Crystal structure of 1,4-bis[5-(2-methoxyphenyl)-2H-tetrazol-2-yl]butane

The diffraction data confirmed the title compound as the main isomer produced in a coupling reaction. The structure and Hirshfeld surface analysis of the formed di-tetrazolyl chelate ligand are reported.


Chemical context
Tetrazole ligands have four nitrogen atoms in their fivemembered rings and the lone pairs of these nitrogen atoms are useful for coordination bonds with metal ions (Zhao et al., 2008). Tetrazole has a variety of binding modes with metal ions, which results in the unusual formation of high-dimensional metal-organic frameworks (MOFs) or coordination polymers (Karaghiosoff et al., 2009;Liu et al., 2013). Valuable mono-, bis-and polytetrazole ligands for the formation of MOFs and coordination polymers have been also reported (Boland et al., 2013;Fan et al., 2016;Tȃ bȃ caru et al., 2018;Zhao et al., 2016). As an extension of a project on the study of selfassembly behaviour in solution, we designed a ditetrazolyl chelate ligand possessing a butane bridge. It is worth noting that tetrazole has two different resonance structures in which the hydrogen atoms are located at either the N1 or N2 positions. In many cases, this results in the formation of several products (Lee et al., 2017). It is therefore essential to study the molecular structure of synthesized tetrazole complexes by X-ray crystallography.
The title compound was isolated as an intermediate in the middle of the synthetic route for a chelate ligand. The reaction between the sodium salt of tetrazole and 1,4-dibromobutane gave three isomeric products (Fig. 1). Using column chromatography, the major product was isolated and its molecular structure was determined unambiguously by X-ray crystal- ISSN 2056-9890 lography. This compound is a useful precursor for the synthesis of dinuclear metal complexes with the expectation of synergetic effects of two metal centers (Fig. 2). Herein, we report the synthesis and crystal structure of this compound.

Structural commentary
The reaction yielded three isomeric products as described in Section 5, Synthesis and crystallization, and the structural analysis confirms the formation of the desired major product. The molecular structure of the title compound is shown in Fig. 3. There are no unusual bond lengths or angles. The title compound possesses two identical phenyl tetrazole fragments, connected by a butyl (C17-C20) bridge. The butyl group is attached to the second N atom of both tetrazole rings (N2 and N6, Fig. 3). The dihedral angles between the phenyl group and tetrazolyl ring are somewhat different in the two phenyltetrazolyl groups. One phenyltetrazolyl group (N1-N4/C1-C7) is almost planar with an angle of 5.32 (6) between the mean planes of the rings. However, the other phenyltetrazolyl group (N5-N8/C9-C15) is tilted with a dihedral angle of 15.37 (7) .
Two intramolecular C-HÁ Á ÁN hydrogen bonds (Table 1) occur, which are shown as yellow dashed lines in Fig. 4. These interactions may contribute to the planarity of the phenyltetrazolyl units.

Figure 2
Synthetic route of the desired dinuclear metal complexes from the title compound (I).

Figure 3
A view of the molecular structure of the title compound, with the atom labelling and 30% probability displacement ellipsoids.

Figure 4
A plot showing the intramolecular C-HÁ Á ÁN hydrogen bonding (dashed yellow lines) and short contacts between molecules (dashed pink, skyblue and blue lines).

Table 1
Hydrogen-bond geometry (Å , ). hand side of Fig. 4 (for clarity a different reference molecule was used for the illustration of this contact). It is interesting that the C1 atom has another close C-HÁ Á ÁC contact from the opposite side of the aromatic plane ( Fig. 4, purple dashed lines), C16-H16AÁ Á ÁC1 v [HÁ Á ÁC = 2.798 (2) Å ; symmetry code: (v) Àx + 1, y + 1 2 , Àz À 1 2 ]. There is one notable close contact, C17-H17AÁ Á ÁO1 i that can be considered a weak hydrogen bond, which is indicated by green dashed line in Fig. 5. This contact forms a dimeric rectangle between two molecules. This rectangle extends in the c-axis direction by the short interactions described above.
To provide an overall view of the weak interactions between the molecules, a Hirshfeld surface analysis (Spackman & Jayatilaka, 2009) was performed with CrystalExplorer17 (Turner et al., 2017). The Hirshfeld surface was calculated using a standard (high) surface resolution with the three-dimensional (3D) d norm surface plotted over a fixed colour scale of À0.1339 (red) to 1.4773 a.u. (blue). The 3D d norm surface of the title complex is shown in Fig. 6a and 6b. The red spots indicate short contacts, i.e., negative d norm values on the surface, which highlight the most important weak interactions: C17-H17AÁ Á ÁO1 i hydrogen bond (green dashed line), C4-H4Á Á ÁC1 iv contact (blue in Fig. 6a), C18-H18BÁ Á ÁC5 iii (pink in Fig. 6a, red in Fig. 6b) and C16-H16AÁ Á ÁC1 v (blue in Fig. 6b).

Synthesis and crystallization
The synthesis scheme for the title compound is represented in Fig. 1. The sodium salt of 5-(2-methoxyphenyl)-1H-tetrazole (495 mg, 2.5 mmol) and dibromobutane (150 ml, 1.25 mmol) were dissolved in acetonitrile and refluxed for 2 d. The resulting white solid was filtered and the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel using hexane:acetone (1:1) as eluent. Three isomeric compounds were obtained, as shown in Fig. 1. The major product (I) (yield = 35%) was recrystallized in ethanol by the slow evaporation method and yielded colourless crystals of the title compound.  152.18, 133.10, 131.20, 120.80, 112.26, 111.91, 55.50, 46.63, 25.57 ppm.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were included in calculated positions using a riding model, with C-H = 0.95-1.00 Å and U iso (H) = 1.5U eq (C) for methyl H atoms and U iso (H) = 1.2U eq (C) for all others. Two reflections (100 and 110) were omitted because of truncation by the beamstop.