Conductive Supramolecular Architecture Constructed from Polyoxovanadate Cluster and Heterocyclic Surfactant

Proton-conductive solid electrolytes are significant for fuel-cell battery technology. Especially for use in motor vehicles, proton conductors which work at intermediate temperatures (373–673 K) under an anhydrous atmosphere are desired to improve the fuel cell stability and efficiency. Inorganic–organic hybrid supramolecular architectures are a promising option for the realization of highly conductive proton conductors. Here, a hybrid layered crystal was synthesized for the first time by using an proton-containing decavanadate (V10) anion and a heterocyclic surfactant cation. A simple ion-exchange reaction led to the formation of an inorganic–organic hybrid of V10 by using dodecylpyridazinium (C12pda) as the heterocyclic surfactant. Single crystal X-ray analyses revealed that four C12pda cations were associated with one V10 anion, which was a diprotonated species forming a one-dimensional infinite chain structure through hydrogen bonds. Anhydrous proton conductivity was investigated by alternating current (AC) impedance spectroscopy in the range of 313–393 K, exhibiting a maximum value of 1.7 × 10−5 S cm−1 at 373 K.


Introduction
Supramolecular chemistry enables the production of a variety of self-organized architectures from the artificial to the biological [1], which include static and dynamic systems [2].In addition, supramolecular chemistry can produce inorganic-organic hybrid materials which exhibit characteristic functions derived from the synergy of inorganic and organic components [3][4][5].Recently, toward the application to fuel-cell batteries [6], crystalline inorganic-organic hybrids such as MOFs (metal-organic frameworks) or PCPs (porous coordination polymers) have been widely investigated as a possible substitution for the present polymer proton conductors [7][8][9][10][11][12].
Here we demonstrate the first syntheses and structural analyses of V 10 -heterocyclic surfactant hybrid crystals.Dodecylpyridazinium ([C 4 H 4 N 2 (C 12 H 25 )] + (C 12 pda), Figure 1) cation was employed as the heterocyclic surfactant.The C 12 pda cation has been rarely reported to form hybrid crystals with POMs [44].In the crystal structure, the V 10 anion formed a diprotonated species, and anhydrous proton conductivity was elucidated.
evaluated for the V10 hybrid crystals comprising decyltrimethylammonium ([(C10H21)N(CH3)3] + , C10) surfactant [47].The anhydrous proton conductivity could be enhanced by the delocalized πelectrons in the heterocyclic moiety of the hybridized surfactant.However, no V10 hybrid crystal comprising the surfactant with a heterocyclic moiety has been reported.
Here we demonstrate the first syntheses and structural analyses of V10-heterocyclic surfactant hybrid crystals.Dodecylpyridazinium ([C4H4N2(C12H25)] + (C12pda), Figure 1) cation was employed as the heterocyclic surfactant.The C12pda cation has been rarely reported to form hybrid crystals with POMs [44].In the crystal structure, the V10 anion formed a diprotonated species, and anhydrous proton conductivity was elucidated.
Conductivity measurements were carried out by alternating current (AC) impedance method in a frequency range from 20 to 1.0 × 10 7 Hz using a Wayne Kerr 6510P inductance-capacitance-resistance (LCR) meter.Pelletized powder samples (10 mm in diameter, 0.81 mm in thickness) were sandwiched with Pt electrodes, and the impedance was measured under a dry N2 atmosphere at 313-393 K.

Synthesis
C12pda-V10 hybrids were synthesized as follows: solid V2O5 (0.40 g, 2.2 mmol) was dispersed in 15 mL of water, and dissolved by adding LiOH•H2O (0.24 g, 5.7 mmol).The solution was adjusted to pH 6.0 or 4.0 with 6M HCl, and the resulting orange solution was added at room temperature to an ethanol solution (15 mL) of C12pda•Br (0.30 g, 0.91 mmol) with stirring for 60 min.Obtained dark green (pH 6.0) or yellow (pH 4.0) precipitates were filtered off, and washed with 10 mL of ethanol to obtain as-prepared product of C12pda-V10 (0.377 g (56% yield) for pH 6.0; 0.179 g (34% yield) for pH 4.0).
Conductivity measurements were carried out by alternating current (AC) impedance method in a frequency range from 20 to 1.0 × 10 7 Hz using a Wayne Kerr 6510P inductance-capacitance-resistance (LCR) meter.Pelletized powder samples (10 mm in diameter, 0.81 mm in thickness) were sandwiched with Pt electrodes, and the impedance was measured under a dry N 2 atmosphere at 313-393 K.

Synthesis
C 12 pda-V 10 hybrids were synthesized as follows: solid V 2 O 5 (0.40 g, 2.2 mmol) was dispersed in 15 mL of water, and dissolved by adding LiOH•H 2 O (0.24 g, 5.7 mmol).The solution was adjusted to pH 6.0 or 4.0 with 6M HCl, and the resulting orange solution was added at room temperature to an ethanol solution (15 mL) of C 12 pda•Br (0.30 g, 0.91 mmol) with stirring for 60 min.Obtained dark green (pH 6.0) or yellow (pH 4.0) precipitates were filtered off, and washed with 10 mL of ethanol to obtain as-prepared product of C 12 pda-V 10 (0.377 g (56% yield) for pH 6.0; 0.179 g (34% yield) for pH 4.0).

X-ray Crystallography
Single crystal X-ray diffraction data for the C 12 pda-V 10 crystals were recorded with an ADSC Q210 CCD area detector with a synchrotron radiation at the 2D beamline in Pohang Accelerator Laboratory (PAL).The diffraction images were processed by using HKL3000 [63], and absorption correction was also performed with HKL3000.The structure was solved by the direct method using SHELXT Version 2014/5 [64] and refined by the full-matrix least-squares method on F 2 using SHELXL Version 2014/7 [65].All calculations were performed using the CrystalStructure software package [66].All non-hydrogen atoms were refined anisotropically.The H atoms attached to the O atoms of V 10 were found in the difference Fourier synthesis and their positional and isotropic displacement parameters were refined.The hydrogen atoms of C 12 pda surfactants and ethanol molecule of crystallization 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 1813634).

Syntheses of C 12 pda-V 10 hybrids
C 12 pda-V 10 hybrids were obtained as insoluble precipitates from aqueous solution of solid V 2 O 5 , the pH of which was adjusted at 6.0 or 4.0.The C 12 pda-V 10 hybrids synthesized at pH 6.0 were dark green, while the C 12 pda-V 10 hybrids synthesized at pH 4.0 were yellow.In this pH range, the V 10 anions or their protonated species are the main species in the solution [67]. Figure 2a,b shows Infrared (IR) spectra of these dark green and yellow C 12 pda-V 10 hybrids, in which the characteristic peaks for the V 10 anion were observed in the range of 400-1000 cm −1 .This means that both C 12 pda-V 10 hybrids synthesized at pH 6.0 and 4.0 contained the V 10 anion, although the sample colors were different (dark green and yellow).The dark green color will be derived from the reduced V 10 species.On the other hand, the C 12 pda-V 10 hybrids synthesized at pH 6.0 and 4.0 exhibited different powder X-ray diffraction (XRD) patterns (Figure 3a,b), indicating that these hybrids had different bulk structures.
The yellow-colored C 12 pda-V 10 crystals were obtained from the synthetic filtrate at pH 6.0.The C 12 pda-V 10 crystals were identified to possess the same molecular and crystal structure as the yellow C 12 pda-V 10 hybrids synthesized at pH 4.0, which was revealed by the IR spectrum (Figure 2b,c) and powder XRD pattern (Figure 3b,c).The yellow crystalline hybrids were employed for the conductivity measurements (see below).The XRD pattern of the C 12 pda-V 10 crystals was different in peak positions from that calculated from the results of single crystal X-ray analyses (Figure 3d).This may be due to the difference in the measurement temperature (powder: ambient temperature, single crystal: 100 K) and the desolvation of solvent molecules of the crystals during the XRD measurements.
The crystal packing of C12pda-V10 was composed of alternating V10 inorganic monolayers and C12pda organic bilayers parallel to the bc plane with an interlayer distance of 25.5 Å (Figure 4a).The dodecyl chains of C12pda were interdigitated straightly with each other.The solvent molecules
The crystal packing of C12pda-V10 was composed of alternating V10 inorganic monolayers and C12pda organic bilayers parallel to the bc plane with an interlayer distance of 25.5 Å (Figure 4a).The dodecyl chains of C12pda were interdigitated straightly with each other.The solvent molecules     4).The crystal consisted of the V 10 anion, being consistent with the IR spectrum (Figure 2c).Four C 12 pda cations (1+ charge) were associated with one V 10 anion (6-charge) due to the charge compensation, suggesting the presence of two protons as counter cation in the C 12 pda-V 10 crystals.The protons were connected to the V 10 anion to form [H 2 V 10 O 28 ] 4− (H 2 V 10 ) as observed in the hybrid crystals of the V 10 anion and C 10 cation (C 10 -V 10 ) [47] (see below).
The crystal packing of C 12 pda-V 10 was composed of alternating V 10 inorganic monolayers and C 12 pda organic bilayers parallel to the bc plane with an interlayer distance of 25.5 Å (Figure 4a).The dodecyl chains of C 12 pda were interdigitated straightly with each other.The solvent molecules (water and ethanol) of crystallization were placed at the interface between the V 10 and C 12 pda layers (Figure 4a).
The V 10 anion in the C 12 pda-V 10 crystals was clearly identified as a diprotonated species by the X-ray structure analyses (Figure 4b).This result was confirmed by the bond valence sum (BVS) calculations [68] giving values of 1.37 (O15) and 1.24 (O24), while the BVS values were within the range 1.63-2.03for other oxygen atoms.Each diprotonated V 10 anion was related by the two-fold screw axis, and connected by the O-H•••O hydrogen bond to form a one-dimensional infinite chain structure (Figure 4c).The O•••O distances were 2.740(2) Å for O15-H66•••O17 i (symmetry code i: 1 − x, −0.5 + y, 0.5 − z) and 2.766(2) Å for O24-H67•••O9 i , respectively.The C 12 pda-V 10 crystals had a zigzag infinite chain structure of V 10 different from the chain structure observed in the C 10 -V 10 crystals [47], which would be caused by the different manner of the protonation.In the C 12 pda-V 10 crystals, one protonated oxygen site (O15) was different from the case of the C 10 -V 10 hybrid crystals, resulting in a different arrangement of the V 10 anions from that in the C 10 -V 10 crystals.These hydrogen-bonded V 10 chains were formed in the inorganic layers, and isolated by the pyridazinium moieties of the C 12 pda cations.The structural features of the C 12 pda cations were then investigated.Although most C-C bonds of the dodecyl chains of C 12 pda had an anti conformation, three C-C bonds (C22-C23, C40-C41, C56-C57) had a gauche conformation (Figure 5a), two of which (C40-C41, C56-C57) were located some methylene groups away from the hydrophilic head, which was similar to the C 12 pda conformation in the hybrid crystal comprising decatungstate ([W 10 O 32 ] 4− , W 10 ) anion (C 12 pda-W 10 ) [44].The hydrophilic heads of C 12 pda penetrated into the V 10 inorganic layers as mentioned above.The penetrated pyridazine rings of C 12 pda had short contacts between the heterocyclic moiety due to the C-H•••π interactions (Figure 5b), being different from the C 12 pda-W 10 hybrid crystal [44].The C 12 pda cation interacted with the V 10 anions by C-H•••O hydrogen bonds [69] with C•••O distances ranging from 2.82 to 3.92 Å (mean value: 3.33 Å), most of which were formed between the V 10 anion and pyridazine rings of the C 12 pda cations.The structural features of the C12pda cations were then investigated.Although most C-C bonds of the dodecyl chains of C12pda had an anti conformation, three C-C bonds (C22-C23, C40-C41, C56-C57) had a gauche conformation (Figure 5a), two of which (C40-C41, C56-C57) were located some methylene groups away from the hydrophilic head, which was similar to the C12pda conformation in the hybrid crystal comprising decatungstate ([W10O32] 4− , W10) anion (C12pda-W10) [44].The hydrophilic heads of C12pda penetrated into the V10 inorganic layers as mentioned above.The penetrated pyridazine rings of C12pda had short contacts between the heterocyclic moiety due to the C-H•••π interactions (Figure 5b), being different from the C12pda-W10 hybrid crystal [44].The C12pda cation interacted with the V10 anions by C-H•••O hydrogen bonds [69] with C···O distances ranging from 2.82 to 3.92 Å (mean value: 3.33 Å), most of which were formed between the V10 anion and pyridazine rings of the C12pda cations.

Anhydrous Proton Conductivity of C12pda-V10
The anhydrous proton conductivity was investigated for the yellow C12pda-V10 hybrids by alternating current (AC) impedance spectroscopy.Figure 6a shows a typical Nyquist spectrum, which was measured at 373 K under a dry N2 atmosphere.The spectrum showed a suppressed half circle in the high-and medium-frequency regions and a slightly inclined line in the low-frequency region.The Nyquist spectrum was fitted based on an equivalent circuit shown in Figure 6a (inset) [45−47].The red line represents simulated data with the equivalent circuit, which successfully reproduced the measured Nyquist spectrum.The estimated value of the bulk resistance, Rb, was 6.05 × 10 3 Ω at 373 K, from which the conductivity of the yellow C12pda-V10 hybrids was calculated to be 1.7 × 10 −5 S cm −1 .This anhydrous conductivity will be owing to the protons which were connected to the V10 anions.
Figure 6b shows the temperature dependence of the conductivity for the yellow C12pda-V10 hybrids at 313−393 K (40−120 °C).The conductivity at 313 K (40 °C) was 1.1 × 10 −8 S cm −1 , increased with the increasing temperature, and reached 1.7 × 10 −5 S cm −1 at 373 K (100 °C).The proton conductivity jumped by three orders of magnitude from that at 313 K to 373 K.However, the conductivity dropped to 9.8 × 10 −6 S cm −1 at 393 K (120 °C), plausibly due to the removal of water molecules of crystallization by the heating.
The activation energy of the proton conductivity was estimated from the Arrhenius plot as shown in Figure 6c.The value of the slope was obtained by the conductivities except for that at 393 K, where the conduction mechanism would have changed.An obtained value of the activation energy was 1.3 eV (125 kJ/mol), suggesting that the proton conduction mechanism in the yellow C12pda-V10 hybrids was more similar to the vehicle mechanism rather than the Grotthuss mechanism [11].However, the detailed mechanism is unclear and under investigation.

Anhydrous Proton Conductivity of C 12 pda-V 10
The anhydrous proton conductivity was investigated for the yellow C 12 pda-V 10 hybrids by alternating current (AC) impedance spectroscopy.Figure 6a shows a typical Nyquist spectrum, which was measured at 373 K under a dry N 2 atmosphere.The spectrum showed a suppressed half circle in the high-and medium-frequency regions and a slightly inclined line in the low-frequency region.The Nyquist spectrum was fitted based on an equivalent circuit shown in Figure 6a (inset) [45][46][47].The red line represents simulated data with the equivalent circuit, which successfully reproduced the measured Nyquist spectrum.The estimated value of the bulk resistance, R b , was 6.05 × 10 3 Ω at 373 K, from which the conductivity of the yellow C 12 pda-V 10 hybrids was calculated to be 1.7 × 10 −5 S cm −1 .This anhydrous conductivity will be owing to the protons which were connected to the V 10 anions.
Figure 6b shows the temperature dependence of the conductivity for the yellow C 12 pda-V 10 hybrids at 313−393 K (40−120 • C).The conductivity at 313 K (40 • C) was 1.1 × 10 −8 S cm −1 , increased with the increasing temperature, and reached 1.7 × 10 −5 S cm −1 at 373 K (100 • C).The proton conductivity jumped by three orders of magnitude from that at 313 K to 373 K.However, the conductivity dropped to 9.8 × 10 −6 S cm −1 at 393 K (120 • C), plausibly due to the removal of water molecules of crystallization by the heating.
The activation energy of the proton conductivity was estimated from the Arrhenius plot as shown in Figure 6c.The value of the slope was obtained by the conductivities except for that at 393 K, where the conduction mechanism would have changed.An obtained value of the activation energy was 1.3 eV (125 kJ/mol), suggesting that the proton conduction mechanism in the yellow C 12 pda-V 10 hybrids was more similar to the vehicle mechanism rather than the Grotthuss mechanism [11].However, the detailed mechanism is unclear and under investigation.

Discussion
Here, the successful crystallization was realized to obtain the single crystals of C12pda-V10.Surfactant-V10 hybrid crystals have often been obtained from the synthetic filtrates [36,47,48].The recrystallization with organic solvents was usually unsuccessful, leading to difficulty in the crystallization of hybrid crystals comprising hydrophobic heterocyclic surfactants.In the case reported here, the different pH values (6.0 and 4.0) were tried to obtain the C12pda-V10 hybrids, and suitable single crystals were obtained from the dark green C12pda-V10 hybrids obtained at pH 6.0.The dark green C12pda-V10 hybrids seemed to contain reduced V10 species (Figure 2a), since the dark green color was plausibly derived from the presence of reduced V atoms [16].The ultraviolet-visible (UV-vis) spectrum of the dark green C12pda-V10 hybrids (Figure 7a) suggests the presence of intervalence charge transfers between reduced and fully-oxidized V atoms (ex.V IV and V V ), while the yellow C12pda-V10 hybrids comprising the fully-oxidized V atoms (BVS values: 5.02-5.07)exhibited no distinct absorption (Figure 7b).However, the detailed oxidation states are unclear.The reduced V10 species in the filtrate were gradually oxidized to cause the slow crystallization of the C12pda-V10 crystals comprising oxidized and yellow-colored V10 species, which had the same molecular and bulk structures as the yellow C12pda-V10 hybrids.

Discussion
Here, the successful crystallization was realized to obtain the single crystals of C 12 pda-V 10 .Surfactant-V 10 hybrid crystals have often been obtained from the synthetic filtrates [36,47,48].The recrystallization with organic solvents was usually unsuccessful, leading to difficulty in the crystallization of hybrid crystals comprising hydrophobic heterocyclic surfactants.In the case reported here, the different pH values (6.0 and 4.0) were tried to obtain the C 12 pda-V 10 hybrids, and suitable single crystals were obtained from the dark green C 12 pda-V 10 hybrids obtained at pH 6.0.The dark green C 12 pda-V 10 hybrids seemed to contain reduced V 10 species (Figure 2a), since the dark green color was plausibly derived from the presence of reduced V atoms [16].The ultraviolet-visible (UV-vis) spectrum of the dark green C 12 pda-V 10 hybrids (Figure 7a) suggests the presence of intervalence charge transfers between reduced and fully-oxidized V atoms (ex.V IV and V V ), while the yellow C 12 pda-V 10 hybrids comprising the fully-oxidized V atoms (BVS values: 5.02-5.07)exhibited no distinct absorption (Figure 7b).However, the detailed oxidation states are unclear.The reduced V 10 species in the filtrate were gradually oxidized to cause the slow crystallization of the C 12 pda-V 10 crystals comprising oxidized and yellow-colored V 10 species, which had the same molecular and bulk structures as the yellow C 12 pda-V 10 hybrids.

Figure 2 . 12 Figure 2 .
Figure 2. IR spectra of C 12 pda-V 10 hybrids: (a) dark green C 12 pda-V 10 hybrids synthesized at pH 6.0; (b) yellow C 12 pda-V 10 hybrids synthesized at pH 4.0; (c) C 12 pda-V 10 crystals obtained from the filtrate of the synthesis at pH 6.0.

Figure 3 .
Figure 3. Powder X-ray diffraction patterns of C 12 pda-V 10 hybrids: (a) dark green C 12 pda-V 10 hybrids synthesized at pH 6.0; (b) yellow C 12 pda-V 10 hybrids synthesized at pH 4.0; (c) C 12 pda-V 10 crystals obtained from the filtrate of the synthesis at pH 6.0; (d) Calculated pattern of C 12 pda-V 10 crystals using the structure obtained by single-crystal X-ray diffraction.

3. 2 .
Crystal Structure of C 12 pda-V 10 The X-ray structure and elemental analyses revealed the formula of the C 12 pda-V 10 crystals to be [C 4 H 4 N 2 (C 12 H 25 )] 4 [H 2 V 10 O 28 ]•H 2 O•C 2 H 5 OH (

Figure 4 .
Figure 4. Crystal structure of C12pda-V10 (C: gray, N: blue, O: red, H: white); (a) packing diagram along b axis (upper) and c axis (lower).V10 anions in polyhedral representation.H atoms of C12pda and ethanol of crystallization are omitted for clarity.Some solvent molecules are highlighted; (b) molecular structure of diprotonated V10 anion.Another V10 anion is generated by the symmetry operation (1 − x, −0.5 + y, 0.5 − z).Symmetry code: (i) 1 − x, −0.5 + y, 0.5 − z; (c) molecular arrangements in the inorganic layers.V10 anions in polyhedral representation.The short contacts derived from O-H•••O hydrogen bonds are represented in red broken lines.The C12pda cations and solvents of crystallization are omitted for clarity.

Figure 4 .
Figure 4. Crystal structure of C 12 pda-V 10 (C: gray, N: blue, O: red, H: white); (a) packing diagram along b axis (upper) and c axis (lower).V 10 anions in polyhedral representation.H atoms of C 12 pda and ethanol of crystallization are omitted for clarity.Some solvent molecules are highlighted; (b) molecular structure of diprotonated V 10 anion.Another V 10 anion is generated by the symmetry operation (1 − x, −0.5 + y, 0.5 − z).Symmetry code: (i) 1 − x, −0.5 + y, 0.5 − z; (c) molecular arrangements in the inorganic layers.V 10 anions in polyhedral representation.The short contacts derived from O-H•••O hydrogen bonds are represented in red broken lines.The C 12 pda cations and solvents of crystallization are omitted for clarity.

Figure 5 .
Figure 5.View of crystallographically-independent surfactant molecules; (a) whole C 12 pda cations in the asymmetric unit together with V 10 anion; (b) pyridazinium moieties of the C 12 pda cations in the vicinity of the V 10 anions.The distances of short contacts are represented in Å unit.Symmetry code: (ii) 1 − x, 1 − y, 1 − z; (iii) x, 1.5 − y, 0.5 + z.

Table 1 .
Crystallographic data of C 12 pda-V 10 crystal.