Crystal Structure and Phase Transition of the C–H···F–H-Bonded Supramolecular Compound with 4-Nitroanilinium Based on 18-Crown-6
A novel inorganic-organic hybrid supramolecular macrocyclic compound, (4-nitroanilinium) (18-crown-6)(PF 6 )(1), was synthesized and characterized by infrared spectroscopy, thermogravimetric analysis, elemental analysis, differential scanning calorimetry (DSC), and single-crystal X-ray diffraction. Crystal 1 is found to comprise 1D C-H···F-P hydrogen-bonded chains of (4-nitroanilinium + ) (18-crown-6) supramolecular cations and PF 6 − anions. DSC measurements confirm that 1 undergoes a
... e phase transition at 255 K with a hysteresis width of 6 K. A strong dielectric response is observed above 250 K at a low frequency (500 Hz), suggesting the occurrence of proton transfer in the C-H···F-P hydrogen bonds. A precise analysis on the main packing and structural differences, as well as the changes in the intermolecular interaction between the low-and high-temperature phases, reveals that C-H···F-P hydrogen bonds are the main factors affecting phase transition and dielectric behavior. Crystals 2017, 7, 276 2 of 11 bonds of such structures are weaker than other chemical bonds, such as covalent bonds [18,19]. During temperature changes, the hybrid crystalline materials constructed by hydrogen bonds can easily undergo transformations, thereby leading to phase transition. Xiong et al. reported a novel mononuclear metal-organic compound, [Cu(Hdabco)(H 2 O)Cl 3 ], which forms a 3D network by N-H···O and O-H···O hydrogen bond interactions. The crystal displays a paraelectric-to-ferroelectric phase transition triggered by the motion of the anions or cations and proton transfer of the hydrogen bonds [    . Intermolecular hydrogen bonds also play an important role in the stabilization of the structure. In most cases, the most widely used hydrogen bond patterns are C-H···O, N-H···O, and O-H···O. By contrast, other heteroatoms with high negativity (e.g., fluorine) functioning as proton acceptors have rarely been explored and compared with oxygen or nitrogen atoms. Although fluorine ion acts as a strong proton acceptor, covalently bound fluorine is a weak intermolecular hydrogen bond acceptor [23, 24] . Preparing suitable materials with variable-temperature dielectric response, especially in relatively high-and low-frequency range, is useful for the identification of phase-transition materials. Pure inorganic compounds are well developed, but supramolecular compounds that possess preponderant inorganic and organic properties as phase transition materials are difficult to prepare [25, 26] . Previously, we reported an organic-inorganic hybrid supramolecular compound, [(4-nitroanilinium)(18-crown-6)][BF 4 ](CH 3 CN), which displays a 1D supramolecular chain structure by weak hydrogen bonds between the −NH 3 + moieties and −NO 2 of adjacent 4-nitroanilinium interaction through the cavity of 18-crown-6 molecule  . The dielectric response of the crystal correlates with the direction of hydrogen bonds, suggesting the occurrence of proton transfer in the hydrogen bonds. In the current study, an inorganic-organic hybrid phase-transition material, (4-nitroanilinium)(18-crown-6)(PF 6 ) (1), was synthesized and characterized. The crystal forms a 1D chain structure via C-H···F-P hydrogen bonds. The differential scanning calorimetry (DSC), single-crystal X-ray diffraction data, and dielectric constant measurements indicate that 1 undergoes a reversible phase transition from high-temperature (HT) phase to low-temperature (LT) phase with the same space group P2 1 /c. The driving force of the phase transition mainly originates from the proton transfer in C-H···F-P hydrogen bonds.