Open access peer-reviewed chapter - ONLINE FIRST
Abstract
The scientific community has given Schiff-based metallic complexes much attention because of their distinctive properties and numerous uses in a variety of fields, including biology and industry. The nature of the ligands and metal ions influences coordination chemistry. The biological activities of these complexes include cytostatic, antiviral, antibacterial, anticancer, and antifungal effects. They also have remarkable catalytic activity. With its diverse uses, coordination complexes serve a vital role in research.
Keywords
- Schiff base
- metal complexes
- applications
- ligand
- synthesis
1. Introduction
A metal complex is composed of one or more ligands, which are molecules or ions that have one or more pairs of electrons that they can share with the metal. Metal complexes can be neutral, positively charged, or negatively charged. A compound that forms a coordination complex by bonding with a central metal atom or ion is called a ligand. The ligand is an electron-rich compound that can be an anion, cation, or neutral. Ionic or covalent bonding is possible in metal-ligand complexes. Depending upon the number of donating atoms, the ligands may be bidentate, tridentate, or polydentate. The condensation of primary amines with aldehydes or ketones gives compounds that contain a carbon-nitrogen double bond with the nitrogen attached to an alkyl group. These are called Schiff bases since their synthesis was reported by Hugo Schiff [1].
Coordination chemistry is one of the most active research areas in inorganic chemistry, which involves the interaction of organic/inorganic ligands with metal centers. Numerous fields, such as dyes, colors, nuclear fuels, catalysis, toxicology, bioinorganic chemistry, medicine, ceramics, materials science, and toxicology, uses various forms of coordination compounds. The application of these complexes as biocides, catalysts, NMR shift reagents, and DNA binders is due to the inclusion of a numerous kinds of ligands [2]. Because of this work, inorganic chemists have been able to significantly advance their understanding of how to modify the concept of chemical bonding [3].
The synthesis, characterization, and uses of various ligands, including Schiff base ligands and chelates, are well documented in coordination chemistry [4]. Many coordination chemists are interested in transition metal complexes with higher nuclearity, and polytopic ligands play a key role in modifying the molecular structures of these complexes. While one-pot syntheses of polynuclear complexes are highly complex, whereas polytopic ligands are superior for the successful creation of homopolynonucleic complexes [5, 6, 7, 8, 9, 10].
The development of synthetic pathways is of greatest significance in order to enable the regulated and predictable tailor-making of heterometallic systems. Among the several methodologies, the use of metal complexes as ligands has shown to be quite effective. If the complexes formed by coordination with metal ions have the tendency to coordinate further or react with other complexes, then they may act as ‘metal organic ligands’ [11]. In a variety of applications, polynuclear complexes are particularly helpful as magnetic materials. Magnetic materials are currently being utilized in biomedical applications, including magnetic separation of cells, DNA, proteins, and other materials, and targeted drug delivery in magnetic resonance imaging (MRI) [12, 13, 14, 15, 16, 17].
1.1 Introduction to metal complexes
Over the past few decades, metal complexes synthesized from different Schiff base ligands served a significant role in coordination chemistry. Because of their simplicity in synthesis and many uses, these ligand derivatives are frequently used. Schiff base ligand metal complexes have become popular due to their remarkable biological, catalytic, and fluorescent properties. Many Schiff base metal complexes have been developed and tested for their antibacterial and antifungal qualities because Schiff bases containing azomethine or imine groups have a variety of biological activities that can advance the field of bioinorganic sciences [18, 19, 20].
Researchers are working harder to create novel antibiotic compounds because of the growing number of bacterial and fungal strains that are becoming resistant to traditional antibiotics. Schiff base has an azomethine (−C〓N−) functional group, which forms bonds with two or more aromatic or heterocyclic compounds that are physiologically active to generate a variety of molecular hybrids with excellent antibacterial capabilities. These Schiff bases are chelating compounds that often create stable chelates by coordinating with metals, particularly lanthanides and d-block metals with a wide range of therapeutic applications [21, 22]. These Schiff bases form a new family of compounds with promising antibacterial and anticancer properties. Schiff base metal complexes, which are derived from halogenated salicylaldehyde, have a broad variety of applications in research involving biocatalytic, anti-HIV, antibacterial, and luminous probes for DNA and RNA breakage reactions [23, 24, 25]. These derivatized complexes are also employed as fungicides, insecticides, and flavoring agents for bouquets and liquor [26, 27, 28, 29, 30, 31, 32].
The structure and vibrational spectra of Schiff base complexes have been investigated by various methods [33, 34, 35, 36, 37, 38, 39, 40], but few studies have investigated symmetric azine compounds and their complexes [41, 42]. Azines are important due to their analogy with Schiff bases, which are considered models of some biological systems [43, 44, 45, 46, 47, 48, 49, 50]. Recently, the stereochemistry and stereo electronics of some substituted (E, E)-azines [51] and the crystal structure of asymmetric acetophenone azines have been studied [52] since these compounds are considered nonlinear optical materials.
2. Synthesis
Schiff base synthesis by green synthetic methods has gained significant attention in recent years because of increasing need for more environmentally friendly and sustainable chemical production methods. Traditional methods of synthesis of Schiff bases involve extreme circumstances, such as high temperatures and acidic or basic catalysts, which can produce hazardous byproducts. Green synthetic techniques, on the other hand, use softer conditions and renewable resources to prepare Schiff bases. General Schiff base formation includes ketone or aldehyde, where the >C〓O group is substituted with the >C〓N−R group (Figure 1) where R may be an alkyl, aryl, halogen, or hydrogen group. Schiff bases containing aryl substituent are significantly more stable than those with alkyl substituents [53].
Figure 1.
General synthesis of Schiff base.
Aromatic aldehydes Schiff bases are more durable because of strong conjugation, while aliphatic aldehyde Schiff bases are highly unstable and easily polymerizable. The stability of Schiff bases is also affected by the substituents, as a result, several methods for producing imines have been reported [54]. A Schiff base can be formed from an aldehyde or ketone in an acidic, basic, or heated environment. In Figure 2, the reversible reaction is depicted.
Figure 2.
Formation of Schiff bases via carbinol formation.
The formation is usually pushed to the finishing touch of these preparations of the product or elimination of water molecules, or both. Many Schiff bases can be hydrolyzed in an aqueous acidic or basic medium to return to their aldehydes or ketones and amines. The reaction of nucleophile addition to the carbonyl group results in Schiff base formation. In this case, the nucleophile is the amine.
In first step of the mechanism, the amine reacts with the aldehyde or ketone to offer a volatile addition compound known as carbinolamine. In next step, carbinolamine loses water by both acid and base catalyzed pathways. Since carbinolamine is an alcohol, it undergoes acid-catalyzed dehydration which is shown in Figure 3.
Figure 3.
Formation of Schiff base in acidic medium.
The Schiff base formation is a series of kinds of reactions that are addition followed by elimination. The chelating cap potential of Schiff bases makes them an intriguing ligand in coordination chemistry combined with their ease of use and flexibility in a variety of chemical environments around the >C〓O group [55]. In addition to being useful in the formation of metallic complexes, the formation of Schiff bases is beneficial in a variety of chemical reactions and has essential medical value [56].
2.1 Chemical properties of Schiff bases
Coordination chemistry has evolved significantly with the help of Schiff bases. Schiff base metal complexes have attracted a lot of interest because of their unique physical and chemical characteristics and numerous applications in a range of scientific fields [57]. Additionally, the heterocyclic compounds provide the fundamental blocks for the synthesis of various Schiff bases. Since Schiff bases physical and biological characteristics have a close relationship to the equilibrium of proton transfer and hydrogen bonding in intermolecular systems, they are of greater importance [58].
Inducing substrate chirality, modifying metal-centered electronic parameters, and enhancing the solubility and stability of homogenous or heterogeneous catalysts are all made feasible by Schiff bases [59]. Schiff base’s catalytic activity is demonstrated by several homogeneous and heterogeneous reaction complexes. Transition metal complexes of Schiff bases are involved in multiple biological reactions that are essential to the life process [60]. The metals are necessary for the verification and functioning of biological macromolecules since they can coordinate with the O or N terminals of proteins in various kinds of models [61].
2.2 Synthesis of Schiff base transition metal complexes
Metals and their compounds are required in almost every factor of daily life. Metals are important elements in numerous organic key functions. These factors are beneficial in unique approaches in technology for revolutionizing the manner of communication, nuclear reactor, renewable energy, and generally in drug synthesis.
Depending on their binding nature, metal ions are divided into two classes with the elements like N, O, F, etc. Thus, metallic ions can be identified as hard Lewis acids and soft Lewis acids. Some of the metal ions having d electrons can be donated to suitable ligands to form π-bonds, and a number of the metal ions having vacant d-orbitals can accept electrons from the ligands to form the metal complexes. In the current chemistry, metal complexes are also recognized as coordination complex. The metallic complex is the molecules, which consists of a central metal atom or ion surrounded by different ions, molecules, or atoms that are recognized as ligands or complexing agents [62].
The transition metal due to their vacant d orbital participated in the complex formation. Alfred Werner published the theory of coordination chemistry in 1893. He described the two possibilities in terms of location in the coordination sphere of the metal ions. He also explained the outer and inner sphere complexes with respect to ligands.
Generally, under suitable experimental conditions, metal salts react with Schiff base ligands to form metal complexes [63, 64, 65]. On the contrary, the Schiff base metal complexes are made in situ for specific catalytic applications. Generally, equimolar quantity of Schiff bases and metal salt solution were required; however, the amount of metal salt solution varied depending on the type of ligand, such as monodentate, tridentate, or tetradentate. Schiff base metal complexes can be synthesized by using various techniques, such as reflux, heating, microwave, grinding, sonication, and solvents used like ethanol, methanol, or ethyl acetate. Solvent-free synthesis has also been accomplished recently.
Transition metals exhibit different oxidation states and interact with numerous negatively charged molecules, and as a result, transition metal complexes are cationic or anionic species [66, 67]. Schiff base ligands containing azomethine group such a ligand are also known as privileged ligands [68]. Mono or poly nuclear complexes of majority of multidentate N−O, N−N, O−N−S, donor Schiff bases, have important applications.
Schiff base metal complexes have been studied in different areas because of their versatile chemical properties like electrical conductivity, magnetic, catalytic, redox, etc. [69] and wide range of usage in medicinal, chemical, and industrial areas [70]. A number of metals like transition metals and some of the F block elements are also used to synthesize complexes. Complex formation reactions are completely based on nature of ligand and reaction conditions. Complex formation reaction is also very useful to determine quantity of heavy metals that are poisonous to living things.
Generally, a Schiff base is mixed with readily available metallic complexes such as metal amides, alkyls, acetates, or halides to yield metallic complexes. For catalytic applications, this process is appropriate and simple [71].
3. Applications of metal complexes
Schiff base metal complexes have been studied in various chemical and biological areas due to their diverse applications in chemical, pharmacological, dyes-paints, medicinal, material sciences, and industrial areas. The variety of metal complexes is used as a catalyst in many essential chemical reactions like organometallic, metathesis reaction, Suzuki coupling, organic reduction reaction, etc. Schiff bases also show pharmacological activities such as antitubercular, antibacterial, antifungal, antimicrobial, antiviral, anticancer, and wide range of organic activities and commercial applications. Schiff base metal complexes have been of a dominant nature for a long time due to their ability to attach oxygen to redox systems exerting their ability to damage. It also shows the antibacterial activity because of the free radical scavenging property.
Because of the ability of metal complexes to provide synthetic models for metal-containing molecules in metalloproteinase and metalloindole, which are widely employed in the manufacture of fragrances, dyes, agrochemicals, and pharmaceuticals, they have drawn attention in the field of bioinorganic chemistry. Furthermore, preparing metal complexes linked to both synthetic and natural oxygen carriers requires a greater understanding of tetra dentate and tridentate Schiff base complexes. Bioinorganic substances are defined as copper complexes with dimeric units, namely those of the tridentate Schiff base type.
The ability of metallodrugs to cause DNA cleavage is one of the most important criteria for the advancement of metallodrugs as chemotherapeutic agents. Recently, a big number of transition metal complexes have been observed to promote DNA cleavage due to their redox properties. Co (II), Fe (III), and Ru (III) complexes of Schiff bases derived from hydroxyl benzaldehyde are used for the oxidation of cyclohexane into cyclohexanol and cyclohexanone in presence of hydrogen peroxide.
The greenest catalysts are the Fe (III) complexes which are uncommon because the cobalt (II) complexes have high interest in alkane oxidation reactions. New Mn (II) and Mn (III) complexes of substituted N, N′- bis (salicylidine)-1,2- diimino-2-methylene appear to be efficient models for peroxidase activity. New Copper (II) complexes of indoxyl thio-semicarbazone (ITSC) show one pair of well-defined reduction peaks at different potential in the forward scan, which represent the reduction of copper.
3.1 Catalyst
Excellent catalytic activity is observed by several Schiff base complexes in a variety of reactions, including oxygenation, hydrolysis, electro-reduction, and decomposition. Alkene oxygenation is catalyzed by Co (II) aromatic Schiff base four coordinated chelate complexes. Certain Cu complexes increase the rate of hydrolysis obtained from amino acids by 10–50 times compared to the simple copper (II) ion. The iron (II) Schiff base complex is catalytically active for oxygen electro-reduction reactions. Certain metal complexes containing a polymer-bound Schiff base show the catalytic activity for oxidation of ascorbic acid and the breakdown of hydrogen peroxide.
Catalytic activity of cyanohydrin cobaltate complexes is observed at high temperature (1000°C) and in the presence of moisture. At low pressure, Schiff base complexes catalyze the carbonylation of alcohols and alkenes to create aryl propionic acid and its esters. Catalysis reactions are more efficient in terms of energy consumption and waste creation, so it is a significant area of study in both academic and industrial research. These reactions involve catalytically active species that lower the activation energy by forming reactive intermediates through the coordination with organic ligands. The renewal of the catalytically active species should lead to the production of the product.
The efficiency of the catalyst can be calculated on the basis of catalytic cycles passed by one molecule of catalyst. The catalyst should form only labile intermediates with the substrate for efficient regeneration. This concept can be realized using transition metal complexes because metal-ligand bonds are generally weaker than covalent bonds. The possibility of switching reversibly between different oxidation states in redox reactions is due to different oxidation state of transition metals with only moderate differences in their oxidation potentials. Thus many transition metals have been applied as catalysts for organic reactions [72].
Iron has not played a dominant role in catalytic processes. The field of organo-iron chemistry began with the discovery of pentacarbonyl iron by Mond and Berthelot in 1891. Another significant event was the 1951 report on ferrocene. Research was done on Reppe synthesis which is based on iron catalysis. The cross-coupling of Grignard reagents with organic halides, catalyzed by iron, was reported by Kochi and associates in 1971. On the other hand, cross-coupling reactions with transition metals like palladium and nickel gained popularity. In recent times, iron complexes have been used in various reactions, showing the importance of this metal in catalysis.
3.2 Dyes
Transition metals are used to prepare the metal complexes, which show different colors due to different oxidation state and d-d transitions of electrons. Chromium azomethine complex and cobalt complex are used as dyes, to give fast color to wool, leathers, food packages, etc. Azo groups containing metal complexes are used for dying cellulose polyester textiles and mass dye poly-fibers. Salicylaldehyde with diamante Schiff base complex with cobalt has excellent light dye properties and it does not degrade. Some Schiff bases act as a tester or chromogenic reagent for determination of Ni or any other metals present in some natural food samples.
3.3 Polymers
The synthesis and manufacture of polymers have become essential in today’s chemical world. Amine-terminated liquid natural rubber (ATNR) is created by photochemically breaking down natural rubber in solution with ethylene-diamine. ATNR and glyoxal are combined, to produce poly Schiff base, which increases antiaging properties. Organo cobalt complexes are used to emulsify and co-polymerize vinyl and divinyl monomers. Tridentate Schiff base serves as an initiator in this process. The alkylation of allylic substrates and the reduction of ketones to alcohols were two important uses of Schiff base complexes.
Polymerization of olefins has increased recently due to catalytic activity of Schiff base complexes in the synthesis of commercially important branched and linear polyethylene’s. High-temperature ring-opening polymerization of cycloalkenes with transition metals like ruthenium, molybdenum, and tungsten in the presence of agents like R4Sn or RAlCl2 is possible without affecting the molecular weight of the polymers. Schiff base complex catalyzed ring-opening polymerization of cycloalkenes at low temperature is allowed with control over the molecular weight of the polymers without any adverse reactions.
3.4 Chirality of Schiff bases
Catalysts used in Michael addition reaction are chiral Schiff base complexes of salen and binaphthyl. A new catalytic route for annulations reaction using Schiff base complexes is a current area of research, although the hetero annulations reaction has been reported.
The complexes of nickel (II) and copper (II) ions have increased enantioselectivity in the alkylation of enolates. According to this research, Schiff base complexes can act as catalysts to affect the yield and selectivity of chemical transformations. Hence, it is important to evaluate the applications of complexes and determine the appropriate function of Schiff base complexes in different processes.
According to the binding site of ligands with metal ions, it is classified as the mono, di, tri, tetra, and multi-dentate chelating ligands. The synthesis of chiral complexes become an important area of current research in coordination chemistry due to stereoselectivity of metal complexes of chiral Schiff base ligands in organic transformation. The chiral binaphthyl Schiff base ligands were potentially valuable in various metal-mediated catalytic reactions.
Chiral Schiff base ligands with titanium (IV), vanadium (IV), copper (II), or zinc (II), transition metal ions were used in various asymmetric chemical transformations. The addition of trimethylsilyl cyanide to benzaldehyde in the presence of titanium (IV) ions resulted in trimethylsilyl cyanohydrin 40–85% enantioselectivity.
3.5 Biological applications
It is known that ternary complexes are essential for the activation of enzymes as well as the storage and movement of active compounds. In the reaction, these components give a lot of surface area for the reactants to be absorbed and move them closer to one another. Biological activity has been revealed by the ternary and binary transition metal complexes [73]. Biological systems often include transition metal-ligand catalysts. Recently, metal complexes of certain N-/O donor ligands have received a lot of attention, due to their stronger antifungal and antibacterial properties than the parent ligands. Ternary complexes with an amino acid as a secondary ligand are important as they can serve as models for complexes between metal ions and enzymes.
A review of the literature indicated that the synthesis of heterogeneous metal complexes has received less attention. Some metal-ligand catalysts have been found to catalyze reactions such as oxidation, oxidative cleavage, and hydroformylation and decomposition of hydrogen peroxide. Phthalocyanines are widely used in diverse areas. Around the world, many scientists working on creating and developing new metal complexes to explore their potential biological and catalytic uses. Additionally, efforts are taken to synthesize the metal complexes to use in agriculture, medicine, and other industries. The study of various transition metal complexes and metal alkylated catalysts has attracted a lot of attention in recent years. The interest in the design and synthesis of novel transition metal complexes lies in their biological and catalytic activity in numerous reactions.
In chemical fields, there are immense applications of Schiff base alkylated groups and ligands, as catalysts or complexes. Schiff bases with variable donation sites could be monodentate, bidentate, tridentate, or tetradentate, forming mono- or polynuclear complexes. Tetradent Schiff base complexes are employed to design metal complexes related to synthetic and natural oxygen carriers.
3.6 Antiviral activities
Gossypol’s Schiff bases and their Ag (I) complexes have strong antiviral activity and inhibition against the Cucumber mosaic virus. In this modern era, producing antiviral medications is a significant task, and our research has overcome those obstacles. The synergistic activity of some tetra dentate Schiff base and its metal complexes with Mn (II), Ni (II), Cu (II), and Zn (II) demonstrate a variety of effects on a membrane in amylose synthesis, as well as therapeutic activities, anticancer, and cytotoxic properties [65]. Amylase transportation through the membrane was enhanced by Zn (II) and Mn (II) complexes, but it was inhibited by Ni (II) and Cu (II) complexes, which also possessed simple harmonic generation activity [74]. In the degradation of aldose-inferred Schiff bases carnosine and serine are effective trans-glycating agents.
3.7 Insecticide, pesticide, and repellent activity
Copper complexes of semicarbazones and phenolic hydrazone have been utilized as herbicides, insect poisons, nematocides, and rodenticides. Phenoxyacetic corrosive and its subordinates are used as herbicides. Aryloxyacetate anions act as corrosive ligands while (carboxylato) copper (II) complex have herbicidal activities. Pesticide exercises of Schiff’s bases like; N-(1-phenyl-2-hydroxy-2- phenylethylidine) etc. were synthesized from benzoin, salicylaldehyde, 2-aminophenol and 2, 4-dinitrophenyl hydrazine and their copper complex has been concentrated against Tribolium castaneum.
Complex of copper and N-salicylidene-O, S-dimethyl thiophosphorylimine (HSMa) showed a lot higher pesticide rates. Metal like aluminum, cobalt, and mercury are also used as insecticide and pesticide chemicals. Many natural extracts that can have chemical composition with aromatic compounds were used as an effective repellent for many insect pests. Schiff base containing metal complexes was used as a contact poison.
4. Conclusion
Many chemists around the globe are busy designing and synthesizing novel metal complexes to test their possible biological/catalytic applications. Efforts are also directed toward the synthesis of metal complexes for other industrial, agricultural, and medicinal applications. Recent years have witnessed a great deal of interest in the research of different types of metal alkylated catalysts and metal complexes of transition elements. The interest in the design and synthesis of novel transition metal complexes containing metals lies in their biological and catalytic activity in many reactions. Many metal complexes possess interesting biological properties, such as antibacterial and antitumor activities.
There are numerous applications for the complexes of Schiff base alkylated groups or catalysts made, depending on the ligands and group characteristics. Mono- or polynuclear complexes can be formed by Schiff bases with varying donation sites, which can be bidentate, tridentate, tetradentate, or monodentate. To synthesize metal complexes attached to artificial and natural oxygen carriers, tetradent Schiff base complexes are used.
Schiff-base ligand-based metallic complexes have diverse and interdisciplinary uses, so it is a very important field of study. Specifically, the study of Schiff base complexes in biology and catalysis has demonstrated their tremendous potential as potent antimicrobial and anticancer agents and efficient catalysts for various chemical reactions.
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