Characterization of Schiff Base Ligand and Its Metal Complexes

Abstract

Schiff bases, derivatives born from the condensation of primary amines with carbonyl compounds, have long captured the attention of chemists due to their intriguing structural, biological, and coordination properties. The versatility of these compounds is evident in their wide array of applications ranging from medicinal chemistry to coordination-driven processes and material science. This chapter provides an exhaustive exploration into the synthesis, characterization, and applications of Schiff base ligands and their metal complexes. Emphasis is laid on the myriad ways Schiff bases can be tailored to design metal complexes with specific properties, opening doors to targeted applications. Through a blend of fundamental principles, modern characterization techniques, and real-world applications, readers will gain a holistic understanding of Schiff base chemistry, its interplay with metals, and its bearing on various scientific and industrial domains. This comprehensive guide serves as a testament to the adaptability and significance of Schiff bases in contemporary chemical research and development.

1. Introduction

Schiff bases, named after Hugo Schiff who first reported them in the nineteenth century, are a class of organic compounds characterized by a functional group containing a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group but not hydrogen. Their general structure is RHC=N-R′, where R and R′ can be a variety of substituents. These compounds are primarily synthesized by the condensation of primary amines with carbonyl compounds, typically aldehydes or ketones. The importance of Schiff bases transcends their straightforward synthesis. They have been a subject of scientific interest due to their diverse range of biological activities, including anticancer, antiviral, antibacterial, antifungal, anti-inflammatory properties, and nematicidal activities [1]. This has led to their study in the realm of medicinal chemistry and their application in drug design.

In the field of coordination chemistry, Schiff bases have carved a niche for themselves as versatile ligands. They can chelate to metal ions through the nitrogen atom of the azomethine group, forming stable ring structures. The resulting metal complexes often exhibit remarkable properties, which make them suitable for a wide array of applications. They can act as catalysts, sensors, and even as materials with magnetic or photophysical properties.

The study of Schiff base metal complexes is not just about understanding their structure, but also about tapping into their potential applications for clarity. Whether it is designing a new catalyst for a challenging chemical reaction or developing a metal-based drug with fewer side effects, Schiff base ligands, and their metal complexes offer a rich tapestry of possibilities.

Moreover, the ease of structural modification in Schiff bases, due to their synthetic accessibility, allows for tuning of the electronic and steric properties of the ligand. This paves the way for designing metal complexes with tailor-made properties, catering to specific applications.

In this chapter, we will journey through the realm of Schiff base ligands from their synthesis to their complexation with metals, diving deep into their characterization, and understanding the nuances that make them special in the world of coordination chemistry.

2. Synthesis of Schiff bases

Schiff bases are condensation products of primary amines and carbonyl compounds, and they were discovered by a German chemist, Nobel Prize winner, Hugo Schiff [2]. Structurally, Schiff base (also known as imine or azomethine) is an analog of a ketone or aldehyde in which the carbonyl group (C=O) has been replaced by an imine or azomethine group (Figure 1) [3]. A Schiff base is a type of chemical compounds containing a carbon-nitrogen double bond as functional group, where the nitrogen atom is connected to aryl group or alkyl group (R) but not hydrogen. The Schiff base is synonymous with an azomethine. These compounds were named after Hugo Schiff on honor and have the following general structure:

Figure 1.

Schiff base.

Where R stands for a phenyl or alkyl group which makes the Schiff base a stable imine. This kind of ligand is able to coordinate metal ions through the imine nitrogen and another group, usually linked to the aldehyde. The chemists still prepare Schiff bases and nowadays active and well-designed Schiff base ligands are considered “privileged ligands” [4]. The bridged Schiff bases have the following structure, which contains many functional groups able to change according to the purpose required (Figure 2).

Figure 2.

Bridged Schiff base.

where R′ = H or alkyl group, R″ = phenyl or substituted phenyl, R″′ = alkyl or aryl group.

In fact, Schiff bases are able to stabilize many different metals in various oxidation states, controlling the performance of metals in a large variety of useful catalytic transformations. Most commonly Schiff bases have NO or ONO donor groups, but the oxygen atoms can be replaced by sulfur or nitrogen atoms.

It is usually formed by condensation of an aldehyde or ketone with a primary amine according to the following Figure 3.

Figure 3.

Formation of Schiff base by condensation reaction.

Where R may be an alkyl or an aryl group. Schiff bases that contain aryl substituent are substantially more stable and more readily synthesized, while those, which contain alkyl substituent are relatively unstable. Schiff bases of aliphatic aldehydes are relatively unstable and readily polymerizable [5]. While those of aromatic aldehydes having effective conjugation are more stable [2].

Schiff bases, commonly represented by the general formula RHC=N-R′, arise from the condensation of a primary amine with a carbonyl compound, typically an aldehyde or ketone. The synthetic process is relatively straightforward and often proceeds under mild conditions, making it a staple in organic synthesis laboratories.

2.1 General Synthetic Procedure

Materials:

• A primary amine: RNH₂

• An aldehyde or ketone: R′CHO or R′CO

Procedure:

1. To a solution of the primary amine in a suitable solvent (e.g., ethanol), add the aldehyde or ketone.

2. The mixture is then stirred at room temperature or by heating.

3. After a few hours (depending on the specific reactants), the reaction typically reaches completion. This can be monitored using thin-layer chromatography (TLC) or other suitable analytical techniques.

4. The Schiff base can then be isolated by filtration, if it is solid, or through solvent removal if it remains in the solution. It is then typically purified using recrystallization.

2.2 Mechanism of formation

The mechanism for the formation of Schiff bases is a classic example of nucleophilic addition followed by a dehydration step:

1. Nucleophilic Addition: The primary amine attacks the carbonyl carbon of the aldehyde or ketone. This results in the formation of a carbinolamine intermediate.

2. Dehydration: The carbinolamine intermediate loses a molecule of water to form the desired Schiff base, characterized by its C=N azomethine bond.

2.3 Factors affecting Schiff base formation

Several factors can influence the formation of Schiff bases:

1. Catalysis: The addition of acid or base can catalyze the reaction. Acid catalysis typically enhances the electrophilicity of the carbonyl carbon, while base catalysis can increase the nucleophilicity of the amine.

2. Solvent: Polar solvents, particularly protic ones, tend to favor the reaction. Common solvents include alcohols and water, although the choice can depend on the solubility of the starting materials [6].

3. Substituents on the Reactants: Electron-withdrawing groups on the carbonyl compound can make it more susceptible to nucleophilic attack, thereby speeding up the reaction.

2.4 Variability and versatility

The ease of synthesis and the plethora of available primary amines and carbonyl compounds mean that a vast array of Schiff bases can be synthesized, each with its unique properties. By changing the R and R′ groups, chemists can tailor-make Schiff bases for specific applications, whether it is to bind a particular metal ion or to exhibit a certain biological activity.

In summary, Schiff bases are a cornerstone in the realm of organic and coordination chemistry, serving as a bridge between the two worlds. Their synthesis is emblematic of the elegant simplicity that can sometimes underpin the most profound chemical phenomena.

3. Characterization method

3.1 Spectroscopic methods

Schiff bases and their metal complexes often exhibit unique spectroscopic features that allow researchers to gain insights into their structure, bonding, and other characteristics. The following are some of the most pertinent spectroscopic techniques used in the study of Schiff base ligands and their metal complexes:

3.1.1 UV-Visible spectroscopy

• Principle: Studies the absorption of ultraviolet (UV) and visible light, which results in electronic transitions in molecules.

• Applications: Used to determine conjugation in organic molecules, assess the electronic environment in metal complexes, and measure concentrations via Beer’s Law.

• Relevance: Schiff bases and their metal complexes typically show distinct UV-visible absorption bands. The electronic transitions often include π→π* transitions (due to the conjugated system) and n→π* transitions (from nonbonding electron pairs). When Schiff bases coordinate to metal ions, new bands associated with metal-to-ligand charge transfer (MLCT) or d-d transitions (in transition metals) may arise. The position, intensity, and nature of these bands can provide information about the electronic environment around the metal center and the nature of the metal-ligand bond.

3.1.2 Infrared (IR) spectroscopy

• Principle: Infrared radiation absorption results in vibrational transitions in molecules. Each functional group has a characteristic vibration frequency.

• Applications: Widely used for identification of functional groups in organic compounds and studying molecular vibrations.

• Relevance: The formation of Schiff bases involves the creation of a C=N bond, which exhibits a characteristic stretching frequency in the IR spectrum. This C=N stretching frequency is a crucial marker for the successful formation of a Schiff base. Upon metal coordination, this frequency might shift, indicating the binding mode of the ligand to the metal ion. Other characteristic peaks related to the ligand can also be observed and monitored for changes upon metal coordination.

3.1.3 Nuclear magnetic resonance (NMR) spectroscopy

• Principle: Explores the magnetic properties of certain atomic nuclei (e.g., 1H, 13C) under an external magnetic field.

• Applications: Provides detailed information about molecular structure, dynamics, reaction state, and environment of molecules.

• Relevance: 1H NMR spectroscopy can give detailed insights into the protons’ environment in the Schiff base ligand. Changes in chemical shifts upon metal complexation can hint at the areas of the ligand involved in binding, especially if certain protons come close to paramagnetic metal centers. 13C NMR can provide similar insights for carbons, especially in elucidating the coordination environment.

3.1.4 Electron spin resonance (ESR) spectroscopy (or electron paramagnetic resonance, EPR)

• Principle: ESR (or EPR, Electron Paramagnetic Resonance) measures the interactions of unpaired electron spins in a magnetic field.

  Relevance to Schiff bases: For Schiff base metal complexes containing transition metals with unpaired electrons, ESR can be an invaluable tool. It provides information about the electronic environment of the metal center, aiding in the identification of the oxidation state and the nature of the metal-ligand bond.

• Relevance: ESR is especially relevant for metal complexes of Schiff bases where the metal is in a paramagnetic state (having unpaired electrons). ESR can give details about the number and nature of unpaired electrons, helping in the identification of the oxidation state, the nature of the metal-ligand bond, and the electronic environment of the metal center.

3.1.5 X-ray photoelectron spectroscopy (XPS)

• Principle: XPS investigates the photoelectric effect where core electrons are emitted upon irradiation with X-rays. The measured kinetic energy of these electrons, combined with the known energy of the X-rays, reveals the binding energy of the electrons. Specific elements have characteristic binding energies, and the slight shifts in these energies can hint at their chemical state or environment.

• Applications;

  1. Identification of elements: XPS can confirm the presence of elements in Schiff base ligands, such as nitrogen from the imine group or other heteroatoms.

  2. Coordination chemistry: For Schiff base metal complexes, XPS can determine the oxidation state of the coordinated metal and provide insights into the nature of the metal-ligand bond.

  3. Chemical environment: Variations in the binding energy can suggest different environments or states of the same element within the Schiff base. For example, nitrogen in a free Schiff base ligand might have a slightly different binding energy than nitrogen coordinated to a metal. Therefore, XPS may also indicate specific C=N’s C or N as both 1 and 3.

  4. Purity and surface analysis: XPS can detect surface contaminants or degradation products, ensuring the purity of synthesized Schiff base compounds or their complexes.

  5. Interactions and binding modes: For Schiff bases used in surface modifications or as linkers in materials, XPS can help in understanding their binding mode and interaction with surfaces or other entities.

  6. In essence, for Schiff bases and their associated metal complexes, XPS serves as a pivotal tool, offering detailed information on elemental composition, oxidation states, and chemical environments, crucial for their thorough characterization and application in various domains.

3.1.6 Mass spectrometry (MS)

• Principle: Measures the mass-to-charge ratio of ions. A sample is ionized, and the resulting ions are separated based on their mass-to-charge ratio.

• Applications: Used to determine molecular weight, elemental composition, and structural information of molecules.

• Relevance: MS can be used to confirm the molecular weight and structure of a synthesized Schiff base or its metal complex. Fragmentation patterns can also help elucidate the structure, especially in cases of complex ligands or metal assemblies.

In summary, the study of Schiff base ligands and their metal complexes often necessitates a multipronged spectroscopic approach. Each technique can provide specific insights, and together they offer a comprehensive picture of the system under study. When characterizing a new Schiff base or its metal complex, researchers typically employ a combination of the above techniques to validate their findings and gain a holistic understanding of the molecule’s nature.

3.2 X-ray diffraction (XRD) and its principle

The technique of X-ray diffraction (XRD) is employed in the study of crystalline materials. X-rays are diffracted or scattered by the crystal lattice when they strike a crystalline sample. This diffraction pattern can be examined to ascertain the substances molecular structure because it provides details about the locations of the atoms within the crystal [7].

3.2.1 Application to Schiff bases and their metal complexes

Determination of Molecular Geometry: Using XRD, one can precisely determine the geometry surrounding a central metal atom or ion, be it tetrahedral, square-planar, octahedral, etc. This is particularly important for Schiff base metal complexes as the geometry can affect their magnetic properties, reactivity, and other properties.

Coordination Mode: Schiff bases have the ability to bind to metal centers in a variety of ways by acting as multidentate ligands. If the Schiff base binds in a chelating way, forming rings with the metal center, XRD can demonstrate whether it functions as a bidentate, tridentate, or even polydentate ligand.

Bond Lengths and Angles: Bond lengths and angles can be measured using XRD. This can reveal details about the strain present in the complex, as well as the type of metal-ligand bond [8]—whether it is more covalent or ionic—in Schiff base metal complexes. Therefore, bond length of C=N in Schiff bases is shorter than that of C-N generally.

Presence of other ligands or molecules: XRD can detect the presence of additional ligands in the metal complex, as well as any solvent molecules or counterions that may be connected to it. These may have an impact on the complex’s stability and characteristics.

Intramolecular and Intermolecular Interactions: XRD can show interactions between and within complexes in the crystal lattice, including hydrogen bonding and π-π stacking. The Schiff base metal complex’s solubility, stability, and other characteristics may be influenced by these interactions.

Verification and characterization: X-ray diffraction (XRD) offers unambiguous evidence of the atomic structure and arrangement, whereas other techniques, such as infrared spectroscopy or nuclear magnetic resonance (NMR), may indicate the formation of a Schiff base and its complexation with a metal.

3.3 Magnetic susceptibility and its basics

A substance’s ability to become magnetized in the presence of an external magnetic field is measured by its magnetic susceptibility (χ). This characteristic provides insight into the number of unpaired electrons present in metal complexes [9].

• Diamagnetic: All electrons are paired. The compound is not attracted to a magnetic field. It has a magnetic susceptibility close to zero.

• Paramagnetic: It contains unpaired electrons. The compound is attracted to a magnetic field. The extent of attraction (or the value of χ) is directly related to the number of unpaired electrons.

Magnetic susceptibility can be used in the elucidation of Schiff base ligands and metal complexes in other to study viz:

1. Metal Oxidation State and Electron Configuration: We can determine the number of unpaired electrons in a metal and possibly its oxidation state and d-electron configuration by measuring its magnetic susceptibility. A Cu(II) ion in a similar environment (d⁹ configuration) is paramagnetic with one unpaired electron, whereas a Ni(II) ion in a square planar environment (d⁸ configuration) is usually diamagnetic.

2. Geometry of the Complex: The number of unpaired electrons and the splitting of d-orbitals can be affected by the geometry surrounding the metal. For example, the d-orbitals are split differently in octahedral and tetrahedral environments. If the geometry is known, magnetic susceptibility can reveal details about the electron configuration.

3. Ligand Field Strength: Because they contain nitrogen donor atoms, Schiff bases can function as potent field ligands. Depending on the strength of the ligand field, a high-spin to low-spin transition may take place, which would be represented in a shift in magnetic susceptibility.

4. Determination of Antiferromagnetic Coupling: There may be magnetic interactions between neighboring metal centers in some dinuclear or polynuclear metal complexes containing Schiff base ligands. Antiferromagnetic coupling may be indicated if the magnetic moments, which are commonly expressed in terms of Bohr magnetons, or μB, are smaller than one would expect given the number of unpaired electrons.

5. Verifying Synthesis: Magnetic susceptibility measurements can help confirm if the desired compound has been synthesized if a targeted metal complex of a Schiff base is expected to have a specific magnetic behavior (based on the metal’s oxidation state, coordination number, and geometry).

3.4 Thermogravimetric analysis (TGA) and differential thermal analysis (DTA)

3.4.1 Thermogravimetric analysis (TGA)

TGA calculates how a sample’s weight changes in response to temperature or time spent in a controlled environment. Usually, the sample is heated steadily, and any variations in weight are noted.

Decomposition temperature: By identifying the temperature at which Schiff base ligands and their metal complexes begin to break down, TGA can shed light on the thermal stability of these ligands.

Identification of coordinated and crystallized solvents: A lot of metal complexes contain solvent molecules that are either crystallized in the lattice or within the coordination sphere. Based on the temperature and weight loss related to their removal, TGA can detect the presence of these solvents.

Stoichiometry: A metal complex’s stoichiometry can be determined using TGA. For example, a complex may indicate the presence of two coordinated or crystallized water molecules if its weight decreases by the amount equivalent to two water molecules.

3.4.2 Differential thermal analysis (DTA)

DTA uses a controlled temperature to measure the difference in temperature between a sample and a reference material, which typically does not go through any phase transitions under the selected conditions.

Phase transitions: Melting points and changes in phase from one crystal to another can be found using DTA. For example, the melting of a Schiff base metal complex might be represented by an endothermic peak on a DTA curve.

Decomposition temperature: Similar to TGA, DTA can be used to ascertain when decomposition begins. On a DTA curve, breakdown events usually show up as exothermic peaks.

Coordination changes: Because these changes may involve energy changes (either endothermic or exothermic), DTA can occasionally be used to detect changes in a metal’s coordination sphere, such as ligand dissociation.

3.4.3 Combined TGA-DTA

Many modern instruments offer simultaneous TGA-DTA measurements. This allows researchers to correlate weight loss events from TGA with endothermic or exothermic events from DTA, providing a more comprehensive understanding of the thermal behavior of the sample.

Simultaneous TGA-DTA can provide a complete picture of the thermal events from desolvation and ligand loss to decomposition. For instance, a weight loss in the TGA curve corresponding to an endothermic peak in the DTA might indicate the loss of a coordinated solvent molecule from a metal complex.

Both TGA and DTA are crucial tools in the characterization of Schiff base ligands and their metal complexes. They provide detailed information about the thermal stability, presence of solvents, phase transitions, and other thermally induced processes in these compounds.

3.5 Electrochemical techniques

Electrochemical techniques are powerful tools to characterize Schiff bases and their metal complexes. They provide valuable information about the redox properties and reactivity of these compounds. Some basic electrochemical techniques include the followings:

3.5.1 Cyclic voltammetry and its role in studying redox features

Cyclic voltammetry (CV) is a powerful electrochemical technique used to study the redox (reduction-oxidation) properties of chemical compounds and materials. It provides valuable information about the electrochemical behavior of species in a solution or on the surface of an electrode. Here is an overview of cyclic voltammetry and its role in studying redox features:

• Basic Principle: Cyclic voltammetry involves applying a potential (voltage) to an electrode and measuring the resulting current as the potential is swept back and forth in a repetitive cycle.

• The potential is scanned linearly, starting from a certain initial value and increasing or decreasing at a defined scan rate.

• Electrochemical Cell: In a typical setup, a working electrode (often a conductive material, such as a metal or modified carbon electrode), a reference electrode (e.g., Ag/AgCl), and a counter electrode (usually a platinum or carbon electrode) are immersed in an electrolyte solution.

• Redox Reactions: Redox reactions occur at the working electrode. When the potential is swept, these reactions lead to the transfer of electrons between the electrode and the species in the solution.

• Oxidation involves the loss of electrons (anodic process), while reduction involves the gain of electrons (cathodic process).

• Voltammogram: The primary output of cyclic voltammetry is a voltammogram, which is a plot of current (I) versus applied potential (E). It typically displays distinct peaks corresponding to different redox processes.

• Information Obtained: Cyclic voltammetry provides information about the redox potentials (E°) of the species involved in the electrochemical reactions. The position of the peaks in the voltammogram indicates the potentials at which redox processes occur.

• The current response is related to the concentration of electroactive species and the kinetics of the redox reactions.

• Scan Direction: Cyclic voltammetry can be performed in two directions: forward (from initial to final potential) and reverse (from final to initial potential). This allows the study of both oxidation and reduction processes.

• Scan Rate: The scan rate is an important parameter that can be adjusted. Faster scan rates can reveal kinetic information about the redox reactions, while slower rates can provide more detailed information about the thermodynamics.

• Applications: Cyclic voltammetry is widely used in various fields, including chemistry, materials science, electrochemistry, and biochemistry. It is used to study the redox behavior of molecules, determine the stability and reactivity of materials, investigate corrosion processes, and examine the electrochemical behavior of biomolecules, among other applications.

3.5.2 The electrochemical window of Schiff base metal complexes

The electrochemical window of Schiff base metal complexes, such as that of Schiff base compounds, refers to the range of electrochemical potentials (voltages) within which the complex can undergo redox reactions without decomposition or undesired side reactions. These metal complexes consist of a Schiff base ligand coordinated to a metal ion. The electrochemical behavior and window of Schiff base metal complexes can be more complex due to the presence of the metal center, which can participate in redox processes.

The determination of the electrochemical window of Schiff base metal complexes is essential when studying their electrochemical properties, such as their redox behavior and stability. Here are some factors that influence the electrochemical window of Schiff base metal complexes:

• Nature of the Metal Center: The choice of metal ions in the complex significantly affects the electrochemical behavior. Different metals have different redox potentials, and the presence of the metal center can introduce additional redox processes. The ligand’s coordinating ability to the metal ion also plays a role.

• Schiff Base Ligand: The specific structure of the Schiff base ligand and the nature of the substituents on it can impact the electrochemical window. The ligand can donate or accept electrons during redox reactions, affecting the overall electrochemical behavior of the complex.

• Solvent: The choice of solvent can influence the electrochemical window. Some solvents can facilitate or hinder redox reactions and affect the stability of the complex.

• pH: The pH of the solution can influence the electrochemical behavior, especially for metal complexes with pH-sensitive ligands. Changes in pH can affect the protonation state and reactivity of the complex.

• Electrode Material: The type of electrode material used in the electrochemical cell can affect the observed redox potentials. The choice of electrode material can impact the kinetics of the redox reactions.

Determining the electrochemical window of Schiff base metal complexes is crucial for designing and optimizing electrochemical processes involving these compounds, such as in catalysis, sensing, or energy storage applications. Researchers typically use cyclic voltammetry and other electrochemical techniques to investigate the redox properties and electrochemical behavior of Schiff base metal complexes, allowing them to understand the complex interplay of factors that influence the electrochemical window and tailor conditions for specific applications.

Other electrochemical techniques include;

3.5.3 Chronoamperometry

This technique involves applying a constant potential to the sample and monitoring the resulting current over time. It is valuable for studying the electrochemical behavior of Schiff bases and their metal complexes in a time-dependent manner. This can reveal information about reaction mechanisms, stability, and electrode processes.

3.5.4 Electrochemical impedance spectroscopy (EIS)

1. EIS is used to investigate the electrical properties of Schiff base compounds and their metal complexes at different frequencies. This technique provides insights into factors such as charge transfer resistance, double-layer capacitance, and reaction kinetics.

4. Formation and applications of Schiff base

4.1 Schiff bases as catalytic agents

Schiff base metal complexes are widely used as catalysts [10] in various organic reactions, including oxidation, reduction, and asymmetric synthesis.

4.2 Biological activity

Schiff bases and their metal complexes have shown diverse biological activities, including antimicrobial, antiviral, and anticancer properties. Some Schiff bases are also used as ligands in coordination chemistry with applications in medicinal chemistry.

4.3 Colorimetric sensors

Schiff bases are known for their sensitivity to metal ions. They are used in the development of colorimetric sensors for detecting specific metal ions in solution [3].

4.4 Dyes and pigments

Some Schiff bases exhibit vibrant colors and are used as dyes or pigments in the textile and printing industries.

4.5 Polymer chemistry

Schiff bases can be incorporated into polymers, contributing to the development of materials with specific properties, such as enhanced mechanical strength or conductivity.

4.6 Photonic and electronic devices

Schiff bases and their metal complexes have been investigated for their potential use in photonic and electronic devices due to their optical and electronic properties.

4.7 Antioxidants

Some Schiff bases possess antioxidant properties and are studied for their potential role in preventing oxidative damage.

4.8 Anthelmintic

Anthelmintics serve as therapeutic agents designed to eliminate parasitic worms from infected hosts. The initial efficacious anthelminthic agents, discovered in the twentieth century, incorporated toxic metals like arsenic (atoxyl) or antimony (tartar emetic). These compounds were employed in the treatment of trypanosome and schistosome infestations.

Contemporary anthelminthic agents typically function by inducing paralysis in the parasite, often by impeding muscular contraction, causing damage to the worm to enable elimination by the host’s immune system, or modifying parasite metabolism, such as affecting microtubule function. Due to variations in the metabolic requirements among different worm species, drugs highly effective against one type of worm may prove ineffective against others [11].

Worm infestation or helminthiasis stands as a significant health concern, particularly in third-world countries, representing a major global public health problem. Infections caused by parasitic worms and protozoa contribute significantly to global mortality and morbidity. Despite the commercial availability of anthelmintic drugs from diverse chemical groups (e.g., imidazole, benzimidazole, and lactone) for treating worm infections, the prevalence and severity of these diseases are on the rise, partly due to the emergence of multidrug resistance [12].

4.9 Nematicidal activity

The nematicidal activity of Schiff base ligands and their metal complexes has garnered interest in agricultural and environmental research. Schiff bases, known for their versatile coordination properties, can form stable complexes with various metal ions. These complexes have demonstrated potential as nematicidal agents, showing efficacy against nematodes, which are parasitic roundworms that can be detrimental to crops.

The mode of action of these Schiff base metal complexes against nematodes involves interactions with the nematode’s biological systems. The metal ions coordinated by Schiff base ligands can disrupt key physiological processes in nematodes, leading to paralysis or death. Additionally, the specific chemical structure of Schiff bases may contribute to their nematicidal activity by targeting essential biological pathways in nematodes.

Research in this area is ongoing, exploring different Schiff base ligands and their metal complexes for their efficacy, selectivity, and environmental impact. The development of nematicidal agents based on Schiff bases holds promise for sustainable and effective strategies in controlling nematode infestations in agriculture.

5. Case studies

The present case study delves into the metal complexes of (E) – N¹- (2 hydroxybenzylidene) nicotinohydrazide Schiff base; synthesis, characterization, and nematicidal activity [1] of Cu(II) and Fe(II) Schiff base ligand complexes; synthesis and characterization [7] marked by its potential significance in the agrochemical sector as it will provide additional chemical pesticides for the prevention and effective control of nematodes. This study explores the intricate chemistry involved in the formation of Schiff base metal complexes and its subsequent applications in chemical pesticides. Schiff base metal complexes have garnered considerable attention due to their diverse applications ranging from catalysis [13, 14] to biological activities. Against this backdrop, our investigation aims to shed light on the unique properties and potential utilities of the synthesized Schiff base metal complex, contributing valuable insights to the ever-expanding field of coordination chemistry. Through a systematic examination of the synthesis route, thorough characterization techniques, and practical applications, this case study seeks to elucidate the compound’s structural nuances and explore avenues for its application in cutting-edge scientific endeavors [14].

The synthesis of the Schiff base metal complex involved a novel approach that not only streamlined the process but also resulted in a higher yield compared to traditional methods. This innovative synthesis strategy addresses longstanding challenges in the field and sets the groundwork for more efficient synthetic routes in the future.

Advanced spectroscopic techniques, including TGA and crystallography, provided detailed structural insights into the metal complex. Of particular significance is the C=N bond, which plays a pivotal role in its influence on reactivity, stability, etc. These revelations mark a substantial contribution to the understanding of Schiff base metal complex structures.

Metal complexes containing Mo(IV) and Mn(II) paired with ligands hydrazine carboxamide and hydrazine carbothiamide exhibit notable antibacterial effects against S. aureus and Xanthomonas campestris. Additionally, antibacterial activities are observed in tridentate Schiff bases and their associated metal complexes against E. coli, S. aureus, B. subtilis, and B. pumilis, as documented in reference [15]. Azan, et al. [16] reported on the antimicrobial potential of Schiff bases derived from naphtha-[1, 2-d]-thiazol-2-amine and their corresponding metal complexes in reference [17].

Schiff bases demonstrate anti-inflammatory properties, act as inhibitors for allergic responses, exhibit activity in reducing radicals, and have analgesic effects [10]. Thiazole-based chitosan and carboxymethyl-chitosan display antioxidant capabilities, including scavenging of superoxide and hydroxyl radicals [18]. Metal complexes of furan semicarbazone show noteworthy anthelmintic and analgesic activities [19].

Transition metal complexes involving Cu, Zn, and Cd hinder the cellular proliferation of MDA-MB-231 breast cancer cells. Among these complexes, Cd(C₁₈H₁₆N₃O₂)₂.2CH₃OH exhibits the most potent antiproliferative activity [20]. In a separate study, Shabani et al. [21] have documented the anti-tumor activity of Iron(III) Schiff base complexes.

Schiff bases, renowned for their diverse biological functions, have undergone extensive investigation regarding their coordination tendencies with metals, leading to the development of highly efficient catalysts [22, 23]. These complexes, especially those involving transition metals, exhibit remarkable catalytic efficacy across various reactions, even in the presence of moisture and at elevated temperatures [24]. The notable thermal and moisture stability of these complexes enhances their utility in catalytic applications, and ongoing research is exploring their potential in the development of novel chemotherapeutic agents [22, 23].

A catalytically active Mn(II) Schiff base complex, featuring a triazole structure, has been identified as an efficient catalyst for the Henry reaction, yielding b-hydroxyl nitroalkanes in high yield [14]. Additionally, Co(II), Fe(III), and Ru(III) complexes derived from Schiff bases, synthesized from hydroxyl benzaldehyde, have demonstrated effective catalysis in the oxidation of cyclohexane to cyclohexanol and cyclohexanone in the presence of hydrogen peroxide [25].

6. Conclusion

In conclusion, the thorough examination of Schiff base ligands and their metal complexes has yielded significant understandings into the fascinating field of coordination chemistry. By thoroughly examining their spectroscopic, chemical, and physical characteristics, we have been able to comprehend these compounds’ coordination modes and structural characteristics on a deeper level.

The utilization of various spectroscopic techniques, including X-ray crystallography, NMR, IR, and UV-visible, has been beneficial in clarifying the molecular structures of the ligands and their metal complexes. These analyses have clarified the coordination environment surrounding the metal centers in addition to confirming the successful formation of the intended compounds.

Additionally, the investigation has brought attention to the effects of various metal ions on the characteristics of Schiff base ligands, underscoring the significance of metal-ligand interactions in controlling the general behavior of these complexes. We now have a better understanding of the variables affecting the stability and reactivity of the complexes thanks to the investigation of the electronic and steric effects of the metal ions.

The importance of this work is highlighted by the potential uses of Schiff base ligands and their metal complexes in a variety of industries such as material science, medicinal chemistry, and catalysis. The information gathered from this research not only broadens our understanding of these compounds but also opens up new avenues for further study and application within the scientific community.

To sum up, the identification of Schiff base ligands and their metal complexes is an important step toward improving our understanding of coordination chemistry. The diverse characteristics of these substances present an abundant opportunity for additional investigation and utilization, rendering them auspicious contenders for an extensive array of scientific and technological pursuits.