ABSTRACT
2-[(E)-[3-[(E)-(2-hydroxyphenyl)methyleneamino]phenyl]iminomethyl]phenol was synthesized from the condensation reaction of 1,3-diaminobenzene and 2-hydroxyzaldehyde in dimethylformaldehyde (DMF). Its coordination characteristic with vanadium(III) and vanadium(V) complexes was studied via, UV/Visible, IR and NMR spectroscopy; stoichiometric, melting point and conductivity determinations. The analytical data of these complexes and the mode of bonding show that, the ligand acted as a tetradentate ligand via coordination through the two azomethine nitrogen and the two hydroxylic oxygen. Mole ratio method indicated a 1:1 ligand to metal ratio for the complexes. Vanadium(III) and vanadium(V) were determined spectrophotometrically by measuring their absorbance at 400 and 405 nm respectively. From the calibration curve, Beer’s law was valid for vanadium(III) and vanadium(V) between 0.488 – 3.904 ppm. The calibration and analytical sensitivity of vanadium(III) complex is 0.074 and 0.32 while that of vanadium(V) is 0.024 and 24 respectively. Optimum pH for the formation of the complex was determined to be 10 and 11 for vanadium(III) and vanadium(V) respectively. Very few elements were found to interfere with the method. The method was successfully applied in the determination of vanadium in steel.
TABLE OF CONTENTS
Title page – – – – – – – – – – i
Approval – – – – – – – – – – ii
Certification page – – – – – – – – – iii
Acknowledgement – – – – – – – – – iv
Table of Content – – – – – – – – – v
List of Tables – – – – – – – – – – ix
List of Figures – – – – – – – – – x
List of Abbreviations – – – – – – – – – xi
Abstract – – – – – – – – – – xii
CHAPTER ONE
- INTRODUCTION – – – – – – – – 1
1.1 Spectrophotometry – – – – – – – – 1
1.2 Beer – Lambert’s Law – – – – – – – 3
1.3 Ultra Violet (UV) Spectrophotometry – – – – – 5
1.4 Infrared (IR) Spectrophotometry – – – – – – 5
1.5 Nuclear Magnetic Resonance (NMR) – – – – – 5
1.6 Schiff Base – – – – – – – – – 6
1.7 Schiff Base Syntheses – – – – – – – – 8
1.8 Synthetic Important of Schiff Base – – – – – – 10
1.9 Phenylenediamine – – – – – – – – 16
1.10 Salicylaldehyde – – – – – – – – 16
1.11 Application of Schiff Base – – – – – – – 17
1.12 Biological Importance of Schiff Base – – – – – – 18
1.13 Schiff Base in Transition metal complexes – – – – – 19
1.13.1 Application of Schiff Base in Transition Metal Complexes – – – 19
CHAPTER TWO
- LITERATURE REVIEW – – – – – – – 22
- Introduction – – – – – – – – – 22
2.2 Methods Based on Complex Formation (With Schiff Base) – – – 23
2.3 Analytical Techniques for the Determination of Vanadium – – – 25
2.4 Spectroscopic Determination of Metals – – – – – 27
2.4.1 Atomic Absorption Spectroscopy – – – – – – 27
2.5 Other Methods for the Determination of Vanadium – – – – 29
2.5.1 Biological method of determining Vanadium metal – – – – 30
2.5.2 Determination of Vanadium in Environmental Samples – – – 30
2.6 The Coordination Chemistry of Vanadium – – – – – 33
2.7 Coordination Complexes of Vanadium(III) – – – – 33
2.8 Coordination Complexes of Vanadium(V) – – – – – 34
2.9 Importance of Vanadium in Living Organism – – – – – 36
2.10 Toxic Effects of Vanadium – – – – – – – 38
2.11 Equilibrium Constant for the Formation of Complexes in Aqueous Solution – 38
2.12 Stability Constant – – – – – – – – 40
2.13 Previous Work Done on Spectrophotometric Determination of Vanadium – 43
2.14 Determination of Stoichiometry of Complexes – – – – 45
2.15 Statement of the Problem – – – – – – – 46
2.16 Aims and Objectives – – – – – – – – 47
CHAPTER THREE
3.0 EXPERIMENTAL – – – – – – – – 49
3.1 Description of Apparatus – – – – – – – 49
3.2 Reagents – – – – – – – – 50
3.3 Preparation of stock solution – – – – – – – 50
3.4 Preparation of pH Solutions – – – – – – – 51
3.5 Synthesis of the Ligand 2-[(E)-[3-[(E)-(2-hydroxyphenyl)methyleneamino]phenyl]
Iminomethyl]phenol – – – – – – – – 53
3.5.1 Synthesis of Vanadium(III) and Vanadium(V) Complexes of 2-[(E)-[3-[(E)
(2-hydroxyphenyl)methyleneamino]phenyl]iminomethyl]phenol – – 53
3.5.2 Spectroscopic Determination of the Stoichiometry of the Complexes By
Slope-Ratio Method – – – – – – – – 53
3.6 General Procedure for the Complexation Studies – – – – 54
3.6.1 Effect of Time on Complexation – – – – – 54
- Effect of the Concentration of Reagent on the Formation of the Complexes 54
- Effect Of Temperature On the Formation of the Complexes – – 55
3.6.4 Effect of pH On the Formation of the Complexes – – – – 55
3.6.5 Effect of Interfering Ions on the Formation of the Complexe – – 55
3.6.6 Calibration Curve- Beer’s Law – – – – – – 55
3.6.7 Determination of Vanadium(III) and Vanadium(V) in Alloys with Flame Atomic
Absorption Spectrophotometry – – – – – – 56
3.6.8 Determination of Vanadium(III) and Vanadium(V) in Standard Alloys with UV
Spectrophotometry – – – – – – – – 56
3.6.9 Determination of Vanadium(V) in Standard Alloys with UV Spectrophotometry 56
3.6.10 Characterization of the Schiff Base Ligands and their Metal Complexes – 57
3.6.11 Melting Points determination – – – – – – 57
3.6.12 Ultraviolet-Visible Spectroscopy – – – – – – 57
3.6.13 Infrared Spectroscopy – – – – – – – 57
3.6.14 Nuclear Magnetic Resonance Spectroscopy – – – – – 58
3.6.15Conductivity Measurements – – – – – – – 58
CHAPTER FOUR
4.0 RESULTS AND DISCUSSIONS – – – – – – 59
4.1 Physical characterization and molar conductivity data of the ligand and its
V(III) and V(V) Complexes – – – – – – – 59
4.2 Spectroscopic Characterization of the ligand and its V(III) and (V) Complexes 59
4.2.1 Electronic Spectra – – – – – – – – 61
4.2.2 Infrared Spectral Properties – – – – – – – 63
4.3 Stoichiometry of the Complexes – – – – – – 63
4.3.1 Metal- Ligand Mole Ratio of V(III) Complexes – – – – 63
4.3.2 Metal- Ligand Mole Ratio of V(V) Complexes – – – – 67
4.3.3 Molecular Formulae and Structures of the Ligand and its Complexes
Vanadium(III) Complex – – – – – – – 70
Vanadium(V) Complex – – – – – – – 71
4.4 Heat of Solvent of Absorption of ligand- – – – – – 72
4.5 Complexation Studies – – – – – – – – 74
- Effect of Time on the Formation of the Complexes – – – – 74
- Effect of the Concentration of the Reagent on Formation of Vanadium(III)
Complex – – – – – – – – – 75
- Effect of Temperature on the Formation of the Complexes – – – 78
4.8 Effect of pH on the Absorbance of the Complexes – – – – 80
4.9 Effect of Interfering Ions on the Formation of V(III) and V(V) Complexes – 82
4.9.1 V(III) Complex – – – – – – – – 82
4.9.2 V(V) Complex – – – – – – – – – 84
4.10 Calibration Plots-Beer’s law Of V(III) and V(V) Complexes – – 86
4.10.1 V(III) Complex – – – – – – – – 86
4.10.2 V(V) Complex – – – – – – – 88
4.11 Analytical Characteristics of the Procedures – – – – – 89
4.12 Determination of Vanadium ions in Alloy – – – – – 89
4.12.1 Determination of vanadium in steel with Flame AAS – – – 89
4.12.2 Determination of V(III) in Steel with the Reagent/UV – – – 90
4.12.3 Determination of V(V) with the Reagent/ UV – – – – 90
4.13 Summary and Conclusion – – – – – – – 91
REFERENCES
APPENDIX A
APPENDIX B
CHAPTER ONE
INTRODUCTION
1.1 Spectrophotometry
In Chemistry, Spectrophotometry is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. Spectrophotometry deals with visible light, near-ultraviolet, and near-infrared, but does not cover time-resolved spectroscopic techniques1.
Spectrophotometry involves the use of a spectrophotometer. A spectrophotometer is a device for measuring light intensity which is a function of the light wavelength. Important features of spectrophotometers are spectral bandwidth and linear range of absorption or reflectance measurement1.
A spectrophotometer is commonly used for the measurement of transmittance or reflectance of solutions, transparent or opaque solids, such as polished glass, or gases. However, they can also be designed to measure the diffusivity on any of the listed light ranges that usually cover around 200nm – 2500nm using different controls and calibrations 2. Within these ranges of light, calibrations are needed on the machine using standards that vary in type depending on the wavelength of the photometric determination.
An example of an experiment in which spectrophotometry is used is in the determination of the equilibrium constant of a solution. For instance, a certain chemical reaction may occur in a forward and reverse direction where reactants form products and products break down into reactants. At some point, this chemical reaction will reach a point of balance called an equilibrium point. In order to determine the respective concentrations of reactants and products at this point, the light transmittance of the solution can be tested using spectrophotometry. The amount of light that passes through the solution is indicative of the concentration of certain chemicals that do not allow light to pass through 1,2 .
The use of spectrophotometers spans various scientific fields, such as physics, materials science, chemistry, biochemistry, and molecular biology.They are widely used in many industries including semiconductors, laser and optical manufacturing, printing and forensic examination, and as well in laboratories for the study of chemical substances. Ultimately, a spectrophotometer is able to determine, depending on the control or calibration, what substances are present in a target and exactly how much through calculations of observed wavelengths.
There are two major classes of devices: single beam and double beam. A double beam spectrophotometer compares the light intensity between two light paths, one path containing a reference sample and the other the test sample. A single beam spectrophotometer measures the relative light intensity of the beam before and after a test sample is inserted. Although comparison measurements from double beam instruments are easier and more stable, single beam instruments can have a larger dynamic range and are optically simpler and more compact3. Additionally, some specialized instruments, such as spectrophotometer built onto microscopes or telescopes, are single beam instruments due to practicality.
Historically, spectrophotometers use a monochromator containing a diffraction grating to produce the analytical spectrum. The grating can either be movable or fixed. If a single detector, such as a photomultiplier tube or photodiode is used, the grating can be scanned stepwise so that the detector can measure the light intensity at each wavelength (which will correspond to each “step”). Arrays of detectors, such as charge coupled devices (CCD) or photodiode arrays (PDA) can also be used. In such systems, the grating is fixed and the intensity of each wavelength of light is measured by a different detector in the array. Additionally, most modern mid-infrared spectrophotometers use a Fourier transform technique to acquire the spectral information. The technique is called Fourier Transform Infrared.
When making transmission measurements, the spectrophotometer quantitatively compares the fraction of light that passes through a reference solution and a test solution. For reflectance measurements, the spectrophotometer quantitatively compares the fraction of light that reflects from the reference and test samples. Light from the source lamp is passed through a monochromator, which diffracts the light into a “rainbow” of wavelengths and outputs narrow bandwidths of this diffracted spectrum. Discrete frequencies are transmitted through the test sample. Then the photon flux density (watts per meter squared usually) of the transmitted or reflected light is measured with a photodiode, charge coupled device or other light sensor. The transmittance or reflectance value for each wavelength of the test sample is then compared with the transmission (or reflectance) values from the reference sample3.
In short, the sequence of events in a modern spectrophotometer is as follows:
The light source is imaged upon the sample
A fraction of the light is transmitted or reflected from the sample
The light from the sample is imaged upon the entrance slit of the monochromator
The monochromator separates the wavelengths of light and focuses each of them onto the photo detector sequentially.
Many older spectrophotometers must be calibrated by a procedure known as “zeroing.” The absorbancy of a reference substance is set as a baseline value, so the absorbance of all other substances are recorded relative to the initial “zeroed” substance. The spectrophotometer then displays percent absorbance (the amount of light absorbed relative to the initial substance).The concentration of the solution can be determined by measuring the amount of light it absorbs which requires a quantitative relationship. This is provided by Beer’s-Lambert’s Law 4.
1.2 Beer-Lambert’s Law
Lambert concluded that the power P of the transmitted light varies exponentially with the path length, b and directly with the power of Po of the incidence light. If P (or I) represents the power (or intensity) of transmitted light and Po (or Io) represents the power incident light, then the change in P is proportional to the power of incident light multiplied by the change in thickness b of the material through which the light passes.
Mathematically,
dP = KPdb
K is proportionality constant, and the negative sign indicates that P becomes smaller when b becomes larger. Rearranging and integrating the above equation 5 .
And
or
or
Log
Beer modified the law to apply to solution. He found that doubling the concentration of light absorbing molecules in a solution produced the same effect as doubling the thickness. The modified form of the above law is logP/Po = bc
In this expression;
C is the concentration of the solution and is expressed in moles per liter 4.
is molar absorptivity (molar extinction coefficient) and
b is the cell width expressed in centimeters,
thus log(P/Po) is directly proportional to concentration of solution.
If log (P/Po) is plotted against concentration for a solution which obeys the Beer’s –Lambert law, a straight line results whose slope is –
P/Po is called the transmittance of the solution.
Beer-Lambert law is a combination of two absorption laws and tells us quantitatively how the amount of transmitted power depends on the concentration of the absorbing molecules and the path length over which absorption occurs 5.
Beer-Lambert law is well obeyed with dilute solution, where there is a linear relationship. In the plot of absorption or transmittance (i.e A or log T) at the wavelength of maximum absorption, max, versus concentration for a series of standard solution. The concentration range in which the Beer-Lambert law is obeyed is known as linear dynamic range, and only quantitative determination done within it can be accurate and reliable.
Beer-Lambert law as expressed in the equation above can be used in several ways. Molar absorptivities of species can be calculated, if the concentration is known. The measured value of absorbance can be used to obtain concentration if absorption and path length are known. The law also applies to solution containing more than one kind of absorbing substance. Provided, that there is no interaction among the various species, the total absorbance substance. The total absorbance for multi-component system at a single wavelength is the sum of individual absorbancies5.
1.3 Ultra Violet (UV) Spectrophotometry
The most common spectrophotometers are used in the UV and Visible regions of the spectrum. Light of wavelength between 400nm and 750 nm is visible and the instrument used to measure it is ultraviolet spectrometer and it absorbs light in the visible and near ultraviolet region, that is in the 200-750 nm range. This light is of higher frequency with respect to the nearby protons3.
In Ultra Violet spectrophotometer, the samples are usually prepared in cuvettes; and fill up to mark, the chosen wavelength is set and the maximum absorbance is taken once the cuvette is placed inside the UV machine 6 .
1.4 Infrared (IR) Spectrophotometry
Spectrophotometers designed for the main infrared region are quite different because of the technical requirements of measurement in that region. One major factor is the type of photosensors that are available for different spectral regions, but infrared measurement is also challenging because virtually everything emits IR light as thermal radiation 7 .
A molecule is constantly vibrating, that is, its bonds stretch and bend with respect to each other, changes in vibrations of a molecule are caused by absorption infra red light.
The infrared spectrum helps to reveal the structure of a new compound by telling us what groups are present in or absent from the molecule 4. IR is a highly characteristic property of an organic compound/ element because a particular group of atoms give rise to characteristic absorption bands.
1.5 Nuclear Magnetic Resonance (NMR)
NMR is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained. It relies on the phenomenon of NMR and can provide detailed information about the structure, dynamics reaction state and chemical environment of molecules 7.
1H, 15N, 13C and 31P are highly abundant isotopes whilst 15N and 13C are present at only low level <1%. In simply terms, when the sample is placed in the magnet the nuclei of the atoms align with the magnetic field. Typically the magnets used in NMR spectroscopy are very strong with pulses of energy in the radio frequency (RF) range, typically 40-800 MHz to the sample. The pluses cause Nuclei to rotate away from their equilibrium position and they start to precise (rotate) around the axis of the magnetic field. The exact frequency at which the nuclei precise is related to both the chemical and physical environment of the atom in the molecule. This results in a spectrum showing many absorption peaks, whose relative position, reflect different environments of portions which can give unbelievable detailed information about molecular structure.
The various aspects of the NMR spectrum are 7
The number of signals which tell us how many different kinds of protons that are in a molecule.
The position of the signals which tells us about the electronic environments of each kind of proton.
The intensities of the signals which tells us how many protons of each kind.
The splitting of a signal into several peaks, which tells us about the environment of the protons.
NMR can also be used to look at dynamic processes. These include internal motions within regions of larger molecules such as loops in a protein or the base pair in DNA or RNA.
1.6 Schiff Base
Schiff base is a term used to describe the product formed when an amine undergoes a condensation reaction with a carbonyl compound or it is said to be the nitrogen analog of an aldehyde or ketone in which the C = O group is replaced by a C = N – R group 8. A German Chemist named Hugo Schiff discovered these bases. He discovered the Schiff bases and other imines, and was responsible for research into aldehydes, the field of amino acids and the Biuret reagent. He also had the Schiff test named after him9. Schiff bases are synonymous with imines, and even Azomethines 10.
An imine is a functional group or chemical compound containing a carbon- nitrogen double bond11 . Imines can be classified further as Aldimines and Ketimines and this mainly depends on the type of carbonyl compound involved in the reaction. Imines derived from aldehydes are called aldimines [Scheme1] while those from ketones arc called ketimines [Scheme 2] 11.
Scheme 1 —Aldimine synthesis
RCHO + R/NH2 RCH = NR/ + H2O
Scheme 2 — Ketimine synthesis
RR/CO + R//NH2 RR/C = NR//+ H2O
The substituent on the nitrogen atom is an alkyl or aryl group and not a hydrogen atom12. The reaction of an aldehyde or a ketone with a secondary amine results in what is called Enamine. These enamines are not Schiff bases, because they do not have a carbon nitrogen double bond [C = N] in their structure13, therefore, for Schiff base to be form, the amine must be of the primary type. Schiff bases obtained from aromatic amines are known as Anils14.
Schiff bases generally are common substrates in a wide variety of trnsformations9. They are used as substrates in the preparation of a number of industrial and biologically active compounds via ring closure, cyclic-addition and replacement reactions. They have numerous applications, which include preparative use, identification, detection and determination of aldehydes or ketones, purification of carbonyl and amino compounds and the protection of these groups during complex or sensitive reactions. These bases show some biological activities, that is to say they can act as herbicides, antimicrobial compounds, antifungal compounds and even anti-tumor compounds. Also industrially, they are employed as dyes and pigments11.
Schiff bases are generally capable of forming very stable complexes with transition metals11,15. Various methods of preparation of these metal derivatives are summarized below as:9
- Direct Reaction of the metal ion and the1igand with special interest in the choice
of the solvent and metal compound to be used 16,17
- Constituent Combination, which consists in mixing in a suitable amine
or ketone, base and metal ion in a suitable solvent. This method eliminates the need for preparation of the ligand.18, 19
- Displacement of the alkyl group of the RN moiety of an imine by an alkyl group
of an amine14 [scheme 3].
Scheme 3
- Chelate Exchange which involve the displacement of a chelate from the coordination sphere of a metal ion by another chelate, HCh/ [scheme 4] 20
MChn+ nHCh/ MChn/ + nHCh
Scheme 4
- Metal Exchange whereby a metal chelate is mixed with a metal salt in solution and a more stable metal-chelate is formed [scheme 5]
MChn + M/n+ M/Chn + Mn+
Scheme 5
- Template Synthesis where a metal-chelate or the coordination sphere of a metal ion can serve as a template and induce different ligand molecules to orient in a manner that is suitable for condensation and complex formation. In short, the metal ion serves to organize the reactants in a form that is suitable for compound formation.21, 22
- Interaction of a soluble salt of a Schiff base and a soluble salt of a metal.
- Reactions of functional groups or replacement of functional groups on the coordination compound.
1.7 Schiff Bases Synthesis
Earlier, we defined a Schiff base as the nitrogen analog of an aldehyde or ketone in which the C = O group is replaced by a C = N -R group It is generally synthesized by the condensation reaction of an aldehyde or a ketone with a primary amine according to the following [scheme 6]
Scheme 6
R in this reaction may be an alkyl (aliphatic) or an aryl (aromatic) group. Schiff bases that contain aryl (aromatic) substituents are substantially more stable and are more readily synthesized while those that contain alkyl (aliphatic) substituents are relatively unstable 23 in general, majority of this category of Schiff bases are used immediately.24
Many Schiff bases can he hydrolyzed back to their aldehydes, ketones and/or amines by aqueous acids or bases.
The mechanism of Schiff base formation involves the nucleophilic addition of amines to the carbonyl group. Thus, the amine is the nucleophile. In the first part of its mechanism, the amine reacts with the aldehyde or ketone to give an unstable addition compound called carbinolamine. This carbinolamine is a neutral amino alcohol25 and it loses water either by acid or base catalyzed pathways.
If it undergoes acid catalyzed dehydration, protonation of the carbinolamine oxygen by the acid catalyst then converts the hydroxyl group into a leaving group and subsequently the loss of water produces an iminium ion. Loss of a proton now gives the final product and regenerates acid catalyst. Scheme 7 explains this acid catalysis21.
Typically the dehydration of the carbinolamine is the rate-determining step of Schiff base formation and this is why acids catalyze the reaction. Yet, the acid concentration can’t be too high because amines are basic compounds. If the amine is protonated and becomes non-nucleophilic, equilibrium is pulled to the left and carbinolamine formation can’t occur 25 .
The base catalyzed dehydration of carbinolamines is analogous to the elimination of alkyl halides except that it is not a concerted reaction, rather it proceeds in two steps through an: anionic intermediate.
Finally, Schiff base formation is really a sequence of two types of reaction, that is, addition reaction followed by elimination reaction.
Generally, Schiff base ligands and their metal complexes have been extensively studied over the past few decades. And various classes of Schiff base have been prepared by condensation of different types of amines and carbonyl compounds.22-32
1.8 Synthetic Importance of Schiff Bases
For organic Chemist, these Schiff bases have very large synthetic uses that are known, some are summarized below:
The addition of hydrogen cyanide to Schiff bases33,34 occurs readily and provides a viable route to α amino nitriles, which can in turn be used as the precursors for the synthesis (via hydrolysis) of amino acids. This is the strecker synthesis. This reaction is usually carried out using anhydrous hydrogen cyanide in inert solvents such as benzene. However, a more convenient procedure, which gives improved yields, is to use sodium cyanide in a phosphate buffer. Recently, trimethyl silyl cyanide has been recommended as a safer alternative to hydrogen cyanide in the Strecker synthesis. Reaction of Schiff bases with primary amines results in adducts which tend to decompose to a new imine and primary amine, the overall process corresponding to imine exchange36,37. The rate of imine exchange increases with increase in the basicity of the primary amine effecting displacement. Sodamide reacts with aldimines with formal replacement of the imidyl hydrogen to give amidines [scheme 8]38.
Scheme 8
Acylation of Schiff bases by acid anhydrides, acid chlorides and acyl cyanides is initiated by attack at the nitrogen atom and leads to net addition of the acylating agent to the carbon-nitrogen double bond. Reaction of this type has been put to good use in natural product synthesis.
Schiff bases serve as intermediates in the preparation of sizeable quantities of secondary amines by the hydrogenation of amides or nitrile35, 39 [scheme 9]. The reactions are believed to be reversible, since the presence of ammonia has been found to inhibit the formation of secondary amine.
| 2 |
Scheme 9
Cycloaddition of Schiff bases to ketones are highly stereoselective implying a concerted process. However, a two step mechanism involving a dipolar intermediate adequately accounts for the observed stereoselectivity and is strongly supported by mechanistic studies of B-1actam formation through intermediate from Schiff bases and ketenes [scheme 10] 40:
Scheme 10
Salts of Schiff bases35 are gotten through the Gatterman aldehyde synthesis whereby in the presence of hydrogen chloride and zinc chloride, hydrogen cyanide will react with phenol (or substituted phenols) to give the salt of an imine. Hydrolysis gives a phenolic aldehyde. Zinc cyanide and hydrogen chloride may be used to generate the active reagent as shown in scheme 11.
Scheme 11
The carbon-nitrogen double bond of Schiff bases like the carbon-oxygen double bond is readily reduced by complex metal hydrides. Reduction of this type is probably the most efficient and convenient method for the conversion of C=N to amines. Thus lithium aluminum hydride at room temperature (or in difficult cases at elevated temperature) smoothly reduces Schiff bases in high yield (> 90%) to secondary amines. Sodium borohydride is an equally effective reducing agent and is preferred to lithium aluminum hydride because of its inertness to a wider range of solvent media and because of its greater specificity. An even more effective reagent of this type is sodium cyanahydridoborate, NaBH3CN41.
Reaction of ketones and phenols which depends on the high reactivity of enol systems toward electrophilic reagents is that with formaldehyde, a secondary amine and hydrogen chloride. The electrophilic reagent may be visualized as an immonium salt [scheme 12]. This is called the Mannich reaction 35 and this reaction finds numerous synthetic applications.
Scheme 12
Schiff bases can generally be isolated only in the form of their cyclic trimers in a reaction called the trimerisation of imines35 [scheme 13].
Scheme 13
Schiff bases derived from any carbonyl compounds are reducible electrolytically over a wide pH range to give the corresponding amines. Thus, the electrolytic reduction of N-substituted aryl aldimines and ketimines occurs by the stepwise addition of two electrons and gives initially a radical V anion, and by protonation a radical intermediate which can be further reduced to a secondary amine or dimerize to a 1,2-diamino compound 42,43 [scheme 14].
Scheme 14
The uncontrolled oxidation of a Schiff base with a peroxy acid results in cleavage of the carbon-nitrogen double bond to give a carbonyl compound and a nitroso Compound, respectively. On the other hand, oxidation using peroxy acids at low temperature (O°C) affords an excellent synthetic route to oxaziridines36 [scheme 15]
Scheme 15
In Vilsmeyer reaction, DMF and POCl3 are used to generate highly reactive Schiff base specie, which is used in formylation reactions33 [scheme 16]
Scheme 16
Schiff base macroligands synthesized from the reaction of dialdehydes and amino compounds are known to contain polyfunctional units, able to bind certain metallic ions are of great interest in environmental chemistry. They are known to be used as modifying agents to produce chemically modified electrodes. They aid the detection of organic pollutants in water by binding these materials to an electrode surface. An example of this macroligand can be N,N’1-bis(2-nitrobenyl)ethylenediimine [Scheme 17]
Scheme 17
Given that these materials form stable complexes, they provide the opportunity to design new systems selective to specific metallic ions. These materials could be applied in areas like electrochemistry, bioinorganic, catalysis, metallic deactivators, separation processes and environmental chemistry among others.
1.9 Phenylenediamine
Phenylenediamine (1,3-diamino benzene) is an organic compound with the formula C6H4(NH2)2. This derivative of aniline is a colorless solid, but typically samples can contain yellowish impurities arising from oxidation. It is mainly used as a component of engineering polymers and composites. It is also an ingredient in hair dyes.
1.10 Salicylaldehyde
Salicylaldehyde (2-hydroxybenzaldehyde) is the organic compound with the formula C6H4CHO-2-OH. Along with 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde, it is one of the three isomers of hydroxybenzaldehyde. This colorless oily liquid has a bitter almond odor at higher concentration. Salicylaldehyde is a key precursor to a variety chelating agents, some of which are commercially important.
salicylaldehyde is prepared from phenol and chloroform by heating with sodium hydroxide or potassium hydroxide in a Reimer-Tiemann reaction.
Alternatively, it is produced by condensation of phenol or its derivatives with formaldehyde to give hydroxybenzyl alcohol, which is oxidized to the aldehyde.
Salicylaldehyde is converted to chelating ligands by condensation with amines.
Scheme 18
Structure and preparation of Schiff Base
1.11 Applications of Schiff Base
Schiff bases derived from aromatic amines and aromatic aldehydes have a wide variety of applications in many fields, e.g., biological, inorganic and analytical chemistry 43, 44. Applications of many new analytical devices require the presence of organic reagents as essential compounds of the measuring system. Schiff bases are used in optical and electrochemical sensors, as well as in various chromatographic methods, to enable detection of enhanced selectivity and sensitivity 45-46. Among the organic reagents actually used, Schiff bases possess excellent characteristics, structural similarities with natural biological substances, relatively simple preparation procedures and the synthetic flexibility that enables design of suitable structural properties 47, 48. Schiff bases are widely applicable in analytical determination, using reactions of condensation of primary amines and carbonyl compounds in which the azomethine bond is formed (determination of compounds with an amino or carbonyl group); using complex formation reactions (determination of amines, carbonyl compounds and metal ions); or utilizing the variation in their spectroscopic characteristics following changes in pH and solvent49. Schiff bases play important roles in coordination chemistry as they easily form stable complexes with most transition metal ions 50, 51. In organic synthesis, Schiff base reactions are useful in making carbon-nitrogen bonds.
1.12 Biological Importance of Schiff Bases
Schiff bases appear to be important intermediates in a number of enzymatic reactions involving interaction of the amino group of an enzyme, usually that of a lysine residue, with a carbonyl group of the substrate 52. Stereochemical investigations 14 carried out with the aid of molecular models showed that Schiff bases formed between methylglyoxal and the amino group of the lysine side chains of proteins can bend back in such a way towards the N- atom of peptide groups that a charge transfer can occur between these groups and the oxygen atoms of the Schiff bases. Schiff bases derived from pyridoxal (the active form of vitamin B6) and amino acids are considered as very important ligands from biological point of view. Schiff bases are involved as intermediates in the processes of non-enzymatic glycosylations. These processes are normal during aging but they are remarkably accelerated in pathogeneses caused by stress, excess of metal ions or diseases such as diabetes, Alzheimer’s disease, and atherosclerosis. Non-enzymatic glycosylation begins with an attack of sugar carbonyls or lipid peroxydation fragments on amino groups of proteins, aminophospholipids and nucleic acid, causing tissue damages by numerous oxidative rearrangements. One of the consequences is cataract of lens proteins 53.
Many biologically important Schiff bases have been reported in the literature possessing, antimicrobial, antibacterial, antifungal, anti-inflammatory, anticonvulsant, antitumor and anti HIV activities 54-59.
Other important role of Schiff base structure is in transamination 60. Transamination reactions are catalyzed by a class of enzymes called transaminases. Transaminases are found in mitochondria and cytosal of eukaryotic cells. All the transaminases appear to have the same prosthetic group, that is, pyridoxal phosphate, which is covalently attached to them via an imino group. Schiff base formation is also involved in the chemistry of vision, where the reaction occurs between the aldehyde function of 11-cis-retinal and amino group of the protein (opsin) 61. The biosynthesis of porphyrin, for which glycine is a precursor, is another important pathway, which involves the intermediate formation of Schiff base between keto group of one molecule of δ-aminolevulinic acid and ε-amino group of lysine residue of an enzyme.
1.13 Schiff Base in Transition Metal Complexes
Transition metals are known species and find applications in biomimetic catalytic reactions. Chelating ligands containing N, S and O donor atoms show broad biological activity and are of special interest because of the variety of ways in which they are bonded to metal ions. It is known that the existence of metal ions bonded to biologically active compounds may enhance their activities 62-65.
1.13.1 Application of Schiff Base In Transition Metal Complexes
The major applications of the Schiff base complexes are in catalysis. The Schiff base complexes do have a number of other applications which are;
i As electroluminescent materials
Organic electroluminescent (EL) devices are useful in novel-type flat-panel displays since Tang and Van Styke first reported on high-performance organic EL devices66Their discovery was based on employing a multilayer device structure containing an emitting layer and a carrier transport layer of suitable organic materials. Organic dyes, chelate metal complexes and polymers are three major categories of materials used in the fabrication of organic EL devices. Out of the three, chelate metal complexes having high-luminance, blue emitting nature find use as materials for RGB (red, green, and blue) emission.
Schiff base complexes, especially those of Zn(II), are nowadays used as electroluminescent materials67,68. Zinc complex of the Schiff base, N,N’-bis(2-hydroxy-1-naphthylidene)-3,6-dioxa-1,8-diaminooctane, emits blue light with an emission peak at 455 nm having maximum brightness of 650 cd m− 2, when it is used as the emitting layer in an electroluminescence device. Fabrication of EL devices employing this kind of zinc complexes as blue electroluminescent material was carried out by thermal vacuum-deposition. Prepared blue luminescent zinc and beryllium complexes of the Schiff bases derived from calixarene69. These Schiff bases complexes have good solubility in normal solvents and can easily form thin films. Xie et al. reported the crystal structure, thermal stability and optoelectronical properties of bis[salicylidene(4-dimethylamino)aniline]zinc(II)68. This complex exhibits very good light emission and charge transporting performance in organic light emitting diodes (OLEDs). These experimental reports point to the possible application of Schiff base complexes as emitting materials in full colour flat-panel displays.
ii Non linear optics (NLO)
Deals with the interactions of applied electromagnetic fields with various materials to generate new electromagnetic fields, altered in frequency, phase, or other physical properties. Such materials that are able to manipulate photonic signals efficiently are of importance in optical communication, optical computing, and dynamic image processing71-75. In this connection, transition metal complexes have emerged as potential building blocks for nonlinear optical materials due to the various excited states present in these systems as well as due to their ability to tailor metal-organic-ligand interactions76-81.
Compared to the more common organic molecules, the metal complexes offer a large variety of novel structures, the possibility of enhanced thermal stability, and a diversity of tunable electronic behaviors by virtue of the coordinated metal center and hence they may find use as NLO materials with unique magnetic and electrochemical properties82-84. The investigations on NLO properties of metal complexes are being pursued by several research groups77,78,85-91. It has been reported by Di Bella et al that bis (salicylaldiminato) metal Schiff base complexes exhibit good second order NLO properties92-98.
iii In electrochemical sensors
Schiff bases have been used as carriers in the preparation of potentiometric sensors for determining cations and anions 92-108. A ruthenium (III) Schiff base complex was used in the fabrication of chloride PVC-based membrane sensor 109.
The sensor with a composition of 30% PVC, 62% benzyl acetate, 5% ruthenium(III) Schiff base complex and 3% hexadecyltrimethyl ammonium bromide displays near-Nernstian behavior over a wide concentration range. It shows high selectivity toward chloride ions over several organic and inorganic anions and was successfully applied for the determination of chloride in serum samples. It could also be used as an indicator electrode in the potentiometric titration of chloride ions with silver nitrate solution. It has been reported a potentiometric aluminium sensor based on the use N,N’-bis(salicylidene)-1,2-cyclohexanediamine as a neutral carrier in poly(vinyl chloride) matrix 110. It was successfully applied for direct determination of aluminum (III) in biological, industrial and environmental samples.
The electrode could be used in the pH range of 2.0–9.0 and mixtures containing up to 20% (v/v) non-aqueous content. It has been used as an indicator electrode in potentiometric titration of aluminium ion with EDTA. The Schiff base, N,N′,N″,N′′′-1,5,8,12-tetraazadodecane-bis(salicylaldiminato), has been used as ionophore for preparing Mn2+ selective sensor 111. The sensor was found to be sufficiently selective for Mn2+ over a number of alkali, alkaline and heavy metal ions and could therefore be used for the determination of manganese in various samples by direct potentiometry.
Do you need help? Talk to us right now: (+234) 08060082010, 08107932631 (Call/WhatsApp). Email: [email protected].
IF YOU CAN'T FIND YOUR TOPIC, CLICK HERE TO HIRE A WRITER»
