Polymeric coordination complex of lithium(I) with aqua and cyanurate ligands

The polymer contains Li+ cations coordinated via oxygen to two cyanuranate anions and three water molecules in a trigonal–bipyramidal geometry and to three water molecules and an oxygen from the cyanuric anion in a tetrahedral geometry. A three-dimensional network of hydrogen bonds serves to hold the structure together.

A number of physical and structural properties, including molecular geometry, metal-ligand bonding and directional supramolecular architecture, control and influence the applications of hybrid metallo-organic coordination compounds (Coubrough et al., 2019). Such compounds find potential applications in catalysis, gas storage, ion exchange, magnetic materials, sensors, optics and batteries (Qu et al., 2016). The various possible metal and linker combinations are endless and have led to the synthesis of thousands of new materials with different metal geometries and functionalities (Chatenever et al., 2019). Among the metals investigated, lithiumbased complexes have unique advantages, exploiting properties of the lithium cation such as small ionic radius, high polarizing power, aqueous solubility and low economic cost (Ge et al., 2018;Wan et al., 2012). In solution, the lithium cation is of great importance because it can bind with selective organic ligands, leading to uses in many areas, including as active cellular components in ion-selective electrodes (ISE) in medicine, in nuclear power and in batteries (Ivanova et al., 2019).
Cyanuric acid (1,3,5-triazine-2,4,6-triol) is an industrially important compound used to make pesticides, dyes, and disinfectants (Cho et al., 2014). The acid is used as a chlorine stabilizer for outdoor swimming pools and sizeable industrial water systems. It is non-toxic to human and aquatic animals. It also has the remarkable property of biodegradability by soil bacteria (Prabhaharan et al., 2015) and was recently found to be an effective nucleating agent during kinetic studies of biodegradable poly(l-lactide) and poly(3-hydroxylbutyrate) co-polyesters (Pan et al., 2013;Weng & Qiu, 2014).
With regard to metallo-organic chemistry, cyanuric acid is an important ligand due to its structural symmetry based on a planar six-membered ring, the existence of canonical structures and the presence of multiple hydrogen-bond-donor centres (Divya et al., 2017). In its neutral, undissociated form, cyanuric acid shows tautomerism and can exist in the keto (I) or enol (II) forms ( Fig. 1) (Abu-Salem et al., 2017). In basic solution, it forms an anion with resonance between the (III), (IV) and (V) forms (Fig. 1).
As cyanuric acid has three hydrogen-bonding-donor amine sites and three hydrogen-bonding-acceptor keto sites, it has been the subject of several structural and crystal-design studies (Shemchuk et al., 2017). In the present work, we report the synthesis of a new lithium complex of cyanuric acid, [Li 4 (C 3 H 2 N 3 O 3 ) 4 (H 2 O) 7 ] n . The complex has been character-ized by single-crystal X-ray diffraction, FTIR and UV-Vis spectroscopy, and TG-DTA analysis.

Structural commentary
The title compound crystallizes in the triclinic space group Pı ". The asymmetric unit comprises two lithium ions, two cyanurate ligands and three and a half coordinated water molecules. An inversion centre lies between the related Li + cations, Li1 and Li1 i , generating a molecular unit of formula [Li 4 (C 3 H 2 N 3 O 3 ) 4 (H 2 O) 7 ] (Fig. 2).
The two crystallographically distinct cyanurate ligands exist in resonance form (IV) (Fig. 1), in which the negative charge is located on a nitrogen atom. Interestingly, for both ligands, coordination to lithium does not involve the deprotonated N2 and N5 atoms, but occurs via the keto oxygen atoms opposite (O1 and O4). This coordination preference may be due to the hard acid, Li + , preferring to bond to the harder base i.e. oxygen. The

Figure 3
Observed bond lengths in the cyanurate anions found in the title compound. The delocalization of the negative charge on the deprotonated nitrogen atoms (N2 and N5) over the adjacent keto groups is shown as dashed lines.
The remaining axial water ligand, H 2 O7, bridges to the second Li + cation, Li2, which is disordered over two sites, Li2A and Li2B, which have approximately equal occupancies. The Li1Á Á ÁLi2A and Li1Á Á ÁLi2B distances are 3.438 (7) and 3.439 (7) Å , respectively. Li2 is coordinated to two more water molecules, H 2 O9, H 2 O10 and an oxygen atom from a cyanurate ligand (either O3 ii for Li2A or O2 iii for Li2B) to complete its distorted tetrahedral coordination geometry. The Li2-O bond lengths lie in the range 1.931 (7)

Figure 4
View of the crystal packing of the title compound along the a axis. Hydrogen-bonding interactions are indicated by green dashed lines.

Fourier transform infrared spectroscopy
The FTIR spectrum of the title compound was measured using a Perkin Elmer Spectrum One instrument over a 450-4000 cm À1 scan range at 1.0 cm À1 resolution (Fig. 5). The bands at 3400 (sh, m) and 3389 cm À1 (br) correspond to (O-H) (Prabhaharan et al., 2015;Bourzami et al., 2018) and those at 3165, 3102 and 2831 cm À1 to (N-H) (Divya et al., 2020;Surinwong et al., 2014). The bands at 1718 and 1675 cm À1 correspond to (C O) (Divya et al., 2020;Vu et al., 2019) and those at 1578 and 1478 cm À1 to sym (C-N) (Surinwong et al., 2014). The wavenumbers of the vibrations involving the N-H, C N and C O groups are affected by the partial delocalization of electron charge density on one part of the ring, as shown in Fig. 2, and by the to coordination of C O oxygen to Li + . Finally the bands at 870, 784 and 559 cm À1 are attributed to the characteristic vibrations of the 1,3,5-triazine ring (Bourzami et al., 2018;Bellardita et al., 2018).

Absorption spectroscopy
The UV-Vis NIR absorption spectrum was measured using a Perkin Elmer lambda 950 UV-Vis-NIR spectrophotometer (Fig. 6). The peaks observed at 290 and 228 nm are due to -* and n-* transitions, respectively (Qiu & Gao, 2005;Moreno-Guerra et al., 2019). The band gap, E g , can be estimated from the maximum absorption at 228 nm using the following equations. The optical absorption coefficient, , is related to the absorbance, A, by the relations: = 2.303 A/t and = A(h -E g ) 1/2 /h, where t is the thickness of the crystal (1 mm) and h is the photon energy. A plot of (h) 2 versus h is shown as the inset in Fig. 7, from which the band gap (E g ) is estimated to be 5.22 eV. The absorption spectrum of the title compound. The direct bandgap, E g , is estimated from the plot in the inset to be 5.22 eV.

Figure 7
TG-DTA of the title compound measured under an N 2 atmosphere using a heating rate of 10 C min À1 .

Figure 5
The infrared spectrum of the title compound. min À1 with a resolution of 1 mg under a dry N 2 atmosphere. The thermogram (Fig. 7) shows four stages of decomposition. The first stage starts at 92 C and ends at 172 C with a derivative peak at 146.26 C and a measured weight loss of 20.13%, which is in reasonable agreement with the loss of the seven coordinated water molecules (calculated weight loss 18.91%). The second and third stages of decomposition, occurring from 298 to 550 C, correspond to the decomposition of the cyanurate ligands with a measured total weight loss of 52.80%, leading to the formation of LiNO 3 (calculated weight loss 51.07%) (Divya et al., 2020). In the fourth decomposition stage, occurring from 550 to 662 C, LiNO 3 decomposes with a measured weight loss of 20.39% to produce Li 2 O as the final solid residue (calculated weight loss 21.67%).

Synthesis and crystallization
Lithium hydroxide (1.25 g, 0.052 mol; LOBA) and cyanuric chloride (1.84 g, 0.01 mol; Sigma-Aldrich) were dissolved in water (100 ml). The resulting solution was stirred for 5 h at ambient temperature (300-301 K) and filtered twice using Whatman filter paper. The solvent was allowed to evaporate in a dust-free environment. After 22 days, good quality colourless crystals were harvested.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Li2 was found to be disordered over two positions, Li2A and Li2B, which were resolved using the PART command (Sheldrick, 2015b) with an occupancy ratio of 0.501 (6):0.499 (6). The N-bound H atoms were placed geometrically and refined using a riding model with respect to their parent atoms using AFIX 43 with N-H = 0.86 Å and U iso (H) = 1.2U eq (N). The hydrogen atoms on the water molecules were located in difference-Fourier maps and each U iso (H) parameter was freely refined with the O-H distance restrained to 0.85 (2) Å using DFIX. The H-O-H angle distances were restrained using DFIX to a target value of 1.39 (2) Å [or 1.41 (2) Å for H9A-O9-H9B] in order to keep the water molecules close to their standard geometries. Data collection: APEX3 (Bruker, 2016); cell refinement: APEX3/SAINT (Bruker, 2016); data reduction: SAINT/XPREP (Bruker, 2016); program(s) used to solve structure: SHELXT2014/5 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2020); software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015b).

Poly[µ 3 -aqua-hexa-µ 2 -aqua-tetra-µ-cyanurato-tetralithium]
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.