A new cadmium coordination polymer based on 4-amino-4H-1,2,4-triazole

Here we report the chemical synthesis and crystal structure of a new hybrid chloro-cadmium coordination polymer based on 4-amino-4H-1,2,4 triazole solved by single-crystal X-ray diffraction. With an unusual architecture, the crystal structure exhibits two distorted octahedral coordinations of CdII joined by edge sharing. The first is composed by four chlorine and two N atoms from the triazole ligands. The second is formed by five Cl atoms and by one N atom from the triazole ligand.


Chemical context
The last decade has seen a large number of investigations of Cd II hybrid coordination polymers (HCPs). Indeed, these materials exhibit a wide variety of polymeric frameworks with attractive properties. The coordination sphere of Cd II is variable, with coordination numbers ranging from four to eight, corresponding to different geometries (tetrahedral, square planar, square pyramidal, trigonal bipyramidal, octahedral, pentagonal bipyramidal, bicapped triangular prismatic and dodecahedral; Li & Du, 2011). Many factors should be considered in the self-assembly processes of HCPs, such as the nature of the organic ligands, temperature, pH values, solvents, and so on (Guo et al., 2013). The choice of the organic ligands is an important factor that greatly influences the structure and stabilization of the coordination architecture formed (Tao et al., 2000;Choi & Jeon, 2003). In this regard, organic building units that are based on five-membered N-heterocycles such as 1,2,4 triazole exhibit a strong and typical property of acting as bridging ligands between two metal centres. These bridges can adopt various different geometries, depending on the donor atoms of the ligand and the properties of the metal (Haasnoot et al., 2000). The reaction of 4-amino-4H-1,2,4 triazole (NH 2 trz) with cadmium dichloride leads to the formation of the title two-dimensional coordination polymer.

Structural commentary
The asymmetric unit of the studied compound, completed by the atoms necessary to achieve the coordination around the Cd ions, is represented in Fig. 1. It comprises one and a half ISSN 2056-9890 Cd II cations [with Cd2 occupying the special position ( 1 2 , 1 2 , 1 2 )], one triazole molecule (NH 2 trz), one triazolium cation (NH 2 trzH) + , four chloride anions and one lattice water molecule. Cd1 and Cd2 are bridged by the coordinated triazole molecule (NH 2 trz) through atoms N1 and N2, and by the two chlorine atoms Cl1 and Cl3.
Both metals show an octahedral coordination geometry. Cd1 is surrounded by the five chloride anions Cl1, Cl2, Cl3, Cl4, Cl2 i [symmetry code: (i) x, 1 2 À y, z À 1 2 ] and the nitrogen N1 of the coordinated triazole ring (NH 2 trz). On the other hand, Cd2 is bonded to four equatorial chloride anions (Cl1, Cl3, Cl1 ii and Cl3 ii ) and two axial nitrogen atoms, N2 and N2 ii , belonging to the coordinated triazole (NH 2 trz) and to its symmetry-related analogue, respectively [symmetry code: (ii) 1 À x, 1 À y, 1 À z). As a result of the bridge formed by atoms N1 and N2 of the triazole ligand, the Cd1Á Á ÁCd2 distance is 3.6145 (7) Å . Selected geometrical parameters are summarized in Table 1, showing that the octahedron around Cd1 is more distorted than the one around Cd2.
When symmetry is applied, a Cd 3 Cl 8 (NH 2 trz) 2 building block is formed. These trinuclear units are connected via the chloride ions Cl2 to build up infinite inorganic corrugated sheets in the bc plane, stacked along the a-axis direction (Fig. 2). The triazolium cations (NH 2 trzH) + and the water molecules are located in the interlayer space (Fig. 3), interacting with the anionic framework by hydrogen bonds. Thus, the overall three-dimensional network consists of alternate organic-inorganic hybrid layers, responsible for the interesting behaviour of this class of materials.

Supramolecular features
The crystal structure of the title compound is mainly stabilized by hydrogen-bonding andstacking interactions. In particular, a number of O-HÁ Á ÁCl, O-HÁ Á ÁN, N-HÁ Á ÁO and N -HÁ Á ÁCl hydrogen bonds is present (Table 2), involving the lattice water molecules, the triazolium cations, the organic ligands and the chlorine anions. These hydrogen bonds connect the organic and inorganic moieties, leading to a self-organized, hydrated hybrid structure.

Figure 2
Crystal packing showing the two-dimensional anionic framework of the title compound.

Figure 1
ORTEP of the asymmetric unit of the studied compound plus the atoms necessary to complete the coordination around the Cd ions. Cd2 is on the special position ( 1 2 , 1 2 , 1 2 ). Displacement ellipsoids are drawn at the at the 50% probability level. [Symmetry codes: acceptor and donor, respectively ( Fig. 6 and Table 2): . Finally, the coordinated triazole rings (NH 2 trz) are connected along the c-axis direction throughstacking interactions, with a centroid-centroid distance of 3.761 (7) Å .

Database survey
Recently, a great deal of attention has been paid to the rational design and synthesis of new hybrid coordination polymers (HCPs) composed of metal ions and bridging ligands due to their fascinating structural diversity and their potential application as functional materials (Xiong et al., 2001;Liao et al., 2004;Gao et al., 2008). These coordination polymers exhibit a wide range of infinite zero-to three-dimensional frameworks with interesting structural features, which result  Hydrogen bonds (red dashed lines) involving the chloride anions around Cd1. Displacement ellipsoids are displayed at the 50% probability level.

Figure 3
Corrugated anionic sheets with the non-coordinating triazolium cations and water molecules located in the interlayer space. Displacement ellipsoids are drawn at the 50% probability level.

Figure 5
Hydrogen bonds (red dashed lines) involving the chloride anions around Cd2. Displacement ellipsoids are displayed at the 50% probability level.
[Symmetry codes: (i) x, 1 2 À y, À 1 2 + z; (ii) 1 À x, 1 À y, 1 À z; (iii) 1 À x, from coordination bonding, hydrogen-bonding and aromatic stacking interactions as well as van der Waals forces (Su et al., 2003). A search of the latest version of the Cambridge Structural Database (Version 5.38; Groom et al., 2016) based on the organic fragment '4-amino-4H-1,2,4-triazole' of the studied compound yielded 70 hits. The structure of the chlorocadmate PEPWIR (Zhai et al., 2006) is probably the nearest to that of the title compound, even if it lacks the water molecules of crystallization and the protonated triazole cations. This is probably due to the difference in the stoichiometry of the initial reagents and to the solvent used in the chemical synthesis. Two other related compounds comprising 4-amino-4H-1,2,4-triazole in combination with chloride ligands are the coordination polymer ROFJED (Wang et al., 2014) and the discrete complex GAVFEP (Xuan-Wen, 2005).

Synthesis and crystallization
The compound was prepared by the reaction of 4-amino-4H-1,2,4 triazole and CdCl 2 ÁH 2 O (molar ratio 1:1) in an equal volume of water and ethanol (10 ml) mixed with 2 ml of hydrochloric acid (37%). The solution was stirred for 1 h.
Colourless crystals suitable for X-ray diffraction were grown in two weeks by slow evaporation at room temperature.