A two-dimensional coordination polymer: poly[[bis[μ2-N-ethyl-N-(pyridin-4-ylmethyl)dithiocarbamato-κ3 N:S,S′]cadmium(II)] 3-methylpyridine monosolvate]

The title compound, {Cd[S2CN(Et)CH2py]2.3-methylpyridine}n, is a two-dimensional coordination polymer with square channels in which reside the 3-methylpyridine molecules.


Chemical context
Despite the relatively recent observations of one-dimensional coordination polymers for some binary cadmium dithiocarbamates (Tan et al., 2013Ferreira et al., 2016), i.e. compounds of general formula Cd(S 2 CNRR 0 ) 2 , for R, R 0 = alkyl, aryl, the overwhelming majority of Cd(S 2 CNRR 0 ) 2 structures are binuclear and zero-dimensional (i.e. molecular). This arises owing to the presence of equal numbers of chelating ligands and tridentate ligands, with the latter chelating one Cd II atom while bridging a second. The coordination geometry defined by the resulting S 5 donor set is invariably highly distorted and intermediate between trigonalbipyramidal and square-pyramidal (Tiekink, 2003). The polymeric motifs of Cd(S 2 CNRR 0 ) 2 have 3 -bridging ligands exclusively and six-coordinate, S 6 , geometries. Systematic crystallization studies indicate these transform to the binuclear motif with the egress of time (Tan et al., 2013, suggesting the zero-dimensional motif is the thermodynamic outcome of crystallization. The addition of monodentate pyridyl-N donor molecules during adduct formation more often than not results in the breakdown of the binuclear motif to form a mononuclear species, e.g. as in the structures of Cd{S 2 CN[CH 2 C(H)Me 2 ] 2 } 2 (pyridine) (Rodina et al., 2011) and Cd[S 2 CN(Me)Ph] 2 (pyridine) 2 (Onwudiwe et al., 2013). The latter structure shows it is possible for the Cd II atom to increase its coordination number to six in the presence of N-donors. Hence, bipyridyl donors with suitably disposed nitrogen atoms might be anticipated to produce coordination ISSN 2056-9890 polymers. This has been realized in several examples, e.g. in the one-dimensional coordination polymers of {Cd(S 2 CNEt 2 ) 2 [1,2-bis(pyridin-4-yl)ethylene]} n (Chai et al., 2003) and in its 1,2-bis(pyridin-4-yl)ethane analogue (Avila et al., 2006). In these instances, the Cd II atom exists within a trans-N 2 S 4 coordination geometry. However, the reaction outcomes are not always as expected.
The varied and interesting structures notwithstanding, it is obvious that Cd II will expand its coordination number in the presence of pyridyl-N donors. Hence, in order to encourage the formation of higher-dimensional aggregates, functionalizing the dithiocarbamate ligand with pyridyl substituents offers an opportunity to increase the dimensionality of the structure. Indeed, Cd II structures with pyridin-4-yl groups included in the dithiocarbamate ligand have appeared in the recent literature, e.g. Cd[S 2 CN(ferrocenylmethyl)CH 2 Py] 2 -(1,10-phenanthroline) (Kumar et al., 2016). Here, the Cd II atom is coordinatively saturated within a cis-N 2 S 4 donor set so the pyridyl-N atoms of the dithiocarbamate ligand are noncoordinating. However, pyridyl-N bridging has been observed in the binuclear structure, [Cd[S 2 CN(1H-indol-3-ylmethyl)-CH 2 (CH 2 py)] 2 } 2 . This structure is in fact very closely related to the common binuclear motif but, instead of a bridging, tridentate dithiocarbamate ligand, via three sulfur donors, the bridges in this structure are provided by the pyridyl-N atoms; the two pendent pyridyl groups are non-coordinating. In a continuation of exploratory work in this field (Arman et al., 2013), herein the crystal and molecular structures of the title two-dimensional coordination polymer, (I), {Cd[S 2 CN(Et)CH 2 py]2.3-methylpyridine} 2 , containing a pyridyl-functionalized dithiocarbamate ligand, is described.

Structural commentary
The asymmetric unit of (I) comprises a molecule of Cd[S 2 CN(Et)CH 2 py] 2 , Fig. 1, and a molecule of 3-methylpyridine. Referring to Table 1, each dithiocarbamate anion is chelating, forming very similar Cd-S bond lengths. This similarity is reflected in the experimental equivalence of the associated C-S bond lengths. Each dithiocarbamate ligand is in fact tridentate, chelating one Cd II atom as just described and simultaneously bridging another via the pyridyl-N atom so that the coordination geometry about the Cd II atom is cis-N 2 S 4 , distorted octahedral,  (17) Symmetry codes: (i) Àx; y À 1 2 ; Àz; (ii) x À 1; y; z.

Figure 1
The Cd[S 2 CN(Et)CH 2 py] 2 component of the asymmetric unit of (I), extended to show the immediate coordination geometry about the Cd II atom, showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level. [Symmetry codes: (i) Àx, À 1 form two interconnected rows of molecules, with those aligned along the a axis being formed via S3/S4-N4 bridges and those along the b axis being sustained by S1/S2-N2 bridges. The result is a two-dimensional architecture in the ab plane, Fig. 2. Square channels are formed in the b-axis direction and these are occupied by the solvent 3-methylpyridine molecules, Fig. 2a and b. The slats along the a axis are defined by the pyridyl residues and these block access along this direction, Fig. 2c.

Supramolecular features
A summary of specific intermolecular interactions contributing to the molecular packing of (I) is given in Table 2. The main interactions between the host framework and the guest 3-methylpyridine molecules are of the type methylene-C-HÁ Á ÁN(3-methylpyridine) and (3-methylpyridine)-C-HÁ Á Á(pyridyl). The connections between layers stacking along the c axis are of the type pyridyl-C-HÁ Á ÁS and dithiocarbamate-methyl-C-HÁ Á Á(pyridyl). Two illustrations of the molecular packing are given in Fig Table 2 Hydrogen-bond geometry (Å , ).

Figure 3
Two representations of the molecular packing in (I), showing (a) a view of the unit-cell contents in projection down the b axis and (b) a simplified view where all H atoms not participating in the specified intermolecular contacts are removed, adjacent layers are coloured in green and brown, and 3methylpyridine molecules are coloured orange. The C-HÁ Á ÁS, C-HÁ Á ÁN and C-HÁ Á Á interactions are shown as orange, blue and purple dashed lines, respectively.

Figure 2
The two-dimensional architecture in (I), showing (a) a view in projection down the a axis, (b) a view slightly off-set from the a axis and (c) a view in projection down the b axis. The 3-methylpyridine molecules are shown in space-filling mode. All H atoms have been removed for reasons of clarity.

Database survey
The dithiocarbamate anion, À [S 2 CN(Et)CH 2 py], found in (I) has been reported in a series of diorganotin bis(dithiocarbamate)s (Barba et al., 2012) but there was no evidence for intermolecular Sn-N(py) interactions, the structures rather conforming to the expected motifs (Tiekink, 2008 (Yadav et al., 2014), i.e. with two pyridyl groups per dithiocarbamate ligand, which adopts a relatively rare one-dimensional coordination polymer with a twisted topology (Jotani, Tan et al., 2016). In the other structures, R is a non-coordinating residue. For example, in the centrosymmetric Zn II compound with R = CH 2 (ferrocenyl) (Kumar et al., 2016), a two-dimensional architecture is found. Reverting back to Hg II structures, when R = CH 2 (furyl) (Kumar et al., 2016), a flat, two-dimensional architecture is found as the Hg II atom lies on a centre on inversion. In the case of {Hg[S 2 CN(Me)CH 2 Py] 2 } n , molecules self-assemble into a one-dimensional coordination polymer as one pyridyl-N atom coordinates a neighbouring Hg II atom while the other is non-coordinating. Finally, when R = CH 2 (1-methyl-1H-pyrrol-2-yl) (Yadav et al., 2014), no Hg-N interactions are found. The Hg II atom has a distorted tetrahedral geometry defined by an S 4 donor set. Such a variety in structures warrants continuing interest in this area.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3. The carbon-bound 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). Owing to interference from the beam-stop, the (100) reflection was removed from the final cycles of refinement.  (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

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. Refinement. Being affected by the beamstop, the (100) reflection was omitted from the final cycles of refinement.