QUANTUM-CHEMICAL ANALYSIS OF Cu, Zn AND Ni COMPLEXES OF 1-METHYLIMIDAZOLE-2-THIOL

ABSTRACT

This article uses density functional theory (DFT) to study the electronic properties and stability of 1-methylimidazole-2-thiol complexes with copper, zinc, and nickel. Complex formation reduces total energy, showing improved stability for complexes than the free ligand, with zinc and nickel complexes having the lowest energy levels. Copper complex has a lower dipole moment, indicating a more symmetrical structure, while zinc and nickel complexes have larger dipole moments due to polar chlorine atoms in their coordination spheres. HOMO-LUMO gap study shows that complexes have smaller gaps than the ligand, indicating stronger stability and lower reactivity. The ligand's nitrogen and sulphur atoms coordinated with metal ions to stabilize complexes.

АННОТАЦИЯ

В этой статье используется теория функционала плотности (DFT) для изучения электронных свойств и стабильности комплексов 1-метилимидазол-2-тиола с медью, цинком и никелем. Образование комплекса снижает общую энергию, показывая улучшенную стабильность комплексов, чем свободный лиганд, при этом комплексы цинка и никеля имеют самые низкие уровни энергии. Комплекс меди имеет более низкий дипольный момент, что указывает на более симметричную структуру, в то время как комплексы цинка и никеля имеют более высокие дипольные моменты из-за полярных атомов хлора в их координационных сферах. Исследование щелей HOMO-LUMO показывает, что комплексы имеют меньшие щели, чем лиганд, что указывает на более высокую стабильность и более низкую реакционную способность. Атомы азота и серы лиганда координируются с ионами металлов для стабилизации комплексов.

Keywords: imidazole derivatives, complexes, molecular orbital, DFT, HOMO, LUMO, total electron density

Ключевые слова: производные имидазола, комплексы, молекулярная орбиталь, DFT, HOMO, LUMO, общая электронная плотность

INTRODUCTION

Due t special structural qualities and capacity to coordinate with a variety of metal ions, imidazole derivatives are important in coordination chemistry. The imidazole ring is a good ligand because it may function as a bidentate ligand and form stable complexes with transition metals. It is distinguished by its five-membered structure, which contains two nitrogen atoms. This coordination capacity is especially useful in the synthesis of coordination polymers and metal-organic frameworks (MOFs), which have attracted a lot of interest due to their various uses in materials research, drug transport, and catalysis [1][2].

The electronic characteristics and steric variables of imidazole derivatives have a significant impact on their coordination chemistry. For example, the imidazole ring’s nitrogen donor atoms enable the creation of robust coordination interactions with metal ions including copper, zinc, and silver. [3][4][5]. In addition to the nitrogen atoms, imidazole can form complexes with these metals because of the ligand’s flexibility in assuming different coordination geometries, which can result in the development of one, two-, or three-dimensional structures [6][7]. The creation of novel materials with specialised qualities, such as increased thermal stability and particular catalytic activity, depends on this adaptability [8][9].

Imidazole derivatives have been shown to be crucial in the development of supramolecular structures in recent research. For instance, it has been demonstrated that using 1H-imidazole-4,5-dicarboxylic acid can help construct a variety of coordination polymers because of its multidentate nature, which enables it to coordinate with several metal centres at once [7][8]. This characteristic is especially helpful in the creation of zeolitic imidazolate frameworks (ZIFs), which are excellent for gas storage and separation because of their exceptional porosity and stability [10][11]. Thanks to the structural diversity that imidazole derivatives provide, scientists can design materials with particular properties, including selective adsorption or catalytic activity. Furthermore, by making chemical changes at different locations on the imidazole ring, imidazole derivatives' coordination behaviour can be adjusted. The stability and geometry of the resultant metal complexes can be affected by substituents by considerably changing the ligand's electrical properties and steric hindrance [5][12]. For example, adding alkyl or aryl groups can increase the imidazole ligand's lipophilicity, which could increase its solubility in organic solvents and make it easier for coordination compounds to form with particular metal ions [4][13]. These alterations have been used to produce new materials with improved biocidal capabilities, as demonstrated by metal azolate frameworks based on silver that take use of imidazole’s coordination chemistry [3][4].

Apart from their synthetic uses, imidazole derivatives hold great significance in biological systems, especially because of their ability to imitate the coordination behaviour of histidine, an amino acid that is essential for both metal ion binding and enzyme catalysis [14]. Designing biomimetic catalysts and medicinal agents is affected by imidazole’s capacity to interact with metal ions in a biological setting. For instance, a great deal of research has been done on the coordination of imidazole with copper ions, which has provided insights into the mechanisms underlying a number of enzyme processes [15]. Because of their biocompatibility and functional flexibility, derivatives of imidazoles are desirable candidates for the creation of novel medications and diagnostic instruments. Imidazole-based coordination compounds are frequently synthesised using solid-state or solution-phase procedures, which, depending on the reaction circumstances and metal ion selected, can produce a range of structural motifs [1][6][12]. Understanding the relationship between structure and function in coordination chemistry requires crystalline materials with well-defined structures, which can be produced particularly effectively via the solid-state synthesis of coordination molecules [1][16]. Moreover, these compounds' coordination modes and stability can be better understood by characterising them using methods like NMR spectroscopy and X-ray crystallography [16][5].

In coordination chemistry, imidazole derivatives play a key role by providing a rich platform for the creation of new materials with a wide range of uses. Because of their distinct structural characteristics and capacity to form stable complexes with a wide range of metal ions, they are essential for the synthesis of coordination polymers and metal-organic frameworks. It is certain that further studies into imidazole ligand modification and coordination behaviour will yield novel insights and advances in the fields of materials science, catalysis, and biochemistry.

MATERIALS AND METHODS

All starting materials were purchased from commercial sources and used without further purification.

All quantum chemical calculations in this work were performed using Hyperchem 8.0 and Gaussian 09 software. Ground state, density functional theory (DTF), B3LYP, 6-31G calculation methods were used to optimize the molecular geometry. Together with these methods, 3-21G, STO-3G basis sets were used to optimize the structure of the ligand molecule, as it is known that they require less processing power and shorter calculation time without losing too much accuracy. The DFT/B3LYP method was combined with the STO-3G basis set to calculate the single point energy. The above method was also used to calculate electron density potentials (ESP) of ligands and complexes, total electron density (TED). Several chemical parameters, such as (E_HOMO), (E_LUMO), energy gap between HOMO and LUMO orbitals (E_gap = E_HOMO − E_LUMO), electronic charges of each atom, electronic density of each molecule and dipole moments (µ), were calculated.

The process of complex compounds (1-3) synthesis.

Synthesis of [Cu(MIT)₂]Cl₂ (1). 50 ml of DMSO and water in a ratio of 7:3 was poured into the flask, and 10 mmol of 1-methylimidazole-2-thiol (MIT) was added. Then a 10 ml solution of 5 mmol of CuCl₂ in water was added to it. HI-7007L buffer was used to keep the medium pH constant. The mixture was stirred on a magnetic stirrer and refluxed at 60°C for 4 hours. The reaction was monitored by TLC. The resulting products were separated, washed with hexane, and dried in a vacuum desiccator. Yield 71%.

Synthesis of [Zn(MIT)Cl₂] (2). 50 ml of 1-methylimidazole-2-thiol (MIT) was added to the flask by pouring 50 ml of DMSO and water in a ratio of 7:3. Then a 10 ml solution of 5 mmol of ZnCl₂ in water was added to it. HI-7007L buffer was used to keep the medium pH constant. The mixture was stirred on a magnetic stirrer and refluxed at 60°C for 4 h. The reaction was monitored by TLC. The resulting products were separated, washed with hexane, and dried in a vacuum desiccator. Yield 74%.

Synthesis of [Ni(MIT)Cl₂] (3). 50 ml of DMSO and water in a ratio of 7:

 was poured into the flask, and 5 mmol of MIT was added. Then a 10 ml solution of 5 mmol of NiCl₂ in water was added to it. HI-7007L buffer was used to keep the medium pH constant. The mixture was stirred on a magnetic stirrer and refluxed at 70°C for 6 hours. The reaction was monitored by TLC. The resulting products were separated, washed with hexane, and dried in a vacuum desiccator. Yield 78%.

RESULTS AND DISCUSSIONS

Figure 1 below graphically depicts the charges of the atoms and the total electron density of the molecule, calculated using the ab initio method of the ligand substance 1-methylimidazole-2-thiol. Here we can observe that there are large negative charges on the nitrogen atom in the 3-position of the imidazole ring and on the sulphur atom attached to the ring in the 2-position. Their values are -0.365 and -0.725, respectively. In nitrogen atom 1, this value is equal to -0.191. The presence of unshared electron pairs in the outer shells of these atoms justifies these values. The abundance of electron densities in the 3rd nitrogen and 2nd sulphur in the picture below confirm this conclusion. The presence of the 3rd nitrogen and 2nd sulphur in the ring allows these atoms to form coordination compounds with intermediate metal atoms.

Figure 1. Graphic representation of charge distribution and total electron density in atoms of 1-Methylimidazole-2-thiol

Table 1 below presents the quantum chemical calculations of dipole moments and total molecular energies for 1-methylimidazole-2-thiol and its complexes with copper, zinc, and nickel. The dipole moments reduce when transitioning from 1-methylimidazole-2-thiol to its copper complex. This signifies that the intricate molecule possesses a wholly symmetrical structure, resulting in diminished polarity, so validating the provided formula. The transition from the ligand to its zinc and nickel complex results in a significant rise in the dipole moment, signifying the existence of highly polar chlorine atoms within the inner sphere of the complex, hence affirming the enhanced polarity of the molecule and supporting the proposed formula. The reduction in total energy during the transition from the ligand to the complex indicates that the stability of the complex molecule exceeds that of the free ligand. The reduced energy levels of zinc and nickel complexes signify their enhanced stability.

Table 1.

Quantum chemically calculated dipole moments and total energies of the molecule of the ligand and its complexes with intermediates

Understanding the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) is crucial for analyzing the electronic properties of ligands and coordination compounds. The HOMO represents the highest energy level of electrons that are occupied in a molecule. It is primarily involved in electron donation during chemical reactions. The LUMO is the lowest energy level that can accept electrons. It plays a critical role in electron acceptance processes [17] (Shown in the Figure 2.).

Table 2 below presents the calculated values of the HOMO and LUMO orbitals for the ligand 1-methylimidazole-2-thiol and its metal complex compounds, utilising quantum chemical calculation methods. Upon examining the table, it is evident that the HOMO values diminish from the ligand to the complex compound, while the LUMO values also show a decrease in the same transition.

The energy gap between these two orbitals is known as the HOMO-LUMO gap. It refers to the energy difference between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO). This gap plays a significant role in determining the electronic, optical, and chemical properties of molecules. A larger HOMO-LUMO gap generally indicates greater stability and lower reactivity of a compound. Conversely, a smaller gap suggests higher reactivity and lower stability [18]. According to the data in the table, the ligand has a value of 3.6552, indicating that it is highly reactive and less stable. This value is demonstrated to be close to each other in the complexes, and their values are lower than those of the ligand, demonstrating that these complexes are more stable than the ligand itself.

 Figure 2 displays a graphical depiction of these values for each ligand and complex. The colour red-brown indicates high electron density in the molecular orbital, whereas green indicates low electron density.

Table 2.

The HOMO and LUMO energies of ligands and coordination compounds and the difference between them, as well as the hardness of the substances, are given

Figure 2. Graphical representation of the calculated HOMO and LUMO orbitals of 1-methylimidazole-2-thiol and its complexes

Conclusion

This work investigated the quantum chemical features of copper, zinc, and nickel complexes with 1-methylimidazole-2-thiol, with the objective of enhancing the comprehension of their coordination behaviour and electronic attributes. The findings demonstrated that complex formation markedly affects the dipole moments, molecule stability, and electronic structures of these compounds. The decrease in total energy from the free ligand to the complex molecules indicates increased stability, especially for zinc and nickel complexes, which displayed the lowest energy values.

The examination of HOMO-LUMO gaps yielded insights into the reactivity and stability of these coordination molecules. The complexes exhibited narrower HOMO-LUMO gaps compared to the free ligand, indicating enhanced stability and decreased reactivity. This discovery corresponds with the identified charge distribution and electron density, wherein nitrogen and sulphur atoms were instrumental in metal coordination.

The study substantiates the efficacy of 1-methylimidazole-2-thiol as a ligand in the formation of stable metal complexes. These discoveries can guide future research in the design of metal-organic frameworks and coordination polymers with targeted features, as well as the creation of innovative materials for catalysis, gas storage, and biological applications.

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