Multi-stimulus linear negative expansion of a breathing M(O₂CR)₄-node MOF

Area of Science:

• Materials Science
• Crystallography
• Chemistry

Background:

• Metal-organic frameworks (MOFs) are porous crystalline materials with diverse applications.
• Understanding the structural dynamics and guest interactions of MOFs is crucial for their targeted design.
• The (Me2NH2)2[Cd(NO2BDC)2] (SHF-81) MOF presents a unique twinned alpha-quartz-type structure.

Purpose of the Study:

• To characterize the structure and properties of the (Me2NH2)2[Cd(NO2BDC)2] (SHF-81) metal-organic framework.
• To investigate the gas adsorption behavior, particularly for CO2, and its relationship with structural changes.
• To explore the negative thermal expansion (NTE) and breathing phenomena in response to temperature and guest molecules.

Main Methods:

• Single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD) were employed to determine the crystal structure and monitor in situ structural changes.
• Gas adsorption isotherms (N2 and CO2) were measured at various temperatures to quantify uptake and determine adsorption enthalpies.
• In situ diffraction experiments under varying temperatures and gas pressures were conducted to observe dynamic structural responses.

Main Results:

• The MOF (SHF-81) crystallizes in a twinned alpha-quartz-type structure, closely related to beta-quartz.
• Significant CO2 uptake was observed at 273-298 K with an isosteric enthalpy of adsorption of -27.4 kJ mol-1.
• The MOF exhibits remarkable flexibility, showing breathing behavior with up to 12% changes in unit cell axes and linear negative thermal expansion (NTE) upon heating or CO2 adsorption.

Conclusions:

• SHF-81 is a flexible MOF with a quartz-like structure capable of significant CO2 adsorption.
• The observed breathing behavior and NTE are coupled with gas adsorption and temperature variations, indicating dynamic structural responses.
• These findings highlight the potential of SHF-81 for gas storage and separation applications, driven by its responsive framework.