Redetermination of the crystal structure of NbF4
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). Inorg. Chem. 4, 182–186; Schäfer et al. (1965). 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.
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). According to Gortsema and coworker, a reduction of NbF5 with silicon is seemingly the best way to obtain pure NbF4 (Gortsema & Didchenko, 1965). The obtained products were reported as dark-blue or black powders, respectively (Gortsema & Didchenko, 1965, Schäfer et al., 1964). 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). 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) or NbF3 (Gortsema & Didchenko, 1965). In a sealed niobium ampoule NbF4 disproportionates at 825 K to NbF5 and Nb2F5 (Chassaing & Bizot, 1980). Infrared spectra (Dickson, 1969), UV/Vis-spectra (Chassaing & Bizot, 1980) and powder X-ray patterns are available for NbF4 (Gortsema & Didchenko, 1965, Schäfer et al., 1965). Magnetic measurements show that NbF4 orders antiferromagnetic in contrast to the other niobium tetrahalides which are reported to be diamagnetic (Chassaing & Bizot, 1980).
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; a = 4.08 (3), c = 8.16 (1) Å; Schäfer et al., 1965).
NbF4 crystallizes in the SnF4 structure type (Hoppe & Dähne, 1962; Bork & Hoppe, 1996), which has been discussed extensively and its structural relationship to the NaCl structure type (Müller, 2013) 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) and 2.042 Å (Schäfer et al., 1965) 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] but shorter than the respective one of Nb2F5 [2.1179 (4) Å; Knoll et al., 2006]. 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) and 2.040 Å (Schäfer et al., 1965). 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 mm (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), an elongation is observed. This is attributed to the higher oxidation state of the Nb atom in NbF5. Fig. 1 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.
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). A selected single crystal of NbF4 was investigated using X-ray diffraction and diffraction data measured at room temperature.
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. One reflection (112) was omitted from the refinement as it was affected by the primary beam stop.
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).
|NbF4||Dx = 4.127 Mg m−3|
|Mr = 168.91||Mo Kα radiation, λ = 0.71073 Å|
|Tetragonal, I4/mmm||Cell parameters from 2534 reflections|
|a = 4.0876 (5) Å||θ = 5.0–42.2°|
|c = 8.1351 (19) Å||µ = 4.32 mm−1|
|V = 135.93 (5) Å3||T = 293 K|
|Z = 2||Plate, green|
|F(000) = 154||0.06 × 0.04 × 0.01 mm|
|Stoe IPDS 2T |
|167 independent reflections|
|Radiation source: sealed X-ray tube, 12 x 0.4 mm long-fine focus||167 reflections with I > 2σ(I)|
|Plane graphite monochromator||Rint = 0.057|
|Detector resolution: 6.67 pixels mm-1||θmax = 42.1°, θmin = 5.0°|
|rotation method scans||h = −7→5|
|Absorption correction: integration |
(X-RED32 and X-SHAPE; Stoe & Cie, 2009)
|k = −7→7|
|Tmin = 0.664, Tmax = 0.925||l = −14→15|
|1392 measured reflections|
|Refinement on F2||Primary atom site location: isomorphous structure methods|
|Least-squares matrix: full||Secondary 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 = 0.98||Δρmax = 0.69 e Å−3|
|167 reflections||Δρmin = −0.58 e Å−3|
|10 parameters||Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4|
|0 restraints||Extinction coefficient: 0.026 (5)|
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.
|F2||0.0000||0.0000||0.2277 (2)||0.0209 (3)|
|Nb||0.00583 (10)||0.00583 (10)||0.01230 (11)||0.000||0.000||0.000|
|F1||0.0211 (7)||0.0056 (5)||0.0235 (6)||0.000||0.000||0.000|
|F2||0.0239 (5)||0.0239 (5)||0.0149 (5)||0.000||0.000||0.000|
|Nb—F2i||1.8524 (19)||Nb—F1iii||2.0438 (3)|
|Nb—F2||1.8524 (19)||Nb—F1iv||2.0438 (3)|
|Nb—F1||2.0438 (3)||F1—Nbv||2.0438 (3)|
|Symmetry codes: (i) −x, −y, −z; (ii) x, y−1, z; (iii) −y, x, z; (iv) −y+1, x, z; (v) x, y+1, z.|
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.
Bork, M. & Hoppe, R. (1996). Z. Anorg. Allg. Chem. 622, 1557–1563. CrossRef CAS Web of Science Google Scholar
Brandenburg, K. (2015). DIAMOND. Crystal Impact GbR, Bonn, Germany. Google Scholar
Chassaing, J. & Bizot, D. (1980). J. Fluor. Chem. 16, 451–459. CrossRef CAS Google Scholar
Dickson, F. E. (1969). J. Inorg. Nucl. Chem. 31, 2636–2638. CrossRef CAS Google Scholar
Edwards, A. J. (1964). J. Chem. Soc. pp. 3714–3718. CrossRef Web of Science Google Scholar
Gortsema, F. P. & Didchenko, R. (1965). Inorg. Chem. 4, 182–186. CrossRef CAS Google Scholar
Hoppe, R. & Dähne, W. (1962). Naturwissenschaften, 49, 254–255. CrossRef CAS Google Scholar
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. CrossRef CAS Google Scholar
Müller, U. (2013). In Symmetry Relationships between Crystal Structures. Oxford University Press. Google Scholar
Schäfer, H., Bauer, D., Beckmann, W., Gerken, R., Nieder-Vahrenholz, H.-G., Niehues, K.-J. & Scholz, H. (1964). Naturwissenschaften, 51, 241–241. Google Scholar
Schäfer, H., von Schnering, H. G., Niehues, K.-J. & Nieder-Vahrenholz, H. G. (1965). J. Less-Common Met. 9, 95–104. Google Scholar
Sheldrick, G. M. (2015). Acta Cryst. C71, 3–8. Web of Science CrossRef IUCr Journals Google Scholar
Stoe & Cie (2009). X-RED32 and X-SHAPE. Stoe & Cie GmbH, Darmstadt, Germany. Google Scholar
Stoe & Cie (2011). X-AREA. Stoe & Cie GmbH, Darmstadt, Germany. Google Scholar
This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.