An unexpected rhenium(IV)–rhenium(VII) salt: [Co(NH3)6]3[ReVIIO4][ReIVF6]4·6H2O

The title hydrated salt, tris[hexaamminecobalt(III)] tetroxidorhenate(VII) tetrakis[hexafluoridorhenate(IV)] hexahydrate, arose unexpectedly due to possible contamination of the K2ReF6 starting material with KReO4. It consists of octahedral [Co(NH3)6]3+ cation (Co1 site symmetry 1), tetrahedral [ReVIIO4]− anions (Re1 site symmetry 1) and octahedral [ReIVF6]2− anions (Re site symmetries, Re2: 1; Re3 and Re4: ). A network of N—H⋯F hydrogen bonds consolidates the structure.

In the process of exploring the coordination chemistry of hexafluororhenate(IV) compounds, the title compound (I), an unexpected mixed-valence rhenium(IV)-rhenium(VII) salt arose in an effort to prepare [Co(NH 3 ) 6 ] 2 [ReF 6 ] 3 by metathesis from K 2 [ReF 6 ] and Co(NH 3 ) 6 Cl 3 in water (353 K). Yellow-orange needle-like crystals of (I) were obtained within two hours by slow evaporation in water at room temperature.
The crystals of (I) are air stable over short periods, but decompose to a black material after six months of storage at ambient temperature.

Database survey
To the best of our knowledge, (I) is the only reported hexahalogenorhenate-perrhenate structure containing both rhenium(IV) and rhenium(VII

Synthesis and crystallization
All chemicals were obtained commercially from Sigma Aldrich1 and used without any further purification. The starting material, K 2 [ReF 6 ], was prepared following the method described in our previous publication (Louis-Jean et al., 2018). K 2 [ReF 6 ] (114 mg, 0.3 mmol) was dissolved in 2 ml of hot water (353 K), and [Co(NH 3 ) 6 ]Cl 3 (53.5 mg, 0.2 mmol) dissolved in 1 ml of H 2 O was added. The solution was allowed to evaporate slowly at room temperature and yellow-orange needle-like crystals of (I) were obtained within two hours. The compound was washed with H 2 O (3 Â 1 ml), followed by isopropanol (3 Â 1 ml) and then diethyl ether (3 Â 1 ml). Single crystals of (I) were grown in H 2 O by slow evaporation at room temperature. Yield: ca 91%. The presence of perrhenate in (I) is probably due to the presence of K[ReO 4 ] in the starting material (i.e. K 2 ReF 6 ). Detail of the hydrogen bonding (blue dotted lines) in (I). Atom colors as in Fig. 2. Table 1 Hydrogen-bond geometry (Å , ).

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. Refinement. Refined as a 2-component twin.