Crystal structure of a cadmium sulfate coordination polymer based on the 3,6-bis(pyrimidin-2-yl)-1,4-dihydro-1,2,4,5-tetrazine ligand

In the title compound, polymeric (100) sheets consisting of [Cd(SO4)(H2O)] units are linked by H2bmtz [H2bmtz is 3,6-bis(pyrimidin-2-yl)-1,4-dihydro-1,2,4,5-tetrazine] into a three-dimensional structure.


Chemical context
Coordination polymers (CPs) are a class of organic-inorganic hybrid materials formed from metal ions or metal clusters and organic linkers through covalent bonds. The structural organization of CPs can result in chains, sheets or three-dimensional frameworks (Batten et al., 2009). These hybrid materials have received extensive attention over the past three decades owing to their structural features and useful applications in the fields of gas storage and separation, catalysis, chemical sensing, magnetism or proton conduction (Furukawa et al., 2010;Ye & Johnson, 2016;Espallargas & Coronado, 2018;Xu et al., 2016;Zhang et al., 2017). Nowadays, many multidimensional CPs with structural and topological diversity have been synthesized through the tremendous possibilities of choices for building blocks, and some of them seem promising as candidate materials, for instance, in gas purification (Duan et al., 2015). In the context of the crystal engineering of CPs, the most feasible strategy for the construction of such infinite hybrid networks is by the careful selection of metal coordination arrangements and suitable organic linkers. Among the most common ligands, the rigid organic carboxylate-and pyridyl-based ligands have by far been the most widely used to control the structural motifs of these solids (Glö ckle et al., 2001).

Structural commentary
The asymmetric unit of the title compound consists of one Cd II cation, one-half of the H 2 bmtz ligand, one sulfate anion and one coordinating water molecule. The 1,4-dihydro-1,2,4,5tetrazine ring of the H 2 bmtz ligand is located about an inversion centre, with the NH group (N4) being equally disordered over two sites. As shown in Fig. 1 Safin et al., 2013). The complete H 2 bmtz molecule is not planar (r.m.s. deviation = 0.111 Å ) with the central six-membered ring of the 1,4-dihydro-1,2,4,5-tetrazine moiety in a twist-boat conformation; the C5-N3-N4A i -C5 i torsion angle is 36.4 (4) [symmetry code: (i) 2 À x, 1 À y, 1 À z]. The sulfate anion acts as a 3 -bridging ligand to connect three Cd II atoms to form a sheet-like structure of [Cd(SO 4 )(H 2 O)] units, propagating parallel to the bc plane, Fig. 2. Adjacent sheets are interconnected across the H 2 bmtz ligands, which exhibit a bisbidentate coordination mode, giving rise to a three-dimensional framework structure, Fig. 3.

Supramolecular features
In the crystal, classical O-HÁ Á ÁO hydrogen bonds exist between the coordinating water molecules and the sulfate groups, and N-HÁ Á ÁO hydrogen bonds involving the disordered tetrazine NH group and sulfate oxygen atoms. In this Molecular structure of (I), showing the atom-labelling scheme. Only one orientation of the disordered N4-H group is shown. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: way, rings with R 1 1 (8) and R 4 4 (16) graph-set motifs are formed, Table 1. Additionally, C-HÁ Á Á [HÁ Á ÁCg = 3.34 (2) Å ; Cg is the centroid of the pyrimidine ring] andstacking [centroid-to-centroid separation = 3.5954 (15) Å , slippage between parallel pyrimidine rings = 1.131 Å ] interactions between the pyrimidine rings of the H 2 bmtz ligand are also observed, Fig. 4.  Chainok et al., 2012). In these structures, the coordination mode of the H 2 bmtz ligands is bis-bidentate through nitrogen atoms.

Synthesis and crystallization
All reagents were of analytical grade and were used as received without further purification. The ligand 3,6-bis(pyrimidin-2-yl)-1,4-dihydro-1,2,4,5-tetrazine was synthesized according to a literature method (Kaim & Fees, 1995). A mixture solution of CdSO 4 Á8/3H 2 O (41.7 mg, 0.2 mmol) and the H 2 bmtz ligand (36.7 mg, 0.1 mmol) in water (5 ml) was added into a 15 ml Teflon-lined reactor, stirred at room temperature for 10 min, sealed in a stainless steel autoclave and placed in an oven. The mixture was heated to 383 K under autogenous pressure for 48 h, and then cooled down to room temperature. After filtration, brown block-shaped crystals were obtained in 80% yield (33.4 mg) based on the cadmium(II) source.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 Partial packing diagram of (I), showing C-HÁ Á Á andstacking interactions (dashed lines) between the H 2 bmtz ligands. Table 1 Hydrogen-bond geometry (Å , ).

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.