Crystal structure of poly[(μ3-4-amino-1,2,5-oxadiazole-3-hydroxamato)thallium(I)]

The thallium(I) atom in the polymeric title compound is bonded to four O atoms, with the lone pair electrons stereochemically active.


Chemical context
Substituted oxadiazoles attract attention because of their wide range of applications in organic synthesis as useful intermediates (Romeo & Chiacchio, 2011;Zlotin et al., 2017) and for drug design (Giorgis et al., 2011;Pal et al., 2017;Stepanov et al., 2015). In addition, molecules with the oxadiazole moiety can be considered for the creation of energetic systems (Zhang et al., 2015) with high thermal stability and mechanical sensitivity. The variety of coordination modes typical for oxadiazole-containing ligands result in the formation of multiple mono-and polynuclear complexes, as well as coordination polymers (Akhbari & Morsali, 2010). Complexes with oxadiazole-based ligands have demonstrated significant biological activity as anti-cancer (Glomb et al., 2018), antiinflammatory (Singh et al., 2013), anti-tuberculosis (De et al., 2019) and anti-malarial (Zareef et al., 2007) agents.
However, the standard synthetic procedures for oxadiazolecontaining scaffolds usually utilizes the dehydrative cyclization of bis-oximes, which is performed at high temperatures (Fershtat & Makhova, 2016;Romeo & Chiacchio, 2011) and often includes the introduction of different activating reagents (Shaposhnikov et al., 2003;Telvekar & Takale, 2013). A convenient procedure for the synthesis of substituted 4-amino-1,2,5-oxadiazoles based on the formation of bis-oximes in situ from the hydroxylamine and cyano-oximes was recently proposed (Neel & Zhao, 2018). The introduction of dehydrating agents allows a significant decrease in the temperature during reaction, gave the possibility to synthesize substituted 1,2,5-oxadiazoles with various side functional groups. In this regard, we have adapted the synthetic procedure for 1,2,5oxadiazole with amino-and hydroxamate groups in the 4-and 3-position of the 1,2,5-oxadiazole ring, respectively, and report here the thallium(I) salt of this compound, 1, Tl(C 3 H 3 N 4 O 3 ). The introduction of a hydroxamic group at the 1,2,5-oxadiazole ring allows the consideration of potentially interesting ligand systems for the synthesis of various polynuclear complexes (Pavlishchuk et al., 2018;Lutter et al., 2018;Ostrowska et al., 2019;Gumienna-Kontecka et al., 2007).

Figure 1
A fragment of the crystal structure of 1 showing the coordination environment of the Tl1 ions with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: Ày + 1 2 , z + 1 2 ] atoms from another three oxadiazole moieties. The closest contact between adjacent Tl1 cations within a zigzag chain is 3.7458 (5) Å .

Supramolecular features
In the crystal, the oxadiazole rings are stacked in a parallel manner with a centroid-centroid distance = 3.746 (3) Å ( Fig. 1). Together with weak intermolecular hydrogen bonds between the amino group (N4) and two nitrogen atoms from the azolo (N3) and the hydroxamic (N1) group (Table 1, Fig. 3) they support the cohesion of the chains along the b-axis direction.

Synthesis and crystallization
The title compound was obtained according to a modification of the procedure reported by Neel & Zhao (2018) (Fig. 4). Solutions containing 5 mmol of hydroxylamine hydrochloride in 10 ml of methanol, and 10 mmol of sodium methoxide in 15 ml of methanol were stirred for 30 min while cooling in an ice bath. The formed precipitate of sodium chloride was filtered off. The methanolic solutions of ethyl-2-cyano-2-(hydroxyimino)acetate (5 mmol) and hydroxylamine were combined and stirred for 5 h at room temperature. The resulting white precipitate was filtered off and dissolved in 5 ml of water, followed by HCl addition to pH = 5. The organic compound was extracted with ethyl acetate; the extract was subsequently dried over anhydrous Na 2 SO 4 , and the solvent was finally removed by rotary evaporation. Colorless crystals Acta Cryst. (2020). E76, 328-331 research communications Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; Ày; Àz þ 1; (ii) Àx þ 1; y À 1 2 ; Àz þ 3 2 .

Figure 3
Packing diagram of 1, with hydrogen bonds indicated by dashed lines.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The H atoms of the amino group were located from a difference-Fourier map; their coordinates were refined freely with U iso (H) = 1.2U eq (N). The hydrogen atom of the hydroxamate function could not be observed in difference-Fourier maps, and a tentative calculated position was in too close vicinity to atom H4B of the amino group. Most probably, the hydroxamate H atom is disordered over the N1-C1-O2 backbone due to the presence of both tautomeric forms. Hence, this H atom is not included in the final model. The highest remaining electron density is located 0.88 Å from Tl1.

Poly[(µ 3 -4-amino-1,2,5-oxadiazole-3-hydroxamato)thallium(I)]
Crystal data 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.