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- Herausgeber: Wiley-VCH GmbH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Presenting an important topic in a concisely and systematically way, this book covers every important aspect from synthesis to applications in catalysis, medicine and industry. It is a high quality resource for every researcher in the field.
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Seitenzahl: 321
Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Title Page
Copyright
Preface
Part I: Introduction
1 Historical Background
1.1 Introduction
1.2 Theories of Coordinate Bond
References
2 Classification
2.1 Ligands
2.2 Schiff Base
2.3 Types of Schiff Base
2.4 Different Bonding Modes of Schiff Bases
References
3 Different Routes of Synthesis
3.1 Formation of Schiff Bases
References
4 Schiff Base Metal Complexes
References
5 Effect of Different Parameters on Schiff Base and their Metal Complex
5.1 Ionic Charge
5.2 Ionic Size
5.3 Nature of Central Metal Ions
5.4 Nature of the Ligand
References
6 Thioether and Chiral Schiff Base
6.1 Thioether Schiff Base
6.2 Chiral Schiff Base
References
Part II: Synthesis
7 General Routes of Synthesis
7.1 Introduction
7.2 Mechanism of the Synthesis of Schiff Base Ligand
7.3 Problems Found in Conventional Method – Hydrolysis of C=N Bond
References
8 Different Route of Synthesis of Schiff Base‐Metal Complexes
8.1 Introduction
8.2 Different Chemical Routes
8.3 Different Methods
References
9 Synthesis and Mechanism of Schiff Base‐Metal Complexes
9.1 Introduction
9.2 Synthesis of Schiff Bases Metal Complexes
9.3 Synthesis of Some of the Schiff Base Metal Complexes
References
10 Synthesis and Mechanism of Chiral and Achiral Schiff Base and Their Metal Complexes
10.1 Introduction
10.2 Synthesis of Chiral and Achiral SB Ligand
10.3 Synthesis of Chiral SB Metal Complexes
10.4 Chiral Schiff Bases of Titanium, Zirconium, and Vanadium
10.5 Chiral Schiff Bases of Main Group Metals
References
11 Synthesis and Mechanism of Thioether: Schiff Base and Their Metal Complexes
11.1 Introduction
11.2 Chemical Synthesis Procedures
References
12 Computational Chemistry
12.1 Introduction
12.2 Application of DFT in the Field of Schiff Base and Their Metal Complexes
References
Part III: Application
13 General Applications of Schiff Bases and Their Metal Complexes
13.1 Catalyst
13.2 Biological and Medicinal Importance
13.3 Coatings
13.4 Analytical Chemistry
13.5 Dyes
13.6 Semi‐conducting Materials
13.7 Solar System
13.8 Photocatalyst
13.9 Polymer Chemistry
13.10 Agrochemical Industry
References
14 Application in Pharmacological Field
14.1 Introduction
14.2 Antimicrobial Activity
14.3 Antifungal Activity of Schiff Bases
14.4 Anticancer Activity of Schiff Bases and Their Metal Complexes
14.5 Antidyslipidemic and Antioxidant Activity
14.6 Anthelmintic Activity
14.7 Antitubercular Activity
14.8 Antidepressant Activity
14.9 Anticonvulsant Activity
14.10 Antioxidant Activity
14.11 Antiviral Activity
14.12 Anti‐inflammatory and Analgesic Activities
References
15 Application as Catalyst
15.1 Introduction
15.2 Coupling Reaction
15.3 Polymerization Reaction
15.4 Oxidation Reaction
15.5 Epoxidation Reaction
15.6 Ring‐Opening Epoxidation Reaction
15.7 Cyclopropanation Reaction
15.8 Hydrosilylation Reaction
15.9 Hydrogenation Reaction
15.10 Aldol Reaction
15.11 Michael Addition Reaction
15.12 Annulation Reaction
15.13 Diels–Alder Reaction
15.14 Click Reaction
15.15 Mannich Reaction
15.16 Ene Reaction
15.17 Summary
References
16 Application as Drug‐Delivery System
References
17 Chemosensors/Bioimaging Applications
17.1 Introduction
17.2 Chemosensors
17.3 Conclusion
References
18 Application in Industrial Field
18.1 Introduction
18.2 Current Status in India
18.3 Conclusion
References
Index
End User License Agreement
List of Tables
Chapter 14
Table 14.1 List of Schiff bases and their metal complexes.
List of Illustrations
Chapter 1
Figure 1.1 Crystal field splitting in tetragonally elongated, octahedral, an...
Figure 1.2 Molecular orbital energy‐level diagram of an octahedral complex....
Chapter 2
Figure 2.1 Structure of EDTA and Heme B.
Figure 2.2 Some examples of salen‐type ligands.
Figure 2.3 Some examples of salophen‐type ligands.
Figure 2.4 General formula of hydrazine‐type ligands.
Figure 2.5 Some examples of hydrazine‐type ligands.
Figure 2.6 Some examples of thiosemicarbazone/carbazone‐type ligands.
Figure 2.7 Some examples of heterocyclic Schiff bases.
Figure 2.8 Example of monodentate Schiff base.
Figure 2.9 Examples of bidentate Schiff bases.
Figure 2.10 Examples of tridentate Schiff base.
Figure 2.11 Examples of tetradentate Schiff base.
Figure 2.12 Example of pentadentate Schiff base.
Figure 2.13 Example of hexadentate Schiff base.
Scheme 2.1 General mechanism for the synthesis of Schiff base.
Scheme 2.2 General methodology for the preparation of Salen ligand.
Scheme 2.3 General scheme for the synthesis of salophen‐type ligand.
Scheme 2.4 General scheme for the synthesis of thiosemicarbazone/carbazone‐t...
Chapter 3
Scheme 3.1 Mechanistic pathway of imine formation.
Scheme 3.2 Different synthetic routes of Schiff base complexes.
Scheme 3.3 Synthesis and proposed structure of macrocyclic complexes.
Scheme 3.4 Synthesis and proposed structure of heterocyclic complexes.
Chapter 4
Figure 4.1 (a) Proposed structure of complexes by Raman et al. [7]. (b) Prop...
Figure 4.2 Schematic representation of some complexes [19, 20].
Figure 4.3 Proposed structure of Ni metal complex [21].
Figure 4.4 Proposed structure of some metal complexes [25–27].
Chapter 6
Figure 6.1 Some sulfur drugs containing thioether.
Figure 6.2 Some examples of organic bonded sulfur.
Figure 6.3 Some examples of sulfur‐containing proteins.
Figure 6.4 Some examples of sulfur‐containing natural products.
Figure 6.5 Examples of thioether Schiff bases.
Figure 6.6 Examples of chiral Schiff bases.
Chapter 7
Figure 7.1 Schiff base.
Scheme 7.1 Schiff base formation (where R
1
, R
2
, R = alkyl or aryl groups).
Scheme 7.2 Mechanism of synthesis of SB ligand.
Scheme 7.3 Reaction mechanism of Schiff base ligand in acid medium.
Scheme 7.4 Reaction mechanism of Schiff base ligand in basic medium.
Scheme 7.5 Aldol‐like condensation of imines with aldehydes.
Scheme 7.6 Synthesis of polymeric compounds with 1° aliphatic aldehydes.
Scheme 7.7 Oxidative synthesis of imines from alcohols and amines.
Chapter 8
Scheme 8.1 Oxidative addition of amines to imines.
Scheme 8.2 Organometallic reagents like RMgX or RLi addition to cyanides.
Scheme 8.3 Reaction of phenol and nitriles to form ketimines.
Scheme 8.4 Synthesis of ketimines from phenols or their ethers.
Scheme 8.5 Synthesis of ketamine from alkali metals and aromatic ketones.
Scheme 8.6 9‐Aminofluorine to imine by ammonia and potassium amide.
Scheme 8.7 Synthesis of α‐cyanoimine by alkali metal amide with dinitriles....
Scheme 8.8 Synthesis of imines and nitrones by active hydrogen compounds.
Scheme 8.9 Synthesis of anil camphor.
Scheme 8.10 Oxidation of metal amines into imines.
Scheme 8.11 Synthesis of ketimines from oximes.
Scheme 8.12 Synthesis of imines from nitriles.
Scheme 8.13 Reduction of aromatic nitriles by LiAlH
4
.
Scheme 8.14 Reduction of α‐nitrostyrenes to produce imine.
Scheme 8.15 Synthesis of Schiff Base from ketals.
Scheme 8.16 Synthesis of imines from olefins and tertiary alcohols with hydr...
Scheme 8.17 Conversion of α‐amino acids to imines.
Scheme 8.18 Synthesis of
N
‐boryl imines and
N
‐aluminum imines.
Scheme 8.19 Synthesis of
N
‐metallo imines of Li and Mg.
Scheme 8.20 Preparation of
N
‐silylimines via reaction of lithium hexalkyldis...
Scheme 8.21
N
‐alkylsilyl imines via hydrosilylation of nitrile.
Scheme 8.22 Synthesis of silylimines from (a)
N
‐chlorosilylamines; (b) α‐cya...
Scheme 8.23 Synthesis of tin imines.
Scheme 8.24 Conventional or classical method of synthesis of Schiff base.
Scheme 8.25 Synthesis of SBs using ethanol as solvent.
Scheme 8.26 Synthesis of SBs using microwave method.
Scheme 8.27 Synthesis of SBs using water as solvent under microwave irradiat...
Scheme 8.28 Synthesis of SB using grindstone method.
Scheme 8.29 SB synthesis using ultrasonication method.
Scheme 8.30 Ultrasound method for SB synthesis using lemon juice as catalyst...
Scheme 8.31 Example of SBs synthesis using ultrasound method.
Scheme 8.32 Ultrasonic method of synthesis of SB.
Scheme 8.33a Reaction of Schiff base synthesis in presence of green acid cat...
Scheme 8.33b General reaction and mechanism for Schiff base formation in aci...
Scheme 8.34a Synthesis of Schiff base by CES.
Scheme 8.34b Possible mechanism of Schiff base synthesis by CES.
Scheme 8.35a Synthesis of
N
‐benzylideneaniline derivatives by kinnow peel po...
Scheme 8.35b Plausible mechanism of SB synthesis by kinnow peel powder.
Chapter 9
Scheme 9.1 Synthesis of Cu‐complex using template method.
Scheme 9.2 Synthesis of Schiff base metal complexes using metal alkoxides.
Scheme 9.3 Synthesis of metal complex using metal amides.
Scheme 9.4 Synthesis of metal complex using metal alkyl complexes.
Scheme 9.5 Synthesis of metal complex using metal acetate salts.
Scheme 9.6 Synthesis of metal complex using metal halides.
Scheme 9.7 Synthesis of mononuclear metal complexes of a SB ligand.
Scheme 9.8 Example of dinuclear SB metal complexes of Cu, Ni, Co, and Zn in ...
Scheme 9.9 Synthesis of mononuclear octahedral Ti(IV) complexes of a SB liga...
Scheme 9.10 Synthesis of mononuclear distorted square‐antiprism Zr(IV) compl...
Scheme 9.11 Octahedral SB metal complexes.
Chapter 10
Figure 10.1 New chiral SB ligands using chiral amines.
Figure 10.2 Second‐generation Salen complexes.
Figure 10.3 Formation of Salen complex in the presence of axial ligand.
Figure 10.4 Fluxional octahedral complex of SB.
Figure 10.5 A chiral Mn‐SB metal complex.
Figure 10.6 Cr‐Salen complex using CrCl
3
in THF.
Scheme 10.1 Synthesis of an unsymmetrical chiral Schiff base ligand.
Scheme 10.2 Synthesis of chiral Schiff base ligands from (1R)‐(+)‐camphor.
Scheme 10.3 Synthesis of achiral SB.
Scheme 10.4 Synthesis of achiral SB.
Scheme 10.5 Synthesis of achiral SB.
Scheme 10.6 Synthesis of achiral SB.
Scheme 10.7 Synthesis of achiral SB.
Scheme 10.8 Synthesis of Al‐SB metal complexes.
Scheme 10.9 Synthesis of a cationic chiral tridentate Schiff base chromium c...
Scheme 10.10 Direct synthesis of Sm‐SB.
Scheme 10.11 Synthesis of Si and Sn Schiff base complexes.
Scheme 10.12a Mechanism of formation of 2A.
Scheme 10.12b Mechanism of synthesis of a racemic mixture of Si‐SB complex....
Chapter 11
Scheme 11.1 Synthesis of SB by Wang et al. [7].
Scheme 11.2 Synthesis of SB by Kalita et al. [8].
Scheme 11.3 Synthesis of SB by Patra et al. [9].
Scheme 11.4 Synthesis of SB by Bhanja et al. [11].
Scheme 11.5 Synthesis of SB by Dehghani‐Firouzabadi et al. [3].
Scheme 11.6 Synthesis of SB metal complexes.
Scheme 11.7 Synthesis of Schiff base ligand.
Scheme 11.8 Synthesis of metal complexes.
Scheme 11.9 Synthesis of SB by Biswas et al. [13].
Scheme 11.10 Synthesis of SB rhodium complex.
Scheme 11.11 Synthesis of SB by rhodium complex [14].
Scheme 11.12 Synthesis of SB by rhodium complex by Dehghani‐Firouzabadi and ...
Scheme 11.13 Synthesis of SB by Pattanayak et al. [16].
Scheme 11.14 Synthesis of SB complex.
Scheme 11.15 Synthesis of SB by Pattanayak et al. [16].
Scheme 11.16 Synthesis of SB by Tamizh et al. [18].
Chapter 12
Figure 12.1 Optimized geometries of the palladium complexes. Source: Reprodu...
Figure 12.2 HOMO and LUMO orbitals of palladium complexes. Source: Reproduce...
Figure 12.3 Optimized geometries of the complexes. Bond lengths are in Å, an...
Figure 12.4 Frontier Kohn–Sham orbitals and the energy separation between th...
Figure 12.5 Optimized geometries of the palladium complexes. Source: Reprodu...
Figure 12.6 FMOs and the energy difference between the highest SOMO and LUMO...
Chapter 13
Figure 13.1 General applications of Schiff base and their metal complexes.
Chapter 14
Figure 14.1 Various applications of Schiff base and their metal complexes in...
Chapter 15
Scheme 15.1 Suzuki–Miyaura reaction using amino‐salicyldimmine‐Pd(II) comple...
Scheme 15.2 Palladium tridentate complex (
C6
) catalyzed the Suzuki–Miyaura r...
Scheme 15.3 Polystyrene‐supported SB complex for Heck reaction.
Scheme 15.4 Ni‐based SB complex (
C8
) for C–S cross‐coupling reaction.
Scheme 15.5 Ti‐based SB complex (
C9
) for Buchwald–Hartwig amination reaction...
Scheme 15.6 Polythene formation using pyridine bis(imine) complexes.
Scheme 15.7 ROP of
ɛ
‐caprolactone for the formation of polyester.
Scheme 15.8 Oxidation of sulfide to sulfoxide.
Scheme 15.9 Oxidation of alcohol to aldehyde.
Scheme 15.10 Asymmetric Baeyer−Villiger oxidation reaction.
Scheme 15.11 Asymmetric epoxidation reaction.
Scheme 15.12 Asymmetric epoxidation catalyzed by di‐μ‐oxo‐titanium‐Salen cat...
Scheme 15.13 Asymmetric meso epoxide ring‐opening reaction catalyzed by Cr‐S...
Scheme 15.14 Enantioselective ring‐opening epoxidation catalyzed by tridenta...
Scheme 15.15 Enantioselective Ir‐Salen complex catalyzed cycloproparation re...
Scheme 15.16 Copper SB‐complex‐catalyzed olefin cycloproparation reaction.
Scheme 15.17 Hydrosilylation of ketones catalyzed zinc SB complexes.
Scheme 15.18 Fe‐catalyzed asymmetric hydrosilylation reaction.
Scheme 15.19 Hydrosilylation of alkynes catalyzed by Co‐SB complexes.
Scheme 15.20 Transfer hydrogenation of acetophenone catalyzed by Ru‐based SB...
Scheme 15.21 Rh‐complex catalyzed transfer hydrogenation of ketones.
Scheme 15.22 Vinylogous aldol reaction of dienolate catalyzed by Ti‐SB compl...
Scheme 15.23 Direct catalytic asymmetric aldol reaction promoted by dinuclea...
Scheme 15.24 Heterobimetallic Ga/Yb‐SB complex catalyzed aldol reaction.
Scheme 15.25 Bimetallic Co
2
‐catalyzed asymmetric Michael addition reaction o...
Scheme 15.26 Michael addition reaction of malonates to cyclic
α
,
β
‐...
Scheme 15.27 Vanadium‐SB complex catalyzed annulation reaction.
Scheme 15.28 Chiral Cr‐Salen complexes catalyzed Diels–Alder reaction.
Scheme 15.29 Asymmetric Diels–Alder reaction catalysed by cobalt‐Salen compl...
Scheme 15.30 Copper‐SB complexes catalytic click reaction.
Scheme 15.31 Catalytic asymmetric Mannich reaction.
Scheme 15.32
Syn
‐selective catalytic asymmetric nitro‐Mannich reactions.
Scheme 15.33 Ene reaction of alkoxy or silylopropene with aldehydes.
Scheme 15.34 Enantioselective carbonyl‐ene reactions catalyzed by Salen comp...
Chapter 16
Figure 16.1 Types of DNA binding of small molecules.
Figure 16.2 Proposed structures compounds exhibiting DNA‐binding activity pr...
Figure 16.3 Proposed structures of Schiff base ligands exhibiting DNA‐bindin...
Figure 16.4 Proposed structures of Schiff base ligands and metal complexes e...
Chapter 17
Figure 17.1 Types of chemosensing.
Figure 17.2 (a) Explosives sensing, (b) oxygen sensing, (c) high pH sensing,...
Figure 17.3 Schematic diagram to trace metal levels in human body using fluo...
Figure 17.4 Chemosensors for biological applications.
Figure 17.5 Schematic diagram of the preparing CdSe/ZnS QDs 21 and its detec...
Figure 17.6 Schematic diagram of the preparing fluorophore hybridization and...
Figure 17.7 Schematic diagram of the preparing 54 and its detection method t...
Figure 17.8 Confocal microscopy images of live HeLa cells. [30] / Reproduced...
Figure 17.9 In vivo imaging of Sn
2+
by CK on mouse. [30] / Reproduced fr...
Figure 17.10 In vivo imaging of living
C. elegans
. [16] / Reproduced from Sp...
Figure 17.11 Modifications in the absorption and fluorescence of 1o (2.0 × 1...
Figure 17.12 In vivo imaging of Sn
2+
by CK on mouse. [31] / Reproduced f...
Figure 17.13 Live HeLa cells fluorescing while being treated in various envi...
Guide
Cover Page
Title Page
Copyright
Preface
Table of Contents
Begin Reading
Index
Wiley End User License Agreement
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Schiff Base Metal Complexes
Synthesis and Applications
Edited by Pranjit Barman and Anmol Singh
Editors
Prof. Pranjit Barman
National Institute of Technology Silchar
Department of Chemistry
788010 Cachar, ASSAM
India
Anmol Singh
National Institute of Technology Silchar
Department of Chemistry
788010 Cachar, ASSAM
India
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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<http://dnb.d-nb.de>.
© 2023 WILEY‐VCH GmbH, Boschstraße 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978‐3‐527‐35070‐4
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ePub ISBN: 978‐3‐527‐83946‐9
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Cover Images: © KC_Woon/pixabay; Courtesy of Pranjit Barman
Cover Design: Wiley
Preface
Schiff bases (SBs) are extensively used as organic compounds that coordinate with metal ions through imine or azomethine nitrogen (C=N) and are subjected to subsidizing an ample spectrum of biological behaviors to these azomethinic compounds. SBs have wide applications in various fields including organic and inorganic chemistry, analytical, catalysis, pharmaceutical, biological, food and dye industry, pharmacological such as anti‐inflammatory, antitumor, anticonvulsant, antifungal, analgesic, antibacterial, antimalarial, antiviral, anthelmintic, anti‐oxidant, and so forth. SB effortlessly forms stable complexes with main group elements, transition metals, lanthanides, and actinides as they easily retain tenability of their stereo‐electronic structures, which is why it is also regarded as “privileged ligand.”
Many coordination complexes have been used in medicine contain metals such as platinum (as cisplatin, anticancer chemotherapy drug); gold (as auranofin, used for rheumatoid arthritis); technetium and rhenium (as radiopharmaceuticals used in imaging and radiotherapy); manganese and gadolinium (used in magnetic resonance imaging); ruthenium (as an anticancer drug); lithium (as Li2CO3 to treat prophylaxis of manic‐depression behavior); bismuth (bismuth salicylate used as an antacid); iron, vanadium, and titanium (used to treat cancer as they react with DNA specifically in tumor cells); lanthanum (lanthanum carbonate used as a phosphate binder in patients suffering from chronic kidney disease); gold, silver, and copper (as anticancer drugs); gallium; lutetium.
Over the past decade, substantial emphasis has concentrated on the evolution of SB metal complexes owing to their DNA‐binding properties, catalytic activities, electroluminescent properties, sensors, organic photovoltaic materials, fluorescence properties, energy materials, synthesis of polymeric materials, and biological activities. This area has been the subject of numerous reviews due to its distinct and peculiar structural properties, particularly its distinguished catalytic activity, selectivity, and stability, along with various physicochemical properties. The coordination of both essential and nonessential elements with these versatile ligands can lead to alterations in their physiochemical and biological properties, which depicts them as competent contenders to use judiciously in various fields.
This book is a collection of the experiences assembled during the experiments performed in the laboratory. Since the author has more than fifteen years of experimental and theoretical experience on this topic as well as published more than 50 research articles in reputed journals. So, all the information, as well as the experience gathered from the students and research scholars, is going to be highlighted in detail.
This book covers up‐to‐date information and discussion based on the authors' experiences and references. The difficulties and technicalities faced by students and research scholars in the laboratory can be solved through this book.
The authors express the data and information concisely, systematically, and orderly so that the readers will get benefitted in all aspects related to SBs and their metal complexes.
The book is written for undergraduates, Ph.D. students, postdocs, and researchers in academia and industry.
This book highlights the SB metal complexes and includes synthesis and applications of SBs and their metal complexes.
This book also covers the biological applications and stereochemistry‐related information of SBs and their metal complexes.
The methodologies discussed in this book are uncomplicated, easy, and unconventional. We have discussed green methodologies like a solid support system, microwave synthesis, and solvent‐free methodology for the synthesis of SBs and their metal complexes.
This book also discusses various applications, such as DNA binding and fluorescence, of SBs and their metal complexes. This book highlights computational or theoretical studies on the reactivity of the SBs and their metal complexes depending on the steric effect, electronic effect, and field effect. This book also focuses on some of the tips or drawbacks of the methods and applications so far, so that research scholars as well as industry people can do more work in the near future.
Silchar, October 2022
Pranjit BarmanAnmol Singh
Part IIntroduction
1Historical Background
Anmol Singh, Himadri Priya Gogoi, and Pranjit Barman
National Institute of Technology Silchar, Department of Chemistry, Silchar, Assam, 788010, India
1.1 Introduction
Over the past few decades, chemistry of coordination compounds has expanded and diversified considerably. Coordination chemistry accomplished a significant interdisciplinary place at the interface of numerous fields, which serves as a connecting link between the different classes of chemical, physical, and biological sciences. The precise date that the first coordination compound was prepared is challenging to ascertain. Johann Jacob Diesbach, a color manufacturer, discovered the blue pigment known as Prussian blue in the seventeenth century, which is the beginning of coordination complex chemistry [1]. Chromium was discovered in 1798 by the German researchers Lowitz and Klaproth separately in a sample of a heavy black rock located near north of the Beres of Mines. The chromium in the same mineral from a small deposit in the Var region of South‐eastern France was discovered by another German chemist Tassaert in 1799. He identified this mineral as the Cr–Fe spinel, also known as chromite (FeO·Cr2O3) [2].
Alfred Werner's findings marked the beginning of an insight into coordination compounds and their varied properties. Werner's coordination theory in 1893 was the first effort which focused on the quantity and type of groups connected to the central metal ion to characterize the bonding and interpretation of coordination complexes. In 1913, Werner's idea and meticulous work over the ensuing 20 years earned him the Noble Prize in Chemistry. Due to the development of sophisticated physicochemical techniques of high accuracy and precision, his fundamental theories regarding the stereochemistry of metal complexes, mechanisms of isomerization, and racemization, etc. prevail indisputable even now despite all the advancements that have occurred since his time and through the last 50 years [3, 4]. These have greatly improved our knowledge of the attributes of metal–ligand connection, the configuration and stereochemistry of metal complexes, and their stabilities, liabilities, and other characteristics [5, 6]. Werner's theory became a key component of “The Electronic Theory of Valency” proposed by G.N. Lewis (1916) and extensively related to coordination compounds by N.V. Sidgwick (1927), replacing the majority of the earlier concepts presented by Berzelius (1819), Grohen (1837), Claus (1856), Blomstr (1869), and Jorgenson (1894).
Coordination chemistry has expanded in three dimensions, taking into consideration its breadth, depth, and applications. The five Noble awards that significantly impact the topic reflect the sustained respect for the emerging science (A. Werner, 1913; M. Eigen, 1967; Wilkinson and Fischer, 1973; H. Taube, 1983; Cram, Lehn and Pedersen, 1987). In the list, Werner and Cram, Lehn & Pedersen acknowledged the ancient and new domains of coordination chemistry [7].
A coordination compound, also known as a metal complex, comprises an ensemble of bonded ions or molecules, regarded as ligands, and a central ion or atom called a coordination center, which is typically a metal ion. There are reasonably profound changes in the properties of a metal ion on the complex formation, which reflects in structures, stereochemistry, stability, and many other properties. It is necessary to understand the properties of bond between the metal ion and ligand to interpret the design of complex.
1.2 Theories of Coordinate Bond
1.2.1 Valence Bond Theory
According to Lewis and Sidgwick theory, ligands donate an electron pair to a metal ion to establish a coordinate bond [8, 9]. In 1931, L. Pauling expanded this theory to comprehend and predict various coordination complex traits, including magnetic behavior, stereochemistry, kinetics, and other physical and other chemical characteristics [10]. This hypothesis states that a coordinate bond between the metal and ligand leads to the complex formation of the duo. It describes how s, p, or d orbitals, as well as hybridized orbitals, i.e. sp3, d2sp3 or sp3d2, or dsp [2], etc. generate strong bonds. Tetrahedral, octahedral, and square planar stereochemistry, each dealt by coordinate covalent bonds, formed by metal atoms having sp3, d2sp3/sp3d2, or dsp [2] hybrid orbitals with the ligand orbitals. Through semi‐quantitative calculations, Pauling and Gould elucidated the stability of complexes and concluded that the formation of sigma bonds among ligand and metal from electron donation occurs in carbonyl and nitrosyl compounds [10]. Back donation, i.e. dπ–pπ linkage amid metal and ligand, justifies the stability of carbonyl and nitrosyl compounds.
1.2.2 Crystal Field Theory
The splitting of d or f orbitals is explained by crystal field theory (Figure 1.1). Degeneracies of electron orbital states are typically brought on by a static electric field induced by the distribution of charges nearby. Hans Bethe and John Hasbrouck van Vleck, two scientists, proposed a theory that describes several spectroscopies of transition metal coordination complexes, specifically optical spectra (colors) [11, 12]. Although it makes an effort to characterize and explain bonding, crystal field theory (CFT) is effective in defining colors, magnetic characteristics, and hydration enthalpies of transition metal complexes. The attraction among the positively charged metal cation and ligands leads to the interaction between a transition metal and ligands. Owing to the attraction between like charges, the electrons in the ligand's d‐orbital resist one another, splitting the energy of the d‐orbital. The following factors have an impact on this splitting:
Figure 1.1 Crystal field splitting in tetragonally elongated, octahedral, and square planar filled.
Concerning the spherical field, the distribution of ligands encircling metal, the coordination number of the metal ion, such as tetrahedral or octahedral, and the type of ligands, a greater oxidation state of the metal causes considerable splitting.
Tetrahedral symmetry hinders the ligand electrons from approaching d‐orbitals directly. The higher energy orbitals are d
z
2
and d
x
2
−
y
[2]
, while lower energy orbitals are d
xy
, d
yz
, d
xz
. The energy splitting will be less than in an octahedral field. CFT can also be used to explain complex geometry.
1.2.3 Molecular Orbital Theory
The electronic structure of molecules is described using quantum mechanisms. The electrons are not allocated to specific atom‐to‐atom linkage; instead, they are regarded as moving under the influence of the nuclei in the entire molecule. In 1935, molecular orbital theory (MOT) stated that molecular orbitals were formed by linear combinations of atomic orbitals. Atomic orbitals must overlap within space and cannot form molecular orbitals if they are too apart. The electrons in an atom may be either waves or particles; therefore, electrons can be considered as accommodating an atomic orbital or a wave function ψ, which is a solution of Schrodinger wave equation. The overlapping atomic orbitals must have nearly the same energy; maximum overlapping must have symmetry concerning the molecular bonding axis [13, 14].
1.2.4 Ligand Field Theory
The orbital configuration, bonding, and other properties of coordinating complexes are explained by ligand field theory (Figure 1.2). To formulate the more extensive and realistic ligand field theory, which describes the process of chemical bonding in transition metal complexes, the MOT and CFT were later integrated. This theory which elucidates the loss of degeneracy of metal d‐orbitals in transition metal complexes was developed by incorporating the tenets of CFT and MOT. Ligand field theory originated in the 1930s by John Hasbrouck Van Vleck and Orgel to explain the results using CFT and visible spectra of transition metal complexes [12,15–18]. The vacant d‐orbitals of transition metals facilitate bonding, which affects the colors they absorb in solution. Depending on the intensity of the surrounding ligands, the various d‐orbitals are influenced differentially and have their energy raised or diminished. The symmetry orbitals of the ligands in octahedral complexes generate bonding and antibonding combinations through the dz2 and dx2−y2 orbitals as a result of approaching along the x‐, y‐, and z‐axes. In contrast, the dxy, dxz, and dyz orbitals remain nonbonded [19, 20].
Figure 1.2 Molecular orbital energy‐level diagram of an octahedral complex.
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2Classification
Anmol Singh, Himadri Priya Gogoi, and Pranjit Barman
National Institute of Technology Silchar, Department of Chemistry, Silchar, Assam, 788010, India
2.1 Ligands
The terminology “ligand” has two distinct meanings. At first, ligands can act as donor atoms by being present in molecules and connected to the core metal ions, or the entire ligand can be employed as a molecule [1]. Second, a ligand coupled to a central metal ion forms an initial ionic or covalent interaction with one or more donor atoms.
The number of donor atoms in a ligand can be used to distinguish between various ligands.
One donor atom is considered to be a monodentate ligand.
Two donor atoms are considered to be a bidentate ligand.
One or more donor atoms are considered to be a polydentate ligand. These ligands are coupled to the central metal ion to generate complexes with cyclic structures.
The chelating agents that arise to ring‐structured complexes are known as bidentate and polydentate ligands.
Additionally, bidentate ligands are divided into two categories:
Symmetrical bidentate ligand with comparable donor atoms,
Unsymmetrical bidentate ligand with two different donor atoms.
The ability to generate complexes exists for almost all of the metals in the periodic table. Numerous chelating substances can utilize nonmetallic elements from groups V and VI as donor atoms to bind with metal ions. The most prevalent examples of donor atoms include halides, oxygen, nitrogen, and phosphorus (Figure 2.1) [2].
2.2 Schiff Base
German chemist Schiff Hugo Josef first discovered the Schiff base in 1864. It has the general formula R—CH=N—R′, where R and R′ may be aryl or alkyl groups [3]. Schiff bases are the condensation product of an aldehyde or ketone with a primary aromatic amine that contains an electron‐donating group known as azomethine group (—CH=N—) group [4, 5]. They are characterized by the occurrence of azomethine group [6, 7]. Through azomethine nitrogen, these molecules coordinate with metal ions, and these complexes have undergone substantial research [8]. Schiff bases are prominent ligands due to the relevance of these complexes in analytical, chemical, biological, and industrial fields. These complexes comprising of oxygen, nitrogen, sulfur, etc. have a wide range of biological characteristics [9].
Figure 2.1 Structure of EDTA and Heme B.
The following chemical scheme is illustrated for the synthesis of Schiff bases:
First, this mechanism (Scheme 2.1) was recognized by Laurent and Gerhart in 1850. Schiff bases are weak bases that can only be synthesized by using mineral acid to remove water. Alkyl substituents are significantly less stable in the Schiff bases than aryl substituents [10]. Compared to Schiff bases derived from aliphatic aldehydes and having efficient conjugation, those formed from aromatic aldehydes are more stable [11–13]. Aliphatic aldehydes can easily polymerize Schiff bases, which renders it challenging to isolate them in their pure state and results in a product with a complicated structure [14, 15].
Scheme 2.1 General mechanism for the synthesis of Schiff base.
Schiff bases have a multitude of bonding sites as well as can donate a proton or electron to generate high polyhedrons or complexes. Examining the coordination compound geometries that exhibit higher kinetic and thermodynamic stability with different transition metal ions is crucial. It serves as the framework of structural chemistry. Polydentate Schiff bases are more efficient in achieving high coordination structures because of their compact nature. Consequently, coordination compounds with metal ions are prepared using a bidentate, tridentate, or tetradentate of ligands. A five‐ or six‐membered ring structure with a metal ion can be formed when functional groups like —NO2, —SH, and —OH are in close proximity with the azomethine group to act as a chelating agent.
The use of Schiff bases in optical and electrochemical sensors and in various chromatographic techniques allows for better selectivity and sensitivity [16–18]. Among the organic reagents used, Schiff bases have excellent properties, a comparatively simple preparation method, and the flexibility of synthesis that enables to design of appropriate structural properties [19, 20].
Numerous organic, inorganic, biological, and analytical chemistry domains utilize Schiff bases synthesized from aromatic aldehydes and aromatic amine [21]
