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 cluster entity and alkali metal cations led to the new crystalline phase Na8[B12(BSe3)6] which consists of a icosahedral B12 cluster completely saturated with trigonal-planar BSe3 units and sodium counter-ions. The new chalcogenoborate was prepared in a solid state reaction from sodium selenide, amorphous boron and selenium in evacuated carbon coated silica tubes at a temperature of 850 °C.
Na8[B12(BSe3)6] crystallizes in the monoclinic space group P21/c ˚ , b ⫽ 16.279(1) A ˚ , c ⫽ 11.490(1), β ⫽ (no. 14) with a ⫽ 10.990(1) A 117.82(1)° and Z ⫽ 2.
Keywords: Selenoborates; Boron; Cluster compounds; Crystal structures
Na8[B12(BSe3)6]: Ein neues Selenoborato-closo-dodecaborat Inhaltsübersicht. Gezielte Versuche zur Synthese neuer Selenoborato-closo-dodecaborate führten im ternären Phasengebiet Na/B/Se zu der neuen Verbindung Na8[B12(BSe3)6], in der ein vollständig durch Chalkogenatome abgesättigtes B12-Ikosaeder vorliegt. Das neue Chalkogenoborat wurde in einer Hochtemperatur-Feststoffre-
aktion aus Natriumselenid, amorphem Bor und Selen in graphitierten, evakuierten Quarzglasampullen hergestellt. Na8[B12(BSe3)6] kristallisiert in der monoklinen Raumgruppe P21/c (Nr. 14) mit a ⫽ ˚ , b ⫽ 16,279(1) A ˚ , c ⫽ 11,490(1), β ⫽ 117,82(1)° und 10,990(1) A Z ⫽ 2.
1 Introduction
thioborate-closo-dodecaborates could be synthesized and characterized [26]. In all these structures [B12(BQ3)6]8⫺ anions (Q ⫽ S, Se) occur which are formed by B12-closoclusters which are completely saturated with six BQ3 units. With the synthesis and structure analysis of the selenoborate-closo-dodecaborate Na6[B18Se17] we were able to characterize the first boron selenium compound with a polymeric anionic cluster chain [27]. Now we report the synthesis and crystal structure of Na8[B12(BS3)6] which strikingly is not isotypic to its heavier homologues.
Numerous thio- and selenoborates have been synthesized and characterized in recent years due to improved preparation techniques [1, 2]. Binary boron sulfides and selenides [1, 3⫺5] as well as ternary and quaternary thio- and selenoborates contain boron atoms in a trigonal-planar coordination, for which various novel types of chalcogenoborate anions have been observed. Typical examples are the small, highly charged anion entities BS33⫺ [6⫺8], BSe33⫺ [9], B2S42⫺ [10], B2S52⫺ [11], and B3S63⫺ [8, 12⫺14], which are characteristic structural features in non-oxide chalcogenoborates of alkali and alkaline earth metals. In contrast to a tetrahedral chalcogen coordination which is found not only in thio- but also extensively in selenoborates [15⫺22], examples for boron atoms in a trigonal-planar coordination with selenium atoms are scarce. Apart from the aforementioned BSe33⫺ anion present in the thallium compound Tl3BSe3 [9] as well as in Ba7(BSe3)4Se [23], such a boronselenium coordination sphere is also observed in the recently discovered cluster phases of general formulae M8[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. Krebs Wilhelm-Klemm-Str. 8 D-48149 Münster Fax: ⫹49 (0)251/8338366 e-mail:
[email protected] Z. Anorg. Allg. Chem. 2004, 630, 2299⫺2303
DOI: 10.1002/zaac.200400291
2 Experimental Synthesis The synthesis of well-defined and highly pure boron chalcogen compounds is rather difficult because of the high reactivity of in situ formed boron chalcogenides towards a variety of container materials at elevated temperatures. The fused silica tubes usually employed for solid-state reactions are attacked by boron chalcogenides at temperatures above 400 °C forming silicon chalcogen compounds by B-Si exchange at the surface of the ampoules. For the synthesis of pure samples the reaction vessel must either be made of boron nitride or graphite, or silica tubes coated with glassy carbon must be used. The latter are prepared by slowly turning a silica ampoule filled with acetone vapour through the flame of an oxygen-hydrogen operated welding torch at about 1000 °C. In some cases, especially when longer annealing is necessary, the former type of crucibles are employed. To protect them against oxidation they are encapsulated in steel or tantalum ampoules under an ar 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
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A. Hammerschmidt, M. Döch, S. Pütz, B. Krebs gon atmosphere, and these again are enclosed in evacuated silica tubes. As starting materials the following products were used: sodium selenide (prepared following a method by Thiele et al. [28]), amorphous boron (Alfa, amorphous powder, 95%), and selenium (Strem, powder, 99.5%). For the synthesis of Na8[B12(BSe3)6] appropriate amounts of the starting compounds were mixed and filled into a carbon-coated silica tube which was thereafter sealed at a pressure of 6 Pa and inserted into a horizontal one-zone furnace. Heating and cooling procedures were performed as follows: h 4h 150 h 10 h 20 °C ᎏ8 씮 850 °C (6h) ᎏ씮 700 °C ᎏ᎐씮 300 °C ᎏ씮 20 °C.
Colourless plate-shaped crystals suitable for single crystal X-ray diffraction were obtained. The product is air and moisture sensitive and 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 capillary under an argon atmosphere. The X-ray diffraction data for Na8[B12(BSe3)6] were collected on a Bruker AXS Smart CCD diffractometer. The structure solution, which was possible in space group P21/c, was achieved by applying direct statistical methods of phase determination using the SHELXTL PLUS program [29], and full-matrix least-squares refinements were performed using the SHELXL-97 software programs [30]. The complete data collection parameters and details of the structure solutions and refinements are given in Table 1. Nine selenium sites were directly obtained from the electron density map and subsequent refinements gave the sodium and boron positions, all atoms occupy the general position 4e in P21/c. Table 2 gives the coordinates of all atoms, average temperature factors and their estimated standard deviations. Details of the crystal structure may be obtained from the Fachinformationszentrum Karlsruhe, Gesellschaft für wissenschaftlichtechnische Zusammenarbeit, D-76344 Eggenstein-Leopoldshafen, on quoting the depository number CSD-414010, the name of the authors and this journal.
3 Results and Discussion Isolated monomeric cluster entities are known from the recently published alkali metal selenoborato-closo-dodecaborates M8[B12(BSe3)6] and the mercury containing compounds M4Hg2[B12(BSe3)6] (M ⫽ Rb, Cs) [24, 25] in which the characteristic structural features are B12 icosahedra which are also observed in elementary boron as well as in various closo-boranes. Examples of completely substituted closo-dodecaboranes apart from halogenide or hydroxide substitued B12 clusters [32⫺36] are rare. Although a hydrogen-selenium exchange succeeded in the formation of the cluster anion [B6(SeCN)6]2⫺ [37] without any lower substitution side products, only monosubstitution by a selenocyanate ligand is observed for a B12 cluster even under basic conditions [38]. Another example of selenium bonding to a B12 icosahedron is the binary boron selenide B12Se2⫺xBx 2300
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Table 1 Crystallographic data and refinement parameters for Na8[B12(BSe3)6] Formula Formula weight Crystal dimensions Crystal system Space group Lattice parameters
Na8[B12(BSe3)6] 1799.78 g·mol-1 0.20 x 0.13 x 0.10 mm3 monoclinic P21/c (no. 14) ˚ a ⫽ 10.990(1) A ˚ b ⫽ 16.279(1) A ˚ c ⫽ 11.490(1) A β ⫽ 117.82(1) ˚3 1817.9(8) A 2 3.288 g·cm-3 Smart CCD Bruker AXS φ-ω-scan; ∆φ ⫽ 0.3° 2.1 to 28.02° ⫺14 ⱕ h ⱕ 14 ⫺21 ⱕ k ⱕ 21 ⫺15 ⱕ l ⱕ 15 Sadabs [31] 18.156 mm-1 18340 4405 [R(int) ⫽ 0.045] 4405 / 0 / 199 1.024 R1 ⫽ 0.0248 wR2 ⫽ 0.0507 R1 ⫽ 0.0328 wR2 ⫽ 0.0529 0.6095 ˚ ⫺3 0.74 / ⫺0.55 e·A
Cell volume Z Density calculated Measurement device Scan mode Range in θ Indices
Absorption correction Absorption coefficient Reflections observed Independent reflections Data / Restraints / Parameter (n) Goodness-of-fit (F2) Residuals [Reflections with I > 2σ(I)] Residuals [all Reflections] Weighting factor (h) Largest diff. peak/hole
兺 储F 兩⫺兩F 储 R ⫽ 兺 兩F 兩 o
1
兺 w(F ⫺F ) 兺 w(F )
冪
c
hkl
wR2 ⫽
o
hkl
2 o
2 2 o
兺 w(F ⫺F )
冪
2 2 c
hkl
Goof ⫽ S ⫽
hkl
2 o
2 2 c
hkl
m⫺n
Weighting scheme: 1/w ⫽ [σ2 (Fhkl)2 ⫹ (g·P)2 ⫹ h·P] P ⫽ ((max.(Fhklo · 0))2 ⫹ 2·Fhklc2)/3
Table 2 Atom coordinates and isotropic thermal displacement pa˚ 2 with standard deviations for Na8[B12(BSe3)6] rameters /A Atom
WyckoffPosition
x
y
z
Ueqa)
Se(1) Se(2) Se(3) Se(4) Se(5) Se(6) Se(7) Se(8) Se(9) Na(1) Na(2) Na(3) Na(4) B(1) B(2) B(3) B(4) B(5) B(6) B(7) B(8) B(9)
4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e
0.15870(4) 0.25910(4) 0.40569(4) 0.77620(4) 0.35027(4) 0.20311(4) 0.52092(4) 0.01896(4) 0.79533(4) 0.0333(2) 0.0651(2) 0.1861(2) 0.4651(3) 0.5004(4) 0.6334(4) 0.4387(4) 0.2808(4) 0.3436(4) 0.4630(4) 0.3820(4) 0.6969(4) 0.1752(4)
0.51571(2) 0.34198(2) 0.31373(2) 0.41142(2) 0.69858(2) 0.16721(2) 0.50563(2) 0.69646(2) 0.42354(2) 0.3794(2) 0.7411(2) 0.0766(2) 0.6597(2) 0.5109(2) 0.4576(2) 0.5899(2) 0.2745(2) 0.4981(2) 0.4156(2) 0.4338(2) 0.4458(2) 0.6362(2)
0.48293(4) 0.29045(3) 0.61960(4) 0.79918(3) 0.55146(4) 0.41684(4) 0.83012(3) 0.49530(4) 1.09984(4) 0.0405(2) 0.7639(2) 0.2056(3) 0.9313(2) 0.6463(4) 0.6338(4) 0.5254(4) 0.4404(4) 0.4946(4) 0.5634(4) 0.3916(4) 0.9139(4) 0.5131(4)
0.01947(8) 0.02094(8) 0.02487(9) 0.02015(8) 0.02195(9) 0.02503(9) 0.02036(8) 0.02637(9) 0.02674(9) 0.0464(5) 0.0536(5) 0.0885(9) 0.0704(7) 0.0184(8) 0.0150(7) 0.0163(8) 0.0155(8) 0.0153(7) 0.0193(8) 0.0155(8) 0.0183(8) 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 close structural resemblance to the herein described selenoborates is present. zaac.wiley-vch.de
Z. Anorg. Allg. Chem. 2004, 630, 2299⫺2303
Na8[B12(BSe3)6]: A Novel Selenoborato-closo-dodecaborate ˚ with standard deviaTable 3 B ⫺ B and B ⫺ Se bond lengths /A tions for Na8[B12(BSe3)6] B1 B1 B1 B1 B1 B3 B3 B3 B3 B5 B5 B5 B5 B7 B7 B7 B7 a)
Fig. 1 [B12(BSe3)6]8- anion in Na8[B18Se18], symmetry equivalent atoms are not labelled
⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺
B2 B3 B5 B6 Se7 B2a B6a B7a Se5 B2a B6 B7 Se1 B1a B2a B3a Se2
The significant stability of the B12 icosahedron is based on the ability of boron to form multicentre bonds. Wade⬘s rules [40] apply for the [B18Se18]8⫺ entities as well: with six negative charges located on the terminal selenium atoms, two negative charges remain on the cluster core giving 2n⫹2 binding electrons per B12 moiety. In the novel sodium selenoborato-closo-dodecaborate Na8[B18Se18] isolated B12 cluster entities saturated with BSe3-ligands occur as depicted in Fig. 1 (Figs. 1⫺3 were created using the DIAMOND program system [41]). Due to the perpendicular arrangement of the bidentate BSe3ligands the [B18Se18]8⫺ entities show nearly Cmmm symmetry as becomes clear from the projection of the unit cell along [100] given in Fig. 2. The monomeric [B12(BSe3)6]8⫺ unit exhibits two types of endocyclic 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
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B2 B2 B2 B2 B2 B4 B4 B4 B6 B6 B6 B8 B8 B8 B9 B9 B9
⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺
B3a B5a B6 B7a Se4 Se2 Se3 Se6 B3a B7 Se3 Se4 Se7 Se9 Se1 Se5 Se8
1.798(5) 1.766(5) 1.796(5) 1.788(5) 1.967(4) 1.964(4) 1.975(4) 1.911(4) 1.805(5) 1.772(5) 1.989(4) 1.974(4) 1.973(4) 1.927(4) 1.989(4) 1.968(4) 1.907(4)
1-x; 1-y; 1-z
˚ with standard deviations for Table 4 Na ⫺ Se bond lengths /A Na8[B12(BSe3)6] Na1 Na1 Na1 Na1 Na1 Na1
⫺ ⫺ ⫺ ⫺ ⫺ ⫺
Se2 Se4a Se4b Se6c Se8d Se9b
2.855(2) 3.977(2) 2.943(2) 2.929(2) 3.029(2) 3.085(2)
Na3 Na3 Na3 Na3 Na3 Na3 Na3
⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺
Se1c Se1d Se3c Se6 Se7c Se8d Se9e
2.863(2) 3.525(2) 3.501(2) 2.774(2) 3.538(3) 3.064(2) 3.294(2)
a) 1-x; 1-y; 1-z d) ⫺x; -0.5⫹y; 0.5-z g) x; 1.5-y; 0.5⫹z
Fig. 2 Projection of the unit cell of Na8[B18Se18] along [100]
1.764(5) 1.782(5) 1.804(5) 1.767(5) 2.021(4) 1.798(5) 1.805(5) 1.789(5) 1.987(4) 1.766(5) 1.787(5) 1.774(5) 1.997(4) 1.791(5) 1.788(5) 1.789(5) 1.991(4)
Na2 Na2 Na2 Na2 Na2 Na2 Na2
⫺ ⫺ ⫺ ⫺ ⫺ ⫺ ⫺
Se2h Se4f Se5g Se6h Se8 Se8g Se9l
3.590(2) 3.532(2) 3.519(2) 3.090(2) 2.978(2) 3.105(2) 3.126(1)
Na4 Na4 Na4 Na4 Na4 Na4
⫺ ⫺ ⫺ ⫺ ⫺ ⫺
Se3f Se5 Se6f Se7 Se7l Se9l
3.074(2) 3.971(2) 3.234(2) 2.951(2) 3.798(3) 3.039(2)
b) ⫺1⫹x; y; -1⫹z e) 1-x; -0.5⫹y; 1.5-z h) ⫺x; 1-y; 1-z
c) x; 0.5-y; -0.5⫹z f) 1-x; 0.5⫹y; 1.5-z l) 1-x; 1-y; 2-z
˚ (B2⫺Se4) to 2.021(4) A ˚ (B1⫺Se7) and from 1.967(4) A B⫺Se bonds within the trigonal planar BSe3 units vary ˚ (B4⫺Se6) to 1.989(4) A ˚ (B9⫺Se1). In confrom 1.911(4) A trast, the B⫺Se bonds to the exocyclic selenium atoms are ˚ (B8⫺Se9) and 1.908(4) significantly shorter with 1.927(4) A ˚ A (B9⫺Se8). All bond lengths are very similar to the 1D polymeric selenoborato-closo-dodecaborate Na6[B18Se17] which shows the same substitution pattern concerning the BSe3 ligands. Strikingly, the heavier homologues with potassium, rubidium and cesium exhibit different substitution patterns [26] with a lower symmetry. The B⫺B bonds in the B12 cluster entity show only slight deviations from the mean value for all distances (comput˚ ). Therefore the boron icosahedra are nearly able to 1.78 A ideal platonic solids (for an aesthetic mathematical treatment of such polyhedra in crystal structures see [42]) with ˚ . These values are a mean antipodal B···B distance of 3.38 A in very good agreement with those found for the aforementioned isolated cluster compounds (a detailed listing of B⫺Se and B⫺B bond lengths is given in Table 3). 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
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A. Hammerschmidt, M. Döch, S. Pütz, B. Krebs Acknowledgements. We would like to thank the Deutsche Forschungsgemeinschaft (SFB 458) and the Fonds der Chemischen Industrie for substantial support of this work.
References
Fig. 3 Sodium selenium coordination in Na8[B18Se18] in an ellipsoidal representation (50% probability)
Four crystallographically distinguishable sodium cations are located between the isolated anion entities. As illustrated in Fig. 3 the sodium cations are in a six-fold and seven-fold selenium coordination, respectively. Selenium atoms forming the coordination polyhedra of these Na sites ˚ (Na3 ⫺ Se6) to 3.798(3) are found at distance of 2.774(2) A ˚ A (Na4 ⫺ Se7l). A detailed listing of all sodium ⫺ selenium distances is given in Table 4. With the synthesis of Na8[B18Se18] a further building block was found in selenoborate chemistry. Since polymerization of selenoborato-closo-dodecaborates has recently proven to be possible at least in one dimension with the synthesis of Na6[B18Se17] the formation of three-dimensional networks could be possible even within the system Na/B/Se. Here the Cmmm symmetry of the anion possibly enables a connection in three dimensions without striking distortions. Further studies will aim at the two- and threedimensional linkage of [B12(BSe3)6] entities which can be seen as the basic building units of these selenoborato-closododecaborate cluster compounds. 2302
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