Conductive Hybrid Crystal Composed from Polyoxomolybdate and Deprotonatable Ionic-Liquid Surfactant

A polyoxomolybdate inorganic-organic hybrid crystal was synthesized with deprotonatable ionic-liquid surfactant. 1-dodecylimidazolium cation was employed for its synthesis. The hybrid crystal contained δ-type octamolybdate (Mo8) isomer, and possessed alternate stacking of Mo8 monolayers and interdigitated surfactant bilayers. The crystal structure was compared with polyoxomolybdate hybrid crystals comprising 1-dodecyl-3-methylimidazolium surfactant, which preferred β-type Mo8 isomer. The less bulky hydrophilic moiety of the 1-dodecylimidazolium interacted with the δ-Mo8 anion by N–H···O hydrogen bonds, which presumably induced the formation of the δ-Mo8 anion. Anhydrous conductivity of the hybrid crystal was estimated to be 5.5 × 10−6 S·cm−1 at 443 K by alternating current (AC) impedance spectroscopy.


Syntheses of C12im-Mo Hybrids
As-prepared C12im-Mo hybrids were obtained as insoluble precipitates from aqueous solution of sodium molybdate (pH = 3.6) in 50%-65% yield (based on Mo). Figure 2 shows infrared (IR) spectra of as-prepared C12im-Mo hybrids. The structure of the C12im-Mo hybrids depended on the ionic-liquid species employed in the syntheses. Using neutral 1-dodecylimidazole (C3H3N2(C12H25), denoted as C12im-N) as surfactant source resulted in the formation of C12im-Mo hybrid of 1. The IR spectrum of 1 (Figure 2a) showed characteristic peaks in the range of 400-1000 cm −1 , indicating conceivable presence of heptamolybdate ([Mo7O24] 6− , Mo7) in the hybrid [58,59]. C12im-N was acidified to form the C12im cation when added into the acidified sodium molybdate solution, and pH value will rise to cause the formation of the Mo7 anion [58,59]. On the other hand, utilizing the C12im cation prepared by prior neutralization of C12im-N with hydrochloric acid led to C12im-Mo hybrid of 2, which contained α-or δ-type Mo8 anion (Figure 2b) [47,58,59]. The α-and δ-type Mo8 isomers are difficult to distinguish only by IR spectra, since they have similar molecular structures except for some elongated Mo-O bonds of the δ-Mo8 anion (represented in broken lines in Figure 1b). These C12im-Mo hybrids of 1 and 2 exhibited distinct powder X-ray diffraction (XRD) patterns (Figure 3a,b), indicating the formation of pure crystalline compounds having different structures.
Recrystallization of both 1 and 2 enabled us to obtain single crystals, which were identified to possess the same molecular and crystal structure as 2, revealed by IR spectrum (Figure 2c) and powder XRD pattern (Figure 3c) of the single crystals. During the recrystallization process, the dissolved Mo7 anion from 1 will change to α-or δ-Mo8 in the solution [59,60], which reprecipitated into the single crystals of 2 (Figures 2a,c and 3a,c). On the other hand, the structure of 2 was retained before and after the recrystallization process (Figures 2b,c and 3b,c). Interestingly, the presence of AlCl3·6H2O under the recrystallization process was necessary to obtain suitable single crystals, as in the case when 1-dodecyl-3-methylimidazolium cation ([(C12H25)C3H3N2(CH3)] + , C12mim) and β-type Mo8 anion were hybridized to form C12mim-β-Mo8 (referred to as 3) [45]. No presence of AlCl3·6H2O resulted in the formation of precipitates or crystals with worse quality. This implies that the hydrated

Syntheses of C 12 im-Mo Hybrids
As-prepared C 12 im-Mo hybrids were obtained as insoluble precipitates from aqueous solution of sodium molybdate (pH = 3.6) in 50%-65% yield (based on Mo). Figure 2 shows infrared (IR) spectra of as-prepared C 12 im-Mo hybrids. The structure of the C 12 im-Mo hybrids depended on the ionic-liquid species employed in the syntheses. Using neutral 1-dodecylimidazole (C 3 H 3 N 2 (C 12 H 25 ), denoted as C 12 im-N) as surfactant source resulted in the formation of C 12 im-Mo hybrid of 1. The IR spectrum of 1 ( Figure 2a) showed characteristic peaks in the range of 400-1000 cm´1, indicating conceivable presence of heptamolybdate ([Mo 7 O 24 ] 6´, Mo 7 ) in the hybrid [58,59]. C 12 im-N was acidified to form the C 12 im cation when added into the acidified sodium molybdate solution, and pH value will rise to cause the formation of the Mo 7 anion [58,59]. On the other hand, utilizing the C 12 im cation prepared by prior neutralization of C 12 im-N with hydrochloric acid led to C 12 im-Mo hybrid of 2, which contained αor δ-type Mo 8 anion (Figure 2b) [47,58,59]. The αand δ-type Mo 8 isomers are difficult to distinguish only by IR spectra, since they have similar molecular structures except for some elongated Mo-O bonds of the δ-Mo 8 anion (represented in broken lines in Figure 1b). These C 12 im-Mo hybrids of 1 and 2 exhibited distinct powder X-ray diffraction (XRD) patterns (Figure 3a,b), indicating the formation of pure crystalline compounds having different structures.
Recrystallization of both 1 and 2 enabled us to obtain single crystals, which were identified to possess the same molecular and crystal structure as 2, revealed by IR spectrum (Figure 2c) and powder XRD pattern (Figure 3c) of the single crystals. During the recrystallization process, the dissolved Mo 7 anion from 1 will change to αor δ-Mo 8 in the solution [59,60], which reprecipitated into the single crystals of 2 (Figures 2a,c and 3a,c). On the other hand, the structure of 2 was retained before and after the recrystallization process (Figures 2b,c and 3b,c). Interestingly, the presence of AlCl 3¨6 H 2 O under the recrystallization process was necessary to obtain suitable single crystals, as in the case when 1-dodecyl-3-methylimidazolium cation ([(C 12 H 25 )C 3 H 3 N 2 (CH 3 )] + , C 12 mim) and β-type Mo 8 anion were hybridized to form C 12 mim-β-Mo 8 (referred to as 3) [45]. No presence of AlCl 3¨6 H 2 O resulted in the formation of precipitates or crystals with worse quality. This implies that the hydrated Al 3+ ion allows slow crystallization. In addition, the crystallization of 2 from 1 also requires the presence of AlCl 3¨6 H 2 O, which may promote the structural conversion from Mo 7 to αor δ-Mo 8 . Al 3+ ion allows slow crystallization. In addition, the crystallization of 2 from 1 also requires the presence of AlCl3·6H2O, which may promote the structural conversion from Mo7 to α-or δ-Mo8.   Al 3+ ion allows slow crystallization. In addition, the crystallization of 2 from 1 also requires the presence of AlCl3·6H2O, which may promote the structural conversion from Mo7 to α-or δ-Mo8.   The powder XRD patterns of as-prepared and recrystallized 2 measured at ambient temperature (Figure 3b,c) were almost the same in the peak position as the pattern calculated from the results of single crystal X-ray analysis (Figure 3d). This indicates that 2 was obtained as a single phase, being consistent with the results of elemental analyses. Slight differences in the peak intensity and position of the patterns may be due to the difference in the measurement temperature (powder: ambient temperature, single crystal: 93 K), and to preferred orientation derived from the predominant layered structure of 2.

Crystal Structure of C 12 im-δ-Mo 8 (2)
The X-ray structure and elemental analyses revealed the formula of 2 to be [ Table 1). The crystal structure contained δ-type Mo 8 anion with no solvent of crystallization, which was consistent with the IR spectrum ( Figure 2c). Four C 12 im cations (1+ charge) were associated with one δ-Mo 8 anion (4-charge) due to the charge compensation. 2 contained only the C 12 im cation as counter cation, being similar to the hybrid crystal of 3 [45]. The IR spectra of 2 exhibited the characteristic peaks of δ-Mo 8 (Figure 2b,c), which contrasted with that of 3, which consists of the β-Mo 8 anion (Figure 2d). This difference in the Mo 8 isomer structures is notable, since C 12 mim cation preferred βor γ-type Mo 8 anion [45,46]. The difference in the Mo 8 isomers seems to depend on the difference in the hydrophilic moiety of ionic-liquid surfactants. C 12 im has no methyl group in the imidazole ring, while C 12 mim has one methyl group. The charged imidazolium moiety of C 12 im or C 12 mim strongly interacts with Mo 8 anions. The difference in the bulkiness of the hydrophilic moiety and in the ability to form a strong N-H¨¨¨O hydrogen bond (see below) may result in the formation of different Mo 8 isomer structures in 2 and 3. Figure 4 shows the crystal structure of 2. The crystal packing consisted of alternating δ-Mo 8 inorganic monolayers and C 12 im organic bilayers with an interlayer distance of 19.9 Å (Figure 4a,b). The hydrophilic heads of C 12 im penetrated into the δ-Mo 8 layers to isolate each δ-Mo 8 anion (Figure 4c), being similar to that in the crystal of 3 [45]. The two crystallographically independent C 12 im cations formed a paired structure ( Figure 5). They had a slight overlap of the imidazole rings, indicating the presence of a π-π stacking interaction (distance of C2-C17 bond between the imidazole rings: 3.38 Å).
All C-C bonds of the C 12 im in 2 had anti conformation. These conformations of the imidazole rings and long alkyl chain were similar to the crystal of 3 [45]. indicating the presence of a π-π stacking interaction (distance of C2-C17 bond between the imidazole rings: 3.38 Å). All C-C bonds of the C12im in 2 had anti conformation. These conformations of the imidazole rings and long alkyl chain were similar to the crystal of 3 [45].  In the crystal structure of 2, two types of hydrogen bond were observed, namely N-H···O and C-H···O hydrogen bonds [61]. Most hydrogen bonds were formed at the interface between the δ-Mo8 and C12im layers. The N-H···O hydrogen bonds were derived from protonated nitrogen atom of imidazole ring in the C12im cation. The N···O distances of the N-H···O hydrogen bond in 2 were 2.89-3.09 Å (mean value: 2.97 Å), indicating the presence of strong hydrogen bonds. The C-H···O indicating the presence of a π-π stacking interaction (distance of C2-C17 bond between the imidazole rings: 3.38 Å). All C-C bonds of the C12im in 2 had anti conformation. These conformations of the imidazole rings and long alkyl chain were similar to the crystal of 3 [45].  In the crystal structure of 2, two types of hydrogen bond were observed, namely N-H···O and C-H···O hydrogen bonds [61]. Most hydrogen bonds were formed at the interface between the δ-Mo8 and C12im layers. The N-H···O hydrogen bonds were derived from protonated nitrogen atom of imidazole ring in the C12im cation. The N···O distances of the N-H···O hydrogen bond in 2 were 2.89-3.09 Å (mean value: 2.97 Å), indicating the presence of strong hydrogen bonds. The C-H···O In the crystal structure of 2, two types of hydrogen bond were observed, namely N-H¨¨¨O and C-H¨¨¨O hydrogen bonds [61]. Most hydrogen bonds were formed at the interface between the δ-Mo 8 and C 12 im layers. The N-H¨¨¨O hydrogen bonds were derived from protonated nitrogen atom of imidazole ring in the C 12 im cation. The N¨¨¨O distances of the N-H¨¨¨O hydrogen bond in 2 were 2.89-3.09 Å (mean value: 2.97 Å), indicating the presence of strong hydrogen bonds. The C-H¨¨¨O hydrogen bond in 2 exhibited C¨¨¨O distances of 2.87-3.86 Å (mean value: 3.42 Å), which was similar to the C¨¨¨O distances in 3 (3.04-3.85 Å, mean value: 3.42 Å) [45,62].
2.3. Conductivity of C 12 im-δ-Mo 8 (2) Figure 6 shows an impedance spectrum for as-prepared 2 at 443 K under anhydrous atmosphere. As mentioned above, 2 retained both molecular and crystal structures before and after the recrystallization process. The spectrum showed a suppressed half circle in the high-and medium-frequency regions and an inclined line in the low-frequency region. The suppressed half circle will be derived from two overlapped semicircles due to bulk and grain boundary elements [48,49]. The linear part in the low-frequency region would result from combination of charge transfer resistance and Warburg impedance related to the diffusion of the carrier. The equivalent circuit employed here is shown in Figure 6 (inset). It consists of bulk resistance and capacitance (R b and C b ), grain boundary resistance and capacitance (R gb and C gb ), and charge transfer resistance (R ct ) along with double layer capacitance (C dl ). Z W represents the Warburg impedance. The red line in Figure 6 represents simulated data with the equivalent circuit, which successfully reproduced the measured impedance spectrum. The estimated value of R b was 1.85ˆ10 4 Ω, from which the conductivity of 2 was calculated to be 5.5ˆ10´6 S¨cm´1. This anhydrous conductivity is due to the residual proton in the bulk solid of 2 derived from the deprotonatable C 12 im cation, since 2 contained no molecule of crystallization nor small counter cation as a plausible source of carrier. The proton attached to the imidazole ring in C 12 im will be dissociated at the intermediate temperature of 443 K. Although the value of the anhydrous conductivity is not high enough, conductive polyoxometalate-surfactant hybrid crystals would pave a way to another class of anhydrous proton conductors at intermediate temperatures.

Conductivity of C12im-δ-Mo8
(2) Figure 6 shows an impedance spectrum for as-prepared 2 at 443 K under anhydrous atmosphere. As mentioned above, 2 retained both molecular and crystal structures before and after the recrystallization process. The spectrum showed a suppressed half circle in the high-and mediumfrequency regions and an inclined line in the low-frequency region. The suppressed half circle will be derived from two overlapped semicircles due to bulk and grain boundary elements [48,49]. The linear part in the low-frequency region would result from combination of charge transfer resistance and Warburg impedance related to the diffusion of the carrier. The equivalent circuit employed here is shown in Figure 6 (inset). It consists of bulk resistance and capacitance (Rb and Cb), grain boundary resistance and capacitance (Rgb and Cgb), and charge transfer resistance (Rct) along with double layer capacitance (Cdl). ZW represents the Warburg impedance. The red line in Figure 6 represents simulated data with the equivalent circuit, which successfully reproduced the measured impedance spectrum. The estimated value of Rb was 1.85 × 10 4 Ω, from which the conductivity of 2 was calculated to be 5.5 × 10 −6 S·cm −1 . This anhydrous conductivity is due to the residual proton in the bulk solid of 2 derived from the deprotonatable C12im cation, since 2 contained no molecule of crystallization nor small counter cation as a plausible source of carrier. The proton attached to the imidazole ring in C12im will be dissociated at the intermediate temperature of 443 K. Although the value of the anhydrous conductivity is not high enough, conductive polyoxometalate-surfactant hybrid crystals would pave a way to another class of anhydrous proton conductors at intermediate temperatures.
Conductivity measurements were carried out by alternating current (AC) impedance method in a frequency range from 20 to 1.0 × 10 7 Hz using a Wayne Kerr 6510P inductance-capacitance-

Materials and Genaral Methods
All chemical reagents except for imidazolium surfactant were obtained from commercial sources (Wako, Osaka, Japan and TCI, Tokyo, Japan, the highest grade). 1-dodecylimidazole (C 12 im-N) and its hydrochloric-acid salt ([C 3 H 4 N 2 (C 12 H 25 )]Cl, C 12 im¨Cl) were prepared according to the literature [63].
Conductivity measurements were carried out by alternating current (AC) impedance method in a frequency range from 20 to 1.0ˆ10 7 Hz using a Wayne Kerr 6510P inductance-capacitance-resistance (LCR) meter. Pelletized powder samples (10 mm in diameter, 0.79 mm in thickness) were sandwiched with Pt electrodes, and the impedance was measured under a dry N 2 atmosphere at 443 K.

Synthesis
As-prepared C 12  Colorless platelet crystals of 2 were obtained as follows: acetonitrile solution (15 mL) of the as-prepared C 12 im-Mo hybrid (1 or 2, 0.03 g) and AlCl 3¨6 H 2 O (0.02 g) was kept at 323 K for one day. The resulting supernatant was kept at 303 K for a few days, and then evaporated at room temperature to obtain colorless plates of 2 in ca. 30% yield. Anal.: Calcd for C 60

X-ray Crystallography
Single crystal XRD measurements were performed on a Rigaku Saturn70 diffractometer (Tokyo, Japan) using graphite monochromated Mo-Kα radiation (λ = 0.71075 Å). Diffraction data were collected and processed with CrystalClear [64]. The structure was solved by direct methods [65]. The refinement procedure was performed by the full-matrix least-squares using SHELXL Version 2014/7 [66]. All calculations were performed using the CrystalStructure [67] software package. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms on C atoms were refined using the riding model. Further details of the crystal structure investigation may 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: deposit@ccdc.cam.ac.uk (CCDC 1472277).

Conclusions
Polyoxomolybdate hybrid crystals were successfully obtained by employing deprotonatable ionic-liquid cation, 1-dodecylimidazolium ([C 3 H 4 N 2 (C 12 H 25 )] + , C 12 im). The crystal contained δ-type octamolybdate ([Mo 8 O 26 ] 4´, Mo 8 ), being different from the case of crystals comprising methylimidazolium surfactant having no dissociative proton. The crystal structure of C 12 im-δ-Mo 8 consisted of alternate stacking of the δ-Mo 8 layers and C 12 im layers. The hydrophilic moiety of the C 12 im cation formed N-H¨¨¨O and C-H¨¨¨O hydrogen bonds between the Mo 8 anions, and the presence of the N-H¨¨¨O hydrogen bonds suggests the formation δ-type Mo 8 in the C 12 im-δ-Mo 8 crystal. The C 12 im-δ-Mo 8 crystal exhibited anhydrous conductivity of 5.5ˆ10´6 S¨cm´1 at 443 K presumably due to the proton dissociated from the protonated C 12 im cation, which is promising for the exploration of anhydrous proton conductors working at an intermediate temperature region.