Syntheses, Raman spectroscopy and crystal structures of alkali hexafluoridorhenates(IV) revisited

The A 2[ReF6] (A = K, Rb, Cs) salts are isotypic and crystallize in the K2[GeF6] structure type.


Chemical context
The hexafluoridorhenate(IV) anion has been known for 80 years but its chemistry is understudied with respect to the heavier halogen analogs (Ruff & Kwasnik, 1934). The scarcity of [ReF 6 ] 2À salts is attributed to the difficulties in their preparation and purification. K 2 [ReF 6 ] was the first hexafluoridorhenate(IV) salt to be reported; it was prepared from the solid-state melting reaction (SSMR) of K 2 [ReBr 6 ] with KHF 2 (Ruff & Kwasnik, 1934 (Peacock, 1956;Weise, 1956;Pedersen et al., 2014;Brauer & Allardt, 1962). Those salts were prepared by cation metathesis starting from (NH 4 (Clark & Russell, 1978;Pedersen et al., 2014). Similarly, the synthesis of the K 2 [TcF 6 ] congener, which was reported in 1963, involves the SSMR of K 2 [TcBr 6 ] with KHF 2 followed by an aqueous work-up (Schwochau & Herr, 1963). However, [TcF 6 ] 2À salts have been reinvestigated recently (Balasekaran et al., 2013), and various routes for the different salts of A 2 [TcF 6 ] [A = Na, K, Rb, Cs and N(CH 3 ) 4 ] were reported. These salts were characterized by Raman and IR spectroscopy and by SCXRD. The A 2 [ReF 6 ] salts could serve as suitable precursors to explore the chemistry of rhenium in the oxidation state IV. ISSN 2056-9890 Here, we revisited the synthesis of A 2 [ReF 6 ] (A = K, Rb, Cs) salts and report their crystal structures determined from single crystal data, and their Raman spectra.
In A 2 [ReF 6 ] (A = K + , Rb + , Cs + ), each cation is located on a position with site symmetry 3m. (Wyckoff position 2d) and is surrounded by twelve neighboring F atoms resulting in a [3 + 6 + 3] arrangement with three groups of fluoride ligands with distances of 3.0955 (19) Å (three of such), 3.1655 (6) Å (six of such), and 3.224 (2) (Balasekaran et al., 2013;Hoard & Vincent, 1939). In comparison with the previous structure determination of K 2 [ReF 6 ] (Clark & Russell, 1978), the current redetermination resulted in better reliability factors, together with a more precise determination of lattice parameters and atomic coordinates.

Synthesis and crystallization
Ammonium perrhenate, ammonium bifluoride, potassium fluoride, rubidium fluoride, cesium fluoride, and hydrobromic acid (48%) were purchased from Sigma Aldrich and used without any further purification. This work was performed in a well-ventilated fume hood due to the corrosive nature of bifluoride. K 2 [ReBr 6 ] was prepared as described in the literature (Watt et al., 1963) ] was prepared by melting K 2 [ReBr 6 ] (2 g, 2.69 mmol) with excess KHF 2 (14 g, 0.18 mol) in a nickel crucible at 673 K for 30 min in a box furnace. The resulting greyish solid product formed was allowed to cool to room temperature and was washed first with MeOH (4 Â 10 ml). Subsequently, the product was washed with several aliquots of an H 2 O/MeOH mixture (3 Â 5 ml, 1:4 volume ratios) and centrifuged. The pink solid obtained was dissolved in warm water (5-10 ml, 353 K) and evaporated slowly at room temperature. The resultant pink crystals of K 2 [ReF 6 ] were recrystallized from warm water (5 ml, 353 K) and colorless  Computer programs: APEX3 and SAINT (Bruker, 2015), SHELXS (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015), DIAMOND (Brandenburg, 2007) and publCIF (Westrip, 2010 IR spectra were measured on a Shimadzu IR Affinity-1 spectrometer between 400 and 4000 cm À1 . Raman spectra were recorded on a HORIBA T64000 triple spectrometer operating at 30 mW in subtractive mode. The spectra were taken from pure single crystals at room temperature using the 514.5 nm (Kr/Ar) laser line.

Dipotassium hexafluoridorhenate(IV) (SMB_K2ReF6_1)
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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) x y z U iso */U eq Re1 0 0 0 0.00863 (14)   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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Re1 0 0 0 0.0157 (5)  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.