Anisotropic thermal expansion in a metal-organic framework

Area of Science:

• Materials Science
• Crystallography
• Chemistry

Background:

• Metal-organic frameworks (MOFs) are advanced porous materials with tunable properties.
• Understanding the thermal expansion behavior of MOFs is crucial for their application in various environments.
• Ionic liquids offer unique reaction media for synthesizing novel MOF structures.

Purpose of the Study:

• To synthesize and characterize a new manganese-based metal-organic framework (MOF).
• To investigate the thermal expansion properties of the synthesized MOF.
• To analyze the structural interactions within the MOF, including framework-cation interactions.

Main Methods:

• Ionothermal synthesis using Mn(II)(acetate)2·4H2O and 1,3,5-benzenetricarboxylic acid (H3BTC) in ionic liquids.
• Single-crystal and powder X-ray diffraction for structural determination and thermal expansion analysis.
• Multi-temperature studies (15–400 K) to observe anisotropic thermal expansion.
• Hirshfeld surface analysis to investigate framework-cation interactions.

Main Results:

• Formation of a new MOF, EMIM[Mn(II)BTC], crystallizing in the orthorhombic space group Pbca.
• Demonstration of strongly anisotropic thermal expansion, with significant expansion along the a- and b-axes and negative thermal expansion along the c-axis at elevated temperatures.
• Observation of coupled framework deformation during thermal expansion, leading to expansion in two directions and contraction in the third.
• Identification of weak interactions and governing hydrogen-bonding interactions between the framework and the EMIM cation via Hirshfeld surface analysis.

Conclusions:

• The synthesized MOF, EMIM[Mn(II)BTC], exhibits complex anisotropic thermal expansion behavior.
• The rigidity of the BTC linker plays a key role in coupling expansion and contraction within the framework.
• Understanding these thermal properties is essential for designing MOFs for specific applications.