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. Issue 7 (4th January 2018) - Duarte Rodrigues, Alysson
Abstract: Solvothermal reaction in N, N ‐dimethylformamide (DMF) between 1, 6‐bis(1‐imidazolyl)‐2, 4‐hexadiyne monohydrate (L1⋅ H2 O), isophthalic acid (H2 L2 ), and Zn(NO3 )2 ⋅ 6 H2 O gives the diacetylene‐based mixed‐ligand coordination polymer {[Zn(L1 )(L2 )](DMF)2 } n (UMON‐44 ) in 38 % yield. Combination of DSC with variable‐temperature single‐crystal X‐ray diffraction revealed the occurrence of two phase transitions spanning the ranges 129–144 K and 158–188 K. Furthermore, the three structurally similar phases ofUMON‐44 show giant negative and/or colossal positive thermal expansions. These unusual phenomena exist without any change in the contents of the unit cell. DFT calculations using the PBE+D3 dispersion scheme were able to distinguish between these polymorphs by accurately reproducing their salient structural features, although corrections in the size of the unit cell turned out to be necessary for the high‐temperature phase to account for its large thermal expansion. In addition, the infrared spectra (vibration frequencies and peak intensities) of these theoretical models were calculated, allowing for univocal identification of the corresponding polymorphs. Last, the limits of our computational method were tested by calculating the phase transition temperatures and their associated enthalpies, and the derived figures compare favorably with the values determined experimentally. Abstract : UMON‐44, a diacetylene‐based mixed‐ligand coordination polymer of empirical formula {[Zn(L1 )(L2 )](DMF)2 } n (L1 =1, 6‐bis(1‐imidazolyl)‐2, 4‐hexadiyne;L2 =isophthalate anion), has been investigated by means of X‐ray diffraction, IR spectroscopy, DSC, and DFT methods. This MOF exhibits a phase transition from Pn 21 a to P 21 21 21 in the range 129–144 K, and a second phase transition from P 21 21 21 to Pnma in the range 158–188 K. These three structurally similar phases show giant negative and/or colossal positive thermal expansions. These phenomena exist in the material without changing the amount of entrapped solvent or its nature, a situation that is not widespread in MOF chemistry.