supplementary materials

As part of an ongoing research programme, we are investigating the synthesis and structural characterization of organic ammonium tetrasulfidometalates of the group VI metals Mo and W (Srinivasan, Naik et al., 2007). In earlier work we have structurally characterized several [WS₄]²⁻ compounds derived from chiral amines (Srinivasan, Naik et al., 2007), diamines (Srinivasan et al., 2002; 2003a; Srinivasan et al., 2006a), triamines (Srinivasan et al., 2006b), cyclic amines (Srinivasan et al., 2003b; Srinivasan et al., 2006) and a tetraamine (Srinivasan et al., 2005). All the organic ammonium tetrasulfidotungstates exhibit several weak hydrogen bonding interactions between the organic cations and [WS₄]²⁻ anions. We have also shown that in some organic [WS₄]²⁻ compounds the amines are partially protonated (Srinivasan et al., 2006b). The secondary amine benzyl(methyl)amine used for the synthesis of the title compound is an isomer of the chiral primary amine α-methylbenzylamine used in an earlier study (Srinivasan, Naik et al., 2007).

The title compound is isostructural with the corresponding Mo analogue (C₈H₁₂N)₂[MoS₄] (Srinivasan, Girkar & Raghavaiah 2007). The structure of (I) consists of a discrete tetrahedral [WS₄]²⁻ ion and two crystallographically independent benzyl(methyl)ammonium cations (Fig. 1) with all atoms located in general positions. The bond lengths and bond angles of the organic cations are in good agreement with the reported values for the isotypic Mo compound. The (WS₄) tetrahedron is slightly distorted with S—W—S angles between 108.89 (3) and 110.18 (4) ° (Table 1). The W—S bond lengths range from 2.1703 (8) to 2.2083 (7) Å with an average value of 2.1865 Å which is comparable to the bond lengths observed in the related chiral [WS₄]²⁻ compound synthesized from the isomeric chiral primary amine (Srinivasan, Naik et al., 2007). The W1—S1 and W1—S2 bond distances are indistinguishable within experimental error and are shorter than the average Mo—S bond length while the other two W—S bonds are longer. The weak H-bonding interactions between the cations and anions can explain the observed short and long W—S bond distances. An analysis of the structure reveals that the organic cations and tetrasulfidotungstate anions are linked with the aid of several N—H···S and C—H···S hydrogen bonding interactions. Thus each [WS₄]²⁻ is hydrogen bonded to five different organic cations with the aid of six N—H···S bonds and two weak C—H···S interactions (Fig.2). An examination of the surroundings of the cations reveals that one organic cation (N1) is H-bonded to two different [WS₄]²⁻ ions while the second organic cation (N2) is surrounded by three different [WS₄]²⁻ ions. One H atom on each N atom functions as a singly shared donor with the other functioning as a bifurcated donor (Table 2). A benzilic H atom from each unique cation is involved in a weak C—H···S interaction. S4 atom which makes the longest W—S bond at 2.2083 (7) Å is involved in three N—H···S bonds, two of which are singly shared. S4 also makes the shortest singly shared N—H···S bond at 2.39 Å, which can explain the elongation of this bond. In contrast, S1 atom involved in the shortest W—S bond makes a bifurcated N—H···S bond at a longer S···H distance accompanied by a small NH—S angle. S1 also makes a very weak C—H···S contact. The observed difference Δ between the longest and the shortest W—S bonds of 0.0380 Å in (I) is slightly longer than the Δ value of 0.0356 Å in the tetrasulfidotungstate compound containing the R-form of the monoprotonated isomeric chiral primary amine α-methylbenzylamine (Srinivasan, Naik et al., 2007).