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Crystals 2014, 4(1), 42-52; doi:10.3390/cryst4010042
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.
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 . 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).
|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)|
|µ (Mo Kα) (mm−1)||1.472||1.500|
|No. of reflections measured||70784||21352|
|No. of independent reflections||15939||7869|
|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  (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 . 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 .
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).
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  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).
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)  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 . 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 . The structure was solved by direct methods  for C10im-Na-Mo8 and by heavy-atom Patterson methods for C12im-Na-Mo8 . The refinement procedure was performed by the full-matrix least-squares using SHELXL97 . All calculations were performed using the CrystalStructure  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: firstname.lastname@example.org (CCDC 980232 and 980233).
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.
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.
- Welton, T. Room-temperature ionic liquids. Solvents for synthesis and catalysis. Chem. Rev. 1999, 99, 2071–2083. [Google Scholar]
- Wasserscheid, P.; Keim, W. Ionic liquids-new “solutions” for transition metal catalysis. Angew. Chem. Int. Ed. 2000, 39, 3772–3789. [Google Scholar] [CrossRef]
- Haumann, M.; Riisager, A. Hydroformylation in room temperature ionic liquids (RTILs): Catalyst and process developments. Chem. Rev. 2008, 108, 1474–1497. [Google Scholar]
- Honma, I.; Yamada, M. Bio-inspired membranes for advanced polymer electrolyte fuel cells. Anhydrous proton-conducting membrane via molecular self-assembly. Bull. Chem. Soc. Jpn. 2007, 80, 2110–2123. [Google Scholar] [CrossRef]
- Coronado, E.; Gómez-García, C.J. Polyoxometalate-based molecular materials. Chem. Rev. 1998, 98, 273–296. [Google Scholar] [CrossRef]
- Coronado, E.; Giménez-Saiz, C.; Gómez-García, C.J. Recent advances in polyoxometalate-containing molecular conductors. Coord. Chem. Rev. 2005, 249, 1776–1796. [Google Scholar] [CrossRef]
- Pope, M.T. Heteropoly and Isopoly Oxometalates; Springer: Berlin, Germany, 1983. [Google Scholar]
- Hill, C.L. Introduction: Polyoxometalates multicomponent molecular vehicles to probe fundamental issues and practical problems. Chem. Rev. 1998, 98, 1–2. [Google Scholar] [CrossRef]
- Long, D.-L.; Burkholder, E.; Cronin, L. Polyoxometalate clusters, nanostructures and materials: From self assembly to designer materials and devices. Chem. Soc. Rev. 2007, 36, 105–121. [Google Scholar] [CrossRef]
- Proust, A.; Matt, B.; Villanneau, R.; Guillemot, G.; Gouzerh, P.; Izzet, G. Functionalization and post-functionalization: A step towards polyoxometalate-based materials. Chem. Soc. Rev. 2012, 41, 7605–7622. [Google Scholar] [CrossRef]
- Okuhara, T.; Mizuno, N.; Misono, M. Catalytic chemistry of heteropoly compounds. Adv. Catal. 1996, 41, 113–252. [Google Scholar] [CrossRef]
- Sadakane, M.; Steckhan, E. Electrochemical properties of polyoxometalates as electrocatalysts. Chem. Rev. 1998, 98, 219–237. [Google Scholar] [CrossRef]
- Song, Y.-F.; Long, D.-L.; Ritchie, C.; Cronin, L. Nanoscale polyoxometalate-based inorganic/organic hybrids. Chem. Rec. 2011, 11, 158–171. [Google Scholar] [CrossRef]
- Qi, W.; Wu, L. Polyoxometalate/polymer hybrid materials: Fabrication and properties. Polym. Int. 2009, 58, 1217–1225. [Google Scholar] [CrossRef]
- Clemente-León, M.; Coronado, E.; Soriano-Portillo, A.; Mingotaud, C.; Dominguez-Vera, J.M. Langmuir–Blodgett films based on inorganic molecular complexes with magnetic or optical properties. Adv. Colloid Interface Sci. 2005, 116, 193–203. [Google Scholar] [CrossRef]
- Stein, A.; Fendorf, M.; Jarvie, T.P.; Mueller, K.T.; Benesi, A.J.; Mallouk, T.E. Salt-gel synthesis of porous transition-metal oxides. Chem. Mater. 1995, 7, 304–313. [Google Scholar] [CrossRef]
- Janauer, G.G.; Dobley, A.; Guo, J.; Zavalij, P.; Whittingham, M.S. Novel tungsten, molybdenum, and vanadium oxides containing surfactant ions. Chem. Mater. 1996, 8, 2096–2101. [Google Scholar] [CrossRef]
- Taguchi, A.; Abe, T.; Iwamoto, M. Non-silica-based mesostructured materials: Hexagonally mesostructured array of surfactant micelles and 11-tungstophosphoric heteropoly anions. Adv. Mater. 1998, 10, 667–669. [Google Scholar]
- Landsmann, S.; Lizandara-Pueyo, C.; Polarz, S. A new class of surfactants with multinuclear, inorganic head groups. J. Am. Chem. Soc. 2010, 132, 5315–5321. [Google Scholar] [CrossRef]
- Zhang, G.; Ke, H.; He, T.; Xiao, D.; Chen, Z.; Yang, W.; Yao, J. Synthesis and characterization of new layered polyoxometallates-1,10-decanediamine intercalative nanocomposites. J. Mater. Res. 2004, 19, 496–500. [Google Scholar] [CrossRef]
- Janauer, G.G.; Dobley, A.D.; Zavalij, P.Y.; Whittingham, M.S. Evidence for decavanadate clusters in the lamellar surfactant ion phase. Chem. Mater. 1997, 9, 647–649. [Google Scholar] [CrossRef]
- Spahr, M.E.; Nesper, R. Anhydrous octamolybdate with trimethyl hexadecyl ammonium cations. Z. Anorg. Allg. Chem. 2001, 627, 2133–2138. [Google Scholar] [CrossRef]
- Nyman, M.; Ingersoll, D.; Singh, S.; Bonhomme, F.; Alam, T.M.; Brinker, C.J.; Rodriguez, M.A. Comparative study of inorganic cluster-surfactant arrays. Chem. Mater. 2005, 17, 2885–2895. [Google Scholar]
- Nyman, M.; Rodriguez, M.A.; Anderson, T.M.; Ingersoll, D. Two structures toward understanding evolution from surfactant-polyoxometalate lamellae to surfactant-encapsulated polyoxometalates. Cryst. Growth Des. 2009, 9, 3590–3597. [Google Scholar] [CrossRef]
- Yin, P.; Wu, P.; Xiao, Z.; Li, D.; Bitterlich, E.; Zhang, J.; Cheng, P.; Vezenov, D.V.; Liu, T.; Wei, Y. A double-tailed fluorescent surfactant with a hexavanadate cluster as the head group. Angew. Chem. Int. Ed. 2011, 50, 2521–2525. [Google Scholar] [CrossRef]
- Ito, T.; Sawada, K.; Yamase, T. Crystal structure of bis(dimethyldioctadecylammonium) hexamolybdate: A molecular model of Langmuir-Blodgett films. Chem. Lett. 2003, 32, 938–939. [Google Scholar] [CrossRef]
- Ito, T.; Mikurube, K.; Abe, Y.; Koroki, T.; Saito, M.; Iijima, J.; Naruke, H.; Ozeki, T. Hybrid inorganic-organic crystals composed of octamolybdate isomers and pyridinium surfactant. Chem. Lett. 2010, 39, 1323–1325. [Google Scholar] [CrossRef]
- Ito, T.; Mikurube, K.; Hasegawa, K.; Kurasawa, M.; Naruke, H.; Ozeki, T. Polyoxomolybdate-surfactant hybrid layered crystal with unusually long periodicity. Chem. Lett. 2011, 40, 626–628. [Google Scholar] [CrossRef]
- Ito, T.; Ide, R.; Kosaka, K.; Hasegawa, S.; Mikurube, K.; Taira, M.; Naruke, H.; Koguchi, S. Polyoxomolybdate-surfactant layered crystals derived from long-tailed alkylamine and ionic-liquid. Chem. Lett. 2013, 42, 1400–1402. [Google Scholar] [CrossRef]
- Ito, T. Polyoxometalate-surfactant hybrids as building strategy for two-dimensional molecular arrays. Polyoxometalate Chem. 2012, 1, 6–14. [Google Scholar]
- Bourlinos, A.B.; Raman, K.; Herrera, R.; Zhang, Q.; Archer, L.A.; Giannelis, E.P. A liquid derivative of 12-tungstophosphoric acid with unusually high conductivity. J. Am. Chem. Soc. 2004, 126, 15358–15359. [Google Scholar] [CrossRef]
- Leng, Y.; Wang, J.; Zhu, D.; Ren, X.; Ge, H.; Shen, L. Heteropolyanion-based ionic liquids: Reaction-induced self-separation catalysts for esterification. Angew. Chem. Int. Ed. 2009, 48, 168–171. [Google Scholar] [CrossRef]
- Rafiee, E.; Evani, S. Polyoxometalate-based acid salts with tunable separation properties as recyclable Brönsted acid catalysts for the synthesis of β-keto enol ethers. Catal. Commun. 2012, 25, 64–68. [Google Scholar] [CrossRef]
- Chen, X.; Souvanhthong, B.; Wang, H.; Zheng, H.; Wang, X.; Huo, M. Polyoxometalate-based ionic liquid as thermoregulated and environmentally friendly catalyst for starch oxidation. Appl. Catal. B 2013, 138–139, 161–166. [Google Scholar]
- Jiang, Y.; Liu, S.; Li, S.; Miao, J.; Zhang, J.; Wu, L. Anisotropic ionic liquids built from nonmesogenic cation surfactants and Keggin-type polyoxoanions. Chem. Commun. 2011, 47, 10287–10289. [Google Scholar]
- Rickert, P.G.; Antonio, M.R.; Firestone, M.A.; Kubatko, K.-A.; Szreder, T.; Wishart, J.F.; Dietz, M.L. Tetraalkylphosphonium polyoxometalate ionic liquids: Novel, organic-inorganic hybrid materials. J. Phys. Chem. B 2007, 111, 4685–4692. [Google Scholar] [CrossRef]
- Desiraju, G.R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: New York, NY, USA, 1999; pp. 12–16. [Google Scholar]
- McCarron, E.M., III; Harlow, R.L. Synthesis and structure of Na4[Mo8O24(OCH3)4]·8MeOH: A novel isopolymolybdate that decomposes with the loss of formaldehyde. J. Am. Chem. Soc. 1983, 105, 6179–6181. [Google Scholar] [CrossRef]
- Niven, M.L.; Cruywagen, J.J.; Heyns, J.B.B. The first observation of γ-type octamolybdates: Synthesis, crystal and molecular structure of [Me3N(CH2)6NMe3]2[Mo8O26]·2H2O. J. Chem. Soc. Dalton Trans. 1991, 20, 2007–2011. [Google Scholar]
- Inoue, M.; Yamase, T. Synthesis and crystal structures of γ-type octamolybdates coordinated by chiral lysines. Bull. Chem. Soc. Jpn. 1995, 68, 3055–3063. [Google Scholar] [CrossRef]
- Khan, M.I.; Zubieta, J. Oxovanadium and oxomolybdenum clusters and solids incorporating oxygen-donor ligands. Prog. Inorg. Chem. 1995, 43, 1–149. [Google Scholar] [CrossRef]
- Aupoix, A.; Pégot, B.; Vo-Thanh, G. Synthesis of imidazolium and pyridinium-based ionic liquids and application of 1-alkyl-3-methylimidazolium salts as pre-catalysts for the benzoin condensation using solvent-free and microwave activation. Tetrahedron 2010, 66, 1352–1356. [Google Scholar] [CrossRef]
- CrystalClear; Rigaku Corporation: Tokyo, Japan, 1999.
- Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M.; Polidori, G.; Camalli, M. SIR92—A program for automatic solution of crystal structures by direct methods. J. Appl. Cryst. 1994, 27, 435–436. [Google Scholar]
- Beurskens, P.T.; Admiraal, G.; Beurskens, G.; Bosman, W.P.; Garcia-Granda, S.; Gould, R.O.; Smits, J.M.M.; Smykalla, C. The DIRDIF Program System; University of Nijmegen: Nijmegen, The Netherlands, 1992. [Google Scholar]
- Sheldrick, G.M. A short history of SHELX. Acta Cryst. Sect. A 2007, 64, 112–122. [Google Scholar] [CrossRef]
- Crystal Structure 4.0; Rigaku Corporation: Tokyo, Japan, 2010.
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