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

A hybrid layered crystal containing polyoxomolybdate was successfully synthesized from reduced Mo-blue species as starting material. The hybrid crystal, [C 5 H 5 N(C 16 H 33 )] 2 [β-H 2 Mo 8 O 26 ]⋅2C 2 H 5 OH (C 16 py-H 2 Mo 8 ), was obtained as a single phase by the gradual oxidation of hexadecylpyridinium-Mo-blue (C 16 py-Mo-blue) hybrid. The X-ray structure analysis revealed that C 16 pyH 2 Mo 8 comprised β-type octamolybdate anion with two protons (β-H 2 Mo 8 ). The β-H 2 Mo 8 anions and ethanol molecules of crystallization formed two-dimensional anionic layers. The pyridine rings of C 16 py did not attend to form the two-dimensional inorganic layers, and the interdigitated C 16 py bilayers were sandwiched by the β-H 2 Mo 8 anionic layers with periodicity of 18.2 Å. These C 16 py-H 2 Mo 8 hybrid layered crystals possibly work as a new class of proton conductor.


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][2][3][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][6][7].
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][25][26][27]. Another strategy uses precursor surfactant-POM hybrid crystals as synthetic starting material [21][22][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][22][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 Moblues 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 (C 16 py) cation formed hybrid materials (C 16 py-Mo-blue).The gradual oxidation of the C 16 py-Mo-blue resulted in the formation of C
C 16 py-H 2 Mo 8 was synthesized by using gradual oxidation of C 16 py-Mo-blue hybrids.0.10 g of C 16 py-Mo-blue was dispersed as starting material in hot ethanol/N,Ndimethylformamide (15 mL, 2 : 1 (v/v)), and the supernatant was kept at 293 and 288 K to give colorless needles of C 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 K radiation ( = 1.54056Å) at ambient temperature.
Single crystal X-ray diffraction measurements for C 16 py-H 2 Mo 8 were made on a Rigaku VariMax with RAPID imaging plate area detector using CuK radiation (crystalto-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 C 16 py cation were refined isotropically, and H atoms of ethanol C atoms were located in calculated positions.H atoms of H 2 Mo 8 and hydroxyl group in the ethanol were not included in the refinement procedure.CCDC-922099.

Results and Discussion
Polyoxomolybdate is well known to form reduced Mo-blue species [14][15][16].The dark-blue precipitates were isolated from reduced polyoxomolybdate solution by adding C 16 py cation.The IR spectrum of the precipitates (Figure 1(a)) revealed the presence of C 16 py 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 C 16 py-Mo-blue hybrids.The Mo-blue species are gradually oxidized to form conventional polyoxomolybdate anion, which means that C 16 py-Mo-blue works as starting material for the C 16 py-POM hybrid crystals.Here colorless crystals of C 16 py-H 2 Mo 8 were reproducibly synthesized by using the C 16 py-Mo-blue hybrid during the recrystallization process.The IR spectra of C 16 py-H 2 Mo 8 exhibited similar pattern in the 400-1200 cm −1 range to that of -octamolybdate (-[Mo 8 O 26 ] 4− , -Mo 8 ) anion (Figure 1(b)).The molecular structure was confirmed by Xray structure analysis as described below.However, single broad band in the 600-800 cm −1 range for the usual -Mo 8 species [38,39] was split into two bands for C 16 py-H 2 Mo 8 , suggesting the presence of attaching protons to -Mo 8 anion (see below).
Powder X-ray diffraction pattern of C 16 py-H 2 Mo 8 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 C 16 py-H 2 Mo 8 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:    than 2, being similar to the literature [40].The crystals of C 16 py-H 2 Mo 8 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 C 16 py-H 2 Mo 8 as mentioned above.Thus, the composition of C 16 py-H 2 Mo 8 was revealed to be . 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 C 16 py-H 2 Mo 8 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].C 16 py-H 2 Mo 8 possibly exhibits moderate proton conductivity in the intermediate temperature region over 373 K as observed in another surfactant-POM hybrid layered crystal [42].Symmetry codes: i −x, −, −; ii −1 + , , ; iii −x, 1 −, −; iv 1 − , −0.5 + , −0.5 − ; v , 0.5 − , −0.5 + .
However, the molecular arrangement in the inorganic layer was quite different from that for other C 16 py-POM hybrid layered crystal [21][22][23][24][25].The hydrophilic heads of C 16 py usually penetrated into the POM inorganic layers, and the pyridine rings of C 16 py interacted to form a - stacking pair or were located in the adjacent positions [21][22][23][24][25]. On the contrary in C 16 py-H 2 Mo 8 , the hydrophilic heads of C 16 py were completely excluded from the -H 2 Mo 8 inorganic layers (Figure 4(a)).-H 2 Mo 8 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 These hydrogen bonds as well as electrostatic interaction between C 16 py and -H 2 Mo 8 would stabilize the layered crystal structure of C 16 py-H 2 Mo 8 with rigid packing.

Figure 2 :
Figure 2: Powder X-ray diffraction patterns of C 16 py-H 2 Mo 8 from (a) observed data measured at ambient temperature and (b) calculated data using the structure obtained by single-crystal X-ray diffraction.

Figure 4 :
Figure 4: (a) Molecular arrangements in the inorganic layers of C 16 py-H 2 Mo 8 .The short contacts including C-H⋅ ⋅ ⋅ O hydrogen bonds are represented in broken lines.(b) Molecular arrangements of C 16 py-H 2 Mo 8 at the interface between the -H 2 Mo 8 inorganic and C 16 py organic layers.The hexadecyl groups and ethanol molecules are omitted for clarity.
A hybrid layered crystal, [C 5 H 5 N(C 16 H 33 )] 2 [-H 2 Mo 8 O 26 ]⋅ 2C 2 H 5 OH (C 16 py-H 2 Mo 8 ), was successfully synthesized as single phase by using Mo-blue species as starting precursor material.C 16 py-H 2 Mo 8 consisted of -octamolybdate anion with two protons (-H 2 Mo 8 ), being the first example of proton-comprising surfactant-polyoxomolybdate hybrid crystals.The -H 2 Mo 8 anions and ethanol molecules of crystallization formed two-dimensional anionic layer, which sandwiched interdigitated bilayers of C 16 py cation.The hydrophilic head of C 16 py did not attend to form the inorganic -H 2 Mo 8 layers.Such protonated surfactantpolyoxomolybdate hybrid crystals would be promising for a new class of proton-conducting materials.