Decisive Interactions between the Heterocyclic Moiety and the Cluster Observed in Polyoxometalate-Surfactant  Hybrid Crystals

Saki Otobe; Natsumi Fujioka; Takuro Hirano; Eri Ishikawa; Haruo Naruke; Katsuhiko Fujio; Takeru Ito

doi:10.3390/ijms16048505

,

and

¹

Department of Chemistry, School of Science, Tokai University, 4-1-1 Kitakaname, Hiratsuka 259-1292, Japan

²

Department of Applied Chemistry, College of Engineering, Chubu University, 1200 Matsumoto, Kasugai, Aichi 487-8501, Japan

³

Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-23, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan

^*

Author to whom correspondence should be addressed.

Int. J. Mol. Sci.2015, 16(4), 8505-8516;https://doi.org/10.3390/ijms16048505

This article belongs to the Special Issue Chemical Bond and Bonding 2015

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Abstract

Inorganic-organic hybrid crystals were successfully obtained as single crystals by using polyoxotungstate anion and cationic dodecylpyridazinium (C₁₂pda) and dodecylpyridinium (C₁₂py) surfactants. The decatungstate (W₁₀) anion was used as the inorganic component, and the crystal structures were compared. In the crystal comprising C₁₂pda (C₁₂pda-W₁₀), the heterocyclic moiety directly interacted with W₁₀, which contributed to a build-up of the crystal structure. On the other hand, the crystal consisting of C₁₂py (C₁₂py-W₁₀) had similar crystal packing and molecular arrangement to those in the W₁₀ crystal hybridized with other pyridinium surfactants. These results indicate the significance of the heterocyclic moiety of the surfactant to construct hybrid crystals with polyoxometalate anions.

Keywords:

inorganic-organic; polyoxometalate; surfactant; heterocyclic; hybrid crystal

1. Introduction

Weak chemical interactions, such as hydrogen bonding or van der Waals interactions, are crucial for the construction and function of biological molecules, such as proteins, DNA or RNA []. Employing these weak chemical interactions also provides effective options for building up synthetic molecular architectures [,,,]. To build up such molecular architectures, organic molecules or ligands are often used due to synthetic flexibility to control the directions and intensity of the weak chemical interactions. However, all-organic molecular architectures are less stable in the intermediate temperature (>100 °C) regions.

Inorganic-organic hybrid materials are more structurally stable than purely organic compounds owing to inorganic components, and the synergy of inorganic and organic characteristics will benefit constructing functional materials []. Conductive hybrid compounds composed of organic cations and inorganic anions have been reported, where the emergence of conductive functions is prompted by precise control of the molecular structures and arrangements of the components []. The precisely controlled inorganic-organic materials have been obtained as crystalline materials.

As a molecular inorganic component, polyoxometalates (POMs) are promising candidates with respect to their structural and functional controllability [,,,,,,,,]. POMs with various physicochemical properties have been successfully hybridized by structure-directing surfactants [,,] to construct inorganic-organic crystalline hybrids [,,,,] and single crystals [,,,,,,,,,]. Among several POM-surfactant single crystals, utilizing surfactants with a heterocyclic moiety enables the precise control of the composition and structure [,,]. However, the variation of the surfactants has been limited to pyridinium and imidazolium cations.

Here, we report the syntheses and structures of polyoxotungstate hybrid crystals containing heterocyclic surfactants, including the first example of a POM-surfactant crystal comprised of a pyridazinium surfactant. The decatungstate ([W₁₀O₃₂]⁴⁻, W₁₀) anion was hybridized with dodecylpyridazinium ([C₄H₄N₂(C₁₂H₂₅)]⁺, C₁₂pda) and dodecylpyridinium ([C₅H₅N(C₁₂H₂₅)]⁺, C₁₂py) to form the crystals of C₁₂pda-W₁₀ (1) and C₁₂py-W₁₀ (2), respectively, and their chemical interactions and molecular arrangements were compared by X-ray structure analyses.

2. Results and Discussion

2.1. Crystal Structure of C₁₂pda-W₁₀ (1)

C₁₂pda-W₁₀ (1) was obtained by the cation exchange reaction of sodium salt of the W₁₀ anion (Na-W₁₀). The retention of the W₁₀ structure before and after the recrystallization was confirmed by infrared (IR) spectra, which exhibited the characteristic peaks in the range of 400–1000 cm⁻¹ (Figure 1a–c). Suitable single crystals for X-ray crystallography were obtained by employing acetone as the crystallization solvent.

The X-ray structure and elemental analyses revealed the formula of 1 to be [C₄H₄N₂(C₁₂H₂₅)]₄[W₁₀O₃₂]·2(CH₃)₂CO (Table 1). Four C₁₂pda cations (1+ charge) were associated with one W₁₀ anion (4− charge) due to the charge compensation. Figure 2 shows the crystal structure of 1. The crystal packing consisted of alternating W₁₀ inorganic monolayers and C₁₂pda organic bilayers with a periodicity of 24.5 Å (Figure 2a). The acetone molecules were placed at the interface between the W₁₀ and C₁₂pda layers, being excluded from the inorganic layers. Although most C-C bonds of the dodecyl chains of C₁₂pda had the anti conformation, three C-C bonds (C8-C9, C41-C42, C54-C55) had the gauche conformation (Figure 2b), two of which (C8-C9, C41-C42) were located at some methylene groups far away from the hydrophilic head.

Figure 1. IR spectra of W₁₀ compounds. (a) Na-W₁₀ as a starting material; (b) As-prepared sample of 1; (c) 1 after recrystallization; (d) As-prepared sample of 2.

Table 1. Crystallographic data.

**Table 1.** Crystallographic data.
Compound	1	2
Chemical formula	C₇₀H₁₂₈N₈W₁₀O₃₄	C₇₆H₁₄₀N₄W₁₀O₃₆
Formula weight	3464.31	3524.45
Crystal system	triclinic	triclinic
Space group	P $\bar{1}$ (No.2)	P $\bar{1}$ (No.2)
a (Å)	10.55918(19)	10.813(7)
b (Å)	18.7700(3)	11.339(7)
c (Å)	25.4318(5)	23.610(13)
α (°)	74.4842(7)	99.415(9)
β (°)	86.5737(7)	91.558(5)
γ (°)	85.6363(7)	115.588(9)
V (Å³)	4838.62(15)	2560(3)
Z	2	1
ρ_calcd (g·cm⁻³)	2.378	2.286
T (K)	193	173
μ (Mo·Kα) (mm⁻¹)	11.924	11.272
No. of reflections measured	78,035	18,275
No. of independent reflections	22,146	11,774
R_int	0.0900	0.1384
No. of parameters	1106	563
R₁ (I > 2σ(I))	0.0452	0.0983
wR₂ (all data)	0.1162	0.3237

Figure 2. Crystal structure of 1 (C: gray, N: blue; W₁₀ anions in polyhedral representations. H atoms are omitted for clarity). (a) Packing diagram along the a axis. Some acetone molecules are highlighted; (b) View of crystallographically-independent surfactant molecules.

The hydrophilic heads of C₁₂pda penetrated into the W₁₀ inorganic layers and isolated each W₁₀ anion (Figure 3a) in a similar way to that in the crystal of hexadecylpyridinium ([C₅H₅N(C₁₆H₃₃)]⁺, C₁₆py) and W₁₀ (C₁₆py-W₁₀, 3) []. However, the conformations of the heterocyclic moiety were different. In 1, the pyridazine rings of the C₁₂pda cations were not in the vicinity of each other (Figure 3b), as in the case of the POM crystal comprising the pyridazinium cation without a long alkyl chain []. This indicates that there were no interactions, such as π–π stacking or the C-H···π interaction, between the heterocyclic moiety, being different from 2 (see below) and 3. On the other hand, the pyridazine rings of C₁₂pda interacted rather more directly with the W₁₀ anions. The crystals of 1 had several short contacts between of O atoms of W₁₀ and C or N atoms of the pyridazine ring (2.88–3.22 Å (mean: 3.09 Å); Table 2), indicating the direct interactions between W₁₀ and the heterocyclic moiety of C₁₂pda. The alignment of two crystallographically-independent W₁₀ anions was not parallel, and the angle between the molecular C₄ axes was 8.3° (Figure 3b).

Figure 3. Molecular arrangements in the inorganic layers of 1 (C: gray, N: blue; W₁₀ anions in polyhedral representations. H atoms and the dodecyl groups are omitted for clarity). (a) Packing diagram along the c axis; (b) View of crystallographically-independent pyridazinium moieties of surfactants in the vicinity of the W₁₀ anions.

Table 2. Short contacts between W₁₀ and the heterocyclic moiety of C₁₂pda in 1.

**Table 2.** Short contacts between W₁₀ and the heterocyclic moiety of C₁₂pda in 1.
Contact ^a	Distance (Å)	Contact ^a	Distance (Å)
C20ⁱ···O1	3.206	C2···O13	3.118
C20ⁱ···O2	3.085	C51^iv···O13	3.201
C4···O5	3.210	C33ⁱ···O20	3.177
C19ⁱ···O5	3.147	C49···O22	2.914
C33ⁱⁱ···O6	3.140	C4···O24	2.886
C34ⁱⁱ···O6	3.180	C17···O25	2.894
C50ⁱⁱⁱ···O7	2.876	C3ⁱⁱ···O27	3.216
C51ⁱⁱⁱ···O7	3.110	C19ⁱⁱ···O27	3.163
C34ⁱⁱⁱ···O8	3.134	C33ⁱ···O27	3.142
C4···O9	3.183	C17ⁱ···O28	3.135
C3···O9	2.890	C20···O29	3.119
N8ⁱⁱⁱ···O12	3.046	N6···O30	2.909

^a Contact between O atoms of W₁₀ and C or N atoms of the pyridazine ring of C₁₂pda. Symmetry codes: (i) 1 + x, y, z; (ii) −x, −y, 2 − z; (iii) 1+x, −1 + y, z; (iv) x, −1 + y, z.

In the crystal of 1, The C₁₂pda cations had weak C-H···O hydrogen bonds []. Some C-H···O bonds were present in the vicinity of the gauche C-C bonds. The C···O distances were in the range of 2.89–3.74 Å (mean value: 3.32 Å), being much shorter than those of 2 (see below). In addition, most C-H···O hydrogen bonds were formed between W₁₀ and the hydrophilic head of C₁₂pda, i.e., the pyridazine ring of C₁₂pda. This suggests stronger interactions between W₁₀ and the heterocyclic moiety of the surfactant in 1 than in 2. These hydrogen bonds, as well as the electrostatic interaction between W₁₀ and the heterocyclic moiety of C₁₂pda stabilized the layered crystal structure of C₁₂pda-W₁₀ with rigid packing.

2.2. Crystal Structure of C₁₂py-W₁₀ (2)

C₁₂py-W₁₀ (2) was also synthesized by the cation exchange reaction of Na-W₁₀ (Figure 1d). The molecular structure of W₁₀ was retained before and after the recrystallization, as in the case of C₁₆py-W₁₀ (3) []. Although the recrystallization of 2 was difficult, some crystals obtained from ethanol were able to be analyzed by X-ray crystallography.

The formula of 2 was revealed to be [C₅H₅N(C₁₂H₂₅)]₄[W₁₀O₃₂]·4C₂H₅OH (Table 1). Four C₁₂py cations (1+ charge) were associated with one W₁₀ anion (4− charge), being similar to 1 and 3. The crystal packing of 2 consisted of alternating W₁₀ inorganic monolayers and C₁₂py organic bilayers with a distance of 23.2 Å (Figure 4a). Surprisingly, the cell parameters and the layered distances of 2 and 3 [] were quite similar, even if the length of the pyridinium surfactants were changed to C₁₂py (2) from C₁₆py (3). The vacant spaces produced by changing to the shorter surfactant for 2 were filled by the ethanol molecules located at the interface between the W₁₀ and C₁₂py layers. This similarity of the layered structures led to the similar molecular arrangements of W₁₀ and the pyridine rings (see below). All C-C bonds in the dodecyl chains showed the anti conformation except one C-C bond (C7-C8) near the hydrophilic head of C₁₂py, being similar to that in 3 (Figure 4b).

Figure 4. Crystal structure of 2 (C: gray, N: blue; W₁₀ anions in polyhedral representations. H atoms are omitted for clarity). (a) Packing diagram along the b axis. Some ethanol molecules are highlighted; (b) View of crystallographically-independent surfactant molecules in 2.

The hydrophilic heads of C₁₂py penetrated into the W₁₀ inorganic layers in 2 (Figure 5a), which was almost the same manner as observed in 3. The alignment of each W₁₀ anion was parallel, being similar to 3, but different from 1. The penetrated C₁₂py cations formed two crystallographically-independent pairs (Figure 5b). The two C₁₂py cations interacted weakly through the C-H···π interaction, where the shortest interatomic distance was 2.91 Ǻ for C3···H20 (Figure 5b). In the crystal of 2, there were short contacts between W₁₀ and the pyridine ring (2.93–3.22 Å (mean: 3.07 Å), Table 3). 2 also had C-H···O hydrogen bonds at the interface between the W₁₀ and C₁₂py layers (C···O distance: 3.03–3.88 Å; mean value: 3.52 Å), being much longer than those observed in 1. These longer C-H···O hydrogen bonds were formed around W₁₀ and the pyridine ring of C₁₂py, indicating that the interactions between W₁₀ and the heterocyclic moiety in 2 were weaker than in 1. The structure of the heterocyclic moiety in the surfactants is the significant factor to construct the POM-surfactant hybrid crystals.

Figure 5. Molecular arrangements in the inorganic layers of 2 (C: gray, N: blue, H: white; W₁₀ anions in polyhedral representations. The dodecyl groups are omitted for clarity). (a) Packing diagram along the c axis. H atoms are omitted for clarity; (b) View of crystallographically-independent pyridinium moieties of surfactants in the vicinity of the W₁₀ anions. The selected short contact is presented as a pink dotted line.

Table 3. Short contacts between W₁₀ and the heterocyclic moiety of C₁₂py in 2.

**Table 3.** Short contacts between W₁₀ and the heterocyclic moiety of C₁₂py in 2.
Contact ^a	Distance (Å)	Contact ^a	Distance (Å)
C22···O5	3.153	C20ⁱⁱ···O11	3.026
N2ⁱ···O9	2.929	N1ⁱⁱⁱ···O14	2.960
C18ⁱ···O9	2.980	C5ⁱⁱⁱ···O14	3.203
C22ⁱ···O9	3.108	C21^iv···O14	3.091
C2···O11	3.220	C18^v···O16	3.031

^a Contact between O atoms of W₁₀ and C or N atoms of the pyridine ring of C₁₂py. Symmetry codes: (i) −1 + x, y, z; (ii) 1 − x, 1 − y, −z; (iii) x, −1 + y, z; (iv) 1 − x, −y, −z; (v) −1 + x, −1 + y, z.

3. Experimental Section

3.1. Syntheses and Methods

All chemical reagents were obtained from commercial sources (Wako, Osaka, Japan). [C₄H₄N₂(C₁₂H₂₅)]Br (C₁₂pda·Br) was synthesized by using pyridazine and 1-bromododecane based on the literature []. Na₄[W₁₀O₃₂] (Na-W₁₀) was synthesized by combining a boiled solution of Na₂WO₄·2H₂O (0.5 M, 25 mL) and boiled HCl (1 M, 25 mL) [].

C₁₂pda-W₁₀ (1) was synthesized by the cation exchange of Na-W₁₀. Na-W₁₀ (0.50 g, 0.20 mmol) was dissolved in 20 mL of HCl (pH = 2), and ethanol solution containing 0.27 g (0.81 mmol) of C₁₂pda·Br was added. After stirring for 10 min, the obtained white precipitates were filtered and dried. Recrystallization of the crude product from hot acetone gave colorless plates of 1. The crystals of 1 were efflorescent, and its elemental composition was calculated for the formula without the solvent of crystallization. Data for 1: anal. calcd. for C₆₄H₁₁₆N₈W₁₀O₃₂: C, 22.96; H, 3.49; N, 3.35%; found: C, 22.77; H, 3.36; N, 3.33%. IR (KBr disk): 996 (w), 958 (s), 889 (m), 800 (s), 772 (s), 589 (w), 437 (m), 407 (m) cm⁻¹.

C₁₂py-W₁₀ (2) was synthesized by a similar procedure as for 1. [C₅H₅N(C₁₂H₂₅)]Br (C₁₂py·Br, 0.28 g (0.81 mmol)) was employed as the cationic surfactant. The crude product was recrystallized from ethanol to obtain colorless prisms of 2, which were efflorescent. Data for 1: anal. calcd. for C₇₆H₁₂₀N₄W₁₀O₃₂: C, 24.42; H, 3.62; N, 1.68%; found: C, 24.01; H, 3.42; N, 1.62%. IR (KBr disk): 997 (w), 959 (s), 895 (m), 796 (s), 683 (m), 584 (w), 440 (m), 410 (m) cm⁻¹.

3.2. X-ray Diffraction Measurements

Single-crystal X-ray diffraction measurements for 1 were made on a Rigaku RAXIS RAPID imaging plate diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71075 Å). Diffraction data were collected and processed with PROCESS-AUTO []. The structure of 1 was solved by the charge flipping method [] and expanded using Fourier techniques. The refinement procedure was performed by the full-matrix least-squares using SHELXL97 []. The measurements for 2 were made on a Rigaku Saturn70 diffractometer using graphite monochromated Mo-Kα radiation. Diffraction data were collected and processed with CrystalClear []. The structure of 2 was solved by direct methods [] and expanded using Fourier techniques. The refinement procedure was performed by full-matrix least-squares using SHELXL2013 []. All calculations were performed using the CrystalStructure [] software package. In the refinement procedure, all non-hydrogen atoms, except for C37 of 2, were refined anisotropically, and the hydrogen atoms on C atoms were located in the calculated positions. The weak reflection intensities from the crystals of 2 may result in relatively high R₁ and wR₂ values. The results of checking cif files are available as supplementary materials. 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 1055676-1055677).

4. Conclusions

Inorganic-organic hybrid crystals comprised of polyoxotungstate and surfactants having a heterocyclic moiety, [C₄H₄N₂(C₁₂H₂₅)]₄[W₁₀O₃₂]·2(CH₃)₂CO (1) and [C₅H₅N(C₁₂H₂₅)]₄[W₁₀O₃₂]·4C₂H₅OH (2), were successfully synthesized. Dodecylpyridazinium (C₁₂pda) and dodecylpyridinium (C₁₂py) were utilized as organic cations, and the decatungstate anion (W₁₀) was used for the inorganic component, which enabled the discussion of the effect of the heterocyclic moiety for the construction of the hybrid crystals. Although both hybrid crystals contained alternate stacking of W₁₀ monolayers and interdigitated surfactant bilayers, pyridazine rings in 1 interacted more strongly with the W₁₀ anions. 1 contained shorter C-H···O hydrogen bonds than those observed in 2, indicating that these hydrogen bonds, as well as the electrostatic interaction between the C₁₂pda cation and the W₁₀ anion stabilized the layered structure of C₁₂pda-W₁₀ with rigid packing. On the other hand, 2 has similar cell parameters and molecular arrangements as those in the crystals of hexadecylpyridinium (C₁₆py) and W₁₀ (3), also indicating the significance of the heterocyclic moiety for the construction of the hybrid crystals. The difference between the heterocyclic moieties led to different arrangements of W₁₀ anions in 1 and 2, which will contribute to the precise control of molecular arrangements and the emergence of characteristic conductivity.

Supplementary Materials

Supplementary materials can be found at http://www.mdpi.com/1422-0067/16/04/8505/s1.

Acknowledgments

This work was supported in part by the JSPS Grant-in-Aid for Scientific Research (No. 26410245) and the Research and Study Project of Tokai University Educational System General Research Organization.

Author Contributions

Saki Otobe and Takeru Ito conceived of and designed the experiments. Saki Otobe, Natsumi Fujioka and Takuro Hirano performed the experiments. Saki Otobe and Takeru Ito analyzed the data. Eri Ishikawa and Haruo Naruke contributed analysis tools. Katsuhiko Fujio contributed materials. Takeru Ito wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

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Decisive Interactions between the Heterocyclic Moiety and the Cluster Observed in Polyoxometalate-Surfactant Hybrid Crystals

Abstract

1. Introduction