Polyoxotungstate-surfactant Layered Crystal toward Conductive Inorganic-organic Hybrid

A polyoxotungstate-surfactant hybrid layered compound was synthesized as a single phase by using decatungstate ([W 10 O 32 ] 4− , W 10) and hexadecylpyridinium (C 16 py). The X-ray structure analysis combined with infrared spectroscopy and elemental analysis revealed the formula to be (C 16 py) 4 [W 10 O 32 ] (C 16 py-W 10). The layered structure consisted of alternative stacking of W 10 inorganic monolayers and interdigitated C 16 py bilayers with layered periodicity of 23.3 Å. Each W 10 anion in the W 10 inorganic monolayers was isolated by the hydrophilic heads of C 16 py. The hybrid crystals of C 16 py-W 10 decomposed at around 500 K. The conductivity of the hybrid layered crystal was estimated to be 4.8 × 10 −6 S cm −1 at 423 K by alternating current (AC) impedance spectroscopy.

Here we report the synthesis of a hybrid compound of C 16 py and W 10 (C 16 py-W 10 ).The crystal structure was successfully determined by the X-ray structure analysis.The conductive property was also investigated.

Crystal Structure of C 16 py-W 10
IR spectra of as-prepared precipitates, recrystallized samples, and crystals prepared with Na-W 10 showed the presence of C 16 py cations and W 10 anions.The single crystal X-ray structure analysis combined with the elemental analysis revealed the formula of C 16 py-W 10 to be [C 5 H 5 N(C 16 H 33 )] 4 [W 10 O 32 ] (Table 1).Four C 16 py cations (1+ charge) were associated with one W 10 anion (4− charge), and C 16 py-W 10 did not contain any H + .Figure 1 shows the crystal structure of C 16 py-W 10 .The crystal packing consisted of alternating W 10 inorganic layers and C 16 py organic layers with periodicity of 23.3 Å (Figure 1).The W 10 anions formed monolayers, while the hexadecyl chains of C 16 py interdigitated to form a bilayer structure, which is a typical structure for most POM-surfactant hybrid crystals [26][27][28][29][30][31][32].All C-C bonds in the hexadecyl chains showed anti conformation except one C-C bond (C7-C8  The hydrophilic heads of C 16 py penetrated into the W 10 inorganic monolayers and isolated each W 10 anion (Figure 2) in a similar way to that in the crystal of C 16 py-hexamolybdate (C 16 py-Mo 6 ) [31] or C 16 py-α-octamolybdate (C 16 py-α-Mo 8 ) [32].On the other hand, two independent C 16 py cations were not parallel (angle: 52.6°) without π-π stacking, different from C 16 py-Mo 6 [31] and C 16 py-α-Mo 8 [32].The C-H•••π(centroid) distance was 3.32 Ǻ, and the nearest C-H bond (C24-H24) was not directed to the center of the pyridine ring.The shortest interatomic distance (C3•••H24, 2.88 Ǻ) between the pyridine rings was almost the same as the sum of the van der Waals radii (2.90 Ǻ).Therefore, C-H• • • π interaction [40,41] was hardly observed in the present C 16 py-W 10 .C 16 py-W 10 had two-dimensionally confined monolayers of the W 10 anions (Figure 2).The distance between the nearest W 10 anions was 3.23 Å, much shorter than that for W 10 crystals composed of n-butyl- [42,43] or n-propylammonium [44] (5.4-5.8Å).The close distance between W 10 would contribute to the emergence of conductivity for C 16 py-W 10 .
C 16 py-W 10 had C-H•••O hydrogen bonds [40] at the interface between the W 10 and C 16 py layers.The C•••O distances were 3.28-3.88Å (Table 2).The mean value was 3.51 Å, and was shorter than the mean C•••O distances (~3.6 Å) in other POM hybrid crystals containing the C 16 py cation [31,32].These shorter hydrogen bonds as well as electrostatic interactions between C 16 py and W 10 would stabilize the layered crystal structure of C 16 py-W 10 with rigid packing.Most hydrogen bonds were formed between oxygen atoms of W 10 and the hydrophilic head of C 16 py (i.e., pyridine rings or methylene groups near nitrogen).

Powder X-Ray Diffraction (XRD) Patterns of C 16 py-W 10
Powder XRD patterns of C 16 py-W 10 were measured at room temperature (Figure 3).The XRD pattern of as-prepared C 16 py-W 10 exhibited weak and broad peaks (Figure 3a).The XRD pattern of recrystallized C 16 py-W 10 showed much sharper and stronger peaks (Figure 3b), while the peak positions were close to those of the as-prepared C 16 py-W 10 (Figure 3a).This demonstrates that the structure of as-prepared C 16 py-W 10 is not changed by the recrystallization from hot acetonitrile.The pattern of the recrystallized C 16 py-W 10 (Figure 3b, a = 10.7588,b = 11.5068,c = 24.7480Å, α = 99.914,β = 93.577,γ = 116.664°,V = 2662.9Å 3 [45]) was almost the same as that calculated with the single crystal X-ray analysis data (Figure 3c), indicating that the recrystallized C 16 py-W 10 is a single phase.

Conductivity of C 16 py-W 10
Figure 4 shows an impedance spectrum for the recrystallized C 16 py-W 10 at 423 K.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 consisted of two partially overlapped semicircles due to bulk and grain boundary resistances.The linear part in the low-frequency region would result from a combination of charge transfer resistance and Warburg impedance related to the diffusion of the carrier.The equivalent circuit [46][47][48] is shown in Figure 4: R b and C b are the resistance and capacitance of the bulk, respectively.R gb and C gb are the resistance and capacitance, respectively, of the grain boundary.R ct and C dl are the charge transfer resistance and double layer capacitance, respectively.Z W is the Warburg impedance.The red line in Figure 4 represents fitted data with the equivalent circuit described above (Figure 4, inset), which successfully reproduces the measured impedance spectrum.
The value of R b obtained by the fitting was 2.25 × 10 4 Ω, from which the conductivity of the bulk C 16 py-W 10 was estimated to be 4.8 × 10 −6 S cm −1 considering uncertainty.The estimated value of C b was 1.01 × 10 −6 F, resulting in the time constant for the process in the bulk (R b × C b ) of 2.27 × 10 −2 s.This short time constant suggests that the bulk process occurs by electronic conduction [47], in good agreement with the fact that C 16 py-W 10 contains no easily moving ion such as H + .Figure 5 shows the thermogravimetric (TG) curve and IR spectra of recrystallized C 16 py-W 10 .No weight loss was observed below 523 K.The weight of C 16 py-W 10 decreased by 36% from 523 K to 723 K (Figure 5a), which was attributed to the decomposition and removal of the C 16 py cations.The crystallinity of C 16 py-W 10 revealed by powder XRD decreased after the impedance spectroscopy measurements (not shown).However, IR spectra before and after the measurements at 423 K (Figure 5b) exhibited characteristic peaks for the W 10 anion in the range of 400-1000 cm −1 [49], demonstrating that the molecular structure of W 10 was retained after heating at 423 K.These results indicate that C 16 py-W 10 is thermally stable below 423 K.The conductivity of C 16 py-W 10 was much lower than the radical salts of POM containing organic donor such as bis(ethylenedithio)tetrathiafulvalene [2,3].These radical salts have conductive layers of organic donor, which possibly leads to three-dimensional conduction.On the other hand, the conductivity of C 16 py-W 10 is considered to be two-dimensional along the inorganic layers composed of W 10 and pyridinium hydrophilic heads (ab plane in the crystal).The anisotropy of the conductivity was difficult to investigate because large single crystals were not obtained.The conductivity of C 16 py-W 10 reported here was measured for pelletized ground powder, and is considered to be averaged and overall conductivity.Although the conductivity of C 16 py-W 10 was not so high, these results suggest that appropriate combination of POMs as electron reservoirs and surfactants with π-electrons would pave the way to another class of hybrid conductors.

Syntheses and Methods
All chemical reagents were obtained from commercial sources.C 16 py-W 10 was synthesized according to a modified procedure of the preparation of tetrabutylammonium salt of W 10 [49] IR spectra (as KBr pellet) were recorded on Jasco FT-IR 5000 and Horiba FT-710 spectrometers.Thermogravimetric and differential thermal analyses (TG-DTA) were performed on an ULVAC MTS9000 + TGD9600 system.Conductivity measurements were carried out by the alternating current (AC) impedance method in a frequency range from 5 Hz to 13 MHz using an Agilent 4192A inductance-capacitance-resistance (LCR) meter.Pelletized powder samples of recrystallized C 16 py-W 10 (10 mm in diameter, 0.854 mm in thickness) were sandwiched with Pt electrodes, and the impedance was measured under a dry Ar atmosphere at 423 K. Bulk resistances and conductivities of C 16 py-W 10 were estimated by the fitting of typical Nyquist plots.

X-ray Diffraction Measurements
Single crystal X-ray diffraction measurements for C 16 py-W 10 were made on a Rigaku RAXIS RAPID imaging plate diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71075 Å).Diffraction data were collected for a platelet crystal (0.30 × 0.30 × 0.02 mm) and processed with PROCESS-AUTO [50].The structure was solved by heavy-atom Patterson methods [51] and expanded using Fourier techniques [52].The refinement procedure was performed by the full-matrix least-squares using SHELXL97 [53].All calculations were performed using the CrystalStructure [54] software package.Numerical absorption correction was performed for the observed data.In the refinement procedure, all non-hydrogen atoms were refined anisotropically, and the hydrogen atoms on C atoms were located in calculated positions.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-865932).
Powder X-ray diffraction (XRD) patterns for C 16 py-W 10 were measured with a XRD-DSCII (Rigaku Corporation) diffractometer by using Cu Kα radiation (λ = 1.54056Å, 50 kV-300 mA) at ambient temperature.A powder C 16 py-W 10 sample was sieved in a 200 mesh sieve to remove large particles and to avoid preferred orientation.Diffraction data were collected in the range of 2θ = 2-30° at 0.01° point and 5 s/step.The lattice parameters were calculated using Materials Studio Softwares (Accelrys Inc.) by the peak profile fitting using the Pawley refinement [55].

Conclusions
Decatungstate-hexadecylpyridinium hybrid layered crystal, [C 5 H 5 N(C 16 H 33 )] 4 [W 10 O 32 ] (C 16 py-W 10 ), was successfully synthesized by a simple cation-exchange reaction.C 16 py-W 10 was obtained as a single phase, and the crystal structure was determined by single crystal X-ray diffraction.C 16 py-W 10 contained the stacking of W 10 monolayers and C 16 py interdigitated bilayers.The alternating current (AC) impedance spectroscopy measurements revealed the conductivity of C 16 py-W 10 to be 4.8 × 10 −6 S cm −1 at 423 K.Although the conductivity was considerably lower than the radical salts of POM [2,3] or other layered materials [5,6], C 16 py-W 10 shows the potential of polyoxometalate-surfactant hybrid crystals as conductive materials.

Figure 1 .
Figure 1.Crystal structure of C 16 py-W 10 .(a) Packing diagram along b axis (W 10 in polyhedral representations); (b) Asymmetric unit together with atoms generated by the symmetry operation (−x, −y, −z,) to complete W 10 anion.Displacement ellipsoids are drawn at the 30% probability level, and H atoms are omitted for clarity.

Figure 2 .
Figure 2. Molecular arrangements in the inorganic layers of C 16 py-W 10 .The hexadecyl groups are omitted for clarity.

Figure 3 .
Figure 3. Powder X-ray diffraction patterns of (a) as-prepared C 16 py-W 10 and (b) recrystallized C 16 py-W 10 , and that (c) calculated with single crystal data.

Figure 5 .
Figure 5. (a) TG curve of recrystallized C 16 py-W 10 ; (b) IR spectra of recrystallized C 16 py-W 10 before and after the impedance spectroscopy measurements at 423 K.

Table 1 .
).While both C 16 py-W 10 and C 16 pyCl• H 2 O [39] contained interdigitated bilayers of C 16 py with the pyridine ring inserted into the hydrophilic layers, the packing of hydrophilic layers was different; the W 10 monolayers for C 16 py-W 10 and the Cl --H 2 O bilayers for C 16 pyCl• H 2 O. Crystallographic data for C 16 py-W 10 . iii ii