucial for host-guest chemistry.
• Phase transitions in molecular solids can significantly alter their properties.

Purpose of the Study:

• To investigate the crystallographic and structural properties of a Li+ complex with 222-cryptand and perchlorate.
• To characterize the phase transition behavior of the Li+-222-cryptand-ClO4- complex.
• To explore the influence of temperature on the positional disorder of the Li+ cation within the cryptand cavity.

Main Methods:

• Single-crystal X-ray diffraction at 100 K and 200 K.
• Variable-temperature crystallographic analysis to identify phase transitions.
• Density Functional Theory (DFT) computations to model cation-cryptand interactions.

Main Results:

• A reversible second-order phase transition was observed at 253(2) K.
• The crystal structure transitions from an ordered P2(1)2(1)2(1) space group at 100 K to a disordered P2(1)2(1)2 (2c'=c) unit cell above 253 K.
• Significant dynamic positional disorder of the Li+ cation and perchlorate anion was observed at room temperature, which is gradually frozen out upon cooling.

Conclusions:

• The Li+-222-cryptand-ClO4- complex undergoes a temperature-induced structural phase transition.
• The observed positional disorder of the Li+ cation is temperature-dependent, with dynamic disorder persisting even at 100 K.
• DFT calculations suggest that the solid-state cation position below 253 K represents a local minimum, while the gas phase favors a more symmetrical conformation with the cation centered.