SPECTROSCOPIC INVESTIGATION OF THE Cu(II) HETEROLIGAND COMPLEXES WITH 2,4-DIHYDROXYBENZOIC ACID

ABSTRACT

This study is dedicated to the investigation of complexes formed by 2,4-dihydroxybenzoic acid (2,4-DHBA) with urea, monoethanolamine, and biuret in the presence of Cu(II) ions using ultraviolet (UV) and electron paramagnetic resonance (EPR) spectroscopic methods. The electronic and structural properties of the complexes resulting from the interaction of 2,4-dihydroxybenzoic acid and biuret with copper(II) ions were thoroughly analyzed. UV spectroscopy was employed to examine electronic transitions within the ligands and their corresponding complexes. EPR spectroscopy provided insights into the paramagnetic properties of the Cu(II) ions and their interactions with the ligands. As a result, the study offers an in-depth understanding of the structure and electronic characteristics of the heteroligand Cu(II) complexes formed with 2,4-DHBA, urea, monoethanolamine, and biuret.

АННОТАЦИЯ

Данное исследование посвящено исследованию комплексов, образуемых 2,4-дигидроксибензойной кислотой (2,4-ДГБК) с мочевиной, моноэтаноламином и биуретом в присутствии ионов меди(II), с использованием методов ультрафиолетовой (УФ) и электронной парамагнитной резонансной спектроскопии (ЭПР). Были тщательно проанализированы электронные и структурные свойства комплексов, образующихся в результате взаимодействия 2,4-дигидроксибензойной кислоты и биурета с ионами меди(II). Для изучения электронных переходов внутри лигандов и соответствующих им комплексов использовали УФ-спектроскопию. ЭПР-спектроскопия позволила получить представление о парамагнитных свойствах ионов Cu(II) и их взаимодействиях с лигандами. В результате исследования было получено глубокое представление о структуре и электронных характеристиках гетеролигандных комплексов Cu(II), образованных с 2,4-DHBA, мочевиной, моноэтаноламином и биуретами.

Keywords: 2,4-dihydroxybenzoic acid, urea, monoethanolamine, biuret, Cu(II) complexes, UV spectroscopy, EPR spectroscopy, paramagnetic property, coordination compound.

Ключевые слова: 2,4-дигидроксибензойная кислота, мочевина, моноэтаноламин, биурет, комплексы Cu(II), УФ-спектроскопия, ЭПР-спектроскопия, парамагнитные свойства, координационное соединение.

Introduction.

Copper (II) ions (Cu²⁺) play a crucial role in both biological and chemical systems due to their unique complexation properties [1–3]. As one of the key transition metals, copper is actively involved in numerous enzymatic processes, particularly in redox reactions and catalytic activities [5–7]. Copper ions  can form coordination complexes with a wide variety of ligands, thereby altering their physicochemical and biological characteristics. Among these ligands, 2,4-dihydroxybenzoic acid (2,4-DHBA) is notable for containing two hydroxyl and one carboxyl functional group, enabling it to form stable chelate complexes with metal ions [8]. Urea, monoethanolamine (MEA), and biuret are relatively simple yet highly reactive ligands that can coordinate to metal centers through their nitrogen atoms. The heteroligand complexes formed by these ligands with Cu (II) ions are of particular interest due to their complex electronic structures and potential biological relevance. To investigate the electronic and paramagnetic properties of such complexes, ultraviolet (UV) and electron paramagnetic resonance (EPR) spectroscopy are widely employed analytical techniques [4,9]. UV spectroscopy provides insight into electronic transitions within the molecular framework, while EPR spectroscopy offers detailed information on the spin state of Cu (II) ions and their interactions with the surrounding environment. In this study, heteroligand Cu (II) complexes involving 2,4-DHBA, urea, MEA, and biuret were synthesized, and their spectral properties (UV and EPR) were systematically analyzed. The main objective of the research is to gain a deeper understanding of the electronic structure, determine the magneto chemical characteristics, and assess structural and conformational changes in these complexes. Ultimately, this study aims to lay the groundwork for the development of novel biologically active compounds based on these coordination systems.

Experimental part

Materials and Methods

Considering the critical importance of reagent purity and the appropriate selection of synthesis and analytical techniques, the following chemicals were employed: 2,4-dihydroxybenzoic acid (2,4-DHBA – L¹), monoethanolamine (MEA – L²), urea (L³), biuret (L⁴), and copper (II) chloride (CuCl₂). All reagents were of analytical grade and conformed to established purity standards, prepared for laboratory use. Ethanol (C₂H₅OH) was utilized as the solvent during synthesis procedures. Ultraviolet-visible (UV-Vis) and electron paramagnetic resonance (EPR) spectroscopic analyses were conducted using appropriate spectrometers. UV-Vis spectra of the complexes, prepared in 10⁻³ M solutions, were recorded on a UV-Vis-1900i spectrophotometer within the wavelength range of 190–1100 nm, with primary absorption maxima measured in the 270–400 nm region. EPR spectra were obtained using an X-band Spinscan X spectrometer.

Synthesis of Cu (II) Heteroligand Complexes:

Synthesis of the [CuL¹·2L²] Complex

The synthesis of the first ligand complex was carried out as follows: Copper (II) chloride (CuCl₂), weighing 0.0136 g, was dissolved in 2 ml of ethanol at 50°C under ultrasonic treatment in a water bath until fully dissolved. Subsequently, 0.0308 g of 2,4-dihydroxybenzoic acid (2,4-DHBA) and monoethanolamine (MEA) dissolved in 2 ml of ethanol were added dropwise to the solution. Upon addition, the solution color gradually changed from light green to brown. The synthesis proceeded according to the following scheme.

Synthesis of the [CuL¹·2L³] Complex

The synthesis of the complex was carried out under precisely controlled reaction conditions to optimize pH and reaction time for the formation of the Cu (II) complex with 2,4-dihydroxybenzoic acid and urea. Copper (II) chloride (CuCl₂), weighing 0.0136 g, was dissolved in 2 ml of ethanol at 40°C using ultrasonic treatment in a water bath until fully dissolved. Then, 0.0308 g of 2,4-DHBA (L¹) and 0.006 g of urea (L³), dissolved in 2 ml of ethanol, were added dropwise to the solution. Upon addition, the solution color changed to light green. The synthesis proceeded according to the following scheme:

During the synthesis process, pH, temperature, and reaction time were considered critical factors. The resulting complexes were characterized using spectroscopic techniques.

Synthesis of the [CuL¹·2L⁴] Complex

The reaction conditions for synthesizing the complex of 2,4-dihydroxybenzoic acid and biuret with Cu (II) ions were precisely optimized. The pH and reaction duration were adjusted to achieve the optimal formation of the complex. Copper (II) chloride (CuCl₂), weighing 0.0136 g, was dissolved in 2 ml of ethanol at 40°C in an ultrasonic water bath until completely dissolved. Subsequently, 0.0308 g of 2,4-DHBA (L¹) and biuret (L⁴), dissolved in 2 ml of ethanol, were added dropwise to the solution. The solution color gradually changed from light green to brown. The synthesis proceeded according to the following scheme.

Results and Discussion

The UV-Vis spectroscopic analysis revealed that the maximum absorption peaks of the Cu (II) complexes were located within the 400–500 nm range. These peaks correspond to the electronic transitions of the copper ion and its interactions with the coordinated ligands. The high intensity of these absorption bands indicates the stability of the complexes and the strong binding affinity between the copper ion and the ligands.

The EPR spectroscopy results demonstrated the anisotropic nature of the Cu (II) complexes’ EPR spectra. The g-factors and linewidths observed in the spectra reflect parameters related to the electronic structure of the copper ion within the complex. Changes in the EPR spectra indicate variations in the coordination environment and interactions between the copper ion and ligands. The g-factors and spin properties were confirmed as primary factors influencing the paramagnetic behavior of the complexes [10–12].

Using EPR spectroscopy, it was determined that urea and 2,4-dihydroxybenzoic acid ligands coordinate with the Cu (II) ion through O, O- and O, N-type donor atoms. The electronic donor capabilities of the ligands and their interactions with the copper ion were analyzed to assess the complex stability. These findings provided insight into the heteroligand nature of the complexes and clarified the interaction mechanisms of the copper ion within these coordination compounds.

Figure 1. EPR Spectrum of the [CuL¹·2L²] Complex

UV-Vis spectral analysis allowed for the identification of absorption bands characteristic of d–d electronic transitions in the Cu (II) complex. The EPR spectrum exhibited signals corresponding to the d⁹ electronic configuration of the copper ion. The g-tensor values were determined, enabling the assessment of the complex’s geometric structure and the influence of the coordinated ligands.

Based on EPR analysis and key conceptual criteria, the structure of the synthesized [CuL¹·2L²] complex was elucidated as follows:

1.When g∥>g⊥ and g∥≈2.2−2.4, the Cu (II) ion is consistent with a square planar or D₂d symmetric complex geometry. Examples include CuCl₄^2-, Cu (Acac)_2.

2.If g∥>g⊥ va g∥≫2.3, the complex adopts an elongated octahedral or axial (tetragonal) distorted geometry, typically found in complexes such as Cu (H₂O)₆)²⁺, Cu (NH₃)₄)²⁺ Cu²⁺.In these cases, both g∥ and A∥ values are significantly larger.

3.When g∥≈g⊥ va g∥≈2.1−2.2, this is indicative of the copper (+2) ion residing in an octahedral or nearly spherical coordination environment.

The EPR spectral data of the complex synthesized with the 2,4-DHBA ligand support these conclusions. The observed values g∥=2.71 and g⊥=2.03 are significantly large, consistent with a tetragonally distorted octahedral geometry. This suggests that the complex predominantly exhibits an octahedral structure but is distorted along the z-axis. Such distortion is characteristic of Jahn–Teller effects, a common phenomenon in Cu (II) complexes.

According to the UV-Vis spectral analysis, the ligand exhibited an absorption maximum at λ_max=296 nm, whereas the complex showed a bathochromic shift with λ_max=312 nm. This shift is theoretically attributed to π→π∗ electronic transitions. Additionally, a hypochromic effect was observed, characterized by a decrease in the molar absorptivity coefficient from ε=0.968 for the free ligand to ε=0.145 in the complex, likely due to changes in the refractive index of the solution.

In the UV-Vis spectrum of the [CuL¹·2L⁴] complex, new absorption maxima were detected, demonstrating the influence of Cu (II) ions on the electronic structure of the complexes.

The EPR spectra of Cu (II) ions provided valuable information regarding the electronic structure of the complexes. The g-factors and principal lines in the EPR spectra aided in characterizing the paramagnetic properties within the coordination environment of the complexes. In particular, the g-factor is a critical parameter for elucidating the electronic structure of Cu (II) complexes. A g-factor near 2.0 typically corresponds to a free electron; however, for Cu (II) ions, the characteristic relationship is g∥>g⊥>2.0. In the synthesized mixed-ligand complex, a g-value of 2.0357 was observed, indicating that the Cu (II) ions in the complex exhibit a distorted geometry in solution. This result suggests that the complex adopts a geometry close to a distorted octahedral configuration. Based on the EPR spectral analysis, it was concluded that complexes with the structures presented in the scheme were successfully synthesized.

CONCLUSION

Preliminary analyses of the Cu (II) complexes revealed potential antimicrobial activity, antioxidant properties, and other biological effects. The results suggest that these complexes may exhibit efficacy against certain microorganisms and could act as effective catalysts in accelerating various chemical reactions.

The complex formed between monoethanolamine and 2,4-dihydroxybenzoic acid with Cu (II) was successfully synthesized, and its electronic structure was investigated using UV-Vis and EPR spectroscopic methods. The findings confirm significant interactions between the Cu (II) ions and the ligands, indicating a distorted octahedral geometry of the complex.

Furthermore, the study demonstrated that complexes of 2,4-dihydroxybenzoic acid and biuret with Cu (II) possess distinct and noteworthy properties. Based on UV-Vis and EPR spectroscopy data, the electronic structure, and paramagnetic characteristics of the Cu (II) ions were elucidated, confirming a geometry closely resembling a distorted octahedron. These complexes hold considerable importance for the development of novel materials in biological and chemical processes. Future investigations focusing on their catalytic and antioxidant activities could provide a foundation for extended research and practical applications.

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