Hybrid Layered Crystal Comprising Polyoxometalate and Surfactant Synthesized from Reduced Mo-Blue Species

Abstract

A hybrid layered crystal containing polyoxomolybdate was successfully synthesized from reduced Mo-blue species as starting material. The hybrid crystal, [C₅H₅N(C₁₆H₃₃)]₂[𝛽-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 𝛽-type octamolybdate anion with two protons (𝛽-H₂Mo₈). The 𝛽-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 𝛽-H₂Mo₈ anionic layers with periodicity of 18.2 Å. These C₁₆py-H₂Mo₈ hybrid layered crystals possibly work as a new class of proton conductor.

1. Introduction

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 [20]. 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 (C₁₆py) cation formed hybrid materials (C₁₆py-Mo-blue). The gradual oxidation of the C₁₆py-Mo-blue resulted in the formation of C₁₆py-octamolybdate hybrid crystal, [C₅H₅N(C₁₆H₃₃)]₂[β-H₂Mo₈O₂₆]⋅2C₂H₅OH (C₁₆py-H₂Mo₈), which was the first example of proton-containing surfactant-polyoxomolybdate hybrid crystal.

2. Experimental

All chemical reagents were obtained from commercial sources. C₁₆py-Mo-blue hybrid was prepared as follows; Na₂MoO₄⋅2H₂O (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 C₁₆pyCl⋅H₂O (0.54 g, 1.5 mmol) was added to the solution, and stirred for 10 min. Obtained dark-blue precipitates (C₁₆py-Mo-blue) were filtered and washed by water/ethanol solution followed by drying in dark place. IR (KBr disk) for C₁₆py-Mo-blue: 953 (m), 908 (m), 876 (m), 845 (m), 791 (m), 764 (m), 661 (s), 644 (s), 625 (s), and 559 (s) cm⁻¹.

C₁₆py-H₂Mo₈ was synthesized by using gradual oxidation of C₁₆py-Mo-blue hybrids. 0.10 g of C₁₆py-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 C₁₆py-H₂Mo₈. Dimethylamine molecules formed by the decomposition of N,N-dimethylformamide probably adsorbed to the crystals of C₁₆py-H₂Mo₈ employed for the elemental analysis. Anal. for C₁₆py-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 C₁₆py-H₂Mo₈: 943 (s), 914 (s), 837 (m), 723 (m), 687 (w), 656 (m), 552 (w), and 519 (w) cm⁻¹.

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 C₁₆py-H₂Mo₈ were made on a Rigaku VariMax with RAPID imaging plate area detector using CuK𝛼 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 [34] and expanded using Fourier techniques. The refinement procedure was performed by the full-matrix least squares using SHELXL97 [34]. All calculations were performed using the CrystalStructure [35] software package. Empirical absorption correction was performed for the observed data. All nonhydrogen atoms were refined anisotropically. H atoms of C₁₆py 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 C₁₆py cation. The IR spectrum of the precipitates (Figure 1(a)) revealed the presence of C₁₆py cation (1400–1600 and 2800–3200 cm⁻¹ range). The IR bands in the 400–1200 cm⁻¹ range were similar to those typical for Mo-blue nanorings [36, 37], indicating the formation of C₁₆py-Mo-blue hybrids. The Mo-blue species are gradually oxidized to form conventional polyoxomolybdate anion, which means that C₁₆py-Mo-blue works as starting material for the C₁₆py-POM hybrid crystals. Here colorless crystals of C₁₆py-H₂Mo₈ were reproducibly synthesized by using the C₁₆py-Mo-blue hybrid during the recrystallization process. The IR spectra of C₁₆py-H₂Mo₈ exhibited similar pattern in the 400–1200 cm⁻¹ range to that of β-octamolybdate (𝛽-[Mo₈O₂₆]⁴⁻, 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 cm⁻¹ range for the usual β-Mo₈ species [38, 39] was split into two bands for C₁₆py-H₂Mo₈, suggesting the presence of attaching protons to β-Mo₈ anion (see below).

Figure 1 

IR spectra of (a) C16py-Mo-blue (starting material) and (b) C16py-H2Mo8 (obtained product).

Powder X-ray diffraction pattern of C₁₆py-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 C₁₆py-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 C₁₆py-H₂Mo₈.

Figure 2 

Powder X-ray diffraction patterns of C16py-H2Mo8 from (a) observed data measured at ambient temperature and (b) calculated data using the structure obtained by single-crystal X-ray diffraction.

X-ray structure analysis (Table 1) revealed that the colorless crystals of C₁₆py-H₂Mo₈ contained conventional 𝛽-[Mo₈O₂₆]⁴⁻ (β-Mo₈) anions. Two C₁₆py cations and two ethanol molecules were identified per β-Mo₈ of 4− charge (one C₁₆py, 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 C₁₆py-H₂Mo₈ for balancing the charge, confirmed by the bond valence sum (BVS) calculation. All the Mo atoms of β-Mo₈ in C₁₆py-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 𝜇₅ bridging oxygen atoms (O4) of β-Mo₈ had relatively low BVS value (1.69) than 2, being similar to the literature [40]. The crystals of C₁₆py-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 C₁₆py-H₂Mo₈ as mentioned above. Thus, the composition of C₁₆py-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 C₁₆py-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]. C₁₆py-H₂Mo₈ possibly exhibits moderate proton conductivity in the intermediate temperature region over 373 K as observed in another surfactant-POM hybrid layered crystal [42].

Table 1 

Crystallographic data for C₁₆py-H₂Mo₈.

(a)

(b)

Figure 3 

Crystal structure of C16py-H2Mo8. (a) Asymmetric unit together with atoms generated by the symmetry operation (−𝑥,−𝑦,−𝑧) to complete half β-H2Mo8 anion (symmetry code: (i)  −𝑥,−𝑦,−𝑧). Displacement ellipsoids are drawn at the 30% probability level. (b) Packing diagram along b-axis (β-H2Mo8 in polyhedral representations).

The crystal packing of C₁₆py-H₂Mo₈ consisted of alternating inorganic β-H₂Mo₈ monolayers and organic C₁₆py bilayers (Figure 3(b)). The hexadecyl chains of C₁₆py interdigitated in the C₁₆py 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 C₁₆py and hexamolybdate [24].

However, the molecular arrangement in the inorganic layer was quite different from that for other C₁₆py-POM hybrid layered crystal [21–25]. The hydrophilic heads of C₁₆py usually penetrated into the POM inorganic layers, and the pyridine rings of C₁₆py interacted to form a π-π stacking pair or were located in the adjacent positions [21–25]. On the contrary in C₁₆py-H₂Mo₈, the hydrophilic heads of C₁₆py 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–H⋯O 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 C₁₆py bilayers were sandwiched by the β-H₂Mo₈ anionic layers (Figure 3(b)).

Table 2 

C--H⋯O hydrogen bonds in C₁₆py-H₂Mo₈.

(a)

(b)

Figure 4 

(a) Molecular arrangements in the inorganic layers of C16py-H2Mo8. The short contacts including C–H⋯O hydrogen bonds are represented in broken lines. (b) Molecular arrangements of C16py-H2Mo8 at the interface between the β-H2Mo8 inorganic and C16py organic layers. The hexadecyl groups and ethanol molecules are omitted for clarity.

C₁₆py-H₂Mo₈ had C–H⋯O hydrogen bonds [43] (Table 2). The C–H⋯O hydrogen bonds between β-H₂Mo₈ and C₁₆py were 3.45–3.99 Å in C⋯O distance. The mean value was 3.67 Å, similar to the mean C⋯O distances (~3.6 Å) in other C₁₆py-POM hybrid crystals [21, 22, 24]. These hydrogen bonds as well as electrostatic interaction between C₁₆py and β-H₂Mo₈ would stabilize the layered crystal structure of C₁₆py-H₂Mo₈ with rigid packing.

4. Conclusions

A hybrid layered crystal, [C₅H₅N(C₁₆H₃₃)]₂[β-H₂Mo₈O₂₆]·2C₂H₅OH (C₁₆py-H₂Mo₈), was successfully synthesized as single phase by using Mo-blue species as starting precursor material. C₁₆py-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 C₁₆py cation. The hydrophilic head of C₁₆py 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.

Acknowledgments

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.