Synthesis, structure and properties of cetyltrimethylammonium polyiodides

Polyhedron 37 (2012) 14–20

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Synthesis, structure and properties of cetyltrimethylammonium polyiodides Ganesh Chandra Das a, Babulal Das b, Neelotpal Sen Sarma c, Okhil Kumar Medhi a,⇑ a

Department of Chemistry, Gauhati University, Guwahati 781014, Assam, India Department of Chemistry, Indian Institute of Technology, Guwahati 781039, Assam, India c Physical Sciences Division, Institute of Advanced Study in Science and Technology, Guwahati 781035, Assam, India b

a r t i c l e

i n f o

Article history: Received 14 December 2011 Accepted 22 January 2012 Available online 4 February 2012 Keywords: Polyiodides Electrical conductivity Crystal structures

a b s t r a c t A series of compounds of the cetyltrimethylammonium cation with polyiodides were synthesized and the crystal structures were determined by single crystal X-ray diffraction. The newly synthesized compounds C16H33N(CH3)3I3 and C16H33N(CH3)3I5 exist as ionic species with a cation consisting of a linear hydrocarbon chain, C16H33N(CH3)3+. How such a large cation influences the crystal packing, shape symmetry and inter molecular associations of the polyiodide ion is of interest. The electrical conductivities were determined from 30 to 65 °C in the solid state. The conductivities of these compounds rises with a rise in temperature and the compounds C16H33N(CH3)3I3 and C16H33N(CH3)3I5 behaves as intrinsic conducting materials. Observations indicate that the addition of iodine increases the ionic conductivity of these compounds. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Solid polyiodides are of interest since they form low-dimensional materials with electrical conductivity ranging from that of insulators to that of metals, depending on their structure and composition [1–4]. Polyiodides containing conducting linear organic compounds are found to contain the triiodide species, which is known to influence the conducting properties of such compounds [5,6], and they have possible technical applications in electronic and electrochemical devices [7–9]. These solid electrolytes are of growing importance in solid-state electrochemistry due to their potential value for high energy density batteries. The main advantage of these electrolytes are their mechanical properties, ease of fabrication of thin films of desired sizes and their ability to form proper electrode–electrolyte contacts. Polyiodides also exhibit an unusually rich structural chemistry, showing homoatomic catenation or clustering, and they form extended inorganic networks [9–12]. They have also received attention in template synthesis and molecular engineering [10–13]. Several new crystal structures containing the triiodide species have been published recently [3,4,14–20]. The rich diversity of polyiodide structures indicates that the possible orientations and configurations of the extended polyiodide structures are close in energy, and that small differences in the counter ion, packing effects, iodine:iodine ratio or experimental conditions during the synthesis have a large influence on the structure obtained [3]. ⇑ Corresponding author. Tel.: +91 9954011962; fax: +91 0361 2675515. E-mail address: [email protected] (O.K. Medhi). 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2012.01.030

This work demonstrates the synthesis of crystalline triiodide and pentaiodide salts with a large linear hydrocarbon chain. The synthesis, crystal structure and electrical conductivity measurements of these solid polyiodides are reported in this paper. 2. Experimental 2.1. Materials and measurements Solvents for the synthesis, potassium iodide and iodine were obtained from Merck chemical company. Cetyltrimethylammonium bromide was obtained from Sigma Chemicals, USA. All materials were used without further purification. UV–Vis spectra were recorded on a Hitachi U-3210 double beam spectrophotometer. Elemental analyzes were done using a Perkin Elmer Series II CHNS/O 2400 analyzer. NMR spectra were recorded on a Bruker Ultrashield – 300 FT NMR spectrometer. The electrochemical measurements were performed using a standard three electrode configuration with a glassy carbon electrode (GCE) as the working electrode and an Ag–AgCl reference (which gave the FeCp/FeCp couple at 0.55 V, DEp = 80 mV) using a BioAnalytical Systems, USA, BAS 100 W electrochemical analyzer. All measurements were made in a nitrogen purged solution of either CH2Cl2 or ethanol using 0.5 mol dm3 n-Bu4NClO4. The bulk electrical conductivity of the compounds was evaluated from the complex impedance-admittance plots, recorded at different temperatures ranging from 30 to 65 °C at intervals of 5 °C using a Hioki 3532-50 frequency response analyzer. The plots were recorded in the frequency range 42–100 kHz, keeping a signal amplitude of 20 mV.

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G.C. Das et al. / Polyhedron 37 (2012) 14–20 Table 1 Crystallographic data for the complexes. Compound

1

2

3

Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z T (K) Crystal size (mm)

C19H42I3N

C19H42IN

C19H42I5N

665.24

411.44

919.04

triclinic

monoclinic

triclinic

 P1 7.7146 (3) 7.8066 (3) 22.7155 (9) 81.825 (2) 82.588 88.731 1342.81 (9) 2 293 (2) 0.50  0.32  0.10

P21/m 5.7843 (2) 7.5439 (3) 25.8873 (11) 90 95.403 (2) 90 1124.60 (8) 2 293 (2) 0.48  0.34  0.12

 P1 9.5504 (2) 9.7625 (2) 32.6824 (8) 90.5820 (10) 90.2620 (10) 91.4890 (10) 3045.95 (12) 4 296 (2) 0.50  0.30  0.16

1.645

1.215

2.004

18 027

14 976

48 287

6619

2958

14 856

212/0

127/0

459/0

3.493

1.422

5.110

0.0526/0.956

0.0317/1.083

0.047/0.995

0.1508

0.0921

0.1316

qcalc (mg m3) Reflections collected Independent reflections Parameters/ restraints l (Mo Ka) (mm1) R1a/Goodness of fitb wR2c[I > 2r(I)] a b

c

P P Observation criterion: I > 2r(I). R1 = ||F0|  |Fc||/ |F0|. P 2 2 2 12 ½wðF 0 F c Þ  GOF = . ðnpÞ wR2 =

P 2 2 2 12 ½wðF 0 F c Þ  P . 2 2 ½wðF 0 Þ 

2.2. Synthesis 2.2.1. Preparation of [C16H33N(CH3)3I3] (1) KI (2.0 g, 12.04 mmol) was taken in 20 ml of water in a glass stoppered 100 ml conical flask. Freshly resublimed iodine (1.27 g, 5 mmol) was transferred to the concentrated KI solution by means of a small dry funnel. This solution was added slowly with continuous stirring using a Teflon coated magnetic bar to an aqueous solution of cetyltrimethylammonium bromide (1.84 g, 5 mmol). The reaction mixture was then stirred for 20 min at 25 °C, yielding the dark brown colored precipitate of 1. The resulting brown product was filtered off, washed three times with water and dried in vacuum. The product was recrystallized from ethanol, Yield:

98%. Anal. Calc. for C16H33N(CH3)3I3 (1): C, 34.3; H, 6.3; N, 2.1. Found: C, 34.4; H, 6.4; N, 2.2%. 1H NMR (300 MHz, CDCl3) d: 3.5– 3.45 (m, 2H, CH2), 3.35 (s, 9H, CH3), 1.85–1.24 (Br, m, 28 H, CH2), 0.86 (t, 3H, CH3). 2.2.2. Preparation of [C16H33N(CH3)3I] (2) An aqueous KI (5 g, 30.1 mmol) solution (100 ml) was slowly added to an aqueous C16H33N(CH3)3Br (2.9 g, 7.9 mmol) solution at room temperature with continuous stirring with a Teflon coated magnetic bar. The stirring was continued for another 20 min after the addition of the iodide solution. The resulting white product was filtered off, washed three times with water and dried in vacuum. Yield: 96%. X-ray quality white crystals of C16H33N(CH3)3I (2) were obtained by slow evaporation of the solvent from an ethanol solution. Anal. Calc. for C16H33N(CH3)3I (2): C, 55.5; H, 10.2; N, 3.4. Found: C, 55.7; H, 10.3; N, 3.6%. 1H NMR (300 MHz, CDCl3) d: 3.6 (t, 2H, CH2), 3.45 (s, 9H, CH3), 1.66–1.24 (Br, m, 28 H, CH2), 0.86 (t, 3H, CH3). 2.2.3. Preparation of [C16H33N(CH3)3I5] (3) Solid 2 (1.23 g, 3 mmol) was dissolved in dichloromethane (25 ml). Then a solution of iodine (1.54 g, 6.1 mmol) in dichloromethane (25 ml) was added slowly with continuous stirring. The stirring was continued for another 20 min after the addition of the iodine solution, whereupon a dark green product was obtained. The resulting green product was dried in a vacuum and kept in a desiccator in the dark. Slow evaporation of a concentrated dichloromethane solution afforded X-ray quality single crystals. Yield: 65%; Anal. Calc. for C16H33N(CH3)3I5 (3): C, 24.8; H, 4.6; N, 1.5. Found: C, 25.1; H, 4.8; N, 1.6%. 1H NMR (300 MHz, CDCl3) d: 3.48 (t, 2H, CH2), 3.35 (s, 9H, CH3), 1.84–1.24 (Br, m, 28 H, CH2), 0.86 (t, 3H, CH3). 2.2.4. Crystal structure determination and refinement The crystal structures were determined by single crystal X-ray analyses. A brown single crystal of 1, white crystal of 2 and dark green crystal of 3 were mounted on a glass fibre for intensity data collection using graphite monochromatized Mo Ka radiation (k = 0.71073 Å) on a Bruker Nonius SMART platform CCD diffractometer. The crystals were found to be stable against thermal and oxidative decomposition and X-radiation induced decay. Final cell constants were obtained from least squares fits of a subsets of several thousand strong reflections. Crystallographic data of the compounds are listed in Table 1. The space groups for the crystals were established by successful solution and refinement of the structures. The structures were solved by the direct method (SHELXS) and refined by full-matrix least squares techniques (SHELXL) using the SHELX-97 package of crystallographic programs [21] using the WinGX [22] platform available for personal computers. Structural

Table 2 Selected bond distances (Å) and angles (°) of complexes 1, 2 and 3. Complex 1 N(1)–C(19) N(1)–C(17) N(1)–C(18) N(1)–C(16) I(2)–I(3) I(2)–I(1) C(19)–N(1)–C(17) C(19)–N(1)–C(18) C(17)–N(1)–C(18) C(19)–N(1)–C(16) C(17)–N(1)–C(16) C(18)–N(1)–C(16) I(3)–I(2)–I(1) #1 x, y + 1/2, z.

Complex 2 1.475(8) 1.492(8) 1.517(8) 1.530(8) 2.8550(6) 2.9688(6) 110.0(6) 109.4(5) 109.1(6) 111.5(5) 110.7(5) 105.9(5) 178.724(17)

N(1)–C(18) N(1)–C(17) N(1)–C(16) C(18)–N(1)–C(17) C(18)–N(1)–C(16) C(17)–N(1)–C(16)

Complex 3 1.498(3) 1.500(4) 1.518(4) 109.11(18) 111.69(17) 106.2(3)

I(4)–I(5) I(4)–I(3) I(2)–I(1) I(2)–I(3) N(1)–C(17) N(1)–C(18) N(1)–C(19) N(1)–C(16) I(5)–I(4)–I(3) I(1)–I(2)–I(3) I(2)–I(3)–I(4)

2.7978(9) 3.1183(8) 2.7771(10) 3.1014(9) 1.483(9) 1.484(8) 1.500(9) 1.513(8) 175.09(3) 176.73(3) 90.24(2)

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G.C. Das et al. / Polyhedron 37 (2012) 14–20

Fig. 1. Structure of the molecules in crystals of 1.

Fig. 2. Crystal packing diagram of compound 1 showing sheets of triiodide ions and the position of the hydrocarbon chains.

diagrams were drawn using ORTEP-3 for Windows [23] and PLUTON [24]. While all non-hydrogen atoms were refined with anisotropic thermal parameters, the hydrogen atoms belonging to the cation were refined isotropically after placing them at calculated positions using a riding model of refinement. In compound 3 all the methyl carbon atoms (C35, C36 and C37) associated with the nitrogen atom (N2) of one of the C16H33N(CH3)3+ units are observed as being highly disordered. 3. Results and discussion 3.1. Crystal structure analysis The crystallographic data of 1, 2 and 3 are summarized in Table 1. The most significant bond angles and bond lengths for all three compounds are listed in Table 2.

Table 3 Comparison of the geometry of the triiodide ion in crystals containing large cations. Compound

(C6H5)4AsI3 [26] [(C6H5)CONH2]2HI3 [27] [C10H9N2]I3 [28] [1-Me Cytosinium]I3 [29] [Pyrene]10(I3)4(I2)10 [30] (a-Cyclodextrin) LiI3I2 [31] Me(C6H5)3PI3 [19] C16H33N(CH3)3I3 [This work]

Bond angle (°) \I1–I2–I3

Bond distances (Å) I1–I2

I2–I3

Difference (Å) I12–I23

175.61(5) 177.5(5)

2.919(1) 2.900(8)

2.919(1) 2.959(8)

0.000 0.059

177.88(5) 177.6(1)

2.902(1) 2.794(1)

2.920(1) 3.123(1)

0.018 0.329

175.9(9) 176(1) 176.8(5)

2.775(1) 2.840(1) 2.882(5)

3.167(1) 2.983(1) 2.973(10)

0.329 0.143 0.091

– 178.72(2)

2.782 2.855(6)

3.222 2.969(6)

0.442 0.114

3.2. Structure of [C16H33N(CH3)3I3] (1) Cetyltrimethylammonium triiodide crystallizes in the triclinic system. The unit cell consists of two identical molecules which consist of C16H33N(CH3)3+ cations and triiodide (I3) anions. The compound exists as an ionic species with discrete cations and anions, as shown in Fig. 1. The cation is packed in the crystal as a linear chain with bond distances and bond angles as expected for a linear C16 hydrocarbon chain. The triiodide anion is near to linear, with a bond angle of 178.72(4) Å. The I–I bond distances are 2.855(6) and 2.969(6) Å, which differ by about 0.114 from a sym-

metrical triiodide ion. The triiodide ions are packed as sheets in the crystal, as depicted in Fig. 2. In crystals containing small cations, an unsymmetrical triiodide ion with different I(1)–I(2) and I(2)–I(3) bonds are expected [25], as in NH4I3 or CsI3. On the other hand, in crystals of large cations, a symmetrical triiodide ion with two equal I–I bonds is expected [25,26], as in Ph4AsI3 or (C2H5)4NI3. The linear symmetric triiodide ion is associated with large cations, while the bent and unsymmetrical triiodide ion is found with small highly charged cations [25,26].

G.C. Das et al. / Polyhedron 37 (2012) 14–20

17

Fig. 3. Structure of the molecules in crystals of 2.

Fig. 4. Structure of the molecules in crystals of 3.

Fig. 5. (a) Electronic spectrum of compound 1 (1  103 mol dm3) in ethanol recorded at room temperature. (b) Absorption spectrum of solid (powder) compound 1 recorded at room temperature.

A comparison of the geometry of the triiodide ion, particularly the difference in bond distances between I(1)–I(2) and I(2)–I(3), in C16H33N(CH3)3+I3 with that in the crystal structures of related compounds containing large cations show a considerable deviation from the expected results (Table 3). It is seen that in crystals of large cations, including this work, the triiodide ion deviates from linearity and symmetry. The slight deviation from the expected symmetrical triiodide in C16H33N(CH3)3+I3 may probably be due to the effect of crystal packing forces.

3.4. Structure of [C16H33N(CH3)3I5] (3) Fig. 4 exhibits the structure of compound 3. The unit cell consists of two molecules which consists of a [C16H33N(CH3)3]+ cation and an I5 anion. The pentaiodide anion, I5, is an L shaped molecule which may be considered to be two iodine molecules coordinated to a single iodide ion [3]. Alternatively, it can be considered to be two unsymmetrical triiodide ions sharing a common iodide atom. The bond distances of the I5 species, as listed in Table 2, correspond very well to the bond lengths in unsymmetrical triiodide.

3.3. Structure of [C16H33N(CH3)3I] (2) 3.5. UV–Vis spectroscopy Compound 2 crystallizes in the monoclinic system. Fig. 3 exhibits the crystal structure of compound 2. The compound exists as an ionic species with a discrete cation (C16H33N(CH3)3+) and anion (I). Table 2 summarizes the selected bond lengths and angles of this compound.

The UV–Vis spectrum of compound 1 in CH2Cl2 solution shows absorption features at 290 (2 = 1.8  105 mol dm3) and 359 nm (2 = 1.0  105 mol dm3) (Fig. 5a). These may be ascribed to charge-transfer transitions in the triiodide anion [31] which exists

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G.C. Das et al. / Polyhedron 37 (2012) 14–20 Table 4 Conductivities (Scm1) of compounds 1, 2, 3 and 4. Temperature (K)

1

2 6

303 308 313 318 323 328

8.01  10 9.29  106 1.55  105 5.53  105 7.17  105 6.91  105

3 9

6.40  10 6.22  109 6.52  109 7.35  109 2.15  108 1.93  108

4 5

2.11  10 2.58  104 5.35  104 1.61  104 5.93  104 9.22  104

1.07  108 1.07  108 1.02  109 9.55  109 8.69  109 7.69  109

I3 ! I2 þ I I2 þ 2e ! 2I 3.7. AC conductivity measurement

Fig. 6. Osteryoung Square Wave Voltammogram of the compound (1  103 M) C16H33N(CH3)3I3 in ethanol. Working electrode: Glassy Carbon; Reference electrode: Ag–AgCl; Supporting electrolyte: 0.5 mol dm3 n-Bu4NClO4.

in solution as isolated ions. Measurement of the absorption spectrum of the solid (powder) at room temperature shows a broad band near 600 nm (Fig. 5b) indicating a possible interaction of the triiodide ions in the solid state, as observed for the aggregated polyiodides in the amylase–I2 complex [31]. The spectrum of compound 3 in CH2Cl2 solution displays similar absorption features as observed for compound 1. This may be due to the interesting property of releasing the triiodide ion in non-polar solvents. 3.6. Electrochemical studies Measurement of the electrode potential at a glassy carbon electrode by Osteryoung Square Wave Voltammetry (OSWV) of the triiodide compound shows E½ at +764 mV in ethanol versus the Ag– AgCl reference (Fig. 6). However, a cyclic voltammogram of a reversible process was not found for the triiodide ion over the scan rate 50–1000 mV/s. The observed peak may be assigned to the I3/ I couple, and the electrochemical process may be described as a coupled chemical–electrochemical reaction:

The bulk electrical conductivities of the compounds were evaluated from the complex impedance-admittance plots recorded at different temperatures ranging from 303 to 338 K at an interval of 5 K using a Hioki 3532-50 frequency response analyzer. The plots were recorded in the frequency range 42–100 kHz, keeping the signal amplitude at 20 mV. The geometry of the cell for the measurement of conductivity was SS| electrolyte film |SS, where SS plate (SS stands for Stainless Steel) was used as electrodes. The total ionic transport number, tion, was evaluated by the standard Wagner polarization technique [32]. The cell SS| electrolyte film |SS was polarized by a step potential (about 1.0 V) and the resulting potentiostatic current was monitored as a function of time. The stainless steel acts as blocking electrodes for the above cell. The tion was evaluated using the formula: tion = (iT  ie)/iT, where iT and ie are total and residual current respectively. The ionic conductivity of cetyltrimethylammonium bromide, 4, and compounds 1, 2 and 3 were determined from the relation r ¼ Rlb r2 p, where l is the thickness of the sample, r is the radius of the sample and Rb is the bulk resistance of the electrolyte obtained from the complex impedance plot [33]. Fig. 7 shows the conductivity versus temperature plot for the electrolytes. The curves indicate that basically compound 4 is not at all an electrolyte as this compound has no scope of delivering ions to the surroundings. However, addition of iodine into the system improves the character of the molecules dramatically. With the addition of iodine, compound 3 shows the maximum improvement with a sigma value of

0

-1

Log of σT

-2

d c

-3

-4

-5

-6 2.9

a b 2.95

3

3.05

3.1

3.15

3.2

3.25

3.3

3.35

103/T Fig. 7. Log of rT vs. 1/T plots of: (a) compound 4 (j), (b) compound 2 (), (c) compound 1 (N), (d) compound 3 (d).

19

G.C. Das et al. / Polyhedron 37 (2012) 14–20

135

115

Current (μA)

95

75 d

55 c

35

15 a b

-5 0

10

20

30

40

50

60

Time (min)

Fig. 8. Time vs. current plots of: (a) C = ompound 4, (b) compound 2, (c) compound 1, (d) compound 3.

1  103 Scm1 at 328 K. The curvature of the plots in Fig. 7 shown by these improved electrolytes indicates that the ionic conduction in these electrolyte systems obeys the VTF (Vogel–Tamman–Fulcher) relation [34–37]. Conductivity of these compounds rises with a rise of temperature (Table 4) and these can be understood in terms of the free-volume model [38]. As the temperature increases the free volume also increases and this leads to an increase in ion mobility and segmental mobility that assist ion transport. The activation energy computed from the Arrhenius plot (Fig. 7a–d) of log (rT) versus 1/T of the compounds 4, 2, 1 and 3 are 0.15, 0.14, 0.64 and 0.89 eV, respectively. The higher values of the activation energies for compounds 1 and 3 indicate that the ionic conductions are predominant in these two compounds compared to compounds 4 and 2. Fig. 8 gives the ion transport number, as evaluated by the Wagner polarization technique, and this gives information on the percentage of ionic transportation against electronic conduction. The variation of electrical conductivity with time has been taken as a measure of the ion transport number. This graph indicates a fast exponential type decrease in electrical conductivity initially, saturating later to almost constant values, which could be separated as electronic and ionic parts by extrapolating the linear part to zero time for electronic and point wise subtraction for ionic conduction. The transport number for compounds 1 and 3 are 0.63 and 0.46, respectively. The plots indicate that the original compound 4 and also compound 2 have no ionic conduction, which means that there are practically no ions present to conduct the current. On the other hand, with the addition of iodine the ionic conductivity increases prominently, and compounds 1 and 3 behave like intrinsically conducting materials. 4. Conclusion Crystalline triiodide and pentaiodide salts with a large amphipathic organic carrier as the counter cation were synthesized and the crystal structures show that the triiodide anion is nearly linear, but deviates from the symmetric triiodide ion expected for a salt with a large cation [e.g. (C6H5)4AsI3]. The reduction potential of the I3/I couple is +764 mV versus Ag–AgCl. The compounds have the interesting property of releasing the triiodide ion in non-polar solvents. The maximum ionic conductivity for these electrolyte systems at 328 K is of the order of 1  103 Scm, shown by

C16H33N(CH3)3I5. Both the electrolyte C16H33N(CH3)3I3 and C16H33N(CH3)3I5 show comparable conductivities and transport numbers. However, though the compound C16H33N(CH3)3I5 shows the maximum conductivity, the percentage of ionic conduction, i.e. the transport number, is maximum for C16H33N(CH3)3I3. Acknowledgements Funding for this work was provided by the University Grants Commission (UGC), New Delhi. The authors thank the Indian Institute of Technology (IIT), Guwahati, India for help in the crystallographic data collection. Appendix A. Supplementary data CCDC 679105, 766103, 766104 contain the supplementary crystallographic data for compounds 1, 2 and 3, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2012.01.030. References [1] P. Coppens, Extended Linear Chain Compounds, in: J.S. Miller (Ed.), Extended Linear Chain Compounds, vol. 19, Plenum Press, New York, 1982, pp. 333–356. [2] K.P. Tebbe, in: A.J. Rheingold (Ed.), Homoatomic Rings, Chains and Macromolecules of Main Group Elements, Elsevier, Amsterdam, 1977, pp. 551–606. [3] P.H. Svensson, L. Kloo, J. Chem. Soc., Dalton Trans. (2000) 2449. [4] S. Lee, B. Chen, D.C. Fredrickson, F.J. Disalvo, E. Lobkeovsky, J.A. Adams, Chem. Mater 15 (2003) 1420. [5] Y.J. Cheng, Z.M. Wang, C.S. Liao, C.H. Yan, New J. Chem. 26 (2002) 1360. [6] A.D. Bandrauk, K.D. Truong, C. Carlone, A.W. Hanson, J. Chem. Phys. 91 (1987) 2063. [7] K. Kawabata, M. Nakano, H. Tamura, G. Matsubayashi, Inorg. Chem. Acta. 358 (2005) 2082. [8] B.B. Owens, B. Pate, P.M. Skarstad, D.L. Worburton, Solid State Ionics 9 (10) (1983) 1241. [9] I. Tatsuro, K. Megumi, S. Takashi, Mol. Cryst. Liq. Cryst. 455 (2006) 65. [10] K.F. Tebbe, R. Buchem, Angew Chem., Intl. Ed. Engl. 37 (1997) 1345. [11] A.J. Blake, R.O. Gould, S. Parsons, C. Radek, M. Schroder, Angew Chem., Intl. Ed. Engl. 34 (1995) 2374. [12] A.J. Blake, R.O. Gould, W.S. Li, V. Lippolis, S. Parsons, C. Radel, M. Schroder, Angew. Chem., Intl. Ed. Engl. 37 (1998) 293.

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