The first compound with an unusual type of anion, [Li(SR)₂]^-: bis­(₂-aqua-d₂)tetra­kis(aqua-d₂)dilithium(I) bis­[bis­(tri-tert-butoxy­silanethiol­ato-²O,S)lithate(I)] dihydrate-d₂

Thiolates, selenolates and tellurolates of the s-block elements are very often used as precursors of chalcogenolates of other metals. Special attention has therefore been devoted to their structure, which can sometimes be controlled by an appropriate method of synthesis (Englich & Ruhlandt-Senge, 2000). The most common lithium thiolates are monomeric (Chadwick & Ruhlandt-Senge, 1998; Ives et al., 2003) or dimeric (Bjernemose et al., 2004; Komuro et al., 2004). However, trimeric (Ruhlandt-Senge et al., 1996; Niemeyer & Power, 1996), hexameric (Janssen et al., 1996) and polymeric forms (Ruth et al., 2005; Hampe et al., 2005) are also known. In the case of lithium compounds, where the most common coordination number is four, the central atoms are frequently coordinated by solvent molecules, with O– and N-donor solvents preferred.

Silanethiolates of the s-block metals are much less numerous and the structural data of only a few alkali metal derivatives have been described to date. Compounds with Si—S—Li (Baranowska et al., 2002; Komuro et al., 2004; Jesionka et al., 2005) and Si—S—Na bond systems (Wojnowski et al., 1986; Chadwick et al., 1997) have been reported. None of them, however, is similar to the title compound, (I), described in the present paper. This lithate is not only the first compound with an S—Li bond within the [M(SR)₂]⁻ type of anion {where M is an alkali metal and R is Si[OC(CH₃)₃]₃}, but is also the first ionic thiolate where both ions are metal complexes and both are based on lithium.

Compound (I) can be obtained using the procedure described in the Experimental section. Fig. 1 shows the complete parts of complex (I), along with the atom-numbering scheme. Selected bond lengths and angles are presented in Table 1. Because of the doubly charged complex bimetallic lithium cation, two singly charged complex lithate anions are required to balance the charge. Two solvent heavy water molecules complete the structure.

The central atom in the lithate [Li(SR)₂]⁻ anion is coordinated by two silanethiolate S atoms and two siloxy O atoms from two undoubtedly chelating (^tBuO)₃SiS⁻ ligands. This leads to the central LiO₂S₂ core which, due to the O—Li—S chelate bite angle, shows a severely distorted tetrahedral arrangement of atoms, with angles between 80.29 (12) and 148.2 (2)°. The Li—S and Li—O bond lengths within the lithate anion seem to be unexceptional, being comparable with those found in two other known neutral lithium tri-tert-butoxysilanethiolates (Jesionka et al., 2005). Moreover, the Li—O bond lengths in the lithate anion fall within the limits set by the bonds present in the accompanying [Li₂(D₂O)₆]²⁺ cation. The cation contains two Li atoms, both coordinated only by O atoms from D₂O molecules. Two of these D₂O ligands are in bridging and four in terminal positions. The geometry of the resulting LiO₄ core can also be described as a distorted tetrahedron. Moreover, the bimetallic cation system created by the bridging molecules of heavy water leads to the formation of a planar centrosymetric Li₂(µ-O)₂ ring.

Outside the bimetallic complex cation, two other solvent heavy water molecules are present within the crystal structure. Their presence has important consequences. Although all heavy water molecules are engaged in the formation of hydrogen bonds, each ligating D₂O forms only two such bonds. On the other hand, each solvating D₂O molecule forms four hydrogen bonds. It is simultaneously a donor of O—D···O and O—D···S hydrogen bonds to one of the [Li(SR)₂]⁻ lithate anions and an acceptor of two O···D—O hydrogen bonds from two different [Li₂(D₂O)₆]²⁺ cations. Both ion types are therefore held within an extended network of O—D···O and O—D···S hydrogen bonds. This is illustrated in Fig. 2 and data for eight different hydrogen bonds are summarized in Table 2. The hydrogen-bond network in some respect `covers' the bimetallic cations, which are placed in the space between two lithate anions as shown in Fig. 3. Because of these interactions between the complex cations and the solvent D₂O molecules, the one-dimensional hydrogen-bond network which is formed runs in the a direction, as shown in Fig. 4.

Isolation of the distinct ionic domains in compound (I) is further evidenced by the intermetallic distances. The bimetallic [Li₂(D₂O)₆]²⁺ cation may serve as an example of a lithium complex, with an Li···Li distance of 2.876 (7) Å. A similar distance can be found in the Li₂(µ-S)₂ ring in two neutral tri-tert-butoxysilanethiolates of lithium (Jesionka et al., 2005). In the case of (I), the distances between the Li atom of the lithate anion and either of the two Li atoms within the cation are much larger, at 5.279 (5) and 5.261 (5) Å. In fact, the structure of (I) shown in Fig. 5 can be roughly described as side-by-side packed tetragonal columns interacting only by van der Waals forces. These columns, composed of negative lithate anions, have in their interior another columnar structure formed of complex cations, embedded with and covered by the hydrogen-bond network.