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

Abstract

Solvothermal reaction in N,N-dimethylformamide (DMF) between 1,6-bis(1-imidazolyl)-2,4-hexadiyne monohydrate (L1⋅H O), isophthalic acid (H L2), and Zn(NO ) ⋅6 H O gives the diacetylene-based mixed-ligand coordination polymer {[Zn(L1)(L2)](DMF) } (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 of UMON-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.

Linker Substituents Control the Thermodynamic Stability in Metal–Organic Frameworks

“…Metal–organic frameworks (MOFs) are advanced materials whose modular structure permits combining high microporosity with a range of functional properties. − Whereas MOF development has led to sophisticated functional materials, achieving thermodynamic stability by design remains a challenge. − For example, while the sensitivity of MOFs to hydrolytic degradation by water is a topic of high interest, , most studies have focused on kinetic aspects of the process, e.g., by introducing hydrophobic substituents − or changing MOF design to increase the strength of metal–linker bonds. − In contrast, thermodynamic driving forces behind MOF stability remain largely unexplored and poorly understood. We previously explored the relationship between the thermodynamic stability and the topology of zeolitic imidazolate frameworks (ZIFs), − a class of MOFs with topological diversity akin to zeolites. − This enabled quantitative evaluation of the structure-related energy differences between MOF polymorphs and indicated periodic density functional theory (DFT) − with semiempirical dispersion correction (SEDC) − as an accurate tool to calculate the relative thermodynamic stabilities of MOFs.…”

Colossal Positive and Negative Axial Thermal Expansion Induced by Scissor-like Motion of a Two-Dimensional Hydrogen Bonded Network in an Organic Salt

“…Therefore, materials with controlled thermal expansion properties is highly desired in a variety of technological applications. , However, the design of molecules with controlled thermal expansion is very challenging because positive thermal expansion (PTE) of materials is a normal phenomenon in which most of the materials expand upon heating owing to the increase of the anharmonic vibration of bonds. There are some materials that contract upon heating or vice versathis phenomenon is commonly known as negative thermal expansion (NTE). ,, Recent development in NTE material has attracted significant interest due its potential applications in various fields such as thermomechanical actuators, cookware, sensors, and other applications. , In addition, NTE materials have been used to achieve controlled thermal expansion property in composite materials by mixing of NTE with PTE materials in a certain stoichiometric ratio. − Although NTE has been mostly observed in a number of oxide-based compounds, ,, framework materials including some metal cyanides − and three-dimensional (3D) and isotropic NTE has not been found in any organic material until date. Very few single-component organic materials are known for one- or two-dimensional (1D or 2D) NTE. − The fundamental problem to design new molecules with NTE properties is the understanding of the underlying mechanism which varies from material to material.…”

Pentafluorophenylphosphonic Acid as a New Building Block for Molecular Crystal Fabrication

“…Then, the overall quality of the calculations relative to the X-ray structure can be evaluated by optimization of all atom positions in the experimental unit cell, starting from the H-optimized structure. In this approach, the energy variation comes solely from the heavier atoms that are correctly positioned by X-ray diffraction, a large relaxation implying a poor description of the chemical bonds by DFT (provided that complicating factors such as phase transitions 87 are absent). This optimization leads to structures that are only 300 meV/f.u.…”