Crystal structure of a Zn complex with terephthalate and 1,6-bis(1,2,4-triazol-1-yl)hexane

A new Zn coordination polymer with bitopic rigid terephthalate and flexible 1,6-bis(1,2,4-triazol-1-yl)hexane was synthesized and structurally characterized.


Chemical context
Coordination polymers with flexible bitopic ligands have attracted great interest as prospective materials for gas separation, sensing materials, electrochemical devices or catalysis (Pettinari et al., 2016). One of the favoured classes of bitopic ligands are bis(azol-1-yl) alkanes, which have been used for the preparation of various transition metals coordination polymers with different topologies (Alkorta et al., 2017;Pellei et al., 2017;Manzano et al., 2016;Liu et al., 2012). Bitopic bis(azol-1-yl)alkanes have two separated metalbinding sites that allow them to form a wide variety of polymeric structures. Thus, coordination compounds based on these ligands could be applied in the design of various functional materials with a wide range of potential applications. Recently, we have synthesized three new Zn coordination polymers based on bis(triazol-1-yl)propane and terephtalate anions (Semitut et al., 2017). By varying the conditions, it was possible to synthesize three different polymeric compounds, which have interesting luminescent properties. As part of our studies with the aim of preparing new coordination polymers with flexible bis(triazol-1-yl)alkane ligands, we report herein the synthesis and crystal structure of [Zn(bdc)(btrh)]ÁDMF (bdc = benzene-1,4-dicarboxylate, btrh = 1,6-bis(1,2,4-triazol-1-yl)hexane, DMF = dimethylformamide).
The btrh ligand ( Fig. 1) was prepared by the reaction of 1,2,4-triazole with 1,6-dibromohexane in a superbasic dimethyl sulfoxide-potassium hydroxide medium using our modified procedure reported for bis(triazolyl)propane (Semitut et al., 2017). Our proposed procedure does not require the use of toxic solvents and gives higher yields compared to the literature procedure (Liu et al., 2012). The title complex was prepared by the reaction of zinc nitrate, btrh and terephthalic acid under solvothermal conditions (368 K) in DMF. The product was formed after 48 h as a crystalline colourless solid of plate-like shape. The single crystal used for structure determination was collected from the filtered product. The polycrystalline compound was characterized by elemental (C, H, N) and powder XRD analysis (Fig. S1, Supporting information), indicating formation of this complex as a main phase.

Thermal stability
The thermal stability of the synthesized coordination polymer was studied in oxidative O 2 /Ar (21%) atmosphere. Thermogravimetric measurements were carried out on a NETZSCH thermobalance TG 209 F1 Iris. Open Al 2 O 3 crucibles were used (loads 7-10 mg, heating rate 10 K min À1 ). The thermal analysis of [Zn(btrh)(bdc)]ÁnDMF revealed that the synthesized compound has three thermolysis stages in an oxidative atmosphere (Fig. 2). The first stage of thermolysis is the process of the loss of solvate molecules that runs in the range of 373-453 K and has a well-defined step on the TG curve. The mass loss of solvate molecules corresponds to a composition with n ' 1, which is in good agreement with the crystal data. The desolvated compound is stable up to 503 K. The second and third stages run in the ranges 503-573 and 633-773 K, respectively. The second stage corresponds to partial degradation of btrh and terephtalate and third to further decomposition and the burning process of the formed carbon products, resulting in the formation of ZnO according to powder XRD analysis.

Structural commentary
The structure is a 2D coordination polymer crystallizing in space group P1. The central Zn atom has a distorted tetrahedral environment comprising two oxygen and two nitrogen atoms. It is coordinated by two crystallographically independent (bdc) 2À ligands (halves), forming zigzag chains along the [210] direction, which are linked by btrh ligands (Fig. 3). Contrary to our recently reported Zn complexes with 1,3bis(1,2,4-triazol-1-yl)propane containing a shorter alkyl bridge (Semitut et al., 2017), 1,3-bis(pyrazol-1-yl)propane (Potapov et al., 2012) and bis(imidazol-1-yl)alkanes , the title compound is a 2D polymer, because the Zn atoms are connected by btrh ligands in pairs, not in chains, thus preventing the formation of a 3D net. Each Zn atom is linked with three others via (1) the first bdc 2ligand, (2) a second bdc 2ligand and (3) a pair of btrh ligands. The layers of the title compound are arranged perpendicular to the [122] direction in such a way that the {Zn 2 (btrh) 2 } units lie between the hollows of neighboring layers (Figs. S2, S3).

Supramolecular features
Layers of the complex are packed tightly, revealing only one DMF solvent molecule per formula unit. Analysis of the residual electron-density map clearly indicates the presence of a not or very slightly disordered DMF molecule (Fig. S4). After refining DMF, only one peak of 0.60 e Å À3 (attributed to a C atom of occupancy ca 0.15) is observed, while the densities of other peaks coincide with those of holes (ca AE0.3 e Å À3 ). Thus, the DMF molecule is rather not disordered. Besides disorder, atomic displacement parameters that are larger than Curves of thermal analysis for [Zn(btrh)(bdc)]ÁDMF in O 2 /Ar (21%) atmosphere; 1 TG, 2 DTG, 3 c-DTA.

Figure 1
Synthesis of 1,6-bis(1,2,4-triazol-1-yl)hexane. those for other atoms can be due to partial loss of the solvent during the experiment. DMF molecules are located in the channel voids, which occupy 26.4% of the structure (Fig. S5). As a result of the lack of H-donor groups, hydrogen bonds are not observed in the structure of the complex; however, intermolecular C-HÁ Á Á contacts of 3.07 Å (Table 1) occur between the aromatic rings of bdc ligands (Fig. S6). These contacts connect neighbouring layers.

Database survey
A database survey showed that the majority of the known structures of polymers with flexible bis(azol-1-yl)alkanes are compounds based on relatively short linkers (from methane to pentane) but that the number of polymers based on longer linkers (having a CH 2 -chain higher than six) is relatively low. The lack of structural information on long flexible ligands can be due to the fact that it is more difficult to obtain single crystals of good quality for these compounds. Such ligands tend to form interpenetrated polymers with disorder and a variety of modifications. A search of the Cambridge Structural Database (CSD, Version 5.38, update May 2017; Groom et al., 2016) for compounds containing btrh and any metal gave 51 hits, of which only one contains both btrh and bdc ligands (refcode ETAKAM; Zhang et al., 2011). This Cd polymer also has a 2D structure, but the {Cd(bdc)} chains are linear and are intersected by {Cd(btrh)} chains. Thus, contrary to our case, the two central metal atoms are connected by only one btrh ligand.

Starting materials and experimental procedures
The starting reagents used for the synthesis of the coordination compound -Zn(NO 3 ) 2 Á6H 2 O (chemical grade), dimethyl formamide (analytical grade) and terephthalic acid (analytical grade) -were used as received.
NMR spectra were recorded on a Bruker AV300 instrument operating at 300 MHz for 1 H and 75 MHz for 13 C, solvent residual peaks were used as internal standard. Elemental analyses were carried out on a Eurovector EuroEA 3000 analyser. Infrared (IR) spectra of solid samples as KBr pellets were recorded on a FT-801 spectrometer (4000-550 cm À1 ). The powder XRD data were collected with a DRON RM4 powder diffractometer equipped with a Cu K source ( = 1.5418 Å ) and graphite monochromator for the diffracted beam.
Synthesis Displacement ellipsoid plot of a single layer of the coordination polymer showing ellispoids drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were refined as riding atoms (C-H = 0.97 Å with U iso (H) = 1.5U eq (C) for methyl H atoms and C-H = 0.93 Å 1.2U eq (C) for all others. Methyl H atoms were refined as rotating groups.

Funding information
This study was supported by the Russian Science Foundation, grant No. 15-13-10023 and the X-ray structure analysis was carried out with support from the Ministry of Education and Science of the Russian Federation (a project of the Joint Laboratories of the Siberian Branch of the Russian Academy of Sciences and the National Research Universities).  program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).