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Short Note

Crystal Structure of Na3MoCl6

Department für Chemie, Universität zu Köln, Greinstraße 6, D-50939 Köln, Germany
*
Author to whom correspondence should be addressed.
Crystals 2011, 1(3), 99-103; https://doi.org/10.3390/cryst1030099
Submission received: 10 May 2011 / Revised: 25 May 2011 / Accepted: 23 June 2011 / Published: 4 July 2011
(This article belongs to the Special Issue Feature Paper)

Abstract

: The ternary chloride Na3MoCl6 is obtained as red crystals from a disproportionation reaction of molybdenum dichloride, {Mo6}Cl12, in an acidic NaCl/AlCl3 melt at 350 °C. The crystal structure (trigonal, P-31c, a = 687.1(1), c = 1225.3(2) pm, Z = 2, V = 501,0(1) 106 pm3) is that of Na3CrCl6: within a hexagonal closest-packing of chloride ions two thirds of the octahedral voids are filled between the AB double layers with Na+/Mo3+, and between the BA layers with Na+.

1. Introduction

In their lower oxidation states, the early transition metals of the fourth and fifth periods tend to form metal clusters {Mx} for two reasons. One, 4d and 5d orbitals are larger than 3d orbitals and are, thus, capable of forming metal-metal bonds. Two, the sublimation enthalpies of the metals are high; part of it is saved when metal clusters are retained. The virtually simple binary halide MoCl2, obtained by a synproportionation reaction, features a crystal structure [1,2] which contains octahedral molybdenum clusters {Mo6} which are surrounded by eight inner (i) and six outer (a) chloride ligands; four of the latter bridge to neighboring clusters producing a layer structure, according to the Niggli formulation, {Mo6}Cli8Cla2Cla-a4/2.

2 MoCl 5 + 3 Mo 5 MoCl 2

In attempts to synthesize ternary chlorides containing the [{Mo6}Cl14]2- cluster-complex anion in a molten-salt system, MoCl2 faced a disproportionation reaction and red crystals of Na3[MoCl6] were obtained.

2. Results and Discussion

Red single crystals of Na3MoCl6 were obtained from the attempted dissolution of MoCl2 = {Mo6}Cli8Cla2Cla-a4/2 in a NaCl/AlCl3 flux (45:55 mol%, close to the eutectic [3]) at 350 °C in a sealed Pyrex ampoule. In this melt the {Mo6} cluster must have been disrupted during a disproportionation reaction, under the influence of the acidic flux. Hexagonal red crystals were embedded in essentially white crystalline material (Figure 1); some black powder (molybdenum) could also be recognized.

{ Mo 6 } Cl 12 + 12 NaCl ( from xs . NaCl / AlCl 3 ) 4 Na 3 [ MoCl 6 ] + 2 Mo

Na3MoCl6 crystallizes with the Na3CrCl6 type of structure [4], trigonal, space group P-31c (No. 163) with a = 687.1(1), c = 1225.3(2) pm, Z = 2. Previous data from powder diffraction, a = 692.0(8), c = 1222.2(5) pm, agree sufficiently well. It was also assumed that Na3MoCl6 and Na3VCl6 are isotypic with Na3CrCl6 [4]. Other hints at the existence of Na3MoCl6 are from preparative investigations or from phase diagram determinations where the crystal structure was apparently of no concern [5,6].

The structure of Na3MoCl6 consists of hexagonally closest-packed layers of chloride anions, 4H–…B | ABAB | A…. Octahedral voids between these layers are filled in a way that half of them are filled between double layers BA by Na+ cations, and half of the voids are filled by Na+ and Mo3+ in an ordered fashion between double layers AB, see Figure 2. Thus, chains of face-sharing octahedra run parallel [001] and are filled with Na+, Cr3+, Na+, □, where □ denominates a void. Neighboring chains are displaced by ½c in the [001] direction. Therefore, the Mo3+–Mo3+ distance is 729.9(1) pm. In the triple octahedron (Cl)3Na+(Cl)3Mo3+(Cl)3Na+(Cl)3, Mo3+ resides in a perfect octahedron when distances are concerned, 245.2(1) pm, 6×, but the octahedron is somewhat compressed along the −3 axis giving rise to Cl-Mo-Cl angles of 88.71(3)° and 93.75(3)°, respectively. The Na+ ions are, however displaced from the octahedral center with Na+-Cl distances of 274.8(1) to 291.4(2) pm, 3× each. The Cr3+–Cl distances in Na3CrCl6 are with 235.3(2) pm 10 pm smaller, roughly in accord with Shannon's ionic radii for Cr3+ (CN 6, 62 pm) and Mo3+ (CN 6, 69 pm) [7].

It is interesting to note that the Na3CrCl6 type of structure is only adopted with M = V, Cr, Mo, wheras the lighter and larger M = Sc, Ti, Y [8-11] as well as the lanthanides R = Dy−Lu [8,12,13] adopt the cryolite type of structure, Figure 3. The cryolite type of structure (Na3AlF6 type) is a monoclinic structure in which Na+ and F in a 1:3 ratio form layers between which octahedral voids are occupied by Na+ and Al3+. The Na3GdCl6 structure, on the other hand, is a stuffed LiSbF6 type structure [14] in which Cl ions form, again, a hexagonal closest packing and Na+ and Gd3+ occupy octahedral voids. One Na+ and Gd3+ center rather regular octahedra, the remaining two Na+ are statistically distributed over the remaining four octahedral voids. There is a close relationship between the cryolite and the Na3GdCl6 type [11]; Na3GdCl6, for example, undergoes a reversible first-order phase transition from Na3GdCl6-I (stuffed LiSbF6) to Na3GdCl6-II (cryolite type) at 205 °C [8].

3. Experimental Section

All reactions and handling were carried out under a dry nitrogen atmosphere using dry box equipment (MBraun, Garching, Germany). MoCl2 was prepared by synproportionation of Mo (Chempur, Karlsruhe, Germany, 99.95%) and MoCl5 (Sigma-Aldrich, Munchen, Germany, 99.99%) in a 3:2 molar ratio with a slight excess of MoCl5. MoCl2 was filled into a Pyrex ampoule together with an excess AlCl3 (Sigma-Aldrich, München, Germany, 99.99%) / NaCl (Chempur, Karlsruhe, Germany, 99.99%) flux, 55:45 mol%. The Pyrex ampoule was sealed under reduced pressure. The following temperature program was applied in a tubular furnace: heated to 623 K with 20 K/h, kept at that temperature for 3 days, then cooled slowly to 298 K (2 K/h). The Pyrex tube was transferred to a dry-box and the contents inspected with the aid of a microscope.

Na3MoCl6 forms well-faceted, polygonal red crystals. Some of these were selected under a microscope and sealed in thin-walled glass capillaries. After their quality had been checked by Laue diffraction patterns, the single crystals were transferred to a single-crystal X-ray diffractometer (Stoe Image Plate Diffraction System, IPDS I) to collect a complete intensity data set at ambient temperature. Structure solution and refinement was performed with the programs SHELXS-97 (direct methods) [15] and SHELXL-97 [16], scattering factors were from International Tables for X-ray Crystallography [17]. Data corrections were carried out for Lorentz and polarization factors and absorption (numerical with the aid of the programs X-RED [18] and X-SHAPE [19]). Further details of the crystal structure determination may be obtained from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247-808-666; e-mail: [email protected]), on quoting the depository number ICSD-422981, the authors and the journal citation.

Crystal data for Na3MoCl6 (377.64 g mol−1); diffractometer IPDS-I, Stoe, Darmstadt; Mo-Kα (graphite monochromator, λ = 71.073 pm); T = 293(2) K; 2θmax = 56.3°; 100 images, 0° ≤ φ ≤ 200°; Δφ = 2°; indices: −9 ≤ h ≤ 9, −9 ≤ k ≤ 9, −15 ≤ l ≤ 16; transmission (min, max) = 0.0872, 0.1363; ρcalc = 2.503 g cm−3; 4490 reflection intensities measured of which 416 were symmetrically independent, Rint = 0.0543, F(000) = 354, μ = 17.76 mm−1. Trigonal, P-31c (no. 163), a = b = 687.1(1), c = 1225.3(2) pm, V = 501.0(1) × 106 pm3, Z = 2. R values: R1/wR2 for 318 reflections with [I0 > 2σ(I0)]: 0.0238/0.0671 and for all data: 0.0350/0.0706; Sall = 1.062.

4. Conclusions

Red single crystals of Na3MoCl6 were obtained from the solution of the cluster chloride {Mo6}Cl12 in a slightly acidic NaCl/AlCl3 melt at 350 °C upon cooling. The crystal structure was first observed for Na3CrCl6; in a hexagonal closest-packing of chloride spheres, half of the octahedral voids are occupied by Na+ and one sixth by Mo3+ ions such that these are 729.92(7) pm apart. Mo3+–Cl distances (245.2(1) pm) are 10 pm longer than for homologous Cr3+-Cl.

Figure 1. Red single crystals of Na3MoCl6.
Figure 1. Red single crystals of Na3MoCl6.
Crystals 01 00099f1 1024
Figure 2. Views of the crystal structure of Na3MoCl6. Left: A [1-10] projection showing the hexagonal closest packing of chloride ions (green) and the occupation of octahedral voids by sodium (yellow) and molybdenum (red) ions. Middle: A [110] projection. Right: A sequence of triple octahedra {Cl3NaCl3MoCl3NaCl3} as they appear in the [001] direction.
Figure 2. Views of the crystal structure of Na3MoCl6. Left: A [1-10] projection showing the hexagonal closest packing of chloride ions (green) and the occupation of octahedral voids by sodium (yellow) and molybdenum (red) ions. Middle: A [110] projection. Right: A sequence of triple octahedra {Cl3NaCl3MoCl3NaCl3} as they appear in the [001] direction.
Crystals 01 00099f2 1024
Figure 3. Na3MCl6 type compunds and their structures. M on a colored field denominates existence and defines the crystal structure at ambient temperature. Yellow: Na3AlF6 (cryolite) type; red: Na3CrCl6 type; green: Na3GdCl6 (stuffed LiSbF6) type.
Figure 3. Na3MCl6 type compunds and their structures. M on a colored field denominates existence and defines the crystal structure at ambient temperature. Yellow: Na3AlF6 (cryolite) type; red: Na3CrCl6 type; green: Na3GdCl6 (stuffed LiSbF6) type.
Crystals 01 00099f3 1024

Acknowledgments

This work was supported by the Deutsche Forschungsgemeinschaft, Bonn, within the framework of the Sonderforschungsbereich 608 (Complex transition metal compounds with spin and charge degrees of freedom and disorder).

References

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MDPI and ACS Style

Beran, M.; Meyer, G. Crystal Structure of Na3MoCl6. Crystals 2011, 1, 99-103. https://doi.org/10.3390/cryst1030099

AMA Style

Beran M, Meyer G. Crystal Structure of Na3MoCl6. Crystals. 2011; 1(3):99-103. https://doi.org/10.3390/cryst1030099

Chicago/Turabian Style

Beran, Martin, and Gerd Meyer. 2011. "Crystal Structure of Na3MoCl6" Crystals 1, no. 3: 99-103. https://doi.org/10.3390/cryst1030099

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