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.

1. Introduction

Ionic-liquids exhibit characteristic conductive or catalytic properties, and enable us to construct functional hybrid materials [1,2,3]. Ionic-liquid species often contain imidazolium moiety in their molecular structures. Inorganic-organic materials comprising imidazolium ionic-liquid have been explored as ionic or proton conductors [4,5]. As for inorganic components in hybrid conducting materials, polyoxometalate anions are effective candidates [6,7,8,9,10,11,12,13,14,15,16,17]. Polyoxometalates and ionic-liquids have been successfully hybridized [18,19,20,21,22], and some of them exhibit promising conductive properties [15,18].

In such polyoxometalate hybrids with ionic-liquids, the structure and arrangement of molecular components should be precisely controlled for the emergence of characteristic functions. To construct well-ordered polyoxometalate-ionic liquid hybrids, utilizing structure-directing species such as surfactant molecules [23,24,25] is advantageous. Polyoxometalate anions have been organized by surfactant cations to form inorganic-organic hybrids [26,27,28,29,30,31,32,33,34,35,36,37,38,39,40] and single crystals [41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57]. These polyoxometalate-surfactant hybrids allow flexible selection of the ionic components, which leads to precise engineering of the structure and function. In addition, polyoxometalate single crystals hybridized with ionic-liquid surfactants have also been reported [45,46,52,53,54].

We report here synthesis and structure of a polyoxomolybdate-ionic liquid hybrid crystal. Deprotonatable 1-dodecylimidazolium ([C3H4N2(C12H25)]+ (C12im), Figure 1a) cations were employed to obtain C12im-polyoxomolybdate (C12im-Mo) hybrids (referred to as 1). Recrystallization of C12im-Mo hybrids resulted in the formation of single crystals comprising δ-type isomer of octamolybdate ([Mo8O26]4− (Mo8), Figure 1b) anion, C12im-δ-Mo8 (referred to as 2). The weak interactions between C12im cation and δ-Mo8 anion were investigated, and anhydrous conductivity was estimated.

Figure 1

Molecular structures of ionic-liquid surfactants and polyoxomolybdates: (a) 1-dodecylimidazolium (C12im) and 1-dodecyl-3-methylimidazolium (C12mim); (b) Octamolybdate (Mo8) isomers and heptamolybdate (Mo7) in ball and stick (green: Mo, red: O) and polyhedral representations.

2. Results and Discussion

2.1. 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.

Figure 2

Infrared (IR) spectra of C12im-Mo hybrids: (a) C12im-Mo hybrid (1) obtained from C12im-N; (b) C12im-δ-Mo8 (2) obtained from C12im·Cl; (c) 2 after recrystallization; (d) C12mim-β-Mo8 (3) obtained by using C12mim.

Figure 3

Powder X-ray diffraction (XRD) patterns of C12im-Mo hybrids: (a) 1 obtained from C12im-N; (b) 2 obtained from C12im·Cl; (c) 2 after recrystallization; (d) Calculated pattern of 2 using the structure obtained by single-crystal XRD.

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 (Figure 2a,c and Figure 3a,c). On the other hand, the structure of 2 was retained before and after the recrystallization process (Figure 2b,c and Figure 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 Al3+ 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.

2.2. Crystal Structure of C12im-δ-Mo8 (2)

The X-ray structure and elemental analyses revealed the formula of 2 to be [C3H4N2(C12H25)]4[δ-Mo8O26] (Table 1). The crystal structure contained δ-type Mo8 anion with no solvent of crystallization, which was consistent with the IR spectrum (Figure 2c). Four C12im cations (1+ charge) were associated with one δ-Mo8 anion (4-charge) due to the charge compensation. 2 contained only the C12im cation as counter cation, being similar to the hybrid crystal of 3 [45].

Table 1

Crystallographic data.

Compound	C12im-δ-Mo8 (2)
Chemical formula	C60H116N8Mo8O26
Formula weight	2133.13
Crystal system	monoclinic
Space group	P21/c (No. 14)
a (Å)	21.859(4)
b (Å)	10.0395(18)
c (Å)	20.683(4)
α (°)	90.0000
β (°)	114.307(2)
γ (°)	90.0000
V (Å3)	4136.7(14)
Z	2
ρcalcd (g·cm−3)	1.712
T (K)	93
μ (Mo·Kα) (mm−1)	1.244
No. of reflections measured	27938
No. of independent reflections	7550
Rint	0.0463
No. of parameters	460
R1 (I > 2σ(I))	0.0422
wR2 (all data)	0.1292

The IR spectra of 2 exhibited the characteristic peaks of δ-Mo8 (Figure 2b,c), which contrasted with that of 3, which consists of the β-Mo8 anion (Figure 2d). This difference in the Mo8 isomer structures is notable, since C12mim cation preferred β- or γ-type Mo8 anion [45,46]. The difference in the Mo8 isomers seems to depend on the difference in the hydrophilic moiety of ionic-liquid surfactants. C12im has no methyl group in the imidazole ring, while C12mim has one methyl group. The charged imidazolium moiety of C12im or C12mim strongly interacts with Mo8 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 Mo8 isomer structures in 2 and 3.

Figure 4 shows the crystal structure of 2. The crystal packing consisted of alternating δ-Mo8 inorganic monolayers and C12im organic bilayers with an interlayer distance of 19.9 Å (Figure 4a,b). The hydrophilic heads of C12im penetrated into the δ-Mo8 layers to isolate each δ-Mo8 anion (Figure 4c), being similar to that in the crystal of 3 [45]. The two crystallographically independent C12im 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 C12im in 2 had anti conformation. These conformations of the imidazole rings and long alkyl chain were similar to the crystal of 3 [45].

Figure 4

Crystal structure of 2 (C: gray, N: blue; δ-Mo8 anions in polyhedral representations). H atoms are omitted for clarity; (a) Packing diagram along b axis; (b) Packing diagram along c axis; (c) Molecular arrangements in the inorganic layers. The dodecyl groups are omitted for clarity.

Figure 5

View of crystallographically independent C12im cations. Symmetry code: (i) −x, −0.5 + y, 0.5 − z.

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 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 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 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 (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 × 104 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.

Figure 6

Nyquist spectrum (open circles) of as-prepared C12im-δ-Mo8 (2) at 443 K and simulated spectrum (red line) based on an equivalent electronic circuit in the figure. The parameters obtained by the fitting (see text) are as follows: Rb = 1.85 × 104 Ω, Rgb = 1.2 × 104 Ω, Rct = 1.1 × 104 Ω, Cb = 1.0 × 10−8 F, Cgb = 6.0 × 10−9 F, Cdl = 3.0 × 10−6 F, σ = 2.2 × 104 Ω·s−1/2 (Zw = ).

3. Materials and Methods

3.1. Materials and Genaral Methods

All chemical reagents except for n class="Chemical">imidazolium surfactant were obtained from commercial sources (Wako, Osaka, Japan and TCI, Tokyo, Japan, the highest grade). 1-dodecylimidazole (C12im-N) and its hydrochloric-acid salt ([C3H4N2(C12H25)]Cl, C12im·Cl) were prepared according to the literature [63].

IR spectra (as KBr pellet) were recorded on a Jasco FT/IR-4200ST spectrometer (Tokyo, Japan). Powder X-ray diffraction (XRD) patterns were measured with a Rigaku MiniFlex300 diffractometer by using Cu Kα radiation (λ = 1.54056 Å) at ambient temperature.

Conductivity measurements were carried out by alternating current (AC) impedance method in a frequency range from 20 to 1.0 × 107 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 N2 atmosphere at 443 K.

3.2. Synthesis

As-prepared C12im-Mo hybrid of 1 was precipitated by adding n class="Chemical">ethanol solution of C12im-N (0.2 M, 10 mL) to Na2MoO4·2H2O aqueous solution (0.4 M, 10 mL), which was adjusted to pH 3.6 with 6 M HCl. The precipitates were isolated by filtration, and dried in the ambient atmosphere to obtain colorless powder of 1 in a yield of 64%. Melting point: 463 K. IR (KBr disk): 936 (m), 905 (s), 889 (m), 853 (s), 829 (w), 761 (w), 722 (w), 665 (vs), 644 (m), 563 (w), 523 (w), 481 (w), 444 (w), 428 (w) cm−1.

C12im-δ-Mo8 hybrid of 2 was prepared as follows: to aqueous solution of Na2MoO4·2H2O (0.4 M, 10 mL) acidified to pH 3.6 with 6 M HCl was added ethanol solution of C12im-N (0.2 M, 10 mL) neutralized by 1 M HCl (1.7 mL). The resulting precipitates were isolated by filtration, and dried in the ambient atmosphere to obtain colorless powder of 2 in a yield of 59%. Using ethanol solution of C12im·Cl (0.2 M, 10 mL) instead of the acidified C12im-N solution gave the same hybrids (yield: 52%). Anal.: Calcd for C60H116N8Mo8O26: C: 33.78, H: 5.48, N: 5.25%. Found: C: 33.14, H: 5.21, N: 5.11%. Melting point: 501 K. IR (KBr disk): 960 (w), 930 (s), 912 (s), 898 (m), 856 (m), 795 (s), 721 (w), 661 (s), 620 (w), 552 (w), 499 (w), 464 (w), 422 (w) cm−1.

Colorless platelet crystals of 2 were obtained as follows: acetonitrile solution (15 mL) of the as-prepared C12im-Mo hybrid (1 or 2, 0.03 g) and AlCl3·6H2O (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 C60H116N8Mo8O26: C: 33.78, H: 5.48, N: 5.25%. Found: C: 34.49, H: 5.66, N: 5.30%. Melting point: 493 K. IR (KBr disk):960 (w), 930 (s), 912 (s), 898 (m), 855 (m), 795 (s), 707 (w), 661 (s), 620 (w), 553 (w), 498 (w), 472 (w), 457 (w) cm−1.

3.3. 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).

4. Conclusions

Polyoxomolybdate hybrid crystals were successfully obtained by employing deprotonatable ionic-liquid cation, 1-dodecylimidazolium ([C3H4N2(C12H25)]+, C12im). The crystal contained δ-type octamolybdate ([Mo8O26]4−, Mo8), being different from the case of crystals comprising methylimidazolium surfactant having no dissociative proton. The crystal structure of C12im-δ-Mo8 consisted of alternate stacking of the δ-Mo8 layers and C12im layers. The hydrophilic moiety of the C12im cation formed N–H···O and C–H···O hydrogen bonds between the Mo8 anions, and the presence of the N–H···O hydrogen bonds suggests the formation δ-type Mo8 in the C12im-δ-Mo8 crystal. The C12im-δ-Mo8 crystal exhibited anhydrous conductivity of 5.5 × 10−6 S·cm−1 at 443 K presumably due to the proton dissociated from the protonated C12im cation, which is promising for the exploration of anhydrous proton conductors working at an intermediate temperature region.