Joint Experimental and Computational Investigation of the Flexibility of a Diacetylene-Based Mixed-Linker MOF: Revealing the Existence of Two Low-Temperature Phase Transitions and the Presence of Colossal Positive and Giant Negative Thermal Expansions.

摘要: Solvothermal reaction in N,N-dimethylformamide (DMF) between 1,6-bis(1-imidazolyl)-2,4-hexadiyne monohydrate (L1·H2O), isophthalic acid (H2L2), and Zn(NO3)2·6 H2O gives the diacetylene-based mixed-ligand coordination polymer {[Zn(L1)(L2)](DMF)2}n (UMON-44) 38% yield. Combination of DSC with variable-temperature single-crystal X-ray diffraction revealed occurrence two phase transitions spanning ranges 129-144 K 158-188 K. Furthermore, three structurally similar phases UMON-44 show giant negative and/or colossal positive thermal expansions. These unusual phenomena exist without any change contents unit cell. DFT calculations using PBE+D3 dispersion scheme were able to distinguish these polymorphs by reproducing accurately their salient structural features, although corrections size cell turned out be necessary for high-temperature account its large expansion. In addition, infrared spectra (vibration frequencies peak intensities) theoretical models calculated, allowing univocal identification corresponding polymorphs. Last, limits our computational method tested calculating transition temperatures associated enthalpies, derived figures compare favorably values determined experimentally.