Crystal structures, phase transitions, and switchable dielectric behaviors: comparison of a series of N-heterocyclic ammonium perchlorates.

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DOI: 10.1039/C5DT00263J

Crystal Structures, Phase Transitions, and Switchable Dielectric Behaviors: Comparison of a Series of

Published on 24 March 2015. Downloaded by Northern Illinois University on 25/03/2015 05:39:29.

Ping-Ping Shi, Qiong Ye*, Qiang Li, Hui-Ting Wang, Da-Wei Fu, Yi Zhang and Ren-Gen Xiong* Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P. R. China

ABSTRACT: Three

analogue

N-heterocyclic

complexes,

1-propyl-1-methylpiperidinium

perchlorate (1, [PMpip][ClO4]), 1-cyanomethyl-1-methylpiperdinium perchlorate (2, [CMpip][ClO4]),

and

1-cyanomethyl-1-methylmorpholinium

perchlorate

(3,

[CMmor][ClO4]) are discovered as phase transition materials displaying switchable dielectric behaviors. Despite the common [ClO4]– anion and the closely related cations, compound 1 crystallizes in the orthorhombic space group P212121, but compounds 2 and 3 belong to the monoclinic space group P21/n with distinct cell dimensions. Compounds 1, 2 and 3 undergo reversible phase transitions around 199, 387 and 416 K, respectively, accompanied by the notable step-like dielectric anomalies which could be switched by the phase transition and be tuned in distinct dielectric states. The dielectric constants in the high dielectric states are respective 1.2, 2.2 and 3.2 times that in the low dielectric states for compounds 1, 2 and 3. Generally, these differences in phase transition and dielectric properties are caused by the distinct molecular structures as well hydrogen-bonding conformations resulting from the

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N-heterocyclic Ammonium Perchlorates

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DOI: 10.1039/C5DT00263J



structural variations in the side-chain and the ring structure.

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INTRODUCTION

DOI: 10.1039/C5DT00263J

Triggered by the external stimuli like temperature, pressure and light, solid-state


structures and some physical properties such as thermal, optical, dielectric, magnetic, and even ferroelectric properties.1−9 Consequently, phase transition materials have recently attracted a great deal of attention owing to the wide applications in data storage, signal processing, switchable dielectric device and so on.10−14 Although much progress has been achieved in the development of phase transition materials, the prediction and design of them are still challenging presently due to the lack of full understandings of the complicated structure-property relationships.15−23 However, for the reason that dielectric responses are sensitive to the internal motions and structural variations of materials, constructing molecular compounds containing motional or flexible moieties is a potential strategy to obtain phase transition materials coupled with functional dielectric properties.24−32 Specifically, as one class of attractive dielectric materials playing an important role in photo-electronic fields, switchable molecular dielectrics generally have dielectric constants which could be switched by the reversible phase transitions and be tuned by the different dielectric states.33−39 Such characteristic dielectric behaviors always result from the motional changes of the polar moieties between “melt-like” and “frozen” states during the phase transition processes. 40−44 For example, a representative work has been reported by Zhang et al., in which the tunable and switchable dielectric response accompanied by a phase transition below room temperature was generated by the order−disorder transitions of


phase transitions usually occur accompanied by the dramatic variations of crystal

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DOI: 10.1039/C5DT00263J the polar [(CH3)2NH2]+ cations.40 Lately, we also obtained a class of multifunctional

materials, [(CH3)4P][FeX4] (X = Cl− and Br−), displaying simultaneously switchable


transitions above room temperature.2 Molecular-ionic compounds have been extensively investigated for the explorations of novel phase transition materials as well switchable molecular dielectrics.33−48 Nevertheless, their practical applications are often restricted by the relatively low phase transition temperatures observed in most of the present switchable dielectrics. Thus, the significant challenge is how to obtain new above-room-temperature switchable dielectric materials. During the past decade, a family of organic salts containing anions like I−, BF4−, PF6−, TFSI− (bis(trifluoromethanesulfonyl)imide) and organic N-heterocyclic cations like N-alkyl-N-methylpyrrolidinium, piperidinium and morpholinium has been discovered as ionic liquids with potential applications in electrochemical devices and biocompatible solvents.49−59 They are composed of flexible cationic moieties with cyclohexane-type rings easily generating solid-state phase transitions, which should be good candidates for potential dielectric phase transition materials.3−5,60 However, the relevant reports about reversible phase transitions and dielectric properties of such salts are scarce, limited by the low melting points generally below 100℃. Bearing these in mind, the emphasis of our work is to investigate the effects of suitable structural variations on the crystal structures and physical properties like melting points, phase transition properties and dielectric behaviors, and thus to obtain functional dielectric materials coupled with phase


dielectric, magnetic, and optical properties, accompanied by the sequential phase

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DOI: 10.1039/C5DT00263J transitions which are more suitable for applications. Here we present three compounds

with a common [ClO4]– anion and the closely related N-heterocyclic cations, that is, (PMpip+),

1-cyanomethyl-1-methylpiperdinium


(CMpip+), and 1-cyanomethyl-1-methyl morpholinium (CMmor+) in compounds 1, 2 and 3, respectively (Scheme 1). As expect, the structural modifications lead to distinct crystal structures, hydrogen-bonding conformations, phase transition temperatures and dielectric properties. [PMpip][ClO4] (1), [CMpip][ClO4] (2), and [CMmor][ClO4] (3) undergo reversible phase transitions around 199, 387 and 416 K, respectively, accompanied by the step-like dielectric responses in good accordance with the switchable molecular dielectrics. Meanwhile, the thermal hystereses and magnitudes of the dielectric anomalies for compounds 1, 2 and 3 are also different.

Scheme 1. Chemical structures of (a) 1-propyl-1-methylpiperidinium (PMpip+), (b) 1-cyanomethyl-1-methylpiperdinium

(CMpip+),

and

(c)

1-cyanomethyl-1-

methylmorpholinium (CMmor+) cations in compounds 1, 2 and 3. EXPERIMENTAL SECTION

Syntheses. All of the reagents were of analytical grade and used without any further purification.


perchlorate

(1,

[PMpip][ClO4]).

1-bromopropane (0.11 mol) was added dropwise to 1-methylpiperidine (0.1 mol) in 50 ml dry chloroform. Then the resulting mixed solution was stirred at room



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DOI: 10.1039/C5DT00263J temperature for 24 h. After the solvent was removed under reduced pressure, the

white solid product, 1-propyl-1-methylpiperidinium bromide ([PMpip][Br]), was


[PMpip][Br] (0.01 mol) and HClO4 (70%, 0.01 mol) at room temperature. Colorless crystals suitable for the X-ray crystal structural analysis were easily obtained by slow evaporation of the mixed aqueous solution, mp ~110℃. 1-cyanomethyl-1-methylpiperdinium

perchlorate

(2,

[CMpip][ClO4]).

1-methylpiperidine (0.1 mol) was dissolved in 50 ml dry acetonitrile, and then bromoacetonitrile (0.11 mol) was added dropwise to it. The reaction mixture was stirred overnight at room temperature and after reduce pressure distillation, [CMpip][Br] was obtained as white solids. Colorless crystals of [CMpip][ClO4] were obtained by evaporation of an aqueous solution containing equimolar [CMpip][Br] and HClO4, mp ~285℃. 1-cyanomethyl-1-methylmorpholinium perchlorate (3, [CMmor][ClO4]). The synthesis of [CMmor][Br] was similar to that of [CMpip][Br], by combining 1-methylmorpholine (0.1 mol) and bromoacetonitrile (0.11 mol) in dry acetronitrile. Reaction of [CMmor][Br] (0.01 mol) and HClO4 (70%, 0.01 mol) in deionized water gave colorless crystals of [CMmor][ClO4], mp ~260℃. Caution! Although our samples never exploded during handing, perchlorates are potentially explosive. Only a small amount of material should be prepared, and it should be handled with caution. For compounds 1, 2 and 3, elemental analyses for C, H and N contents were carried


collected and dried in vacuo. [PMpip][ClO4] was synthesized by the reaction of

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DOI: 10.1039/C5DT00263J out on the Elementar Vario MICRO (Germany) analyzer. Anal. Calcd for

C9H20NClO4 (1): C, 44.72%; H, 8.34%; N, 5.80%. Found: C, 44.70%; H, 8.36%; N,


40.41%; H, 6.46%; N, 11.71%. Anal. Calcd for C7H13N2ClO5 (3): C, 34.94%; H, 5.45%; N, 11.64%. Found: C, 34.80%; H, 5.50%; N, 11.59%. Infrared spectra (as shown in Figure S1, Supporting Information) were obtained at room temperature on a Shimadzu model IR-60 spectrometer, with all samples prepared as KBr-diluted pellets. The peaks at approximately 1100 cm−1 confirm the presences of [ClO4]– anions in compounds 1, 2 and 3, and the peaks at approximately 2280 cm−1 confirm the presences of cyano-groups in compounds 2 and 3. Single-Crystal X-ray Diffractions. The room-temperature single-crystal structural analyses of compounds 1, 2 and 3 were carried out on a Rigaku Saturn 924 diffractometer with Mo Kα radiation (λ = 0.71073 Å). The suitable crystals used for data collection had approximate dimensions of 0.20×0.20×0.20 mm. The structures were solved by direct methods using the SHELXS program and refined by the full-matrix least-squares refinements on F2 using the SHELXTL package. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were generated geometrically at idealized positions. For compounds 1, 2 and 3, a summary of crystallographic data and detailed structure refinements at room temperature are given in Table 1. Table1. Crystallographic data and structure refinements for [PMpip][ClO4] (1), [CMpip][ClO4] (2), and [CMmor][ClO4] (3) at room temperature


5.60%. Anal. Calcd for C8H15N2ClO4 (2): C, 40.26%; H, 6.33%; N, 11.74%. Found: C,

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compound

1

2

3

moiety formula crystal system, space group temperature (K) a (Å) b (Å) c (Å)

C9H20N, ClO4 orthorhombic, P212121 293 8.0231(16) 12.525(3) 12.668(3)

C8H15N2, ClO4 monoclinic, P21/n 293 7.7088(15) 11.933(2) 12.458(3)

C7H13N2O, ClO4 monoclinic, P21/n 293 6.6726(13) 19.552(4) 8.2906(17)

α (deg)

90.00(0)

90.00(0)

90.00(0)

β (deg) γ (deg) V (Å3)

90.00(0) 90.00(0) 1273.0(5)

94.94(3) 90.00(0) 1141.7(4)

96.50(3) 90.00(0) 1074.7(4)

Z

4

4

4

μ (mm ) Tmin/Tmax F(000) number of measured, independent and observed [I > 2(I)] reflections

1.261 0.943/0.943 520

1.388 0.936/0.936 836

1.487 0.931/0.931 504

7225, 2243, 1602

9546, 2007, 1785

10975, 2470, 2058

Rint

0.0556

0.0499

0.0321

refinement R[F2 > 2(F2)], wR(F2), GOF

0.1015, 0.2802, 1.163

0.0751, 0.2120, 1.071

0.0637, 0.1679, 1.116

−1

DOI: 10.1039/C5DT00263J

Powder X-ray Diffractions. For compounds 1, 2 and 3, variable-temperature powder X-ray diffraction (PXRD) measurements were performed on a Rigaku D/MAX 2000 PC X-ray diffractometer. PXRD patterns were recorded in the 2θ range of 5°−52° with a step size of 0.02°. As shown in Figure S2 (Supporting Information), PXRD patterns obtained at 298 K coincide fairly well with the simulated patterns based on the crystal structures at room temperature, indicating the phase purity of as-grown crystals of the three compounds. DSC Measurements. Differential scanning calorimetry (DSC) experiments were carried out on a Perkin-Elmer Diamond DSC instrument. The crystalline samples of compounds 1 (8.2 mg), 2 (11.2 mg) and 3 (20.1 mg) in aluminum crucibles were



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DOI: 10.1039/C5DT00263J heating and cooling with a rate of 10 K min−1 during the temperature ranges of

182−273 K, 315−425 K and 320−445 K, respectively.


complex dielectric constants (ε = ε′ − iε″, where ε″ represent the imaginary part) and the dielectric losses (tanδ = ε″/ε′) were measured on the Tonghui TH2828 analyzer at the respective frequencies of 100 kHz and 1 MHz within the temperature ranges of 134−290 K, 306−424 K and 302−451 K, respectively. The pressed-powder pellets deposited with carbon conducting glue were used as electrodes in the dielectric measurements under an applied AC electric field of 1 V, with a heating and cooling rate of about 10 K min−1. RESULTS AND DISCUSSION Thermal Properties. For the purpose of detecting the reversible phase transitions triggered by temperature, DSC measurements were carried out on compounds 1, 2 and 3. As shown in Figure 1a, 1b and 1c, respectively, DSC curves of compounds 1, 2 and 3 were obtained in the cooling and heating cycles. Despite the analogue chemistry formulas of [PMpip][ClO4] (1), [CMpip][ClO4] (2) and [CMmor][ClO4] (3), they displayed distinct phase transition temperatures (TC). For [PMpip][ClO4] (1), upon cooling and heating, both the pair of sharp anomalies appearing around TC(1) ≈ 199 K and the large thermal hysteresis of 13 K are indicative of a first-order reversible phase transition. Based on the DSC curves of compound 1, the average enthalpy change ΔH(1) is estimated to be 0.93×103 J mol−1, and given that the entropy change ΔS =


Dielectric Measurements. For compounds 1, 2 and 3, the real parts (ε′) of the

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DOI: 10.1039/C5DT00263J ΔH/TC, ΔS(1) is obtained as 4.67 J mol−1 K−1. Interestingly, different from the

low-temperature phase transition occurring in compound 1, [CMpip][ClO4] (2)


Considering the shape of the observed endothermic and exothermic peaks accompanying the relatively large thermal hysteresis of about 21 K, the phase transition around TC(2) is attributed to a first-order type, too. In view of the DSC curves, ΔH(2) and ΔS(2) are estimated as 7.14×103 J mol−1 and 18.45 J mol−1 K−1, respectively. For [CMmor][ClO4] (3), the sharp anomalies centered at TC(3) ≈ 416 K and the thermal hysteresis of 28 K also indicate a reversible first-order phase transition. Furthermore, the calculated values of ΔH(3) and ΔS(3) are 7.59×103 J mol−1 and 18.24 J mol−1 K−1, respectively. According to the Boltzmann equation ΔS = R lnN, where R means the gas constant and N represents the ratio of numbers of possible configurations, N(1), N(2) and N(3) are estimated to be 1.76, 9.19 and 8.97 for compounds 1, 2 and 3, respectively. Generally, the large N values suggest severe structural transformations or molecular dynamics accompanying the phase transitions, especially for compounds 2 and 3. For compounds 1 and 2, the phase transition temperature is significantly improved by replacing the propyl side-chain with a cyanomethyl one, that is, about 188 K. With respect to compounds 2 and 3, modifying the six-membered ring by the substitution of a single ring CH2 segment with an ether O atom also results in the increase of the phase transition temperature. However, the relative increment of about 29 K is comparatively smaller than that in the cases of compounds 1 and 2, suggesting that the variation of the side-chain may make more


undergoes a reversible phase transition above room temperature, i.e. TC(2) ≈ 387 K.

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DOI: 10.1039/C5DT00263J influence on the thermal property. As a matter of convenience, the phases below and

above TC(1) are labeled as the low temperature phase (LTP) and room temperature


above TC(2) and TC(3) as the high temperature phase (HTPs).

Figure 1. DSC curves of (a) [PMpip][ClO4] (1), (b) [CMpip][ClO4] (2), and (c) [CMmor][ClO4] (3) obtained in the cooling and heating cycles. Crystal Structure Discussions. Why are there so notable differences between the phase transition properties of compounds 1, 2 and 3, especially for compounds 1 and 2? To gain further insight into the effects of structural variations on the thermal properties or the other physical properties, for compounds 1, 2 and 3, the crystal structures at RTP are compared as follows. At RTP, [PMpip][ClO4] (1) crystallizes in the point group 222 and orthorhombic space group P212121, with cell parameters of a = 8.0231(16) Å, b = 12.525(3) Å, c = 12.668(3) Å, α = β = γ = 90.00 (0)°, and V = 1273.0(5) Å3. Despite the similar shape and size, the modification of the side-chain results in that [CMpip][ClO4] (2) belongs to the point group of 2/m and monoclinic space group P21/n at RTP, with a = 7.7088(15) Å, b = 11.933(2) Å, c = 12.458(3) Å, α = 90.00 (0)°, β = 94.94(3)°, γ = 90.00 (0)°, and V = 1141.7(4) Å3. For [CMmor][ClO4] (3), the


phase (RTP), respectively, the phases below TC(2) and TC(3) as RTPs, and the phases

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modification of the six-membered ring does not change the space group, butDOI: the10.1039/C5DT00263J unit cell dimensions are different, with a = 6.6726(13) Å, b = 19.552(4) Å, c = 8.2906(17)


in Figure 2, the asymmetric units of compounds 1, 2 and 3 are all composed of one [ClO4]− anion and one [PMpip]+, [CMpip]+ or [CMmor]+ cation which are located at the general positions. As shown in Figure 3, the six-membered rings in the three heterocyclic compounds all adopt chair conformations, in which the C1, C2, C3 and C4 atoms lie nearly in the same plane labeled as C4 plane. Interestingly, compared to the modification of the six-membered ring, the change of the side-chain made more differences between the molecular geometries. The N1 and C8 atoms deviate by respective 0.6291(32) and 0.6634(74) Å from the C4 plane in [PMpip][ClO4] (1), 0.6708(23) and 0.6561(43) Å in [CMpip][ClO4] (2). As for [CMmor][ClO4] (3), the deviations of N1 and O5 atoms from the C4 plane are 0.6605(20) and 0.6625(21) Å, respectively. Moreover, for comparison purposes, the key bond parameters of the six-membered rings of compounds 1, 2 and 3 are listed in Table 2. Apparently, not only the bond lengths but also the bond angles of the piperidinium ring in compound 2 are remarkably dissimilar to those found in compound 1. By contrast, except for the C−O bond lengths, the other bond parameters of the heterocycles in compounds 2 and 3 are nearly the same. Significant differences can also be found between the side-chains in addition to the ring structures. For instance, the methyl groups occupy the pseudo-axial positions in compounds 1 and 3, accordingly, the propyl group in compound 1 and the cyanomethyl group in compound 3 occupy the pseudo-equatorial


Å, α = 90.00 (0)°, β = 96.50(3)°, γ = 90.00 (0)°, and V = 1074.7(4) Å3. As illustrated

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DOI: 10.1039/C5DT00263J positions. Surprisingly, in compound 2, the methyl group occupies the

pseudo-equatorial position, but the more bulky cyanomethyl group is located in the


(methyl), N1−C6 (propyl or cyanomethyl) bonds and the C4 planes are 82.484(284) and 13.572(264)° in [PMpip][ClO4] (1), 6.803(184) and 77.561(171)° in [CMpip][ClO4] (2), as well 80.673(129) and 10.287(142)° in [CMmor][ClO4] (3). The C5−N1−C6 bond angle of 110.9(4)° in compound 1 is obviously larger than that of 109.1(3)° in compound 2, while in compound 3 the value is 109.6(2)°. Besides, more significant is the difference of the methyl substituent, that is, the N1−C5 bond distance in compound 1 (1.440(6) Å) is shorter than those in compounds 2 and 3 (1.500(4) and 1.501(3) Å, respectively). With respect to the other side-chain, for [PMpip][ClO4] (1), the propyl group displays an energetically preferred trans zig-zag chain conformation with a C6−C7−C9 bond angle of 120.6(7)° and a N1−C6−C7−C9 torsion angle of −179.7(6)°. Correspondingly, for [CMpip][ClO4] (2) and [CMmor][ClO4] (3), the cyanomethyl groups were almost linear with the essentially identical C6−C7≡N2 bond angles (178.1(4) and 177.8(3)°), while the N1−C6−C7≡N2 torsion angles of respective 144(13) and 148(11)° in compounds 2 and 3 are distinct. The [ClO4]− anions in compounds 1, 2 and 3 adopt distorted tetrahedral geometries. In particular, for [PMpip][ClO4] (1), [CMpip][ClO4] (2) and [CMmor][ClO4] (3), the Cl1−O bond lengths range from 1.293(6) to 1.438(5) Å, 1.408(4) to 1.459(4) Å, and 1.414(3) to 1.437(2) Å, respectively, with the O−Cl−O bond angles differ in the respective ranges of 105.6(4) to 111.1(3)°, 104.4(3) to 113.7(3)°, and 106.60(19) to


pseudo-axial one. To be specific, the corresponding angles between the N1−C5

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DOI: 10.1039/C5DT00263J 113.1(2)°. It is noteworthy that, for both the cations and anions, the equivalent

isotropic displacement parameters (or atomic temperature factors) of [PMpip][ClO4]


S1 (Supporting Information). The large and abnormal ellipsoids as depicted in Figure 2 always suggest the presence of disorder. That is, the RTP of [PMpip][ClO4] (1) can be corresponded to a disordered state, whereas the RTPs of [CMpip][ClO4] (2) and [CMmor][ClO4] (3) are more likely to be corresponded to the ordered states.

Figure 2. Molecular structures of (a) [PMpip][ClO4] (1), (b) [CMpip][ClO4] (2) and (c) [CMmor][ClO4] (3), showing atom labeling scheme with 30% probability thermal ellipsoids.

Figure 3. Cationic structures of (a) [PMpip][ClO4] (1), (b) [CMpip][ClO4] (2) and (c)


(1) are nearly twice or even more than those of compounds 2 and 3, as listed in Table

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DOI: 10.1039/C5DT00263J

[CMmor][ClO4] (3). The hydrogen atoms are omitted for clarity.

Table 2. Selected bond lengths (Å) and bond angles (deg) in PMpip][ClO4] (1),

Bond lengths

1

2

3

Bond angles

1

2

3

N1−C1 C1−C2 C2−C8 C2−O5 C8−C3 O5−C3 C3−C4 C4−N1 N1−C5 N1−C6 C6−C7 C7−C9

1.501(6) 1.401(9) 1.517(11)

1.516(4) 1.510(5) 1.530(5)

1.514(3) 1.510(4)

C4−N1−C1 N1−C1−C2 C1−C2−C8 C1−C2−O5 C2−C8−C3 C2−O5−C3 C8−C3−C4 O5−C3−C4 C3−C4−N1 C5−N1−C6

108.6(4) 115.0(5) 107.9(6)

109.0(2) 112.6(3) 111.6(3)

108.08(19) 111.0(2)

C7≡N2

1.426(4) 1.531(9) 1.419(8) 1.458(6) 1.440(6) 1.531(6) 1.407(8) 1.386(7)

1.519(6) 1.510(5) 1.528(4) 1.500(4) 1.506(4) 1.459(5)

1.420(3) 1.505(4) 1.515(3) 1.501(3) 1.509(3) 1.468(4)

1.143(5)

1.124(4)

111.2(2) 111.5(5)

110.5(3) 110.2(2)

109.2(5)

111.5(3) 111.4(2)

116.0(4) 110.9(4)

112.7(3) 109.1(3)

109.6(2)

In many cases, only small variations of the cations will directly impact on not only the molecular geometries but also both the packing structures and the hydrogen bond conformations.51,52,55,57,58 Given that the lack of acidic protons on the [PMpip]+, [CMpip]+ and [CMmor]+ cations, no classical hydrogen bond interaction is expected to play a prominent role in the crystal packing. However, several kinds of weak hydrogen bonds involving the carbon bound hydrogen atoms are observed in compounds 1, 2 and 3. Consequently, the diagrams and details of these weak C−H···A interactions (A represents the acceptor atom of the hydrogen bond) are outlined in Figure 4 and Table 3, respectively. For [PMpip][ClO4] (1), each [PMpip]+ cation is hydrogen bonded to two [ClO4]− anions, and the methyl substituent and one methylene group of the piperidinium ring act as hydrogen-bonding donors of the respective C5−H5C···O4 (3.469(7) Å) and C2−H2A···O4 (3.422(9) Å) interactions.



[CMpip][ClO4] (2) and [CMmor][ClO4] (3)

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DOI: 10.1039/C5DT00263J Replacing the propyl side-chain by a cyanomethyl one changes the hydrogen bond

architecture significantly. In the case of [CMpip][ClO4] (2), five weak C−H···O


anions. Except for the three C−H···O interactions within the range of 3.359(5) to 3.454(5) Å originating from the methylene group of the piperidinium ring, the methylene group of the cyanomethyl side-chain also participates in two C−H···O interactions with the generally shorter distances of 3.287(5) and 3.290(5) Å. In contrast, [CMmor][ClO4] (3) displays more complicated hydrogen-bonding formations resulting from the presence of the morpholinium ring. Specifically speaking, there are also five weak C−H···O interactions linking the [ClO4]− anions with the methylene hydrogen atoms of the morpholinium ring and the side-chain, while the C−H···O distances differ in the respective ranges of 3.262(4) to 3.384(4) Å and 3.177(4) to 3.279(5) Å. In addition, it is quite distinct from compound 2 that the methyl substituent and the methylene hydrogen atoms of the side-chain participate in two weak C5−H5C···O5 (3.227(3) Å) and C6−H6A···N2 (3.201(4) Å) interactions with the ether oxygen atom and the cyanomethyl nitrogen atom of the neighboring [CMmor]+ cations, respectively. Consequently, as shown in Figure 5, the cations and anions in compound 1 are interconnected by the zig-zag cahins of weak C−H···O hydrogen bonds, contrasting with the three-dimensional hydrogen-bonding networks found in compounds 2 and 3. Furthermore, both the packing structures of compounds 1, 2 and 3 display alternating two-dimensional sheets of cations and anions (Figure S3, Supporting Information). In the cases of [PMpip][ClO4] (1) and [CMpip][ClO4] (2),


hydrogen bonds are found between the [CMpip]+ cation and the neighboring [ClO4]−

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DOI: 10.1039/C5DT00263J the respective sheets of cations and anions are arranged to be nearly parallel to the ac

plane, however, the corresponding sheets of cations and anions in [CMmor][ClO4] (3)


In summary, only the slight variations of the cations lead to the distinct crystal structures of compounds 1, 2 and 3, which also make notable impacts on the thermal properties as indicated by the DSC results. Compared to the modification of the ring structure, the modification of the side-chain results in more significant differences between [PMpip][ClO4] (1) and [CMpip][ClO4] (2), reflected in the more distinct phase transition temperatures between them. On the one hand, taking into account the weak hydrogen bonds which are sensitive to heat and thus will affect the melting point,51,52 the extremely higher melting points of [CMpip][ClO4] (2) and [CMmor][ClO4] (3) may be closely related to the cyanomethyl substituent and the resultantly increased hydrogen bonding interactions. On the other hand, the more flexible groups like the propyl group in [PMpip][ClO4] (1) always result in lower melting points and make the compound more easily to undergo structural transitions, compared to the relatively rigid cyanomethyl group.52,57,58 Consequently, not only the melting point but also the phase transition temperature of [CMpip][ClO4] (2) are extremely higher than those of compound 1. In the cases of [CMpip][ClO4] (2) and [CMmor][ClO4] (3), the difference of the phase transition temperatures is probably due to the unusual configurations of the methyl (occupying the pseudo-equatorial position) and another bulky cyanomethyl substituent (occupying the pseudo-axial position) observed in compound 2, which may lead to the instable structure and thus


are approximately 45° to the ac plane.

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Figure 4. For (a) [PMpip][ClO4] (1), (b) [CMpip][ClO4] (2) and (c) [CMmor][ClO4] (3), weak C−H···O and C−H···N hydrogen bonds in the surroundings of the cations are shown with dashed lines, and the distances are indicated in Å. The other atoms not involved in the hydrogen-bonding interactions are omitted for clarity.

Figure 5. Packing diagrams of (a) [PMpip][ClO4] (1), (b) [CMpip][ClO4] (2) and (c) [CMmor][ClO4] (3) viewed down the c axis, with the weak hydrogen bonds shown as dashed lines. The hydrogen atoms not involved in any hydrogen-bonding interactions



the lower phase transition temperature.

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are omitted for clarity.

Table 3. Parameters of the weak hydrogen bonds in PMpip][ClO4] (1),

Compound

D−H···A

H···A (Å)

D···A (Å)

D−H···A (°)

2.55 3.422(9) 150.4 C2−H2A···O4#1 2.57 3.469(7) 156.3 C5−H5C···O4#2 2.48 3.359(5) 151.1 2 C1−H1A···O3#3 2.56 3.402(5) 145.0 C1−H1B···O3#4 2.58 3.454(5) 150.0 C3−H3A···O2#5 2.36 3.287(5) 159.4 C6−H6A···O1#6 2.34 3.290(5) 166.9 C6−H6B···O2#7 2.46 3.346(4) 151.1 3 C1−H1A···O1#8 2.44 3.262(4) 143.0 C3−H3B···O2#9 3.384(4) 144.9 C4−H4A···O3#10 2.54 3.227(3) 129.2 C5−H5C···O5#11 2.53 2.45 3.177(4) 131.4 C6−H6A···O3 3.201(4) 119.3 C6−H6A···N2#12 2.61 3.279(5) 149.8 C6−H6B···O1#13 2.40 Symmetry code: #1 1/2+x, 3/2−y, 1−z. #2 x, 1+y, z. #3 1/2−x, −1/2+y, 1/2−z. #4 1/2+x, 3/2−y, −1/2+z. #5 x, −1+y, z. #6 1+x, −1+y, z. #7 1−x, 1−y, −z. #8 x, y, −1+z. #9 1−x, −y, 1−z. #10 1+x, y, z. #11 −1+x, y, z. #12 1/2+x, 1/2−y, 1/2+z. #13 1/2+x, 1/2−y, −1/2+z.


1

Variable-temperature PXRD Results. Due to the difficulties of obtaining the single-crystal structures at LTP and HTP, variable-temperature PXRD measurements were performed on compounds 1, 2 and 3 to further reveal the phase transitions. As depicted in Figure 6a, for PMpip][ClO4] (1), upon cooling, the PXRD patterns recorded at 223 and 203 K match fairly well with that recorded at 298 K (i.e. RTP). At 183 K, with the exception of the other diffraction peaks in good agreement with those obtained at 203 K, the diffraction peak at 22.13° displayed somewhat shift and split. When the temperature decreased to 153 K (corresponding to the LTP), in comparison with the PXRD pattern at RTP, the


[CMpip][ClO4] (2) and [CMmor][ClO4] (3)

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DOI: 10.1039/C5DT00263J diffraction peaks at 22.13°, 23.37°, 26.36°, 27.32°, 29.36° and 29.93° disappeared,

and seven new diffraction peaks were observed at 22.53°, 22.91°, 26.98°, 28.66°,


during the cooling process demonstrate the occurrence of the phase transition agreeing well with the DSC result. In the case of [CMpip][ClO4] (2), with the temperature increasing, both the PXRD patterns recorded at 363, 373 and 383 K are consistent with the PXRD pattern at 298 K (RTP), as shown in Figure 6b. However, above 393 K (corresponding to the HTP), the five diffraction peaks at 13.84°, 20.02°, 23.70°, 27.52° and 28.03° were still observed, additionally, a new diffraction peak appeared at 15.69°. Generally speaking, the sharp decrease of the number of diffraction peaks always suggests a phase transition from low symmetry to high symmetry, matching well with the DSC result. With respect to [CMmor][ClO4] (3), the variable-temperature PXRD patterns within the temperature range of 298−438 K depicted in Figure 6c further confirm the phase transition revealed by the above-mentioned DSC result. Upon heating to 398 K, the PXRD patterns keep essentially same with that recorded at 298 K (RTP). In addition to the diffraction peaks at 16.82°, 18.05°, 21.11°, 25.45° and 29.17°, all the other diffraction peaks disappeared above 413 K (corresponding to the HTP). Moreover, a new diffraction peak was also observed at 29.72° contrasting with the RTP.


29.06°, 30.53° and 34.69°. Obviously, the notable variations of the PXRD patterns

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Figure 6. Variable-temperature PXRD patterns of PMpip][ClO4] (1), [CMpip][ClO4] (2) and [CMmor][ClO4] (3) measured in the heating modes during 153−298 K, 298−403 K and 298−438 K, respectively. Dielectric Properties. Generally speaking, variety of physical properties including the dielectric property will undergo abrupt changes in the vicinity of the phase transition, while the variable magnitude is related to the characteristic of the phase transition. In consideration of the above-mentioned reversible phase transitions confirmed by the DSC and PXRD results, compounds 1, 2 and 3 are expected to display interesting dielectric responses triggered by temperature. As a consequence, the respective temperature dependences of the real parts (ε') of the dielectric constants and the dielectric losses taken at 1 MHz are depicted in Figure 7 and Figure 8. For [PMpip][ClO4] (1), as illustrated in Figure



DOI: 10.1039/C5DT00263J

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DOI: 10.1039/C5DT00263J 7a, during the cooling process, the ε' maintained at approximately 6.2 above the TC(1)

of 199 K, corresponding to the RTP. And it then displayed an abrupt decrease down to


dielectric state (RTP) is about 1.2 times that at the low dielectric state (LTP). For [CMpip][ClO4] (2), as shown in Figure 7b, the temperature-dependent ε' increased sharply from 4.8 at RTP to 10.5 at HTP around TC(2) of 387 K upon heating. The value of ε' in the high dielectric state (HTP) is almost 2.2 times that in the low dielectric state (RTP), demonstrating a notable step-like anomaly. With the temperature increasing, a step-like dielectric anomaly similar to that of compound 2 was also observed in [CMmor][ClO4] (3), as depicted in Figure 7c. In the vicinity of TC(3) of about 416 K, the ε' exhibited a prominent increase from 7.4 (RTP) to 23.5 (HTP). The RTP and HTP correspond to the low and high dielectric states, respectively, that is, the ε' in the high dielectric state is approximately 3.2 times that in the low dielectric state. In addition, for compounds 1, 2 and 3, the curves of the temperature-dependent ε' obtained in the cooling modes match well with those recorded during the heating processes, suggesting the occurrences of the reversible phase transitions. And the relatively large temperature hystereses further confirm the first-order features of the phase transitions of compounds 1, 2 and 3, in good agreement with the DSC results mentioned above. Besides, similar to those of the ε', the temperature dependences of the dielectric losses present in Figure 8 also display obvious anomalies in the cooling and heating cycles, further confirming the reversible phase transitions. What is noteworthy is that,


about 5.1 (LTP) showing as a step-like anomaly around TC(1). The ε' at the high

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DOI: 10.1039/C5DT00263J taking into account the ε' and dielectric losses measured at frequencies of 100 kHz

and 1 MHz (as shown in Figure S4 and S5, Supporting Information), it is obvious that


2 and 3. Overall, in the cases of [PMpip][ClO4] (1), [CMpip][ClO4] (2) and [CMmor][ClO4] (3), the dielectric constants could be switched by the reversible phase transitions and be tuned in distinct dielectric states. These typical step-like dielectric responses afford the three N-heterocyclic compounds potentials to be used as switchable molecular dielectrics. Furthermore, for [PMpip][ClO4] (1), both the increment of the dielectric constant and the thermal hysteresis are relatively small. However, by the modifications of the side-chain and then the ring structure, not only the phase transition temperatures but also the corresponding increments of the dielectric constants for [CMpip][ClO4] (2) and [CMmor][ClO4] (3) exhibited notable increases contrasting with those observed in compound 1. It means that the chemical variations can affect the physical properties intensively.

Figure 7. Temperature dependences of the real parts of dielectric constants (ε') for (a) [PMpip][ClO4] (1), (b) [CMpip][ClO4] (2) and (c) [CMmor][ClO4] (3), measured in the heating and cooling cycles at 1 MHz.


the magnitudes of the dielectric anomalies are frequency-dependent in compounds 1,

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Figure 8. Temperature dependences of the dielectric losses for (a) [PMpip][ClO4] (1), (b) [CMpip][ClO4] (2) and (c) [CMmor][ClO4] (3), measured in the heating and cooling cycles at 1 MHz. CONCLUSION In conclusion, three novel N-heterocyclic complexes [PMpip][ClO4] (1), [CMpip][ClO4] (2) and [CMmor][ClO4] (3), have been obtained and characterized as potential switchable dielectrics coupled with reversible phase transitions. Compound 1 undergoes a reversible phase transition below room temperature, TC(1) ≈ 199 K. However, with the [ClO4]– anion unchanged, the phase transition temperatures of compounds 2 and 3 are greatly increased by the modifications on the side-chain and ring structure, TC(2) ≈ 387 K and TC(3) ≈ 416 K. Accompanying the phase transitions, compounds 1, 2 and 3 also display notable step-like dielectric anomalies, which are characteristics of switchable dielectrics. Besides, the corresponding dielectric constants in the high dielectric states are 1.2, 2.2 and 3.2 times that in the low dielectric states for compounds 1, 2 and 3, respectively. Facts proved that, not only the phase transition temperatures but also the dielectric responses have been successfully tuned by the structural variations and the resultant distinct crystal structures as well hydrogen-bonding conformations. Our findings will not only open a new avenue for the design of novel above-room-temperature switchable dielectrics and phase


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transition

materials

but

also

gain

further

insight

into

the

DOI: 10.1039/C5DT00263J complicated

structure-property relationships.


For compounds 1, 2 and 3, IR spectra and PXRD patterns obtained at room temperature, and the complementary dielectric results are available in the Supporting Information. Crystallographic data are available from the Cambridge Crystallographic Data Centre, with CCDC No. 1011869 for compound 1, No. 1011867 for compound 2, and No. 1011866 for compound 3. Copies of these data can be obtained free of charge from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT This work was financially supported by the Project 973 (2014CB848800), National Natural Science Foundation of China (21471032 and 21422101). Jiangsu Province NSF (BK20140056, BK20130600), Program for NCET and Ph.D. Programs Foundation of Ministry of Education of China (20130092120013). REFERENCES 1 D. W. Fu, H. L. Cai, Y. M. Liu, Q. Ye, W. Zhang, Y. Zhang, X. Y. Chen, G. Giovannetti, M. Capone, J. Y. Li and R. -G. Xiong, Science, 2013, 339, 425−428. 2 P. P. Shi, Q. Ye, Q. Li, H. T. Wang, D. W. Fu, Y. Zhang and R. -G. Xiong, Chem.


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The table of contents entry

Crystal Structures, Phase Transitions, and Switchable


N-heterocyclic Ammonium Perchlorates Ping-Ping Shi, Qiong Ye*, Qiang Li, Hui-Ting Wang, Da-Wei Fu, Yi Zhang and Ren-Gen Xiong*

1-propyl-1-methylpiperidinium,

1-cyanomethyl-1-methylpiperdinium

and

1-cyanomethyl-1-methylmorpholinium perchlorates display switchable dielectric responses accompanied by phase transitions around 199, 387 and 416 K, respectively.


Dielectric Behaviors: Comparison of a Series of

Crystal Structures of New Ammonium 5-Aminotetrazolates.

Structural characterization, phase transition and switchable dielectric behaviors in a new zigzag chain organic-inorganic hybrid compound: [C3H7NH3]2SbI5.

Formamidinium iodide: crystal structure and phase transitions.

Phase transitions, prominent dielectric anomalies, and negative thermal expansion in three high thermally stable ammonium magnesium-formate frameworks.

Phase structures and transitions in fully hydrated diacyltrehalose.

Dielectric and Raman investigations of structural phase transitions in (C2H5NH3)2CdCl4.

Phospholipid phase transitions. Effects of n-alcohols, n-monocarboxylic acids, phenylalkyl alcohols and quaternary ammonium compounds.

Synthesis, crystal structures, and two-photon absorption of a series of cyanoacetic acid triphenylamine derivatives.

Evolution of crystal structures in GeTe during phase transition.

Ab initio molecular crystal structures, spectra, and phase diagrams.

A Low Voltage Liquid Crystal Phase Grating with Switchable Diffraction Angles.

Generation of crystal structures using known crystal structures as analogues.

Phase transitions and twinned low-temperature structures of tetraethylammonium tetrachloridoferrate(III).

Overall dielectric study on the odd members of a highly nonsymmetric pyrene-based series of liquid crystal dimers.

Frequency splitter based on the directional emission from surface modes in dielectric photonic crystal structures.

Phase diagram of ammonium nitrate.

Crystal structure of bis-(allyl-ammonium) oxalate.

Crystal structure of the monoclinic phase (phase IV) of bis-(tetra-methyl-ammonium) tetra-chlorido-cuprate(II).

A Tetraferrocenyl-Resorcinarene Cavitand as a Redox-Switchable Host of Ammonium Salts.

Dielectric Breakdown of Single-Crystal Strontium Titanate.

Dielectric and phase behavior of dipolar spheroids.

Strain induced topological phase transitions in monolayer honeycomb structures of group-V binary compounds.

Elastic anomalies associated with structural and magnetic phase transitions in single crystal hexagonal YMnO3.

Switchable dielectric, piezoelectric, and second-harmonic generation bistability in a new improper ferroelectric above room temperature.