Synthesis and crystal structure of a heterobimetallic cadmium–sodium complex of 1,3,5-triazine-2,4,6-trione, [CdNa2(C3H2N3O3)4(H2O)8]

In the heterobimetallic cadmium–sodium complex, heptaaqua-1κ3 O,2κ2 O,3κ2 O-bis(μ-4,6-dioxo-1,4,5,6-tetrahydro-1,3,5-triazin-2-olato)-1:2κ2 O 2:N 1;2:3κ2 N 1:O 2-bis(4,6-dioxo-1,4,5,6-tetrahydro-1,3,5-triazin-2-olato)-1κO 2,3κO 2-2-cadmium-1,3-disodium, the ligand coordination around the Cd and Na atoms leads to the formation of a two-dimensional coordination polymer in the (110) plane, which is supported by means of a variety of N—H⋯O, O—H⋯O and O—H⋯N intermolecular and intramolecular interactions owing to different substitution patterns.


Chemical context
Chelation is considered as the preferred method for the reduction of toxic effects of heavy metals, in which the metals are removed in the form of stable complex chelates. Cadmium, one of the most toxic heavy metals, can accumulate in the human body, leading to renal dysfunction, lung cancer, etc. In addition, chelation reactions are utilized in the determination of cadmium toxicity (Flora & Pachauri, 2010) with 1,3,5-triazine-2,4,6-trione, also known as cyanuric acid, being the preferred ligand used for the chelation as it has multiple hydrogen-bond donor centres (Mistri et al., 2014). 1,3,5-Triazine-2,4,6-trione exists in either the keto or enol form but the most stable isomer is the keto form (Reva, 2015). In this work, we report the crystal structure of a heterobimetallic cadmium and sodium complex of 1,3,5-triazine-2,4,6-trione.

Structural commentary
The title complex crystallizes in the triclinic space group P1. Fig. 1 shows the asymmetric unit of the crystal, which consists of four cyanuric acid ligands, two sodium atoms (Na1 and Na2) and one cadmium atom. Of the four ligands, two are monodentately coordinated to Na1 and Na2 atoms each. The third ligand is coordinated bidentately to Na1 and Cd1 atoms and the fourth one also coordinated bidentately to Na2 and Cd1 atoms. The sodium atom Na2 is coordinated to oxygen atoms of two cyanuric acid ligands  ]. The Na1 atom is also coordinated to oxygen atoms of two cyanuric acid ligands [O10-Na1-O13 = 173.39 (7) ]. The Cd1 atom is coordinated to nitrogen atoms of two cyanuric acid ligands ] . In addition to the ligand coordination, atoms Na1, Na2 and Cd1 are also coordinated by two, three and four water molecules, respectively.
The title compound forms a two dimensional coordination polymer. In the coordination environment of the polymer, the Na1 atom is six-coordinate with the coordination angle varying from 90 [78.82 (6)-90.60 (6) ], forming a distorted octahedral geometry. Atom Na2 also exhibits a distorted octahedral geometry, with the coordination angles ranging from 77.17 (6) to 91.77 (6) . The cadmium atom also shows a distorted octahedral geometry with coordination angles in the range 87.94 (5) to 95.46 (6) . A similar geometry is observed for the Cd atom in the heterobimetallic compound tetraaquabis(malonato)cadmium(II)copper(II) (Dhanya et al., 2014) Hashemian & Mangeli, 2017). The water molecules are tetrahedrally coordinated to the cadmium atom, forming bond angles ranging from 81.59 (5) to 111.45 (5) . Three water molecules are coordinated to Na2, with bond angles ranging from 85.43 (6) to 102.25 (7) . Two water molecules are coordinated to Na1, forming a bond angle of 170.94 (7) . The two water molecules coordinated to Na1 bridge adjacent Na1 atoms on both sides, forming a coordination polymer chain along the a axis. These chains are interconnected by means of two Cd1 and Na2 coordinations through the cyanuric acid ligands present in the Na1 coordination polymer chain on both sides to build a 2D coordination polymer in the (110) plane. The asymmetric unit of the title compound with displacement ellipsoids drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ). water molecules with DÁ Á ÁA distances ranging from 2.716 (2) to 3.236 (2) Å (Table 1). Two types ofinteractions (Table 2) occur between the cyanurate rings of different units, having centroid-centroid distances of 3.5174 (12) and 3.4893 (11) Å , and two types of C-OÁ Á Á interactions (Table 3) with different cyanurate rings with XÁ Á ÁCg distances of 3.6086 (2) and 3.4783 (2) Å are also present in the complex (Fig. 2). A packing diagram is presented in Fig. 3.

Synthesis and crystallization
Needle-shaped transparent single crystals were obtained by the single gel diffusion method (Chandran et al., 2017). 1,3,5-Triazine-2,4,6-trione, acetic acid, sodium metasilicate and cadmium chloride hydrate were used for the growth in a single glass test tube of length 20 cm and diameter 2.5 cm. The preparation of silica gel of specific gravity 1.03-1.05 g cm À3 involved dissolution of sodium metasilicate (SMS) in doubledistilled water to which 1,3,5-triazine-2,4,6-trione (0.01-0.02 M concentration) was added. The resulting SMS solution was acidified with drops of glacial acetic acid to adjust the pH to within the range 4-7. The test tubes were filled with 30 ml of the above solution for gel setting. Over the set gel, cadmium chloride solution (0.25-1 M) was added carefully along the sides of the test tube to prevent the gel breakage. Finally, the test tube was sealed with a transparent plastic sheet to prevent contamination and kept undisturbed for crystal growth. Crystals formed within the gel after two weeks and growth was completed in a month. A series of trials was undertaken to obtain the optimum conditions to grow well-defined single crystals. 1,3,5-Triazine-2,4,6-trione (0.02 M) was used as inner reactant and cadmium chloride (0.25 M) as the top solution.
Well-defined good-quality single crystals suitable for singlecrystal XRD studies were grown in a gel medium of pH 6 and density 1.03 g cm À3 .

Refinement
Crystal data, data collection and structure refinement details are summarized in  Table 2 Analysis ofinteractions (Å , ).

Figure 3
The packing, viewed down the a axis, showing the coordination polyhedra.

cadmium-1,3-disodium
Crystal data 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.