Pressure-Induced Structural Effects in the Square Lattice (sql) Topology Coordination Network Sql-1-Co-NCS·4OX

Area of Science:

• Materials Science
• Crystallography
• Supramolecular Chemistry

Background:

• Switching coordination networks (sql) exhibit phase transitions between porous and nonporous states.
• These materials are of interest for gas adsorption due to high working capacity and recyclability.
• Previous studies showed significant interlayer expansion in sql-1-Co-NCS·4OX under ambient conditions.

Purpose of the Study:

• To investigate pressure-induced structural changes in a tetragonal phase (Phase I) of sql-1-Co-NCS·4OX up to 1 GPa.
• To understand the behavior of adsorbed o-xylene (OX) under high pressure.
• To elucidate the relationship between framework conformation, voids, and molecular packing under compression.

Main Methods:

• High-pressure X-ray diffraction studies up to approximately 1 GPa.
• Use of penetrable pressure transmitting media, including o-xylene (OX) and methanol/ethanol mixtures.
• Analysis of electron counts to track the evacuation of OX molecules.

Main Results:

• Phase I of sql-1-Co-NCS·4OX, under pressure, underwent compression and released adsorbed OX molecules, rather than further framework separation.
• Tetragonal symmetry was maintained, but interlayer voids significantly reduced between 0.3 and 0.5 GPa, with disorder above 0.5 GPa.
• Multistep evacuation of OX molecules was confirmed, suggesting the existence of two intermediate phases (Ia and Ib) with varying OX loading.

Conclusions:

• High pressure induces compression and guest molecule (OX) release in sql-1-Co-NCS·4OX, rather than framework expansion.
• The study provides insights into the pressure-dependent structural dynamics of porous coordination networks.
• The findings contribute to understanding the behavior of these materials for potential applications in separation and storage.