Synthesis and redetermination of the crystal structure of NbF5

NbF5 was synthesized in high purity by direct fluorination. IR and Raman spectroscopy confirms the high purity. The crystal structure was redetermined at 100 K with higher precision.


Chemical context
NbF 5 was first synthesized by Ruff and Schiller (Ruff, 1909;Ruff & Schiller, 1911) from the reaction of Nb metal with elemental fluorine or from the reaction of NbCl 5 with anhydrous HF.By now, several alternative ways for its synthesis have also been described in the literature (Scha ¨fer et al., 1965;O'Donnell & Peel, 1976).Niobium pentafluoride is a colorless, hygroscopic solid that melts at 352.1 K and has a boiling point of 506.5 K (Junkins et al., 1952).The vapor pressure (Junkins et al., 1952;Fairbrother & Frith, 1951), the enthalpy of fusion (Junkins et al., 1952), and the electrical conductivity (Fairbrother et al., 1954) of liquid NbF 5 have also been determined.Infrared and Raman spectra of the solid were measured (Preiss & Reich, 1968;Beattie et al., 1969) and the structure of NbF 5 in the (supercooled) liquid, glassy state and the gas phase have been investigated by Raman spectroscopy (Boghosian et al., 2005;Papatheodorou et al., 2008).In a search for a suitable laboratory synthesis of NbF 5 , we investigated several methods for its preparation.During our efforts, single crystals of several millimeters in size were obtained when hot NbF 5 re-sublimed at colder parts of our reaction setup (see Synthesis and crystallization).The former crystal structure published by Edwards (1964) is of lower precision compared to structure determinations possible nowadays and displacement parameters had not been refined anisotropically.
The single-crystal structure determination was performed at 100 K and thus resulted in smaller lattice parameters by about 1-3% compared to those determined at room temperature (see Table 1).Otherwise, there are no significant structural differences compared to the RT structure.The slight contraction of the lattice parameters is mainly due to the shortening of the distances between the Nb 4 F 20 molecules, while the intramolecular F-Nb distances determined at 100 K differ only insignificantly from those determined at room temperature.
NbF 5 crystallizes in the space group C2/m (No. 12, Pearson code mC48, Wyckoff sequence j 4 i 3 h) with the lattice parameters a = 9.4863 (12), b = 14.2969 (12), c = 4.9892 (6) A ˚, � = 97.292(10) � , Z = 8 at 100 K. NbF 5 crystallizes in the MoF 5 structure type (Edwards et al., 1962;Stene et al., 2018).The structure consists of NbF 5 units forming tetrameric molecules that can be described by the Niggli (Niggli, 1945) formula    10) and 1.8157 (11) A ˚.This phenomenon was observed to a similar extent for the structure of MoF 5 (Stene et al., 2018) and can be attributed to the structural trans effect (Coe & Glenwright, 2000;Shustorovich et al., 1975).The Nb atoms in a molecule lie in a flat, nearly square plane and the crystallographic point group of the Nb 4 F 20 molecule is 2/m (C 2h ).The intramolecular Nb1� � �Nb2 distance is 4.1275 (4) A ˚while the Nb1� � �Nb2� � �Nb1 angle measures 89.62 (1) � .The distances between diagonally opposite Nb atoms in the ring are 5.8565 (8), and 5.8179 (6) A ˚. Thus, the four Nb atoms of the Nb 4 F 20 molecule do not form an ideal square.It is distorted in a diamond shape, which corresponds to a compression along the twofold axis of rotation.An overview of interatomic distances and angles in the structure of NbF 5 is given in Tables 2 and 3.The global crystal structure can be approximately described by a cubic closepacking of the fluorine atoms, in which 1/5th of the octahedral voids are occupied by Nb atoms in such a way that the Nb 4 F 20 molecules are obtained (Edwards, 1964;Mu ¨ller, 2009).
In addition to X-ray powder diffraction, the bulk phase was also investigated by IR and Raman spectroscopy.The obtained spectra, which are given in the supporting informa-  Symmetry codes: (i) x, À y, z; (ii) À x, y, 1 À z, (iii) À x, À y, 1 À z.

Table 3
Selected interatomic angles ( � ) for the crystal structure of NbF 5 .

Conclusion
NbF 5 was synthesized from F 2 and Nb metal and obtained as a colorless, phase-pure solid and by sublimation as single crystals.The previous structure model was significantly improved with much more precise atomic coordinates and all atoms refined anisotropically, giving much better bond lengths and angles for the Nb 4 F 20 molecules.

Synthesis and crystallization
Niobium pentafluoride was synthesized from the elements directly using the apparatus sketched in Fig. 4. Therein, niobium metal sheets (17.28g, 185.9mmol,TANIOBIS GmbH) were loaded in a corundum boat, which was placed inside a tube furnace.One side of the inner corundum tube of the furnace was connected to a metal Schlenk line via a PTFE sealed copper fitting, allowing control of the fluorine supply, as well as evacuating and purging the system with argon.The other side was connected to a U-shaped, 3/4-inch PFA tube via a copper pipe, followed by a PFA gas wash bottle filled with perfluoro polyether (Hostinert 216) and an absorber column filled with soda lime (Carl Roth).The copper pipe, all fittings and valves were surrounded by heating sleeves or wires and heated to 473 K to prevent resublimation of solid NbF 5 inside.Before use, the apparatus was thoroughly baked out and passivated using diluted fluorine (F 2 /Ar, 20:80 v/v, Solvay).For the reaction a stream of diluted fluorine (F 2 /Ar, 20:80 v/v, approx.36 mL min À 1 ) was applied and the furnace temperature was set to 473 K.The first single crystals of resublimed NbF 5 were obtained within several minutes in the U-shaped PFA tube.After 16 h the reaction was complete, giving 34.2 g (182.0 mmol, 98%) NbF 5 as a colorless, crystalline solid (see Fig. 5).

Structure determination
5.1 Single crystal structure determination: A crystal of NbF 5 was selected under pre-dried perfluorinated oil (Fomblin YR 1800) and mounted using a MiTeGen loop.Intensity data of a suitable crystal were recorded with an IPDS 2 diffractometer (Stoe & Cie).The diffractometer was operated with Mo K� radiation (0.71073 A ˚, graphite monochromator) and equipped with an image plate detector.Evaluation, integration and reduction of the diffraction data was carried out using the X-AREA software suite (X-AREA V1.90; Stoe & Cie, 2020).A numerical absorption correction was applied with the modules X-SHAPE and X-RED32 of the X-AREA software suite.The structures were solved with dual-space methods (SHELXT; Sheldrick, 2015a), and refined against F 2 (SHELXL) within the ShelXle GUI (Sheldrick, 2015b;Hu ¨bschle et al., 2011).All atoms were refined with anisotropic displacement parameters.
The highest residual electron density after the final refinement was 0.80 A ˚distant from atom F6.Representations of the crystal structures were created with the DIAMOND software (Brandenburg & Putz, 2022).

Powder X-ray diffraction:
For powder X-ray diffraction, the sample was ground using a glassy carbon mortar and filled into a quartz capillary with a diameter of 0.3 mm.The powder X-ray pattern was recorded with a StadiMP diffractometer (Stoe & Cie) in Debye-Scherrer geometry.The diffractometer was operated with Cu K� 1 radiation (1.5406A ˚, germanium monochromator) and equipped with a MYTHEN 1K detector.
Rietveld refinements (Rietveld, 1969) were performed using the TOPAS-Academic software (version 7; Coelho, 2018).The structural model derived from single-crystal X-ray diffraction was used as the starting point for the refinement.A shifted Chebyshev polynomial was used to describe the background of the powder pattern, the peak profiles were fitted with a modified Thompson-Cox-Hastings pseudo-Voigt ('TCHZ') function as implemented in TOPAS, and the zero offset was refined.To account for absorption, an intensity correction for cylindrical samples was applied as implemented in TOPAS.A weak preferential orientation of the crystallites was taken into account by means of a fourth-order sphericalharmonics function.The final refinement cycles converged with free refinement of all background, profile, and lattice  parameters, including the coordinates of all atoms, the isotropic displacement parameters of the F atoms and anisotropic displacement parameters of the Nb atoms.Further details concerning the Rietveld refinement are given in Table 1 and in the supporting information.Crystal data, data collection and structure refinement details are summarized in Table 4. Computer programs: X-AREA and X-RED32 (Stoe & Cie, 2020), SHELXT2014/5 (Sheldrick, 2015a), SHELXL2018/3 (Sheldrick, 2015b) and DIAMOND (Brandenburg & Putz, 2020).

Figure 1
Figure 1 Powder X-ray diffraction pattern and Rietveld refinement of NbF 5 : measured data points (black dots), calculated diffraction pattern (red line), background (green line) and difference curve (gray).The calculated reflection positions are indicated by the vertical bars at the bottom.R p = 3.08, R wp = 4.25%, R Bragg = 1.32%, S = 1.77.

Figure 3
Figure 3Crystal structure of NbF 5 viewed along the c axis.Displacement ellipsoids are shown at 70% probability level at 100 K.

Figure 5
Figure 5Photo of colorless crystalline NbF 5 accumulated in the U-shaped PFA tube during the reaction (left, photo was taken inside a glove box) and corundum boat containing niobium metal: before (top right) and during the reaction (bottom right).

Figure 4
Figure 4 Scheme of the apparatus used for the synthesis of NbF 5 .(a) Connection to a metal Schlenk line for evacuation, purging with inert gas, and fluorine supply, (b) tube furnace, (c) copper pipe surrounded by a heating sleeve, (d) PFA U-trap for product collection equipped with Monel connectors and diaphragm valves (Hoke), (e) PFA gas wash bottle with steel fitting filled with perfluoro polyether, (f) outlet connected to the absorber.

Table 2
Selected interatomic distances (A ˚) for the crystal structure of NbF 5 .

Table 4
Experimental details.