Phase transition and switchable dielectric behaviours in an organic–inorganic hybrid compound: (3-nitroanilinium)2(18-crown-6)2(H2PO4)2(H3PO4)3(H2O)

An organic–inorganic hybrid compound with an extensive three-dimensional (3D) crystal structure, (3-nitroanilinium)2(18-crown-6)2(H2PO4)2(H3PO4)3(H2O) (1), was synthesized under slow evaporation conditions. Differential scanning calorimetry measurements indicated that 1 underwent a reversible phase transition at ca 231 K with a hysteresis width of 10 K. Variable-temperature X-ray single-crystal diffraction revealed that the phase transition of 1 can be ascribed to coupling of pendulum-like motions of its nitro group with proton transfer in O–H···O hydrogen bonds of the 3D framework. The temperature dependence of its dielectric permittivity demonstrated a step-like change in the range of 150–280 K with remarkable dielectric anisotropy, making 1 a promising switchable dielectric material. Potential energy calculations further supported the possibility of dynamic motion of the cationic nitro group. Overall, our findings may inspire the development of other switchable dielectric phase transition materials by introducing inorganic anions into organic–inorganic hybrid systems.


Introduction
Phase transition compounds with electric, magnetic, and thermally switchable ON/OFF states usually provide an efficient strategy for the design of multifunctional switching materials such as ferroelectrics [1][2][3][4][5][6][7][8][9]. Generally, such physical responses always exhibit abrupt changes in the vicinity of a phase transition, giving rise to phase transition materials with potential applications in energy storage, temperature controlling, data communications and phase shifters [10][11][12][13][14][15][16][17][18][19]. Molecular switchable dielectrics typically have dielectric constants that vary between high and low states upon temperature change. For this reason, they are one of the most attractive and promising phase transition materials for application in modern multifunctional materials [20 -26]. In this regard, organic-inorganic hybrid compounds that combine desirable characteristics from both types of constituents, especially those that can be prepared into large-size crystals, may be an effective strategy for obtaining ideal phase transition materials [27 -33].
Several novel organic-inorganic hybrid phase transition materials with electrical switches have been synthesized [34 -38]. Previous studies have also demonstrated that freezing and reorientation of the organic cations and deformation of the anionic framework can easily induce phase transitions. To obtain molecular crystals that exhibit such phase transitions, complexes based on crown ether and its derivatives have recently been investigated [39 -46]. These materials are crucial not only because they comprise diverse molecule motions, thereby providing favourable conditions for reversible phase transitions; but also for their extensive utilization in supramolecular chemistry and crystal engineering for assembling multiple ionic compounds to produce an assortment of host -guest molecules [47 -49]. Tang et al. successfully demonstrated a class of perovskite structure clathrates, [Hcpa- (18-crown-6)] þ [ClO 4 ] 2 , where Hcpa represents the protonated cyclopentylamine cation, that display unusual multisequential reversible phase transitions accompanied by switchable dielectric behaviour. Sequential reversible phase transitions and symmetry breaking occurs in response to the stepwise synergistic disordering of the Hcpa cations and ClO -4 anions. Shi and co-workers reported the supramolecular phase change material (PCM) potassium hydrogen bis(dichloroacetate)-18-crown-6, which undergoes a reversible second-order phase transition at 181.8 K (Tc). The phase transition was ascribed to order-disorder transformations of pendulum-like motions of the CHCl 2 COO/ CHCl 2 COOH units between the low-and room-temperature phases [50][51][52].
We have previously reported molecular rotators based on supramolecules formed by crown ether and anilinium derivatives that showed reversible phase transitions via hydrogen bonding. For example, a layered crystal of (2-nitroanilinium)(18-crown-6)(IO 4 ) showed a pair of reversible peaks at 260.5 K (cooling) and 264.5 K (heating) with heat hysteresis of approximately 4 K. Structural analysis and potential energy calculations indicated that the dielectric anomaly and phase transition occur because of intra-and intermolecular proton transfer of -NH þ 3 in 2-nitroanilinium as a consequence of -NO 2 motion in the cation [53][54][55]. It was suggested that hydrogen-bonding interactions and/or molecular motion give rise to symmetry breaking, thus leading to the dielectric anomaly behaviour as well as the phase transition. With a view to expand this research, a protonated 3-nitroanilinium cation that can form supramolecular rotator structures with 18-crown-6 through hydrogen-bonding interactions was adopted as a new type of molecular phase transition material. Compared with 2-nitroanilinium, 3-nitroanilinium provides a larger volume for molecular motion within the crystal, thereby reducing the potential energy required for molecular motion. Meanwhile, H 2 PO -4 is used as the counter-ion in the supramolecular cation assembly. By providing active H atoms, H 2 PO -4 easily forms hydrogenbonding interactions that greatly contribute to the phase transition behaviour and dielectric and ferroelectric properties of the material. In this study, we successfully synthesized a new organicinorganic hybrid compound, (3-nitroanilinium) 2 (18-crown-6) 2 (H 2 PO 4 ) 2 (H 3 PO 4 ) 3 (H 2 O) (1), which not only shows sequential reversible structure phase transition at ca 231 K, but also exhibits switchable dielectric anisotropy behaviour along the a-, b-and c-axes. The molecular motions are discussed in terms of X-ray crystallographic analyses and potential energy calculations.

Material and methods
All reagent-grade chemicals and solvents were of analytical grade and used without any further purification. The IR spectra of compound 1 were recorded on an Affinity-1 spectrophotometer (400-4000 cm 21 ) at room temperature, with all samples prepared as diluted KBr pellets (electronic supplementary material, figure S1). Elemental analysis was carried out on a Vario EL Elementar Analysen systeme GmbH at the Collaboration Center of TRW Research, Shandong. Thermogravimetric analysis (TG) and differential thermal analysis (DTA) measurements were performed on a TA Q50 instrument under flowing nitrogen at a heating rate of 10 K min 21 . Differential scanning calorimetry rsos.royalsocietypublishing.org R. Soc. open sci. 5: 180738 (DSC) measurements were performed on a TA Q2000 DSC instrument by heating and cooling the crystal samples (12.3 mg) at a rate of 10 K min 21 within the temperature range of 210-260 K under nitrogen at atmospheric pressure. Single crystals of 1 deposited with a silver conducting glue were used for the dielectric measurements.

Preparation of 1
The title compound 1 was prepared by slow evaporation of a mixture solution at room temperature, as shown in scheme 1. An acetonitrile solution (20 ml) of 3-nitroaniline (1.4 mol, 0.208 g) was added slowly to an acetonitrile solution (20 ml) of 18-crown-6 (1.4 mol, 0.400 g), followed by the dropwise addition of 0.174 g phosphoric acid (85%). The solvent was evaporated for five days at room temperature to give a block crystal of 1 in 65.5% yield. Elemental analysis calcd (%) for compound 1 (C 36

Single-crystal structure determination
Crystallographic data of single crystals of 1 were obtained using a Bruker AXS CCD area-detector diffractometer with Mo-Ka radiation (l ¼ 0.71073 Å ) at 100 and 296 K. The data collection and cell refinements were obtained using the CrystalClear software package (Rigaku). The structures were solved by direct methods and successive Fourier synthesis and refined on F 2 by the full-matrix leastsquares method (SHELXTL-97). All non-hydrogen atoms were refined anisotropically using reflections with I . 2d (I ). Hydrogen atoms were added geometrically and refined using the riding model with Uiso(H) ¼ 1.2Ueq. The packing views were drawn with Crystal Make, and selected bond distances and angles were calculated using SHELXTL. Table 1 provides a summary of the crystallographic data and details of the data collection and refinement. CCDC: 1569746 (100 K) and CCDC: 1569747 (296 K) for 1 contain the supplementary crystallographic data from this study. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.

Calculations
To characterize the molecular motions of the 3-nitroanilinium cation, potential energy calculations were performed at the RHF/6-31G(d) level of theory. The structural units of compound 1 used in the calculations were (3-nitroanilinium) 3 (18-crown-6) 3 (H 2 PO 4 ) 2 and (3-nitroanilinium) 3 (18-crown-6) 3 , which differ from the actual stoichiometry of the compound to make the calculations computationally tractable. These computations were based on the fixed atomic coordinates obtained from the crystal structure at 296 K. The relative energy was calculated by evaluating the rigid rotation of the C (25) -N (1) bond axes in 308 increments (electronic supplementary material, figure S9a). The nearest-neighbour molecules around the nitro-group ( -NO 2 ) of the 3-nitroanilinium cation were included in the calculation of the potential energy curves (electronic supplementary material, figure S9b and S9c). The relative energy of the structure was obtained by evaluating the rigid rotation of the nitro group along the C (27) -N (2) and C (33) -N (4) bonds, with the rotation performed every 58 in the range of 2358 to 358.

Differential scanning calorimetry and thermogravimetric analysis
DSC is a thermodynamic method that detects the temperature dependence of reversible phase transitions. When a compound is subjected to a disorder -order transition of molecular or proton motion, reversible heat anomalies upon heating and cooling can be detected in DSC measurements. In such measurements of 1, a pair of reversible heat anomalies at 231/241 K (cooling/heating) was clearly observed, indicating that 1 exhibits a phase transition at Tc ¼ 241 K (figure 1). The relatively large heat hysteresis of 10 K and the peak shapes indicate the occurrence of a reversible phase transition with a probable first-order feature. Entropy changes DS in the cooling and heating processes were estimated to be 6.32 and 8.19 J (mol K) 21 , respectively. According to Boltzmann's rsos.royalsocietypublishing.org R. Soc. open sci. 5: 180738 where R is the universal gas constant and N is the ratio of the number of geometrically distinguishable orientations, the values of N 1 and N 2 are calculated as 2.14 and 2.68, respectively. These values suggest that the number of independent orientations of disordered moieties in the crystal structure of 1 changed during the phase transition, which will be confirmed by structural analysis (vide infra). The thermal behaviour of 1 was investigated through TG and DTA measurements from 300 to 850 K (electronic supplementary material, figure S2). The DTA curve shows a relatively weak endothermic peak at 421.5 K, which corresponds to the melting point of 1. The TG curve reveals two main weight loss regions. The structure of compound 1 remains ordered up to 421.5 K. The first weight loss comprises approximately 61.98% of the total weight, and appears to involve the loss of (3-nitroanilinium) 2 (18crown-6) 2 (H 2 O) (calcd. at 63.54%). The second weight loss step of 38.02% (calcd. at 37.84%) involves three H 3 PO 4 molecules and two H 2 PO -4 anions.

Crystal structure of 1
The phase transition of 1 was further confirmed at 100 K (low-temperature phase, LTP) and 296 K (roomtemperature phase, RTP) by variable-temperature single-crystal X-ray structure determination. The RTP structure was solved in the monoclinic P2 1 /n space group with cell parameters of a ¼ 13.    figure S6). By comparing the distances of the p -p interactions in the two LTP and RTP structures, an average Cg 0 -Cg 00 distance of 3.608 Å in the RTP structure is determined, which is slightly longer than that in the LTP structure (3.484 Å , table 2). The dihedral angles between the benzene rings in Cg 0 -Cg 00 are 1.958 and 0.868 in the LTP and RTP structures, respectively (electronic supplementary material, figure S4). These results suggest that the 3-nitroanilinium cations move slightly along the C-N bond in the space between the supramolecular cations A and B.
According to the law of charge conservation, the inorganic anions in the non-supramolecular cation part of 1 consist of two H 2 PO

Potential energy calculations
To confirm whether the motions of the 3-nitroanilinium cations and nitro groups make significant contributions to the phase transitions, the potential energies for (i) rotation of the phenyl ring and (ii) pendulum-like motion of the nitro group were calculated with the RHF/6-31(d) level of theory. These calculations were based on the fixed atomic coordinates obtained from the crystal structure at 296 K. The model used for these calculations is shown in electronic supplementary material, figure S9a. For the phenyl ring motion along the C 25 -N 1 axis, the atomic coordinates of the -NH þ 3 groups were kept fixed while rigid motion of the phenyl ring was applied to calculate the rotational potential energy. Three 18-crown-6 molecules, two 3-nitroanilinium cations, and two H 2 PO -4 anions were included in the models to evaluate the effects of steric hindrance. The rotational angle (V) dependencies of the potential energies (DE) for the phenyl ring in compound 1 from 08 to 3608 at 308 increments are provided in figure 8a. The initial atomic coordinates obtained from the crystal structure at 296 K correspond to the first potential energy minimum at V ¼ 08, whose relative energy was defined as zero. The second and third potential energy minima appear at V ¼ 1208 and 2408, respectively, for the phenyl ring in 1, and the maxima were calculated at V ¼ 908, 1808 and 2708, with corresponding DE values of 965.90, 993.05 and 958.24 kJ mol 21 , respectively. The magnitude of the maximum DE for the phenyl ring (%1000 kJ mol 21 ) was far greater than that in (mfluoroanilinium þ )(DB[18]-crown-6)[Ni(dmit) 2 ] 2 (%270 kJ mol 21 ) [8]. The high potential energy barriers for rotation of the phenyl ring suggest that it is almost impossible above room temperature.  Table 2. Dihedral angles and average Cg 0 and Cg 00 distances with p-p interactions along the a-axis at 100 and 296 K.
angle (8) /Cg 0 -Cg 00 /Cg 00 -Cg 00 /Cg 00 -Cg 0 /Cg 0 -Cg 0 The pendulum-like motion of the nitro group was evaluated with fixed atomic coordinates of the C 6 H 4 -NO 2 moiety in the supramolecular cations. The models used for these calculations comprise three supramolecular cations [(3-nitroanilinium)(18-crown-6)] þ (electronic supplementary material, figure S9b and S9c). Because rotation of the nitro group by 3608 is very likely to be impossible because of the large steric hindrance of the neighbouring phenyl rings, the relative energy of pendulum-like motion of the nitro group was calculated at 58 increments within the range of 2358 to 358. Two symmetrical potential energy profiles were calculated for this motion in the supramolecular cations A and B (figure 8b). The magnitude of the maximum DE was ca 140 kJ mol 21 (less than 270 kJ mol 21 ), indicating that pendulum-like motion of the nitro group around the C 27 -N 2 and C 33 -N 4 bonds is possible at 296 K. Consequently, such motion is likely to be restricted at 100 K but easily activated as the temperature is increased. Overall, these results suggest that the pendulum-like motion of the nitro group may well be related to the observed phase transitions.

Dielectric properties
It is well known that physical properties usually display abrupt changes in the vicinity of a phase transition, and the magnitude is correlated to characteristic features of the phase transition. The dielectric properties of 1 were measured in three directions of a single-crystal sample. The temperature dependence of the real parts (1 0 ) of the dielectric constants taken at 5 kHz, 10 kHz, 100 kHz and 1 MHz are depicted in figure 9 and electronic supplementary material, figure S10. The T-1 0 curve shows two obvious step-like dielectric anomalies in the temperature range of 150-280 K, which is consistent with the results determined using DSC. At 5 kHz, 1 0 remains at approximately 11-12 from   Figure 9b and electronic supplementary material, figure S10b display two clear step-like dielectric anomalies at approximately 225 K on cooling and 242 K on heating. For the c-axis, two step-like dielectric anomalies similar to that observed for the a-and b-axes were also observed (figure 9c and electronic supplementary material, figure S10c). In the heating mode, 1 0 remains stable (approx. 10) until the temperature reaches ca 242 K at different frequencies, corresponding to the low dielectric state. With rising temperature, the 1 0 exhibited a prominent increase from ca 10 to 25, 20, 14, and 12 at 5 kHz, 10 kHz, 100 kHz, and 1 MHz, respectively. Upon cooling again, the curves plateau with a slight decrease and the 1 0 values change from ca 12.8 to 11.8 at a frequency of 5 kHz. The changes of 1 0 at lower frequencies are more pronounced than those at higher frequencies, indicating that the dielectric constant is very sensitive to external frequencies. From analysis of the crystal structure of 1, the dielectric anisotropy was related to the anisotropy of proton transfer in the 3D framework through O-H ... O hydrogen bonds. In addition, the temperature-dependent curves of 1 0 along the a-, b-and caxes obtained in the cooling mode match well with those obtained during the heating process, suggesting the occurrence of a reversible phase transition. Distinct step-like anomalies around 241 K can be viewed along both the a-and c-axes, while relatively smaller anomalies are apparent along the b-axis. These phenomena can be explained by examining the dynamic motion of the nitro group in the cation. Pendulum-like motion of this group occurs in the acplane, which enhances the 1 0 along the a-and c-axes. The fluxional frequency of the nitro group motion coupling with proton transfer is suppressed as the temperature decreases, when they become frozen in a given state. Therefore, there is no obvious change in the value of 1 0 with changing frequency; it does increase with increasing temperature, however. Further dielectric enhancement was observed at temperatures above 260 K, where the low-frequency and 1 0 was larger than the highfrequency values. Importantly, the phase transition of compound 1, which can be tuned between low and high dielectric states at low and room temperature, makes it an excellent candidate for molecular switchable dielectrics.

Conclusion
In conclusion, a new three-dimensional organic-inorganic hybrid compound, (3-nitroanilinium) 2 (18crown-6) 2 (H 2 PO 4 ) 2 (H 3 PO 4 ) 3 (H 2 O) (1), was synthesized. Compound 1 undergoes a reversible phase transition at ca 231 K, which is anisotropy in the vicinity of Tc. The dynamic behaviours between the pendulum-like motions of the nitro group in the organic cations and proton transfer in the O -H ... O hydrogen bonds of the 3D framework produce a synergistic effect that gives rise to the observed phase transition and distinct dielectric anisotropy along various crystallographic axes. Notably, the dielectric constants could be switched by continuous phase transitions and tuned in the low and high dielectric states, indicating the potential application of structures like 1 in switchable dielectrics. We believe that this example will inspire new strategies for the successful design of other organic-inorganic hybrid multifunctional materials.
Ethics. We were not required to complete an ethical assessment before conducting the research. No data were collected from human or animal subjects for the purpose of this study.
Data accessibility. This article does not contain any additional data. Authors' contributions. Z.-q.L. designed the method and wrote the manuscript; X.-y.Z. synthesized the crystal materials.