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The crystal structure of Nb3O2Cl5, an original Nb3 cluster oxyhalide
Pergamon
Materials Research Bulletin 35 (2000) 253–262
The crystal structure of Nb3O2Cl5, an original Nb3 cluster oxyhalide Fakhili Gulo1, Christiane Perrin* Laboratoire de Chimie du Solide et Inorganique Mole´culaire, UMR 6511, Universite´ de Rennes 1, Avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France (Refereed) Received 6 April 1999; accepted 6 April 1999
Abstract
The new Nb3O2Cl5 oxychloride was synthesized at 700°C in a sealed silica tube from a stoichiometric mixture of NbCl5, Nb2O5, and Nb. The crystal structure of this new compound was determined by single crystal X-ray diffraction (Pnnm, a ⫽ 8.060(2), b ⫽ 14.496(3), c ⫽ 6.695(2) Å, V ⫽ 782.2(4) Å3, Z ⫽ 4; dcalc ⫽ 4.14 g/cm3, R ⫽ 0.036, R ⫽ 0.047). It consists of [Nb3(3-Cli)(2-Cli)(3-Oi–a)2/2(3-Oa–i)2/2(2-Cla–a)4/2(3-Cla–a–a)3/3] units, in which the Nb3 triangle is face-capped by one chlorine atom and edge-capped by one chlorine and two oxygen atoms. In addition, each of the two oxygens is linked to an adjacent Nb3 cluster, while Cla–a and Cla–a–a bridge two and three Nb3 clusters, respectively. The new Oi–a ligand gives relatively short Nb–Nb intercluster distances (3.50 Å). The linkages between the clusters lead to zigzag chains along two directions of the space, building layers bridged together by chlorine atoms. In this compound, six valence electrons remain for the Nb–Nb bonding states, a good agreement with previous molecular orbital calculations performed on Nb3 cluster compounds. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic compounds; B. Chemical synthesis; C. X-ray diffraction; D. Crystal structure
* Corresponding author. Tel.: ⫹33-2-99-28-62-53; fax: ⫹33-2-99-63-57-04. E-mail address: [email protected] (C. Perrin). 1 Permanent address: FKIP Universitas Sriwijaya, Kampus-Inderalaya, Palembang, Indonesia. 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 2 0 0 - 2
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1. Introduction In solid state chemistry, the field of both niobium cluster halides and niobium cluster oxides has been well developed for some time. Combining halogen and oxygen in niobium based clusters could yield new structure types with novel chemical and physical properties [1]. Recently, the first families of ternary and quaternary Nb6 oxyhalides were isolated namely, MRENb6X17O [2], M2RENb6X15O3 [3], RENb6X13O3 [4] (M ⫽ monovalent cation, RE ⫽ rare earth metal, X ⫽ Cl or Br), and Ti2Nb6Cl14O4 [5]. No pseudo-binary Nb6 oxyhalide has been obtained up to now. The chemistry of triangular Nb3 cluster compounds has been extensively developed in recent years by solid state and solution routes. These developments cover a wide field of cluster chemistry [6 – 8]. The binary halides Nb3X8 (X ⫽ Cl, Br, I) obtained by solid-state synthesis have been well known for a long time [9,10]. They are based on (Nb3X13) units linked together by sharing X ligands. In this unit, the Nb3 triangular cluster is face-capped by one halogen atom (3-Xi) and edge-capped by three other halogen atoms (2-Xi), while three additional halogens coordinated to each apex of the Nb3 triangle link together adjacent clusters to form layers. Various arrangements of the latter layers give different structure types. More recently, Nb3YX7 (Y ⫽ chalcogen, X ⫽ halogen) [11–13] pseudo-binary and ANb3YX7 (A ⫽ Rb, Cs, X ⫽ Cl, Br and Y ⫽ S, Se) [14] pseudo-ternary chalcohalides based on the same (Nb3L13) (L ⫽ ligand) units have been isolated. From molecular orbital calculations, such Nb3 cluster compounds should exist with six, seven, and eight electrons in the metal–metal bonding states [15–18]. Some examples are Nb3SBr7 [13], Nb3SI7 [12], and the organo-mineral (PEt3H)[Nb3Cl10(PEt3)3] [19] with six electrons per Nb3 cluster, Nb3Cl8 [9], Nb3Br8, Nb3I8 [10], and CsNb3SBr7 [14] with seven electrons per cluster, and Nb3Cl7(PMe2Ph)6 [19] with eight electrons per cluster. Until now, no pseudo-binary or ternary (Nb3L13) based oxyhalide has been obtained by solid state routes. In this paper we present the synthesis and the crystal structure of the novel Nb3O2Cl5 oxyhalide, which exhibits a new Nb3 cluster stacking. This original structure type is due to the presence of the Oi–a ligand, bridging the edge of the Nb3 cluster and the apex of an adjacent one, which is encountered for the first time in this type of (Nb3L13)-based compounds.
2. Experimental 2.1. Synthesis of Nb3O2Cl5 All starting materials were handled under dry atmosphere in a glove box. The Nb3O2Cl5 oxychloride was synthesized from a stoichiometric mixture of NbCl5 (Ventron, purity 99.998%), Nb2O5 (Merck, Optipur), and Nb (Ventron, purity 99.8%) heated at 700°C for 48 hours in a silica tube sealed under vacuum. The X-ray powder pattern of the final product was recorded on an INEL CPS 120 diffractometer using Cu K␣1 radiation. It exhibits diffraction peaks corresponding to the new Nb3O2Cl5 oxychloride and some additional secondary phases; mainly NbOCl2 [20] and other unidentified phases. These additional phases cannot
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be completely avoided: the compound is difficult to obtain free of any impurities even if synthesis conditions are changed. The Nb3O2Cl5 is stable in ambient atmosphere. It is obtained in the form of needle-like crystals or very narrow platelets of black color, but is dark brown when finely ground. 2.2. Crystal data and structural determination of Nb3O2Cl5 Preliminary X-ray diffraction studies performed on single crystals lead to an orthorhombic unit cell. Conditions for the possible reflections are 0kl: k ⫹ l ⫽ 2n and h0l: h ⫹ l ⫽ 2n (h00: h ⫽ 2n, 0k0: k ⫽ 2n, 00l: l ⫽ 2n) which correspond to Pnn2 or Pnnm space groups. The intensities of a suitable Nb3O2Cl5 single crystal obtained during synthesis were recorded at room temperature with a Enraf-Nonius CAD-4 four-circle diffractometer using graphite monochromatized Mo K␣ radiation. The experimental conditions for data collection are summarized in Table 1. Intensity data treatment and refinement calculations were performed using the MOLEN programs from Enraf-Nonius [21] on a Digital Micro VAX 3100. The measured intensities were corrected for Lorentz and polarization effects. Semi-empirical absorption corrections were applied using ⌿SCAN from five reflections. The structure was solved by the direct method using MULTAN 11/82 [24] and successive difference Fourier synthesis. Refinements in the Pnn2 space group gave strong correlations between variables; consequently, the Pnnm space group was chosen for the structural determination. All the atoms were anisotropically refined. All the atomic positions were found to be fully occupied. The final Fourier difference map did not exhibit significant residual electron density. The details of the structural refinement and the final reliability factors are given in Table 1. The positional parameters and the equivalent thermal factors are given in Table 2. The main interatomic distances and angles are summarized in Table 3. Additional materials, anisotropic thermal parameters and observed and calculated structure factors can be obtained from the authors on request.
3. Results Nb3O2Cl5 crystallizes in an original structure type based on Nb3 triangles linked to six inequivalent ligands: Cl5 capping the Nb3 triangle (3-Cli), Cl2 bridging one edge of the Nb3 cluster (2-Cli), O bridging one edge of the cluster and one Nb of an adjacent cluster (3-Oi–a and 3-Oa–i), Cl1 and Cl4, each bridging two adjacent clusters (2-Cla–a), and Cl3 bridging three adjacent clusters (3-Cla–a–a). The three-dimensional structure of this oxychloride can be described on the basis of Nb3(3-Cli)(2-Cli)(3-Oi–a)2/2(3-Oa–i)2/2(2-Cla–a)4/2(3Cla–a–a)3/3, in which i denotes edge bridging or face bridging ligands and a denotes apex bridging ligands of the cluster, and i–a, a–i, a–a, and a–a–a indicate that the corresponding ligands are shared between two or three adjacent clusters, according to the notation developed by Scha¨fer and von Schnering [25].
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Table 1 Crystal data and experimental parameters used for the intensity data collection Crystal data Formula: Nb3O2Cl5 M ⫽ 487.98 g/mol Crystal system: Orthorhombic Space group: Pnnm, No. 58 Parameters: a ⫽ 8.060(2), b ⫽ 14.496(3), c ⫽ 6.695(2) Å V ⫽ 782.2(4) Å3 Unit cell refined from 25 reflections (4° ⬍ ⬍ 15°) cal ⫽ 4.14 g 䡠 cm⫺3 Z⫽4 Crystal size: 0.02 ⫻ 0.02 ⫻ 0.14 mm3 Linear absorption factor: 57.98 cm⫺1 Data collection Temperature: 295 K Wavelength: Mo K␣ radiation Diffractometer: Enraf-Nonius CAD-4 Scan mode, scan width: -2, 1.20 ⫹ 0.35 tg Graphite monochromator 0 ⬍ h ⬍ 11, 0 ⬍ k ⬍ 20, 0 ⬍ l ⬍ 9 max ⫽ 30° ⌿Scan with 5 reflections Tmin ⫽ 0.976, Tmax ⫽ 0.999 3 standard reflections 1358 measured independent reflections Structure determination Lorentz and polarization corrections Refinement on F 854 independent reflections with I ⬎ 3(I) Refined parameters: 55 Unweighted agreement factor R ⫽ 0.036 Weighted agreement factor Rw ⫽ 0.047 w ⫽ 4F02/[2(F02) ⫹ (0.07 F02)2] S ⫽ 0.948 (⌬/)max ⬍ 0.01 (⌬)max ⫽ 1.7(3) e⫺/Å3 (⌬)min ⫽ ⫺1.4(3) e⫺/Å3 Extinction correction [22] Extinction coefficient: 2.96 ⫻ 10⫺9 Atomic scattering factors from Internal Table for X-ray Crystallography [23]
3.1. The (Nb3O4Cl9) unit As shown in the two perpendicular views displayed in Fig. 1, the Nb3 cluster is surrounded by 13 ligands to complete the common unit usually present in the (Nb3L13)n⫺ or Nb3L8 structure types. In this (Nb3O4Cl9) unit, the Nb3 cluster is formed from two inequivalent niobium atoms, Nb1 and Nb2, each of which is linked to six ligands: 4 Cl and 2 O. The average Nb–Nb distance in the Nb3 cluster is within the range of average distances observed in other compounds based on triangular niobium clusters. For example, some Nb–Nb distances previously reported include {Nb3SO3(NCS)9}6⫺ (2.763(3) Å) [26] and Nb3Cl8 (2.810 Å) [9]. However, the Nb1–Nb2 bond bridged by an oxygen atom is significantly
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Table 2 Positional parameters and equivalent isotropic displacements for Nb3O2Cl5 Atom
Position
x
y
z
Beq(Å2)
Nb1 Nb2 Cl1 Cl2 Cl3 Cl4 Cl5 O
4g 8h 4e 4g 4g 4g 4g 8h
0.0253(1) 0.23970(8) 0 0.2109(3) 0.2050(3) 0.2745(3) 0.3269(3) 0.9912(6)
0.13481(6) 0.24009(4) 0 0.3736(2) 0.3591(2) 0.1287(2) 0.1111(2) 0.2259(3)
0 0.2126(1) 0.2418(4) 0 1/2 1/2 0 0.2215(8)
0.37(1) 0.39(1) 0.80(4) 0.74(4) 0.59(4) 0.72(4) 0.63(4) 0.41(8)
Beq ⫽ 4/3 ⌺⌺ai䡠ajBij. Standard deviations are given in parentheses.
shorter (2.709(1) Å) than the Nb2–Nb2 bond bridged by a chlorine atom (2.847(1) Å) due to steric effects related to the respective sizes of O and Cl. Similar effects have been encountered in other quaternary niobium cluster oxychlorides, for which the oxygen-capped Nb–Nb bonds are systematically shorter than the chlorine-capped bonds. For instance, in Cs2UNb6Cl15O3, the Nb–Nb bond length is 3.024 and 2.777 Å for chlorine- and oxygencapped bonds, respectively [3]. All the other Nb–Cl and Nb–O distances are in the ranges usually observed for Nb–Cl and Nb–O distances encountered in cluster oxychloride chemistry [2–5]. The Nb–Cl distances are significantly affected by the type of intra-unit linkage and by the linkages with adjacent clusters (Fig. 2). Indeed, the Nb–(3-Cli) and Nb–(2-Cli) distances are significantly shorter Table 3 Interatomic distances (Å) and angles (°) for Nb3O2Cl5 Intracluster Nb1–Nb2 Nb2–Nb2 Nb3O4Cl9 intraunit Nb1–Cl1 Nb1–Cl3 Nb1–Cl5 Nb1–Oi-a Nb2–Cl2 Nb2–Cl3 Nb2–Cl4 Nb2–Cl5 Nb2–Oi-a Nb2–Oa-i Intercluster Nb1–Nb2 Nb1–Nb1 Nb2–Nb2
2.709(1) 2⫻ 2.847(1)
Nb1–Nb2–Nb2 Nb2–Nb1–Nb2
58.20(2) 2⫻ 63.40(4)
2.546(2) 2⫻ 2.583(3) 2.455(3) 2.005(2) 2⫻ 2.414(2) 2.599(2) 2⫻ 2.527(2) 2⫻ 2.453(2) 2⫻ 2.014(5) 2⫻ 2.132(5) 2⫻
Nb2–Cl2–Nb2 Nb1–O–Nb2 Nb2–C15–Nb2 Nb1–Cl5–Nb2
72.30(8) 84.8(2) 2⫻ 70.95(8) 67.01(7) 2⫻
3.506(1) 3.930(2) 4.071(1)
Nb1–Cl3–Nb2 Nb1–O–Nb2 Nb1–Cl1–Nb1 Nb2–Cl3–Nb2 Nb2–Cl4–Nb2 Nb2–O–Nb2
Standard deviations are given in parentheses.
85.13(7) 115.1(3) 101.03(9) 95.49(9) 99.16(9) 158.1(3)
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Fig. 1. Coordination of a Nb3 cluster in two perpendicular directions.
than the Nb–(3-Cla–a–a) and Nb–(2-Cla–a) distances respectively, as is usually observed in niobium cluster chemistry. 3.2. Linkages of the adjacent (Nb3O4Cl9) units In this compound, the Nb3 clusters are linked to each other forming zigzag chains in two directions, resulting in the formation of layers. Along the a axis (Fig. 3), these chains are formed through bent Nb–O–Nb and Nb–Cl–Nb bridges between the clusters, involving Oi–a and Cla–a–a ligands (Nb2–O–Nb2 ⫽ 158.1(3)° and Nb1–O–Nb2 ⫽ 115.1(3)°), (Nb1–Cl3– Nb2 ⫽ 85.13(7)°). In these chains, the intercluster Nb1–Nb2 distance is only 3.506(1) Å. A second set of zigzag chains runs parallel to the c axis. In these chains, the clusters are linked to each other by Nb–Cl–Nb bridges involving Cla–a–a and Cla–a ligands (Nb2–Cl3–Nb2 ⫽ 95.49(9)° and Nb2–Cl4 –Nb2 ⫽ 99.16(9)°) with Nb2–Nb2 intercluster distances of 3.848(1) Å. These layers of clusters are connected along the b axis through bent Cla–a bridges
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Fig. 2. Connection between the unit and adjacent Nb3 clusters. Black, gray, and white circles represent Nb, O, and Cl atoms, respectively.
Fig. 3. Projection of the Nb3O2Cl5 structure on the (001) plane (perspective view). Black, gray, and white circles represent Nb, O, and Cl atoms, respectively.
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Fig. 4. Projection of the Nb3O2Cl5 structure on the (100) plane (perspective view). Black, gray, and white circles represent Nb, O, and Cl atoms, respectively.
(Nb1–Cl1–Nb1 ⫽ 101.03(9)°) and the intercluster Nb1–Nb1 distance in this direction is 3.930(2) Å. It must be pointed out that the needle-like single crystals grow along the c-direction, which should correspond to the direction of stronger linkage between the clusters (Fig. 4).
4. Discussion The new Nb3O2Cl5 oxychloride described in this paper is strongly related to the other (M3L13)-unit-based compounds previously reported, in which the units were either discrete or condensed by sharing some ligands. Usually, in these compounds, the ligand bridging the edge of the M3 triangle is not linked to an adjacent cluster. In contrast, in Nb3O2Cl5, the presence of such 3-Oi–a ligands, original in this M3 cluster chemistry, leads to a new Nb3 stacking. This type of 3-Li–a ligand has already been found in other Nb6 edge-capped halides and oxides, such as in Nb6Cl14 [27] and LaNb7O12 [28], but not in the Nb6 oxyhalides in which the oxygen always acts as 2-Li ligand [2–5]. In the Nb3SI7 thioiodide, a Li–a ligand is observed, but this inner ligand caps the face of the Nb3 triangle and not its edge [7]. The latter ligand is of the 4-Li–a type, also found in several M6 face-capped cluster compounds (M ⫽ Mo, W, Re) [29]. The tridimensional arrangement of the Nb3 clusters in Nb3O2Cl5 leads to relatively short Nb–Nb intercluster distances, mainly in the connection via the 3-Oi–a ligand (dNb–Nb ⫽ 3.506(1) Å). However, these distances are too long to correspond to any metal–metal bonding between the Nb3 clusters. Shorter Nb–Nb intercluster distances have been found in the CsNb3SBr7 thiobromide (dNb–Nb ⫽ 3.11(1) Å) [14]. In this compound, the Nb3 clusters
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are linked together by two additional Nb–Nb bonds to form infinite chains, which are linked together by Cs⫹ ions. Several molecular orbital calculations have been performed on the (Nb3L13)n⫺ anions and Nb3X8 layer compounds [15–18]. The highest energy levels that can be occupied are constituted of three metal–metal bonding orbitals (e, a1) and another a1 metal–metal nonbonding orbital of higher energy. There are non-negligible metal–ligand antibonding contributions at these levels. The ideal electron count per Nb3 cluster is six; however, one or two more electrons can be accommodated at the a1 level without destabilizing the structure. This is the case for the Nb3X8 binary halides, which have seven electrons per Nb3 cluster. The substitution of halogens for two oxygens should destabilize the a1 HOMO level and consequently lead to the new Nb3O2Cl5 oxychloride presented here, which is a six-electron cluster compound. Finally, the substitution of halogens for two oxygens leads to the new Nb3L7 stoichiometry versus the usual Nb3X8, which retains the most stable six-electron count per Nb3 cluster.
Acknowledgments This work was supported in part by the Fondation Langlois, which is warmly acknowledged. This work was undertaken as part of the “Proyek PGSM Dikti Depdikbud Republik Indonesia,” supported by a grant for F.G.
References [1] A. Simon, in: G. Schmid (Ed.), Clusters and Colloids: From Theory to Applications, VCH, Weinheim, 1994, pp. 374 – 458. [2] S. Cordier, C. Perrin, M. Sergent, Mater Res Bull 31 (6) (1996) 683– 690. [3] S. Cordier, C. Perrin, M. Sergent, Mater Res Bull 32 (1) (1997) 25–33. [4] S. Cordier, C. Perrin, M. Sergent, Euro J Solid State Inorg Chem 31 (11) (1994) 1049 –1060. [5] E.V. Anokhina, M.W. Essig, A. Lachgar, Angew Chem Int Ed Engl 37 (4) (1998) 522–525. [6] A. Mu¨ller, R. Jostes, F.A. Cotton, Angew Chem Int Ed Engl 19 (11) (1980) 875– 882. [7] G.J. Miller, J Alloys Compd 229 (1995) 93–106. [8] E.B. Kibala, F.A. Cotton, M. Shang, Inorg Chem 29 (26) (1990) 5148 –5156. [9] H.G. von Schnering, H. Wo¨hrle, H. Scha¨fer, Naturwissenschaften 48 (1961) 159. [10] A. Simon, H.G. von Schnering, J. Less-Common Met 11 (1966) 31– 46. [11] S. Furuseth, W. Ho¨nle, G. Miller, H.G. von Schnering, Abstracts of 9th International Conference of Transition Elements, Royal Society of Chemistry, London, 1988 (abstract). [12] G.J. Miller, J. Lin, Angew Chem Int Ed Engl 33 (3) (1994) 334 –336. [13] G.V. Khvorykh, A.V. Shevelkov, V.A. Dolgikh, B.A. Popovkin, J Solid State Chem 120 (1995) 311–315. [14] H.-J. Meyer, Z Anorg Allg Chem 620 (5) (1994) 863– 866. [15] F.A. Cotton, T.E. Haas, Inorg Chem 3 (1) (1964) 10 –17. [16] B.E. Bursten, F.A. Cotton, G.G. Stanley, Isr J Chem 19 (1– 4) (1980) 132–142. [17] B.E. Bursten, F.A. Cotton, M.B. Hall, R. Najjar, Inorg Chem 21 (1) (1982) 302–307. [18] H.-J. Meyer, Z Anorg Allg Chem 620 (1) (1994) 81– 84. [19] F.A. Cotton, M.P. Diebold, X. Feng, W.J. Roth, Inorg Chem 27 (19) (1988) 3413–3421. [20] D.V. Drobot, E.A. Pisarev, Zh Neorg Khim 29 (11) (1984) 2723–2727.
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[21] C.K. Fair, MOLEN: An Interactive Intelligent System for Crystal Structure Analysis, Enraf-Nonius, Delft, The Netherlands, 1990. [22] G. Stout, L.H. Jensen, X-ray Structure Determination, Macmillan, London, 1968. [23] International Tables for X-ray Crystallography, IV, Kynoch Press, Birmingham/D. Reidel, Dordrecht, 1995. [24] P. Main, S.J. Fiske, S.E. Hull, L. Lessinger, G. Germain, J.P. Declercq, M.M. Woolfson, MULTAN 11/82: A System of Computer Programs for the Automatic Solution of Crystal Structure Analysis, Enraf-Nonius, Delft, The Netherlands, 1990. [25] H. Scha¨fer, H.G. von Schnering, Angew Chem 20 (1964) 833– 847. [26] F.A. Cotton, M.P. Diebold, R. Llusar, W.J. Roth, J Chem Soc, Chem Commun (16) (1986) 1276 –1278. [27] A. Simon, H.G. von Schnering, H. Wo¨hrle, H. Scha¨fer, Z Anorg Allg Chem 339 (3– 4) (1965) 155–170. [28] J. Xu, T. Emge, M. Greenblatt, J Solid State Chem 123 (1996) 21–29. [29] C. Perrin, J Alloys Compd 262 (1997) 10 –21.
Materials Research Bulletin 35 (2000) 253–262
The crystal structure of Nb3O2Cl5, an original Nb3 cluster oxyhalide Fakhili Gulo1, Christiane Perrin* Laboratoire de Chimie du Solide et Inorganique Mole´culaire, UMR 6511, Universite´ de Rennes 1, Avenue du Ge´ne´ral Leclerc, 35042 Rennes Cedex, France (Refereed) Received 6 April 1999; accepted 6 April 1999
Abstract
The new Nb3O2Cl5 oxychloride was synthesized at 700°C in a sealed silica tube from a stoichiometric mixture of NbCl5, Nb2O5, and Nb. The crystal structure of this new compound was determined by single crystal X-ray diffraction (Pnnm, a ⫽ 8.060(2), b ⫽ 14.496(3), c ⫽ 6.695(2) Å, V ⫽ 782.2(4) Å3, Z ⫽ 4; dcalc ⫽ 4.14 g/cm3, R ⫽ 0.036, R ⫽ 0.047). It consists of [Nb3(3-Cli)(2-Cli)(3-Oi–a)2/2(3-Oa–i)2/2(2-Cla–a)4/2(3-Cla–a–a)3/3] units, in which the Nb3 triangle is face-capped by one chlorine atom and edge-capped by one chlorine and two oxygen atoms. In addition, each of the two oxygens is linked to an adjacent Nb3 cluster, while Cla–a and Cla–a–a bridge two and three Nb3 clusters, respectively. The new Oi–a ligand gives relatively short Nb–Nb intercluster distances (3.50 Å). The linkages between the clusters lead to zigzag chains along two directions of the space, building layers bridged together by chlorine atoms. In this compound, six valence electrons remain for the Nb–Nb bonding states, a good agreement with previous molecular orbital calculations performed on Nb3 cluster compounds. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Inorganic compounds; B. Chemical synthesis; C. X-ray diffraction; D. Crystal structure
* Corresponding author. Tel.: ⫹33-2-99-28-62-53; fax: ⫹33-2-99-63-57-04. E-mail address: [email protected] (C. Perrin). 1 Permanent address: FKIP Universitas Sriwijaya, Kampus-Inderalaya, Palembang, Indonesia. 0025-5408/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 0 ) 0 0 2 0 0 - 2
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1. Introduction In solid state chemistry, the field of both niobium cluster halides and niobium cluster oxides has been well developed for some time. Combining halogen and oxygen in niobium based clusters could yield new structure types with novel chemical and physical properties [1]. Recently, the first families of ternary and quaternary Nb6 oxyhalides were isolated namely, MRENb6X17O [2], M2RENb6X15O3 [3], RENb6X13O3 [4] (M ⫽ monovalent cation, RE ⫽ rare earth metal, X ⫽ Cl or Br), and Ti2Nb6Cl14O4 [5]. No pseudo-binary Nb6 oxyhalide has been obtained up to now. The chemistry of triangular Nb3 cluster compounds has been extensively developed in recent years by solid state and solution routes. These developments cover a wide field of cluster chemistry [6 – 8]. The binary halides Nb3X8 (X ⫽ Cl, Br, I) obtained by solid-state synthesis have been well known for a long time [9,10]. They are based on (Nb3X13) units linked together by sharing X ligands. In this unit, the Nb3 triangular cluster is face-capped by one halogen atom (3-Xi) and edge-capped by three other halogen atoms (2-Xi), while three additional halogens coordinated to each apex of the Nb3 triangle link together adjacent clusters to form layers. Various arrangements of the latter layers give different structure types. More recently, Nb3YX7 (Y ⫽ chalcogen, X ⫽ halogen) [11–13] pseudo-binary and ANb3YX7 (A ⫽ Rb, Cs, X ⫽ Cl, Br and Y ⫽ S, Se) [14] pseudo-ternary chalcohalides based on the same (Nb3L13) (L ⫽ ligand) units have been isolated. From molecular orbital calculations, such Nb3 cluster compounds should exist with six, seven, and eight electrons in the metal–metal bonding states [15–18]. Some examples are Nb3SBr7 [13], Nb3SI7 [12], and the organo-mineral (PEt3H)[Nb3Cl10(PEt3)3] [19] with six electrons per Nb3 cluster, Nb3Cl8 [9], Nb3Br8, Nb3I8 [10], and CsNb3SBr7 [14] with seven electrons per cluster, and Nb3Cl7(PMe2Ph)6 [19] with eight electrons per cluster. Until now, no pseudo-binary or ternary (Nb3L13) based oxyhalide has been obtained by solid state routes. In this paper we present the synthesis and the crystal structure of the novel Nb3O2Cl5 oxyhalide, which exhibits a new Nb3 cluster stacking. This original structure type is due to the presence of the Oi–a ligand, bridging the edge of the Nb3 cluster and the apex of an adjacent one, which is encountered for the first time in this type of (Nb3L13)-based compounds.
2. Experimental 2.1. Synthesis of Nb3O2Cl5 All starting materials were handled under dry atmosphere in a glove box. The Nb3O2Cl5 oxychloride was synthesized from a stoichiometric mixture of NbCl5 (Ventron, purity 99.998%), Nb2O5 (Merck, Optipur), and Nb (Ventron, purity 99.8%) heated at 700°C for 48 hours in a silica tube sealed under vacuum. The X-ray powder pattern of the final product was recorded on an INEL CPS 120 diffractometer using Cu K␣1 radiation. It exhibits diffraction peaks corresponding to the new Nb3O2Cl5 oxychloride and some additional secondary phases; mainly NbOCl2 [20] and other unidentified phases. These additional phases cannot
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be completely avoided: the compound is difficult to obtain free of any impurities even if synthesis conditions are changed. The Nb3O2Cl5 is stable in ambient atmosphere. It is obtained in the form of needle-like crystals or very narrow platelets of black color, but is dark brown when finely ground. 2.2. Crystal data and structural determination of Nb3O2Cl5 Preliminary X-ray diffraction studies performed on single crystals lead to an orthorhombic unit cell. Conditions for the possible reflections are 0kl: k ⫹ l ⫽ 2n and h0l: h ⫹ l ⫽ 2n (h00: h ⫽ 2n, 0k0: k ⫽ 2n, 00l: l ⫽ 2n) which correspond to Pnn2 or Pnnm space groups. The intensities of a suitable Nb3O2Cl5 single crystal obtained during synthesis were recorded at room temperature with a Enraf-Nonius CAD-4 four-circle diffractometer using graphite monochromatized Mo K␣ radiation. The experimental conditions for data collection are summarized in Table 1. Intensity data treatment and refinement calculations were performed using the MOLEN programs from Enraf-Nonius [21] on a Digital Micro VAX 3100. The measured intensities were corrected for Lorentz and polarization effects. Semi-empirical absorption corrections were applied using ⌿SCAN from five reflections. The structure was solved by the direct method using MULTAN 11/82 [24] and successive difference Fourier synthesis. Refinements in the Pnn2 space group gave strong correlations between variables; consequently, the Pnnm space group was chosen for the structural determination. All the atoms were anisotropically refined. All the atomic positions were found to be fully occupied. The final Fourier difference map did not exhibit significant residual electron density. The details of the structural refinement and the final reliability factors are given in Table 1. The positional parameters and the equivalent thermal factors are given in Table 2. The main interatomic distances and angles are summarized in Table 3. Additional materials, anisotropic thermal parameters and observed and calculated structure factors can be obtained from the authors on request.
3. Results Nb3O2Cl5 crystallizes in an original structure type based on Nb3 triangles linked to six inequivalent ligands: Cl5 capping the Nb3 triangle (3-Cli), Cl2 bridging one edge of the Nb3 cluster (2-Cli), O bridging one edge of the cluster and one Nb of an adjacent cluster (3-Oi–a and 3-Oa–i), Cl1 and Cl4, each bridging two adjacent clusters (2-Cla–a), and Cl3 bridging three adjacent clusters (3-Cla–a–a). The three-dimensional structure of this oxychloride can be described on the basis of Nb3(3-Cli)(2-Cli)(3-Oi–a)2/2(3-Oa–i)2/2(2-Cla–a)4/2(3Cla–a–a)3/3, in which i denotes edge bridging or face bridging ligands and a denotes apex bridging ligands of the cluster, and i–a, a–i, a–a, and a–a–a indicate that the corresponding ligands are shared between two or three adjacent clusters, according to the notation developed by Scha¨fer and von Schnering [25].
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Table 1 Crystal data and experimental parameters used for the intensity data collection Crystal data Formula: Nb3O2Cl5 M ⫽ 487.98 g/mol Crystal system: Orthorhombic Space group: Pnnm, No. 58 Parameters: a ⫽ 8.060(2), b ⫽ 14.496(3), c ⫽ 6.695(2) Å V ⫽ 782.2(4) Å3 Unit cell refined from 25 reflections (4° ⬍ ⬍ 15°) cal ⫽ 4.14 g 䡠 cm⫺3 Z⫽4 Crystal size: 0.02 ⫻ 0.02 ⫻ 0.14 mm3 Linear absorption factor: 57.98 cm⫺1 Data collection Temperature: 295 K Wavelength: Mo K␣ radiation Diffractometer: Enraf-Nonius CAD-4 Scan mode, scan width: -2, 1.20 ⫹ 0.35 tg Graphite monochromator 0 ⬍ h ⬍ 11, 0 ⬍ k ⬍ 20, 0 ⬍ l ⬍ 9 max ⫽ 30° ⌿Scan with 5 reflections Tmin ⫽ 0.976, Tmax ⫽ 0.999 3 standard reflections 1358 measured independent reflections Structure determination Lorentz and polarization corrections Refinement on F 854 independent reflections with I ⬎ 3(I) Refined parameters: 55 Unweighted agreement factor R ⫽ 0.036 Weighted agreement factor Rw ⫽ 0.047 w ⫽ 4F02/[2(F02) ⫹ (0.07 F02)2] S ⫽ 0.948 (⌬/)max ⬍ 0.01 (⌬)max ⫽ 1.7(3) e⫺/Å3 (⌬)min ⫽ ⫺1.4(3) e⫺/Å3 Extinction correction [22] Extinction coefficient: 2.96 ⫻ 10⫺9 Atomic scattering factors from Internal Table for X-ray Crystallography [23]
3.1. The (Nb3O4Cl9) unit As shown in the two perpendicular views displayed in Fig. 1, the Nb3 cluster is surrounded by 13 ligands to complete the common unit usually present in the (Nb3L13)n⫺ or Nb3L8 structure types. In this (Nb3O4Cl9) unit, the Nb3 cluster is formed from two inequivalent niobium atoms, Nb1 and Nb2, each of which is linked to six ligands: 4 Cl and 2 O. The average Nb–Nb distance in the Nb3 cluster is within the range of average distances observed in other compounds based on triangular niobium clusters. For example, some Nb–Nb distances previously reported include {Nb3SO3(NCS)9}6⫺ (2.763(3) Å) [26] and Nb3Cl8 (2.810 Å) [9]. However, the Nb1–Nb2 bond bridged by an oxygen atom is significantly
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Table 2 Positional parameters and equivalent isotropic displacements for Nb3O2Cl5 Atom
Position
x
y
z
Beq(Å2)
Nb1 Nb2 Cl1 Cl2 Cl3 Cl4 Cl5 O
4g 8h 4e 4g 4g 4g 4g 8h
0.0253(1) 0.23970(8) 0 0.2109(3) 0.2050(3) 0.2745(3) 0.3269(3) 0.9912(6)
0.13481(6) 0.24009(4) 0 0.3736(2) 0.3591(2) 0.1287(2) 0.1111(2) 0.2259(3)
0 0.2126(1) 0.2418(4) 0 1/2 1/2 0 0.2215(8)
0.37(1) 0.39(1) 0.80(4) 0.74(4) 0.59(4) 0.72(4) 0.63(4) 0.41(8)
Beq ⫽ 4/3 ⌺⌺ai䡠ajBij. Standard deviations are given in parentheses.
shorter (2.709(1) Å) than the Nb2–Nb2 bond bridged by a chlorine atom (2.847(1) Å) due to steric effects related to the respective sizes of O and Cl. Similar effects have been encountered in other quaternary niobium cluster oxychlorides, for which the oxygen-capped Nb–Nb bonds are systematically shorter than the chlorine-capped bonds. For instance, in Cs2UNb6Cl15O3, the Nb–Nb bond length is 3.024 and 2.777 Å for chlorine- and oxygencapped bonds, respectively [3]. All the other Nb–Cl and Nb–O distances are in the ranges usually observed for Nb–Cl and Nb–O distances encountered in cluster oxychloride chemistry [2–5]. The Nb–Cl distances are significantly affected by the type of intra-unit linkage and by the linkages with adjacent clusters (Fig. 2). Indeed, the Nb–(3-Cli) and Nb–(2-Cli) distances are significantly shorter Table 3 Interatomic distances (Å) and angles (°) for Nb3O2Cl5 Intracluster Nb1–Nb2 Nb2–Nb2 Nb3O4Cl9 intraunit Nb1–Cl1 Nb1–Cl3 Nb1–Cl5 Nb1–Oi-a Nb2–Cl2 Nb2–Cl3 Nb2–Cl4 Nb2–Cl5 Nb2–Oi-a Nb2–Oa-i Intercluster Nb1–Nb2 Nb1–Nb1 Nb2–Nb2
2.709(1) 2⫻ 2.847(1)
Nb1–Nb2–Nb2 Nb2–Nb1–Nb2
58.20(2) 2⫻ 63.40(4)
2.546(2) 2⫻ 2.583(3) 2.455(3) 2.005(2) 2⫻ 2.414(2) 2.599(2) 2⫻ 2.527(2) 2⫻ 2.453(2) 2⫻ 2.014(5) 2⫻ 2.132(5) 2⫻
Nb2–Cl2–Nb2 Nb1–O–Nb2 Nb2–C15–Nb2 Nb1–Cl5–Nb2
72.30(8) 84.8(2) 2⫻ 70.95(8) 67.01(7) 2⫻
3.506(1) 3.930(2) 4.071(1)
Nb1–Cl3–Nb2 Nb1–O–Nb2 Nb1–Cl1–Nb1 Nb2–Cl3–Nb2 Nb2–Cl4–Nb2 Nb2–O–Nb2
Standard deviations are given in parentheses.
85.13(7) 115.1(3) 101.03(9) 95.49(9) 99.16(9) 158.1(3)
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Fig. 1. Coordination of a Nb3 cluster in two perpendicular directions.
than the Nb–(3-Cla–a–a) and Nb–(2-Cla–a) distances respectively, as is usually observed in niobium cluster chemistry. 3.2. Linkages of the adjacent (Nb3O4Cl9) units In this compound, the Nb3 clusters are linked to each other forming zigzag chains in two directions, resulting in the formation of layers. Along the a axis (Fig. 3), these chains are formed through bent Nb–O–Nb and Nb–Cl–Nb bridges between the clusters, involving Oi–a and Cla–a–a ligands (Nb2–O–Nb2 ⫽ 158.1(3)° and Nb1–O–Nb2 ⫽ 115.1(3)°), (Nb1–Cl3– Nb2 ⫽ 85.13(7)°). In these chains, the intercluster Nb1–Nb2 distance is only 3.506(1) Å. A second set of zigzag chains runs parallel to the c axis. In these chains, the clusters are linked to each other by Nb–Cl–Nb bridges involving Cla–a–a and Cla–a ligands (Nb2–Cl3–Nb2 ⫽ 95.49(9)° and Nb2–Cl4 –Nb2 ⫽ 99.16(9)°) with Nb2–Nb2 intercluster distances of 3.848(1) Å. These layers of clusters are connected along the b axis through bent Cla–a bridges
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Fig. 2. Connection between the unit and adjacent Nb3 clusters. Black, gray, and white circles represent Nb, O, and Cl atoms, respectively.
Fig. 3. Projection of the Nb3O2Cl5 structure on the (001) plane (perspective view). Black, gray, and white circles represent Nb, O, and Cl atoms, respectively.
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Fig. 4. Projection of the Nb3O2Cl5 structure on the (100) plane (perspective view). Black, gray, and white circles represent Nb, O, and Cl atoms, respectively.
(Nb1–Cl1–Nb1 ⫽ 101.03(9)°) and the intercluster Nb1–Nb1 distance in this direction is 3.930(2) Å. It must be pointed out that the needle-like single crystals grow along the c-direction, which should correspond to the direction of stronger linkage between the clusters (Fig. 4).
4. Discussion The new Nb3O2Cl5 oxychloride described in this paper is strongly related to the other (M3L13)-unit-based compounds previously reported, in which the units were either discrete or condensed by sharing some ligands. Usually, in these compounds, the ligand bridging the edge of the M3 triangle is not linked to an adjacent cluster. In contrast, in Nb3O2Cl5, the presence of such 3-Oi–a ligands, original in this M3 cluster chemistry, leads to a new Nb3 stacking. This type of 3-Li–a ligand has already been found in other Nb6 edge-capped halides and oxides, such as in Nb6Cl14 [27] and LaNb7O12 [28], but not in the Nb6 oxyhalides in which the oxygen always acts as 2-Li ligand [2–5]. In the Nb3SI7 thioiodide, a Li–a ligand is observed, but this inner ligand caps the face of the Nb3 triangle and not its edge [7]. The latter ligand is of the 4-Li–a type, also found in several M6 face-capped cluster compounds (M ⫽ Mo, W, Re) [29]. The tridimensional arrangement of the Nb3 clusters in Nb3O2Cl5 leads to relatively short Nb–Nb intercluster distances, mainly in the connection via the 3-Oi–a ligand (dNb–Nb ⫽ 3.506(1) Å). However, these distances are too long to correspond to any metal–metal bonding between the Nb3 clusters. Shorter Nb–Nb intercluster distances have been found in the CsNb3SBr7 thiobromide (dNb–Nb ⫽ 3.11(1) Å) [14]. In this compound, the Nb3 clusters
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are linked together by two additional Nb–Nb bonds to form infinite chains, which are linked together by Cs⫹ ions. Several molecular orbital calculations have been performed on the (Nb3L13)n⫺ anions and Nb3X8 layer compounds [15–18]. The highest energy levels that can be occupied are constituted of three metal–metal bonding orbitals (e, a1) and another a1 metal–metal nonbonding orbital of higher energy. There are non-negligible metal–ligand antibonding contributions at these levels. The ideal electron count per Nb3 cluster is six; however, one or two more electrons can be accommodated at the a1 level without destabilizing the structure. This is the case for the Nb3X8 binary halides, which have seven electrons per Nb3 cluster. The substitution of halogens for two oxygens should destabilize the a1 HOMO level and consequently lead to the new Nb3O2Cl5 oxychloride presented here, which is a six-electron cluster compound. Finally, the substitution of halogens for two oxygens leads to the new Nb3L7 stoichiometry versus the usual Nb3X8, which retains the most stable six-electron count per Nb3 cluster.
Acknowledgments This work was supported in part by the Fondation Langlois, which is warmly acknowledged. This work was undertaken as part of the “Proyek PGSM Dikti Depdikbud Republik Indonesia,” supported by a grant for F.G.
References [1] A. Simon, in: G. Schmid (Ed.), Clusters and Colloids: From Theory to Applications, VCH, Weinheim, 1994, pp. 374 – 458. [2] S. Cordier, C. Perrin, M. Sergent, Mater Res Bull 31 (6) (1996) 683– 690. [3] S. Cordier, C. Perrin, M. Sergent, Mater Res Bull 32 (1) (1997) 25–33. [4] S. Cordier, C. Perrin, M. Sergent, Euro J Solid State Inorg Chem 31 (11) (1994) 1049 –1060. [5] E.V. Anokhina, M.W. Essig, A. Lachgar, Angew Chem Int Ed Engl 37 (4) (1998) 522–525. [6] A. Mu¨ller, R. Jostes, F.A. Cotton, Angew Chem Int Ed Engl 19 (11) (1980) 875– 882. [7] G.J. Miller, J Alloys Compd 229 (1995) 93–106. [8] E.B. Kibala, F.A. Cotton, M. Shang, Inorg Chem 29 (26) (1990) 5148 –5156. [9] H.G. von Schnering, H. Wo¨hrle, H. Scha¨fer, Naturwissenschaften 48 (1961) 159. [10] A. Simon, H.G. von Schnering, J. Less-Common Met 11 (1966) 31– 46. [11] S. Furuseth, W. Ho¨nle, G. Miller, H.G. von Schnering, Abstracts of 9th International Conference of Transition Elements, Royal Society of Chemistry, London, 1988 (abstract). [12] G.J. Miller, J. Lin, Angew Chem Int Ed Engl 33 (3) (1994) 334 –336. [13] G.V. Khvorykh, A.V. Shevelkov, V.A. Dolgikh, B.A. Popovkin, J Solid State Chem 120 (1995) 311–315. [14] H.-J. Meyer, Z Anorg Allg Chem 620 (5) (1994) 863– 866. [15] F.A. Cotton, T.E. Haas, Inorg Chem 3 (1) (1964) 10 –17. [16] B.E. Bursten, F.A. Cotton, G.G. Stanley, Isr J Chem 19 (1– 4) (1980) 132–142. [17] B.E. Bursten, F.A. Cotton, M.B. Hall, R. Najjar, Inorg Chem 21 (1) (1982) 302–307. [18] H.-J. Meyer, Z Anorg Allg Chem 620 (1) (1994) 81– 84. [19] F.A. Cotton, M.P. Diebold, X. Feng, W.J. Roth, Inorg Chem 27 (19) (1988) 3413–3421. [20] D.V. Drobot, E.A. Pisarev, Zh Neorg Khim 29 (11) (1984) 2723–2727.
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[21] C.K. Fair, MOLEN: An Interactive Intelligent System for Crystal Structure Analysis, Enraf-Nonius, Delft, The Netherlands, 1990. [22] G. Stout, L.H. Jensen, X-ray Structure Determination, Macmillan, London, 1968. [23] International Tables for X-ray Crystallography, IV, Kynoch Press, Birmingham/D. Reidel, Dordrecht, 1995. [24] P. Main, S.J. Fiske, S.E. Hull, L. Lessinger, G. Germain, J.P. Declercq, M.M. Woolfson, MULTAN 11/82: A System of Computer Programs for the Automatic Solution of Crystal Structure Analysis, Enraf-Nonius, Delft, The Netherlands, 1990. [25] H. Scha¨fer, H.G. von Schnering, Angew Chem 20 (1964) 833– 847. [26] F.A. Cotton, M.P. Diebold, R. Llusar, W.J. Roth, J Chem Soc, Chem Commun (16) (1986) 1276 –1278. [27] A. Simon, H.G. von Schnering, H. Wo¨hrle, H. Scha¨fer, Z Anorg Allg Chem 339 (3– 4) (1965) 155–170. [28] J. Xu, T. Emge, M. Greenblatt, J Solid State Chem 123 (1996) 21–29. [29] C. Perrin, J Alloys Compd 262 (1997) 10 –21.