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Journal of Chemistry
Volume 2013 (2013), Article ID 208075, 6 pages
Hybrid Layered Crystal Comprising Polyoxometalate and Surfactant Synthesized from Reduced Mo-Blue Species
1Department of Chemistry, School of Science, Tokai University, 4-1-1 Kitakaname, Hiratsuka 259-1292, Japan
2X-Ray Research Laboratory, Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima 196-8666, Japan
3Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-23 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Received 30 January 2013; Accepted 21 March 2013
Academic Editor: Jean-Luc Blin
Copyright © 2013 Keisuke Mikurube et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
A hybrid layered crystal containing polyoxomolybdate was successfully synthesized from reduced Mo-blue species as starting material. The hybrid crystal, [C5H5N(C16H33)]2[-H2Mo8O26]2C2H5OH (C16py-H2Mo8), was obtained as a single phase by the gradual oxidation of hexadecylpyridinium-Mo-blue (C16py-Mo-blue) hybrid. The X-ray structure analysis revealed that C16py-H2Mo8 comprised -type octamolybdate anion with two protons (-H2Mo8). The -H2Mo8 anions and ethanol molecules of crystallization formed two-dimensional anionic layers. The pyridine rings of C16py did not attend to form the two-dimensional inorganic layers, and the interdigitated C16py bilayers were sandwiched by the -H2Mo8 anionic layers with periodicity of 18.2 . These C16py-H2Mo8 hybrid layered crystals possibly work as a new class of proton conductor.
Crystalline layered materials are superior to soft layered materials with respect to the structural stability and homogeneity. The two-dimensional anisotropy of layered materials leads to the emergence of properties such as electronic conductivity or intercalation [1–4], while the precise control of the layered periodicity and component arrangement is required. Inorganic-organic hybrid materials have wider options to select compositions and structures owing to organic components than purely inorganic compounds. Hybrid crystals composed of conductive organic molecules and inorganic anions have been reported [5–7].
Surfactant molecules to self-assemble into lamellar phase are an effective organic component as a structure-directing reagent for layered structures [8–10]. The layered distance can be controlled by changing the length of long alkyl chains. Polyoxometalate anions (POMs) having various physicochemical properties [11–17] can be used as inorganic component, which enables to design the composition, structures, and functions of hybrid layered crystals. Surfactant-POM hybrid layered crystals have been rarely reported [18–28] rather than surfactant-POM hybrid materials [29–33].
Surfactant-POM hybrid layered crystals can be synthesized by direct cation-exchange reaction, in which cationic surfactants are added to aqueous POM solution [18, 19, 24–27]. Another strategy uses precursor surfactant-POM hybrid crystals as synthetic starting material [21–23]. In this precursor method, the reaction of isopolymolybdate dissolved in the solution has been employed. The precursor isopolymolybdate anions isomerize or react with the solvent during the recrystallization process to give other types of surfactant-POM hybrid crystal having different compositions and structures [21–23].
Isopolymolybdate can form several types of dark-blue reduced species, so-called “molybdenum-blue” (Mo-blue) species [15, 16]. These Mo-blues are relatively stable and have various structural diversity. After gradual oxidation, the Mo-blues change to conventional isopolymolybdate (colorless or pale yellow). The reoxidation of the Mo-blue species can be employed for the syntheses of surfactant-POM hybrid crystals . Using Mo-blue species as starting precursor material provides another strategy to control of compositions and structures of surfactant-POM hybrid crystals.
We report here a synthesis of surfactant-POM hybrid crystal by using Mo-blue species as starting precursor material. The Mo-blue species and hexadecylpyridinium () cation formed hybrid materials (-Mo-blue). The gradual oxidation of the -Mo-blue resulted in the formation of -octamolybdate hybrid crystal, [C5H5N(C16H33)]2[β-H2Mo8O26]2C2H5OH (-H2Mo8), which was the first example of proton-containing surfactant-polyoxomolybdate hybrid crystal.
All chemical reagents were obtained from commercial sources. -Mo-blue hybrid was prepared as follows; Na2MoO42H2O (2.6 g, 11 mmol) was dissolved in H2O (10 mL), and the pH level was adjusted to 3.0–4.5 with 6 M HCl. To the colorless acidified solution was added solid ascorbic acid (0.06 g, 0.3 mmol) as reducing reagent. After stirring for 40 min, the solution color turned dark blue indicating the formation of Mo-blue species, and then a water/ethanol (10 mL, 1 : 1 (v/v)) solution of ClH2O (0.54 g, 1.5 mmol) was added to the solution, and stirred for 10 min. Obtained dark-blue precipitates (-Mo-blue) were filtered and washed by water/ethanol solution followed by drying in dark place. IR (KBr disk) for -Mo-blue: 953 (m), 908 (m), 876 (m), 845 (m), 791 (m), 764 (m), 661 (s), 644 (s), 625 (s), and 559 (s) .
-H2Mo8 was synthesized by using gradual oxidation of -Mo-blue hybrids. 0.10 g of -Mo-blue was dispersed as starting material in hot ethanol/N,N-dimethylformamide (15 mL, 2 : 1 (v/v)), and the supernatant was kept at 293 and 288 K to give colorless needles of -H2Mo8. Dimethylamine molecules formed by the decomposition of N,N-dimethylformamide probably adsorbed to the crystals of -H2Mo8 employed for the elemental analysis. Anal. for -H2Mo8: Calcd for C46H90N4Mo8O26: C, 29.31; H, 4.92; N, 2.97%. Found: C, 29.11; H, 4.43 N, 2.96%. IR (KBr disk) for -H2Mo8: 943 (s), 914 (s), 837 (m), 723 (m), 687 (w), 656 (m), 552 (w), and 519 (w) .
IR spectra (as KBr pellet) were recorded on a Horiba FT-710 spectrometer. Powder X-ray diffraction (XRD) patterns were measured with a Rigaku Geigerflex RAD-2X diffractometer by using Cu Kα radiation (λ = 1.54056 Å) at ambient temperature.
Single crystal X-ray diffraction measurements for -H2Mo8 were made on a Rigaku VariMax with RAPID imaging plate area detector using CuK radiation (crystal-to-detector distance: 127.40 mm) at Rigaku Corporation. Diffraction data were collected for a needle crystal (0.16 × 0.08 × 0.04 mm). The structure was solved by direct methods using SHELXS97  and expanded using Fourier techniques. The refinement procedure was performed by the full-matrix least squares using SHELXL97 . All calculations were performed using the CrystalStructure  software package. Empirical absorption correction was performed for the observed data. All nonhydrogen atoms were refined anisotropically. H atoms of cation were refined isotropically, and H atoms of ethanol C atoms were located in calculated positions. H atoms of H2Mo8 and hydroxyl group in the ethanol were not included in the refinement procedure. CCDC-922099.
3. Results and Discussion
Polyoxomolybdate is well known to form reduced Mo-blue species [14–16]. The dark-blue precipitates were isolated from reduced polyoxomolybdate solution by adding cation. The IR spectrum of the precipitates (Figure 1(a)) revealed the presence of cation (1400–1600 and 2800–3200 range). The IR bands in the 400–1200 range were similar to those typical for Mo-blue nanorings [36, 37], indicating the formation of -Mo-blue hybrids. The Mo-blue species are gradually oxidized to form conventional polyoxomolybdate anion, which means that -Mo-blue works as starting material for the -POM hybrid crystals. Here colorless crystals of -H2Mo8 were reproducibly synthesized by using the -Mo-blue hybrid during the recrystallization process. The IR spectra of -H2Mo8 exhibited similar pattern in the 400–1200 range to that of β-octamolybdate (, β-Mo8) anion (Figure 1(b)). The molecular structure was confirmed by X-ray structure analysis as described below. However, single broad band in the 600–800 range for the usual β-Mo8 species [38, 39] was split into two bands for -H2Mo8, suggesting the presence of attaching protons to β-Mo8 anion (see below).
Powder X-ray diffraction pattern of -H2Mo8 measured at ambient temperature (Figure 2(a)) was almost the same in the peak position as the pattern calculated from the results of single crystal X-ray analysis (Figure 2(b)). This indicates that -H2Mo8 was obtained as a single phase. Slight differences in the peak intensity and position of the patterns will be owing to the difference in the measurement temperature (powder: ambient temperature, single crystal: 173 K) and to preferred orientation derived from the predominant layered structure of -H2Mo8.
X-ray structure analysis (Table 1) revealed that the colorless crystals of -H2Mo8 contained conventional (β-Mo8) anions. Two cations and two ethanol molecules were identified per β-Mo8 of 4− charge (one , one ethanol, and half β-Mo8 in the asymmetric unit, Figure 3(a)), and there was no residual atom as counter cation. This suggests the presence of two protons in the -H2Mo8 for balancing the charge, confirmed by the bond valence sum (BVS) calculation. All the Mo atoms of β-Mo8 in -H2Mo8 exhibited the averaged BVS values of 5.92, indicating that the Mo atoms are in the highest oxidation state (+6) without reduction. The two bridging oxygen atoms (O4) of β-Mo8 had relatively low BVS value (1.69) than 2, being similar to the literature . The crystals of -H2Mo8 contained no water of crystallization to which proton can be attached. These results show the presence of two separate protons located to O4 (Figure 3(a)), supported by the IR spectrum of -H2Mo8 as mentioned above. Thus, the composition of -H2Mo8 was revealed to be [C5H5N(C16H33)]2[β-H2Mo8O26]·2C2H5OH. This is the first surfactant-polyoxomolybdate hybrid crystals to comprise protons, while sodium cation has been introduced in the surfactant-polyoxomolybdate hybrid crystals [21, 22]. The reduced Mo-blue species tend to contain protons for the charge compensation, which may result in the formation of proton-containing surfactant-POM hybrid crystals. The -H2Mo8 hybrid crystal containing isopolyoxometalate probably does not behave as a strong acid, being different from heteropoly acids (heteropolyoxometalates comprising protons as counter cations) [11, 41]. -H2Mo8 possibly exhibits moderate proton conductivity in the intermediate temperature region over 373 K as observed in another surfactant-POM hybrid layered crystal .
The crystal packing of -H2Mo8 consisted of alternating inorganic β-H2Mo8 monolayers and organic bilayers (Figure 3(b)). The hexadecyl chains of interdigitated in the bilayers. The inorganic monolayers were composed of β-H2Mo8 and ethanol molecules of crystallization. Such alternate stacking of inorganic and organic layers was similar to that of most POM-surfactant hybrid crystals [18–28]. The periodicity of the layers was 18.2 Å. All C–C bonds in the hexadecyl chains showed anti conformation as observed in a hybrid crystal of and hexamolybdate .
However, the molecular arrangement in the inorganic layer was quite different from that for other -POM hybrid layered crystal [21–25]. The hydrophilic heads of usually penetrated into the POM inorganic layers, and the pyridine rings of interacted to form a π-π stacking pair or were located in the adjacent positions [21–25]. On the contrary in -H2Mo8, the hydrophilic heads of were completely excluded from the β-H2Mo8 inorganic layers (Figure 4(a)). β-H2Mo8 anions and ethanol molecules of crystallization had short contacts (the distance between two atoms shorter than the sum of van der Waals radii) including one C–HO hydrogen bond (Table 2), which forms a densely packed anionic layer. This two-dimensional anionic layer has no space for the penetration of the pyridine rings (Figure 4(b)), and the interdigitated bilayers were sandwiched by the β-H2Mo8 anionic layers (Figure 3(b)).
-H2Mo8 had C–HO hydrogen bonds  (Table 2). The C–HO hydrogen bonds between β-H2Mo8 and were 3.45–3.99 Å in CO distance. The mean value was 3.67 Å, similar to the mean CO distances (~3.6 Å) in other -POM hybrid crystals [21, 22, 24]. These hydrogen bonds as well as electrostatic interaction between and β-H2Mo8 would stabilize the layered crystal structure of -H2Mo8 with rigid packing.
A hybrid layered crystal, [C5H5N(C16H33)]2[β-H2Mo8O26]·2C2H5OH (-H2Mo8), was successfully synthesized as single phase by using Mo-blue species as starting precursor material. -H2Mo8 consisted of β-octamolybdate anion with two protons (β-H2Mo8), being the first example of proton-comprising surfactant-polyoxomolybdate hybrid crystals. The β-H2Mo8 anions and ethanol molecules of crystallization formed two-dimensional anionic layer, which sandwiched interdigitated bilayers of cation. The hydrophilic head of did not attend to form the inorganic β-H2Mo8 layers. Such protonated surfactant-polyoxomolybdate hybrid crystals would be promising for a new class of proton-conducting materials.
This work was financially supported in part by JSPS KAKENHI Grant no. 23750246, Tokai University Supporters Association Research and Study Grant, and the Noguchi Institute.
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