ARTICLE

DOI: 10.1002/zaac.200800302

Room-Temperature Synthesis of Bismuth Clusters in Ionic Liquids and CrystalGrowth of Bi5(AlCl4)3

Ejaz Ahmed,[a] Daniel Köhler,[a] and Michael Ruck*[a]

Keywords: Bismuth; Polycation; Cluster compounds; Ionic liquids

Abstract. The viability of Lewis-acid ionic liquids for the synthesisof low-valent bismuth compounds is demonstrated. At room tem-perature, elemental bismuth and bismuth(III) cations synpropor-tionate in the ionic liquid [BMIM]Cl/AlCl3 ([BMIM]�: 1-n-butyl-3-methylimidazolium) within minutes. The existence of bismuth po-lycations in the dark colored solution was proven by Raman spec-troscopy. Dark-red crystals of Bi5(AlCl4)3 were isolated from the

Introduction

The formation of ligand-free polycationic clusters is awell-known feature of bismuth [1]. Textbook examples areBi53� [2�5], Bi82� [6] and Bi95� [7]. The two new speciesBi5� and Bi62� [8] as well as the stuffed or capped bismuthpolycations Bi10

4� [9] have been characterized in the lastyears. Compounds with higher bismuth content proved toconsist of condensed clusters, which form bismuth substruc-tures ranging from one- to three-dimensional networks [10].There are three known routes for the synthesis of bismuthpolycations:

(i) the high temperature reaction of melts containing bis-muth and BiX3 (X � Cl, Br, I), and in some cases a strongLewis acid such as AlX3 [2, 3, 6�10],

(ii) the oxidation of elemental bismuth by (highly toxic)AsF5 or SbF5 in liquid SO2 at very low temperature [4], and

(iii) the reduction of BiX3 in GaX3�benzene media atroom temperature [5].

On the search for new and, optimistically, larger bismuthclusters we were interested in a convenient low-temperaturesynthesis and focused on ionic liquids as reaction media.Ionic liquids are well-established in organic synthesis, ca-talysis and electrochemistry [11]. Their potential for thesynthesis of inorganic materials is currently explored [12].Very often the organic cation of the ionic liquid is found tobe an integral part of the crystalline product [13]. Despitetheir redox-stability ionic liquids have hardly ever been util-

* Prof. Dr. M. RuckFax: �49-351-463-37287E-Mail: Michael.Ruck@chemie.tu-dresden.de

[a] Professur für Anorganische Chemie IITechnische Universität Dresden01062 Dresden, Germany

Z. Anorg. Allg. Chem. 2009, 635, 297�300 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 297

ionic liquid and characterized by Raman spectroscopy andX-ray crystallography (rhombohedral space-group R3c, a �

1187.1(2) pm, c � 3012.0(3) pm). The method allows the synthesisof bismuth cluster compounds under milder conditions than inhigh-temperature melts and more conveniently and environmentalfriendly than in liquid SO2 with strongly oxidizing, toxic agents likeSbF5 or AsF5.

ized for the chemical synthesis of low-valent compounds.The first spectacular success was the preparation of theclathrate-II allotrope of germanium by mild oxidation ofalkali-metal germanides in an ionic liquid [14]. In this arti-cle, we describe a fast and convenient route for the roomtemperature synthesis of bismuth polycations in ionicliquids.

Experimental SectionChemicals: Starting materials were commercial BiCl3 (Alfa Aesar,anhydrous, 99.999 %), 1-methylimidazole (Merck, 99 %), 1-chloro-butane (Merck, 98.5 %) and AlCl3 (Fluka, anhydrous, 98 %). Bi(chemical pure, Riedel de Haen) was treated with hydrogen at220 °C prior to use in synthesis. BiCl3 and AlCl3 were sublimatedthree times and organic reagents were distilled before use. Becauseof the high moisture sensitivity of the anhydrous metal halides usedin this work, all manipulations were performed under dry argon(99.999 %) atmosphere in standard Schlenk tubes.

Synthesis: The ionic liquid [BMIM]Cl/AlCl3 (where [BMIM]�:1-n-butyl-3-methylimidazolium) was prepared according to a litera-ture procedure [15]. Bismuth polycations have been obtained byreacting BiCl3 (0.315 g, 1.0 mmol) and Bi metal (0.626 g, 3.0 mmol)in the Lewis acidic [BMIM]Cl/AlCl3 (1:1.3) ionic liquid at 298 K.

Raman spectroscopy: Raman spectra were recorded usingFT-Raman spectrometers equipped with Nd-YAG-laser (1064 nm).A resolution of 4.0 cm�1 was used in all measurements.

X-ray Crystallography

The powder diffraction pattern was measured with a STOE STADIP powder diffractometer, equipped with a position sensitive detec-tor (PSD) covering 90° in 2θ and using Cu-Kα1 radiation. Thesamples, mixed with 50 % amorphous boron, were in sealed in0.3 mm Hilgenberg capillaries and kept spinning during the 21 h of

E. Ahmed, D. Köhler, M. RuckARTICLETable 1. Crystallographic data and details of the structure determi-nation for Bi5(AlCl4)3.

Formula Bi5(AlCl4)3

Crystal system, rhombohedral, R3c (no. 167)space group

Cell parameters from a � 1187.1(2) pm, c � 3012.0(3) pm,powder V � 3675.9(9) · 106 pm3

Formula units per cell Z � 6

Calculated density ρcalc � 4.21 g · cm�1

Crystal dimensions 0.08 � 0.10 � 0.14 mm

Temperature 293(2) K

Measurement device imaging-plate diffractometer (Stoe IPDS-I)

Radiation graphite-monochromated MoK�(λ � 71.073 pm)

Measurement limits 4.0° � 2θ � 47.9°; �13 � h, k � 13,�34 � l � 34

Scan type 0 � φ � 270°, Δφ � 1.5°Absorption correction numerical, on the basis of a crystal

description optimized using equivalentreflections [16, 17]

Absorption coefficient μ(MoK�) � 372 cm�1

Transmission factors 0.03 to 0.11

Number of reflections 10389 measured, 638 independent

Data averaging Rint � 0.134, Rσ � 0.041

Structure refinement full-matrix least-squares on Fo2 [18];

anisotropic displacement parameters;reverse-obverse twin with domain ratioof 0.91(2) : 0.09(2).

Number of parameters 33

Residual electron density �1.19 to �0.99 e · 10�6 pm�3

Figures of merit R1 (all Fo) � 0.071R1 (465 Fo > 4σ(Fo)) � 0.042wR2 (all Fo) � 0.077

Goodness of fit 1.09

Table 2. Wyckoff positions, coordinates and equivalent isotropicdisplacement parameters. Ueq is one third of the trace of the ortho-gonalized Uij tensor.

atom W. p. x y z Ueq / pm2

Bi1 4f 0.16179(8) 0 1/4 600(3)Bi2 2e 0 0 0.32705(4) 705(5)Al 4f 0.5220(5) 1/4 1/2 368(15)Cl1 4f 0.4197(5) 0.0081(6) 0.3075(1) 469(10)Cl2 2e 0.7124(4) 0.1662(5) 0.2421(3) 563(17)

data collection at 293(2) K. The PSD was calibrated with a siliconstandard. Intensity data of a twinned single-crystal were collectedat 293(2) K on an imaging plate diffraction system IPDS-I (Stoe)using graphite-monochromated Mo-Kα radiation (Table 1). Theraw-data were corrected for background, polarization and Lorentzfactor. The microscopic description of the crystal shape, which waslater used in the numerical absorption corrections [16], was optim-ized using sets of reflections that are equivalent in the trigonal Laueclass 3m [17]. Based on the structure solution in the rhombohedralspace group R3c (no. 167) by Krebs [3] the structural parameterswere refined [18]. Final values of the positional and displacementparameters are given in Table 2. Further details of the crystal struc-ture investigation can be obtained from the Fachinformations-zentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany,on quoting the depository number CSD-420082. Graphics weregenerated with the program Diamond [19].

www.zaac.wiley-vch.de © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2009, 297�300298

Results and Discussion

BiCl3 and bismuth reacted immediately in the Lewisacidic ionic liquid [BMIM]Cl/AlCl3 at room temperature.Within minutes after mixing, the solution turned darkbrown. It is indispensable to use Lewis acidic ionic solventsfor metal cluster synthesis, because excess AlCl3 picks upthe free chloride ions from the solution to form tetrachlo-roaluminate ions (Lewis acidity), and these anions stabilizethe polycations. Consequently, in case of neutral or basicionic liquids no color change was observed. Probably dueto the high viscosity of the reaction medium, crystallizationof dark-red Bi5(AlCl4)3 occurred only after a couple ofweeks. The addition of some polar organic co-solvents likeacetonitrile, dichloromethane and tetrahydrofuran did notspeed up crystallization.

Besides the color of the solution and the precipitation ofcrystals, Raman spectra (Fig. 1) indicate the presence ofbismuth polycations in the ionic liquid as well. The Ramanspectrum of bismuth polycations in solution suffered fromstrong fluorescence and only three bands with weak inten-sities were identified in the characteristic range up to400 cm�1. The band at 134 cm�1 matches the most inten-sive mode ν1(A1�) of the Bi53� polycation [10]. The bandsat 183 cm�1 and 345 cm�1, which have also been observedin melts of Bi5(AlCl4)3/NaAlCl4 at 180 °C [4], can be attri-buted to ν3(F2) and ν1(A1) modes of the tetrahedralAlCl4� anions.

Figure 1. Raman spectra of single crystals of Bi5(AlCl4)3, and ofbismuth polycations (predominantly Bi53�) in the ionic liquid(inset).

For comparison the Raman spectrum of a Bi5(AlCl4)3

crystal grown from the ionic liquid was recorded. The spec-trum nicely matches with the previously reported results ofGillespie [4]. The representation of the Raman- and infra-red-active normal vibrations for the trigonal bipyramidBi53� having D3h symmetry is: Γvib � 2A1� � A2� � 2E� �2E�. Of these only A1�, E� and E� modes are Raman active.

Room-Temperature Synthesis of Bismuth Clusters in Ionic Liquids

In the spectrum of crystalline Bi5(AlCl4)3 the Raman bands(relative intensity in parentheses) for Bi53� polycation arefound at 63 cm�1 (0.9), 99 cm�1 (0.41), 122 cm�1 (0.35) and138 cm�1 (1) for the Raman active ν4(E�), ν6(E�), ν2(A1�),and ν1(A1�) vibrational modes. The bands at 86 cm�1 (0.43)and at 110 cm�1 (0.2) are attributed to lattice modes. Bandsfor AlCl4� anions are found at 166 cm�1 (0.18) and346 cm�1 (0.3).

In order to check whether the Bi5(AlCl4)3 crystals grownfrom ionic liquid differ from those obtained by high-tem-perature melt reactions, X-ray diffraction at room tempera-ture was performed on the powder (Fig. 2) and a single-crystal. The unit cell dimensions (a � 1187.1(2) pm, c �3012.0(3) pm) as well as the atomic parameters correspondwithin the accuracy of the experiments with those deter-mined by Krebs on a crystal grown by the high-temperatureroute [3]. The rhombohedral crystal structure of Bi5(AlCl4)3

is built from Bi53� trigonal bipyramids and AlCl4� tetra-hedra (Fig. 3). The essential interatomic distances ared(Bi1�Bi2) � 301.2(1) pm, d(Bi1�Bi1) � 332.7(2) pm,d(Al�Cl1) � 214.4(5) pm, and d(Al�Cl2) � 214.5(6) pm.

Figure 2. Powder diffraction pattern of Bi5(AlCl4)3 crystals grownfrom an ionic liquid.

Conclusion

The presented method is based on the extraordinarilyhigh solubility of metal halides in Lewis acidic systems,which renders pseudo-melt behavior to the saturated solu-tion. Thus, it has been possible to repeat, at room tempera-ture, several of the reactions previously performed inmolten AlCl3 at substantially higher temperature [2, 3]. Themajor advantages of performing reactions in ionic liquidsat low temperatures are:

i) Fast reaction under mild conditions.ii) Mobile clusters in solution may be used for deposition

on (sensitive) substrates or for further reactions.iii) Easier study of reactions and products in solution,

using for instance spectroscopy.

Z. Anorg. Allg. Chem. 2009, 297�300 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.zaac.wiley-vch.de 299

Figure 3. Rhombohedral structure of Bi5(AlCl4)3, determined froma crystal grown from an ionic liquid. Ellipsoids represent 70 %probability.

iv) Additional options to modify the reaction conditionsby varying the ionic liquid, the Lewis acid, and the tempera-ture.

v) No need for expensive ampoules and furnaces.Compared to other low-temperature synthetic routes,

which make use of cancer causing or toxic organic and inor-ganic substances like benzene [5], SO2 and AsF5 [4], ionicliquids are more convenient and environmental friendly sol-vents [11, 20].

We have evidence that this synthetic route can be ex-tended to elements of other groups of the periodic table,e.g. tellurium.

Acknowledgement

We thank Prof. Dr. T. Straßner (TU Dresden) for his valuablesuggestions especially regarding the designing and synthesis ofionic liquids, and Mr. C. Ziegler (TU Dresden) for performingRaman spectroscopy measurements. The authors also gratefullyacknowledge the Higher Education Commission of Pakistan andthe German Research Foundation for the financial support.

References

[1] a) J. D. Corbett, in: Fused Salts, B. R. Sundheim (eds),McGraw-Hill, New York 1964, chapter 6; b) S. Ulvenlund,

E. Ahmed, D. Köhler, M. RuckARTICLEL. Bengtsson-Kloo in: Metal Clusters in Chemistry, P.Braunstein, L. A. Oro, P. R. Raithby (eds), Wiley-VCH,Weinheim, 1999, vol. 1, p. 561.

[2] a) J. D. Corbett, Inorg. Chem. 1968, 7, 198; b) L. Heerman, W.D’Olieslager, J. Electrochem. Soc. 1991, 138, 1372; c) S. Ulven-lund, K. Sta;anhl, L. Bengtsson-Kloo, Inorg. Chem. 1996, 35,223; d) K. Ichikawa, T. Yamanaka, A. Takamuku, Inorg.Chem. 1997, 36, 5284; e) A. N. Kuznetsov, B. A. Popovkin, K.Sta;anhl, M. Lindsjö, L. Kloo, Eur. J. Inorg. Chem. 2005, 4907;f) M. Ruck, F. Steden Z. Anorg. Allg. Chem. 2007, 633, 1556.

[3] B. Krebs, M. Mummert, C. Brendel, J. Less-Common Met.1986, 116, 159.

[4] R. C. Burns, R. J. Gillespie, W. C. Luk, Inorg. Chem. 1978,17, 3596.

[5] a) S. Ulvenlund, A. Wheatley, L. A. Bengtsson, J. Chem. Soc.,Chem. Comm. 1995, 59; b) M. Lindsjö, A. Fischer, L. Kloo,Eur. J. Inorg. Chem. 2005, 670; c) M. Lindsjö, A. Fischer, L.Kloo, Angew. Chem. 2004, 116, 2594; Angew. Chem. Int. Ed.Engl. 2004, 43, 2540.

[6] a) R. M. Friedman, J. D. Corbett, Inorg. Chim. Acta 1973, 7,525; b) B. Krebs, M. Hucke, C. J. Brendel, Angew. Chem. 1982,94, 453; Angew. Chem. Int. Ed. 1982, 21, 445; c) J. Beck, T.Hilbert, Eur. J. Inorg. Chem. 2004, 2019; d) A. N. Kuznetsov,B. A. Popovkin, Z. Anorg. Allg. Chem. 2002, 628, 2179.

[7] a) A. Hershaft, J. D. Corbett, Inorg. Chem. 1963, 2, 979; b) R.M. Friedman, J. D. Corbett, Inorg. Chem. 1973, 12, 1134; c)H. von Benda, A. Simon, W. Bauhofer, Z. Anorg. Allg. Chem.1978, 438, 53; d) J. Beck, C. J. Brendel, L. Bengtsson-Kloo, B.Krebs, M. Mummert, A. Stankowski, S. Ulvenlund, Chem. Ber.1996, 129, 1219; e) A. N. Kuznetsov, A. V. Shevel’kov, S. I.Troyanov, B. A. Popovkin, Zh. Neorg. Khim. 1996, 41, 958;Russ. J. Inorg. Chem. 1996, 41, 920; f) A. N. Kuznetsov, A. V.Shevel’kov, B. A. Popovkin, Koord. Khim. 1998, 24, 919; Russ.J. Coord. Chem. 1998, 24, 861; g) M. Ruck, V. Dubenskyy, Z.Anorg. Allg. Chem. 2003, 629, 375; h) A. N. Kuznetsov, P. I.Naumenko, B. A. Popovkin, L. Kloo, Russ. Chem. Bull. 2003,52, 2100; i) S. V. Savilov, A. N. Kuznetsov, P. A. Donets, B. A.Popovkin, Zh. Neorg. Khim. 2003, 48, 400; Russ. J. Inorg.Chem. 2003, 48, 335; j) S. Hampel, P. Schmidt, M. Ruck, Z.Anorg. Allg. Chem. 2005, 631, 272; k) B. Wahl, M. Ruck, Z.Anorg. Allg. Chem. 2009, 635, in press.

www.zaac.wiley-vch.de © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2009, 297�300300

[8] a) M. Ruck, Z. Anorg. Allg. Chem. 1998, 624, 521; b) M. Ruck,S. Hampel, Polyhedron 2002, 21, 651; c) S. Hampel, M. Ruck,Z. Anorg. Allg. Chem. 2006, 632, 1150.

[9] a) M. Ruck, V. Dubenskyy, T. Söhnel, Angew. Chem. 2003, 115,3086; Angew. Chem. Int. Ed. 2003, 42, 2978; b) V. Dubenskyy,M. Ruck, Z. Anorg. Allg. Chem. 2004, 630, 2458; c) B. Wahl,L. Kloo, M. Ruck, Angew. Chem. 2008, 120, 3996; Angew.Chem. Int. Ed. 2008, 47, 3932.

[10] a) M. Ruck, Angew. Chem. 2001, 113, 1222; Angew. Chem. Int.Ed. 2001, 40, 1182; b) M. Ruck, Z. Anorg. Allg. Chem. 1997,623, 1535; c) M. Ruck, R. M. Heich, Z. Anorg. Allg. Chem.2000, 626, 2449; d) M. Ruck, Solid State Sci. 2001, 3, 369; e)F. Steden, P. Schmidt, B. Wahl, A. Isaeva, M. Ruck, Z. Anorg.Allg. Chem. 2008, 634, 69.

[11] a) P. Wasserscheid, W. Keim, Angew. Chem. 2000, 112, 3926;Angew. Chem. Int Ed. 2000, 39, 3772; b) P. Wasserscheid, T.Welton, Ionic Liquids in Synthesis, Wiley-VCH 2002; c) P. J.Dyson, T. J. Geldbach, Metal Catalysed Reactions in IonicLiquids, Springer 2005.

[12] G. Bühler, D. Thölmann, C. Feldmann, Adv. Mater. 2007, 19,2224.

[13] a) A. Okrut, C. Feldmann, Z. Anorg. Allg. Chem. 2006, 632,409; b) J. B. Willems, H. W. Rohm, C. Geers, M. Köckerling,Inorg. Chem. 2007, 46, 6197; c) A. Getsis, A.-V. Mudring, Z.Krist. Suppl. 2007, 041-01-id177.

[14] A. M. Guloy, R. Ramlau, Z. Tang, W. Schnelle, M. Baitinger,Yu. Grin, Nature 2006, 443, 320.

[15] J. S. Wilkes, J. A. Levisky, R. A. Wilson, C. L. Hussey, Inorg.Chem. 1982, 21, 1263.

[16] X-RED32, Data Reduction Program, Version 1.01, Stoe & CieGmbH, Darmstadt 2001.

[17] X-SHAPE, Crystal Optimisation for Numerical AbsorptionCorrection Program, Version 1.06, Stoe & Cie GmbH, Darm-stadt 1999.

[18] a) G. M. Sheldrick, SHELX97, Programs for crystal structuredetermination, Univ. of Göttingen, 1997; b) G. M. Sheldrick,Acta Crystallogr. 2008, A64, 112.

[19] K. Brandenburg, Diamond 3.1e, Crystal and Molecular Struc-ture Visualization, Crystal Impact GbR, Bonn 2007.

[20] J. Wright, Environmental Chemistry, Taylor Francis, New York2003, p. 74, 208.

Received: October 24, 2008