Single-crystal XRD measurements
The single-crystal XRD patterns of [NH3(CH2)6NH3]ZnCl4 were obtained at 250, 300, and 350 K. The SCXRD result at three temperatures showed a triclinic system with P1(bar) space group and the cell constants of a=7.2844 (2) Å, b=10.1024 (3) Å, c=10.1051 (3) Å, α=74.3060 (10)°, β=85.9270 (10)°, γ=88.0170 (10)°, and Z=2 at 300 K. These results were in good agreement with data at 300 K reported by Mostafa and El-khiyami.32 On the other hand, the structures and lattice parameters were unchanged at 250, 300, and 350 K, as shown in Table 1, and it between 300 K and 350 K does not change to be significantly at the phase
Table 1. Crystal data and structure refinement for [NH3(CH2)6NH3]ZnCl4 at 250 K, 300 K, and 350 K.
Temperature
Chemical formula
|
250 K
C6H18N2ZnCl4
|
300 K
C6H18N2ZnCl4
|
350 K
C6H18N2ZnCl4
|
Weight
|
325.39
|
325.39
|
325.39
|
Crystal System
|
Triclinic
|
Triclinic
|
Triclinic
|
Space group
|
P1(bar)
|
P1(bar)
|
P1(bar)
|
a (Å)
|
7.2757 (4)
|
7.2844 (2)
|
7.2972 (3)
|
b (Å)
|
10.0165 (6)
|
10.1024 (3)
|
10.1542 (4)
|
c (Å)
α (°)
|
10.0348 (6)
75.028 (2)
|
10.1051 (3)
74.3060 (10)
|
10.1812 (3)
73.8420 (10)
|
β (°)
γ (°)
|
86.496 (2)
88.354 (2)
|
85.9270 (10)
88.0170 (10)
|
87.9390 (10)
85.6380 (10)
|
Z
|
2
|
2
|
2
|
V (Å3)
|
705.10 (7)
|
705.10 (7)
|
705.10 (7)
|
Radiation type
|
Mo-Kα
|
Mo-Kα
|
Mo-Kα
|
Wavelength (Å)
|
0.71073
|
0.71073
|
0.71073
|
Reflections collected
|
14848
|
26702
|
27217
|
Independent reflections
|
3539 (Rint = 0.0242)
|
3538 (Rint = 0.0272)
|
3595 (Rint = 0.0295)
|
Goodness-of-fit on F2
|
1.039
|
1.042
|
1.021
|
Final R indices [I > 2sigma(I)]
|
R1 = 0.0283, wR2 = 0.0687
|
R1 = 0.0359, wR2 = 0.0964
|
R1 = 0.0446, wR2 = 0.1405
|
R indices (all data)
|
R1 = 0.0369, wR2 = 0.0740
|
R1 = 0.0480, wR2 = 0.1046
|
R1 = 0.0656, wR2 = 0.1613
|
transition temperature of 340 K previously reported.32 Fig. 1 shows the structure of the [NH3(CH2)6NH3] cation and ZnCl4 anion, the atomic numbering scheme, and the thermal ellipsoids of the H atoms in the [NH3(CH2)6NH3]ZnCl4 crystal. In ZnCl4, the Zn atom is surrounded by four Cl atoms, forming a tetrahedron. Infinite chains consisting of face-shared ZnCl4 tetrahedra and four doubly bridging chloride ions link the adjacent Zn centers. This crystal is characterized by the N‒H∙∙∙Cl hydrogen bonds, connecting the cation to the anion. Bond lengths for Zn−Cl, N−C, C−C, C−H, N−H at three temperatures, are shown in Table 2. At 250 and 300 K, three N‒H∙∙∙Cl hydrogen bonds were formed, however, one N‒H∙∙∙Cl was broken at 350 K, and two N‒H∙∙∙Cl hydrogen bonds remained. The CIF files results of SCXRD for the crystal structures at 250, 300, and 350 K are shown in the Supplementary Information.
Phase transitions and powder XRD measurements
The phase transition temperature of the [NH3(CH2)6NH3]ZnCl4 crystal was investigated using a DSC experiment which involved heating the crystal from 200 K to 550 K at a rate of 10°C/min, with a sample size of 8.8 mg. The DSC thermogram, shown in Fig. 2, revealed the presence of two weak endothermic peaks at 340 K and 408 K, respectively, along with a strong endothermic peak at 473 K. The endotherm peak at 340 K was relatively small, making it challenging to accurately measure the enthalpy. However, at 408 K and 473 K, the enthalpy values of 796 J/mol and 21 kJ/mol were obtained, respectively.
The temperature-dependent PXRD experiment was conducted, and the corresponding results are presented in the inset of Fig. 2, covering a range of 8–60° (2θ). The PXRD patterns below 400 K, indicated by red color, exhibited slight differences compared to those recorded at 410 K, represented by the olive color. This difference was attributed to TC (=408 K). Notably, the PXRD results were well consistent with the DSC findings. The PXRD patterns did not exhibit any changes corresponding to the small endothermic peak near 340 K observed in the DSC analysis. Also, the triclinic structure derived from SCXRD results at 300 K and 350 K remained unchanged. The temperature-induced variations in the single crystal were examined using an optical polarization microscope to accurately confirm the three peaks seen in DSC. It was observed that the crystal exhibited minimal changes in the temperature range of 300–450 K but gradually became opaque at higher temperatures. Melting of the single crystal began at near 470 K (i.e., Tm=470 K). Moreover, the phase transition temperature (TC) determined from DSC, PXRD, SCXRD, and optical polarizing microscopy experiments, was found to be 408 K. These results were not consistent with the previously reported values of 289 K, 342 K, and 385 K.32
Thermal properties
The thermal characteristics of the [NH3(CH2)6NH3]ZnCl4 crystal, including thermal stability, were assessed through TGA and DTA experiments at the heating rate of 10 °C/min with a method similar to that of the DSC experiment. As shown in Fig. 3, the DTA curve exhibited a peak around 474 K, which correlates with
the melting temperature determined from the DSC and optical polarizing microscope analyses. The TGA and DTA results indicated that the crystal displayed thermal stability up to approximately 584 K, referred to as the partial thermal decomposition temperature (Td). Beyond this temperature, [NH3(CH2)6NH3]ZnCl4 experienced weight loss, and the amount of solid residue was determined using Eq. (1) and (2) using the molecular weight. The weight loss rapidly declined between 600 and 800 K, and approximately 22 % weight loss occurred near 636 K due to the decomposition of 2HCl. Around 800 K, a weight loss of 90 % was observed, resulting from the decomposition of NH2(CH2)6NH2·2HCl, leaving behind only the inorganic ZnCl2 anions.40
[NH3(CH2)6NH3]ZnCl4 (MW = 325.39 mg) → [NH2(CH2)6NH2∙2HCl]ZnCl2
First step
[NH2(CH2)6NH2∙HCl]ZnCl2 (s) + HCl (g) / [NH3(CH2)6NH3]ZnCl4 = 88.79 % (1)
Second step
[NH2(CH2)6NH2]ZnCl2 (s) + 2HCl (g) / [NH3(CH2)6NH3]ZnCl4 = 77.59 % (2)
1H and 13C MAS NMR spectra
The 1H MAS NMR spectra of the [NH3(CH2)6NH3]ZnCl4 crystal were acquired in situ as a function of temperature, and the corresponding 1H chemical shifts are shown in Fig. 4. The TMS reference signal was used as the standard for 1H chemical shifts. At lower temperatures, a single resonance signal with an asymmetric shape was observed, resulting from the overlapping 1H resonance lines of NH3 and CH2 in the organic [NH3(CH2)6NH3] cations. The left (A) and right (B) sides of the resonance signal exhibited unequal half-full width at half maximum (FWHM). At 158 K, a single resonance line was detected at a chemical shift of 6.07 ppm. With increasing the temperature to 410 K, the NMR line displayed a separation into two distinct resonance lines at chemical shifts of 6.79 and 2.13 ppm for NH3 and CH2, respectively. The NH3 spinning sidebands are marked with crosses, and the sidebands for CH2 are marked with open circles in Fig. 4. Above 392 K, near TC, the 1H NMR signals for NH3 and CH2 began to separate. In addition, the narrow line width of the left signal (i.e., NH3) and the broader line width of the right signal (i.e., CH2) aligned well with the respective number of 1H in the NH3 and CH2 groups. The 1H chemical shifts for NH3 remained almost unchanged as the temperature increased, whereas those of CH2 experienced changes. These observations suggest that the surrounding environments of 1H of the NH3 groups remained relatively stable, whereas those of 1H of the CH2 group changed.
The variations of 13C chemical shifts in the MAS NMR spectra with temperature are shown in Fig. 5. The adamantane reference signal was used as the standard for the 13C chemical shifts. In the [NH3(CH2)6NH3] cation illustrated in Fig. 1, the CH2 groups positioned adjacent to the NH3 are designated as C(1) and C(1)#, while the centrally located CH2 groups are denoted as C(3) and C(3)#. The CH2 groups situated between C(1) and C(3) are labeled as C(2) and C(2)#, respectively. At 292 K, three distinct 13C NMR signals were observed; C(1) at 41.76 ppm, C(2) at 27.23 ppm, and C(3) at 25.71 ppm. As the temperature increased, the chemical shifts of C(1) remained largely unchanged, while those of C(2) and C(3) shifted positively. Near the TC temperature, C(1) was resolved into C(1)# with a different environment, and C(2) was separated into C(2)# with a different environment. No notable changes were observed in C(1), C(2), and C(3) around 340 K.
14N static NMR spectrum
The 14N static NMR spectrum of [NH3(CH2)6NH3]ZnCl4 single crystal was obtained in the temperature range of 180–420 K (Fig. 6). The 14N chemical shift range exhibited a significantly wider span compared to the 1H chemical shift range, making it valuable for determining the surrounding environment of 14N. In this study,
the external magnetic field was measured in an arbitrary direction of the single crystal. Considering the spin number of 14N as I=1, the NMR spectrum predicts two resonance lines due to the quadrupole interaction.41 However, due to the very low Larmor frequency of 28.90 MHz for obtaining the 14N NMR spectrum, the signal acquisition was challenging. The 14N NMR chemical shifts shown in Fig. 6 disappeared above 340 K. The SCXRD results listed in Table 2 indicated the formation of three N‒H∙∙∙Cl hydrogen bonds at 250 and 300 K. However, one N‒H∙∙∙Cl at 350 K was broken, leaving two N‒H∙∙∙Cl hydrogen bonds intact. This observation aligned with the disappearance of the 14N NMR signal above 340 K. The continuous change in the 14N chemical shifts with increasing temperature indicated a variation in the local environment and coordination geometry of the 14N atoms.
1H and 13C spin-lattice relaxation times
The spin-lattice relaxation time T1ρ for 1H and 13C was determined by acquiring an FID after applying the spin-lock pulse. The intensity changes of the measured magnetization are described by the following equation:42
S(t)/S(0) = exp(‒τ/T1ρ), (3)
where S(t) is the intensity of the resonance line at the delay time t, S(0) is the intensity of the NMR spectrum at the delay time t=0. The experiment was conducted with various τ values, and the corresponding T1ρ values were determined from the slopes of the resulting intensities plotted against the delay time according to Eq. (3). The T1ρ values of the 1H of NH3 and CH2 in [NH3(CH2)6NH3]ZnCl4 were obtained, and are represented in Fig. 7 as a function of the 1000/temperature. The T1ρ value for 1H in NH3 and CH2 decreased slightly with increasing temperature, followed by a rapid increase above 177 K. Moreover, the 1H T1ρ values in NH3 and CH2 reached a maximum near 340 K, followed by a subsequent decrease in a similar manner. At 177 K, the minimum T1ρ value for 1H in CH2 and NH3 were 4.6 ms and 6.2 ms, respectively. These patterns in T1ρ values indicate active molecular motion. According to the Bloembergen-Pound-Purcell (BPP) theory,43, 44 the experimental T1ρ value is directly related to the correlation time τC as shown in Eq. (4).
(T1ρ)–1 = R[4f1(ω1) + f2(ωC ‒ ωH) + 3f3(ωC) + 6f4(ωC + ωH) + 6f5(ωH)], (4)
f1=τC/[1 + ω12τC2], f2=τC/[1 + (ωC ‒ ωH)2τC2], f3=τC/[1 + ωC2τC2],
f4=τC/[1 + (ωC + ωH)2τC2], f5=τC/[1 + ωH2τC2],
where R(γH γC ħ/r3)2; constant, ω1; the spin-lock field, and ωC and ωH; the Larmor frequencies for carbon and proton. When ω1τC = 1, a minimum value is obtained for T1ρ, enabling the determination of the constant R using τC. R, ω1, ωH, ωC, and T1ρ are then used to calculate τC at a given temperature change. Local field fluctuations around 1H can be explained by the thermal motion of thermally activated protons. For the correlation time τC, the activation energy Ea can be obtained using Eq. (5) shown below41
τC = τC (0) exp(‒Ea/kBT) (5)
where τC(0); the pre-correlation time, Ea; the activation energy, kB; the Boltzmann constant, and T; the temperature. The magnitude of Ea is related to molecular kinetics. When 1000/T for 1H is plotted against τC on a logarithmic scale, as shown in Fig. 7, Ea values of 16.82 ± 1.71 kJ/mol and 19.61 ± 1.71 kJ/mol for 1H in NH3 and for 1H in CH2 can be obtained from the slopes of the solid lines, which were almost similar within the error range.
Furthermore, the T1ρ values of C(1), C(2), C(2)#, and C(3) for 13C in [NH3(CH2)6NH3]ZnCl4 were measured, and their results are represented in Fig. 8 as a function of the 1000/temperature. The T1ρ values for 13C of all three types kinds exhibited a slight decrease as the temperature rose and showed a slow increase at temperatures above 216 K. Notably, the T1ρ values of C(1), C(2), C(2)#, and C(3) demonstrated trends similar to each other. Regarding the T1ρ value of 1H, C(3) exhibits a minimum value of 22 ms, and their 13C T1ρ patterns underwent active molecular motion. The logarithmic plot of the τC versus 1000/T for 13C is shown as an inset in Fig. 8. The slope of the dotted line in the figure yielded Ea=6.56 ± 0.63 kJ/mol, and the values for C(1), C(2), and C(2)# were nearly identical within the margin of error.