Synthesis, Structure, and Dielectric Properties of (3-Nitroanilinium) (18-Crown) (PF6)

(3-Nitroanilinium) (18-crown) (PF6) (1), which is an organic-inorganic hybrid containing one-dimensional chains of hydrogen-bonded supramolecular cations, was synthesized under slow evaporation conditions and subjected to differential scanning calorimetry, temperature-dependent dielectric measurements, and variable-temperature single-crystal X-ray diffraction analysis. These analyses revealed the occurrence of a reversible structural phase transition [P21/n P21/c] at 223 K and a dielectric anomaly, which, based on the results of structural analysis and potential energy calculations, was attributed to the synergistic effect between the pendulum-like motion of the nitro group in the supramolecular cation, (3-nitroanilinium)(18-crown), and the order-disorder motion of PF6− anions.

Crown-type ethers containing S or O heteroatoms as acceptors can effectively bind cations (Na + , K + , Ba 2+ , H 3 O + , RNH 3 + , etc.) via covalent coordination bonding or weak hydrogen bonding and are widely used in supramolecular chemistry because of their appealing structural features, phase transitions, and thermochromic behavior. Crown ethers containing repeating -OCH 2 CH 2 O-units exhibit high flexibility, i.e., can adopt different shapes [17][18][19][20][21][22][23]. Therefore, cavity size and shape control is expected to allow the development of macrocycles capable of specifically recognizing certain guest molecules [24][25][26][27][28][29]. For example, the combination of 18-crown-6 with Ar-NH 3 + guests in a 1:1 stoichiometry affords [(Ar-NH 3 )·(18-crown-6)] + supramolecular cation units in which the Ar-NH 3 + bond is almost perpendicular to the mean plane of the crown ether ring, which allows the cation to freely rotate in the crown ether cavity and may result in dielectric anomalies and structural phase transitions. Moreover, the multi-component [(Ar-NH 3 )·(18-crown-6)] + unit allows for various cation/anion and order-disorder motions in the crystal structure that significantly change the conductive, magnetic, and dielectric properties of the bulk [30][31][32][33]. For example, the synergistic disordering of cations and anions triggers dielectric anisotropy and phase transitions in [Hcpa (18-crown-6)] [ClO 4 ] crystals. In this case, molecular motion leads to sequential reversible phase transitions, thermal anomalies, and abrupt dielectric anomalies during heating and cooling [34]. Previously, we showed that the 3-nitroanilinium cation forms an N-H···O hydrogen bond with the oxygen atom of 18-crown-6 in [(3-nitroanilinium)(18-crown-6)][IO 4 ](CH 3 OH), revealing that the order-disorder rotation of CH 3 OH results in a dielectric anomaly and a phase transition at 220 K [35]. Herein, we build on the results of our previous work and report (3-nitroanilinium) (18-crown) (PF 6 ) (1), which contains one-dimensional hydrogen-bond chains and exhibits a phase transition and a dielectric anomaly originating from the pendulum-like motion of nitro groups and the order-disorder motion of PF 6 − anions. Moreover, the synthesis of 1 as well as the results of its analysis by variable-temperature single-crystal X-ray diffraction, infrared (IR) spectroscopy, differential scanning calorimetry (DSC), and dielectric property measurements are discussed in detail.

IR Spectroscopic Analysis
The IR spectrum of 1 ( Figure S1)

Hermogravimetric (TG) and DSC Analyses
The thermal stability of 1 was probed by TG and DTA measurements in the range of 300-890 K ( Figure S2). The DTA curve showed a weak endothermic peak at 355.4 K and a sharp endothermic peak at 455.6 K corresponding to the melting point of 1. Thus, 1 was concluded to be stable at temperatures below its melting point and was additionally characterized by DSC, as this technique can effectively detect reversible phase transitions occurring upon heating and cooling. The DSC curve of 1 revealed the occurrence of a reversible phase transition at~211.2 K and~215.3 K in the cooling mode and at around 218.4 K and 223.7 K in the heating mode (thermal hysteresis = 7.2 K and 8.4 K, Figure 1), suggesting that this transition was of a second-order type, according to the broad peaks with narrow thermal hysteresis. The total transition enthalpy ∆H equaled 297.12 J/mol in the cooling mode and 259.93 J/mol in the heating mode. The entropy change was estimated as ∆S = ∆H/T c = 1.38 and 1.16 J/(mol K) for both the cooling and heating runs. On the basis of the Boltzmann equation, ∆S = RlnN, where R is the universal gas constant, the ratio of the numbers of respective geometrically distinguishable orientations (N) was obtained as 1.18 and 1.15. The fact that the values were close to unity suggests that the phase transition involved proton transfer or dominant molecular movement accompanying the order-disorder transition.

Crystal Structure Analysis
The above phase transition was further characterized by single-crystal X-ray analysis of 1 at 128 K (low-temperature phase, LTP) and 293 K (room-temperature phase or RTP) ( Table 1). The RTP was found to crystallize in a monoclinic crystal system with a centrosymmetric space group of The slight change of b, α, and γ upon going from the RTP to the LTP was ascribed to the effect of thermal contraction. However, much larger changes were concomitantly observed for a (decrease of~1.49%), c (increase of~1.08%), β (decrease of~0.489%), and V (decrease of~1.27%). Similarly to that of the RTP, the asymmetric unit of the LTP contained one 18-crown-6 molecule, one 3-nitroanilinium cation, and one PF 6 − anion ( Figure 2b). However, the phenomenon of the PF 6 − disorder was not observed for the LTP, which suggested that the reversible LTP to RTP transition is caused by the order-disorder transition of the above anion.
[a] :  The molecules of 18-crown-6 had a nearly ideal conformation with a close to-D 3d symmetry, and the six oxygen atoms of the crown ring were alternately located above and below its median plane, forming two approximately parallel and nearly equilateral triangles. The dihedral angles between the upper-oxygen-atom (O3, O5, O7) and lower-oxygen-atom (O4, O6, O8) planes equaled 0.60 • and 0.48 • for the RTP and LTP, respectively, and indicated a small twist in the crown conformation. The 3-nitroanilinium cation was connected to the 18-crown-6 molecule via intermolecular N-H·O hydrogen bonds between the 3-nitroanilinium nitrogen and 18-crown-6 oxygens to form a supramolecular rotor-stator structure. The N-H·O hydrogen bond distances in the RTP and LTP were within the normal ranges of 2.860-2.906 and 2.846-2.924 Å, respectively (Table S1). Although the LTP distance range was slightly shorter than the RTP one, the average hydrogen bond lengths were not clearly different, which suggested the absence of distinct proton transfer in N-H···O hydrogen bonds. In the (3-nitroanilinium) (18-crown) supramolecular cation, the benzene ring of 3-nitroanilinium was almost perpendicular to the molecular plane of 18-crown-6 with dihedral angles of 87.38 • and 87.80 • observed for the LTP and RTP, respectively. Analysis of the interrelation between two adjacent (3-nitroanilinium)(18-crown) cations revealed that, besides the previously mentioned six N-H···O hydrogen bonds, a seventh hydrogen bond (N 1 -H·O 2 ) passing through the cavity of 18-crown-6 exists between the -NH 3 + group and the -NO 2 group of the adjacent 3-nitroanilinium cation, linking (3-nitroanilinium)(18-crown) cations to form one-dimensional hydrogen bond chains (Figure 3, Figures S3 and S4). Further analysis showed that the distance between the donor (N 1 ) and acceptor atoms (O 2 ) is longer than that in the other six hydrogen bonds, which means that protons move more easily in one-dimensional hydrogen bond chains along the (a + c)-axis (in the RTP) and the c-axis (in the LTP) [N 1 -H·O 2 : 3.130 Å at 128 K; N 1 -H·O: 3.144 Å at 296 K] (Table S1). As shown in Figure 3, the distance between two adjacent (3-nitroanilinium)(18-crown) cations was shorter in the RTP (15.158 Å) than in the LTP (15.386 Å). Hence, the concertina movement of the one-dimensional hydrogen bond chain with the change of temperature was expected to benefit proton transfer in this chain and the motion of the nitro group in the supramolecular cation. In addition, the anisotropic thermal factors of O 1 exceeded those of other atoms in the 3-nitroanilinium cation, which suggested the pendulum-like motion of the nitro group in the (3-nitroanilinium) (18-crown) cation ( Figure S5). The most notable difference between the two phases was observed for dihedral angles between the nitro group and the aromatic ring of the 3-nitroanilinium cation (15.24 • for LTP vs. 15.42 • for RTP) ( Figure S6) and was ascribed to molecular interactions between the adjacent supramolecular cations in the one-dimensional hydrogen bond chain. Thus, the pendulum-like motion of the nitro group could easily occur and was possibly the driving force for the reversible phase transition in 1. In the inorganic part of 1, the PF 6 − anion was disordered over two staggered orientations in the RTP, but was ordered in the LTP. Further analysis showed that the PF 6 − anion, acting as a counter-ion of the supramolecular cation assembly, featured average F-P bond distances of 1.547 (LTP) and 1.567 Å (RTP), i.e., no significant change in this average distance was observed during the phase transition. However, the bond angle formed by the F and P atoms in the RTP (170.47 • ) was lower than that in the LTP (179.02 • ) (Table S2). Structural factor analysis of PF 6 − suggests that the octahedral structure of this anion was distorted with increasing/decreasing temperature. The molecular twist at different temperatures could lead to the structural phase transition and electrical property changes along different axes. The arrangement of PF 6 − anions in the space of two one-dimensional hydrogen bond chains shows that monovalent assemblies of [(3-nitroanilinium)(18-crown)] + and PF 6 − at a molar ratio of 1:1 were alternately stacked together along the c-axis in the RTP and along the (a + c)-axis in the LTP to build the supramolecular self-assembly structure (Figure 4).

Potential Energy Calculation
To confirm whether the molecular motions of the 3-nitroanilinium cation and the nitro group significantly contributed to the structural phase transition and abnormal electrical properties, we evaluated potential energy changes associated with the rotation of the phenyl ring and the pendulum-like motion of the nitro group in the supramolecular cation. As the -NH 3 + group of the 3-nitroanilinium cation was fixed in the cavity of 18-crown-6 by N-H·O hydrogen bonding, the rotation of the phenyl ring occurred around the C 18 -N 1 bond of the 3-nitroanilinium cation, causing the rotation of the 3-nitroanilinium dipole in the ac plane. Notably, the rotation of the 3-nitroanilinium cation was restricted by the steric hindrance of one neighboring 18-crown-6 molecule and two PF 6 − anions. A theoretical calculation using the RHF/6-31(d) basis set was carried out to examine the (3-nitroanilinium)(18-crown) 2 (PF 6 ) 2 model structure at 296 K ( Figure S7a). Figure 5a presents the effects of the rotational angle (Φ = 0-360 • , step = 30 • ) on the potential energy (∆E) for the phenyl ring of the 3-nitroanilinium cation. The initial atomic coordinates obtained from the RTP crystal structure correspond to the first potential energy minimum at Φ = 0 • of which the relative energy was defined as zero, while the second energy minimum appeared at Φ = 180 • . The potential energy maxima occurred at Φ = 90 • (762.24 kJ/mol) and 270 • (1189.34 kJ/mol). The potential energy barrier of phenyl rotation in 1 was far greater than that in (m-fluoroanilinium) (DB18-crown-6) [Ni(dmit) 2 ], i.e., increased upon the replacement of fluorine with the nitro group [36], and suggested that phenyl ring rotation in the 3-nitroanilinium cation is unlikely to be above 296 K. The pendulum-like motion of the nitro group was also evaluated for fixed atomic coordinates of the (3-nitroanilinium)(18-crown) (PF 6 ) model structure ( Figure S7b). Although the nitro group rotation around the C-N bond in the 0-360 • range is unlikely because of the large steric hindrance of neighboring 18-crown-6 and PF 6 − units, we calculated symmetrical potential energy profiles for this pendulum-like motion at 5 • increments within the range of −35 • to 35 • (Figure 5b). The maximum ∆E of 180 kJ/mol (<270 kJ/mol) indicated that the pendulum-like motion of the nitro group around the C 14 -N 2 bond is possible at 296 K and can, therefore, affect the phase transition and electrical property changes.

Dielectric Properties
The frequency-dependent dielectric response at different temperatures can be used to detect thermally activated molecular motion and structural changes, e.g., when the change of polarity due to molecular motion or order-disorder change in the crystal is of the order of the measurement frequency. Clear dielectric responses are expected. With increasing temperature, the molecular motion of the disordered PF 6 − anion in the molecular rotor was severely restricted, as higher temperatures favored the more disordered RTP and were expected to induce a change of molecular polarity due to order-disorder motion. The potential energy curve of the nitro-group pendulum motion is very shallow, and the dielectric response for this motion should be large. Therefore, the pendulum motion of the nitro group associated with order-disorder anion motion significantly affects the dipole moment of which the response should be mainly observed by monitoring temperature-dependent and frequency-dependent dielectric properties. Given the difficulty of obtaining large crystals of 1, pressed powders comprising plate-shaped particles were used for dielectric measurements. The dielectric constant (ε ) of 1 was monitored in the range of 100-300 K at frequencies of 0.5, 1, 5, 10, 100, and 1000 kHz ( Figure 6). A sharp dielectric phase transition peak (dielectric anomaly) was observed around 221.75 K upon cooling, which is in line with the phase transition observed at~223.7 K. Dielectric constants at temperatures below 190 K exhibited temperature-dependent and frequency-dependent behavior with a constant ε of 14.2, whereas a large dielectric constant enhancement was observed above 190 K and below 10 kHz. The dielectric constant (~50) obtained at 500 Hz and 221.75 K was four times larger than that obtained at 500 Hz and 100 K. This low-frequency response suggests a relatively slow pendulum-like motion of the nitro group rather than fast electronic motion, possibly reflecting the fact that the 3-nitroanilinium cation and the PF 6 − anion are present in orientationally ordered states at low temperature. Above 200 K, the pendulum motion of the nitro group and the order-disorder motion of PF 6 − were coupled to yield a strong dipole moment component and, thus, increase the dielectric constant. In addition, the dielectric loss constant (ε ') also displays a sharp change and remarkable frequency-dependent phenomenon at selected frequencies. The anomalies of ε and ε ' strongly confirm the existence of phase transitions of 1, which is consistent with the results of DSC and structural analysis. Thus, the crystal structures of 1 at low and room temperatures indicate that the pendulum motion of the nitro group and the order-disorder motion of PF 6 − may be the driving force of the LTP→RTP phase transition and anomalous dielectric behavior.

Materials and Measurements
All reagents and solvents used for the synthesis of 1 were purchased from TCI (Tokyo Chemical Industry, Shanghai, China) and were of an analytical grade and used without further purification. IR spectra were recorded on an Affinity-1 spectrophotometer (400-4000 cm −1 ). Elemental analyses were carried out using a Vario EL instrument (Elementar Analysensysteme GmbH) at the TRW research Collaboration Center. DSC experiments were performed on a TA Q2000 instrument by heating and cooling crystalline samples (7.5 mg) under nitrogen at a rate of 5 K/min. TG analysis was carried out on a TA Q50 instrument in a flow of nitrogen at a heating rate of 10 K/min. The temperature-dependent dielectric constants of 1 along the three axis directions were measured using a Tonghui 2828 impedance analyzer in the frequency range of 500 Hz-1 MHz using an applied electric field of 0.5 V. The powder-pressed and single-crystal pellets were then sandwiched between two parallel copper electrodes by the carbon paste for dielectric measurements.

Single-Crystal X-ray Diffraction Measurements
Variable-temperature (128 and 296 K) X-ray single-crystal diffraction measurements were carried out for the same crystal on a Rigaku Saturn 924 diffractometer equipped with a Rigaku low-temperature gas-spray cooler using Mo K α radiation (λ = 0.71073 Å). Data collection, cell refinement, and data reduction were performed using Rigaku Crystal Clear 1.3.5 software. The structure of 1 was solved by direct methods and refined by the full-matrix method based on F 2 using the SHELXL97 software package. All non-hydrogen atoms were refined anisotropically, and the positions of all hydrogen atoms were generated geometrically. Supplementary crystallographic data for 1 were deposited at the Cambridge Crystallographic Data Centre (CCDC, Nos. 1,997,720 (128 K) and 1,997,721 (293 K) and can be obtained free of change at www.ccdc.cam.ac.uk/data_request/cif.

Calculation of Rotational Potential Energy
The relative energies of crystal structures were calculated based on a semi-empirical method using the RHF/6-31(d) basis set [37]. Calculations of the rotation potential energy of the 3-nitroanilinium phenyl ring were performed including the nearest-neighbor molecules around this ring and using the atomic coordinates of 1 determined by a low-temperature X-ray crystal structure analysis. The structural units of 1 used in the calculation were (3-nitroanilinium) (18-crown) 2 (PF 6 ) 2 ( Figure S7). The relative energies of different structures were obtained by evaluating the rigid rotation of the phenyl ring around the C 18 -N 1 bond ( Figure S7a). Rotation was performed in 30 • steps in the range of 0-360 • . Calculations of the rotational potential energy of the nitro group were performed by including the nearest-neighbor molecules around the nitro group ( Figure S7b). The relative energies of different structures were obtained by evaluating the rigid rotation (in 5 • steps in the range of −35 • to 30 • ) of the nitro group around the C 14 -N 2 bond.

Conclusions
Compound 1 underwent a reversible structural phase transition at~223 K (from the space group P2 1 /n at a low temperature to P2 1 /c at room temperature). Potential energy calculations of the nitro group revealed the important role of pendulum-like motion for a physical performance change. The combined results of DSC, single-crystal X-ray diffraction, and dielectric constant measurements revealed that a synergistic effect between the pendulum-like motion of the nitro group and the order-disorder motion of PF 6 − gives rise to the above phase transition and a sharp dielectric anomaly peak.