In situ synthesis, crystal structures, topology and photoluminescent properties of poly[di-μ-aqua-diaqua[μ3-4-(1H-tetrazol-1-id-5-yl)benzoato-κ4 O:O,O′:O′′]barium(II)] and poly[μ-aqua-diaqua[μ3-4-(1H-tetrazol-1-id-5-yl)benzoato-κ4 O:O,O′:O′]strontium(II)]

The crystal structures of two alkaline-earth-based coordination compounds containing bifunctional tetrazolecarboxylate ligands reveal a one-dimensional chain structure with the carboxylate-tetrazole anionic ligand. The structures of both compounds are dominated by hydrogen bonds involving water coordination ligands, with an underlying 3.6 − c network.


Chemical context
In recent years, studies on a wide variety of tetrazolyl-5substituted coordination compounds have proliferated (Klapö tke & Stierstorfer, 2009;Fischer et al., 2011). The extension from the synthetic approach developed by Demko and Sharpless (2001) to that of Zhao and colleagues (Zhao et al., 2008) is the main reason for this new interest. Chemists have focused on transition-metal compounds, while studies with alkaline-earth metal-tetrazol coordination compounds remain scarce. This led us to further explore this type of compound, and to study their topological and physical properties.
The choice of ligand is essential in the design of new coordination compounds. In our study we selected a (tetrazolcarboxylate) bifunctional ligand, which is able to adopt several coordination modes, resulting in a variety of crystal structures (Ouellette et al., 2012;Sun et al., 2013;Wei et al., 2012).

Structural commentary
Compound (I) crystallizes in the orthorhombic space group Imma while compound (II) crystallizes in Pmna. In these two coordination compounds, the asymmetric unit comprises half of a crystallographically independent alkaline-earth metal ion, half of a deprotonated 4-(tetrrazol-5-yl)benzoate anion (ttzbenz), and two halves of water molecules in compound (I) and three halves of water molecules in compound (II) (Fig. 1). The bond distances and angles of the ligands are comparable to those found in the literature for similar systems (Zheng et al., 2009;Jiang et al., 2007;Yu et al., 2009).
The ttzbenz ligand can adopt several coordination modes by involving the tetrazole ring (Yao et al., 2013), or the carboxylate group as in our case, where the two compounds use the ttzbenz anion to coordinate two adjacent Ae 2+ cations in a bidentate chelate manner, thus forming a polyatomic bridge and binding neighboring Ae 2+ ions in a zigzag manner, resulting in the formation of binuclear units [Ae-O1-Ae-O1] with a BaÁ Á ÁBa distance of 4.0089 (4) Å for compound (I) and an SrÁ Á ÁSr distance of 3.866 (2) Å for compound (II) (Fig. 3).

Figure 3
Coordinating polymers along the b axis.

Topological study
To simplify the crystalline structure of the title compounds, we used the standard representation of valence-bound CPs (CP = coordination polymer) to obtain the underlying network. In such models, only metal centers and the centroids of organic ligands are considered as structural units (Alexandrov et al., 2011). The simplification of the crystal structure of the two compounds by this procedure and the topological classification of the two studied compounds led to the same topological network, identified as a 3.6-c net with stoichiometry (3-C) 2 (6-C), which can be represented by the point symbol {4 3 } 2 {4 6 .6 6 .8 3 }. Thus the two structures consist of planar layers running parallel to (100) (Fig. 5).

Synthesis and crystallization
Colorless crystals suitable for X-ray diffraction were obtained by hydrothermal synthesis in an aqueous solution according to a literature procedure (Demko & Sharpless, 2001;Zhao et al., 2008), where an aqueous solution (10 ml) of sodium azide (0.065 g, 1 mmol) and 4-cyanobenzoyl chloride (0.165 g, 1 mmol) was added dropwise to an aqueous solution (5 ml) of BaCl 2 Á2H 2 O (0.244 g, 1mmol) for (I) and SrCl 2 Á6H 2 O (0.266g, 1 mmol) for (II) under constant stirring for a few minutes. The resulting solution was sealed in a 25ml teflon-lined stainless steel autoclave and heated at 453 K for 3 d.

Figure 4
Hydrogen bonds (blue dashed lines) and -stacking interactions (green dashed lines) in the crystal packing of compounds (I) and (II).
The thermogravimetric analysis (TGA) was performed in the range 25-600 C under air atmosphere at a flow rate of 5 C/ min (Fig. 6). The pyrolytic processes for compound (I) occurs in two main steps. The first step corresponds to the release of four water molecules (2 bridging water molecules and 2 monodentate) (scheme1) between 90 C and 200 C, which corresponds to approximately 18% of the weight of (I). Subsequently, the ligands undergo pyrolysis to result in decomposition (32% by weight) in the range of 200 to 600 C. In compound (II), the pyrolytic processes also go through two stages. The first step corresponds to the release of three water molecules (1 bridging water molecule and 2 monodentate) (scheme1) between 100 C and 160 C, which corresponds to approximately 16% of the weight of (II). The second step corresponding to a weight loss of 44% of (II) is attributed to the decomposition of the ligand 160 and 600 C.

Thermogravimetric analysis
The thermogravimetric analysis (TGA) was performed in the range 25-600 C under an air atmosphere at a flow rate of 5 C min À1 (Fig. 6). The pyrolytic processes for compound (I) occur in two main steps. The first step corresponds to the release of four water molecules (two bridging water molecules and two monodentate) between 90 C and 200 C, which corresponds to approximately 18% of the weight of (I). Subsequently, the ligands undergo pyrolysis to result in decomposition (32% by weight) in the range 200-600 C. In compound (II), the pyrolytic processes also go through two stages. The first step corresponds to the release of three water molecules (one bridging water molecule and two monodentate) between 100 C and 160 C, which corresponds to approximately 16% of the weight of (II). The second step corresponding to a weight loss of 44% of (II) is attributed to the decomposition of the ligand between 160 and 600 C.

Fluorescence properties
The fluorescence properties of compounds (I) and (II) were determined from the emission spectra at the same excitation wavelength (eX = 322 nm) on an Agilent Cary Eclipse Fluorescence Spectrophotometer at room temperature. Excitation of the two compounds after dissolution in DMSO leads to similar fluorescence emission spectra. The emission maximum of (I) is observed to shift from 368 to 377 nm and from 371 to 378 nm for II (see Fig. S2 in the supporting information), probably corresponding to * ! or *!n electronic transition of the aromatic ring ttzbenz ligands (Koşar et al., 2012), due to the close resemblance of the emission band of the two compounds. We also note downward absorption values ranging from compound (I) to (II), which may be due to the increase in the atomic number from Sr 2+ to Ba 2+ .

Crystal data
[Ba (C 8  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.91 e Å −3 Δρ min = −0.31 e Å −3 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Ba1 0.5000 0.7500 0.462655 (6) 0.02050 (7)   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.65 e Å −3 Δρ min = −0.44 e Å −3 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.