An unexpected oxidation: NaK5Cl2(S2O6)2 revisited

The redetermined structure of NaK5Cl2(S2O6)2 shows a super-cell, which accommodates subtle changes in the orientation of the O atoms of the dithionate groups compared to the previously reported structure.


Chemical context
As well as their large-scale industrial use in reducing and solublizing vat dyes such as indigo (Božič & Kokol, 2008), dithionites containing the S 2 O 4 2À anion (sulfur oxidation state = +3) have long found use as moderately strong reducing agents in organic synthesis (De Vries & Kellogg, 1980, and references therein). The title mixed-cation, mixed-anion compound, NaK 5 Cl 2 (S 2 O 6 ) 2 (I), containing S 2 O 6 2À dithionate ions (sulfur oxidation state = +5), arose as a completely unexpected side product from an attempt to oxidize hexamethyl benzene to mellitic acid as a precursor of synthetic mellite (Plater & Harrison, 2015): sodium dithionite was added to the reaction to destroy excess permanganate ions (as KMnO 4 ) and the source of the chloride ions was added HCl. To our slight surprise, the structure of the title compound, along with that of the non-isostructural Na 2 K 4 Cl 2 (S 2 O 6 ) 2 , was established over 60 years ago (Stanley, 1953). This re-determination presents a superstructure of the earlier reported structure, which arises from subtle orientational changes for the dithionate anions.

Structural commentary
Compound (I) comprises two sodium ions (Na1 site symmetry = 4, Na2 site symmetry = 4), four potassium ions (site symmetries = 4, 4, 1 and 1 for K1, K2, K3 and K4, respectively), three chloride ions (Cl1 and Cl2 with site symmetry 4, Cl3 with site symmetry 2) and two half-dithionate ions (all atoms on general positions) in the asymmetric unit. Selected ISSN 2056-9890 geometrical data are given in Table 1. Both S 2 O 6 2À dithionate ions are completed by crystallographic inversion symmetry at the mid-points of their S-S bonds [S1-S1 i = 2.1227 (9), S2-S2 xiii = 2.1176 (9) Å ; see Table 1 for symmetry codes] and both exhibit almost ideal staggered conformations about their S-S bonds. The mean S-O bond length (both unique ions) is 1.45 Å and the narrow spread of individual S-O bond lengths from 1.4465 (11) to 1.4526 (13) Å indicates that the negative charges of the anion are delocalized over the three O atoms attached to each S atom (i.e.: we cannot identify localized S O double bonds and S-O single bonds). In terms of the orientation of the dithionate ions in the unit cell, the S1-O1 bond deviates from the (001) plane by 12.5 and the S2-O5 bond deviates by 10.6 (vide infra).

Figure 1
View down [001] of a slice (À0.15 z 0.15) of the structure of (I). Displacement ellipsoids are displayed at the 50% probability level.

Figure 2
View down [001] of a slice (0.35 z 0.65) of the structure of (I). Displacement ellipsoids are displayed at the 50% probability level.

Figure 3
View down [001] of a slice (0.25 z 0.35) of the structure of (I). Displacement ellipsoids are displayed at the 50% probability level.
The structure of (I) is completed by the K3 and K4 potassium ions, which occupy interstices in the framework described in the preceding paragraph. The K3 coordination polyhedron approximates to an extremely distorted KO 6 Cl 2 square anti-prism. The coordination for K4 is slightly ambiguous, with six shorter K-O bonds [2.8429 (11)-3.0664 (12) Å ] and two K-Cl links [3.1168 (5) and 3.1291 (5) Å ] forming a squashed and distorted square anti-prism. There are two further K4Á Á ÁO close contacts at 3.2273 (12) and 3.3822 (12) Å [the next-nearest K4Á Á ÁO separation after these is 4.3935 (13) Å ] but given that these K4Á Á ÁO contacts are longer than the K4-Cl bonds and have bond valences (Brown & Altermatt, 1985) of less than 0.05 (Brown, 2002), we regard them as not significant. The three chloride ions each adopt almost regular ClK 5 Na octahedral geometries.
The previously-reported structure of NaK 5 Cl 2 (S 2 O 6 ) 2 (Stanley, 1953) was modelled in space group P4/mnc [a S = 8.5621 (6), c S = 11.5288 (6) Å , V S = 845.2 Å 3 ; S = Stanley], thus it may be seen that the present unit cell is a p 2a S Â p 2a S Â c S super-cell of the Stanley structure with doubled volume. The relative dispositions of the sodium, potassium and chloride ions in the Stanley structure are almost the same as in (I); the main difference occurs in the orientation of the dithionate ions with respect to the (001) plane; in the Stanley structure, this species, which is built up from one unique S atom and two unique O atoms, has 2/m (C 2h ) point-group symmetry about the mid-point of the S-S bond with the S atom and one of the O atoms lying on the z = 0 mirror plane [compare the deviations from the (001) plane noted above for the S1-O1 and S2-O5 bonds in (I)].

Database survey
As already noted, this structure (ICSD reference number 24676) was previously reported by Stanley (1953) Polyhedral view of the structure of (I), showing the [001] chains of NaO 4 Cl 2 octahedra (blue) and KO 4 Cl 2 (K1 and K2) octahedra (lilac) cross-linked by the dithionate groups. Atoms K3 and K4 are shown as purple spheres.

Synthesis and crystallization
In an attempt to prepare mellitic acid (C 6 H 6 O 12 ) as a precursor of synthetic mellite (Plater & Harrison, 2015), hexamethylbenzene (2.0 g, 0.0123 moles) and KMnO 4 (23.4 g, 0.148 moles, 12 equiv.) were refluxed in water for 24 h (Friedel & Crafts, 1884): the organic starting material had a tendency to sublime into the condenser. After cooling, the mixture was treated with excess Na 2 S 2 O 4 to decompose the unreacted permanganate, which turned the solution brown. It was filtered and then treated with conc. HCl to give a pH of 1. After leaving to crystallize, the solid product (1.34 g) was collected by filtration as colourless blocks of (I). Evidently, dithionite has been oxidized by permanganate to dithionate by an unknown pathway and the sodium and potassium cations and chloride ions (from the hydrochloric acid) present in the mixture serendipitiously combine with the dithionate ions to form (I).

Sodium pentapotassium dichloride bis(dithionate)
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. Refinement. Refined as a 2-component twin.