1. INTRODUCTION

α-Hydroxycarboxylic acids are organic compounds with two functional groups, carboxylic acid and an alcoholic in α-position concerning the carboxylic acid group. These types of organic compounds containing a hydroxyl (-OH) and carboxylic (-COOH) moieties constitute suitable building blocks in generating a variety of supramolecular assemblies through strong O--H···O hydrogen bond interactions¹. Meaningfully, the hydrogen bond is one of the most important intermolecular interactions, responsible for several of the main characteristics of these systems, and provides a powerful way to generate supramolecular architectures from simple building blocks².

Both functional groups in α-hydroxycarboxylic acids are capable of coordinating metal ions in a variety of modes and with numerous possibilities leading to the formation of assembly of supramolecular arrays through intermolecular interactions³. When these α-hydroxycarboxylic acids act as a single ligand, three forms of coordination to a single cation have been observed: bidentate chelate through the carboxylate and alcoholic hydroxyl groups, bidentate chelate through the carboxylate group alone, and monodentate through the carboxylate³.

For some metal complexes of α-hydroxy carboxylate ligands have been reported medicinal uses as antibacterial and antiulcer activities^4,5 and more recently as an anticancer agent⁶.

Benzilic acid, an aromatic α-hydroxycarboxylic acid, has long been used in the preparation of antimuscarinic agents^7–10 and treatment against Alzheimer's disease since it can be a considerable inhibitor of acetylcholinesterase enzyme^11,12.

From the supramolecular point of view, the benzilate moiety has two phenyl rings oriented as propeller blades, and hydroxyl and carboxylate groups, which have the potential to form hydrogen bonds and interact with the charged field of a counter-ion. This fact has been evidenced in the structural characterization of some metal complexes studied with benzylic acid acting as the only ligand with Li⁺¹³, K⁺¹⁴, Tl⁺¹⁵ and Pb²⁺¹⁶, or in conjunction with other ligands^6,17–20. In these complexes, packing in the solid-state should comprise diverse cohesive interactions such as O--H···O and aromatic (π···π, C--H···π), which allow the formation of coordination polymer around the metal ion.

Precisely, in recent years, significant attention had been paid in designing and development of coordination polymers (CPs) which are certainly very promising as multifunctional materials^21–23. The possibilities of packaging self-assembly mean that polymers can have unlimited structures and these varied structures can be prepared by choosing different metal ions or/and different ligands, and therefore these materials can have multiple applications in gas storage, heterogeneous catalysis, chemical sensors, energy conversion, drug delivery, among others²³.

In continuation of our previous investigation on coordination polymers^{13,14,24–27}, in this work, we present the synthesis, structural characterization and energetic properties of the new coordination polymer strontium benzilate. Hirshfeld surface analysis²⁸ was used for visually analyzing intermolecular interactions in the crystal structure.

2. EXPERIMENTAL SECTION

2.2 X-ray powder diffraction

The X-ray powder diffraction data of benzylic acid and strontium benzilate was collected at room temperature 293(1) K, in θ/θ reflection mode using a Siemens D5005 diffractometer with CuKα radiation (λ= 1.5418 Å). The diffractometer was worked at 40 kV and 25 mA. A small quantity of each compound was ground mechanically in an agate mortar and pestle and mounted on a flat holder covered with a thin layer of grease. The samples were scanned from 5° to 55° 2θ, with a step size of 0.02° and counting time of 10 s per step. Quartz was used as an external standard.

X-ray powder patterns showed in Figure 1a evidence the formation of a new compound. For the complex powder pattern, the 20 first measured reflections were completely indexed, using Dicvol04 program²⁹, which gave a unique solution in a monoclinic cell with parameters a = 15.05 Å, b = 7.48 Å, c = 25.01 Å, β = 94.6° in a P-type cell. This cell was confirmed employing single-crystal analysis. In order to check the unit cell parameters, a Le Bail refinement³⁰ was carried out using the Fullprof program³¹. The Figure 2b shows a very good fit between the observed and calculated patterns. This analysis is also indicative of the homogeneity of the crystallized sample and the single crystals appear to be representative of the bulk samples.

Figure 1 a) X-ray powder diffraction patterns of benzylic acid and strontium benzilate. b) Le Bail refinement of the complex. 

Figure 2 The coordination environment of the Sr ion in strontium benzilate. Displacement ellipsoids are shown at a 30% probability level. 

3. RESULTS AND DISCUSSION

The structure of strontium benzilate unit is depicted in Figure 2. This complex crystallizes in the monoclinic system with P2₁/n as space group, with 4 entities per unit cell. Table 2 shows the fractional atomic coordinates and equivalent isotropic displacement parameters for the non-hydrogen atoms. Each of the central α-C atoms bonds tetrahedrally to a hydroxyl, two phenyls, and a carboxylate with bond distances and angles shown in Table 3. All bond distances and angles are normal⁴² and agree with the average values found in 90 entries with benzilate fragments found in the Cambridge Structural Database (CSD, version 5.41, Aug. 2020)³⁷.

The orientations of the phenyl rings in the benzilate moieties are those of propeller blades, as found in lithium benzilate¹³, potassium benzilate¹⁴, and the various benzilate derivatives found in the CSD Database³⁷. This conformation minimizes the repulsive interactions between the geminal phenyl rings.

In the structure, the metallic ion is coordinated to eight oxygen atoms, six from benzilate molecules, and two from water molecules, forming a distorted tetragonal antiprism. One of the benzilates is coordinated to the metal in a monodentate fashion (carboxylate only), while the other benzilate molecule does it in the bidentate from carboxylate and hydroxide. Strontium ions lies close to screw 2₁ axis (with x, y, z = 0.746, 0.976, 0.275, respectively) generating an infinite zig-zag chains along the [010] direction with traslational symmetry: ³/₂-x, ¹/₂+y, ¹/₂-z (Figure 3). The resulting one-dimensional chains are supramolecular related by O--H···O hydrogen-bond interactions between the coordinated water molecules and the O atoms of the carboxylate groups with graph set motifs⁴³ R(6) and C22 (7) along those (see Table 4).

Figure 3 Part of the crystal structure showing the zig-zag chains of {SrO8} polyhedra in the structure of strontium benzilate. 

The Hirshfeld surfaces mapped over d_norm (Figure 4) indicates the locations of the strongest intermolecular contacts (dark areas), anf fingerprint plots indicates the contributions of interatomic contacts to the Hirshfeld surface. Table 5 summarizes the main intermolecular contacts and their percentage distributions on the Hirshfeld surface for the strontium benzilate.

Figure 4 Hirshfeld d_norm surface map and the bi-dimensional fingerprint plots of the title compound [d_e and d_i represent the distances from a point on the Hirshfeld surface to the nearest atoms outside (external) and inside (internal) the surface, respectively]. 

The weak intermolecular interactions are mainly constituted by H···O, H···N, H···C and H···H, where the reciprocal contacts appear as a symmetrical wings for H···O, with de + di ∼ 2.4 Å, The H···C as symmetrical clamp with de + di ∼ 2.7 Å. The interatomic contacts of H···H have a majority of the all contribution in the surface generated showing a wide stain with de + di ∼ 2.4 Å, denoting H···H short contacts generating no significant effect over molecular packing in the crystal structure stabilization.

CONCLUSIONS

The new coordination polymer strontium benzilate has been synthetized and it crystal structure was determinate using X-ray single-crystal diffraction. The complex crystallizes in the monoclinic P2₁/n spsce group. The strontium ion is coordinated to eight oxygen atoms in a distorted tetragonal antiprism and forming an infinite zig-zag chains along the [010] direction. These chains form a three-dimensional network via O--H···O hydrogen-bond interactions between the coordinated water molecules and the O atoms of the carboxylate groups. Two dimensional fingerprint plot calculations displayed the H⋯H, C⋯H, and O⋯H pair of contacts that were the most significant interaction to the Hirshfeld surface.