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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.66 no.1 Concepción Jan. 2021 



Gerzon E. Delgado1  2  * 

Asiloé J. Mora1 

Belkis Ramírez1 

Graciela Díaz de Delgado1 

Jonathan Cisterna2 

Alejandro Cárdenas3 

Iván Brito2 

1Laboratorio de Cristalografía, Departamento de Química, Facultad de Ciencias, Universidad de Los Andes, Mérida 5101, Venezuela.

2Departamento de Química, Facultad de Ciencias Básicas, Universidad de Antofagasta, Avda. Universidad de Antofagasta, Campus Coloso, Antofagasta 1240000, Chile.

3Departamento de Física, Facultad de Ciencias Básicas, Universidad de Antofagasta, Avda. Universidad de Antofagasta, Campus Coloso, Antofagasta 1240000, Chile.


A novel coordination polymer was synthesized from strontium carbonate and benzylic acid in aqueous solution and is formulated as Sr[(C6H5)4(COCOO)2·2H2O]. This compound was characterized by FTIR spectroscopy and powder X-ray diffraction. The crystal structure was determined by single-crystal X-ray diffraction. The complex crystallizes in the monoclinic P21/n space group, with unit cell parameters a = 15.0224(9) Å, b = 7.5038(6) Å, c = 25.000(2) Å, β = 94.764(2)°, V = 2808.4 Å3, Z = 4. 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 form infinite zig-zag chains along the [010] direction, which 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. The intermolecular interactions were analyzed using Hirshfeld surface analysis.

Keywords: Benzilate; coordination polymer; X-ray diffraction; hydrogen bonds; Hirshfeld surface


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 activities4,5 and more recently as an anticancer agent6.

Benzilic acid, an aromatic α-hydroxycarboxylic acid, has long been used in the preparation of antimuscarinic agents710 and treatment against Alzheimer's disease since it can be a considerable inhibitor of acetylcholinesterase enzyme11,12.

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 ligands6,1720. 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 materials2123. 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 polymers13,14,2427, 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.


2.1 Synthesis and FT-IR spectroscopy

Strontium benzilate was prepared by reaction of benzylic acid (C14H12O3) and strontium carbonate (SrCO3) in a 1:1 ratio in water. The mixture was maintained under continuous stirring for 24 h. The resulting solution was filtered and allowed to evaporate slowly at ambient temperature. Colorless plates grew in the solution over one month.

The FT-IR absorption spectrum was obtained as KBr pellet using a Perkin-Elmer 1600 spectrometer. The FT-IR showed O-H stretching bands from coordination water molecules at 3650 cm−1, and lattice water at 3358 y 3180 cm−1. The aromatic ring sp2 carbons stretching band is at 3059 cm−1, while the νa (CO2 ) and νs (CO2 ) stretchings are located at 1596 and 1389 cm−1, respectively. The C-O stretching band appears at 1053 cm−1.

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. 

2.3 X-ray single-crystal crystallography

A colorless rectangular crystal (0.50, 0.40, 0.20 mm) was used for data collection. X-ray diffraction data were collected at 295 K on a Bruker SMART CCD area-detector diffractometer32 equipped with MoKα radiation (λ= 0.71073 Å). The data were corrected for Lorentz-polarization and absorption effects33. The structure of were solved by direct methods34 using the OLEX2 program35 and refined by a full-matrix least-squares calculation on F2 using SHELXL36. All H atoms were placed at calculated positions and treated using the riding model with C-H distances of 0.97-0.98 Å. Non-hydrogen atoms were refined with anisotropic displacement parameters. The Cambridge Structural Database (CSD, version 5.41, Aug. 2020) was used for structure analysis37. Hydrogen bond interactions were verified using PLATON Software38. Diagrams were prepared using DIAMOND software39.

Table 1 summarizes the crystal data, intensity data collection, and refinement details for the title compound. Crystallographic data for the structure reported here have been deposited with the Cambridge Crystallographic Data Centre (Deposition No. CCDC-2002517). These data can be obtained free of charge via

Table 1 Crystal data, data collection, and structure refinement of strontium benzilate. 

Chemical formula C28H20O7Sr CCDC 2002517
Formula weight 556.06 Radiation [Å] 0.71073
Crystal system monoclinic θ range [°] 1.6-23.3
Space group P21/n (14) hkl range -16, 6; −8, 8;-27, 22
a [Å] 15.0224(9) Reflections
b [Å] 7.5038(6) Collected 12184
c [Å] 25.000(2) Unique (Rint) 4058 (0.058)
β [°] 94.764(2) With I > 2σ(I) 3039
V [Å3] 2808.4(4) R(F2) [I > 2σ(I)] 0.043
Z 4 wR(F2) [I > 2σ(I)] 0.123
dx [g·cm−3] 1.315 Refinement method Full-matrix least-squares on F2
μ [mm−1] 1.960 Number of parameters 352
Crystal size [mm] 0.5 × 0.4 × 0.2 Goodness of fit on F2 0.92
F [000] 1128 Max/min Δρ [e·Å−3] 0.73/-0.64

2.4 Hirshfeld surfaces analysis

For the title compound, Hirshfeld surfaces analysis28 was performed with the aid of CrystalExplorer program40. The two-dimensional fingerprint plots were calculated for the crystal, as were the electrostatic potentials41. The electrostatic potentials were mapped on the Hirshfeld surfaces using the 3-21G basis set at the level of Hartree-Fock theory. The crystallographic information file (CIF) of the complex was used as input for the analysis. For the generation of fingerprint plots, the bond lengths of hydrogen atoms involved in interactions were normalized to standard neutron values (C-H = 1.083 Å, O-H = 0.983 Å).


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.

Table 2 Fractional atomic coordinates (×104) and Equivalent isotropic displacement parameters (Å2×103) for title compound. Ueq is defined as ⅓ of of the trace of the orthogonalised UIJ tensor. 

Atoms x y z Ueq
Sr 0.74624(3) 0.52442(5) 0.27496(1) 0.0315(2)
O1 0.8450(2) 0.5615(4) 0.3640(1) 0.042(1)
O2 0.9391(2) 0.5437(4) 0.1916(1) 0.051(1)
O3 0.7283(2) 0.3367(4) 0.1839(1) 0.038(1)
O4 0.8408(2) 0.7317(4) 0.2214(1) 0.042(1)
O5 0.6159(2) 0.4966(3) 0.3359(1) 0.037(1)
O6 0.6548(2) 0.5346(4) 0.1318(1) 0.054(1)
O7 0.6428(2) 0.7477(5) 0.2184(2) 0.073(2)
C1 0.8739(3) 0.7282(6) 0.3884(2) 0.034(1)
C2 0.5622(3) 0.3410(5) 0.3433(2) 0.034(2)
C3 0.9036(3) 0.6933(6) 0.1924(2) 0.033(2)
C4 0.6750(3) 0.3771(5) 0.1458(2) 0.033(2)
C11 0.8428(3) 0.7418(6) 0.4452(2) 0.038(2)
C12 0.7689(3) 0.6488(7) 0.4584(2) 0.051(2)
C13 0.7378(4) 0.6634(8) 0.5079(3) 0.069(2)
C14 0.7809(5) 0.7741(9) 0.5466(2) 0.074(3)
C15 0.8541(5) 0.8669(8) 0.5341(2) 0.065(2)
C16 0.8849(4) 0.8539(6) 0.4836(2) 0.050(2)
C17 0.9745(3) 0.7423(6) 0.3857(2) 0.041(2)
C18 1.0332(4) 0.6664(7) 0.4255(2) 0.056(2)
C19 1.1248(4) 0.6687(8) 0.4198(3) 0.075(3)
C21 0.5775(3) 0.2849(6) 0.4023(2) 0.035(2)
C22 0.5170(4) 0.3153(8) 0.4397(2) 0.065(2)

Table 3 Selected geometrical parameters (Å, °).for the strontium benzilate. 

O1-C1 1.442(5) C1-C11 1.535(6) Sr-O1 2.587(3)
O2-C3 1.244(5) C2-C21 1.532(6) Sr-O3 2.672(3)
O3-C4 1.230(5) C1-C17 1.522(6) Sr-O4 2.558(3)
O4-C3 1.271(5) C2-C27 1.529(6) Sr-O5 2.587(3)
O5-C2 1.440(5) C1-C4 1.554(6) Sr-Ow 2.619(4)
O6-C4 1.263(5) C210-C211 1.364(12) Sr-Ow 2.659(4)
O1-Sr-O3 143.18(9) O1-Sr-O4 94.67(10) O1-Sr-O5 84.84(9)
O1-Sr-O7 131.95(11) Sr-O4-C3 129.2(3) O3-Sr-O4 84.02(9)
Sr-O3-C4 122.9(3) Sr-O1-C1 126.0(2) Sr-O5-C2 127.0(2)
O3-Sr-O5 115.55(9) O1-C1-C11 110.1(3) O1-C1-C17 107.9(3)
C11-C1-C17 114.6(4) O5-C2-C21 107.9(3) O5-C2-C27 110.2(3)

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. 

Table 4 Hydrogen bond interactions of the title compound. 

Atoms D – H H ···A D ··· A D – H ···A
O1 – H1···O7i 0.75 1.97 2.174(4) 170
O1 – H7A···O2ii 0.85 1.94 2.782(4) 173
O12 – H12B··O8iii 0.97 1.96 2.820(18) 147
O12A – H12C··O8i 0.97 1.91 2.555(17) 121
i x, 1-y, z; ii 3/2-x, ½+y, ½-z; iii 3/2-x, -½+y, ½-z

The Hirshfeld surfaces mapped over dnorm (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 dnorm surface map and the bi-dimensional fingerprint plots of the title compound [de and di represent the distances from a point on the Hirshfeld surface to the nearest atoms outside (external) and inside (internal) the surface, respectively]. 

Table 5 Contributions of interatomic contacts to the Hirshfeld surface for the title compound*

Contact Contribution H···H C···H O···H C···C Sr···O
(%) 42.2 22.9 15.8 4.9 3.6

*Reciprocal contacts

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.


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.


This work was partially done into G.E. Delgado visit at the Universidad de Antofagasta, supported by MINEDUC-UA project, code ANT 1856. G.E. Delgado thank to FONACIT (grant LAB-97000821).


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