Temperature-induced reversible structural phase transition of 1,4-dimethyl-1,4-diazabicyclo[2.2.2]octane bis(perchlorate)

Abstract

1,4-Dimethyl-1,4-diazabicyclo[2.2.2]octane bis(perchlorate), C₈H₁₈N₂²⁺·2ClO₄⁻, was synthesized and separated as colorless rodlike crystals. Differential scanning calorimetry (DSC) measurements revealed that this compound underwent a reversible phase transition at ca. 201.7 K with a hysteresis of 5.1 K width, which was also confirmed by dielectric measurements. Single crystal X-ray diffraction data suggested that there was a transition from a room temperature phase with the space group of Pnma (a = 15.5417(14) Å, b = 13.3677(12) Å, c = 20.7728(19) Å, V = 4315.7(7) ų and Z = 12) to a low temperature one with a space group of P2₁/n (a = 13.331(3) Å, b = 15.185(4) Å, c = 20.477(5) Å, β = 90.895(3)°, V = 4144.8(17) ų and Z = 12), and symmetry breaking occurred with an Aizu notation of mmmF2/m. The order–disorder transition of ClO₄⁻ anions and the ordering of twisting motions of the dabco ring may drive the phase transition.

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Introduction

Due to the discovery of the phase transition materials including the ferroelectrics, substantial attention has recently been focused on the simple molecular-ionic salts, which contain an organic cation and acid radicals, owing to the fact that they have potential applications in signal processing, data storage, sensing, and switchable dielectric devices, etc.¹ It is crucial to prepare new temperature-triggered molecular-based phase transition materials, not only for the theoretical study of the relationship of structure–property, but also for exploring the novel physical properties.^2–4 Constructing new types of compounds is the key problem in developing phase transition materials, which requires deep understanding of the origin of phase transitions. Up to now, the order–disorder type is always the most commonly employed mechanism in the theory of phase change materials.⁵ We have chosen 1,4-diazabicyclo-[2.2.2]octane (dabco) as an organic cation, not only due to it’s high order symmetry, but also because it allows for the design of molecular-based phase transition materials (which has been verified recently) due to ‘frozen’ ordering or molecular rotation in the crystal lattice, which provides lots of room for us to explore phase transition materials from the viewpoint of structural engineering.^6–8 Due to their moderate size and highly symmetric shape, the well-known ball-like anions such as ClO₄⁻ are stable and prone to position changes with varying temperature and weak interactions in crystals. The interaction of the cations with the monovalent tetrahedral counter anions (e.g. ClO₄⁻, BF₄⁻) is expected to generate the phase transition materials. 1,4-Diazabicyclo-[2.2.2]octane (dabco) forms monosalts with mineral acids, which have exceptional dielectric properties. These monosalts can be described by a general formula dabcoHX where X = ClO₄⁻ and BF₄⁻.⁹ Taking all this into consideration, we report a new compound herein with reversible structural phase transition properties, 1,4-dimethyl-1,4-diazabicyclo[2.2.2]octane bis(perchlorate), as a continuation of our systematic studies of phase transitions.¹⁰ It is based on the potential bridging ligand, 1,4-dimethyl-1,4-diazabicyclo[2.2.2]octane-1,4-diiumdibromide. The complex can be described by a general formula (CH₃–dabco–CH₃)₂⁺·2Br⁻, which was further characterized by variable-temperature single crystal X-ray diffraction, differential scanning calorimetry (DSC) and dielectric constant measurements.

Experimental

Materials and measurements

All reagent-grade chemicals and solvents were obtained from commercial sources and used without further purification. Infrared spectra were recorded on a SHIMADZU IR prestige-21 FTIR-8400S spectrometer in the range of 4000–500 cm⁻¹ with samples in the form of potassium bromide pellets. X-ray powder diffraction (XRPD) data were collected by a Siemens D5005 diffractometer with Cu-Kα radiation (λ = 1.5418 Å). Elemental analyses were carried out on a Perkin-Elmer 240C elemental analyzer. Thermogravimetric analyses (TGA) were conducted on a NETZSCH TG 209 F3 thermo gravimeter with a heating rate of 10 K min⁻¹ in a N₂ atmosphere.

Synthesis of compound 1

1,4-Dimethyl-1,4-diazabicyclo[2.2.2]octane-1,4-diiumdibromide (3.02 g, 10 mmol) and perchloric acid (2 g, 20 mmol) were mixed in aqueous solution (30 ml) (Scheme 1). After being stirred for 30 min in air, the reaction mixture solution was evaporated slowly at room temperature for 3 days, and colorless rodlike crystals were obtained in 53% yield (based on perchloric acid). IR spectra of compound 1: 3438(s), 3041(vs), 2999(vs), 2898(w), 2018(s), 1629(w), 1468(vs), 1375(s), 1335(s), 1105(vs), 919(w), 885(s), 843(s), 622(w). Anal. (%) calcd for C₈H₁₈Cl₂N₂O₅: C, 32.78; H, 6.19; N, 9.56; found: C, 32.71; H, 6.13; N, 9.49 (Caution! Although no problems have been encountered herein, perchlorates are potentially explosive and should be handled with care and only in small quantities).

Scheme 1 Synthesis of compound 1.

Single-crystal X-ray crystallography

Single-crystal X-ray data were collected on a Bruker SMART-APEX II CCD with Mo-Kα radiation (λ = 0.71073 Å). A colorless rodlike crystal of approximate dimensions 0.30 × 0.30 × 0.20 mm was used for the data collection at 298 K, 150 K and 100 K. The data processing including empirical absorption correction was performed using SADABS. The structures of 1 were solved by direct methods and refined by the full-matrix method based on F² by means of the SHELXLTL software package.¹¹ Non-H atoms were refined anisotropically using all reflections with I > 2σ(I). All H atoms were found in the difference maps. However, carbon-bond H atoms were added geometrically and refined using the riding model with U_iso = 1.2U_eq. Asymmetric units and packing views were drawn with DIAMOND (Brandenburg and Putz, 2005). Distances and angles between some atoms were calculated using DIAMOND and other calculations were carried out using SHELXLTL. Crystallographic data and structure refinement of the 298 K, 150 K and 100 K phases are listed in Table 1.

Table 1 Crystallographic data for 1 at 298 K, 150 K and 100 K

T (K)	298	150	100
Empirical formula	C8H18Cl2N2O8	C8H18Cl2N2O8	C8H18Cl2N2O8
Formula weight	341.14	341.14	341.14
Crystal system	Orthorhombic	Monoclinic	Monoclinic
Space group	Pnma	P21/n	P21/n
a (Å)	15.5417(14)	13.365(3)	13.331(3)
b (Å)	13.3677(12)	15.240(3)	15.185(4)
c (Å)	20.7728(19)	20.431(4)	20.477(5)
α (°)	90	90	90
β (°)	90	90.719(2)	90.895(3)
γ (°)	90	90	90
V (Å^3)	4315.7(7)	4161.2(14)	4144.8(17)
Z	12	4	12
Dc (g m^−3)	1.575	1.634	1.640
μ (mm^−1)	0.489	0.507	0.509
F (000)	2136	2136	2136
θ range [°]	1.64 to 25.68	1.00 to 25.60	0.99 to 25.68
Collected reflections	32 689	31 453	31 339
Unique reflections	4284	7776	7783
R1, wR2 [I > 2σ(I)]	0.0787, 0.1863	0.0643, 0.1542	0.0737, 0.1834
R1, wR2 [all data]	0.1060, 0.2117	0.0747, 0.1639	0.0890, 0.2039
GOF	1.048	1.104	1.059
Largest peak and hole (e Å^−3)	0.564 and −0.632	1.478 and −0.768	1.428 and −0.775

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Dielectric measurements

The complex dielectric permittivity ε (ε = ε′ − iε′′) was measured on a Tonghui TH2828A in the frequency range from 5 KHz to 1 MHz with the temperature from 100 to 300 K, at the AC voltage of 1 V. A pellet sample was prepared at 10 MPa and the pressed powder pellet deposited with silver-conducting glue was used for the dielectric studies.

DSC measurement

The differential scanning calorimetry (DSC) analysis of crystal 1 (19.74 mg) was performed using a Perkin-Elmer Diamond DSC instrument in the temperature range of 160 to 260 K in aluminum crucibles, accompanied by a rate of 10 K min⁻¹ on cooling–heating at atmospheric pressure. DSC curves of 1 were obtained in a heating–cooling mode at 5 K min⁻¹, 10 K min⁻¹, 15 K min⁻¹ and 20 K min⁻¹ respectively, as shown in Fig. S4.†

XRPD

The phase purity of 1 was determined using XRPD. The XRPD patterns of 1 at 100 K, 120 K, 150 K, 183 K, 223 K, 248 K and 298 K, were shown in Fig. 1. The peak positions of the experimental and simulated XRPD patterns are in good agreement, as shown in Fig. S5–S7.† The intensity differences may be attributed to the preferred orientation of the powder sample.

Fig. 1 The XRPD patterns of 1 at 100 K, 120 K, 150 K, 183 K, 223 K, 248 K and 298 K, respectively.

DSC

Differential scanning calorimetry (DSC) is well-known as one of the thermodynamic methods that was used to confirm the phase transitions triggered by temperature. Heat anomalies can be observed when the compound undergoes structural phase transitions during heating and cooling, accompanied by thermal entropy change. Reversible heat anomalies can be detected by DSC measurement upon heating and cooling; these may be caused by the disorder–order of ClO₄⁻. Upon cooling and then heating the crystalline sample, the DSC measurement shows a main exothermic peak at 201.7 K and a main endothermic peak at 206.8 K respectively, as shown in Fig. 2. This couple of exothermic and endothermic peaks represents a reversible phase transition. This phase transition is accompanied by the release of heat and the thermodynamic quantities (internal energy, enthalpy, volume etc.) are discontinuous; this is known as a first order phase transition.

Fig. 2 DSC curves of 1 obtained in a heating–cooling mode.

Crystal structure of 1

In order to confirm the phase transition of 1, the crystal structures were further resolved at 298 K, 150 K and 100 K respectively, for the forthcoming comparative investigation (Fig. 3a–c). Variable-temperature X-ray single-crystal diffraction reveals that 1 belongs to the orthorhombic crystal system with a centrosymmetric space group of Pnma (no. 62) and the point group D_2h, at room temperature (298 K). When the temperature decreases to 100 K, the crystal structure of 1 changes to a monoclinic crystal system with the centrosymmetric space group P2₁/n (no. 14) and the point group C_2h. During the cooling process, the symmetry breaking occurs with an Aizu notation of mmmF2/m.¹²

Fig. 3 View of the coordination environment of 1 with an atomic numbering scheme at (a) 298 K, (b) 150 K, (c) 100 K. Symmetry codes: ^#1 x, −y + 1/2, z.

At room temperature (298 K), the crystals are in the orthorhombic space group Pnma (no. 62), with cell parameters of a = 15.5417(14) Å, b = 13.3677(12) Å, c = 20.7728(19) Å, V = 4315.7(7) ų and Z = 12. At 150 K, the crystals transform to a monoclinic crystal system with the space group P2₁/n (no. 14). As for the 100 K structure, the crystals adopt the same structure as those at 150 K and a slight difference can be seen by comparing the crystal data, which suggests that there is no occurrence of a phase transition between 150 K and 100 K. The cell parameters of 1 measured at 100 K are as follows: a = 13.331(3) Å, b = 15.185(4) Å, c = 20.477(5) Å, β = 90.895(3)°, V = 4144.8(17) ų and Z = 12. Visual inspection reveals no obvious changes at the three temperatures. The molecular (ion pair) volume decreases from 4315.7(7) ų in the orthorhombic cell to 4144.7(18) ų in the monoclinic cell, despite no change in the lengths of the a-axis and b-axis.

In the RT phase (298 K) crystal structure of 1, as shown in Fig. 3a, interestingly it is found that the ClO₄⁻ anions are all disordered, which may result in the formation of a higher symmetry (Pnma). Meanwhile, the atoms N1 and N2, which come from the C₈H₁₈N₂²⁺ cations combined with the atoms of Cl3 and Cl4 from the discrete ClO₄⁻ anions, lie on a common crystallographic mirror plane in the crystal of 1 (Fig. 3a). The ClO₄⁻ anions are seriously disordered with the distances of Cl–O ranging from 1.28(3) Å to 1.505(15) Å; these are in good agreement with those observed in similar compounds.¹³

In the LTP crystal structure at 150 K, the disordered ClO₄⁻ anions are frozen and become relatively ordered, as shown in Fig. 3b. Some of the ClO₄⁻ anions adopt an ideal tetrahedral geometry with the distances of Cl–O from 1.401(5) Å to 1.454(5) Å and the angles of O–Cl–O from 107.2(3)° to 112.8(4)°; these lengths of bonds are similar to those observed in the higher-temperature phase. However, there is still the presence of seriously disordered ClO₄⁻ anions with O atoms over two positions.

In the lower-temperature phase (LTP, 100 K), as shown in Fig. 3c, some of the ClO₄⁻ anions adopt an ideal tetrahedral geometry with the distances of Cl–O from 1.413(6) Å to 1.456(5) Å and the angles of O–Cl–O from 107.7(3)° to 114.0(4)°, which are comparable to those in the HTP at 298 K. In addition, there were still some disordered ClO₄⁻ anions as there were in the LTP (150 K). Meanwhile, the conformations of the rings of dabco were significantly different between the HTP (298 K) and LTP (150 K, 100 K) phases. It is notable that in the LTP (100 K), the bonds of N–C–C–N in the three dabco rings exhibit large twisting conformations with the torsion angles from 14.156° to 23.260°.

X-ray crystal structures of 1 were measured at 298 K, 270 K, 250 K, 230 K, 210 K, 200 K, 190 K, 180 K, 170 K, 160 K, 150 K, 140 K, 130 K, 120 K, 110 K and 100 K. The cell parameters of 1 measured from 270 K to 210 K are slightly different from those measured at room temperature (298 K), i.e. in other words, the influence of thermal expansion and contraction can be excluded. Thus, we can conclude that there is no phase transition at 210 K. Particularly compared with those at 210 K, the crystal cell parameters of 1 change dramatically at 200 K. In addition, there were no obvious changes in the crystal cell parameters of 1 measured from 200 K to 100 K, indicating the absence of a phase transition (Fig. 4). The LTP structure undergoes a modest change in a axis lengths compared to those in the RTP. However, a distinct change occurred in the length of the a-axis, b-axis (Fig. 4a) and β angle (Fig. 4b). A phase transition between 200 and 210 K was confirmed by X-ray diffraction at various temperatures. At higher temperatures (210–298 K) no phase transitions occurs and the crystals are in the orthorhombic space group Pnma; at lower temperatures the crystals transform to a monoclinic crystal system. These measurements are in agreement with the DSC data.

Fig. 4 (a) Temperature dependence of the unit-cell length parameters of 1. (b) Temperature dependence of angle beta and the cell volume of 1.

Dielectric properties

It is well-known that the phase transition will accompany an anomaly of the physical properties near the structure phase transition point, such as the dielectric constant.¹⁴ Herein, we measured the temperature- and frequency-dependent dielectric properties to confirm the phase transition and molecular motions in complex 1, and plots of the dielectric constant (ε′) versus temperature at different frequencies are displayed in Fig. 5.

Fig. 5 Temperature-dependent dielectric constant of 1 at different frequencies.

As expected, a clear dielectric anomaly occurred at about 200 K in the dielectric constant upon heating as shown in Fig. 5, which corresponds to the results of DSC and the mutations of cell parameters very well. Thus, the phase transition triggered by temperature results in dielectric anomaly. Furthermore, these figures show the following features: (i) 1 has an invariant ε′ (about 2.7) in the low-temperature region below 190.0 K; (ii) the dielectric constant rapidly increases when the temperature rises above 200.0 K, and no maximum peak appears in the plots of ε′ versus T below 220.3 K; (iii) the increases in the dielectric constant with temperature depend strongly on the frequency of the ac electric filed, which is obvious when the frequency is 5 KHz. In addition, continuously increasing the frequency from 5 × 10³ → 10⁴ → 10⁵ → 10⁶ Hz gradually decreases the dielectric constant, which is 27.9, 20.6, 8.4, 4.5 at these frequencies, respectively. These features of the dielectric properties reveal that compound 1 is a potential tunable dielectric material.

Conclusions

In summary, DSC, dielectric measurements and variable-temperature structural analysis revealed that 1,4-dimethyl-1,4-diazabicyclo[2.2.2]octane bis(perchlorate) underwent a reversible phase transition at ca. 201.7 K. Crystal structures of 1 obtained at 298 K, 150 K and 100 K revealed that it reversibly transformed from the HTP space group of Pnma to the LTP space group of P2₁/n. with an Aizu notation of mmmF2/m.¹² A high symmetry of ClO₄⁻ anions, which were prone to the transition of order–disorder under different temperatures, along with the ordering of twisting motions of the dabco ring probably drove the phase transition.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21201087), NSF of Jiangsu Province (BK20131244), the Foundation of Jiangsu Educational Committee (11KJB150004), the Qing Lan Project of Jiangsu Province and the Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teacher and Presidents, and the Innovation Program of Graduate Students in Jiangsu Province (SJZZ_0142).

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Footnote

† Electronic supplementary information (ESI) available: In addition, IR, PXRD, and TGA of complex 1. CCDC 1044458–1044460. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra07526b

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