Anisotropic Thermal Expansion of a Cationic Copper‐Based Metal–Organic Framework

ABSTRACT

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)(H ₂ O)](NO ₃ )·(H ₂ O) _0.5 } _n (Cu‐MOF), assembled from the flexible hinge‐like ligand 1,1’‐[1,4‐phenylenebis(methylene)]bis(3,5‐dicarboxypyridinium) (H ₄ LCl ₂ ) 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. NO ₃ ⁻ motion alters the hydrogen bond with coordinated water, thereby affecting Cu─O coordination. Changes in L ²⁻ , bpe, and NO ₃ ⁻ are linked through changes in the CuO ₃ N ₂ 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.