Schiff Bases: Contemporary Synthesis, Properties, and Applications

Abstract

Schiff bases, a versatile class of organic compounds with a (>C〓N▬) functional group, have gained significant attention in contemporary chemistry due to their remarkable synthesis methods, diverse properties, and wide-ranging applications. This book chapter focused on Schiff bases, emphasizing their versatility in contemporary chemistry. It covers synthesis methods, structural aspects, and applications across multiple domains. The historical development of Schiff bases is traced, emphasizing their pivotal role in modern chemistry as key intermediates in various processes. The chapter delves into synthesis methods, including traditional, sustainable, and green chemistry approaches, and discusses structural aspects, spectroscopic characterization, thermodynamic properties, and kinetics. Schiff bases are categorized into various types, highlighting their diversity. Applications are explored in catalysis, asymmetric synthesis, coordination chemistry, medicinal chemistry, materials science, and environmental contexts. The chapter concludes with recent advances, emerging trends, and future directions, positioning it as a valuable resource for researchers, students, and practitioners interested in Schiff bases. The proposed chapter aims to explore the latest developments in Schiff bases, providing an in-depth review of their synthesis, properties, and applications across various scientific fields.

1. Introduction

1.1 Definition and historical background of Schiff bases

1.1.1 Definition

The term “Schiff bases” refers to a class of organic compounds resulting from the condensation reaction between a primary amine and a carbonyl compound, typically an aldehyde or a ketone. Named after the German chemist Hugo Schiff, who first reported their synthesis in 1864 [1], these compounds have since become integral to the fabric of organic chemistry.

Schiff bases have significantly influenced the advancement of coordination chemistry, playing a pivotal role in the development of inorganic and bioinorganic chemistry, as well as optical materials. Their widespread use extends across various branches of chemistry, including inorganic, organic, and analytical fields, making them a significant component of commonly employed organic molecules.

Schiff bases are characterized by a central carbon-nitrogen double bond, also known as an imine or azomethine group, formed during the reaction. The general structure can be represented as R₁R₂C〓NR₃, where R₁, R₂, and R₃ denote organic substituents.

In coordination chemistry, Schiff bases are common ligands that are classified based on the number of donor atoms, including uni-, di-, tri-, and tetra-dentate ligands, and are derived from aromatic aldehydes and alkyl diamines (Figures 1 and 2) [2].

Figure 1.

General structure of Schiff bases and few examples of Schiff bases.

Figure 2.

ORTEP diagram (thermal ellipsoids at 40% probability level) of S-benzyl-β-N-(4-hydroxy-3-nitrophenyl)methylenedithiocarbazate with atom numbering scheme.

1.1.2 Historical background

The historical trajectory of Schiff bases is intricately linked with the evolution of organic chemistry in the nineteenth century. Hugo Schiff’s pioneering work marked the initial strides into the synthesis and exploration of these compounds [3]. Schiff himself discovered the condensation reaction while investigating the reactions of amines with aldehydes and ketones. In the decades following Schiff’s groundbreaking discovery, chemists delved deeper into understanding the underlying principles governing Schiff bases. Noteworthy contributions came from eminent figures such as Emil Fischer, who not only expanded the scope of Schiff base reactions but also provided insights into their stereochemistry [4]. The historical significance of Schiff bases extends beyond their fundamental role in chemical reactions; they played a crucial role in advancing the broader field of organic synthesis. The synthesis of these compounds has undergone refinement and diversification over time, resulting in a broad array of derivatives with varying properties and applications.

The Schiff bases derived from the dithiocarbazates, thiosemicarbazides, and dithiocarbamates form an interesting series of compounds whose properties can be greatly modified by introducing organic substituents into the molecule thereby causing variation in the ultimate donor properties and inducing different stereochemistry in the resultant metal complexes. S-Benzyl dithiocarbazate (SBDTC) Schiff bases comprise an essential part of nitrogen-sulfur donor ligands [5, 6].

Since the synthesis of dithiocarbamic acids by Curtius more than a century ago, a significant number of S-alkyl/aryl-esters, their related complexes, and the Schiff bases of these compounds have been developed for potential use in antibacterial, anticancer, antifungal, antitumor, antiamoebic therapy, anti-inhibitory cell migration, analgesic and anti-inflammatory activities, or merely for their diverse coordination geometries [7, 8, 9, 10].

Synthetic chemists extensively utilize Schiff bases and related complexes for diverse processes, ranging from the oxidation of alkenes to the catalytic transformation of hydrocarbons. These processes yield valuable oxygenated derivatives such as alcohols, aldehydes, and epoxides.

As we navigate the historical landscape of Schiff bases, it is essential to recognize how their discovery marked a pivotal moment, setting the stage for continuous exploration and contributing to the rich tapestry of organic chemistry. Today, the historical journey of Schiff bases serves as the foundation for the contemporary understanding and applications explored in subsequent sections of this chapter.

1.2 Significance of Schiff bases in modern chemistry

In the vast and dynamic realm of modern chemistry, Schiff bases have emerged as compounds of profound significance, playing pivotal roles in various facets of scientific exploration. Their versatility, reactivity, and distinctive properties have positioned them as indispensable building blocks across diverse fields.

Schiff bases exhibit a diverse range of biological activities, including antibacterial, antimycobacterial, antifungal, anti-inflammatory, antioxidant, anticancer, etc. [11, 12, 13, 14]. Initially discovered by Hugo Schiff in the nineteenth century, these compounds have evolved to become integral components of contemporary research and synthesis. Schiff bases find extensive use in coordination chemistry, catalysis, and medicinal chemistry. Their ability to form stable complexes with metal ions has led to their widespread adoption in the design of novel catalysts and coordination compounds [15]. Additionally, Schiff bases serve as crucial intermediates in the synthesis of various pharmaceuticals, agrochemicals, and dyes, contributing significantly to the development of new therapeutic agents [16]. In the realm of materials science, Schiff bases play a key role in the design and fabrication of functional materials. Their unique electronic and optical properties make them valuable in the development of sensors, molecular switches, and advanced materials for electronic devices [17]. Furthermore, the use of Schiff bases in supramolecular chemistry has opened avenues for the creation of self-assembled structures and molecular machines, showcasing their importance in the nanotechnology field [18]. The navigation of the complexities of modern chemistry and the enduring relevance of Schiff bases is a testament to their adaptability and multifaceted contributions to scientific advancements.

1.3 Overview of the chapter’s content

This chapter is a comprehensive exploration of Schiff bases, spanning their historical roots to their contemporary significance in modern chemistry. The content is structured to provide a thorough understanding of the synthesis, properties, and applications of Schiff bases, highlighting their diverse roles across various scientific domains.

1. Introduction to Schiff bases: the journey begins with a detailed introduction, defining Schiff bases and tracing their historical background. We revisit the foundational work of Hugo Schiff and the subsequent developments that have shaped our understanding of these compounds.

2. Synthesis of Schiff bases: the chapter then delves into the contemporary methods of synthesizing Schiff bases. From traditional condensation reactions to innovative strategies, this section explores the diverse synthetic routes that enable the creation of Schiff bases with tailored structures and functionalities.

3. Properties of Schiff bases: a critical examination of the unique properties exhibited by Schiff bases follows the synthesis section. Their electronic configuration, structural flexibility, and other distinctive characteristics are dissected to provide a nuanced understanding of how these properties influence their behavior in various chemical contexts.

4. Catalytic and synthetic applications: Schiff bases have found wide-ranging applications in catalysis and synthetic methodologies. This section investigates their catalytic prowess and their role in facilitating diverse transformations, showcasing how they contribute to the advancement of synthetic chemistry.

5. Medicinal chemistry and biological activities: the discussion then shifts to the medicinal chemistry aspects of Schiff bases. Their significant impact on drug design and development is explored, emphasizing their potential as therapeutic agents with antibacterial, antifungal, antiviral, and anticancer properties.

6. Materials science and coordination chemistry: Schiff bases play a central role in materials science and coordination chemistry. This section illuminates their contributions to the design of functional materials, including coordination polymers and metal-organic frameworks, with applications ranging from sensors to catalytic supports.

7. Supramolecular chemistry and analytical applications: the chapter concludes by examining the involvement of Schiff bases in supramolecular chemistry and their utility in analytical applications. Their role in the construction of supramolecular assemblies and their use as fluorescent probes and sensors for chemical detection are explored.

Throughout this journey, we aim to provide readers with a comprehensive and insightful overview of Schiff bases, showcasing their historical significance, contemporary synthesis methods, unique properties, and diverse applications. By the chapter’s end, readers will gain a profound appreciation for the multifaceted nature of Schiff bases and their enduring impact on the landscape of modern chemistry.

2. Synthesis of Schiff bases

The synthesis of Schiff bases has been the subject of extensive investigation, with diverse methodologies explored to obtain Schiff bases featuring varied structures and functionalities. Numerous studies have been conducted to undertake the synthesis of Schiff bases [19].

Historically, the condensation reaction between primary amines and aldehydes or ketones has been a cornerstone in Schiff base synthesis. Pioneered by Hugo Schiff, this classical approach involves the nucleophilic attack of the amino group on the electrophilic carbon of the carbonyl group, resulting in the characteristic carbon-nitrogen double bond. Contemporary methods in Schiff base synthesis have progressed to incorporate inventive strategies, leading to enhanced reaction rates, increased yields, and diminished environmental impact.

The synthesis of Schiff bases encompasses a diverse array of methods that have evolved over time.

2.1 Traditional methods for Schiff base formation

The synthesis of Schiff bases, a class of organic compounds with diverse applications, has a rich historical foundation rooted in traditional methods. Historically, the condensation reaction between primary amines and aldehydes or ketones has been the cornerstone of Schiff base formation. This classical approach, pioneered by Hugo Schiff himself, has provided a fundamental understanding of the reaction principles and laid the groundwork for further exploration [1, 20].

The traditional method of Schiff base formation has been extensively studied and documented. Classic examples include the reaction between aromatic amines and aldehydes. For instance, the condensation of aniline with benzaldehyde leads to the formation of the Schiff base known as Schiff’s base (Figure 3).

Figure 3.

General synthesis reaction of Schiff base.

Example:

$${\text{C}}_6{\text{H}}_5\text{CHO}+{\text{C}}_6{\text{H}}_5{\text{NH}}_2\to {\text{C}}_6{\text{H}}_5\text{CH}〓{\text{NC}}_6{\text{H}}_5+{\text{H}}_2\text{O}$$

This foundational method has been applied to a broad range of amine and carbonyl reactants, contributing to the synthesis of numerous Schiff bases with diverse structural motifs.

2.2 Modern approaches and green chemistry aspects in Schiff base synthesis

The synthesis of Schiff bases has undergone a transformative evolution with the integration of modern approaches and green chemistry principles. Embracing green chemistry, these modern approaches prioritize sustainable practices by utilizing eco-friendly solvents, such as water, and employing catalysis with recyclable metal complexes or enzymes, which reduces the environmental footprint of the synthesis process. Microwave-assisted synthesis has gained prominence as an energy-efficient method for accelerating chemical reactions, including Schiff base formation. This approach not only reduces reaction times but also enhances overall efficiency, aligning with the principles of green chemistry. Modern approaches often seek metal-free catalysis or the use of eco-compatible metal catalysts, ensuring a more sustainable and greener synthetic process [21, 22]. The reference to green chemistry aspects is exemplified by the work of Anastas and Warner in “Green Chemistry: Theory and Practice” [23]. Green synthesis methods prioritize sustainability, minimizing environmental impact and resource consumption. Several innovative strategies have emerged to enhance the eco-friendliness of Schiff base synthesis.

These modern strategies are reflected in recent literature and contribute to the ongoing advancement of Schiff base synthesis with a focus on sustainability and environmental responsibility.

2.3 Catalysts and conditions for efficient Schiff base formation

Efficient formation of Schiff bases, a pivotal process in organic synthesis, relies on judicious selection of catalysts and reaction conditions. Traditional methods often involve metal catalysts, but contemporary approaches prioritize eco-friendly alternatives to mitigate environmental impact. Metal-free catalysis has emerged as a prominent strategy for green and sustainable Schiff base synthesis. Organic or bio-inspired catalysts are preferred due to their effectiveness and reduced environmental footprint. The solvent can impact reaction rates, selectivity, and the overall sustainability of the synthetic process.

Additionally, the optimization of reaction conditions plays a crucial role. Mild conditions, such as ambient temperature and environmentally benign solvents, contribute to the efficiency and eco-friendliness of Schiff base formation.

2.4 Preparation methods of some important recent synthesized Schiff bases

The synthesis of Schiff bases encompasses a historical journey rooted in classical methods and a contemporary evolution marked by innovative strategies. However, some important Schiff bases have been recently synthesized, which are described here.

2.4.1 1,2-bis((benzylsulfanyl){2-[1-(2-hydroxyphenyl)ethylidene]hydrazin-1-ylidene}-methyl)disulfane

The title compound consists of two Schiff base moieties, namely two S-benzyl-β-N-(2-hydroxyphenylethylidene)dithiocarbazate groups, which are connected through an S▬S single bond. These two moieties are twisted with respect to each other, with a dihedral angle of 87.88 (4)° between the S2C N planes (Figure 4).

Figure 4.

The molecular structure of the 1,2-bis((benzylsulfanyl){2-[1-(2-hydroxyphenyl)ethylidene]hydrazin-1-ylidene}-methyl)disulfane.

2.4.1.1 Synthesis and crystallization

The ligand precursor, S-benzyl dithiocarbazate (SBDTC), was prepared using the literature method [20]. The title compound was prepared as follows: to the ligand precursor, SBDTC (0.99 g, 5 mmol) dissolved in ethanol (40 ml) was added 2-hydroxy acetophenone (0.68 g, 5 mmol), and the aliquot was heated under reflux for 1 h. The resultant yellow solution was cooled to room temperature. The light-yellow precipitate that formed was filtered off, washed with hot ethanol, and dried under vacuum over anhydrous CaCl₂ (yield: 1.23 g, 73.65%). The prepared compound (0.17 g) was dissolved in acetonitrile (20 ml) on warming and mixed with ethanol (10 ml). Light-yellow platelet single crystals of the title compound (m.p. 386–387 K) suitable for X-ray study were obtained after 17 days, along with colorless needle-shaped crystalline solids (m.p. 413–418 K) (Figure 5) [24].

Figure 5.

A packing diagram of the title compound. The C▬H…π interactions are shown as green lines.

2.4.2 Synthesis of the Schiff base S-benzyl-β-N-(4-benzyloxyphenyl)methylenedithiocarbazate

The reaction of 4-benzyloxybenzaldehyde with S-benzyl dithiocarbazate (SBDTC) afforded the Schiff base S-benzyl-β-N-(4-benzyloxyphenyl)methylenedithiocarbazate (Figure 6).

Figure 6.

S-Benzyl-β-N-(4-benzyloxybenzyl)methylenedithiocarbazate.

2.4.2.1 Synthesis of S-benzyl-β-N-(4-benzyloxyphenyl)methylenedithiocarbazate

4-Benzyloxybenzaldehyde (1.96 g, 10 mmol) in ethanol (25 mL) was added to a boiled solution of S-benzyldithiocarbazate (1.98 g, 10 mmol) in ethanol (40 mL), and the mixture was refluxed for 1h. The white solid that formed was separated by filtration and recrystallized from methanol as white crystalline solid (Figure 7). Yield: 3.74 g (95%). m.p. 140°C [25].

Figure 7.

Preparation of Schiff bases S-benzyl-β-N-(4-benzyloxyphenyl)methylenedithiocarbazate.

The ligand S-benzyl-β-N-(4-benzyloxyphenyl)methylenedithiocarbazate exists in thionetautomeric form both in the solid state and in solution. The Schiff base is bonded with the metal ions in a uni-negative bi-dentate mode, thereby forming four-coordinate inner complexes. Both the ligand and its complexes showed moderate antibacterial activity.

2.4.3 Synthesis of S-benzyl-β-N-(4-hydroxy-3-nitrophenyl)-methylenedithiocarbazate

Briefly, 4-hydroxy-3-nitrobenzaldehyde (1.67 g, 10 mmol) in acetonitrile (20 mL) was added in hot SBDTC (1.98 g, 10 mmol) in acetonitrile (40 mL) and refluxed for about 0.5 h. The resulting solution was cooled to room temperature (20–25°C). The orange-yellow solid that formed was separated, washed with ethanol, and dried in a vacuum over anhydrous CaCl₂—yield: 2.96g (81%), m.p. 171–172°C. The synthesized compound was recrystallized from a mixture of chloroform and petroleum ether (2:1; v/v) as large orange pyramid-shaped single crystals after 4 days of dissolution and slow evaporation at room temperature (Figure 8).

Figure 8.

Schiff bases S-benzyl-β-N-(4-hydroxy-3-nitrophenyl)-methylenedithiocarbazate.

The synthesized ligand was characterized using conventional techniques like nuclear magnetic resonance (NMR), UV-visible, infrared spectroscopy (IR), mass spectroscopic techniques, magnetic susceptibility measurement, and molar conductance. A single crystal X-ray crystallography data approved the proposed crystal structure of the ligand. Both in the solid and solution phase, the ligand continues to exist in its thione tautomeric form (Figure 9) [26].

Figure 9.

Synthesis of Schiff base (HL) and its metal complexes.

3. Structural aspects and properties of Schiff bases

3.1 Molecular structure and bonding in Schiff bases

The molecular structure and bonding in Schiff bases are fundamental in their properties and reactivity [27]. Schiff bases are a class of organic compounds with a specific functional group, commonly represented as R₁R₂C〓NR₃, where R₁, R₂, and R₃ can be various organic groups. Let us delve into the key aspects of the molecular structure and bonding in Schiff bases:

3.1.1 Functional group

The central feature of a Schiff base is the azomethine or imine functional group (C〓N). This double bond (π bond) results from the linkage between a carbon atom (C) and a nitrogen atom (N). The carbon atom is often part of an organic group (R₁ or R₂), while the nitrogen atom can be part of another organic group (R3).

3.1.2 Geometry

The C〓N double bond exhibits a planar geometry, with the carbon and nitrogen atoms lying in the same plane. This planar arrangement is a consequence of the π bond formed between C and N.

3.1.3 Bonding

The bonding in Schiff bases involves both sigma (σ) and pi (π) bonds. The sigma (σ) bond forms between the carbon atom (C) and one of the nitrogen atom’s lone pairs, creating a single bond (C▬N) and sp² hybridization around the carbon atom. In contrast, the combination of carbon and nitrogen atoms in the C〓N double bond create the pi (π) bond.

3.1.4 Resonance structure

Schiff bases can exhibit resonance structures due to the presence of the π bond. The electrons in the π bond can delocalize, leading to resonance forms. This delocalization results in a partial double bond character in the C〓N bond. The resonance forms contribute to the stability of the molecule.

3.1.5 Tautomeric forms

Schiff bases can exist in tautomeric forms. The most common tautomeric form is the imine tautomer, which has the C〓N double bond. In some cases, Schiff bases can undergo tautomerization to form enamine tautomers.

3.1.6 Electron density and reactivity

The C〓N double bond is polarized, with the carbon atom bearing a partial positive charge (δ+) and the nitrogen atom having a partial negative charge (δ−). This polarization makes the carbon atom electrophilic and the nitrogen atom nucleophilic, which influences the reactivity of Schiff bases. The electrophilic carbon can undergo additional reactions with nucleophiles, such as nucleophilic attack by various reagents.

3.1.7 Stereochemistry

The stereochemistry around the C〓N bond can be important in certain reactions, particularly in the context of asymmetric synthesis. The orientation of substituents around the C〓N bond can lead to different stereoisomers.

3.2 Spectroscopic characterization (UV-vis, IR, NMR, etc.) and analytical techniques

Spectroscopic characterization is a fundamental aspect of understanding the structural features and properties of Schiff bases. Various techniques, including UV-vis spectroscopy, infrared spectroscopy (IR), and nuclear magnetic resonance (NMR), play crucial roles in unraveling the molecular intricacies of Schiff bases [28].

3.2.1 UV-vis spectroscopy

UV–vis spectroscopy provides valuable insights into the electronic transitions of Schiff bases. It allows researchers to analyze the absorption of ultraviolet and visible light, providing information about the conjugation and electronic configuration of the carbon-nitrogen double bond. UV-vis spectroscopy is particularly useful for studying the chromophoric properties of Schiff bases, aiding in their identification and quantification.

3.2.2 Infrared spectroscopy (IR)

IR spectroscopy is employed to examine the vibrational modes of molecules, offering details about functional groups and molecular structure. In the context of Schiff bases, IR spectroscopy is instrumental in identifying characteristic peaks associated with the C〓N stretching vibration, providing a fingerprint for the presence of the imine group.

3.2.3 Nuclear magnetic resonance (NMR)

NMR spectroscopy is a powerful technique for elucidating the structural aspects of Schiff bases. Proton NMR (¹H NMR) and carbon NMR (¹³C NMR) offer information about the connectivity of atoms within the molecule, allowing precise determination of substituent positions and aiding in the verification of the proposed structure.

3.2.4 Analytical techniques

Beyond spectroscopy, other analytical techniques contribute to the comprehensive characterization of Schiff bases. Mass spectrometry (MS) can confirm molecular weight and elucidate fragmentation patterns, while elemental analysis provides information about the elemental composition of the compound. These techniques collectively contribute to a thorough understanding of the composition and structure of Schiff bases.

In summary, the integration of UV-vis spectroscopy, IR spectroscopy, NMR spectroscopy, and various analytical techniques enables a multifaceted characterization of Schiff bases. This holistic approach is essential for establishing their structural features, verifying synthetic outcomes, and laying the foundation for exploring their diverse properties and applications.

3.3 Thermodynamic and kinetic properties of Schiff bases

The study of thermodynamic and kinetic properties of Schiff bases is crucial for understanding their stability, reactivity, and potential applications. Researchers have delved into comprehensive investigations to unveil the intricacies of these properties [29].

3.3.1 Thermodynamic properties

Thermodynamic studies provide insights into the energetics of Schiff base formation and stability. The investigation encompasses enthalpy changes, Gibbs free energy, and entropy alterations during the reaction processes. Understanding these thermodynamic parameters is essential for predicting the feasibility and spontaneity of Schiff base reactions.

3.3.2 Kinetic properties

Kinetic analyses focus on the rate of Schiff base formation or decomposition. Experimental techniques, such as reaction progress monitoring and determination of reaction rates under varying conditions, are employed. These studies unravel the reaction mechanisms, activation energies, and the influence of different factors on the kinetics of Schiff base reactions.

The comprehension of thermodynamic and kinetic properties holds significance in diverse applications. Whether applied in catalysis, materials science, or medicinal chemistry, a thorough understanding of these properties contributes to the design and optimization of Schiff bases for specific purposes.

4. Applications of Schiff bases

Schiff bases, versatile compounds derived from the condensation of a primary amine and an aldehyde or ketone, find a multitude of applications across various scientific disciplines. Their diverse properties make them valuable in catalysis, medicinal chemistry, coordination chemistry, materials science, photovoltaics, sensing, corrosion inhibition, and environmental and industrial applications.

4.1 Catalysis and asymmetric synthesis using Schiff base ligands

Schiff bases play a pivotal role in catalysis and asymmetric synthesis, leveraging their versatile structures to influence the stereochemistry of reactions [15, 30, 31]. As ligands, Schiff bases form coordination complexes with various metal ions, imparting unique catalytic properties. These applications are integral in the synthesis of chiral compounds and are crucial in the pharmaceutical and fine chemical industries. They can coordinate with metal ions to form catalytically active complexes.

Catalysis and asymmetric synthesis:

1. Catalysis: Schiff base ligands exhibit catalytic activity in various reactions, including oxidation, reduction, and cross-coupling reactions. The coordination of metal ions with Schiff bases enhances the catalytic efficiency and selectivity of these processes.

2. Asymmetric synthesis: Schiff base complexes are widely employed in asymmetric synthesis, where the control of stereochemistry is essential. The chiral environment created by the Schiff base ligand in coordination with a metal center allows for the enantioselective formation of chiral compounds.

4.2 Medicinal chemistry: schiff bases as drug intermediates and bioactive compounds

Schiff bases have gained significant importance in medicinal chemistry, serving as versatile compounds with applications as drug intermediates and bioactive molecules [7, 8, 32, 33]. The unique structural features of Schiff bases, combined with their diverse pharmacological activities, make them valuable candidates for drug development.

Medicinal chemistry applications:

1. Drug intermediates: Schiff bases often act as intermediates in the synthesis of pharmaceutical compounds. They provide a versatile platform for introducing specific functionalities and optimizing the pharmacokinetic properties of drugs.

2. Bioactive compounds: Schiff bases exhibit a wide range of biological activities, including antibacterial, antifungal, antiviral, anti-inflammatory, and anticancer properties. These bioactive characteristics make Schiff bases attractive for designing novel therapeutic agents.

4.3 Applications of Schiff bases in coordination chemistry and metal-organic frameworks (MOFs)

Schiff bases play a crucial role in coordination chemistry, forming coordination complexes with metal ions and contributing to the construction of metal-organic frameworks (MOFs). These applications have far-reaching implications in catalysis, sensing, and materials science [34, 35].

Coordination chemistry and MOFs applications:

1. Catalysis: Schiff bases form coordination complexes with various metal ions, acting as ligands that influence the catalytic activity of the metal center. These complexes are employed in a variety of catalytic reactions.

2. Sensing and detection: the unique properties of Schiff bases make them suitable for sensing applications. Coordination complexes involving Schiff bases can act as sensors, detecting specific analytes through changes in their electronic or optical properties.

3. Materials science: Schiff bases contribute to the construction of metal-organic frameworks (MOFs), porous materials with diverse applications in gas storage, separation, and catalysis. MOFs incorporating Schiff bases can be tailored for specific functionalities.

4.4 Applications of Schiff bases in polymer science and materials development

Schiff bases play a significant role in polymer science and materials development, contributing to the design of innovative materials with tailored properties. Their versatile structures and reactivity make them valuable building blocks in the synthesis of polymers for various applications [36].

1. Functional polymers: Schiff bases can be incorporated into polymer chains to impart specific functionalities. These functional polymers find applications in drug delivery, sensors, and other advanced materials.

2. Responsive polymers: Schiff bases can be designed to respond to external stimuli such as pH, temperature, or the presence of specific ions. These responsive polymers are valuable in smart materials and controlled-release systems.

3. Conducting polymers: Schiff bases can be utilized in the development of conducting polymers, which find applications in electronic devices, sensors, and flexible electronics.

4. Coating materials: Schiff bases contribute to the synthesis of coatings with anti-corrosive properties. These coatings protect surfaces from degradation in harsh environments.

4.5 Applications of Schiff bases in dye-sensitized solar cells and photovoltaics

Schiff bases play a significant role in the development of dye-sensitized solar cells (DSSCs) and photovoltaic devices, contributing to the advancement of sustainable and efficient energy conversion technologies [37].

1. Light harvesting: Schiff base dyes can act as sensitizers in DSSCs, absorbing light across a broad spectrum of wavelengths. The absorbed photons generate excited states that facilitate electron injection into the semiconductor material.

2. Electron transport: the imine linkage in Schiff base dyes provides a pathway for efficient electron transport, contributing to the generation of a photocurrent in the solar cell.

3. Tunable properties: the chemical versatility of Schiff bases allows for the design of dyes with tunable properties, including absorption spectra and redox potentials, enabling optimization for specific solar cell architectures.

4. Stability and durability: Schiff base dyes, when appropriately designed, can enhance the stability and durability of DSSCs, ensuring prolonged and efficient performance under varying environmental conditions.

4.6 Applications of Schiff bases as fluorescent probes and sensors

Schiff bases act as fluorescent probes and sensors, detecting specific analytes. Their fluorescence properties make them valuable in bioimaging and sensing applications [38, 39, 40, 41].

1. Metal ion sensing: Schiff bases can selectively coordinate with metal ions, leading to changes in their fluorescence properties. This property is harnessed for the sensitive detection of metal ions in various samples.

2. pH sensing: Schiff bases can undergo protonation or deprotonation in response to changes in pH, resulting in alterations in their fluorescence intensity or emission wavelength. This property is exploited for pH-sensing applications.

3. Biological molecule detection: Schiff bases can be designed to interact selectively with specific biomolecules such as amino acids, proteins, or nucleic acids. Their fluorescence changes serve as indicators of the presence of these biological entities.

4. Environmental monitoring: Schiff base-based fluorescent probes are utilized for environmental monitoring, detecting pollutants, and assessing the quality of water or air based on specific chemical interactions.

4.7 Application of Schiff bases in corrosion inhibition

Schiff bases have corrosion-inhibitive properties, protecting metals from corrosion. They form a protective layer on metal surfaces, preventing the degradation of materials [42, 43, 44].

1. Formation of protective films: Schiff bases, characterized by their versatile molecular structures, have the capability to chelate with metal ions present on the surface of metals. This chelation process leads to the formation of protective films, effectively preventing corrosive agents from interacting with the metal substrate.

2. Versatility and tailoring for specific applications: the diversity in Schiff base structures allows for tailoring their chemical properties, enabling researchers to design compounds with specific corrosion-inhibiting characteristics. This versatility is advantageous for addressing the varying corrosion challenges posed by different metal substrates and environmental conditions.

3. Enhanced corrosion resistance: Schiff bases contribute to enhanced corrosion resistance by acting as barriers that impede the penetration of corrosive elements. The formation of a stable and protective layer on the metal surface retards the corrosion process, thereby extending the lifespan of the metallic structure.

4. Potential for green corrosion inhibition: the application of Schiff bases aligns with the principles of green chemistry as researchers explore environmentally friendly synthesis routes and sustainable practices for corrosion inhibition. This makes Schiff bases not only effective but also compatible with contemporary demands for eco-friendly solutions.

5. Ongoing research and future prospects: continuous research in the field of Schiff bases and corrosion inhibition aims to uncover new compounds with improved efficacy and application in diverse industries. The synergy of experimental studies and computational approaches contributes to a deeper understanding of the mechanisms involved, paving the way for innovative solutions and broader industrial applications.

In summary, the application of Schiff bases in corrosion inhibition stands as a promising area of research, offering tailored solutions for protecting metallic structures against the deleterious effects of corrosion. The versatility, effectiveness, and potential for environmentally friendly practices make Schiff bases valuable contributors to the field of corrosion science.

4.8 Application of Schiff bases in environmental and industrial contexts

Schiff bases find applications in various environmental and industrial contexts, including water treatment, catalysis for chemical transformations, and components of analytical techniques [45, 46].

1. Metal ion chelation for wastewater treatment: Schiff bases exhibit a remarkable ability to chelate metal ions. In environmental applications, this property is leveraged for wastewater treatment. By forming stable complexes with metal contaminants, Schiff bases aid in the removal of heavy metals from industrial effluents, contributing to the mitigation of environmental pollution.

2. Catalysis: acting as catalysts, Schiff bases enhance reaction efficiency in industrial processes, especially in pharmaceutical and petrochemical sectors.

3. Corrosion inhibition: Schiff bases protect metal surfaces, prolonging the life of industrial infrastructure.

4. Solar cells: their role as dye sensitizers contributes to the development of solar cells for sustainable energy.

5. Antimicrobial coatings: Schiff bases are employed to create coatings with antimicrobial properties on industrial surfaces.

6. Sensor development: used in sensor technology, Schiff bases help monitor environmental pollutants for quality control.

7. Green chemistry initiatives: Schiff bases support eco-friendly practices, aligning with green chemistry principles in industrial processes.

In essence, Schiff bases play a vital role in addressing environmental concerns and optimizing industrial processes for sustainability.

5. Recent advances in Schiff bases

5.1 Innovative and emerging applications

In recent years, Schiff bases have witnessed remarkable developments, leading to innovative and emerging applications across various scientific disciplines [16, 33, 47]. These advancements highlight the versatility and adaptability of Schiff bases in addressing contemporary challenges.

1. Drug delivery systems: Schiff bases have been explored in the development of advanced drug delivery systems, capitalizing on their ability to encapsulate and release therapeutic agents in a controlled manner.

2. Nanomaterials and nanotechnology: Schiff bases are integrated into nanomaterials for applications in nanotechnology, such as nanocatalysis, nanosensors, and drug delivery platforms.

3. Biological imaging: Schiff bases have gained prominence in biological imaging as fluorescent probes, enabling precise visualization of cellular processes and structures.

4. Supramolecular chemistry: recent advances in supramolecular chemistry involve the use of Schiff bases for constructing complex architectures, contributing to the development of functional materials.

5. Photodynamic therapy: Schiff bases have been explored in photodynamic therapy for cancer treatment, utilizing their photosensitive properties to induce cell death in targeted cancer cells.

5.2 Recent research and discoveries

In the dynamic field of Schiff bases, recent research and discoveries have unveiled novel applications, synthetic methodologies, and intriguing properties, contributing to the ongoing evolution of this versatile class of compounds [48, 49, 50, 51, 52].

1. Catalytic applications: recent studies have explored Schiff bases as catalysts in various reactions, showcasing their efficacy in asymmetric synthesis, cross-coupling reactions, and sustainable catalysis.

2. Smart materials: advances in Schiff bases have led to the development of smart materials that are responsive to external stimuli such as light, pH, or temperature, opening avenues for applications in sensing and responsive technologies.

3. Bioorthogonal chemistry: innovations in bioorthogonal chemistry involve the use of Schiff bases for selective labeling of biomolecules in living systems, providing new tools for bioimaging and diagnostics.

4. Computational approaches: recent discoveries leverage computational chemistry to design Schiff bases with tailored properties, accelerating the discovery of new compounds with specific functionalities.

6. Challenges and future directions

6.1 Current challenges and limitations in Schiff base research

Despite the versatility and widespread applications of Schiff bases, several challenges and limitations persist in the field. Addressing these issues is crucial for advancing Schiff base research and unlocking their full potential.

1. Stability and reactivity: Schiff bases, especially certain imine-based Schiff bases, may exhibit limited stability under certain conditions, impacting their long-term applications.

2. Selectivity in metal complexation: achieving high selectivity in metal complexation remains a challenge, particularly when dealing with multiple metal ions present in complex matrices.

3. Biocompatibility: while Schiff bases have promising applications in medicinal chemistry, issues related to biocompatibility and potential toxicity need to be thoroughly addressed for safe biomedical applications.

4. Synthetic accessibility: Some Schiff bases, especially those with intricate structures, may pose challenges in terms of synthetic accessibility, limiting their large-scale production and practical applications.

6.2 Future directions in Schiff base research

1. Development of Stable Schiff Bases: research efforts should focus on designing and synthesizing stable Schiff bases that maintain their integrity under various environmental and physiological conditions.

2. Enhanced selectivity: advancements in ligand design and coordination chemistry can contribute to the development of Schiff bases with improved selectivity for specific metal ions, reducing interference in complex samples.

3. Biocompatible Schiff bases: future research should aim to design and characterize Schiff bases with enhanced biocompatibility, ensuring their safe use in various biomedical applications.

4. Green synthetic routes: exploring sustainable and green synthetic routes for Schiff base synthesis can address concerns related to the environmental impact of traditional methods.

6.3 Future prospects and areas of potential growth in Schiff base chemistry

Schiff base chemistry holds promising avenues for future development, and researchers are actively exploring various areas to overcome current challenges and unlock new opportunities. The following outlines potential growth areas and future prospects in Schiff base research.

1. Functional materials: Schiff bases have shown potential in the design of functional materials for diverse applications, including sensors, catalysis, and drug delivery systems. Future research may focus on tailoring Schiff bases for specific functionalities and enhancing their performance in these applications.

2. Supramolecular chemistry: advances in supramolecular chemistry, involving the self-assembly of Schiff bases into well-defined structures, offer prospects for creating complex architectures with unique properties. This area holds potential for the development of new materials and devices.

3. Biomedical applications: the exploration of Schiff bases in biomedical applications, such as imaging agents, drug delivery systems, and therapeutics, represents a growing field. Future research may aim at improving biocompatibility, stability, and targeting capabilities for enhanced biomedical efficacy.

4. Green chemistry: green synthetic routes and environmentally friendly methodologies are gaining importance. Future prospects in Schiff base chemistry involve the development of sustainable synthesis methods, reducing environmental impact, and making the compounds more applicable in green chemistry initiatives.

6.4 The role of computational chemistry in advancing Schiff Base research

Computational chemistry plays a pivotal role in advancing Schiff base research by providing valuable insights into molecular structures, reaction mechanisms, and properties. The synergy between theoretical calculations and experimental studies enhances our understanding and guides the design of novel Schiff bases with tailored functionalities [52].

1. Drug design and discovery: computational methods aid in virtual screening and de novo drug design, accelerating the discovery of novel Schiff bases with therapeutic potential.

2. Catalysis optimization: computational chemistry contributes to the optimization of Schiff base-based catalysts, predicting reaction pathways and identifying key intermediates.

3. Electronic structure analysis: understanding the electronic structure of Schiff bases through computational methods aids in predicting their optical, redox, and magnetic properties.

4. Biological interaction studies: computational chemistry assists in studying the interaction of Schiff bases with biomolecules, providing insights into their potential applications in medicinal chemistry.

Computational chemistry, with its predictive power and ability to simulate complex molecular behaviors, continues to be instrumental in guiding and advancing Schiff base research across various scientific domains.

7. Expected contributions

This chapter will provide a comprehensive overview of Schiff bases, their synthesis, properties, and applications, with a focus on recent advancements. It will serve as a valuable resource for researchers, students, and professionals in the fields of chemistry, biology, and materials science. By presenting the novelties in Schiff bases, this chapter will contribute to the collective understanding of this versatile class of compounds.

8. Conclusion

In conclusion, this chapter comprehensively explores Schiff bases, revealing their synthesis, properties, and diverse applications across scientific domains. Key points include the historical context dating back to Hugo Schiff’s discovery in the 19th century, examination of synthesis methods with a focus on green chemistry, elucidation of structural aspects, and exploration of versatile applications from catalysis to corrosion inhibition. Recent advances and future prospects, along with the emphasized versatility and importance of Schiff bases, underscore their crucial role in addressing contemporary challenges and shaping the landscape of modern chemistry.