Abstract
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)2·6H2Ogives the diacetylene-based mixed-ligand coordination polymer{[Zn(L1)(L2)](DMF)2}n (UMON-44) in 38% yield. Combinationof DSC with variable-temperature single-crystal X-raydiffraction revealed the occurrence of two phase transitionsspanning the ranges 129–144 K and 158–188 K. Furthermore,the three structurally similar phases of UMON-44 show giantnegative and/or colossal positive thermal expansions. Theseunusual phenomena exist without any change in the contentsof the unit cell. DFT calculations using the PBE+D3dispersion scheme were able to distinguish between thesepolymorphs by accurately reproducing their salient structuralfeatures, although corrections in the size of the unit cellturned out to be necessary for the high-temperature phaseto account for its large thermal expansion. In addition, theinfrared spectra (vibration frequencies and peak intensities)of these theoretical models were calculated, allowing forunivocal identification of the corresponding polymorphs.Last, the limits of our computational method were tested bycalculating the phase transition temperatures and their associatedenthalpies, and the derived figures compare favorablywith the values determined experimentally.