Hexaaquazinc(II) dinitrate bis[5-(pyridinium-3-yl)tetrazol-1-ide]

In hexaaquazinc(II) dinitrate 5-(pyridinium-3-yl)tetrazol-1-ide, the pyridinium and tetrazolide rings in the zwitterion are nearly coplanar. Several O—H⋯N and N—H⋯O hydrogen-bonding interactions exist between the [Zn(H2O)6]2+ cation and the N atoms of the tetrazolide ring, and between the nitrate anions and the N—H groups of the pyridinium ring, respectively, giving rise to a three-dimensional network.


Chemical context
Tetrazole functional groups have attracted increased attention in recent years due to their use in drug design and their employment as isosteric subtitutents of carboxylic acids (Herr, 2002), as well as their ability to produce a large variety of metal-organic frameworks (MOFs) Chi-Duran et al., 2018). Push-pull tetrazole complexes with both electron-donor and electron-acceptor substituents have shown efficient second-order nonlinear optical activity in powdered samples (Masahiko et al., 1994), ferroelectric behaviour (Liu et al., 2015) and strong photoluminescence (Zhang et al., 2014). The in-situ synthesis of tetrazole compounds can be realized by the Demko-Sharpless method, in which zinc salts catalyze the cycloaddition reaction between sodium azide and nitrile compounds to form the tetrazole ring (Demko & Sharpless, 2001). In this work, pyridyltetrazole, synthesized at low pH using the Demko-Sharpless method, is cocrystallized in the presence of [Zn(H 2 O) 6 ] 2+ and NO 3 À ions, to obtain the title compound ( Fig. 1).

Structural commentary
The asymmetric unit of the title compound is composed of one 5-(pyridinium-3-yl)tetrazol-1-ide zwitterion, one NO 3 À anion and one half of a [Zn(H 2 O) 6 ] 2+ cation. The hexaaquazinc(II) ISSN 2056-9890 complex exhibits regular octahedral geometry (Table 1), and the tetrazolide and pyridinium rings of the zwitterion are close to being coplanar, with a dihedral angle of 5.4 (2) (Fig. 2). The geometric parameters of the tetrazolide ring are comparable to those in other reported tetrazole compounds (Mu et al., 2010;Dai & Chen, 2011a,b). The H atom attached to the N atom of the pyridine ring could not be located in the Fourier density map. Therefore, the H atom was placed in accordance with similar reported structures containing [Mg(H 2 O) 6 ]X 2 (X = Cl À , Br À ) cocrystallized with 5-(pyridinium-3-yl)tetrazol-1-ide (Dai & Chen, 2011a,b).

Figure 1
The molecular structure of the asymmetric unit (plus the three water molecules of the hexaaquazinc cation generated by symmetry), showing the atom labelling and displacement ellipsoids drawn at the 50% probability level. [Symmetry code: (i) Àx, 2 À y, 2 À z.]

Figure 2
Partial crystal packing of the title compound, showing the hydrogenbonding interactions between [Zn(H 2 O) 6 ] 2+ and the tetrazolide ring.

Figure 3
The

Synthesis and crystallization
All the reactants and chemicals were purchased from Sigma Aldrich and utilized without further purification. A mixture of 3-cyanopyridine (4 mmol), NaN 3 (6 mmol) and ZnCl 2 (2 mmol) were dissolved in 6 ml of distilled water. This mixture was transferred to a glass bottle and then heated at 378 K for 24 h. The pH was adjusted using a HNO 3 (66%) solution immediately after mixing the reactants, and was monitored with a pH meter (pH2700 Oakton) until reaching a pH of 2.0. The reaction mixture was then cooled to 318 K and kept at this temperature for 16 h. The colourless block-shaped crystals obtained were washed with ethanol to give 353 mg (yield 30%) of the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. All H atoms bonded to C atoms were positioned geometrically and treated as riding atoms, using C-H = 0.93 Å and U iso (H) = 1.2U eq (C