Phase Transition Triggered by Ordering of Unique Pendulum-Like

Article pubs.acs.org/crystal

Phase Transition Triggered by Ordering of Unique Pendulum-Like Motions in a Supramolecular Complex: Potassium Hydrogen Bis(dichloroacetate)-18-Crown‑6 Shigeng Li,†,§ Junhua Luo,*,†,‡ Zhihua Sun,†,‡ Shuquan Zhang,† Lina Li,†,§ Xiaojun Shi,†,§ and Maochun Hong†,‡ †

Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ‡ State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China § Graduate School of the Chinese Academy of Sciences, Beijing 100039, China S Supporting Information *

ABSTRACT: A novel supramolecular phase change material (PCM) potassium hydrogen bis(dichloroacetate)-18-crown-6 (1), which undergoes a reversible second-order phase transition at 181.8 K (Tc), has been successfully synthesized and grown as bulk crystals. DSC measurements confirm its reversible phase transition with a thermal hysteresis of 1.6 K. Dielectric permittivities also display obvious anomalies approaching Tc, being characteristic for the reversible phase transition. Variable-temperature X-ray singlecrystal diffractions demonstrate that 1 behaves as a molecular rotor above room temperature in which the chlorine atoms rotate around the C−C axis of the CHCl2COO−/CHCl2COOH exhibiting distinct pendulum-like motions, and the (18-crown-6)·K+ part acts as a stator. Further studies reveal that the origin of its phase transition was ascribed to the moving close together of the whole set molecules from the original position, which is induced by the order− disorder transformation of the pendulum-like motions of the CHCl2COO−/CHCl2COOH units between the LT phase and RT phase. We believe that all the results would urge the exploration of new phase change functional materials.

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transition material.16 Moreover, a great number of organic− inorganic hybrids with 18-crown-6 units as the stators have also been prepared and investigated.17−20 Therefore, utilization of the 18-crown-6 as a supramolecular building block to explore PCMs arouses our great interest. In the present work, we report a novel supramolecular PCM potassium hydrogen bis(dichloroacetate)-18-crown-6 (1), which undergoes a reversible phase transition at 181.8 K (Tc) confirmed by variable-temperature single-crystal structure analyses, differential scanning calorimetry (DSC), and the dielectric measurements. The ordering of the pendulum-like motions of the chlorine atoms in the molecular rotor has been found to predominate the phase transition (Figure 1). To the best of our knowledge, such pendulum-like molecular motion of the rotor unit in 1 is unprecedented and first utilized to assemble smart PCMs with crown ether, even though there has been intensive research focused on 18-crown-6 in supramolecular chemistry.

INTRODUCTION Recently, phase change materials (PCMs) have attracted great attention since they may be widely used in the tropical regions as telecom shelters and mechanical energy transfer, etc.1 For assembling smart PCMs, molecular motions associated with phase transition have been utilized and explored in a wide range of materials, which may greatly affect material properties. As the well-known components with the modest flexibilities, crown ethers play a significant role in the formation of molecular stator, being considered as one of the symbols in supramolecular chemisty.2,3 Numerous derivatives and complexes based on crown ethers are nowadays synthesized,4,5 not only because of their extensive utilization in supramolecular chemistry and crystal engineering for constructing the complex superstructures6 but also for their abilities to form stable complexes with alkali and transition-metal ions and hydrogenate cations via hydrogen bonds.7 Recently, Braga’s group, Tomoyuki’s group, and Xiong et al. had successfully used crown ethers to capture alkali metal, ammonium cations, and molecular rotors to design PCMs and ferroelectric molecular materials.8−15 For example, Xiong and his co-workers utilized 4-methoxyanilinium as the rotation unit combined with 18crown-6 stator to build a second-order ferroelectric phase© XXXX American Chemical Society

Received: March 27, 2013 Revised: April 26, 2013

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Table 1. Crystal Data and Structure Refinement of 1 at 293 and 123 Ka moiety formula formula weight temperature (K) crystal system space group a (Å) b (Å) c (Å) β (deg) volume (Å3) Z F(000) Goodness-of-fit on F2 Tmin/Tmax R1 [on Fo2, I > 2σ(I)] wR2 [on Fo2, I > 2σ(I)]

Figure 1. (a) Schematic structure of 1. (b) Pendulum-like molecular motion of the chlorine atom in the rotor unit with the crown ether behaving as the stator.

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EXPERIMENTAL SECTION Synthesis. Compound 1 was synthesized through reaction of potassium hydroxide, dichloroacetic acid, and 18-crown-6 with a 1:2:1 molar ratio. The potassium hydroxide (0.56 g, 0.01 mol) was dissolved into 30 mL of water and then the 2.58 g (0.02 mol) dichloroacetic acid was added; finally the 18-crown6 (2.64 g, 0.01 mol) was added with stirring and heating. The synthesized solution was kept at room temperature and colorless block crystals were obtained by slow evaporation from the aqueous solution after several days (see Figure S1 of the Supporting Information). The IR spectrum of 1 definitely reveals the existence of typical strong vibration peaks of carboxyl between 1731 and 1756 cm−1 (Figure S2 of the Supporting Information). Its phase purity is confirmed by elemental analysis (Anal. Calcd (%) for 1: C, 34.29; H, 4.86; O, 28.54. Found (%): C, 34.17; H, 5.02; O, 28.68.) and the powder XRD (PXRD) pattern, which matches very well with the pattern simulated from the single-crystal structure at room temperature (Figure S3 of the Supporting Information). Single-Crystal Structure Determination. Variable-temperature X-ray single-crystal diffraction data at room temperature (RT, 293 K) were collected on UltraX-Saturn 70 diffractometer, while the data sets at low temperature (LT, 123 K) were collected on Saturn 724 HG diffractometer. Both are equipped with the graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The crystal temperature was stabilized within 2−5 K. Data collections were obtained with Crystal Clear (Rigaku), while the crystal structures were solved by direct methods and refined by a full-matrix least-squares method on F2 data, using SHELXL-97. All nonhydrogen atoms were refined anisotropically, and the positions of hydrogen atoms were generated geometrically. Crystallographic data and details of data collection and refinement at 293 and 123 K are given in Table 1. DSC Measurement. DSC experiments were performed by heating and cooling the sample (13.900 mg) in the temperature range from 153 to 213 K on a NETZSCH DSC 200 F3, under nitrogen atmosphere in aluminum crucibles with the rate of 2 K min−1. Dielectric Measurement. In the dielectric experiments, the single-crystal plates of 1 with silver painted as the electrodes were used for measuring the complex dielectric, ε = ε′ − iε″. Its dielectric constants were measured using a TH2828 A at the respective frequencies of 500 kHz and 1 MHz with the measuring AC voltage fixed at 1 V.

a

C16H27Cl4KO10 560.63 293(2) monoclinic C2/c 20.447(4) 9.0676(1) 14.438(3) 99.704(1) 2638.6(9) 4 1160 0.983 0.740/1.000 0.0467 0.1400

C16H27Cl4KO10 560.63 123(2) monoclinic C2/c 28.521(2) 17.446(7) 22.870(1) 118.084(7) 10040(8) 16 4640 1.091 0.768/1.000 0.0890 0.3172

αR1 = Σ∥Fo| − |Fc∥/Σ|Fo|, wR2 = [Σ(|Fo|2 − |Fc|2)/Σ|Fo|2]1/2.

group C2/c (no. 15), and cell parameters are a = 20.45(4), b = 9.07(1), c = 14.44(3) Å, β = 99.70(1)°, V = 2638.6(9) Å3, and Z = 4. The asymmetric unit of 1 contains one-half of the 18-crown-6 molecule, one-half of dichloroacetic acid molecule, one-half of the K+ cation, and one-half of the deprotonated dichloroacetate anion. The 18-crown-6 molecule has a cavity in its center, the size of which makes it easy to accommodate the small K+ cation. In its structure, the K+ cation lies on the symmetric center, which is coordinated to the six oxygen atoms of the crown ether and two oxygen atoms from CHCl2COO−/ CHCl2COOH moieties, as shown in Figure 2a. Within this RT phase, it is proposed that the (18-crown-6)·K+ unit acts as a molecular stator and CHCl2COO−/CHCl2COOH can be treated as the molecular rotor. The molecular rotors link molecular stators through K−O bonds. It is noteworthy that the chlorine atoms of the molecular rotor are disordered and look like a pendulum to swing around the axis of the C7−C8 bond (see Figures 1 and 2a). The bond angles of Cl−C−Cl in the CHCl2COO−/CHCl2COOH for RT structure are given in Table 2. It is clearly shown that the pendulum-like motions of chlorine atoms easily occur, which swing in the range of about −20° to 20°. With the temperature decreasing below Tc, the molecular pendulum-like motions of the disordered chlorine atoms in the molecular rotor were severely restricted, corresponding to its more ordered LT phase (see Figure 2b). Interestingly, they are full of O−H···O hydrogen bonds in 1 (Figure 3a). Along the b-axis direction, the molecular units are head-to-tail linked together by the typical O−H···O hydrogen bond, forming the hydrogen-bonded zigzaglike chains (Figure S4 of the Supporting Information) with the neighbor K···K distances of 9.0676 Å. In the LT phase, 1 also crystallizes in monoclinic with space group C2/c, and cell parameters are a = 28.52(2), b = 17.45(7), c = 22.87(1) Å, β = 118.08(7)°, V = 10040(8) Å3, and Z = 16. The LT structure keeps a little change of the c axis compared to the corresponding a axis in the RT state. However, a great change occurs for the LT a axis, b axis, and the whole crystal volume, in which there shows an approximately doubling of the RT c axis length for the LT a axis, a doubling of the RT b axis length for the LT b axis, and quadrupling of the crystal volume (Figures S5 and S6 of the Supporting Information). The asymmetric unit of 1 at LT is 4-fold compared to that of 293 K;

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RESULTS AND DISCUSSION Crystal Structure of 1. The RT X-ray single-crystal structure determination reveals that 1 crystallizes in the monoclinic crystal system with a centrosymmetric space B

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Figure 2. The asymmetric unit of 1 shown at different temperatures. (a) The room temperature phase (293 K): The chlorine atoms of the molecular rotor are totally disordered, which undergo pendulum-like molecular motions. (b) The low-temperature phase (123 K): The chlorine atoms of the molecular rotor are totally ordered.

anions. There are two independent hydrogen-bonded chains in which the neighbor K···K distances are 9.2353 Å and 8.2161 Å of the LTa chain and 9.4700 Å and 7.9950 Å of the LTb chain, respectively (Figure 2b and 3b). With the temperature decreasing from RT to LT, the neighbor whole set molecules move closely with the K···K distances changing from 9.0676 to 8.2161 and 9.2353 Å in the LTa chain and from 9.0676 to 9.4700 and 7.9950 Å in the LTb chain, respectively, which are triggered by the ordering of the pendulum-like motions of the dichloroacetate anions (Figure 3b) and would suggest that the molecular movements occur during the phase transition. In the LT phase, one O−H···O hydrogen bond bridge and two O−H···O hydrogen bonds links alternate along the b axis, while there is only one O−H···O hydrogen bond between the neighbor molecular units in the RT phase, forming a hydrogen-bonded zigzag chain. Phase Transition of 1. Phase transition temperature of 1 is also determined by the thermal analyses, including DSC. The DSC experiment of 1 clearly displays that a reversible phase transition occurred at about 180.2 K in the cooling mode with a thermal hysteresis of 1.6 K, probably suggesting a second-order phase transition (Figure 4).21,22 From the curve, the total

Table 2. Selected Bond Angles (deg) of Cl−C−Cl in the CHCl2COO−/CHCl2COOH for RT Structure Cl1B−C7−Cl1A Cl1A−C7-Cl1C

18.55(1) 19.54(1)

Cl2B−C7−Cl2A Cl2A−C7-Cl2C

19.93(1) 19.11(1)

Figure 4. The DSC curves of 1. Figure 3. (a) Hydrogen bonding structure of 1 in the RT phase viewed along the c axis. The distance between K and K is 9.6076 Å. (b) There are two independent hydrogen-bonded chains in 1 of the LT phase viewed along the c axis (denoted as LTa chain and LTb chain, respectively), in which the K···K distances are 9.2353 Å and 8.2161 Å in the LTa chain and 9.4700 Å and 7.9950 Å for the LTb chain.

transition enthalpy ΔH is equal to 244.2 J/mol, while ΔS is estimated to be ΔS = ΔH/Tc = 244.2/181.8 = 1.34 J/mol K. On the basis of the Boltzmann equation ΔS = R ln N, in which R is the gas constant and N is the ratio of the numbers of respective geometrically distinguishable orientations, N = 1.17 is obtained, indicating a complicated phase transition. The N value, which is closer to 1, suggests that the phase transition is a dominating molecular movement accompanying the ordered−

that is, consisting of two 18-crown-6 molecules, two dichloroacetic acid molecules, two K+ cations and two deprotonated C

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disordered transition.23 All results are well-consistent with the above-mentioned crystal structure determination. The temperature-dependence dielectric permittivity of 1 was measured in two directions on single-crystal samples in the heating mode with frequencies of 500 kHz and 1 MHz. Figure 5 shows the real part (ε′) of the complex dielectric permittivity

complex molecular movements accompanying the order− disorder transition, which is consistent with the variabletemperature single-crystal X-ray diffractions, DSC, and dielectric analyses.

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CONCLUSION In summary, the present work has successfully demonstrated a novel supermolecule assembled by an (18-crown-6)·K+ cation as a stator and CHCl2COO−/CHCl2COOH as a molecular rotor exhibiting pendulum-like motions, which plays an important role in the construction of supramolecular PCMs. The reversible phase transition around 181.8 K is experimentally confirmed by variable-temperature structural analyses, DSC, and dielectric measurements. The findings indicate that 1 behaves as an active molecular rotor, involving the unique pendulum-like motions of chlorine atoms above the phase transition temperature, showing notable changes in the distinctive structures at 293 and 123 K. Origin of its phase transition was ascribed to the molecule moving, which is induced by the order−disorder transformation of the pendulum-like motions of the anions between the LT and RT phases.26,27 As a new supramolecular PCM, we believe that these findings will open a new avenue for the design of smart materials.

Figure 5. Anisotropic dielectric permittivity (ε′) of the single crystal 1 measured in the heating mode at 500 kHz and 1 MHz.

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(ε). The real part shows relatively large maxima of dielectric anomaly around 181.8 K, corresponding to the phase transition. Another striking feature of the dielectric constant of 1 is anisotropic, especially in the vicinity of the phase transitions; namely, the dielectric constants have a crystal-axis dependence.24 Distinct anomalies are observed during the phase transition in the directions parallel to the crystallographic b axis, while comparatively smaller anomaly is recorded in the direction of the a axis. The dielectric anisotropy can be explained by examining the distances between neighbor K and K cations along the b axis and the placement of the chlorine atoms in 1. It can be seen from the crystal structure that the disordered chlorine atoms lie in the plane perpendicular to the b axis in the RT phase. Its in-plane disordering upon temperature changes leads to no change of the dielectric permittivity along the a axis. The variation of permittivities in the plane perpendicular to the b axis is consistent with dielectric behaviors of some substances composed with chlorine atoms, which exhibit free- and frozen-pendulum phases.25 Furthermore, the neighbor K···K distances in the hydrogen-bonded chains between the RT and LT phases have changed, confirming the molecular movements along the b axis during the phase transition, which result in the distinct dielectric anomaly along the b axis. Origin of Phase Transition for 1. From the above studies, it becomes clear that the CHCl2COO−/CHCl2COOH moieties are molecular rotors and undergo unique pendulum-like motions. Above the phase transition temperature, the anion behaves as an active rotor in which the chlorine atoms rapidly rotate around the C−C axis in the CHCl 2 COO − / CHCl2COOH and the (18-crown-6)·K+ acts as a stator. Below Tc, the pendulum-like motions of rotors are frozen and the units become ordered. Moreover, along the b axis, the molecules move close through the hydrogen-bonded chains with the obvious changing of the neighbor K···K distances during the phase transition, which is induced by the order− disorder transformations of the pendulum-like motions of the rotors between the RT and LT phases. The above-mentioned results support the fact that the phase transition might be

ASSOCIATED CONTENT

S Supporting Information *

IR spectrum, XRD patterns, and packing views of the crystal structures. CCDC reference numbers 918222 [RT (293 K) phase] and 918195 [LT (123 K) phase]. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21222102, 51102231, and 21171166), the One Hundred Talent Program of the Chinese Academy of Sciences, the 973 Key Programs of the MOST (Grants 2010CB933501 and 2011CB935904), and the Key Project of Fujian Province (Grant 2012H0045). Z.S. thanks the support from the “Chunmiao” Project of the Haixi Institute of Chinese Academy of Sciences (Grant CMZX-2013002). We gratefully thank X. Chen and Z. Z. Xue for help on the crystal structure measurement and Prof. D. Q. Yuan for help with the crystal structure and valuable discussions.

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