Structural Variation in Polyoxomolybdate Hybrid Crystals Comprising Ionic-Liquid Surfactants

Ito, Takeru; Mikurube, Keisuke; Hasegawa, Kimiko; Matsumoto, Takashi; Kosaka, Kurato; Naruke, Haruo; Koguchi, Shinichi

doi:10.3390/cryst4010042

Open AccessArticle

Structural Variation in Polyoxomolybdate Hybrid Crystals Comprising Ionic-Liquid Surfactants

by

Takeru Ito

^1,*

,

Keisuke Mikurube

¹,

Kimiko Hasegawa

²,

Takashi Matsumoto

²,

Kurato Kosaka

¹,

Haruo Naruke

³ and

Shinichi Koguchi

¹

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

²

Research Laboratory, Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima 196-8666, 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.

Crystals 2014, 4(1), 42-52; https://doi.org/10.3390/cryst4010042

Submission received: 10 January 2014 / Revised: 11 February 2014 / Accepted: 20 February 2014 / Published: 4 March 2014

(This article belongs to the Special Issue Crystal Engineering Involving Weak Bonds)

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Abstract

:

Polyoxomolybdate inorganic-organic hybrid crystals were synthesized with 1-decyl-3-methylimidazolium and 1-dodecyl-3-methylimidazolium as ionic-liquid surfactants. Both hybrid crystals possessed alternate stacking of surfactant layers and octamolybdate (Mo₈) monolayers, while the molecular structures of Mo₈ were different depending on the surfactants and solvents employed for crystallization. Each Mo₈ anion was connected by two sodium cations to form infinite one-dimensional chain. The surfactant chains in these crystals were arranged in a complicatedly bent manner, which will be induced by the weak C–H···O hydrogen bonds between the Mo₈ anions and ionic-liquid surfactants.

Keywords:

inorganic-organic; layered crystal; polyoxometalate; ionic-liquid; surfactant

Graphical Abstract

1. Introduction

Ionic-liquids enable to construct hybrid materials with functions such as catalytic or conductive properties owing to their specific characteristics [1,2,3,4]. Inorganic-organic hybrid compounds using ionic-liquid surfactants will exhibit higher structural variation than purely inorganic compounds and higher stability than purely organic compounds. In such ionic-liquid hybrids, the structures and arrangements of molecular components should be precisely controlled for the emergence of characteristic functions as realized in functional hybrid molecular conductors [5,6].

Polyoxometalate anions having physicochemical functions such as redox, catalytic or conductive properties [7,8,9,10,11,12] have been successfully organized by structure-directing surfactants to construct inorganic-organic hybrids [13,14,15,16,17,18,19,20] and layered crystals [21,22,23,24,25,26,27,28,29,30]. These polyoxometalate-surfactant hybrids allow flexible selection of the ionic components including ionic liquid [31,32,33,34,35,36], which leads to precise engineering of the structure and function.

We report here structural variation of polyoxomolybdate hybrid crystals synthesized by using long-tailed ionic-liquid surfactants. 1-decyl-3-methylimidazolium ([(C₁₀H₂₁)C₃H₃N₂(CH₃)]⁺, C₁₀im) and 1-dodecyl-3-methylimidazolium ([(C₁₂H₂₅)C₃H₃N₂(CH₃)]⁺, C₁₂im) were employed as cationic surfactants. Both crystals comprised octamolybdate (Mo₈O₂₆⁴⁻, Mo₈) anion and sodium cation (C₁₀im-Na-Mo₈ and C₁₂im-Na-Mo₈). The weak interactions between poloxomolybdates and surfactants are considered to affect the formation of complicated packing structures.

2. Results and Discussion

2.1. Crystal Structure of C₁₀im-Na-Mo₈

C₁₀im-Na-Mo₈ was obtained from as-prepared hybrid composed of C₁₀im and polyoxomolybdate, which contained β-type octamolybdate (β-Mo₈) as in the case that C₁₂im was utilized [29]. C₁₀im-Na-Mo₈ crystals suitable for X-ray crystallography were obtained by employing 1-butanol (1-BuOH) as crystallization solvent.

The single crystal X-ray structure analysis combined with the elemental analysis revealed the formula of C₁₀im-Na-Mo₈ to be [(C₁₀H₂₁)C₃H₃N₂(CH₃)]₂Na₂[β-Mo₈O₂₆]·4[1-BuOH] (Table 1). Figure 1 shows the crystal structure of C₁₀im-Na-Mo₈. The crystal packing consisted of alternating β-Mo₈ inorganic layers and C₁₀im organic layers with periodicity of 19.6 Å (Figure 1b).

Two C₁₀im cations (1+ charge) and two Na⁺ were associated with one β-Mo₈ anion (4− charge) due to the charge compensation. The inorganic layers were composed of infinite chains of β-Mo₈ connected by Na⁺ along b axis (Figure 1c) as observed in β-Mo₈ crystals hybridized with hexadecylpyridinium ([C₅H₅N(C₁₆H₃₃)]⁺, C₁₆py) [27,28]. The space between the β-Mo₈-Na⁺ chains were filled by imidazole rings of C₁₀im, which were located in the vicinity of Na⁺ cations. The imidazole rings of C₁₀im had no π–π stacking interaction. C₁₀im-Na-Mo₈ contained two linker Na⁺ per one β-Mo₈, while C₁₆py-Mo₈ had one linker Na⁺ per one β-Mo₈. This difference may be due to the difference in the surfactant type (imidazolium or pyridinium). All 1-BuOH molecules of crystallization were bonded to Na⁺ cations, and the 1-BuOH molecules were neutral as judged from the charge balance between the cations (two C₁₀im and two Na⁺) and anion (one β-Mo₈).

Table 1. Crystallographic data.

**Table 1.** Crystallographic data.
Compound	C₁₀im-Na-Mo₈	C₁₂im-Na-Mo₈
Chemical formula	C₄₄H₅₅N₄Na₂Mo₈O₃₀	C₄₄H₉₄N₄Na₂Mo₈O₃₀
Formula weight	1933.43	1972.73
Crystal system	monoclinic	triclinic
Space group	P2₁/n (No. 14)	P1 (No. 2)
a (Å)	20.089 (4)	9.5496 (6)
b (Å)	8.8791 (18)	11.3505 (8)
c (Å)	39.931 (9)	16.8466 (12)
α (°)	–	102.283 (8)
β (°)	100.301 (3)	90.927 (7)
γ (°)	–	104.802 (8)
V (Å³)	7008 (3)	1720.2 (3)
Z	4	1
ρ_calcd (g·cm⁻³)	1.832	1.904
T (K)	93	93
µ (Mo Kα) (mm⁻¹)	1.472	1.500
No. of reflections measured	70784	21352
No. of independent reflections	15939	7869
R_int	0.0874	0.0536
No. of parameters	730	381
R₁ (I > 2σ(I))	0.0933	0.0494
wR₂ (all data)	0.2612	0.1276

The organic layers were constructed from decyl groups of C₁₀im cations and butyl groups of 1-BuOH molecules to form bilayer-like structure (Figure 1b). However, these decyl and butyl chains were not interdigitated. Each C₁₀im cation had two gauche C–C bonds in the decyl chain [C7–C8 and C11–C12; C21–C22 and C25–C26 (C26–C27B)], while the other C–C bonds had anti conformation (Figure 1a). The C₁₀im cations had complicatedly bent conformation in their chain structure. The presence of more than two gauche C–C bonds per one alkyl chain was rarely observed for polyoxometalate-surfactant hybrid crystals [21,22,23,24,25,26,27,28,29,30]. The C–C bonds in the butyl groups had anti conformation.

The bent C₁₀im cations had several weak C–H···O hydrogen bonds [37] (Figure 2). The hydrogen bonds were formed mainly at the interface between the β-Mo₈ and C₁₀im layers. In addition, some C–H···O bonds were present in the vicinity of the gauche C–C bonds. The C···O distance was in the range of 3.11–3.78 Å (mean value: 3.40 Å). The gauche C–C bonds near the end of decyl chain [C11–C12 and C25–C26 (C26–C27B)] had weak C···C interactions between other decyl and butyl chains.

2.2. Crystal Structure of C₁₂im-Na-Mo₈

C₁₂im-Na-Mo₈ was obtained from as-prepared hybrid of C₁₂im-Mo₈ containing β-Mo₈ [29]. Suitable crystals of C₁₂im-Na-Mo₈ were obtained from ethanol solution under the presence of Na⁺ or Li⁺, while C₁₂im-β-Mo₈ without Na⁺ was crystallized from acetonitrile [29].

Figure 1. Crystal structure of C₁₀im-Na-Mo₈. H atoms are omitted for clarity. (a) Asymmetric unit drawn by displacement ellipsoids at the 30% probability level. Minor parts of the disordered C atoms are indicated in transparent color; (b) Packing diagram along b axis (Mo₈ in polyhedral representations); (c) Molecular arrangements in the inorganic layers. The decyl and butyl groups are omitted for clarity.

Figure 2. C–H···O hydrogen bonds in C₁₀im-Na-Mo₈ (pink dotted line). Selected hydrogen bonds between Mo₈ moiety and C₁₀im are represented [symmetry code: (i) x, −1 + y, z; (ii) x, 1 + y, z]. Na⁺ and 1-BuOH are omitted for clarity. Minor parts of the disordered C atoms are indicated in transparent color.

The formula of C₁₂im-Na-Mo₈ was revealed to be [(C₁₂H₂₅)C₃H₃N₂(CH₃)]₂Na₂[γ-Mo₈O₂₄(OC₂H₅)₄]·2C₂H₅OH (Table 1). The anion was γ-type Mo₈ coordinated by four ethoxo ligands (Figure 3a) [38,39,40,41]. The dissolved β-Mo₈ from as-prepared C₁₂im-Mo₈ reacted with ethanol to isomerize into ethoxo-grafted γ-Mo₈ anions, which reprecipitated as C₁₂im-γ-Mo₈ crystals. The crystal structure of C₁₂im-Na-Mo₈ was composed of alternating Mo₈ inorganic layers and C₁₂im organic layers with periodicity of 16.4 Å (Figure 3b).

Figure 3. Crystal structure of C₁₂im-Na-Mo₈. H atoms are omitted for clarity. (a) Asymmetric unit together with atoms generated by the symmetry operation to complete Mo₈ anion [symmetry code: (i) −x, −y, −z]; Displacement ellipsoids are drawn at the 50% probability level; (b) Packing diagram along b axis (Mo₈ in polyhedral representations); (c) Molecular arrangements in the inorganic layers. The dodecyl and ethyl groups are omitted for clarity.

Two C₁₂im cations (1+ charge) and two Na⁺ were associated with one γ-Mo₈ anion (4− charge). The inorganic layers were composed of infinite chains of γ-Mo₈ connected by two linker Na⁺ cations along a axis (Figure 3c), being similar to C₁₀im-Na-Mo₈. The space between the γ-Mo₈-Na⁺ chains are filled by imidazole rings of C₁₂im, which are located in the vicinity of Na⁺ cations. The imidazole rings of C₁₂im were parallel, but no π–π stacking interaction was observed. One neutral EtOH molecule of crystallization was bonded to each Na⁺ cation.

The organic layers were composed of dodecyl groups of C₁₂im cations (Figure 3b). The dodecyl chains were bent without interdigitation, being different from other polyoxometalate-surfactant hybrid crystals comprising interdigitated straight surfactant chains. Each C₁₂im cation had three gauche C–C bonds (C6–C7, C11–C12, and C14–C15), resulting in the bent chain conformation of C₁₂im as in the case of C₁₀im-Na-Mo₈.

The complicatedly bent C₁₂im cations had several weak C–H···O hydrogen bonds [37] with γ-Mo₈ anions (Figure 4). The hydrogen bonds were formed mainly at the interface between the γ-Mo₈ and C₁₂im layers, and some C–H···O bonds were present in the vicinity of gauche C–C bonds. The C···O distance was in the range of 2.96–3.70 Å (mean value: 3.32 Å). The gauche C–C bonds (C14–C15) near the end of dodecyl chain had weak C···C interactions between other dodecyl chains and ethyl groups. In addition, there were weak interactions between γ-Mo₈ and ethoxo groups of γ-Mo₈ (intramolecular C–H···O hydrogen bonds) and between γ-Mo₈ and ethanol connected to Na⁺ (intremolecular C–H···O hydrogen bonds).

Figure 4. C–H···O hydrogen bonds in C₁₂im-Na-Mo₈ (pink dotted line). Selected hydrogen bonds between Mo₈ moiety and C₁₂im are represented [symmetry code: (ii) x, −1 + y, z]. Na⁺, ethyl groups, and EtOH are omitted for clarity.

3. Experimental Section

3.1. Syntheses and Methods

All chemical reagents except for imidazolium surfactants were obtained from commercial sources (Wako, Osaka, Japan). As-prepared C₁₀im-Mo₈ was precipitated by adding ethanol solution of C₁₀im·Br (0.2 M, 10 mL) [42] to Na₂MoO₄·2H₂O aqueous solution (0.5 M, 10 mL), which was adjusted to pH 3.8 with 6 M HCl. Colorless needle crystals of C₁₀im-Na-Mo₈ were obtained from 1-BuOH/acetonitrile (13 mL/2 mL) solution of the as-prepared C₁₀im-Mo₈ hybrid (0.10 g) and NaCl (0.02 g) kept at 303–308 K (yield: 21% based on Mo). Some 1-BuOH molecules of crystallization seem to be exchanged with acetonitrile. Anal.: Calculated for C₄₄H₉₀N₆Na₂Mo₈O₂₉: C: 26.68%, H: 4.58%, N: 4.24%. Found: C: 27.08%, H: 4.08%, N: 4.34%. Melting point: 463 K. Infrared (KBr disk): 1165 (w), 937 (s), 914 (s), 901 (m), 839 (m), 707 (s), 669 (m), 619 (w), 577 (w), 555 (w), 525 (w), 480 (w), 453 (w), 413 (m) cm⁻¹.

As-prepared C₁₂im-Mo₈ hybrid, a starting precipitate for C₁₂im-Na-Mo₈ crystals, was obtained according to the literature [29]. Colorless plate crystals of C₁₂im-Na-Mo₈ were crystallized from ethanol (15 mL) solution of the as-prepared C₁₂im-Mo₈ hybrid (0.05 g) containing NaNO₃ or LiNO₃ (0.02–0.03 g) (yield: 15% based on Mo). Some ethoxo ligands of C₁₂im-Na-Mo₈ were probably exchanged for hydroxo ligands by the reaction with water in the air, which seems to cause the isomerization of γ-Mo₈ to β-Mo₈. Anal.: Calculated for C₃₂H₆₆N₄Na₂Mo₈O₂₈: C: 21.73%, H: 3.76%, N: 3.17%. Found: C: 21.27%, H: 3.32%, N: 3.06%. Melting point: 509 K. Infrared (KBr disk): 1167 (w), 1120 (w), 1054 (w), 937 (s), 914 (s), 900 (m), 841 (m), 710 (s), 669 (w), 659 (w), 620 (w), 554 (w), 534 (w), 522 (w), 499 (w), 474 (m), 437 (w), 425 (m), 413 (w), 405 (w) cm⁻¹.

3.2. X-Ray Crystallography

Single crystal X-ray diffraction measurements were performed on a Rigaku Saturn724 diffractometer with multi-layer mirror monochromated Mo-Kα radiation (λ = 0.71075 Å) (Rigaku, Tokyo, Japan). Diffraction data were collected and processed with CrystalClear [43]. The structure was solved by direct methods [44] for C₁₀im-Na-Mo₈ and by heavy-atom Patterson methods for C₁₂im-Na-Mo₈ [45]. The refinement procedure was performed by the full-matrix least-squares using SHELXL97 [46]. All calculations were performed using the CrystalStructure [47] software package. Most non-hydrogen atoms were refined anisotropically, while the rest were refined isotropically. The hydrogen atoms on C atoms were refined using the riding model. C₁₀im-Na-Mo₈ crystals were very fine needles, and the weak reflection intensities may result in relatively high R₁ and wR₂ values. In the refinement of C₁₀im-Na-Mo₈ structure, the hydrogen atoms relevant to the disordered C atoms and on O atoms of 1-BuOH were not included. In the refinement of C₁₂im-Na-Mo₈ structure, a hydrogen atom attached to the O atom of ethanol was found in the difference Fourier synthesis and their positional and isotropic displacement parameters were refined. 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 980232 and 980233).

4. Conclusions

The hybrid crystals composed of polyoxomolybdate and imidazolium ionic-liquid surfactants, [(C₁₀H₂₁)C₃H₃N₂(CH₃)]₂Na₂[β-Mo₈O₂₆]·4[1-BuOH] (C₁₀im-Na-Mo₈) and [(C₁₂H₂₅)C₃H₃N₂(CH₃)]₂Na₂[γ-Mo₈O₂₄(OC₂H₅)₄]·2C₂H₅OH (C₁₂im-Na-Mo₈), were synthesized. Both hybrid crystals contained octamolybdate (Mo₈) anions associated with Na⁺ to form one-dimensonal Mo₈-Na⁺ chain, although the molecular structures of Mo₈ were different (β-Mo₈ for C₁₀im-Na-Mo₈ and ehoxo-modified γ-Mo₈ for C₁₂im-Na-Mo₈). The crystal structures comprised alternate stacking of Mo₈ monolayers and surfactant layers. Both crystals contained complicatedly bent conformation in the surfactant chain, which is considered to be derived from the weak C–H···O hydrogen bonds. These ionic-liquid hybrid crystals are expected to exhibit specific catalytic property such as esterification, oxidation, or phase transfer catalysis, and to exhibit Na⁺-ion conductivity.

Acknowledgments

This work was supported in part by JSPS Grant-in-Aid for Scientific Research (No. 23750246), Tokai University Supporters Association Research and Study Grant, Research and Study Program of Tokai University Educational System General Research Organization, and The Noguchi Institute.

Conflicts of Interest

The authors declare no conflict of interest.

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Ito, T.; Mikurube, K.; Hasegawa, K.; Matsumoto, T.; Kosaka, K.; Naruke, H.; Koguchi, S. Structural Variation in Polyoxomolybdate Hybrid Crystals Comprising Ionic-Liquid Surfactants. Crystals 2014, 4, 42-52. https://doi.org/10.3390/cryst4010042

AMA Style

Ito T, Mikurube K, Hasegawa K, Matsumoto T, Kosaka K, Naruke H, Koguchi S. Structural Variation in Polyoxomolybdate Hybrid Crystals Comprising Ionic-Liquid Surfactants. Crystals. 2014; 4(1):42-52. https://doi.org/10.3390/cryst4010042

Chicago/Turabian Style

Ito, Takeru, Keisuke Mikurube, Kimiko Hasegawa, Takashi Matsumoto, Kurato Kosaka, Haruo Naruke, and Shinichi Koguchi. 2014. "Structural Variation in Polyoxomolybdate Hybrid Crystals Comprising Ionic-Liquid Surfactants" Crystals 4, no. 1: 42-52. https://doi.org/10.3390/cryst4010042

APA Style

Ito, T., Mikurube, K., Hasegawa, K., Matsumoto, T., Kosaka, K., Naruke, H., & Koguchi, S. (2014). Structural Variation in Polyoxomolybdate Hybrid Crystals Comprising Ionic-Liquid Surfactants. Crystals, 4(1), 42-52. https://doi.org/10.3390/cryst4010042

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Structural Variation in Polyoxomolybdate Hybrid Crystals Comprising Ionic-Liquid Surfactants

Abstract

1. Introduction