Na8[B12(BSe3)6]: A Novel Selenoborato-closo-dodecaborate

A. Hammerschmidt, M. Döch, S. Pütz and B. Krebs*

Münster, Institut für Anorganische und Analytische Chemie der Westfälischen Wilhelms-Universität, Sonderforschungsbereich 458

Received May 28th, 2004.

Dedicated to Professor Martin Jansen on the Occasion of his 60th Birthday

Abstract. Systematic studies on selenoborates containing a B12 clus-ter entity and alkali metal cations led to the new crystalline phaseNa8[B12(BSe3)6] which consists of a icosahedral B12 cluster com-pletely saturated with trigonal-planar BSe3 units and sodium coun-ter-ions. The new chalcogenoborate was prepared in a solid statereaction from sodium selenide, amorphous boron and selenium inevacuated carbon coated silica tubes at a temperature of 850 °C.

Na8[B12(BSe3)6]: Ein neues Selenoborato-closo-dodecaborat

Inhaltsübersicht. Gezielte Versuche zur Synthese neuer Selenobo-rato-closo-dodecaborate führten im ternären Phasengebiet Na/B/Sezu der neuen Verbindung Na8[B12(BSe3)6], in der ein vollständigdurch Chalkogenatome abgesättigtes B12-Ikosaeder vorliegt. Dasneue Chalkogenoborat wurde in einer Hochtemperatur-Feststoffre-

1 Introduction

Numerous thio- and selenoborates have been synthesizedand characterized in recent years due to improved prep-aration techniques [1, 2]. Binary boron sulfides and selen-ides [1, 3�5] as well as ternary and quaternary thio- andselenoborates contain boron atoms in a trigonal-planar co-ordination, for which various novel types of chalcogenobor-ate anions have been observed. Typical examples are thesmall, highly charged anion entities BS3

3� [6�8], BSe33�

[9], B2S42� [10], B2S5

2� [11], and B3S63� [8, 12�14], which

are characteristic structural features in non-oxide chalcog-enoborates of alkali and alkaline earth metals. In contrastto a tetrahedral chalcogen coordination which is found notonly in thio- but also extensively in selenoborates [15�22],examples for boron atoms in a trigonal-planar coordinationwith selenium atoms are scarce. Apart from the aforemen-tioned BSe3

3� anion present in the thallium compoundTl3BSe3 [9] as well as in Ba7(BSe3)4Se [23], such a boron-selenium coordination sphere is also observed in the re-cently discovered cluster phases of general formulaeM8[B12(BSe3)6] and M4Hg2[B12(BSe3)6] [24, 25] with M �K, Rb, Cs. With M8[B12(BS3)6] (M � Rb, Cs) the first

* Prof. Dr. B. KrebsWilhelm-Klemm-Str. 8D-48149 MünsterFax: �49 (0)251/8338366e-mail: krebs@uni-muenster.de

Z. Anorg. Allg. Chem. 2004, 630, 2299�2303 DOI: 10.1002/zaac.200400291 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 2299

Na8[B12(BSe3)6] crystallizes in the monoclinic space group P21/c(no. 14) with a � 10.990(1) A, b � 16.279(1) A, c � 11.490(1), β �

117.82(1)° and Z � 2.

Keywords: Selenoborates; Boron; Cluster compounds; Crystalstructures

aktion aus Natriumselenid, amorphem Bor und Selen in graphitier-ten, evakuierten Quarzglasampullen hergestellt. Na8[B12(BSe3)6]kristallisiert in der monoklinen Raumgruppe P21/c (Nr. 14) mit a �

10,990(1) A, b � 16,279(1) A, c � 11,490(1), β � 117,82(1)° undZ � 2.

thioborate-closo-dodecaborates could be synthesized andcharacterized [26]. In all these structures [B12(BQ3)6]8�

anions (Q � S, Se) occur which are formed by B12-closo-clusters which are completely saturated with six BQ3 units.With the synthesis and structure analysis of the selenobor-ate-closo-dodecaborate Na6[B18Se17] we were able to charac-terize the first boron selenium compound with a polymericanionic cluster chain [27]. Now we report the synthesis andcrystal structure of Na8[B12(BS3)6] which strikingly is notisotypic to its heavier homologues.

2 Experimental

Synthesis

The synthesis of well-defined and highly pure boron chalcogencompounds is rather difficult because of the high reactivity of insitu formed boron chalcogenides towards a variety of containermaterials at elevated temperatures. The fused silica tubes usuallyemployed for solid-state reactions are attacked by boron chalcogen-ides at temperatures above 400 °C forming silicon chalcogen com-pounds by B-Si exchange at the surface of the ampoules. For thesynthesis of pure samples the reaction vessel must either be madeof boron nitride or graphite, or silica tubes coated with glassy car-bon must be used. The latter are prepared by slowly turning a silicaampoule filled with acetone vapour through the flame of an oxy-gen-hydrogen operated welding torch at about 1000 °C. In somecases, especially when longer annealing is necessary, the formertype of crucibles are employed. To protect them against oxidationthey are encapsulated in steel or tantalum ampoules under an ar-

A. Hammerschmidt, M. Döch, S. Pütz, B. Krebs

gon atmosphere, and these again are enclosed in evacuated silicatubes.

As starting materials the following products were used: sodium sel-enide (prepared following a method by Thiele et al. [28]), amorph-ous boron (Alfa, amorphous powder, 95%), and selenium (Strem,powder, 99.5%).

For the synthesis of Na8[B12(BSe3)6] appropriate amounts of thestarting compounds were mixed and filled into a carbon-coatedsilica tube which was thereafter sealed at a pressure of 6 Pa andinserted into a horizontal one-zone furnace. Heating and coolingprocedures were performed as follows:

20 °C ��8 h 850 °C (6h) ��4 h 700 °C ���150 h 300 °C ��10 h 20 °C.

Colourless plate-shaped crystals suitable for single crystal X-raydiffraction were obtained. The product is air and moisture sensitiveand was therefore handled under dry argon in a glove box.

Single Crystal Structure Analysis

For the data collection a single crystal was sealed in a Mark capil-lary under an argon atmosphere. The X-ray diffraction data forNa8[B12(BSe3)6] were collected on a Bruker AXS Smart CCD dif-fractometer. The structure solution, which was possible in spacegroup P21/c, was achieved by applying direct statistical methods ofphase determination using the SHELXTL PLUS program [29], andfull-matrix least-squares refinements were performed using theSHELXL-97 software programs [30]. The complete data collectionparameters and details of the structure solutions and refinementsare given in Table 1.

Nine selenium sites were directly obtained from the electron densitymap and subsequent refinements gave the sodium and boron posi-tions, all atoms occupy the general position 4e in P21/c. Table 2gives the coordinates of all atoms, average temperature factors andtheir estimated standard deviations.

Details of the crystal structure may be obtained from the Fachin-formationszentrum Karlsruhe, Gesellschaft für wissenschaftlich-technische Zusammenarbeit, D-76344 Eggenstein-Leopoldshafen,on quoting the depository number CSD-414010, the name of theauthors and this journal.

3 Results and Discussion

Isolated monomeric cluster entities are known from the re-cently published alkali metal selenoborato-closo-dodecabo-rates M8[B12(BSe3)6] and the mercury containing com-pounds M4Hg2[B12(BSe3)6] (M � Rb, Cs) [24, 25] in whichthe characteristic structural features are B12 icosahedrawhich are also observed in elementary boron as well as invarious closo-boranes. Examples of completely substitutedcloso-dodecaboranes apart from halogenide or hydroxidesubstitued B12 clusters [32�36] are rare. Although a hydro-gen-selenium exchange succeeded in the formation of thecluster anion [B6(SeCN)6]2� [37] without any lower substi-tution side products, only monosubstitution by a selenocy-anate ligand is observed for a B12 cluster even under basicconditions [38]. Another example of selenium bonding to aB12 icosahedron is the binary boron selenide B12Se2�xBx

2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2004, 630, 2299�23032300

Table 1 Crystallographic data and refinement parameters forNa8[B12(BSe3)6]

Formula Na8[B12(BSe3)6]Formula weight 1799.78 g·mol-1

Crystal dimensions 0.20 x 0.13 x 0.10 mm3

Crystal system monoclinicSpace group P21/c (no. 14)Lattice parameters a � 10.990(1) A

b � 16.279(1) Ac � 11.490(1) Aβ � 117.82(1)

Cell volume 1817.9(8) A3

Z 2Density calculated 3.288 g·cm-3

Measurement device Smart CCD Bruker AXSScan mode φ-ω-scan; ∆φ � 0.3°Range in θ 2.1 to 28.02°Indices �14 � h � 14

�21 � k � 21�15 � l � 15

Absorption correction Sadabs [31]Absorption coefficient 18.156 mm-1

Reflections observed 18340Independent reflections 4405 [R(int) � 0.045]Data / Restraints / Parameter (n) 4405 / 0 / 199Goodness-of-fit (F2) 1.024Residuals [Reflections with I > 2σ(I)] R1 � 0.0248

wR2 � 0.0507Residuals [all Reflections] R1 � 0.0328

wR2 � 0.0529Weighting factor (h) 0.6095Largest diff. peak/hole 0.74 / �0.55 e·A�3

R1 ��hkl

�Fo���Fc�

�hkl

�Fo�wR2 ���

hkl

w(F2o�F2

c)2

�hkl

w(F2o)2

Goof � S ���hkl

w(F2o�F2

c)2

m�n

Weighting scheme: 1/w � [σ2 (Fhkl)2 � (g·P)2 � h·P]P � ((max.(Fhklo · 0))2 � 2·Fhklc

2)/3

Table 2 Atom coordinates and isotropic thermal displacement pa-rameters /A2 with standard deviations for Na8[B12(BSe3)6]

Atom Wyckoff- x y z Ueqa)

Position

Se(1) 4e 0.15870(4) 0.51571(2) 0.48293(4) 0.01947(8)Se(2) 4e 0.25910(4) 0.34198(2) 0.29045(3) 0.02094(8)Se(3) 4e 0.40569(4) 0.31373(2) 0.61960(4) 0.02487(9)Se(4) 4e 0.77620(4) 0.41142(2) 0.79918(3) 0.02015(8)Se(5) 4e 0.35027(4) 0.69858(2) 0.55146(4) 0.02195(9)Se(6) 4e 0.20311(4) 0.16721(2) 0.41684(4) 0.02503(9)Se(7) 4e 0.52092(4) 0.50563(2) 0.83012(3) 0.02036(8)Se(8) 4e 0.01896(4) 0.69646(2) 0.49530(4) 0.02637(9)Se(9) 4e 0.79533(4) 0.42354(2) 1.09984(4) 0.02674(9)Na(1) 4e 0.0333(2) 0.3794(2) 0.0405(2) 0.0464(5)Na(2) 4e 0.0651(2) 0.7411(2) 0.7639(2) 0.0536(5)Na(3) 4e 0.1861(2) 0.0766(2) 0.2056(3) 0.0885(9)Na(4) 4e 0.4651(3) 0.6597(2) 0.9313(2) 0.0704(7)B(1) 4e 0.5004(4) 0.5109(2) 0.6463(4) 0.0184(8)B(2) 4e 0.6334(4) 0.4576(2) 0.6338(4) 0.0150(7)B(3) 4e 0.4387(4) 0.5899(2) 0.5254(4) 0.0163(8)B(4) 4e 0.2808(4) 0.2745(2) 0.4404(4) 0.0155(8)B(5) 4e 0.3436(4) 0.4981(2) 0.4946(4) 0.0153(7)B(6) 4e 0.4630(4) 0.4156(2) 0.5634(4) 0.0193(8)B(7) 4e 0.3820(4) 0.4338(2) 0.3916(4) 0.0155(8)B(8) 4e 0.6969(4) 0.4458(2) 0.9139(4) 0.0183(8)B(9) 4e 0.1752(4) 0.6362(2) 0.5131(4) 0.0158(8)

a) Ueq is defined as 1/3 of the trace of the orthogonalised Uij Tensor

[39]. Since it crystallizes in the B6P-type structure no closestructural resemblance to the herein described selenobor-ates is present.

Na8[B12(BSe3)6]: A Novel Selenoborato-closo-dodecaborate

Fig. 1 [B12(BSe3)6]8- anion in Na8[B18Se18], symmetry equivalentatoms are not labelled

Fig. 2 Projection of the unit cell of Na8[B18Se18] along [100]

The significant stability of the B12 icosahedron is basedon the ability of boron to form multicentre bonds. Wade�srules [40] apply for the [B18Se18]8� entities as well: with sixnegative charges located on the terminal selenium atoms,two negative charges remain on the cluster core giving2n�2 binding electrons per B12 moiety.

In the novel sodium selenoborato-closo-dodecaborateNa8[B18Se18] isolated B12 cluster entities saturated withBSe3-ligands occur as depicted in Fig. 1 (Figs. 1�3 werecreated using the DIAMOND program system [41]). Dueto the perpendicular arrangement of the bidentate BSe3-ligands the [B18Se18]8� entities show nearly Cmmm sym-metry as becomes clear from the projection of the unit cellalong [100] given in Fig. 2.

The monomeric [B12(BSe3)6]8� unit exhibits two types ofendocyclic B�Se bond lengths in the chelate-type B3Se2

rings: B�Se bonds to boron atoms in the cluster core range

Z. Anorg. Allg. Chem. 2004, 630, 2299�2303 zaac.wiley-vch.de 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 2301

Table 3 B � B and B � Se bond lengths /A with standard devia-tions for Na8[B12(BSe3)6]

B1 � B2 1.764(5) B2 � B3a 1.798(5)B1 � B3 1.782(5) B2 � B5a 1.766(5)B1 � B5 1.804(5) B2 � B6 1.796(5)B1 � B6 1.767(5) B2 � B7a 1.788(5)B1 � Se7 2.021(4) B2 � Se4 1.967(4)B3 � B2a 1.798(5) B4 � Se2 1.964(4)B3 � B6a 1.805(5) B4 � Se3 1.975(4)B3 � B7a 1.789(5) B4 � Se6 1.911(4)B3 � Se5 1.987(4) B6 � B3a 1.805(5)B5 � B2a 1.766(5) B6 � B7 1.772(5)B5 � B6 1.787(5) B6 � Se3 1.989(4)B5 � B7 1.774(5) B8 � Se4 1.974(4)B5 � Se1 1.997(4) B8 � Se7 1.973(4)B7 � B1a 1.791(5) B8 � Se9 1.927(4)B7 � B2a 1.788(5) B9 � Se1 1.989(4)B7 � B3a 1.789(5) B9 � Se5 1.968(4)B7 � Se2 1.991(4) B9 � Se8 1.907(4)

a) 1-x; 1-y; 1-z

Table 4 Na � Se bond lengths /A with standard deviations forNa8[B12(BSe3)6]

Na1 � Se2 2.855(2) Na2 � Se2h 3.590(2)Na1 � Se4a 3.977(2) Na2 � Se4f 3.532(2)Na1 � Se4b 2.943(2) Na2 � Se5g 3.519(2)Na1 � Se6c 2.929(2) Na2 � Se6h 3.090(2)Na1 � Se8d 3.029(2) Na2 � Se8 2.978(2)Na1 � Se9b 3.085(2) Na2 � Se8g 3.105(2)

Na2 � Se9l 3.126(1)Na3 � Se1c 2.863(2)Na3 � Se1d 3.525(2) Na4 � Se3f 3.074(2)Na3 � Se3c 3.501(2) Na4 � Se5 3.971(2)Na3 � Se6 2.774(2) Na4 � Se6f 3.234(2)Na3 � Se7c 3.538(3) Na4 � Se7 2.951(2)Na3 � Se8d 3.064(2) Na4 � Se7l 3.798(3)Na3 � Se9e 3.294(2) Na4 � Se9l 3.039(2)

a) 1-x; 1-y; 1-z b) �1�x; y; -1�z c) x; 0.5-y; -0.5�zd) �x; -0.5�y; 0.5-z e) 1-x; -0.5�y; 1.5-z f) 1-x; 0.5�y; 1.5-zg) x; 1.5-y; 0.5�z h) �x; 1-y; 1-z l) 1-x; 1-y; 2-z

from 1.967(4) A (B2�Se4) to 2.021(4) A (B1�Se7) andB�Se bonds within the trigonal planar BSe3 units varyfrom 1.911(4) A (B4�Se6) to 1.989(4) A (B9�Se1). In con-trast, the B�Se bonds to the exocyclic selenium atoms aresignificantly shorter with 1.927(4) A (B8�Se9) and 1.908(4)A (B9�Se8). All bond lengths are very similar to the 1Dpolymeric selenoborato-closo-dodecaborate Na6[B18Se17]which shows the same substitution pattern concerning theBSe3 ligands. Strikingly, the heavier homologues with pot-assium, rubidium and cesium exhibit different substitutionpatterns [26] with a lower symmetry.

The B�B bonds in the B12 cluster entity show only slightdeviations from the mean value for all distances (comput-able to 1.78 A). Therefore the boron icosahedra are nearlyideal platonic solids (for an aesthetic mathematical treat-ment of such polyhedra in crystal structures see [42]) witha mean antipodal B···B distance of 3.38 A. These values arein very good agreement with those found for the aforemen-tioned isolated cluster compounds (a detailed listing ofB�Se and B�B bond lengths is given in Table 3).

A. Hammerschmidt, M. Döch, S. Pütz, B. Krebs

Fig. 3 Sodium selenium coordination in Na8[B18Se18] in an ellips-oidal representation (50% probability)

Four crystallographically distinguishable sodium cationsare located between the isolated anion entities. As illus-trated in Fig. 3 the sodium cations are in a six-fold andseven-fold selenium coordination, respectively. Seleniumatoms forming the coordination polyhedra of these Na sitesare found at distance of 2.774(2) A (Na3 � Se6) to 3.798(3)A (Na4 � Se7l). A detailed listing of all sodium � seleniumdistances is given in Table 4.

With the synthesis of Na8[B18Se18] a further buildingblock was found in selenoborate chemistry. Since polymer-ization of selenoborato-closo-dodecaborates has recentlyproven to be possible at least in one dimension with thesynthesis of Na6[B18Se17] the formation of three-dimen-sional networks could be possible even within the systemNa/B/Se. Here the Cmmm symmetry of the anion possiblyenables a connection in three dimensions without strikingdistortions. Further studies will aim at the two- and three-dimensional linkage of [B12(BSe3)6] entities which can beseen as the basic building units of these selenoborato-closo-dodecaborate cluster compounds.

2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2004, 630, 2299�23032302

Acknowledgements. We would like to thank the Deutsche For-schungsgemeinschaft (SFB 458) and the Fonds der Chemischen In-dustrie for substantial support of this work.

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