Crystal structures of (2,2′-bipyridyl-κ2 N,N′)bis[N,N-bis(2-hydroxyethyl)dithiocarbamato-κ2 S,S′]zinc dihydrate and (2,2′-bipyridyl-κ2 N,N′)bis[N-(2-hydroxyethyl)-N-isopropyldithiocarbamato-κ2 S,S′]zinc

The ZnII atom in each of [Zn{S2CN(CH2CH2OH)2}2(bipy)]·2H2O, (I), and [Zn{S2CN(iPr)CH2CH2OH}2(bipy)], (II), is coordinated symmetrically by two dithiocarbamate ligands and a 2,2′-bipyridine ligand resulting in an N2S4 donor set that defines a heavily distorted octahedral geometry. The molecular packing features significant hydrogen bonding in each case with supramolecular ladders found in (I) sustained by O—H⋯O hydrogen bonds, and layers in (II) sustained by O—H⋯S hydrogen bonds.

The common feature of the title compounds, [Zn(C 5 H 10 NO 2 S 2 ) 2 (C 10 H 8 N 2 )]Á-2H 2 O, (I), and [Zn(C 6 H 12 NOS 2 ) 2 (C 10 H 8 N 2 )], (II), is the location of the Zn II atoms on a twofold rotation axis. Further, each Zn II atom is chelated by two symmetry-equivalent and symmetrically coordinating dithiocarbamate ligands and a 2,2 0 -bipyridine ligand. The resulting N 2 S 4 coordination geometry is based on a highly distorted octahedron in each case. In the molecular packing of (I), supramolecular ladders mediated by O-HÁ Á ÁO hydrogen bonding are found whereby the uprights are defined by {Á Á ÁHO(water)Á Á ÁHO(hydroxy)Á Á Á} n chains parallel to the a axis and with the rungs defined by 'Zn[S 2 CN(CH 2 CH 2 ) 2 ] 2 '. The water molecules connect the ladders into a supramolecular layer parallel to the ab plane via water-O-HÁ Á ÁS and pyridyl-C-HÁ Á ÁO(water) interactions, with the connections between layers being of the type pyridyl-C-HÁ Á ÁS. In (II), supramolecular layers parallel to the ab plane are sustained by hydroxy-O-HÁ Á ÁS hydrogen bonds with connections between layers being of the type pyridyl-C-HÁ Á ÁS.

Chemical context
The dithiocarbamate ligand À S 2 CNRR 0 , is well known as an effective chelator of transition metals, main group elements and lanthanides (Hogarth, 2005;Heard, 2005). The resulting four-membered MS 2 C chelate ring has metalloaromatic character (Masui, 2001) and may act as an acceptor for C-HÁ Á Á(chelate) interactions (Tiekink & Zukerman-Schpector, 2011) much in the same way as the now widely accepted C-HÁ Á Á(arene) interactions. While other 1,1-dithiolate species may also form analogous interactions -these were probably first discussed in cadmium xanthate ( À S 2 COR) structures (Chen et al., 2003) -dithiocarbamate compounds have a greater propensity to form C-HÁ Á Á(chelate) interactions, an observation related to the relatively greater contribution of the canonical structure 2À S 2 C N + RR 0 to the overall electronic structure that enhances the electron density in the chelate ring (Tiekink & Zukerman-Schpector, 2011). This factor explains the strong chelation ability of the dithiocarbamate ligand and at the same time accounts for the reduced Lewis acidity of the metal cation in metal dithiocarbamates which reduces the ability of these species to form extended architectures in their interactions with Lewis bases. One way of overcoming the relative inability of the metal cation to engage in supramolecular association is to functionalize the dithiocarbamate ligand with, relevant to the ISSN 2056-9890 present report, hydrogen-bonding functionality. In this context and as a continuation of earlier studies of the zinctriad elements with dithiocarbamate ligands featuring hydroxyethyl groups capable of forming hydrogen-bonding interactions (Benson et al., 2007;Broker & Tiekink, 2011;Zhong et al., 2004;Tan et al., 2013Tan et al., , 2016Safbri et al., 2016;Howie et al., 2009), herein, the crystal and molecular structures of two new zinc dithiocarbamates, Zn[S 2 CN(CH 2 CH 2 OH) 2 ] 2 -(bipy)Á2H 2 O, (I), and Zn[S 2 CN(iPr)CH 2 CH 2 OH] 2 (bipy), (II) where bipy = 2,2 0 -bipyridine are described.

Structural commentary
The molecular structure of the zinc compound in (I) is shown in Fig. 1 and selected geometric parameters are given in Table 1. The zinc cation is located on a twofold rotation axis and is chelated by two symmetry-equivalent dithiocarbamate ligands and the 2,2 0 -bipyridine ligand, which is bisected by the twofold rotation axis. The dithiocarbamate ligand chelates in a symmetric mode with the difference between the Zn-S long and Zn-S short bond lengths being 0.02 Å . The shorter Zn-S bond is approximately trans to a pyridyl-N atom. The N 2 S 4 coordination geometry is based on an octahedron. In this description, one triangular face is defined by the S1, S2 i and N2 i atoms, and the other by the symmetry equivalent atoms [symmetry code: (i) 3 2 À x, 1 2 À y, z]. The dihedral angle between the two faces is 3.07 (4) and the twist angle between them is approximately 35 , cf. 0 and 60 for ideal trigonalprismatic and octahedral angles, respectively. The twist toward a trigonal prism is related in part to the acute bite angles subtended by the chelating ligands (Table 1).
Compound (I) was characterized herein as a dihydrate and may be compared with an unsolvated literature precedent (Deng et al., 2007) for which selected geometric data are also collected in Table 1. First and foremost, the molecular symmetry observed in unsolvated (I) is lacking. Also, the range of Zn-S bond lengths is significantly broader at 0.14 Å , but the trend that the shorter Zn-S bonds are approximately trans to the pyridyl-N atoms persists. The dihedral angle between the trigonal faces is 5.33 (6) and the twist between them is 31 , indicating an intermediate coordination geometry.

Figure 1
The molecular structure of the zinc compound in (I), showing the atomlabelling scheme and displacement ellipsoids at the 70% probability level; the water molecules of crystallization have been omitted. The unlabelled atoms are related by the symmetry operation 3 2 À x, 1 2 À y, z.
mately 30 , again indicating a highly distorted coordination geometry.

Supramolecular features
Geometric parameters characterizing the intermolecular interactions operating in the crystal structures of (I) and (II) are collected in Tables 2 and 3, respectively.

Figure 2
The molecular structure of (II), showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level. The unlabelled atoms are related by the symmetry operation 1 À x, y, 3 2 À z. Table 3 Hydrogen-bond geometry (Å , ) for (II). Symmetry codes: (i) Àx þ 3 2 ; y À 1 2 ; Àz þ 3 2 ; (ii) Àx þ 1; Ày þ 2; Àz þ 1. pyridyl-C-HÁ Á ÁO(water) interactions (Fig. 3b). The connections between layers to consolidate the three-dimensional architecture are of the type pyridyl-C-HÁ Á ÁS (Fig. 3c). Naturally, the molecular packing in the unsolvated form of (I) is distinct (Deng et al., 2007). However, a detailed analysis of the packing is restricted as one of the hydroxy groups is disordered over two sites. Further, there are large voids in the crystal structure, amounting to approximately 570 Å 3 or 19.2% of the available volume (Spek, 2009). This is reflected in the crystal packing index of 59.2% which compares to 71.3% in (I). Globally, the crystal structure comprises alternating layers of hydrophilic and hydrophobic regions with the former arranged as supramolecular rods, indicating significant hydrogen bonding in this region of the crystal structure.
In the molecular packing of (II), hydroxy-O-HÁ Á ÁS hydrogen bonds lead to supramolecular layers parallel to the ab plane (Fig. 4a). Additional stabilization to this arrangement is provided by methyl-C-HÁ Á ÁO(hydroxy) interactions. Connections between layers to consolidate the three-dimensional packing are of the type pyridyl-C-HÁ Á ÁS (Fig. 4b).

Database survey
Binary zinc dithiocarbamates are generally binuclear as a result of the presence of chelating and tridentate, 2 -bridging ligands, leading to penta-coordinate geometries (Tiekink, 2003). The exceptional structures arise when the steric bulk of at least one of the terminal substituents is too great to allow for supramolecular association, e.g. R = cyclohexyl (Cox & Tiekink, 2009) and R = benzyl (Decken et al., 2004). However, there is a subtle energetic balance between the two forms as seen in the crystal structure of Zn[S 2 CN(i-Bu) 2 ] 2 which comprises equal numbers of mono-and bi-nuclear molecules (Ivanov et al., 2005). As the R groups are generally aliphatic, there is limited scope for controlled supramolecular aggregation between the molecules. This changes in the case of the present study as at least one R group has an hydroxyethyl substituent. Indeed, a rich tapestry of structures have been observed for zinc compounds with this family of dithiocarbamate ligands.
The common feature of the molecular structures of the known binary species, Zn[S 2 NC(R)CH 2 CH 2 OH] 2 , is the adoption of a binuclear motif (Benson et al., 2007;Tan et al., 2015). In the molecular packing of these species, when R = CH 2 CH 2 OH, a three-dimensional architecture is constructed based on hydrogen bonding (Benson et al., 2007). When the hydrogen-bonding potential is reduced, as in the case when R = Et, linear supramolecular chains are formed (Benson et al., 2007). When R = Me, and in the 2:1 adduct with the bridging ligand (3-pyridyl)CH 2 N(H)C( O)C( O)-N(H)CH 2 (3-pyridyl), interwoven supramolecular chains are formed based on hydrogen bonding (Poplaukhin & Tiekink, 2010). Extensive hydrogen bonding is also noted in co-crystals, e.g. for R = Me in the 2:1 adduct with (3-pyridyl)CH 2 N(H)-C( S)C( S)N(H)CH 2 (3-pyridyl), a 2:1 co-crystal with S 8 has been characterized in which a two-dimensional array sustained by O-HÁ Á ÁO hydrogen bonding is found (Poplaukhin et al., 2012). From the foregoing, it is clear that a rich structural chemistry exists for these compounds, well worthy of further investigation. Complementing these interests are the observations that zinc compounds with these ligands (Tan et al., 2015), along with gold (Jamaludin et al., 2013)

Synthesis and crystallization
The potassium salts of the dithiocarbamate anions (Howie et al., 2008;Tan et al., 2013) and zinc compounds (Benson et al., 2007) were prepared in accord with the literature methods. The 1:1 adducts with 2,2 0 -bipyridine were prepared in the following manner. Zn[S 2 CN(CH 2 CH 2 OH) 2 ] 2 (0.20 g, 0.47 mmol) and 2,2 0 -bipyridine (Sigma Aldrich; 0.07 g, 0.47 mmol) were dissolved in acetone (30 ml) and ethanol (10 ml), respectively. The solution of 2,2 0 -bipyridine was added dropwise into the other solution with stirring for about 30 mins, resulting in a change from a colourless to a light-yellow solution. The mixture was left to stand to allow for crystallization and crystals of (I) for X-ray analysis were harvested directly. Compound (II) was prepared and harvested similarly from the reaction of Zn[S 2 CN(iPr)CH 2 CH 2 OH] 2 (0.20 g, 0.47 mmol) in chloroform (30 ml) and 2,2 0 -bipyridine (0.07 g, 0.47 mmol) in acetone (10 ml).

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 = 0.95-1.00 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 a difference Fourier map 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).  (Farrugia, 2012), DIAMOND (Brandenburg, 2006) and publCIF (Westrip, 2010

(I) (2,2′-Bipyridyl-κ 2 N,N′)bis[N,N-bis(2-hydroxyethyl)dithiocarbamato-κ 2 S,S′]zinc dihydrate
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.