Physicochemical properties and structural dynamics of organic–inorganic hybrid [NH3(CH2)3NH3]ZnX4 (X = Cl and Br) crystals

The physical properties of the organic–inorganic hybrid crystals having the formula [NH3(CH2)3NH3]ZnX4 (X = Cl, Br) were investigated. The phase transition temperatures (TC; 268K for Cl and 272K for Br) of the two crystals bearing different halogen atoms in their skeletons were determined through differential scanning calorimetry. The thermodynamic properties of the two crystals were investigated through thermogravimetric analysis. The structural dynamics, particularly the role of the [NH3(CH2)3NH3] cation, were probed through 1H and 13C magic-angle spinning nuclear magnetic resonance spectroscopy as a function of temperature. The 1H and 13C NMR chemical shifts did not show any changes near TC. In addition, the 1H spin–lattice relaxation time (T1ρ) varied with temperature, whereas the 13C T1ρ values remained nearly constant at different temperatures. The T1ρ values of the atoms in [NH3(CH2)3NH3]ZnCl4 were higher than those in [NH3(CH2)3NH3]ZnBr4. The observed differences in the structural dynamics obtained from the chemical shifts and T1ρ values of the two compounds can be attributed to the differences in the bond lengths and halogen atoms. These findings can provide important insights or potential applications of these crystals.

and Ishihara et al. 23 , who studied the single crystals of the complexes through X-ray diffraction analysis. Although [NH 3 (CH 2 ) 3 NH 3 ]ZnX 4 (X = Cl and Br) has many applications, the physical properties and structural dynamics of its crystals have not been elucidated.
Herein, the phase transition temperatures and thermodynamic properties of the crystals of [NH 3 (CH 2 ) 3 NH 3 ] ZnCl 4 and [NH 3 (CH 2 ) 3 NH 3 ]ZnBr 4 complexes are determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The structural dynamics of the crystals of [NH 3 (CH 2 ) 3 NH 3 ]ZnX 4 (particularly the role of the [NH 3 (CH 2 ) 3 NH 3 ] cation) were probed by 1 H magic-angle spinning nuclear magnetic resonance (MAS NMR) and 13 C MAS NMR spectroscopy as a function of temperature. The 1 H and 13 C NMR spectral profiles were recorded to determine the changes in the chemical shifts. A change in the chemical shift values reflects a change in the structural environment. In addition, the spin-lattice relaxation times (T 1ρ ) in the rotating frame were discussed according to the change of temperature. Based on the MAS NMR results, the effects of different halogen atoms on the hydrogen bond and carbon atoms in the [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 and [NH 3 (CH 2 ) 3 NH 3 ]ZnBr 4 crystals were investgated. Moreover, comparison of the physical properties of the two crystals revealed important information regarding the basic mechanisms.
The structures of the [NH 3 (CH 2 ) 3 NH 3 ]ZnX 4 (X = Cl and Br) crystals at 298 K were analysed using an X-ray diffraction system equipped with a Cu-Kα radiation source. And, the lattice parameters were determined by single-crystal X-ray diffraction methods at the Western Seoul Center of Korea Basic Science Institute. The crystals were mounted on a Bruker D8 Venture equipped with IμS micro-focus sealed tube Mo-Kα and a PHOTON III M14 detector. DSC (TA, DSC 25) experiments were carried out at a scanning speed of 10 K/min in the temperature range of 190-620 K under an atmosphere of nitrogen. TGA experiments were performed on a thermogravimetric analyzer (TA Instrument) in the temperature range of 300-870 K. The same heating rate was maintained. The amounts of [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 used for the DSC and TGA experiments were 6.04 and 6.72 mg, respectively and the amounts of [NH 3 (CH 2 ) 3 NH 3 ]ZnBr 4 used were 6.40 and 8.96 mg, respectively.
The NMR spectra of the crystals of [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 and [NH 3 (CH 2 ) 3 NH 3 ]ZnBr 4 were recorded on a 400 MHz Avance II + Bruker solid-state NMR spectrometer equipped with 4 mm MAS probes (at KBSI, Western Seoul Center). The MAS 1 H and 13 C NMR experiments were conducted at the Larmor frequency of 400.13 and 100.61 MHz, respectively. It was observed that an MAS rate of 10 kHz can minimize the spinning sidebands. Tetramethylsilane (TMS) was used as the standard to record the NMR spectra. As preparation for the MAS NMR experiments, the single crystal was ground into powder. The T 1ρ values were obtained using a variable length spin lock pulse. For the two compounds, the width of the π/2 pulse for 1 H was 3.5 μs and the width of the π/2 pulse for 13 C was 3.96-4.3 μs. The radiofrequency power of the spin-lock pulses was 71.42 kHz for 1 H and 62.50 kHz for 13 C. An almost constant temperature (error ± 0.5 K) was maintained even when the rate of flow of nitrogen gas and the heater current were adjusted.   Fig. 3. The results obtained from the DSC experiments revealed that the peaks that appeared at ~ 590 K were significantly larger than the other peaks. An additional TGA and differential thermal analysis (DTA) experiments were performed to determine whether these endothermic peaks are related to the structural phase transitions or melting. The results of the TGA and DTA experiments conducted with the two crystals are presented in Fig. 4a www.nature.com/scientificreports/ The amount of residue obtained is determined as follows: The amount of residue obtained is determined as follows: The amount of residue obtained is determined as follows: The amount of residue obtained is determined as follows: [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 was found to lose approximately 13% and 26% of its weight when the temperature was approximately 606 and 621 K, respectively. [NH 3 (CH 2 ) 3 NH 3 ]ZnBr 4 was found to lose approximately 17% and 35% of its weight when the temperature was approximately 622 and 649 K, respectively. The weight loss can   www.nature.com/scientificreports/ be potentially attributed to the decomposition of HX and 2HX (X = Cl and Br) moieties, respectively, as shown in Fig. 4a where I(t) and I(0) are the signal intensities at times t and t = 0, respectively. The 1 H NMR signals were plotted as a function of the delay times at each temperature. The 1 H NMR signals for [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 at 300 K were    values were recorded at higher temperatures. On the slow side of T 1ρ , a decrease in T 1ρ results in smaller valued of the correlation time τ C . The T 1ρ values for Arrhenius-type random motions with τ C are described in terms of slow motions. When τ C < < ω 1 , T 1ρ ∝ τ C = τ 0 exp(-E a /k B T), where ω 1 denotes the spin-lock frequency and E a represents the activation energy. The decrease in T 1ρ values with temperature indicates an increase in proton mobility at higher temperatures.  (Fig. 7, inset). It was observed that for both the crystals, the 13 C chemical shift values remained almost constant with changes in temperature ( Supplementary Fig. 1). The change in FWHM for 13 C NMR spectra with temperature for both crystals is shown in Fig. 7. In the case of [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 , the 13 C line widths (CH 2 -1 and CH 2 -2) decreased monotonically with an increase in temperature. Significant anomaly due to phase transition was not observed. The 13 C line widths in [NH 3 (CH 2 ) 3 NH 3 ] ZnBr 4 changed with the increase in temperature. A transformation from a Gaussian to Lorentzian shape was observed. The line width reduced because of internal molecular motion. The line width of CH 2 -1 was lower than that of CH 2 -2. The decrease in line width with an increase in temperature can be attributed to the internal molecular motion.

MAS
The intensities of the 13 C NMR signals were determined by varying the delay times at each temperature. The 13 C NMR signals (CH 2 -1 and CH 2 -2) recorded for [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 and [NH 3 (CH 2 ) 3 NH 3 ]ZnBr 4 were plotted as a function of delay time. The decay curves for CH 2 -1 and CH 2 -2 were fitted to an exponential equation of Eq. (5). From the slopes of the recovery traces, the 13 C T 1ρ values were obtained (CH 2 -1 and CH 2 -2) as a function of inverse temperature, as shown in Fig. 8. The 13 C T 1ρ values of [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 were higher than those of [NH 3 (CH 2 ) 3 NH 3 ]ZnBr 4 . The 13 C T 1ρ value of CH 2 -1 was slightly higher than the 13 C T 1ρ value of CH 2 -2. The T 1ρ values of the atoms in both the compounds were temperature independent at > 200 K.

Conclusion
The physical properties of organic-inorganic hybrid crystals [NH 3 (CH 2 ) 3 NH 3 ]ZnX 4 (X = Cl, Br) were investigated through DSC, TGA, and NMR spectroscopy. The TGA results revealed that the [NH 3 (CH 2 ) 3 NH 3 ]ZnX 4 (X = Cl, Br) complex exhibited good thermal stability. When the temperature was increased, the 1 H and 13 C chemical shifts  The structural dynamics were discussed in terms of the 1 H T 1ρ and 13 C T 1ρ values. The 1 H T 1ρ values changed with the temperature, whereas the T 1ρ value of the carbon atom located in the middle of the N-C-C-C-N chain did not change significantly with the temperature. The 1 H T 1ρ and 13 C T 1ρ values of the atoms in [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 were higher than those in [NH 3 (CH 2 ) 3 NH 3 ]ZnBr 4 ( Table 1). The reason why 13 C T 1ρ values in [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 are longer than 13 C T 1ρ values in [NH 3 (CH 2 ) 3 NH 3 ]ZnBr 4 are as follows. The 13 C T 1ρ is most likely driven by 1 H-13 C interactions, and hence, the H-C bond lengths are important.
However, the H-C bond lengths of the two materials are unlikely to be different, and also cannot be accurately measured from X-ray results. We assume that the differences in the dynamics between the materials contribute more to the difference in T 1ρ . Although the structures and lattice constants of two crystals are similar, the differences between the local environments and structural dynamics obtained from the chemical shifts and T 1ρ values of the two compounds can be attributed to the different bond lengths and halogen atoms. Thus, the results of this study elucidate, the physical properties of [NH 3 (CH 2 ) 3 NH 3 ]ZnX 4 , which will expand the application scope of this organic-inorganic hybrid crystals.