A one-dimensional bromide-bridged PtII/PtIV mixed-valence complex with a 2-bromoethanesulfonate counter-ion

The characteristic feature in the crystal structure of the title salt is the formation of a columnar chain structure in the cation comprising mixed-valent ⋯Br—PtIV—Br⋯PtII⋯ entities.

The metal-halogen distances in compounds with MX-chain structures characterize the physical properties based on the mixed-valence electronic state. Compound (I) is one of the first examples of such a compound comprising a sulfonate ion having an alkyl group with a halogen atom at the terminal position, as an organic part of the counter-ion. ISSN 2056-9890

Structural commentary
The structures of the molecular components of (I) are displayed in Fig. 1. The asymmetric unit of (I) comprises of a Pt-complex moiety, [Pt II (en) 2 ] 2+ or [Pt IV Br 2 (en) 2 ] 2+ , two 2-BrCH 2 CH 2 SO 3 À anions, and two half-molecules of water, the O atoms of which are located on a site with symmetry ..2. The Pt-complex moiety and the sulfonate anions lie on general positions. As shown in Fig. 2, the structure of (I) is built up of columns extending parallel to the c axis, composed of square-planar [Pt(en) 2 ] 2+ and elongated octahedral trans-[PtBr 2 (en) 2 ] 2+ cations stacked alternately and bridged by the Br atoms. The Pt and Br atoms form an infinite slight zigzag Á Á ÁBr-Pt IV -BrÁ Á ÁPt II Á Á Á chain. The Br atoms are not located at the exact midpoint between adjacent Pt atoms and are equally disordered over two sites close to the midpoint. Thus, the Pt site is occupationally disordered by Pt II and Pt IV atoms. The valence ordering of the Pt site in (I) belongs to one of three different classes of the order-disorder problem pointed out by Keller (1982). The structure of (I) can be regarded as being of an one-dimensionally ordered structure type, with the other two directions being in a disordered state. The structural order-disorder situation of the Pt site in (I) has also been observed in the structures of a number of other MX-chain compounds (Endres et al., 1980;Beauchamp et al., 1982;Cannas et al., 1983;Yamashita et al., 1985;Matsushita et al., 1992;Toriumi et al., 1993;Huckett et al., 1993;Matsushita, 2003Matsushita, , 2005aMatsushita, ,b, 2015Matsushita & Taira, 2015).
The columns are arranged in layers parallel to the ac plane as a result of the intercolumnar hydrogen-bond linkages, connecting in the direction of the a axis, as shown in Figs. 3 and 4. Stacking the layers to the direction of the b axis makes the three-dimensional crystal packing through contacts between the terminal Br atoms of the 2-bromoethane-1-sulfonate ions. The needle-like crystal form, its elongated direction being The crystal packing of compound (I), viewed along the c axis. Light-blue and green dashed lines represent the hydrogen bonds and the short contacts between Br atoms. Orange solid lines indicate the unit cell.

Figure 4
The crystal packing of compound (I), projected on the bc plane. Light-blue and green dashed lines represent hydrogen bonds and the short contacts between Br atoms. Orange solid lines indicate the unit cell.
parallel to the c axis, does not reflect the layer structure but the columnar structure. The crystal form suggests that the BrÁ Á ÁBr contacts contribute as much to binding the layers and constructing the crystal packing as the intercolumnar hydrogen-bond linkages. Such terminal Br atoms of the alkyl chain therefore appear as significant contributors to the crystal packing.

Synthesis and crystallization
The title compound was prepared by a procedure similar to a previous literature protocal (Matsushita & Taira, 1999). To a solution of [Pt(en) 2 ]Cl 2 (0.231 g, 0.598 mmol) solved in a mixture of water (10 ml) and ethanol (2 ml) was added an ethanolic solution (2 ml) of Br 2 (32 ml, 0.62 mmol). After removing excess Br 2 by heating for 2.5 h, to this solution (including the Pt IV complex species) was added an aqueous solution of [Pt(en) 2 ]Cl 2 (0.346 g, 0.896 mmol), and then an aqueous solution of sodium 2-bromoethanesulfonate (3.414 g, 0.0162 mol). The resulting solution was allowed to stand at room temperature for about one month. Metallic lustrous green needle-like crystals of (I) suitable for X-ray analysis were obtained and were collected by filtration (yield 0.553 g, 0.350 mmol, 59%, based on Pt IV ).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.