Crystal structure of dipotassium N-carbodithioato-l-prolinate trihydrate

K2(SSC-NC4H7—COO)·3H2O or C6H13K2NO5S2 exbibits a highly complex supramolecular structure in the crystal, which is governed by bridging O– and S-atom coordination, as well as hydrogen bonding.

The molecular and crystal structure of the l-proline-derived dithiocarbamatecarboxylate compound poly[tri--aqua-(-2-carboxylatopyrrolidine-1-carbodithioato)dipotassium], [K 2 (C 6 H 7 NO 2 S 2 )(H 2 O) 3 ] n or K 2 (SSC-NC 4 H 7 -COO)Á3H 2 O, has been determined. The dithiocarbamate moiety displays a unique coordination mode, comprising a 'side-on' -coordinated K + cation besides a commonly -chelated K + cation. By bridging coordination of the CSS group, COO group and water molecules, the K + cations are linked into a two-dimensional coordination polymer extending parallel to the ab plane. These layers are again interconnected by O-HÁ Á ÁS hydrogen bonds.

Chemical context
Natural amino acids react readily with carbon disulfide in an alkaline environment to give dithiocarbamate-functionalized carboxylates. Since the first report on a series of barium salts in the 1950s (Zahradnik, 1956), numerous transition metal complexes have been explored. More recently, various late transition metal complexes of this family have been investigated due to their biological activity (e.g. Giovagnini et al., 2005;Cachapa et al., 2006;Nagy et al., 2012). In most cases, the dithiocarbamate moiety acts as a classical small-bite chelate ligand, while the carboxylate group (often esterified) does not contribute to metal coordination. The structural chemistry of main group derivatives of dithiocarbamate-derived amino acids is much less explored, even though alkali metal and alkaline earth metal salts are frequently used as precursors for other metal complexes. A key intermediate in our ongoing reasearch on coordination polymers with dithiocarbamatecarboxylates is the l-proline-derived potassium salt K 2 (SSC-NC 4 H 7 -COO). This compound crystallizes from aqueous solution as a trihydrate, which has been structurally characterized in the course of this work.

Structural commentary
The title compound, K 2 (SSC-NC 4 H 7 -COO)Á3H 2 O, crystallized as colourless plates in the orthorhombic space group P2 1 2 1 2 1 , with one formula moiety in the asymmetric unit ( Fig. 1). One K atom (K2) is bonded in a typical chelating fashion by the CSS group, while K1 is coordinated 'side-on' to the CSS group, certainly under participation of the delocalized electrons. This rather uncommon coordination mode might be supported by additional coordination of a carboxylate O atom (O1) to K1. K1 adopts a low-symmetric seven-coordination by four carboxylate O atoms, two H 2 O molecules and the -coordinating CSS group. K2 is eight-coordinated by three S atoms and five H 2 O molecules (Fig. 2). Consequently, the full coordination mode of the carboxylate group is 3 -4 O,O 0 :O:O 0 , and the dithiocarbamate group adopts a 3 -6 S,S 0 ,C:S,S 0 :S coordination. One H 2 O molecule displays a 3coordination (O3) and the remaining two H 2 O molecules are coordinating in a -bridging mode (O4 and O5). The K-S distances at the -chelated K + cation (K2) are 3.2176 (8) and 3.2650 (9) Å , while the K-S separations at the -coordinated K + cation (K1) are significantly longer at 3.2956 (9) and 3.4463 (8) Å . The coordination mode of the dithiocarbamate group in the title compound [see (C) in Fig. 3] is unique, to our knowledge. The most frequently observed coordination pattern in dithiocarbamates of the heavier alkali metals (K, Rb and Cs) is a symmetric double-chelating mode, leading to a puckered S 2 M 2 ring (e.g. Howie et al., 2008;Reyes-Martínez et al., 2009;Mafud, 2012;see (B) in Fig. 3]. Nonetheless, the values of the K-S separations in the title compound cover the same range as observed in the reference compounds. A simple chelating coordination with significantly shorter K-S contacts is realized when the K + cation is coordinatively highly saturated, as has been observed in a crown ether complex [Arman et al., 2013; see (A) in Fig. 3]. In the title compound, three of the four K-O(carboxylate) contacts are in a range 2.676 (2)-2.802 (2) Å , which is consistent with the values observed in other potassium carboxylates (e.g. Ilczyszyn et al., 2009;Liebing et al., 2016). However, one contact (K1 0 -O1) is strongly elongated to 3.358 (2) Å . The K-O(H 2 O) bond lengths cover a range of 2.723 (2)-3.065 (3) Å .

Supramolecular features
As a result of the bridging coordination of the carboxylate group, the dithiocarbamate group and the water molecules, a two-dimensional polymeric structure parallel to the ab plane is Illustration of the coordination environment of the two K + cations.

Figure 3
The different coordination modes of the dithiocarbamate group in potassium complexes: single-chelating (A), symmetric double-chelating (B) and single-chelating combined with -coordination (this work; C).

Figure 1
The asymmetric unit of the title compound. Displacement ellipsoids for non-H atoms are drawn at the 50% probability level and H atoms attached to C atoms have been omitted for clarity. Adjacent symmetryrelated K + cations are illustrated as semi-transparent spheres. Table 1 Hydrogen-bond geometry (Å , ).

Synthesis and crystallization
A slight excess of carbon disulfide (approximately 4 ml, 0.06 mol) was added to a solution of l-proline (5.76 g, 0.05 mol) and potassium hydroxide (5.61 g, 0.10 mol) in 30 ml water and the resulting solution was stirred vigorously overnight. The yellow solution obtained was filtered and reduced to dryness in vacuo. The crystalline residue was washed with several portions of tetrahydrofuran and diethyl ether, and dried in vacuo, providing analytically pure K 2 (SSC-NC 4 H 7 -COO)Á3H 2 O in almost quantitative (>95%) yield as colourless to light-brown low-melting plates, which are very soluble in water. Single crystals suitable for X-ray structure analysis were obtained by slow evaporation of a concentrated aqueous solution at room temperature.

Figure 4
Supramolecular crystal structure comprising polymeric layers extending parallel to (001), viewed in a projection on (100). The bold black lines mark the unit-cell dimensions.

Figure 5
The supramolecular layer illustrated in Fig. 4, viewed in a projection on (010).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms on C atoms were fixed geometrically and refined using a riding model, with U iso (H) = 1.2U eq (C). C-H distances within the CH 2 groups were constrained to 0.99 Å and that within the CH group to 1.00 Å .
The water H-atom sites were located in difference Fourier maps and refined using restraints on the O-H distance [target value = 0.84 (2) Å ]. The corresponding U iso (H) values were set at 1.5U eq (O). The reflection (002) disagreed strongly with the structural model and was therefore omitted from the refinement.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.