catena-Poly[N,N,N′,N′-tetramethylethylendiammonium [[tetrabromidoantimonate(III)]-μ-bromido] hemihydrate]

The asymmetric unit of the title compound {(C6H18N2)2[Sb2Br10]·H2O}n, consists of two tetramethylethylendiammonium cations that are located on centres of inversion, as well as one tetramethylethylendiammonium cation, one water molecule, one distorted octahedral [SbBr6]3−anion and one bisphenoidal [SbBr4]− anion in general positions. The [SbBr6]3− and [SbBr4]− anions are linked together by two long Sb—Br bonds of 3.2709 (8) and 3.5447 (7) Å into {[Sb2Br10]4−}n chains along [001]. One of the three tetramethylethylendiammonium cations is disordered and was refined using a split model (occupancy ratio 0.58:0.42). The cations and the water molecule are connected to the {[Sb2Br10]4−}n polymeric anions by weak N—H ⋯Br and O(water)—H ⋯Br hydrogen bonding.


Comment
The structure determination is part of a larger project related to the synthesis, structure and phase transitions in the group of new ferroic crystals of halogenoantimonates (III) with organic cations of various sizes and symmetries (Bujak & Angel, 2005;Chaabouni et al., 1997;Chaabouni et al., 1998). In these compounds the Sb atom shows a tendency toward distorded octahedral coordination with some rather long Sb-X bonds, which is attributed to the aspherical distribution of the lone pair electron (LP) at the Sb(III) cation.
The asymetric unit of the title compound consists of both, [SbBr 6 ] 3and [SbBr 4 ]anions, a water molecule and three tetramethylethylene diammonium cations of which two are located on centers of inversion ( Fig. 1). One of these cations is disordered and was refined using a split model. The anionic substructure is composed of distordered [Sb(1)Br 6 ] 3octahedra that share two trans corners with two others [Sb(2)Br 4 ]anions that shows a saw-horse coordination. These anions are linked into zig zag {[Sb 2 Br 10 ] 4-} n pseudo chains that elongate along the [001] direction (Fig. 2). Two types of Sb-Br distances are present within these chains: eight short Sb-Br (terminal) distances [2.5405 (7) -2.9906 (7) Å] and two long Sb-Br (bridging) distances [Sb(2)···Br(5) = 3.2709 (8) Å and Sb(2)···Br(6) = 3.5447 (7) Å], all of them are shorter than the sum of the Van der Waals radii (4.1 Å). A similar structural behavior was already reported by Owczarek (Owczarek et al., 2012). By taking into account the sixth-fold coordination of antimony atom, we have proceeded to calculate the bond-valence sum (BVS) of this metal using the parameters given by Brown (Brown & Altermatt, 1985).
The BVS calculation of the Sb(1) and Sb(2) ions confirm the presumed oxidation state of Sb (III). The difference between the longest and the shortest Sb-Br distances in the Sb(1)Br 6 and Sb(2)Br 6 units amount to 0.3353 (7) Å and 1.0042 (7) Å. Differences were also found in the Br-Sb-Br angles involving Br atoms that are mutually cis configurated. The differences are 13.79 (2)° for Sb(1)Br6 and 23.50 (2)° for Sb(2)Br6. Taking into account the differences described above the lone pair electron at the Sb(III) cations may be considered as stereochemical active. The [C 6 H 18 N 2 ] 2+ cations are located between the inorganic chains with their ammonium group facing the oppositely charged {[Sb 2 Br 10 ] 4-} n polyanions.
In the crystal structure the cations, anions and water molecules are linked by weak intermolecular N-H···Br and O(W)-H···Br hydrogen bonding (Table 1).

Experimental
Crystals of the title compound were obtained by dissolving a stoichiometric mixture of antimony (III) oxide Sb 2 O 3 (5 g, 17 mmol) and N, N, N′, N′tetramethylethylendiamine (C 6 H 16 N 2 ) (5 ml, 34 mmol) in 100 ml of a solution of HBr (24%) . The resulting aqueous solution was then kept at room temperature. After several weeks prismatic shaped single crystals of the title compound were obtained by slow evaporation of the solvent at room temperature. They were washed with diethyl ether and dried for 4 h over CaCl 2 .

Refinement
All C-H and N-H H atoms were positioned with idealized geometry and refined with U iso (H) = 1.2 U eq (C,N) (1.5 for methyl H atoms) using a riding model with C-H = 0.96 Å for methyl, C-H = 0.97 Å for methylene and N-H = 0.91 Å for ammonium H atoms. The H atoms of the water molecules were located in difference map, their bond lengths were set to ideal values and finally they were refined using a riding model with U iso (H) = 1.5 U eq (O). The tetramethylethylendiammonium cation in a general position is disordered and was refined using a split model with occupancy ratio 58:42 using restraints. The O atom of the water molecule shows slightly enlarged anisotropic displacement parameters indicating for some disordering that cannot be resolved successfully.

Special details
Experimental. Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. CrysAlis RED (Oxford Diffraction, 2009) Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.