Anisotropic Thermal Expansion of a Cationic Copper-Based Metal-Organic Framework

Metal-organic frameworks (MOFs) provide a versatile platform for investigating negative thermal expansion (NTE), owing to their modular design and inherent framework flexibility. A cationic {[Cu(L)_0.5(bpe)(H₂O)](NO₃)·(H₂O)_0.5}_n (Cu-MOF), assembled from the flexible hinge-like ligand 1,1'-[1,4-phenylenebis(methylene)]bis(3,5-dicarboxypyridinium) (H₄LCl₂) and 4,4'-vinylenedipyridine (bpe), is investigated as a model system for anisotropic thermal expansion. Variable-temperature powder x-ray diffraction (VT-PXRD) from 100 to 300 K demonstrates distinct anisotropy in the thermal expansion behavior of Cu-MOF, with positive expansion along the a- and c-axes and negative expansion along the b-axis. Variable-temperature single-crystal x-ray diffraction (VT-SCXRD) reveals that, with increasing temperature, hinge-like deformation of L^2- shortens the effective length of L^2-, leading to contraction of the lattice along the b-axis and expansion along the a- and c-axes. Transverse bpe vibrations provide flexibility for the framework adjustment along the a and c directions. NO₃ ^- motion alters the hydrogen bond with coordinated water, thereby affecting Cu─O coordination. Changes in L^2-, bpe, and NO₃ ^- are linked through changes in the CuO₃N₂ coordination geometry, collectively contributing to the anisotropic thermal expansion of the Cu-MOF. This work highlights how ligand flexibility and guest-framework interactions influence thermal expansion, offering new guidance for the design of MOFs with tunable thermal expansion properties.