Crystal structures of {μ2-N,N′-bis[(pyridin-3-yl)methyl]ethanediamide}tetrakis(dimethylcarbamodithioato)dizinc(II) dimethylformamide disolvate and {μ2-N,N′-bis[(pyridin-3-yl)methyl]ethanediamide}tetrakis(di-n-propylcarbamodithioato)dizinc(II)

The ZnII atom is each of the title compounds is coordinated by two dithiocarbamate ligands and a pyridyl-N atom. The resultant NS4 donor set approximates a square-pyramid and trigonal-bipyramid, for the solvated and unsolvated structures, respectively. In the solvate, amide-N—H⋯O(dimethylformamide) hydrogen-bonds define a three-molecule aggregate while in the unsolvated structure, amide⋯amide hydrogen-bonding leads to a supramolecular chain.

The title structures, [Zn 2 (C 3 H 6 NS 2 ) 4 (C 14 H 14 N 4 O 2 )]Á2C 3 H 7 NO (I) and [Zn 2 (C 7 H 14 NS 2 ) 4 (C 14 H 14 N 4 O 2 )] (II), each feature a bidentate, bridging bipyridyl-type ligand encompassing a di-amide group. In (I), the binuclear compound is disposed about a centre of inversion, leading to an open conformation, while in (II), the complete molecule is completed by the application of a twofold axis of symmetry so that the bridging ligand has a Ushape. In each of (I) and (II), the dithiocarbamate ligands are chelating with varying degrees of symmetry, so the zinc atom is within an NS 4 set approximating a square-pyramid for (I) and a trigonal-bipyramid for (II). The solvent dimethylformaide (DMF) molecules in (I) connect to the bridging ligand via amide-N-HÁ Á ÁO(DMF) and various amide-, DMF-C-HÁ Á ÁO(amide, DMF) interactions. The resultant three-molecule aggregates assemble into a three-dimensional architecture via C-HÁ Á Á(pyridyl, chelate ring) interactions. In (II), undulating tapes sustained by amide-N-HÁ Á ÁO(amide) hydrogen bonding lead to linear supramolecular chains with alternating molecules lying to either side of the tape; no further directional interactions are noted in the crystal.

Chemical context
The potential of self-association between amide functionalities via amide-N-HÁ Á ÁO(amide) hydrogen-bonding has long been recognized (MacDonald & Whitesides, 1994). In this way, eight-membered {Á Á ÁHNCO} 2 synthons can be formed. Alternatively, extended aggregation patterns based on a single point of contact repeat associations leading to supramolecular chains or double-connections (edge-shared) leading to tapes. In this connection, isomeric di-amide structures of the general formula (n-NC 5 H 4 )CH 2 N(H)C( O)-C( O)N(H)CH 2 -(C 5 H 4 N-n), for n = 2, 3 and 4, hereafter abbreviated as n LH 2 , have long attracted interest for their potential to form supramolecular tapes. For example, as realized in the twodimensional structure formed in the 1:1 co-crystal of 4 LH 2 and the conformer, bi-functional 1,4-di-iodobuta-1,3-diyne (Goroff et al., 2005). Here, the amide tapes are orthogonal to the NÁ Á ÁI halogen bonding. In the realm of metal-containing species, a three-dimensional architecture can be assembled in ISSN 2056-9890 the crystal of {[Ag( 3 LH 2 ) 2 ]BF 4 } n by a combination of Ag N bonds for the tetrahedral silver(I) atom, provided by bidentate bridging ligands, where the latter are also connected via concatenated {Á Á ÁHNC 2 O} 2 synthons (Schauer et al., 1997).

Structural commentary
The molecular structure of the centrosymmetric, binuclear zinc(II) compound in (I) is shown in Fig. 1a and selected geometric parameters are collected in Table 1. The zinc centre is coordinated by two chelating dithiocarbamate ligands and the coordination geometry is completed by a pyridyl-N atom. The dithiocarbamate ligands coordinate differently, with the S1-ligand coordinating almost symmetrically with Á(Zn-S) = (Zn-S long À Zn-S short ) = 0.10 A. By contrast, the S3-ligand coordinates slightly more asymmetrically with Á(Zn-S) = 0.18 Å . These differences are not reflected in the associated C-S bond lengths, which span an experimentally equivalent range of 1.720 (2) to 1.732 (2) Å . The resulting NS 4 donor set defines a distorted square-pyramidal geometry as judged by the value of = 0.18 which compares to = 0.0 for an ideal square-pyramid and 1.0 for an ideal trigonal-bipyramidal geometry (Addison et al., 1984). In this description, the zinc atom lies 0.5011 (3) Å above the plane defined by the four sulfur atoms [r.m.s. deviation = 0.0976 Å with the range of deviations being À0.0990 (3) Å for the S3 atom to 0.0987 (3) Å for S2]. The widest angles are defined by the sulfur atoms forming the shorter of the Zn-S bonds of each dithiocarbamate ligand and by those forming the longer Zn-S bonds. The dihedral angle between the best plane through the four sulfur atoms and that through the pyridyl ring is 87.13 (4) , indicating a near perpendicular relationship. The dihedral angle between the two chelate rings is 27.46 (6) .
The molecular structure of the binuclear zinc(II) compound, (II), is shown in Fig. 1b and again selected geometric parameters are collected in Table 1. The first and most obvious distinction between the binuclear compounds in (I) and (II) relates to the symmetry within the molecules, i.e. the bridging ligand is disposed about a centre of inversion in (I), leading to an extended conformation, but is disposed about a twofold axis in (II), leading to a curved conformation. While to a first approximation the coordination geometry in (II) matches that in (I), some differences are apparent. Each dithiocarbamate ligand coordinates asymmetrically with Á(Zn-S) = 0.26 and 0.22 Å , respectively, and these differ-ences are reflected in the associated C-S bond lengths with those associated with the weakly coordinating sulfur atoms being significantly shorter than those associated with the more tightly bound sulfur atoms, Table 1. There is also a significant difference in the coordination geometry defined by the NS 4 donor set with = 0.76. This difference arises from a reduction, by approximately 25 , of the angle subtended at the zinc atom by the more tightly bound sulfur atoms, Table 1. The change in coordination geometry is reflected in the relatively wide dihedral angle between the chelate rings of 59.41 (3) .
The common feature of (I) and (II) is the relatively long central sp 2 -C-C(sp 2 ) bond, Table 1. This feature for these ligands is well established and is reflected by comparable bond lengths determined by experiment and theory for the two polymorphs known for the uncoordinated ligand, 3 LH 2 (Jotani, Zukerman-Schpector et al., 2016). Interestingly, in one of the polymorphs, both independent molecules are disposed about a centre of inversion and adopt an anti-periplanar form, as in (I), while in the second polymorph, the molecule is twofold symmetric with a U-shaped conformation, i.e. is synperiplanar, as in (II). Computational chemistry indicated no significant energy difference between the two conformations, a result consistent with the literature expectation for the majority of conformational polymorphs (Cruz-Cabeza et al., 2015).

Supramolecular features
The presence of solvent DMF molecules in the crystal of (I) precludes supramolecular self-association between the amide functionality. Instead, three-molecule aggregates are generated via amide-N-HÁ Á ÁO(DMF) hydrogen bonds, Fig. 2a and Table 2. These aggregates are further linked via DMF-C-HÁ Á ÁO(amide) and pyridyl-C-HÁ Á ÁO(DMF) interactions, leading to eight-membered {Á Á ÁOC 2 NHÁ Á ÁOCH} and sevenmembered {Á Á ÁOÁ Á ÁHNC 3 H} synthons, respectively. Connections between these aggregates are of the type methyl-C-HÁ Á Á, where the -systems are either the pyridyl ring or one of the chelate rings. Referring to the latter, such C-HÁ Á Á(chelate) ring interactions are more and more being observed in the structural chemistry of metal dithiocarbamates owing, no doubt, to the effective chelating ability of dithiocarbamate ligands, which leads to significant -electron density within the chelate rings they form (Tiekink & Zukerman-Schpector, 2011;Tiekink, 2017). The net result of the foregoing is a three-dimensional architecture, Fig. 2b. From the view down the b axis, Fig. 2c, there are obvious areas with little or no directional interactions between the residues.
By contrast to the myriad of supramolecular associations identified in the crystal of (I), only conventional amide-N-HÁ Á ÁO(amide) hydrogen bonding is found in the crystal of (II), Table 3, with no other specific interactions identified based on the distance criteria in PLATON (Spek, 2009). The hydrogen bonding leads to linear supramolecular chains along the c axis, Fig. 3a, with alternate binuclear molecules lying above and below the plane defined by the supramolecular tape  Table 1 Geometric data (Å , ) for (I) and (II).

Database survey
The investigation of zinc(II) dithiocarbamates, Zn(S 2 CNRR 0 ) 2 , with at least one of R/R 0 being CH 2 CH 2 OH, has lead to an interesting array of structures owing to hydrogen bonding. Thus, hydroxy-O-HÁ Á ÁO(hydroxy) hydrogen bonding links otherwise molecular species into supramolecular chains in the cases of Zn[S 2 CN(R)CH 2 CH 2 OH] 2 (pyridine)Ápyridine for R = Me and Et (Poplaukhin & Tiekink, 2017) and Zn[S 2 CN(Me)CH 2 -CH 2 OH] 2 (3-hydroxypyridine)  and supramolecular layers via hydroxy-O-HÁ Á ÁS(dithiocarbamate) hydrogen bonds in Zn[S 2 CN(i-Pr)CH 2 CH 2 OH] 2 (2,2 0bipyridine) (Safbri et al., 2016); the propensity for the hydroxy group in dithiocarbamate ligands with R = CH 2 CH 2 OH to form O-HÁ Á ÁS rather than O-HÁ Á ÁO hydrogen bonds has been summarized recently (Jamaludin et al., 2016). With potentially bridging ligands, mixed results have been observed in recent studies: in terms of potentially tetra-coordinate urotropine (hexamethylenetetraamine, hmta), monodentate coordination has been found in each of the four independent molecules comprising the asymmetric unit of Zn[S 2 CN(i-Pr)-CH 2 CH 2 OH] 2 (hmta) (Câ mpian et al., 2016). Supramolecular layers are sustained by hydroxy-O-HÁ Á ÁO(hydroxy) and hydroxy-O-HÁ Á ÁS(dithiocarbamate) hydrogen bonding, as per above, augmented by hydroxy-O-HÁ Á ÁN(hmta) hydrogen bonding. Bidentate bridging has been found in 2:1 adducts of Zn[S 2 CN(CH 2 CH 2 OH) 2 ] 2 } 2 with pyrazine (Jotani et al., 2017) and 4,4 0 -bipyridyl (Benson et al., 2007) in which three-dimensional architectures are sustained by hydroxy-O-HÁ Á ÁO(hydroxy) hydrogen bonding. Apart from the interwoven polymers discussed in the Chemical context, the most closely related compounds to the title compounds are thioamide analogues of 3 LH 2 , i.e. 3 LSH 2 . Some interesting crystal chemistry occurs when {Zn[S 2 CN(Me)CH 2 CH 2 OH] 2 } 2 -( 3 LSH 2 ) is recrystallized from acetonitrile (Poplaukhin et al., 2012). Upon prolonged standing, a one molar ratio of S 8 , a decomposition product, is incorporated in the co-crystal with hydroxy-O-HÁ Á ÁO(hydroxy) hydrogen bonding leading to a two-dimensional array. When DMF is diffused into an acetonitrile solution of the same compound, one hydroxy group hydrogen bonds to the DMF-O while the other hydroxyl group self-associates to form a supramolecular chain. In the present study, when additional hydrogen-bonding functionality is not present, the amide groups are able to self-assemble as shown in Fig. 3. With the foregoing in mind, i.e. variable coordination geometries, flexible conformations of the bridging ligands and different hydrogen-bonding potential, more systematic studies in this area are warranted.

Synthesis and crystallization
Crystals of (I) were grown from liquid diffusion of ether into a 1:1 molar ratio of Zn(S 2 CNMe 2 ) 2 and 3 LH 2 in DMF; m.p. 479-481 K. Crystals of (II) were grown from the slow evaporation of a 2:1 molar ratio of Zn[S 2 CN(n-Pr) 2 ] 2 and 3 LH 2 in a MeOH/ EtOH solution.

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 = 0.95-0.98 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 N-bound H atoms were located in difference-Fourier maps but were refined with a distance restraint of N-H = 0.88AE0.01 Å , and with U iso (H) set to 1.2U eq (N). Owing to poor agreement, two reflections, i.e. (356) and (014), were omitted from the final cycles of refinement of (I).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq