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ISSN: 2056-9890

Redetermination of the crystal structure of NbF4

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aAnorganische Chemie, Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, 35032 Marburg, Germany
*Correspondence e-mail: florian.kraus@chemie.uni-marburg.de

Edited by M. Weil, Vienna University of Technology, Austria (Received 10 July 2016; accepted 25 July 2016; online 29 July 2016)

Single crystals of NbF4, niobium(IV) tetra­fluoride, were synthesized by disproportionation of Nb2F5 at 1273 K in a sealed niobium tube, extracted and studied by single-crystal X-ray diffraction. Previous reports on the crystal structure of NbF4 were based on X-ray powder diffraction data and the observed isotypicity to SnF4 [Gortsema & Didchenko (1965[Gortsema, F. P. & Didchenko, R. (1965). Inorg. Chem. 4, 182-186.]). Inorg. Chem. 4, 182–186; Schäfer et al. (1965[Schäfer, H., von Schnering, H. G., Niehues, K.-J. & Nieder-Vahrenholz, H. G. (1965). J. Less-Common Met. 9, 95-104.]). J. Less Common Met. 9, 95–104]. The data obtained from a single-crystal X-ray diffraction study meant the atomic coordinates could now be refined as well as their anisotropic displacement parameters, leading to a significant improvement of the structural model of NbF4. In the structure, the Nb atom is octahedron-like surrounded by six F atoms of which four are bridging to other NbF6 octa­hedra, leading to a layer structure extending parallel to the ab plane.

1. Chemical context

The first synthesis of niobium tetra­fluoride was reported by Schäfer and co-workers by reduction of niobium penta­fluoride with niobium metal (Schäfer et al., 1964[Schäfer, H., Bauer, D., Beckmann, W., Gerken, R., Nieder-Vahrenholz, H.-G., Niehues, K.-J. & Scholz, H. (1964). Naturwissenschaften, 51, 241-241.]). According to Gortsema and coworker, a reduction of NbF5 with silicon is seemingly the best way to obtain pure NbF4 (Gortsema & Didchenko, 1965[Gortsema, F. P. & Didchenko, R. (1965). Inorg. Chem. 4, 182-186.]). The obtained products were reported as dark-blue or black powders, respectively (Gortsema & Didchenko, 1965[Gortsema, F. P. & Didchenko, R. (1965). Inorg. Chem. 4, 182-186.], Schäfer et al., 1964[Schäfer, H., Bauer, D., Beckmann, W., Gerken, R., Nieder-Vahrenholz, H.-G., Niehues, K.-J. & Scholz, H. (1964). Naturwissenschaften, 51, 241-241.]). However, we obtained green NbF4 single crystals among a green powder. NbF4 is moisture sensitive and deliquesces to a brown suspension. In aqueous medium a brown precipitate is formed. It is reported to be soluble in hydro­chloric acid, sulfuric acid or hydrogen fluoride (Schäfer et al., 1965[Schäfer, H., von Schnering, H. G., Niehues, K.-J. & Nieder-Vahrenholz, H. G. (1965). J. Less-Common Met. 9, 95-104.]). The compound disproportionates under vacuum above 623 K to NbF5 and a fluoride of which the compositions were reported as NbF2.37 (Schäfer et al., 1965[Schäfer, H., von Schnering, H. G., Niehues, K.-J. & Nieder-Vahrenholz, H. G. (1965). J. Less-Common Met. 9, 95-104.]) or NbF3 (Gortsema & Didchenko, 1965[Gortsema, F. P. & Didchenko, R. (1965). Inorg. Chem. 4, 182-186.]). In a sealed niobium ampoule NbF4 disproportionates at 825 K to NbF5 and Nb2F5 (Chassaing & Bizot, 1980[Chassaing, J. & Bizot, D. (1980). J. Fluor. Chem. 16, 451-459.]). Infrared spectra (Dickson, 1969[Dickson, F. E. (1969). J. Inorg. Nucl. Chem. 31, 2636-2638.]), UV/Vis-spectra (Chassaing & Bizot, 1980[Chassaing, J. & Bizot, D. (1980). J. Fluor. Chem. 16, 451-459.]) and powder X-ray patterns are available for NbF4 (Gortsema & Didchenko, 1965[Gortsema, F. P. & Didchenko, R. (1965). Inorg. Chem. 4, 182-186.], Schäfer et al., 1965[Schäfer, H., von Schnering, H. G., Niehues, K.-J. & Nieder-Vahrenholz, H. G. (1965). J. Less-Common Met. 9, 95-104.]). Magnetic measurements show that NbF4 orders anti­ferromagnetic in contrast to the other niobium tetra­halides which are reported to be diamagnetic (Chassaing & Bizot, 1980[Chassaing, J. & Bizot, D. (1980). J. Fluor. Chem. 16, 451-459.]).

2. Structural commentary

The lattice parameters obtained by our single-crystal structure determination of a = 4.0876 (5), c = 8.1351 (19) Å are in good agreement with those obtained previously from powder X-ray diffraction data recorded on film (a = 4.081, c = 8.162 Å; Gortsema & Didchenko, 1965[Gortsema, F. P. & Didchenko, R. (1965). Inorg. Chem. 4, 182-186.]; a = 4.08 (3), c = 8.16 (1) Å; Schäfer et al., 1965[Schäfer, H., von Schnering, H. G., Niehues, K.-J. & Nieder-Vahrenholz, H. G. (1965). J. Less-Common Met. 9, 95-104.]).

NbF4 crystallizes in the SnF4 structure type (Hoppe & Dähne, 1962[Hoppe, R. & Dähne, W. (1962). Naturwissenschaften, 49, 254-255.]; Bork & Hoppe, 1996[Bork, M. & Hoppe, R. (1996). Z. Anorg. Allg. Chem. 622, 1557-1563.]), which has been discussed extensively and its structural relationship to the NaCl structure type (Müller, 2013[Müller, U. (2013). In Symmetry Relationships between Crystal Structures. Oxford University Press.]) deduced. The Nb atom resides on Wyckoff position 2a (site symmetry 4/mmm) and is octahedron-like coordinated by six fluorine atoms of which four are bridging to further octa­hedra, thus corner-sharing connections are obtained. These Nb—(μ-F) distances, with the F1 atoms residing on the 4c (mmm.) position, are observed to be 2.0438 (3) Å and the Nb—F—Nb angle is 180° due to space-group symmetry. The structure models based on powder diffraction data yielded 2.041 (Gortsema & Didchenko, 1965[Gortsema, F. P. & Didchenko, R. (1965). Inorg. Chem. 4, 182-186.]) and 2.042 Å (Schäfer et al., 1965[Schäfer, H., von Schnering, H. G., Niehues, K.-J. & Nieder-Vahrenholz, H. G. (1965). J. Less-Common Met. 9, 95-104.]) for these Nb—F distances. The Nb—(μ-F) distance is similar to the respective ones of NbF5 [2.06 (2) and 2.07 (2) Å; Edwards, 1964[Edwards, A. J. (1964). J. Chem. Soc. pp. 3714-3718.]] but shorter than the respective one of Nb2F5 [2.1179 (4) Å; Knoll et al., 2006[Knoll, R., Sokolovski, J., BenHaim, Y., Shames, A. I., Goren, S. D., Shaked, H., Thépot, J.-Y., Perrin, C. & Cordier, S. (2006). Physica B, 381, 47-52.]]. Two fluorine atoms (F2, 4e, 4mm) of the title compound are not bridging and are trans arranged at the Nb atom. As expected, the non-bridging F2 atoms show shorter Nb—F distances of 1.8524 (19) Å; these values differ significantly from those of 2.0405 (Gortsema & Didchenko, 1965[Gortsema, F. P. & Didchenko, R. (1965). Inorg. Chem. 4, 182-186.]) and 2.040 Å (Schäfer et al., 1965[Schäfer, H., von Schnering, H. G., Niehues, K.-J. & Nieder-Vahrenholz, H. G. (1965). J. Less-Common Met. 9, 95-104.]). The F2 atoms are surrounded by twelve F atoms (eight symmetry-equivalent F1 and four F2 atoms) in the shape of a distorted cubocta­hedron. A `central' F2 atom is displaced by 0.24 Å from the center of this cubocta­hedron towards the Nb atom to which it is bound. Hence the expected deviation from m[\overline{3}]m (Oh) to 4/mmm (D4h) symmetry is much more obvious. In comparison to the Nb—F distances (non-bridging F-atoms) of NbF5, which are reported to be 1.75 (5) and 1.78 (5) Å (Edwards, 1964[Edwards, A. J. (1964). J. Chem. Soc. pp. 3714-3718.]), an elongation is observed. This is attributed to the higher oxidation state of the Nb atom in NbF5. Fig. 1[link] shows a section of the crystal structure displaying the coordination polyhedron around the Nb atom. As in SnF4, infinite layers with Niggli formula 2[NbF4/2F2/1] are present and extend parallel to the ab plane. The crystal structure is shown in Fig. 2[link].

[Figure 1]
Figure 1
A section of the crystal structure of the title compound displaying the coordination polyhedron around the Nb atom. Displacement ellipsoids are shown at the 70% probability level at 293 K. [Symmetry codes: (i) −x, −y, −z; (ii) x, y − 1, z; (iii) −y, x, z; (iv) −y + 1, x, z.]
[Figure 2]
Figure 2
The crystal structure of NbF4 presented as a polyhedron model. Displacement ellipsoids are shown at 70% probability level at 293 K.

3. Synthesis and crystallization

Niobium tetra­fluoride was synthesized by heating brown Nb2F5 (54,4 mg, 0,16 mmol) to 1273 K in a sealed niobium tube (22 mm, 4 mm i.d., 6 mm o.d.) which was placed upright in an evacuated sealed silica tube. The heating rate was 20 K h−1 and the maximum temperature was held for two days. The niobium ampoule had been charged under nitro­gen atmosphere in a glove box and sealed by arc welding. Nb2F5 was also synthesized in a niobium ampoule (33 mm, 4 mm i.d., 6 mm o.d.) starting from niobium metal and niobium penta­fluoride with a heating rate of 16 K h−1. The maximum temperature of 1073 K was held for two days. The ampoules were allowed to cool to room temperature and were opened under inert atmosphere. A powder X-ray diffraction pattern of the green product shows the reflections of NbF4, Nb and an yet unidentified phase. It seems that Nb2F5 disproportionates to NbF5 and Nb, and by cooling NbF4 is formed. This assumption is supported by the observation that high pressure inside the ampoule blew it up. The pressure is likely induced by gaseous NbF5, and the disproportionation of Nb2F5 to Nb and NbF5 is known from the literature (Schäfer et al., 1965[Schäfer, H., von Schnering, H. G., Niehues, K.-J. & Nieder-Vahrenholz, H. G. (1965). J. Less-Common Met. 9, 95-104.]). A selected single crystal of NbF4 was investigated using X-ray diffraction and diffraction data measured at room temperature.

4. Refinement

As a starting model for the structure refinement, the atomic coordinates of the SnF4 structure type were used. Crystal data, data collection and structure refinement details are summarized in Table 1[link]. One reflection (112) was omitted from the refinement as it was affected by the primary beam stop.

Table 1
Experimental details

Crystal data
Chemical formula NbF4
Mr 168.91
Crystal system, space group Tetragonal, I4/mmm
Temperature (K) 293
a, c (Å) 4.0876 (5), 8.1351 (19)
V3) 135.93 (5)
Z 2
Radiation type Mo Kα
μ (mm−1) 4.32
Crystal size (mm) 0.06 × 0.04 × 0.01
 
Data collection
Diffractometer Stoe IPDS 2T
Absorption correction Integration (X-RED32 and X-SHAPE; Stoe & Cie, 2009[Stoe & Cie (2009). X-RED32 and X-SHAPE. Stoe & Cie GmbH, Darmstadt, Germany.])
Tmin, Tmax 0.664, 0.925
No. of measured, independent and observed [I > 2σ(I)] reflections 1392, 167, 167
Rint 0.057
(sin θ/λ)max−1) 0.944
 
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.014, 0.032, 0.98
No. of reflections 167
No. of parameters 10
Δρmax, Δρmin (e Å−3) 0.69, −0.58
Computer programs: X-AREA (Stoe & Cie, 2011[Stoe & Cie (2011). X-AREA. Stoe & Cie GmbH, Darmstadt, Germany.]), X-RED32 (Stoe & Cie, 2009[Stoe & Cie (2009). X-RED32 and X-SHAPE. Stoe & Cie GmbH, Darmstadt, Germany.]), SHELXL2014 (Sheldrick, 2015[Sheldrick, G. M. (2015). Acta Cryst. C71, 3-8.]) and DIAMOND (Brandenburg, 2015[Brandenburg, K. (2015). DIAMOND. Crystal Impact GbR, Bonn, Germany.]).

Supporting information


Computing details top

Data collection: X-AREA (Stoe & Cie, 2011); cell refinement: X-AREA (Stoe & Cie, 2011); data reduction: X-RED32 (Stoe & Cie, 2009); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2015); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015).

Niobium(IV) tetrafluoride top
Crystal data top
NbF4Dx = 4.127 Mg m3
Mr = 168.91Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I4/mmmCell parameters from 2534 reflections
a = 4.0876 (5) Åθ = 5.0–42.2°
c = 8.1351 (19) ŵ = 4.32 mm1
V = 135.93 (5) Å3T = 293 K
Z = 2Plate, green
F(000) = 1540.06 × 0.04 × 0.01 mm
Data collection top
Stoe IPDS 2T
diffractometer
167 independent reflections
Radiation source: sealed X-ray tube, 12 x 0.4 mm long-fine focus167 reflections with I > 2σ(I)
Plane graphite monochromatorRint = 0.057
Detector resolution: 6.67 pixels mm-1θmax = 42.1°, θmin = 5.0°
rotation method scansh = 75
Absorption correction: integration
(X-RED32 and X-SHAPE; Stoe & Cie, 2009)
k = 77
Tmin = 0.664, Tmax = 0.925l = 1415
1392 measured reflections
Refinement top
Refinement on F2Primary atom site location: isomorphous structure methods
Least-squares matrix: fullSecondary atom site location: isomorphous structure methods
R[F2 > 2σ(F2)] = 0.014 w = 1/[σ2(Fo2) + (0.025P)2]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.032(Δ/σ)max < 0.001
S = 0.98Δρmax = 0.69 e Å3
167 reflectionsΔρmin = 0.58 e Å3
10 parametersExtinction correction: SHELXL2014 (Sheldrick, 2015), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
0 restraintsExtinction coefficient: 0.026 (5)
Special details top

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Nb0.00000.00000.00000.00798 (9)
F10.00000.50000.00000.0167 (3)
F20.00000.00000.2277 (2)0.0209 (3)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Nb0.00583 (10)0.00583 (10)0.01230 (11)0.0000.0000.000
F10.0211 (7)0.0056 (5)0.0235 (6)0.0000.0000.000
F20.0239 (5)0.0239 (5)0.0149 (5)0.0000.0000.000
Geometric parameters (Å, º) top
Nb—F2i1.8524 (19)Nb—F1iii2.0438 (3)
Nb—F21.8524 (19)Nb—F1iv2.0438 (3)
Nb—F12.0438 (3)F1—Nbv2.0438 (3)
Nb—F1ii2.0438 (3)
F2i—Nb—F2180.0F1—Nb—F1iii90.0
F2i—Nb—F190.0F1ii—Nb—F1iii90.0
F2—Nb—F190.0F2i—Nb—F1iv90.0
F2i—Nb—F1ii90.0F2—Nb—F1iv90.0
F2—Nb—F1ii90.0F1—Nb—F1iv90.0
F1—Nb—F1ii180.0F1ii—Nb—F1iv90.0
F2i—Nb—F1iii90.0F1iii—Nb—F1iv180.0
F2—Nb—F1iii90.0Nbv—F1—Nb180.0
Symmetry codes: (i) x, y, z; (ii) x, y1, z; (iii) y, x, z; (iv) y+1, x, z; (v) x, y+1, z.
 

Acknowledgements

FK thanks the DFG for his Heisenberg-Professorship, the X-ray facilities of Dr Harms for their services, Professor Dr B. Harbrecht for the kind donation of niobium metal, and Solvay for the generous donations of fluorine.

References

First citationBork, M. & Hoppe, R. (1996). Z. Anorg. Allg. Chem. 622, 1557–1563.  CrossRef CAS Web of Science Google Scholar
First citationBrandenburg, K. (2015). DIAMOND. Crystal Impact GbR, Bonn, Germany.  Google Scholar
First citationChassaing, J. & Bizot, D. (1980). J. Fluor. Chem. 16, 451–459.  CrossRef CAS Google Scholar
First citationDickson, F. E. (1969). J. Inorg. Nucl. Chem. 31, 2636–2638.  CrossRef CAS Google Scholar
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First citationHoppe, R. & Dähne, W. (1962). Naturwissenschaften, 49, 254–255.  CrossRef CAS Google Scholar
First citationKnoll, R., Sokolovski, J., BenHaim, Y., Shames, A. I., Goren, S. D., Shaked, H., Thépot, J.-Y., Perrin, C. & Cordier, S. (2006). Physica B, 381, 47–52.  CrossRef CAS Google Scholar
First citationMüller, U. (2013). In Symmetry Relationships between Crystal Structures. Oxford University Press.  Google Scholar
First citationSchäfer, H., Bauer, D., Beckmann, W., Gerken, R., Nieder-Vahrenholz, H.-G., Niehues, K.-J. & Scholz, H. (1964). Naturwissenschaften, 51, 241–241.  Google Scholar
First citationSchäfer, H., von Schnering, H. G., Niehues, K.-J. & Nieder-Vahrenholz, H. G. (1965). J. Less-Common Met. 9, 95–104.  Google Scholar
First citationSheldrick, G. M. (2015). Acta Cryst. C71, 3–8.  Web of Science CrossRef IUCr Journals Google Scholar
First citationStoe & Cie (2009). X-RED32 and X-SHAPE. Stoe & Cie GmbH, Darmstadt, Germany.  Google Scholar
First citationStoe & Cie (2011). X-AREA. Stoe & Cie GmbH, Darmstadt, Germany.  Google Scholar

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