Bis(methylammonium) tetrasulfidotungstate(VI)

The title compound, (CH6N)2[WS4], was synthesized by the reaction of ammonium tetrasulfidotungstate(VI) with aqueous methylamine. The title compound is isotypic with the corresponding Mo analogue (CH6N)2[MoS4], and its structure consists of a slightly distorted tetrahedral [WS4]2− dianion and two crystallographically independent methylammonium (MeNH3) cations, all of which are located on crystallographic mirror planes. The tetrasulfidotungstate anions are linked to the organic cations via hydrogen-bonding interactions.

The title compound, (CH 6 N) 2 [WS 4 ], was synthesized by the reaction of ammonium tetrasulfidotungstate(VI) with aqueous methylamine. The title compound is isotypic with the corresponding Mo analogue (CH 6 N) 2 [MoS 4 ], and its structure consists of a slightly distorted tetrahedral [WS 4 ] 2À dianion and two crystallographically independent methylammonium (MeNH 3 ) cations, all of which are located on crystallographic mirror planes. The tetrasulfidotungstate anions are linked to the organic cations via hydrogen-bonding interactions.

Related literature
Previous reports give details of the structural characterization of several organic ammonium tetrasulfidotungstates containing organic cations derived from a variety of amines (Srinivasan, Naik et al., 2007 and (Srinivasan, Nä ther et al., 2006). For related literature, see: Bondi (1964);Jeffrey (1997 Table 1 Selected geometric parameters (Å , ).
Data collection: IPDS Program Package (Stoe & Cie, 1998); cell refinement: IPDS Program Package; data reduction: IPDS Program Package; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: CIFTAB in SHELXTL (Bruker, 1998 (Srinivasan,, Näther et al., 2006). The structure consists of a discrete tetrahedral [WS 4 ] 2− ion and two crystallographically independent methylammonium cations ( Fig. 1) all of which are located on crystallographic mirror planes so that each half of these ions make up the asymmetric unit. The bond lengths and bond angles of the organic cations are in good agreement with the reported values for the isotypic Mo compound. The WS 4 tetrahedron is slightly distorted with S-W-S angles between 108.46 (7) and 110.45 (5) ° (Table 1). The W-S bond lengths range from 2.1862 (13) to 2.2010 (18) Å with an average value of 2.1931 Å. The observed difference Δ between the longest and the shortest W-S bonds of 0.0148 Å in the title compound is slightly shorter than the Δ value of 0.0199 Å in the analogous Mo compound (CNH 6 ) 2 [MoS 4 ] (Srinivasan, Näther et al., 2006).
A scrutiny of the structure reveals that each [WS 4 ] 2− is hydrogen bonded to ten symmetry related organic cations via several weak N-H···S and C-H···S interactions (Fig.2). These weak interactions can explain the observed distinct W-S bond distances. In general, unshared H-bonds have a considerably stronger effect than bifurcated ones, while the effect of trifurcated H-bonds is quite weak (Jeffrey, 1997). The atoms H1N1 and H2N1 are involved in bifurcated H-bonding while H1N2 makes a trifurcated H-bond, and H2N2 forms an unshared H-bond. Although short S···H contacts are observed at 2.40 and 2.55 Å for S2 and S1 respectively, these are relatively weaker in view of the bifurcated nature of H-bonding and hence do not cause much lengthening of these W-S bonds. The weakness of the trifurcated H-bond can also be evidenced from the observed longer S···H distances accompanied by smaller N-H···S angles. S3 is involved in a singly unshared H-bond at a distance of 2.64 Å. The effect of this contact is stronger than all the other contacts and can explain the elongation of the W-S3 bond, the longest observed W-S distance for the title compound. In all ten short S···H contacts ranging from 2.40 to 2.97 Å, all of which are shorter than the sum of their van der Waals radii (Bondi, 1964) are observed ( Table 2). As a result of H-bonding, the organic cations are organized such that the ammonium groups always point towards the S atoms of tetrasulfidotungstate as can be seen in the sequence in the crystallographic bc plane viz. WS 4 ··· H 3 NMe··· MeNH 3 ··· WS 4 ··· and so on (Fig. 3).

Experimental
(NH 4 ) 2 [WS 4 ] (1 g) was dissolved in 40% methylamine (5 ml) and water (2 ml) and filtered. The clear yellow filtrate was left undisturbed for crystallization. After a day crystalline blocks of the title compound separated. The product was filtered, washed with ice-cold water (1 ml), followed by 2-propanol (5 ml) and diethyl ether (10 ml) and dried. Yield: 1.1 g.

supplementary materials
sup-2 Refinement All H atoms were located in difference map but were positioned with idealized geometry ((CH 3 and NH 3 allowed to rotate but not to tip) with 0.98 and 0.96 Å (methyl) and N-H = 0.91 Å) and were refined using a riding model, with U iso (H) fixed at 1.5U eq (CH 3 and NH 3 ). The largest peak in the residual electron density map of 1.48 e Å −3 is located at a distance of 0.09 Å from W1 and the deepest hole of −2.34 e Å −3 is located at a distance of 0.83 Å from W1.