Synthesis, crystal structure, Hirshfeld surface and void analysis of bis(μ2-4-aminobenzoato-κ2 O:O′)bis[bis(4-aminobenzoato-κ2 O,O′)diaquathulium(III)] dihydrate

The asymmetric unit of the title compound comprises three 4-aminobenzoate ligands, two coordinated water molecules, a thulium metal ion and a water molecule of crystallization. The crystal structure features O—H⋯N, N—H⋯O, and O—H⋯O hydrogen-bonding interactions as well as C—H⋯π and off-set π–π stacking interactions. Hirshfeld surface analysis indicates that H⋯H contacts are the most significant contributors to the crystal packing.


Chemical context
The coordination chemistry of rare-earth metals has been widely studied, and the structures of a significant variety of complexes with diverse kinds of ligands have been reported (You et al., 2021). In particular, the lanthanide contraction along the series is of interest, and in a detailed analysis of this phenomenon using elements from the lanthanide series, p-aminobenzoic acid (HL) was found to be a very useful and biologically important ligand (Smith & Lynch, 2015). The carboxylate group of HL can be coordinated with the metals simultaneously in three different modes, namely chelating, bridging, and chelating-bridging (Ali et al., 2014). In the complexes of HL with alkali metals such as Na + or K + , the ligand is not directly coordinated to the metal ion, but rather it is surrounded by coordinated water molecules (You et al., 2021). Both the carboxylic and amino groups of the ligand are coordinated to the metal in complexes with Ba 2+ , Ag + , Zn 2+ , Cd 2+ , and Ni 2+ (Mamedov et al., 1982;Amirasłanov et al., 1982a), while only the oxygen atoms of the carboxylic groups are coordinated to the metal ion in complexes of Sr 2+ , Mg 2+ , and Co 2+ with this ligand (Amirasłanov et al., 1982b;Sun et al., 2004). In comparison to the above coordination diversity, in the complexes of HL with rare-earth elements like Nd +3 and Sm +3 (Khiyalov et al., 1981;Mao & Lianq, 2016), only the nitrogen atom of the amino group is coordinated by the central metal atom, while in complexes of Lu +3 and Ho +3 with HL , the nitrogen atom of the amino group is not coordinated while the ligands are attached to the metal atom by the oxygen atoms of the carboxylate moiety. In this context, we report the synthesis, crystal structure, Hirshfeld surface, void, thermogravimetric and FT-IR analysis of the title compound, [Tm 2 (C 7 H 6 NO 2 ) 6 (H 2 O) 4 ]Á2H 2 O, which is closely related to its Lu +3 and Ho +3 analogues .

Figure 1
ORTEP view of 4ABA-Tm with ellipsoids drawn at a 30% probability level with H atoms shown as small circles of arbitrary radii.

Hirshfeld surface analysis
A Hirshfeld surface (HS) analysis was carried out using Crystal Explorer 21.5 (Spackman et al., 2021) in order to explore the non-covalent interactions in terms of the Hirshfeld surface and two-dimensional fingerprint plots. The HS of a molecule is the region in the crystal where the electron density relevant to the promolecule is greater than the electron density relevant to the procrystal (Spackman et al., 2009;Ashfaq et al., 2020). The Hirshfeld surface is constructed by employing colour coding to show the interatomic contacts that are shorter (red areas), equal to (white areas), or longer than (blue areas) the sum of the van der Waals radii (Ashfaq et al., 2021a,b). The red spots on the surface mapped over d norm (Fig. 5a) indicate the involvement of atoms in hydrogenbonding interactions. The HS mapped over shape-index ( Fig. 5b) is used to check for the presence of interactions such as C-HÁ Á Á andstacking (Ashfaq et al., 2021a,b). The existence of adjacent red and blue triangular regions around the aromatic rings conforms to the presence ofstacking interactions in the title compound.
Two-dimensional fingerprint plots provide unique information about the non-covalent interactions and the crystal packing in terms of the percentage contribution of the interatomic contacts (Spackman et al., 2002;Ashfaq et al., 2021a,b). Graphical representation of C-HÁ Á Á interactions in 4ABA-Tm. Selected H atoms are shown while the water molecules are omitted for clarity.

Figure 4
Graphical representation of off-setinteractions in 4ABA-Tm. H atoms and water molecules are not shown for simplicity.

Figure 5
HS plotted over (a) d norm in the range À1.073 to 1.740 a.u. and (b) shapeindex in the range À1 to 1 a.u. The response to applied stress or force mainly depends on the strength of the crystal packing in single crystals, which have a high mechanical strength as the molecules are strongly packed into them. To check whether the title compound is mechanically stable or not, a void analysis was performed. In order to calculate voids in the crystal packing, the electron densities of all of the atoms in the molecules present in the asymmetric unit are added up, the atoms being assumed to be spherically symmetric (Turner et al., 2011;Kargar et al., 2022). The volume of the void in the crystal packing of the title compound is 120.81 Å 3 (Fig. 7), which infers that voids occupy 10.51% of the space and, hence, the molecules are strongly packed in the title compound.

Infra-red spectroscopy
The structure of the newly synthesized complex was also investigated by FT-IR spectroscopy. It was found that the absorption bands of the -NH 2 group appeared in the region of 3200 cm À1 while the absorption bands due to Tm-OH 2 are visible in the region of 325 cm À1 . The aromatic carbons show their absorption band at 1225 cm À1 , while the Tm-O band is visible in the region of 650 cm À1 . The absorption bands observed in the FT-IR spectrum of the free ligand in the regions of 1715 and 1435 cm À1 are caused by symmetric ( s ) and asymmetric ( as ) stretching of the carboxyl group, which are shifted to 1635 and 1436 cm À1 , respectively, upon coordination with the Tm III metal ion. The difference between s and as is 199 cm À1 , indicating that the carboxyl groups are coordinated to the central metal ion by chelate and bidentatebridging coordination modes.

Thermogravimetric analysis
The title complex was further characterized by thermogravimetry. Thermolysis occurs in three stages. In the first stage, at a temperature of 20-200 C, intermolecular and coordinated water molecules are released, with a weight loss of 4.69%. The complex remains stable over the temperature 200-400 C. In the second stage, at a temperature of 400-600 C, the hydrocarbon residues are decomposed and simultaneously burned out. Thulium carbonate is formed in the last stage at a temperature between 600 and 800 C. The final product of decomposition above 800 C is metal oxide.
It is known that lanthanide carboxylates have good spectroscopic characteristics; they have enhanced thermal stability and are also resistant to moisture and oxygen in the air, which is of great importance in the production and operation of photoluminescent and electroluminescent devices based on them.

Synthesis and crystallization
The infra-red spectrum of 4ABA-Tm in the range 4000 to 250 cm À1 was recorded on an FT-IR Prestige 21 spectrophotometer after preparing the samples with KBr pellets. Thermal analysis was carried out using a NETSCHSTA-409 PC/PG derivatograph, TG, DTG and DTA curves were obtained in a static air atmosphere at a heating rate of 10 C min À1 from 20-800 C using platinum crucibles. Highly sintered Al 2 O 3 was used as a reference. The elemental analysis for C, H, and N was performed using a Costech ECS 4010 CHNSO analyzer.
Preparation of the title complex The reaction of aqueous solutions of TmCl 3 and sodium p-aminobenzoate (1:3) yielded single crystals of tris-(paminobenzoato)thulium(III) dihydrate suitable for X-ray diffraction analysis. The mixture was refluxed for 30 minutes and then cooled to room temperature. After filtration, the filtrate was left for several days, covered with aluminum foil, until yellow prismatic crystals appeared. C 42 H 48 N 6 O 18 Tm 2 , M: 1262.72 g mol À1 . Elemental analysis: calculated %: C:41.11; N: 6.25; Tm: 27.57: found %: C:41.24; N:6.72; Tm: 27.41.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms of all the water molecules and the amino groups of 4-aminobenzoate ligands were found by the careful inspection of residual electron-density peaks and positional parameters were refined using bond-length   Computer programs: APEX2 and SAINT (Bruker, 2007), SHELXS97 (Sheldrick, 2008), SHELXL2018/3 (Sheldrick, 2015), ORTEP-3 for Windows WinGX (Farrugia, 2012) and PLATON (Spek, 2020 Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and PLATON (Spek, 2020); software used to prepare material for publication: WinGX (Farrugia, 2012) and PLATON (Spek, 2020). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.80 e Å −3 Δρ min = −1.07 e Å −3 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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.