PHYSICOCHEMICAL CHARACTERIZATION OF Sm(III) COMPLEXES WITH β-DIKETONATE LIGANDS

ABSTRACT

A novel Sm(III) complex was synthesized, containing 1,1,1-trifluoroacetylacetone (TFAA) as an anionic ligand and aminobenzimidazole as a cationic component. The structure of the synthesized complex was confirmed by single-crystal X-ray diffraction and IR spectroscopy. Hirshfeld surface analysis was used to investigate interatomic interactions within the crystal lattice. The results revealed that the central Sm(III) ion is eight-coordinated and adopts a square-antiprismatic geometry. The coordination sphere of the metal center is formed by eight oxygen atoms from four bidentate TFAA ligands, while the protonated aminobenzimidazole is present as the cationic counterion.

АННОТАЦИЯ

Был синтезирован новый комплекс Sm(III), содержащий 1,1,1-трифтороацетилацетон (TFAA) в виде анионного лиганда и аминобензимидазол в виде катионного компонента. Структура комплекса подтверждена методами рентгеноструктурного анализа монокристаллов и инфракрасной спектроскопии. Для изучения межатомных взаимодействий в кристаллической решётке использован анализ поверхности Хиршфельда. Результаты показали, что центральный ион Sm(III) имеет восьми координированную среду и принимает квадратно-антипризматическую геометрию. Координационная сфера металлического центра образована восемью атомами кислорода от четырёх бидентатных лигандов TFAA, тогда как протонированный аминобензимидазол присутствует в виде катионного противоиона.

Keywords. Ligand, Samarium complex, Crystal structure, Fingerprint plots, Hirshfeld surface.

Ключевые слова. Лиганд, комплексы самарий, кристаллическая структура, отпечатки (fingerprint plots), поверхность Хиршфельда.

Introduction. 

In recent decades, rare earths have become vital to a wealth of advanced materials and technologies, including catalysts, alloys, magnets, optics and lasers, electronics, economical lighting, bio-analyses, and imaging [1]. Lanthanide ions are becoming an interesting area of ​​research in coordination chemistry due to their unique spectroscopic and paramagnetic properties [2]. Unlike d-block metals, lanthanide ions typically exhibit high coordination numbers, ranging from 8 or 9 to 12 [3]. The eight-coordinate complexes have many geometries: cubic, hexagonal bipyramid, square antiprismatic, and triangular dodecahedral. Dodecahedral or square antiprismatic geometries are the geometries for most eight-coordinate lanthanide complexes [4].  The synthetic flexibility of β-diketones has led to an explosion in the development of related lanthanide complexes and their coordination chemistry. The β-diketonate lanthanide complexes have been demonstrated to have variable coordination numbers ranging from six to ten, and diverse structural motifs [5]. The β-diketonate complexes are one of the most extensively investigated rare-earth coordination compounds. Complexes of rare earth metals with β-diketonates are divided into three types: the tris complexes, the Lewis base adducts of the tris complexes, and the tetrakis complexes [6]. Rare-earth β-diketonates are complexes of β-diketones (1,3-diketones) with rare-earth ions. These complexes are the most popular and intensively investigated rare-earth coordination compounds [7]. Lanthanide complexes incorporating β-diketone ligands exhibit a range of interesting photophysical phenomena, such as photoluminescence, laser emission, and electroluminescence [8], [9], [10]. It is observed that Ln⁴⁺ ions usually form neutral complexes with the formula M(TTA)₄. However, +3 ions formed anionic M(TTA)₄¹⁻ species [11]. For the tetrakis chelates of β-diketones with trivalent lanthanides, the chelate is an anion, and a monovalent cation is required. In the compound NH₄Pr (TTA)_4, a water molecule was found outside the coordination sphere, effectively increasing the size of the small NH₄⁺ cation. This suggested that a large cation is necessary for the stability of these structures [12]. Sm(III) ions exhibit high luminescent properties, exhibiting orange-red radiation absorption. Due to this property, Sm(III) complexes are used in sensors, displays, and fluoro-immunoassays. [13].

In this study, we synthesized a samarium (III) complex containing 1,1,1-trifluoroacetylacetone and 2-aminobenzimidazole and investigated its structural and physicochemical properties.

Method and materials.

All reagents were commercially available without clearance and purchased from Sigma-Aldrich.

All synthetic procedures were carried out in air. Initially, 1,1,1-Trifluoroacetylacetone (3 mmol) was dissolved in ethanol and stirred with an alcohol solution of an alkali (3 mmol) for 1 hour. Then, an alcohol solution of Sm (NO₃)₃ · nH2O salt (1 mmol) was added dropwise to the mixture. The combined solution was stirred continuously at room temperature for 4 hours. Then, an alcoholic solution of 2-aminobenzimidazole (1 mmol) is added dropwise to this mixture and stirring is continued for another 2 hours. The final mixture was filtered and stored in a dust-free environment at room temperature to allow for single-crystal growth. After two weeks, the grown crystals were washed with alcohol and purified. The resulting crystals were analyzed using X-ray structural analysis.

To study the formation of a complex with a metal ligand, the synthesized complex was recorded on KBr tablets using an IRTracer-100 (SHIMADZU, Japan) IR-Fourier spectrometer in the wavelength range of 400-4000 cm^-1.

A Hirshfeld surface (HS) analysis was carried out using Crystal Explorer [14] to explore non-covalent interactions through Hirshfeld surfaces and two-dimensional fingerprint plots [15].

Scheme 1. Synthesis of the Sm(III) β-diketonate complexes

Results and discussion.

The structure of the synthesized complex compounds was studied using X-ray diffraction analysis, Hirshfeld surface analysis, fingerprint plots, and IR spectroscopy methods.

The IR spectra of the [Sm(TFAA)₄]ABI complex were compared with the IR spectra of the ligands TFAA and ABI. In the IR spectrum of the TFAA ligand, valence asymmetric bonds of aliphatic C-H bonds are observed around the region of 2977 cm^-1. The enol form of the β-diketonate group shows valence symmetric vibrations in the region of 1605 cm^-1. The peaks appearing in the region of 3664 cm^-1 belong to O-H vibrations in the enol form of the β-diketone. The regions of 1107–1148 cm^-1 show a series of symmetric valence vibrations originating from C–F bonds. In addition, deformed vibrations of CH₂ or CH₃ groups can be seen in the regions of 1471, 1420 cm^-1 [13]. In the IR spectrum of the [Sm(TFAA)₄]ABI complex, the region 3314–3685 cm^-1 corresponds to the vibrations of the N–H and O–H bonds in the enol tautomer. The region 3154–2894 cm^-1 indicates the stretching vibrations of the aliphatic and aromatic C–H bonds originating from the TFAA and ABI ligands. The band at 1689 cm^-1 corresponds to the C=O valence vibration and its shift relative to the free ligand indicates that it is coordinated to the Sm³⁺ ion. The deformation vibrations of the aromatic amine groups occur in the region 1292–1385 cm^-1.

Low-frequency vibrations are observed in the 501 cm^-1 range. These vibrations are due to the Sm-O bond, confirming metal-ligand coordination.

Figure 1. IR spectra of a complex and ligands

 Single crystal XRD analysis results show that the samarium ion is coordinated by four bidentate trifluoro acetylacetonate (TFAA⁻) ligands, which can form an inner sphere charge of -1. In the outer sphere, a single protonated 2-aminobenzimidazole (ABI⁺) cation acts as a +1 charge. The TFAA⁻ ligands form two distorted square antiprismatic geometries around the Sm³⁺ center, acting as typical O, O-chelators. The molecular system contains several fluorine groups (CF₃), which contribute to both the p-stacking potential and the electron-withdrawing property. The fluorine atoms are attached to the terminal positions of the diketone ligands, which can affect the overall polarization and crystal packing. The complex is crystallized in the monoclinic system with the P 2₁ space group. The molecular structure of the Sm (III) complex, drawn using Mercury, is shown in Figs. 2.

a)                                                                b)

Figure 2. The structural formula of [Sm (TFAA)₄ ] ABI (a), packing(b)

The Hirshfeld surface of a molecule represents the region in the crystal where the electron density is sensitive to the procrystal. The HS is visualized using color coding to indicate interatomic contacts that are shorter (red areas), equal to (white areas), or longer than (blue areas) the sum of the van der Waals radii. The red spots on the analysis of d_norm indicate the involvement of atoms in hydrogen-bonding interactions. The HS shown using the shape index helps identify correlations such as C−H∙∙∙π and π-π stacking interactions. The presence of adjacent blue and red triangular regions around the aromatic rings corresponds to the existence of π-π stacking interactions in the new compound [16].

Figure 3. HS plotted over [Sm(TFAA)₄]ABI (a)  dnorm in the range – 0.5449to 1.5106 a.u., (b)  shape-index in the range –1.000 to 1.000 a.u.,  (c) curvedness ranging from -4.00 to 0.40 a.u 

Figure 4. Fingerprint plots of [Sm(TFAA)4]ABI  showing interactions (d_i is the closest internal distance from a given point on the HS, and d_e is the closest external contacts)

The dominant intermolecular interactions are clearly visible on the Hirschfeld surfaces and the corresponding fingerprint patterns (Figure 4), where one molecule acts as a donor (with d_e > d_i) and the other as an acceptor (with d_i > d_e). The fingerprint patterns can be decomposed to separate similar contributions from different interactions and highlight specific atom-pair close contacts. In the [Sm(TFAA)₄]ABI complex, the most prevalent intermolecular contacts are the F···H/H···F (47.2%), H···H/H···H (22.3%), and C···H/H···C (8.3%) interactions. Additional important contributions include F···F/F···F (7.3%) and O···H/H···O (6.5%) contacts. Fingerprint patterns and molecular Hirshfeld surfaces provide valuable information about the nature and extent of these intermolecular contacts, which play a crucial role in stabilizing the crystal structure. The percentage values ​​indicate that molecular arrangement is primarily governed by strong electrostatic interactions, which are the main driving force of crystal aggregation [15].

Conclusion.

A lanthanide complex [Sm(TFAA)₄]ABI was structurally characterized and found to crystallize in the monoclinic space group P2₁ with distorted square antiprismatic geometries around the Ln³⁺ centers. Hirshfeld surface analysis revealed that crystal packing is primarily stabilized by F···H, H···H, and C···H interactions, with additional contributions from F···F and O···H contacts. These dominant electrostatic interactions play a key role in the structural organization and stability of the complexes.

References:

1. Eliseeva S. V., Bünzli J. C. G. Rare earths: jewels for functional materials of the future //New Journal of Chemistry. – 2011. – Vol. 35. – №. 6. – PP. 1165-1176.
2. Ahmed Z., Iftikhar K. Solution studies of lanthanide (III) complexes based on 1, 1, 1, 5, 5, 5-hexafluoro-2, 4-pentanedione and 1, 10-phenanthroline Part-I: Synthesis, 1H NMR, 4f–4f absorption and photoluminescence //Inorganica Chimica Acta. – 2010. – Vol. 363. – №. 11. – PP. 2606–2615.
3. Cotton S. A., Harrowfield J. M. Lanthanides: coordination chemistry //Encyclopedia of Inorganic and Bioinorganic Chemistry. – 2011.
4. P. Cheng, ‘Coordination compounds of the lanthanides,’ in Lanthanides, Elsevier, 2023. - PP. 157–229.
5. Huang C. H. Rare earth coordination chemistry: fundamentals and applications. – John Wiley & Sons, 2010.
6. Gschneidner K. A., Bunzli J. C. G., Pecharsky V. K. Handbook on the physics and chemistry of rare earths. – Elsevier, 2004. – Vol. 34.
7. Binnemans K. Rare-earth beta-diketonates //Handbook on the physics and chemistry of rare earths. – Elsevier, 2005. – Vol. 35. – PP. 107-272.
8. Ahmed Z., Dar W. A., Iftikhar K. Synthesis, and luminescence study of a highly volatile Sm (III) complex //Inorganica Chimica Acta. – 2012. – Vol. 392. – PP. 446-453.
9. Ahmed Z., Iftikhar K. Red, orange-red and near-infrared light emitting ternary lanthanide tris β-diketonate complexes with distorted C 4v geometrical structures //Dalton Transactions. – 2019. – Vol. 48. – №. 15. – PP. 4973–4986.
10. Santos H. P. et al. Synthesis, structures, and spectroscopy of three new lanthanide β-diketonate complexes with 4, 4′-dimethyl-2, 2′-bipyridine. Near-infrared electroluminescence of ytterbium (III) complex in OLED //Inorganica Chimica Acta. – 2019. – Vol. 484. – PP. 60-68.
11. Cary S. K. et al. Advancing understanding of the+ 4 metal extractant thenoyltrifluoroacetone (TTA–); synthesis and structure of MIVTTA4 (MIV= Zr, Hf, Ce, Th, U, Np, Pu) and MIII (TTA) 4– (MIII= Ce, Nd, Sm, Yb) //Inorganic Chemistry. – 2018. – Vol. 57. – №. 7. – PP. 3782–3797.
12. Criasia R. T. The crystal structure of tetrabutylammonium tetrakis (4, 4, 4-trifluoro-1-(2-thienyl)-1, 3-butanedione) samarium (III), (C4H9) 4N (C8H4F3O2S) 4Sm (III) //Inorganica chimica acta. – 1987. – Vol. 133. – №. 1. – PP. 161-166.
13. Hooda A. et al. Samarium (III) complexes with fluorinated diketones and heteroaromatic auxiliary moieties: synthesis and spectral analyses //Inorganica Chimica Acta. – 2023. – Vol. 553. – PP. 121543.
14. Spackman P. R. et al. CrystalExplorer: a program for Hirshfeld surface analysis, visualization, and quantitative analysis of molecular crystals //Applied Crystallography. – 2021. – Vol. 54. – №. 3. – PP. 1006–1011.
15. Demircioğlu Z., Büyükgüngör O., Ersanlı C. C. Intermolecular Interactions and Fingerprint Plots with Hirshfeld Surface Analysis //SETSCI-Conference Proceedings. – SETSCI-Conference Proceedings, 2018. – Vol. 3. – PP. 698-700.
16. Mukhammadiev S. et al. Synthesis, crystal structure, and Hirshfeld surface analysis of 1, 3-dihydro-2H-benzimidazol-2-iminium 3-carboxy-4-hydroxybenzenesulfonate //Structure Reports. – 2024. – Vol. 80. – №. 10.