Crystal structure of bis[N-(2-hydroxyethyl)-N-methyldithiocarbamato-κ2 S,S′](pyridine)zinc(II) pyridine monosolvate and its N-ethyl analogue

The ZnII atoms in {Zn[S2CN(R)CH2CH2OH]2(pyridine)·pyridine}, for R = Me (I) and Et (II), are coordinated non-symmetrically by two dithiocarbamate ligands and by a pyridine ligand, resulting in an NS4 donor set that defines a distorted geometry in each case; the non-coordinating pyridine molecules are connected with the Zn-containing molecules via O—H⋯N(pyridine) hydrogen bonds. The molecular packing features significant (hydroxy)O—H⋯O(hydroxy) hydrogen bonding, in each case leading to supramolecular chains with zigzag (I) or helical (II) arrangements.

The common structural feature of the title compounds, [Zn(C 4 H 8 NOS 2 ) 2 (C 5 H 5 N)]ÁC 5 H 5 N (I) and [Zn(C 5 H 10 NOS 2 ) 2 (C 5 H 5 N)]ÁC 5 H 5 N (II), which differ by having dithiocarbamate N-bound methyl (I) and ethyl (II) groups, is the coordination of each Zn II atom by two non-symmetrically chelating dithiocarbamate ligands and by a pyridine ligand; in each case, the non-coordinating pyridine molecule is connected to the Zn-containing molecule via a (hydroxy)O-HÁ Á ÁN(pyridine) hydrogen bond. The resulting NS 4 coordination geometry is closer to a square-pyramid than a trigonal bipyramid in the case of (I), but almost intermediate between the two extremes in (II). The molecular packing features (hydroxy)O-HÁ Á ÁO(hydroxy) hydrogen bonds, leading to supramolecular chains with a zigzag arrangement along [101] (I) or a helical arrangement along [010] (II). In (I), -[inter-centroid distances = 3.4738 (10) and 3.4848 (10) Å ] between coordinating and non-coordinating pyridine molecules lead to stacks comprising alternating rings along the a axis. In (II), weakercontacts occur between centrosymmetrically related pairs of coordinating pyridine molecules [inter-centroid separation = 3.9815 (14) Å ]. Further interactions, including C-HÁ Á Á(chelate) interactions in (I), lead to a three-dimensional architecture in each case.

Chemical context
Potentially multidentate ligands such as dithiocarbamate, À S 2 CNRR 0 , dithiocarbonate (xanthate), À S 2 COR, and dithiophosphate, À S 2 P(OR)(OR 0 ), all belong to the 1,1-dithiolate class of ligands. While many similarities are apparent in their coordination propensities (Hogarth, 2005;Heard, 2005;Tiekink & Haiduc, 2005;Haiduc & Sowerby, 1996), stark differences sometimes occur. As a case in point are species formed with the potentially bidentate ligand trans-1,2-bis(4pyridyl)ethylene (bpe). With Zn(S 2 COEt) 2 , a 1:1 compound can be prepared which crystallography shows to be a onedimensional coordination polymer with a zigzag arrangement (Kang et al., 2010). A similar structure is found for the R = n-Bu species, but in the case of a bulky cyclohexyl (Cy) group only the dimeric aggregate [Zn(S 2 COCy) 2 ] 2 (bpe) could be isolated (Kang et al., 2010). Such steric control over supramolecular aggregation is well established in the structural chemistry of main group 1,1-dithiolate compounds (Tiekink, 2003(Tiekink, , 2006. A similar situation to the above occurs for zinc(II) dithiophosphates, Zn[S 2 P(OR) 2 ] 2 , in that 1:1 one-dimensional coordination polymers can be formed with bpe when R = i-Pr (Welte & Tiekink, 2007), R = i-Bu (Welte & Tiekink, 2006) and R = Cy (Lai et al., 2004). Interestingly, when R is small, a zigzag ISSN 2056-9890 chain is formed in the crystal but larger groups, i.e. R = Cy, lead to linear chains. The situation changes for zinc(II) dithiocarbamates of bpe, where only binuclear species, [Zn(S 2 CNR 2 ) 2 ] 2 (bpe), have been isolated, e.g. R = Me (Poplaukhin & Tiekink, 2009), R = Et (Arman et al., 2009b) and i-Pr (Arman et al., 2009a). When an excess of bpe is introduced into the reaction with R = Et, the dimeric aggregate is again isolated and an additional molecule of bpe is incorporated into the crystal (Lai & Tiekink, 2003). This contrasting behaviour can be explained in terms of an effective chelating mode of the dithiocarabmate ligand owing to a 40% contribution of the 2À S 2 C N + RR 0 canonical form to the overall electronic structure. This reduces the Lewis acidity of the zinc cation and results in an inability to increase its coordination number beyond five in these systems.
In order to overcome the reluctance of zinc(II) dithiocarbamates to generate extended supramolecular architectures, the dithiocarbamate ligands can be functionalized with hydrogen-bonding potential, i.e. À S 2 CN(R)CH 2 CH 2 OH (Howie et al., 2008), and systematic studies conducted. This influence is nicely seen in the crystal of the binary species, Zn[S 2 N(CH 2 CH 2 OH) 2 ] 2 , where the dimeric aggregate, mediated by Zn-S bridges, self-assembles into a three-dimensional architecture based on hydrogen bonding (Benson et al., 2007). Studies have shown that binuclear aggregates with 4,4 0bipyridine (bipy) bridges can be formed with these functionalized dithiocarbamate ligands, consistent with the above, but extended arrays result, being stabilized via hydrogen bonding (Benson et al., 2007), e.g. an open supramolecular layer in the case of {Zn[S 2 CN(Me)CH 2 CH 2 OH] 2 } 2 (bipy), which allows for the construction of a doubly interpenetrated architecture. When the bridge is pyrazine, the three-dimensional architecture is sustained by (hydroxy)O-HÁ Á ÁO(hydroxy) hydrogen bonding exclusively (Jotani et al., 2017). When the bridge is significantly longer, e.g.

Figure 1
The molecular structures of (a) (I) and (b) (II), showing the atomlabelling scheme and displacement ellipsoids at the 70% probability level. Table 1 Geometric data (Å , ) for (I) and (II).

Structural commentary
The molecular structures of {Zn[S 2 CN(R)CH 2 CH 2 OH] 2 -(pyridine)Á.pyridine}, for R = Me (I) and Et (II), are shown in Fig. 1, and selected geometric parameters are given in Table 1. In (I), the dithiocarbamate ligands coordinate with nonsymmetric Zn-S bond lengths which is conveniently quantified by ÁZn-S = Zn-S long -Zn-S short . For the S1-dithiocarbamate ligand, ÁZn-S = 0.23 Å , but this decreases to 0.17 Å for the S3-ligand. From the data in Table 1, there is a tendency for the sulfur atoms forming the shorter Zn-S bonds to be involved in the longer C-S bonds. The pyridine-N atom occupies the fifth position in the five-coordinate zinc cation and forms N-Zn-S angles in the range 99.96 (4) to 111.99 (3) . Further, the pyridine ring is almost orthogonal to the planes through each of the chelate rings, Table 1. The value of , which ranges from 0.0 to 1.0 for ideal square-pyramidal to trigonal-bipyramidal, respectively (Addison et al., 1984), computes to 0.34, suggesting a distortion towards a squarepyramidal geometry. If this was the case, the basal plane comprises the four sulfur atoms (r.m.s. deviation = 0.1841 Å ) and the zinc cation lies 0.6877 (3) Å out of the plane in the direction of the pyridine-N3 atom. The asymmetric unit of (I) is completed by a second pyridine molecule that is connected to the (hydroxy)O2-H atom via a hydrogen bond, Table 2. To a first approximation the structure of (II) resembles that of (I). The values of ÁZn-S, at 0.27 and 0.19 Å for the S1and S3-dithiocarbamate ligands, respectively, are slightly greater than the equivalent values in (I), and this is also seen in the greater disparity in the associated C-S bond lengths, Table 1. The range of N-Zn-S bond angles is also broader, at 93.26 (5)-116.78 (5) Å and there is a disparity in the CS 2 / pyridine dihedral angles of 10.2 , cf. 1.1 for (I). The value of is 0.56, indicating a small tendency towards trigonal-bipyramidal, certainly when compared with the coordination geometry for (I). The widest angle subtended at the zinc cation is by the two less tightly held sulfur atoms, i.e. 166.375 (19) .
As for (I), distortions in the coordination geometry can be traced to the tight chelate angles, disparity in donor sets, bond lengths, etc. The solvent molecule in (II) is also associated with the O2-hydroxy group via a hydrogen bond.
Despite the relatively close agreement between the coordination geometries of the Zn[S 2 CN(R)CH 2 CH 2 OH] 2 molecules in (I) and (II), the overlay diagram shown in Fig. 2 emphasizes the differences in the relative orientations of the less symmetrically coordinating dithiocarbamate ligands and the coordinating pyridine molecules. More striking are the opposite orientations adopted by the hydroxy groups of the more symmetrically coordinating dithiocarbamate ligands and therefore, the pyridine molecules to which they are connected. This impacts significantly upon the molecular packing as described in the next Section.

Supramolecular features
The key geometric parameters characterizing the intermolecular interactions operating in the crystals of (I) and (II) are collated in Tables 2 and 3, respectively. In the molecular packing of (I), (hydroxy)O-HÁ Á ÁO(hydroxy) hydrogen bonding between the two independent hydroxy groups leads to a supramolecular zigzag chain aligned along [101] with the solvent pyridine molecules associated with the chain via (hydroxy)O-HÁ Á ÁN(pyridine) hydrogen bonding, Fig. 3a. While the hydroxy-O2 atom is involved in two conventional hydrogen bonds, the hydroxy-O1 atom is not. Rather, it participates in a (methyl)C-HÁ Á ÁO(hydroxy) interaction, Table 2. Globally, molecules stack along the a axis with solvent pyridine molecules interspersed between coordinating pyridine molecules to form columns connected by (coordinating pyridine)-(solvent pyridine) interactions so that each ring Overlay diagram of the asymmetric units of (I), red image, and (II). The molecules have been overlapped so the more symmetrically coordinating dithiocarbamate ligands are coincident. Table 3 Hydrogen-bond geometry (Å , ) for (II).

D-HÁ
forms two contacts. The separations between the ring centroids are 3.4738 (10) and 3.4848 (10) Å for an angle of inclination = 0.28 (7) ; symmetry operations: 2 À x, 1 À y, Àz and 1 À x, 1 À y, Àz. Further connections between the constituent molecules are of the type (solvent pyridine)C-HÁ Á Á(chelate ring). As seen from Fig. 3a, the solvent pyridine molecules are located in a position proximate to the chelate rings enabling such interactions to form. A view of the unitcell contents is shown in Fig. 3b.
The result of the aforementioned (solvent pyridine)C-HÁ Á Á(chelate ring) interactions is a supramolecular chain aligned along the b axis as shown in Fig. 3c. Such interactions are well known in the supramolecular chemistry of metal 1,1dithiolates in general and dithiocarbamates in particular owing to significant delocalization of -electron density over the chelate rings relative to other 1,1-dithiolate ligands such as  those cited above (Tiekink & Zukerman-Schpector, 2011;Tiekink, 2017). The unusual feature in the present case is that the (pyridine)C-H hydrogen bond forming the interaction is bifurcated (Tan et al., 2016), Table 3. Finally, for completeness, it is noted that analogous but intramolecular (coordinating pyridine)C-HÁ Á Á(chelate ring) interactions also occur, Fig. 3c, with (pyridine)C-HÁ Á Áring centroid separations of 2 x 2.90 Å and C-HÁ Á Áring centroid angles of 113 . These interactions might account for the symmetric disposition of the coordinating pyridine molecule with respect to the ZnS 4 arrangement.

Database survey
As a result of encouraging biological activities, e.g. as anticancer agents (Cvek et al., 2008;Tan et al., 2015) and for applications in tropical diseases (Manar et al., 2017), as well as their utility as single-source precursors for the deposition of ZnS nanomaterials (Hrubaru et al., 2016;Manar et al., 2017), zinc dithiocarbamates continue to be well studied. The compounds are generally binuclear as there are equal numbers of chelating and tridentate, 2 -bridging ligands, leading to distorted square-pyramidal coordination spheres (Tiekink, 2003). The structures of the precursor molecules are readily disrupted by the addition of small donor molecules such as in the present report with pyridine. Indeed, one of the first pyridine adducts of a zinc dithiocarbamate to be described was that of Zn(S 2 CNEt 2 ) 2 (pyridine), which was motivated by the desire to destroy the binuclear structure observed for the binary dithiocarbamate compound to form a lighter (i.e. lower molecular weight) species to facilitate chemical vapour deposition studies (Malik et al., 1999).  adducts reveal similar mononuclear structures with NS 4 coordination geometries, similar to those described above for (I) and (II). Finally, it is interesting to note that the aforementioned Zn(S 2 CNEt 2 ) 2 (pyridine) adduct has also been characterized as a mono-pyridine solvate (Ivanov et al., 1998), indicating that hydrogen bonding of the type observed in (I) and (II) is not a prerequisite for incorporation of solvent pyridine in the crystal.

Synthesis and crystallization
The Zn[S 2 CN(R)CH 2 CH 2 OH] 2 , R = Me and Et, precursors were prepared as per established procedures (Benson et al., 2007). Crystals were of (I) were prepared in the following manner. In a typical experiment, Zn[S 2 CN(R)CH 2 CH 2 OH] 2 , R = Me and Et (50 mg), was dissolved in pyridine (10 ml) and carefully layered with hexanes (10 ml). Crystals were harvested directly from solution and mounted immediately onto the diffractometer to avoid loss of pyridine.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. For each of (I) and (II), carbonbound H atoms were placed in calculated positions (C-H = 0.95-0.99 Å ) and were included in the refinement in the riding model approximation, with U iso (H) set to 1.2-1.5U eq (C). The O-bound H atoms were located in difference-Fourier maps but were refined with a distance restraint of O-H = 0.84AE0.01 Å , and with U iso (H) set to 1.5U eq (O). For (I), the maximum and minimum residual electron density peaks of 0.70 and 1.48 e Å À3 , respectively, were located 1.03 and 1.02 Å from the Zn atom. For (II), the maximum and minimum residual electron density peaks of 0.92 and 1.61 e Å À3 , respectively, were located 1.02 and 0.61 Å from the Zn atom.  (Farrugia, 2012), QMol (Gans & Shalloway, 2001) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Bis[N-(2-hydroxyethyl)-N-methyldithiocarbamato-κ 2 S,S′](pyridine)zinc(II) pyridine monosolvate (I)
Crystal data [Zn(C 4  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.

Bis[N-ethyl-N-(2-hydroxyethyl)dithiocarbamato-κ 2 S,S′](pyridine)zinc(II) pyridine monosolvate (II)
Crystal data [Zn(C 5  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.