SYNTHESIS AND CRYSTAL STRUCTURE OF CS 3 NB 2 BR 9, A DINUCLEAR NIOBIUM CLUSTER

Transcription

1 SYNTHESIS AND CRYSTAL STRUCTURE OF CS 3 NB 2 BR 9, A DINUCLEAR NIOBIUM CLUSTER Fakhili Gulo LCSIM,UMR 6511 CNRS-Université de Rennes 1, Avenue du Général Leclerc, Rennes, France Permanent address: FKIP Universitas Sriwijaya, Palembang, Indonesia ABSTRACT SYNTHESIS AND CRYSTAL STRUCTURE OF CS 3 NB 2 BR 9, A DINUCLEAR NIOBIUM CLUSTER. The niobium bromide cluster compound, Cs 3 Nb 2 Br 9, was obtained by solid-state synthesis techniques in reaction of stoichiometric amounts of CsBr, Nb powder and NbBr 5. It crystallizes in the hexagonal system with unit cell parameters a = (2) Å, c = (7) Å, space group P6 3 /mmc, Z = 2. Its crystal structure was determined by single crystal X-ray diffraction techniques. The full-matrix least squares refinement against F 2 converged to R 1 = (Fo > 4σ(Fo)), wr 2 = (all data). The structure is based on a dinuclear cluster unit Nb 2 (µ 2 -Br i ) 3 Br a 6, separated by Cs + cation. This compound exhibits the electron counts of four in metal-metal bonding state. INTRODUCTION Designing new materials with desirable and even well controlled physical properties is a major goal of synthetic solid-state chemist and materials scientist. These materials have significant importance due to their remarkable a wide range of applications in catalysis, ionic transport and redox intercalation processes. Low dimensional material containing early transition metal provides interesting candidates for catalysis owing in part to their morphologies and electronic configuration [1]. Thus, the study of the synthesis, design and characterization of transition metal-metal bonds is one of the important research areas in contemporary inorganic chemistry [2]. A great number of the ternary phases of dinuclear cluster with simple formula A x M 2 L 9 have been synthesized and structurally characterized for many transition metals [3]. In particular, the compounds in which A is a large cation, M is a transition metal and L is halide or chalcide crystallize in the perovskite or a related structure and the composition A 3 M 2 L 9 has two-thirds of octahedral sites filled with M cations that carry a 3+ charge [4]. Several ternary halides of chromium [5], molybdenum [6], scandium [4], titan [7] and tungsten [8] were isolated by solid-state reaction. Dinuclear cluster of niobium and tantalum was normally synthesized in solution chemistry. Some examples are Nb 2 Br 6 (SC 4 H 8 ) 3 [9], Ta 2 Cl 6 (THT) 3 [10], Nb 2 Cl 6 (C 4 H 8 O) 2 (C 2 H 6 S) [11] in which organic ligands occupy one inner and two terminal positions, and (Bu 4 N)[Nb 2 Cl 7 (PMe 3 ) 2 ](BF 4 ).THF [12] where organic ligands are ordered on apical position. Recently, we began investigating a new approach to the design and preparation of materials containing niobium metal cluster. Our systematic investigation of this field leads to the synthesis of the triangular [13] and 75

2 octahedral [14,] clusters of niobium metal that have a new type of intercluster connectivity. Here, we report the synthesis and structural study of a dinuclear niobium cluster, Cs 3 Nb 2 Br 9 that crystallizes in crystal system of hexagonal with space group, P6 3 /mmc and exhibits the confacial bioctahedral structure. EXPERIMENTAL Synthesis The title compound was prepared from stoichiometric mixtures of CsBr (Polabo, 99.5 %), NbBr 5 (Ventron, 99.9 %) and Nb powder (Ventron, 99.8 %). The mixture (handled in glove box) was placed in silica tube, sealed under vacuum and heated at 700 C for three days by using a vertical furnace. The compound is obtained as a microcrystalline powder stable in ambient atmosphere. Some single crystals suitable for structure determination were obtained during the synthesis. They grow in the form of black hexagon but a brown powder is obtained when they are ground. The composition of the crystal was determined by energy dispersive X-ray spectrometry (EDS) that indicated the presence of cesium, niobium and bromine atoms in ratio very close to 3:2:9. The X-ray diffraction pattern of the final product powder corresponds to the theoretical one calculated from structural data. Data collection A suitable single crystal of Cs 3 Nb 2 Br 9 has been used for structure determination by X-ray diffraction techniques. Data collection was done at room temperature by a Nonius KappaCCD diffractometer (Centre de Diffractomètrie, Université de Rennes 1, France). Strategy of data collection has been performed with help of the COLLECT program [15] to measure all Bragg reflections inside the full sphere until θ = 35 with using a crystal - detector distances of 25 mm. A total frames of 262 were recorded using Φ =1.2 and ω = 1.2 rotation scans to fill the asymmetric unit cell with exposition time of 20 sec./. Finally, reflections have been indexed, Lorentz-polarization corrected and integrated in hexagonal symmetry by DENZO program from the KappaCCD software package [16]. Frames scaling and merging of the equivalent, redundant and Friedel reflections in the P3 point group were performed using the SCALEPACK program [16] leading to 2647 reflections collected that used then for structural solution. Structure determination The structure was solved in the centrosymmetric space group, P6 3 /mmc that is in good agreement with the observed condition limiting the possible reflections using direct methods [17]. The refinement was carried out by fullmatrix least squares methods and Fourier syntheses against F 2 with the aid of 76

3 SHELXL-97 program [18]. The atoms have been refined with anisotropic displacement factors. All of them occupy fully their crystallographic sites. The largest residual difference Fourier peak and hole are close to the cesium and niobium atom positions. The highest peak is e.å -3 at 0.06 Å from Cs2 and the deepest hole is e.å -3 at 0.67 Å from Nb. Crystal data, experimental condition for intensity data collection and structure refinement are summarized in Table 1. Atomic positions and equivalent isotropic displacement factors are reported in Table 2, while anisotropic displacement parameters are given in Table 3. Important interatomic distances and angles are collected in Table 4. RESULTS AND DISCUSSION The compound Cs 3 Nb 2 Br 9 crystallizes in a hexagonal crystal system with space group of P6 3 /mmc built up from diniobium cluster anions, which its configuration represented on Figure 1. The Nb 2 cluster is surrounded by nine bromide ligands, three on inner position and six on outer position, to complete the common unit usually present in the [Nb 2 L 9 ] n- structure type [3]. It is the face-sharing bioctahedral diniobium cluster containing only halogen bridges. In this cluster unit, the Nb 2 cluster is formed by one crystallographic niobium atom on 4f position with point symmetry, 3m in the octahedral cavities of ligands. Niobium metal has coordination sphere of six bromine ligands. Apart from the three bridging bromine atoms (Br i ), each niobium metal is further coordinated terminally by three bromine atoms (Br a ). Thus, the cluster unit of the compound can be written according to the formula: [Nb 2 (µ 2 -Br i ) 3 Br a 6] 3-. This motif is similar to those found in Cs 3 Cr 2 Br 9 and Cs 3 Mo 2 Br 9 [6] in which chromium and molybdenum metals are replaced by niobium metal. In Cs 3 Nb 2 Br 9, the oxidation state of the niobium metal is +3. This cluster exhibits the valence electron concentration of four and therefore a metal-metal bond, formally of bond orders two, must exist in this compound. The Nb-Nb distance, (16) Å is quite comparable to the metal-metal doubly-bonded distances that correspond to the electron counts of four in metal-metal bonding state as found in other face-sharing bioctahedral diniobium derivatives. The effect of the number of electrons formally entering into metal-metal bonding in confacial bioctahedral structures has been studied for some transition metals [19,20]. This study shown that decrease in the electron counts of cluster should leads to longer metal-metal distance. The Nb-Nb bond in Cs 3 Nb 2 Br 9 is slightly longer than that in Nb 2 Br 6 (SC 4 H 8 ) 3 [9]. This fact is due to the presence of bridging (SC 4 H 8 ) group in place of bridging Br atoms and also the presence of Cs + ions in Cs 3 Nb 2 Br 9 while it is absent in Nb 2 Br 6 (SC 4 H 8 ) 3. It appears that metal-metal distance is more responsive to the nature of the ligands, which play a 77

4 significant role in the formation of molecular orbitals. The influence of the presence of cation in the structure on the metal-metal distance has been studied by R. Stanger et al. [19] for A 3 Mo 2 Cl 9 (A = K, Rb, Cs) and A 3 W 2 Cl 9. Their study shows that Mo-Mo and W-W separations depend on the size of the univalent cation (A). A comparison of known structures of A 3 W 2 Cl 9 (A = K, W-W = 2.41 Å; A = Cs, W-W = 2.50 Å) shows that variation in the W-W distance with the A cation size can occur even for apparently strongly interacting metal centers. The halogen ligands are formed by two crystallographic bromine atoms. Three bridging bromine atoms (Br2) are at the h position in mm point symmetry, using the Wyckoff notation while three terminal bromine atoms (Br1) are at the k position in m point symmetry of the space group. This compound has the Nb-bridging bromine distance of (7) Å and the Nbterminal bromine distance of (7) Å. It is of some interest to note that the distance between the niobium atoms and the bridging bromine atoms is approximately the sum of their ionic radii. Here, we can see that bond length between the Nb atom and the terminal Br is shorter than that of the Nb atom and the bridging Br, which is common in face-sharing bioctahedral cluster of transition metals [2]. In contrast, the distance of metal - bridging ligand is shorter than that of metal - terminal ligand in trinuclear [21] and hexanuclear [22] clusters. Finally, the Nb-Br i -Nb angle 62.57(3) is 8.0 less than that of an ideal face-sharing bioctahedral structure (70.5 ), which indicates the existence of compression effect caused by metal-metal bonding interaction. In the crystal structure of Cs 3 Nb 2 Br 9, the cesium atoms occupy two differently crystallographic sites, the b and f positions with point symmetry, 6-m2 and 3m respectively of the Wyckoff notation. The clusters do not directly connect to each other but they are interlinked by cesium cation (Figure 2). Each cesium is coordinated by twelve bromine atoms within 4 Å (Figure 3). Environment of the Cs1 is occupied by six Br1 and six Br2 while the Cs2 is coordinated by nine Br1 and three Br2. The Cs1 is bonded to six bridging bromines (Br1) and six terminal bromines (Br2) belonging to three different clusters while the Cs2 is surrounded by three inner - and nine apical - bromine ligands of four neighboring cluster units. The bond length of the Cs1-terminal bromine atoms (Br1) and Cs1 - bridging bromine atoms (Br2) are (6) Å and (5) Å respectively. The Cs2 - Br1 distances are (9) Å and (1) Å while the Cs2-Br2 distance is (7) Å (see Table 4). The average of the Cs-Br distance of Å is approximately the sum of their ionic radii (3.84 Å) [23]. ACKNOWLEDGEMENTS The author would like to thank Dr. Christiane Perrin, Université de Rennes 1, France, for helpful discussion and Dr. T. Roisnel, Centre de 78

5 Diffractométrie de l'université de Rennes 1, France, for the data collection on the Nonius KappaCCD X-ray diffractometer. REFERENCES 1. G.J. MILLER, J. Alloys Comp. 229 (1995) F.A. COTTON and R.A. WALTON, "Multiple Bonds between Metal Atoms", 2 nd ed, Clarendon Press, Oxford (1993) 3. Z. LIN and M.F. FAN, in D.M.P. MINGOS (ed.), "Structural Electronic Paradigms in Cluster Chemistry", Springer-Verlag, Berlin (1997) K.R. POEPPELMEIER, J.D. CORBETT, T.P. McMULLEN, D.R. TORGESON, and R.G. BARNES, Inorg; Chem. 19. (1980) G.J. WESSEL and D.J.W. IDJO, Acta Cryst., 10 (1957) R. SAILLANT, R.B. JACKSON, W.E. STREIB, K. FOLTING, and R.A.D. WENTWORTH, Inorg. Chem., 10 (1971) B. BRIAT, O. KAHN, I.M. BADARAU, and J.C. RIVOAL, Inorg. Chem., 20 (1981) W.H. WATSON, Jr. and J. WASER, Acta Cryst., 11 (1958) J.L. TEMPLETON, W.C. DORMAN, J.C. CLARDY, and R.E. McCARLAY, Inorg. Chem., 5 (1978) F.A. COTTON and R.C. NAJJAR, Inorg. Chem., 20 (1981), F.A. COTTON, S.A. DURAJ, and W.J. ROTH, Acta Cryst., C41 (1985) F.A. COTTON and M. SHANG, Inorg. Chem., 32 (1993) F. GULO and C. PERRIN, Mater. Res. Bull., 35 (2000) F. GULO and C. PERRIN, J. Mater. Chem., 10 (2000) COLLECT: "KappaCCD software", Nonius BV, Delft, The Netherlands (1998) 16. Z. OTWINOWSKI and W. MINOR, in C.W. Carter, Jr., R.M. Sweet (eds), "Methods in Enzymology", New York, Academic Press, 276 (1997) G.M. SHEDRICK, SHELXS-97, "Program for Solution of Crystal Structures", Gottingen (1990) 18. G.M. SHEDRICK, SHELXL-97, "Program for Refinement of Crystal Structures", Gottingen (1997) 79

6 19. R. STRANGER, S.A. MACGREGOR, T. LOVELL, J.E. McGRADY, and G.A. HEATH, J. Chem., Dalton Trans. (1996) J.E. McGRADY, T. LOVELL, and R. STRANGER, Inorg. Chem., 36 (1997) F. GULO and C. PERRIN, "Procceding The 4 th Indonesian Students Scientific Meeting 1999", Kassel, Germany (1999) S. CORDIER, F. GULO, and C. PERRIN, Solid State Sci., 1 (1999) R.D. SHANON, Acta Cryst., A32 (1975)

7 Table 1. Crystal data and structure refinement for Cs 3 Nb 2 Br 9. Empirical formula Cs 3 Nb 2 Br 9 Formula weight Temperature 293(2) K Wavelength Å Crystal system Hexagonal Space group P6 3 /mmc (No.194) Unit cell dimensions a = (2) Å c = (7) Å Volume (5) Å 3 Z 2 Density (calculated) g.cm -3 Absorption coefficient mm -1 Crystal size 0.10 x 0.7 x 0.05 mm 3 Theta range for data collection 3.05 to deg. Index ranges 0 h 12; -10 k 0; -29 l 29 Reflections collected 2647 Independent reflections 843 (R int = ) Reflections observed (>2σ) 653 Refinement method Full-matrix least-squares on F 2 Parameters 20 Goodness-of-fit Final R indices [I>2σ(I)] R 1 = ; wr 2 = R indices (all data) R 1 = ; wr 2 = Largest diff. peak and hole and e.å -3 81

8 Table 2. Atomic coordinates and equivalent isotropic displacement parameters (Å 2 ) for Cs 3 Nb 2 Br 9. Atom Wyckoff position x y z U(eq) a Cs1 2b 0 0 ¼ 0.032(1) Cs2 Nb 4f 4f 1 / 3 2 / (1) 0.037(1) 1 / 3 2 / (1) 0.021(1) Br1 24l (1) (1) (1) 0.031(1) Br2 12i (1) (1) ¼ 0.028(1) a U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. Table 3. Anisotropic displacement parameters (Å 2 ) for Cs 3 Nb 2 Br 9. Atom U11 U22 U33 U23 U13 U12 Cs (1) 0.032(1) 0.032(1) (1) Cs (1) 0.041(1) 0.030(1) (1) Nb 0.022(1) 0.022(1) 0.020(1) (1) Br (1) 0.026(1) 0.028(1) (1) (1) 0.013(1) Br (1) 0.035(1) 0.025(1) (1) Table 4. Important bonds lengths (Å) and angles ( ) for Cs 3 Nb 2 Br 9. Nb-Nb (16) Nb-Br2-Nb 62.57(3) Nb-Br (7) Br2-Nb-Br (2) Nb-Br (7) Br1-Nb-Br (3) Cs1-Br (6) Br1-Nb-Br (11) Cs1-Br (6) Cs1-Br1-Nb 94.16(2) Cs2-Br (9) Cs1-Br2-Nb (9) Cs2-Br (14) Cs2-Br1-Nb 89.71(2) Cs2-Br (7) Cs2-Br2-Nb (16) 82

9 Br a Br a Nb Nb Br i Figure 1. The cluster unit in C S3 Nb 2 B r9. Figure 2. The unit cell of C S3 Nb 2 B r9. 83

10 Cs1 Cs2 Figure 3. The cesium environment. 84