A one-dimensional coordination polymer, catena-poly[[[[N-ethyl-N-(pyridin-4-ylmethyl)dithiocarbamato-κ2 S,S′]zinc(II)]-μ2-N-ethyl-N-(pyridin-4-ylmethyl)dithiocarbamato-κ3 S,S′:N] 4-methylpyridine hemisolvate]

The title compound, {Zn[S2CN(Et)CH2py]2.(4-methylpyridine)0.5}n, is a one-dimensional coordination polymer with a zigzag topology.


Chemical context
The most recent surveys of the structural chemistry of the binary zinc-triad dithiocarbamates, i.e. molecules of the general formula M(S 2 CNRR 0 ) 2 for M = Zn, Cd and Hg, indicated that up to that point, R and R 0 were generally restricted to alkyl groups, with only rare examples of R being an aryl group (Tiekink, 2003;Hogarth, 2005). However, since around that time there has been increasing interest in elaborating dithiocarbamate ligands to enhance their functionality for systematic structural studies. This enhancement can be achieved in two ways utilizing their facile procedure of synthesis, i.e. the reaction of CS 2 with an amine in the presence of base. Hence, the utilization of diamines can lead to bis(dithiocarbamates), e.g. À S 2 CN-R-CS 2 À , R = alkyl/aryl (e.g. Cookson & Beer, 2007;Knight et al., 2009;Oliver et al. 2011). Alternatively, the chosen amine can carry a functional group capable of additional coordination to a metal cation, typically a pyridyl group (e.g. Barba et al., 2012;Singh et al., 2014) or groups capable of forming hydrogen-bonding interactions (e.g. Benson et al., 2007;Howie et al., 2008). It is the former class of ligand with a pyridyl substituent which forms the focus of the present contribution.
Previous structural studies have revealed a diversity of coordination modes in the zinc-triad elements coordinated by dithiocarbamate ligands functionalized with pyridyl substi- ISSN 2056-9890 tuents. Thus, a two-dimensional architecture is found in centrosymmetric {Zn[S 2 CN(CH 2 ferrocenyl)CH 2 py] 2 } n , with both pyridyl N atoms being coordinating (Kumar et al., 2016). In the cadmium analogue, isolated as a 1,10-phenanthroline (phen) adduct, i.e. Cd[S 2 CN(CH 2 ferrocenyl)CH 2 py] 2 (phen), no additional Cd-N(pyridyl) interactions are formed in the crystal as the cadmium cation is coordinatively saturated (Kumar et al., 2016). However, in {Cd{[S 2 CN(CH 2 Ph)-CH 2 py] 2 } n and related species, all potential donor atoms are coordinating, leading to a two-dimensional coordination polymer . It is interesting to note that zerodimensional aggregation can also occur, as in the case of {Cd[S 2 CN(1H-indol-3-ylmethyl)CH 2 (CH 2 py)] 2 } 2 , where the tridentate mode of coordination of one dithiocarbamate is retained, but aggregation leads to a dimer only . This may be a result of the now well established steric effects in 1,1-dithiolate chemistry (Tiekink, 2003(Tiekink, , 2006. Several related structures are also available for mercury. In {Hg[S 2 CN(CH 2 Py) 2 ] 2 ]} n , with two pyridyl groups per dithiocarbamate ligand, an unusual one-dimensional coordination polymer with a twisted topology is found in the crystal, as one pyridyl N atom is noncoordinating (Yadav et al., 2014;Jotani et al., 2016). When one CH 2 py group is replaced by a methyl substitutent, as in {Hg[S 2 CN(Me)CH 2 Py] 2 } n , a one-dimensional coordination polymer is also found. Again, when one substituent is large, i.e. as in {Hg[S 2 CN-{CH 2 (1-methyl-1H-pyrrol-2-yl)}CH 2 Py] 2 } n (Yadav et al., 2014), no Hg-N(pyridyl) interactions are found. Very recently, the crystal structure of a binary compound, isolated as the 3-methylpyridine monosolvate, i.e. {Cd[S 2 CN(Et)CH 2 py] 2 Á3-methylpyridine} n , was described and found to feature two S,S 0 ,N-tridentate dithiocarbamate ligands, leading to a twodimensional coordination polymer (Arman et al., 2017), as seen earlier in some of the precedents mentioned above ; the 3-methylpyridine solvent molecules reside in square-shaped channels. In continuation of these structural studies, herein, the crystallographic characterization of a closely related zinc compound to the last mentioned species, namely {Zn[S 2 CN(Et)CH 2 py] 2 Á(4-methylpyridine) 0.5 } n , is described.

Structural commentary
The asymmetric unit of (I) comprises two independent Zn[S 2 CN(Et)CH 2 py] 2 residues, shown in Fig. 1, and a 4-methylpyridine solvent molecule. Each of the dithiocarbamate ligands is chelating, forming approximately similar Zn-S bond lengths, see data in Table 1. For the Zn1containing molecule, the disparity in the Zn-S bond lengths, i.e. Á(Zn-S) = [Zn-S(long) À Zn-S(short)], for the S1dithiocarbamate ligand of 0.32 Å is greater than the value of 0.10 Å for the S3-dithiocarbamate ligand. For the Zn2-molecule, these differences diminish to 0.23 and 0.09 Å for the S5and S7-dithiocarbamate ligands, respectively. The similarity of the structures is emphasized in the overlay diagram of Fig. 2, showing minor variations in the orientations of the pyridyl rings and in the relationship between the two chelate rings. In each of the Zn-containing molecules, one dithiocarbamate The molecular structures of the two independent Zn[S 2 CN(Et)CH 2 py] 2 fragments in the asymmetric unit of (I), showing the atom-labelling scheme and displacement ellipsoids at the 50% probability level. ligand coordinates in a 2 -3 mode, chelating one Zn II cation and simultaneously bridging another via the pyridyl N atom. It is noted that it is the dithiocarbamate ligand that forms the more equivalent Zn-S bond lengths in each residue that forms the bridging interactions. The resultant coordination geometry for each Zn II cation is based on an NS 4 donor set.
For five-coordinate species, the value computed for is a useful indicator of the adopted coordination geometry, with the values of ranging from 0 to 1 for ideal square-pyramidal and trigonal-bipyramidal geometries, respectively (Addison et al., 1984). In (I), the values of for Zn1 and Zn2 are 0.33 and 0.23, respectively, indicating that Zn2 is closer to a square pyramid than Zn1. In each case, the pyridyl N atom occupies the approximately apical position, as indicated by the range of N-Zn1-S angles of 97.62 (8)-111.76 (9) and the narrower range of N-Zn2-S angles of 99.72 (9)-110.48 (9) . In this description, the Zn1 cation lies 0.6827 (6) Å above the best plane through the four S atoms, i.e. S1-S4 (r.m.s. deviation = 0.1721 Å ), in the direction of the pyridyl N6 atom. For the Zn2-molecule, the deviation of the Zn2 cation from the S 4 plane is 0.6018 (6) Å and the r.m.s. deviation through the S5-S8 atoms is 0.1273 Å .
The result of the presence of equal numbers of chelating and bridging ligands in (I) is the formation of a supramolecular polymer aligned along [010], as illustrated in Fig. 3. The topology of the chain is zigzag. Finally, the 4-methylpyridine solvent molecule is non-coordinating.
The most closely related structure in the literature for comparison is that of the aforementioned recently reported {Cd[S 2 CN(Et)CH 2 py] 2 Á3-methylpyridine} n , which was also isolated from an experiment attempting to coordinate isomeric methylpyridines to the heavy element (Arman et al., 2017). The crucial difference between the two structures is that in the cadmium crystal, both dithiocarbamates adopt a 2 -3 coordination mode, leading to a cis-N 2 S 4 coordination geometry and a two-dimensional framework with a flat topology. It is highly likely that the disparity in supramolecular aggregation in the zinc and cadmium compounds arises from the greater ability of the larger Cd atom to expand its donor set.

Supramolecular features
As mentioned above, the supramolecular chains in the crystal of (I) are aligned along [010]. In the crystal, these chains are connected into a three-dimensional architecture by a number of weak intermolecular interactions, as summarized in Table 2. There are two distinct C-HÁ Á ÁS interactions, with the donors being methyl-and pyridyl-C-H groups, as well as a methylene-C-HÁ Á ÁN(pyridyl) interaction. The other connection between chains is of the type pyridyl-C-HÁ Á Á(Zn1,S3,S4,C10), an interaction well known in metal dithiocarbamates (Tiekink & Zukerman-Schpector, 2011) and, indeed, other metal systems (Tiekink, 2017). The main connection identified between the 4-methylpyridine solvent molecule and the chain is of the type pyridyl-C-HÁ Á ÁN(4methylpyridine). An illustration of the molecular packing is given in Fig. 4.

Figure 2
A molecular overlay diagram of the two independent molecules of Zn[S 2 CN(Et)CH 2 py] 2 . The Zn1-containing molecule is shown in red and the molecules have been overlapped so that the two more symmetrically chelating dithiocarbamate ligands are coincident. Table 2 Hydrogen-bond geometry (Å , ).
Cg1 is the ring centroid of the Zn1/S3/S4/C10 ring.  (Barba et al., 2012). In only the R = Me compound is there a weak inter-molecular SnÁ Á ÁN(pyridyl) interaction of 2.98 Å between the two molecules comprising the asymmetric unit. This result is consistent with surveys of diorganotin bis(dithiocarbamate)s in general (Tiekink, 2008) which suggest that the Sn atom in these compounds does not usually increase its coordination number by forming secondary bonding interactions (Tiekink, 2017). Specifically, for dimethyltin compounds, R 2 Sn-(S 2 CNR 0 R 00 ) 2 , a recent survey indicated that secondary bonding interactions occur in only 10% of their crystal structures (Zaldi et al., 2017)

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) values set at 1.2-1.5U eq (C).   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).