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 L2- shortens the effective length of L2-, 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 L2-, 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.