SYNTHESIS AND IR CHARACTERIZATION OF A Cu(II) HETEROLIGAND COMPLEX

ABSTRACT

This study aims to synthesize a new copper (II) complex and to investigate its structural features. The complex was obtained using copper (II) chloride in the presence of 4-chloro-2-methylphenoxyacetic acid and diethanolamine. The structure of the synthesized compound was studied by IR spectroscopy through analysis of functional group vibrations. The results indicate that the carboxyl group undergoes deprotonation and coordinates to the copper ion. Analysis of the vibrational features suggests a monodentate coordination mode of the carboxylate group. Comparison with the spectrum of the initial ligand confirms the formation of the complex.

АННОТАЦИЯ

Целью данного исследования является синтез нового комплекса меди(II) и изучение его структурных особенностей. Комплекс был получен на основе хлорида меди(II) в присутствии 4-хлор-2-метилфеноксиуксусной кислоты и диэтаноламина. Структура синтезированного соединения была исследована методом ИК-спектроскопии на основе анализа колебаний функциональных групп. Результаты показали, что карбоксильная группа подвергается депротонированию и координируется с ионом меди. Анализ колебательных характеристик указывает на монодентатный способ координации карбоксилатной группы. Сравнение со спектром исходного лиганда подтверждает образование комплекса.

Keywords: copper (II) complex, 4-chloro-2-methylphenoxyacetic acid, diethanolamine, IR spectroscopy.

Ключевые слова: комплекс меди(II), 4-хлор-2-метилфеноксиуксусная кислота, диэтаноламин, ИК-спектроскопия.

Introduction

Copper (II) ions are known for forming a wide range of complexes with organic acid ligands, and their stability, bioactivity, and coordination diversity are among the most active areas of modern coordination chemistry. In particular, phenoxyacetic acids and their chlorinated derivatives, with their electron-donating ability and steric properties, influence the structure of the Cu (II) ion[1]. MCPA is one of the widely used herbicides in agriculture, used in the form of its salts or ester derivatives. This compound effectively controls broad-leaved weeds found in cereal crops and pastures, especially harmful plants such as thistle and sorrel. According to statistics, although it has been used steadily since 1992, a certain decrease in consumption volumes was observed in the period up to 2017. Currently, this substance is mainly used in wheat cultivation[2].  The widespread use of MCPA as a herbicide is causing serious concerns about its potential environmental impact. Therefore, in recent decades, a large number of studies have been conducted aimed at assessing the level of environmental risk of MCPA. According to the results of the study, MCPA is moderately toxic to mammals and aquatic organisms, while showing less toxicity to birds[3]. MCP (4-chloro-2-methylphenol), which is an intermediate product in the synthesis of phenoxy herbicides, is also formed as a metabolite during the breakdown of MCPA. According to data, approximately 15,000 tons of MCP were produced in the European Union in 1989[4]. MCP is estimated to be highly toxic to aquatic organisms. Nevertheless, the concentrations of MCPA and MCP detected in water and soil environments have been noted to be below the predicted ineffective levels for all environmental components, and they are considered to have a low level of environmental risk[4].

4-chloro-2-methylphenoxyacetic acid (MSPA) is an organic ligand with an aromatic ring, phenoxy oxygen atoms, and a carboxyl group, which can bind to metal ions in various coordination regimes. Phenoxy oxygen usually does not participate in coordination, but carboxylate oxygen can bind to the metal center in one or two places.

Diethanolamine (DEA) has two functional groups as a ligand:

-OH (oxygen donor)

-NH (nitrogen donor)

Therefore, DEA usually forms a bidentate chelate with Cu (II), filling the coordination sphere. The presence of DEA often leads to the tetragonal geometry of Cu (II) (Jan-Teller effect) [5,6].

MSPA-DEA-Cu (II) systems have not been sufficiently studied in the literature, and the synthesis of new complexes and the determination of their spectroscopic properties is one of the urgent tasks.

Materials and methods

The reagents were purchased from Sigma-Aldrich and other certified sources and used without additional purification. In the synthesis process, analytically pure CuCl₂•2H₂O, 4-chloro-2-methylphenoxyacetic acid (MSPA), diethanolamine (DEA), and ≥96% ethanol were used as solvents. For the synthesis of the Cu (II) complex, 0.01 mol of CuCl₂•2H₂O was dissolved in 20 ml of ethanol. An equivalent amount of 4-chloro-2-methylphenoxyacetic acid was added to the resulting solution, and the mixture was heated at a temperature of 50-60 °C for 20 minutes. After complete dissolution of the solution, 0.01 mol of diethanolamine was added dropwise, and the reaction mixture was constantly stirred for 1-2 hours. Then the solution was cooled to room temperature, resulting in blue-green crystals. The obtained crystals were filtered, washed with ethanol, and dried under vacuum. The yield of the formed complex was recorded in the range of 72-78% based on analytical quantitative methods.

Figure 1. Schematic representation of the formation of the copper(II) complex with 4-chloro-2-methylphenoxyacetic acid and diethanolamine

Results and discussions

The IR spectra were recorded using an IRAffinity-1S spectrometer (Shimadzu, Japan) at Termez State University. The samples were prepared as KBr pellets in the solid state, and the spectra were measured in the range of 600–4000 cm⁻¹. The observed vibrational bands were analyzed to identify functional groups and to investigate the coordination characteristics of the complex. The IR spectra of the 4-chloro-2-methylphenoxyacetic acid (MCPA) ligand and its heteroligand complex compound with Cu (II) ion and diethanolamine (DEA) allow determining which donor atoms of the ligand are connected to the metal during the coordination process. When comparing the spectra of ligands and complexes, significant shifts in the oscillation frequencies of functional groups are observed. In the ligand spectrum, the C=O valence vibration characteristic of the carboxyl group is observed at 1739 cm⁻¹. In the complex spectrum, the disappearance of this signal and the appearance of 1649 cm⁻¹ and 1431 cm⁻¹ bands in its place indicates the deprotonation of the carboxyl group and its transition to the carboxylate form (COO⁻). These bands correspond to the asymmetric ν_as (COO⁻) and symmetric ν_sym (COO⁻) vibrations of the carboxylate group, respectively. To determine the coordination method of the carboxylate group, the difference (Δν) between the ν_as (COO⁻) and ν_sym (COO⁻⁾ vibrations is calculated:

Δν=ν_as (COO⁻) −ν_sym (COO⁻)

In the complex spectrum: Δν=1649−1431=218 cm⁻¹

According to the theory of coordination chemistry, if the value of Δν is greater than 200 cm⁻¹, the carboxylate group is monodentately coordinated with the metal [7]. Therefore, it can be concluded that the MCPA ligand is monodentately coordinated with the Cu (II) ion through one oxygen atom of the carboxylate group. In the high-frequency region of the spectrum, wide bands are observed around 3564 cm⁻¹. This absorption is due to the valence vibrations of the hydroxyl (O-H) and partially amino (N-H) groups in the diethanolamine ligand. The 2980 cm⁻¹ and 2924 cm⁻¹ bands correspond to aliphatic C-H valence vibrations. Skeletal vibrations of the aromatic ring are observed in the complex spectrum at around 1490 cm⁻¹. These bands are associated with the C=C valence vibrations of the benzene ring, indicating that the aromatic fragment in the ligand is preserved during the complex formation process.

The participation of the DEA ligand in coordination is also confirmed by C-N valence vibrations. In the complex spectrum, bands of 1330, 1294, and 1244 cm⁻¹ are observed, which correspond to the vibrations of C-N bonds belonging to the amine group. In addition, the 1102 and 1039 cm⁻¹ bands represent the valence vibrations of the Ar-O-CH₂ bond, characteristic of the phenoxy fragment. In the low-frequency region of the spectrum, the bands around 648 and 619 cm⁻¹ indicate the formation of metal-ligand bonds, which are interpreted as belonging to the vibrations of the Cu-O and Cu-N coordination bonds, respectively. Thus, analysis of the IRF spectrum shows that the Cu (II) ion forms monodentate coordination with the MCPA ligand through carboxylic acid, and the diethanolamine ligand participates in the coordination process through the amino and hydroxyl donor atoms. As a result, it can be concluded that a heteroligand coordination complex with the participation of O,N donor atoms is formed around the Cu (II) center.

Figure 2. IR spectrum of a complex compound containing 4-chloro-2-methylphenoxyacetic acid and [Cu(MSPA)₂(DEA)₂]

Conclusion

Thus, the combined heteroligand complex compound based on 4-chloro-2-methylphenoxyacetic acid (MCPA), Cu (II) ion, and diethanolamine was studied by the FTIR spectroscopic method. In the complex spectrum, v_as (COO⁻) and v_sym (COO⁻) absorption bands characteristic of the carboxyl group were observed at 1649 and 1431 cm⁻¹, respectively. Their external difference is equal to D = 218 cm⁻¹, which indicates the monodentate coordination of the carboxylate group with the metal.

References:

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