Structure-Directing Role of Hydrogen-Bonded Dimers of Phenylenediammonium Cations: Supramolecular Assemblies of Octamolybdate-Based Organic-Inorganic Hybrids
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 5 1837-1843
Shailesh Upreti and Arunachalam Ramanan* Department of Chemistry, Indian Institute of Technology, Delhi, Hauz Khas, India Received March 18, 2005
ABSTRACT: Crystallization of three new organic/inorganic hybrid solids (1-3) based on octamolybdate under selfassembly conditions has demonstrated the structure-directing role of three hydrogen-bonded organic dimers (supramolecular synthons) in the construction of multidimensional networks. In acidified aqueous molybdate solution, isomeric phenylenediammonium ions form dimers through strong hydrogen-bonding and/or π-π interactions, which in turn dictate the supramolecular assemblies between themselves or with octamolybdate anions that eventually result in the growth of hybrid solids. Introduction Hybrid solids are an intense area of contemporary research, due to their potential applications in the area of catalysis, sensors, molecular filters, etc.1 In the context of the inorganic-organic hybrid solids, polyoxomolybate (POM) clusters are attractive nano building blocks for supramolecular assembly, as they can exhibit diverse self-assembly properties, thus controlling the formation of multidimensional organic-inorganic hybrid networks in self-organization processes.2 In the past, POM-based hybrid solids have been constructed either by electrostatic interactions between inorganic and organic components or by formation of covalent bonds between organic and inorganic moieties, though noncovalent interactions do influence the crystal packing.3 A major hurdle, however, is to correctly design an organic component. Unfortunately, POM often precipitates quickly and nonselectively in the presence of multiply charged organic countercations, leading to the formation of insoluble amorphous solids or powders that are difficult to characterize. In addition, since the negative charge is distributed around the oxomolybdate cluster anion, prediction of the directionality of the POM anion-organic cation based interactions and hence the structure to be obtained is challenging. Octamolybdate is one of the well-characterized and well-understood POM cluster anions which crystallize in the presence of a large number of organic cations from acidified aqueous molybdate solution.3-5 Quite often, the organic molecule occurs as an innocent countercation or forms extended networks through bridging with cations and water molecules exhibiting medium to strong hydrogen bonding. In our earlier work, we observed the occurrence of an eight-membered hydrogenbonded cluster between ammonium ions and water molecules in a solid containing octamolybdate cluster.6 The β-Mo8O26 cluster rarely gets protonated7 and com* To whom correspondence should be addressed. E-mail: aramanan@ chemistry.iitd.ac.in. Fax: 91 11 2658 2277. Tel: 91 11 2659 1507.
petes less for H bonding with water and/or organic cations. Thus, we explored the formation of hydrogenbonded clusters between organic diammonium molecules and water crystallized along with a stable POM cluster anion such as β-Mo8O26 and its influence on the resulting hybrid structures. We preferred aromatic diamines, as phenyl groups can act as rigid spacers. We chose isomeric phenylenediamines, as these protonated amines can participate in hydrogen bonding among themselves as well as with water molecules; in addition, they can exhibit π-π and CH-π interactions.8 In acidified aqueous solution p-phenylenediamine (ppda) and m-phenylenediamine (mpda) invariably exist in diprotonated form, while o-phenylenediamine (opda) is either monoprotonated or diprotonated. In this study, we crystallized three new hybrid solids, (C6N2H10)2[Mo8O26]‚6H2O (1), (C6N2H10)2[Mo8O26]‚4H2O (2), and (C6N2H10)4[Mo8O26]‚H2O (3), in the presence of ppda, mpda, and opda, respectively. A detailed crystal structure analysis of the three solids revealed that dimers formed between phenylenediammonium cations and/or water dictate further interaction between such dimers (in the case of ppda) or link the cluster anions (in the case of opda or mpda), forming novel supramolecular assemblies. These dimers appear to have recognizable functional groups in the solid-state structure and, hence, can very well be considered as supramolecular synthons (see Chart 1) in view of the definition and strategies proposed by Desiraju.8c It is important to note that only a few crystal structures of phenylenediammonium salts are known in the literature with discrete anions such as Cl-, ClO4-, H2P2O72-, P6O186-, HSO4-, [C6(COO)6H6-n]n-, and [AlF5(H2O)]2-.9 In the two examples in which monoprotonated o-phenylenediammonium crystallizes along with perchlorate or phosphomolybdate anion, a strong hydrogen bonding between the organic cation and anion is observed.10,11 With other anions, opda is invariably diprotonated and, hence, the nonbonding interactions observed are different. mpda and ppda cations are
10.1021/cg050100m CCC: $30.25 © 2005 American Chemical Society Published on Web 08/19/2005
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Crystal Growth & Design, Vol. 5, No. 5, 2005 Chart 1.
Upreti and Ramanan
Representative Supramolecular Synthons in (a) 1, (b) 2, and (c) 3
known only with polymeric metal halides. Importantly, none of these examples show the hydrogen-bonded dimers between the organic cations and/or water molecules as seen in our case. We also examined several hybrid salts based on diphosphopentamolybdate anions ([HxP2Mo5O23]-(6-x)) crystallized with phenylenediammonium cations.10 In all the cases, the isomeric phenylenediammonium cations interact strongly with cluster oxygens. Thus, we strongly believe synthesis of 1-3 from aqueous solution opens new avenues to explore the use of structure-directing supramolecular synthons such as those reported here (Chart 1) to assemble new hybrid solids. Experimental Section General Methods and Materials. The reagents were purchased commercially and used without further purification. Elemental analyses (C, H, and N) were performed on a PerkinElmer 2400 CHN elemental analyzer. FTIR spectra of 1-3 were recorded on a Nicolet 5DX spectrophotometer as pressed KBr pellets in the 4000-400 cm-1 region. Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D-8 Advance diffractometer with Cu KR radiation (λ ) 1.5418 Å). TGA and DTA analyses were carried out using Perkin-Elmer TGA7 and DTA7 systems on well-ground samples under a nitrogen atmosphere with a heating rate of 5 °C/min. SEM photographs were recorded on a Cambridge Streoscan 360 microscope. Synthesis of 1-3. Crystals of 1-3 were grown from an aqueous solution containing ammonium heptamolybdate (0.5 mmol, 0.6175 g) with ppda, mpda, and opda (1.5 mmol, 0.162 g), respectively, in the mole ratio 1:3. Solutions were then subjected to heating at 60 °C in a microwave oven for complete dissolution. Solutions were acidified with 20% HCl to bring the pH down to 2.3-2.5. In all of the cases, colored crystals (1, pale yellow; 2, green; 3, dark red) appeared within 16 h.
Similar experiments were repeated with sodium molybdate as a precursor, and different acids such as H2SO4, HNO3, CH3COOH, and HOOCCH2CH2COOH were also employed to obtain acidified aqueous molybdate solutions with pHs in the range between 2 and 5; in all of the cases we ended up with a good yield of single-phase solids (1, 81%; 2, 75%; 3, 82%; based on molybdenum). Crystallizations of 1-3 in the presence of ppda, mpda, and opda, respectively, irrespective of the nature of the molybdate precursor and the source of acid employed for acidification (phosphate is an exception, as it yielded the phosphomolybdate anion, P2Mo5O23) suggest that β-[Mo8O26] cluster based salts are the most stable solids under these conditions. The experimental and simulated powder XRD patterns of 1-3 revealed that their peak positions are in good agreement with each other, indicating the phase purity of the products. Characteristic FTIR bands at 3436 (br), 2902 (w), 1626, 1552, 1513, 1482 (w), 1435 (w), and 1078 cm-1 confirm the presence of phenylenediammonium cation and water, and bands at 947, 912, 846, 715, 668, and 553 cm-1 are characteristic of octamolybdate anion. CHN analysis gave the following results. Anal. Found for compound 1: C, 9.75; H, 2.32; N, 3.81. Calcd: C, 9.52; H, 2.11; N, 3.7. Found for compound 2: C, 9.65; H, 1.79; N, 3.82. Calcd: C, 9.76; H, 1.89; N, 3.79. Found for compound 3: C, 17.20; H, 2.20; N, 6.86. Calcd: C, 17.43; H, 2.17; N, 6.77. Single-Crystal X-ray Diffraction. Single-crystal diffraction studies were carried out on a Bruker Smart Apex CCD diffractometer with a Mo KR sealed tube. Crystal structures were solved by direct methods and in anisotropic approximation refined using the SHELXTL package.12 Hydrogen atoms were constrained to the organic part and for the water molecule first located from the difference Fourier map and were fixed during subsequent refinement cycles. Further details of the X-ray structural analysis for 1-3 are provided in Table 1. Selected hydrogen bond lengths and angles are listed in Table 2.
Table 1. Crystal Data and Structure Refinement Details for 1-3 formula formula wt temp/K cryst syst space group a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z Fcalcd/g cm-3 µ/mm-1 R1, wR2 (I > 2σ(I)) R1, wR2 (all) CCDC no.
1
2
3
[H3NC6H4NH3]2[Mo8O26]‚6H2O 1511.94 298 monoclinic P21/n 8.3431(9) 21.468(3) 10.276(1) 90 99.415(2) 90 1815.8(4) 2 2.765 2.784 0.0313, 0.0657 0.0390, 0.0686 233414
[H3NC6H4NH3]2[Mo8O26]‚4H2O 1475.90 298 monoclinic P21/n 10.814(1) 14.132(1) 11.141(1) 90 97.630(2) 90 1687.7(4) 2 2.904 2.987 0.0205, 0.0517 0.0216, 0.0522 260160
[H2NC6H4NH3]4[Mo8O26]‚2H2O 1652.13 298 monoclinic C2/c 27.502(5) 11.622(2) 14.138(2) 90 109.442(3) 90 4261.0(1) 4 2.575 2.381 0.0251, 0.0623 0.0266, 0.0630 260157
H-Bonded Dimers of Phenylenediammonium Cations
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Table 2. Selected Hydrogen-Bond Distances and Angles for 1-3
{MoO6} edge-shared octahedra corresponds to the β form found in many solids.3-6 Interestingly, the structural analysis showed an extensive hydrogen bonding between the organic cations and water molecules, forming a hydrogen-bonded network. A careful analysis of the structure revealed the occurrence of a dimer (supramolecular synthon), as shown in Figure 1a. The terminal nitrogen atoms (N2) from two different organic cations form a strong eight-membered hydrogen-bonded cluster with two symmetrically equivalent water molecules (O3w) with N‚‚‚O distances of 2.7-2.8 Å; a center of inversion occurs in the middle of this dimer. Such dimers are further linked to each other through hydrogen bonding occurring between the two protons of the other terminal nitrogen (N1) and the two asymmetric water molecules (O1w and O2w), as shown in Figure 2a. These extended sheets formed through H-bond (Figure 2b) interactions occurring between N1‚‚‚O1w‚‚‚N2 lie around [001]. The open channels occurring in the sheets are of an ideal size to host β-octamolybdate clusters (Figure 3). In addition, there are a few insignificant close contacts between the anion and organic cation or anion and water molecule, O2w. The packing of the anions along [110] are essentially driven due to the strong H-bonding interactions occurring between the organic dimers and water molecules (Figure 4). X-ray diffraction analysis of 2 revealed the presence of 4 β-octamolybdate units, 4 mpda cations, and 12 water molecules per unit cell (Table 1). The β-octamolybdate cluster is similar to that present in 1 and 3. Like ppda in 1, mpda in 2 is also diprotonated. A structural
D-H
d(H‚‚‚A)/Å
∠DHA/deg
d(D‚‚‚A)/Å
A
2.801 2.822 3.184 2.804 2.729
O3W O3W O1W O1W O2W
N2-H6 N2-H8 N2-H7 N1-H3 N1-H2
1.930 1.982 2.918 1.913 1.921
Compound 1 172.47 168.12 101.50 169.95 164.41
N1-H1A N1-H1B N1-H1C N2-H2A N2-H2A N2-H2B N2-H2
2.336 1.888 1.990 2.398 2.444 2.102 2.064
Compound 2 161.48 160.34 173.32 133.52 119.66 164.07 160.24
3.165 2.776 2.931 3.056 2.966 3.025 2.837
O9 O1W O3 O11 O9 O1W O8
N1-H1A N1-H1B N1-H1C N4-H4A N4-H4B N4-H4C N4-H4C N2-H2A N2-H2B N3-H3B
2.236 2.150 2.359 2.149 2.134 2.237 2.319 2.345 2.266 2.326
Compound 3 166.01 165.36 143.51 171.73 161.97 147.07 128.69 125.30 151.06 164.24
3.107 3.019 3.120 3.033 2.993 3.023 2.958 2.910 3.028 3.081
O13 O10 N3 O6 O12 N2 O9 O1W O11 O12
Crystal Structures Single-crystal X-ray diffraction analysis of 1 revealed the presence of 2 β-octamolybdate units, 4 diprotonated ppda cations, and 12 water molecules per unit cell (Table 1). The centrosymmetric [Mo8O26]4- built from 8
Figure 1. Dimers of phenylenediammonium cations (supramolecular synthons): (a) H bonding among ppda and water molecules; (b) H bonding and π‚‚‚π stacking among mpda and water molecules, and (c) H-bonding and π‚‚‚π stacking among opda molecules.
Figure 2. (a) Supramolecular assembly as a result of H-bonded networks between water and ppda molecules in 1. (b) Packing diagram of (C6N2H10)[Mo8O26]‚6H2O viewed around [001].
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Figure 3. CPK representation of water-ppda sheets in 1 and arrangement of octamolybdate clusters between the cavities.
analysis of 2 revealed the occurrence of the dimer (Figure 1b) formed between two pairs of ammonium groups of two different mpda molecules linked via water molecules (O1w). A further network is formed between the ammonium groups of this dimer and the cluster oxygens of four different octamolybdates through H bonding, as shown in Figure 5. Two protons of each ammonium group (H2B and H2C) are connected to one water molecule (O1w) and a cluster oxygen (O8), while the third proton (H2A) is left free. Also, each anion is surrounded by four such dimers, leading to a threedimensional network as shown in Figure 6. The complex H bonding taking place between the organic dimers and the anions and water molecules is in fact responsible for a compact network (with high density; see Table 1); the anions come much closer to each other than in 1, as revealed by O5-O12 distances (∼3.2 Å). Figure 4. Water-organic sheets forming porous channels along the c axis in 1.
X-ray diffraction analysis of 3 showed the presence of four β-octamolybdate units, eight opda cations, and four water molecules per unit cell. β-Octamolybdate clusters are similar to those present in 1. Unlike ppda
Figure 5. Supramolecular assembly of the mpda dimer with four octamolybdate anions though H bonding in 2.
H-Bonded Dimers of Phenylenediammonium Cations
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dictated by the dimers, forcing the aggregation of cluster anions along the bc plane (Figure 8b). In 3, the anions come closer to each other along the c axis (O-O distance ∼6.36 Å). The morphology of the crystals 1-3 as shown by SEM photographs (Figure 9) suggest that the shapes of these hybrids are also probably directed by their solidstate structures. Thermal Stability of 1-3
Figure 6. Packing of mpda dimers around the octamolybdate anions in 2.
and mpda, opda cations are monoprotonated. In this structure also one can recognize a dimer formed between amine and ammonium groups of two adjacent opda cations (Figure 1c) involved in strong H bonding. In addition, the two opda molecules showed a strong π‚‚‚π interaction. Four such dimers decorate the octamolybdate anion from above and below, forming a supramolecular assembly as shown in Figure 7. These supramolecular assemblies are further linked into twodimensional sheets (bc plane) through hydrogen bonding between the dimers and adjacent octamolybdate anions (Figure 8a). The sheets are kept away from each other along the a axis due to hydrophobic interactions between organic molecules. Incidentally, the crystal packing analysis suggests that 3 is less dense (Table 1) than 1 and 2; in other words, 3 has a more open structure. In a way, the structure of 3 is a result of the H bonding
The thermal dehydrations of 1-3 are quite different (Figure 10). All samples showed the weight loss in two distinct steps, corresponding to the decomposition of water molecules and organic groups, respectively: for 1, obsd 7.50, 16.4% and calcd 7.14, 15.67%; for 2, obsd 2.89, 17.40% and calcd 4.87, 14.67%; for 3, obsd 2.76, 25.81% and calcd 1.09, 27.11%). While the organic groups are removed almost around the same temperature (∼280 °C), the water loss for each sample occurs at different temperatures, reflecting the strength of hydrogen-bonding interactions taking place in the solids. The water loss for 1 occurs at a lower temperature (∼100 °C) in comparison to 2 and 3, as the bonding is mostly between the organic dimers and water molecules. For 2, the water loss occurs at much higher temperature (∼150 °C) than for 1, as the water molecules are also bonded strongly to the cluster anions. In the case of 3, the water loss occurs significantly at higher temperature (∼200 °C), as water is strongly bonded to the cluster anions and also to the dimers. In all of the cases, the weight loss above 400 °C is due to decomposition as well as volatility of MoO3, as confirmed by powder XRD and FTIR studies. Also, the loss of crystallinity in all samples after removal of water suggested that networks responsible for the stability of all three structures are weaker and are broken at ∼200 °C. Conclusions pH is important for the synthesis of 1-3, in order to obtain an octamolybdate-based solid. The hydrogenbonded structure-directing dimers (supramolecular syn-
Figure 7. Decoration of octamolybdate anion by opda dimers through H-bonding interactions in 3 (the H-bonding interaction between octamolybdate cluster and water molecule is not shown for clarity).
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Figure 8. (a) Linking of decorated octamolybdate anions through H bonding along the c axis, forming sheets that are separated from each other along the a axis in 3. (b) The sheets viewed along [101].
Figure 9. SEM photographs of (a) platelike crystals of 1, (b) rodlike crystals of 2, and (c) blocklike crystals of 3. Optical photographs of the respective crystals are shown in the insets.
Figure 10. TGA/DTG diagrams of (a) 1, (b) 2, and (c) 3.
thons) are responsible for the supramolecular assembly and the crystal packing of hybrid salts based on octamolybdate anions. In conclusion, recognition of supramolecular synthons described in this study can be exploited to rationally design new organic-inorganic hybrid solids based on stable anions such as octamolybdate that compete less for H-bonding interactions.
Acknowledgment. S.U. thanks the DST for a research fellowship, and A.R. acknowledges the DST, Government of India, for financial support. Thanks are also due to the DST for funding an X-ray powder diffractometer under IRHPA and a single-crystal diffractometer under FIST to the Department of Chemistry, IIT Delhi, India.
H-Bonded Dimers of Phenylenediammonium Cations Supporting Information Available: CIF files giving crystallographic data and figures giving DTA data for 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) (a) Pope, M. T.; Muller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34-48. (b) Pope, M. T.; Muller, A. Polyoxometalates: From Platonic Solids to Anti-retroviral Activity; Kluwer: Dordrecht, The Netherlands, 1994. (c) Rhule, J. T.; Hill, C. L.; Judd, D. A.; Schinazi, R. F. Chem. Rev. 1998, 98, 327-357. (d) Muller, A.; Roy, S. Russ. Chem. Rev. 2002, 71, 981-991. (e) Peters, F.; Michael, T. P.; Gatteschi, D.; Muller, A. Chem. Rev. 1998, 98, 239-271. (2) (a) Lin, X.; Meng, L.; Bubin, X.; Yongge, W.; Zhonghua, P.; Powell, D. R. Angew. Chem., Int. Ed. 2002, 41, 4129-4132. (b) Wu, C. D.; Zhong, L. C.; Yang W. B.; Lu, S. F.; Zhuang, H. H.; Huang, J. S. J. Cluster Sci. 2002, 13, 55-62. (c) Qiang, L.; Pingfan, W.; Yongge, W.; Yuan, W.; Ping, W.; Hongyou, G. Inorg. Chem. Commun. 2004, 7, 524-527. (3) (a) Hagman, P. J.; Hagrman, D.; Zubieta, J. Angew. Chem., Int. Ed. 1999, 38, 2638-2684. (b) Peng, Z.; Lu, M.; Xu, B.; Xu, L.; Wei, Y.; Xie, B.; Katabathini, N. R. Polym. Prepr. 2003, 44, 671-672. (c) Wu, C. D.; Lu, C. Z.; Zhuang, H. H.; Huang, J. S. Inorg. Chem. 2002, 41, 5636-5637. (d) Yang, W.; Lu, C.; Zhuang, H. Dalton Trans. 2002, 14, 2879-2884. (e) Rarig, R. S.; Zubieta, J. Inorg. Chim. Acta 2000, 312, 188-196. (f) Wu, C. D.; Lu, C. Z.; Zhuang, H. H.; Huang, J. S. Inorg. Chem. 2002, 41, 5636-5637. (4) (a) Kroenke, W. J.; Fackler, J. P., Jr.; Mazany, A. M. Inorg. Chem. 1983, 22, 2412-2416. (b) McCarron, E. M., III; Whitney, J. F.; Chase, D. B. Inorg. Chem. 1984, 23, 32753280. (c) Kamenar, B. Kem. Ind. 1990, 39, 185-191. (d) McCarron, E. M., III.; Harlow, R. L. I. Acta Crystallogr. 1984, C40, 1140-1141. (e) LaDuca, R. L., Jr.; Rarig, R. S., Jr.; Zapf, P. J.; Zubieta, J. Inorg. Chim. Acta 1999, 292, 131-136. (f) Wu, C. D.; Zhan, X. P.; Lu, C. Z.; Zhuang, H. H. Jiegou Huaxue 2002, 21, 525-529.
Crystal Growth & Design, Vol. 5, No. 5, 2005 1843 (5) (a) Hagrman, D.; Hagrman, P.; Zubieta, J. Inorg. Chim. Acta 2000, 300-302, 212-224. (b) Harrison, W. T. A.; Dussack, L. L.; Jacobson, A. Acta Crystallogr. 1996, C52, 1075-1077. (c) Gutierrrez, Z. J. M.; Martinez, R. M.; Garcia, B. S.; Roman, P. Polyhedron 1986, 5, 1799-1803. (d) Martin, Z. P.; Reyes, G. M.; Arrieta, J. M.; Madariaga, G.; Gili, P. Polyhedron 1992, 11, 115-121. (e) Zheng, L. M.; Yongsheng, W.; Wang, X.; Korp, J. D.; Jacobson, A. J. Inorg. Chem. 2001, 40, 1380-1385. (6) Duraisamy, T.; Ramanan, A.; Vittal, J. J. J. Mater. Chem. 1999, 9, 763-767. (7) (a) Wang, W.; Xua, L.; Weic, Y.; Lia, F.; Gaoa, G.; Wanga, E. J. Solid State Chem. 2005, 178, 606-612. (b) Attanasio, D.; Bonamico, M.; Fares, V.; Suber, L. J. Chem. Soc. Dalton Trans. 1992, 2523-2528. (8) (a) Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311-2327. (b) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: Oxford, U.K., 1999. (c) Desiraju, G. R. Crystal Engineering: The Design of Organic Solids; Elsevier: Amsterdam, 1989. (9) (a) Kobayashi, N.; Naito, T.; Inabe, T. CrystEngComm 2004, 6, 189-196. (b) Soumhi, E. H.; Jouini, T. Acta Crystallogr. 1995, C51, 1457-1459. (c) Schroeder, L.; Frenzen, G.; Massa, W.; Menz, D. H. Z. Anorg. Allg. Chem. 1993, 619, 1307-1314. (d) Soumhi, E. H.; Jouini, T. Acta Crystallogr. 1996, C52, 2802-2805. (10) Raghavaiah, P.; Supriya, S.; Das, S. K. CrystEngComm 2005, 7, 167-170. (11) Upreti, S.; Ramanan, A. Manuscript in preparation. (12) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467. (b) Sheldrick, G. M. SHELXTL-NT 2000, Version 6.12, Reference Manual; University of Go¨ttingen: Go¨ttingen, Germany.
CG050100M
