Redetermination of the crystal structure of NbF4

We present the first single-crystal X-ray structure analysis of NbF4 and compare some structural details with those obtained from previous powder X-ray diffraction studies. NbF4 crystallizes in the SnF4 structure type.


Chemical context
The first synthesis of niobium tetrafluoride was reported by Schä fer and co-workers by reduction of niobium pentafluoride with niobium metal (Schä fer et al., 1964). According to Gortsema and coworker, a reduction of NbF 5 with silicon is seemingly the best way to obtain pure NbF 4 (Gortsema & Didchenko, 1965). The obtained products were reported as dark-blue or black powders, respectively (Gortsema & Didchenko, 1965, Schä fer et al., 1964. However, we obtained green NbF 4 single crystals among a green powder. NbF 4 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 (Schä fer et al., 1965). The compound disproportionates under vacuum above 623 K to NbF 5 and a fluoride of which the compositions were reported as NbF 2.37 (Schä fer et al., 1965) or NbF 3 (Gortsema & Didchenko, 1965). In a sealed niobium ampoule NbF 4 disproportionates at 825 K to NbF 5 and Nb 2 F 5 (Chassaing & Bizot, 1980). Infrared spectra (Dickson, 1969), UV/Vis-spectra (Chassaing & Bizot, 1980) and powder X-ray patterns are available for NbF 4 (Gortsema & Didchenko, 1965, Schä fer et al., 1965. Magnetic measurements show that NbF 4 orders antiferromagnetic in contrast to the other niobium tetrahalides which are reported to be diamagnetic (Chassaing & Bizot, 1980).
NbF 4 crystallizes in the SnF 4 structure type (Hoppe & Dä hne, 1962;Bork & Hoppe, 1996), which has been discussed extensively and its structural relationship to the NaCl structure type (Mü ller, 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-(-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 spacegroup symmetry. The structure models based on powder diffraction data yielded 2.041 (Gortsema & Didchenko, 1965) and 2.042 Å (Schä fer et al., 1965) for these Nb-F distances. The Nb-(-F) distance is similar to the respective ones of NbF 5 [2.06 (2) and 2.07 (2) Å ; Edwards, 1964] but shorter than the respective one of Nb 2 F 5 [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 (Schä fer 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 m3m (O h ) to 4/mmm (D 4h ) symmetry is much more obvious. In comparison to the Nb-F distances (non-bridging F-atoms) of NbF 5 , 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 NbF 5 . Fig. 1 shows a section of the crystal structure displaying the coordination polyhedron around the Nb atom. As in SnF 4 , infinite layers with Niggli formula 2 1 [NbF 4/2 F 2/1 ] are present and extend parallel to the ab plane. The crystal structure is shown in Fig. 2.

Synthesis and crystallization
Niobium tetrafluoride was synthesized by heating brown Nb 2 F 5 (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 À1 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. Nb 2 F 5 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 À1 . 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 NbF 4 , Nb and an yet unidentified phase. It seems that Nb 2 F 5 disproportionates to NbF 5 and Nb, and by cooling NbF 4 is formed. This assumption is supported by the observation that high pressure 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) Àx, Ày, Àz; (ii) x, y À 1, z; (iii) Ày, x, z; (iv) Ày + 1, x, z.]

Figure 2
The crystal structure of NbF 4 presented as a polyhedron model. Displacement ellipsoids are shown at 70% probability level at 293 K.
inside the ampoule blew it up. The pressure is likely induced by gaseous NbF 5 , and the disproportionation of Nb 2 F 5 to Nb and NbF 5 is known from the literature (Schä fer et al., 1965). A selected single crystal of NbF 4 was investigated using X-ray diffraction and diffraction data measured at room temperature.

Refinement
As a starting model for the structure refinement, the atomic coordinates of the SnF 4 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.  (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
Crystal data Special details 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.