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Essay: Thesis: Coordination chemistry and schiff bases

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Coordination chemistry is the rapid growing branch of chemistry and deals with the interaction between the metal and ligand. It was Werner, a Swiss chemist who first recognized such a class of compounds and awarded Nobel Prize in chemistry in 1913 for his invaluable contribution to coordination chemistry. Metal complexes possess wide variety of applications in pharmaceutical, agricultural and various industrial fields. Cis’platin is a platinum based chelate and acts as a well known anti cancer drug due to the high DNA binding capacity of the metal chelate. Oxygen has inevitable role in the life of every living organism and it is carried by haemoglobin present in blood, which is an iron complex. Right from textile to petroleum industry numerous metal chelates have got wide range of applications.
Schiff Bases
Among the metal chelates studied, Schiff base complexes have got great attention. A Schiff base is a compound with a functional group that consists of an azomethine linkage or an imino group. It is formed by the condensation reaction between an aldehyde or a ketone with a primary amino group as shown below.
R-NH2 + >C=O ‘ > C=N-R + H2O
Primary amine Aldehyde / Ketone Schiff Base
where R may be an alkyl or aryl group. Schiff bases possessing aryl groups are easier to synthesize due to their extra stability through conjugation effect. Comparatively less stable alkyl Schiff bases may polymerize or decompose to their parent compounds in the presence of moisture. The formation of Schiff base from an aldehyde or ketone is a reversible process and takes place generally in the presence of acidic or basic or neutral media. In the first part of the mechanism, the amine reacts with the aldehyde or ketone to give an unstable addition compound called carbinolamine. The carbinolamine loses water by either acid or base catalyzed pathways. In the second step carbinolamine undergoes acid catalyzed dehydration. The second step is the rate determining step of the process which is catalyzed by acid. But too much acid concentration will adversely affect the nucleophilicity of amine and hence the synthesis is to be best carried out in mild acidic condition. Many Schiff bases may be hydrolyzed back to their parent compounds in the presence of acid or base. In some cases it is better to remove the water formed during the reaction by distillation using azeotrope forming solvent [1].
In most cases, especially the condensation between the aromatic aldehyde or ketones with various amines are not reversible and the resultant Schiff bases can be easily separated from the reaction mixture. The mechanism for the formation of Schiff base is depicted in Figure 1.1.
Figure 1.1 Mechanism of Schiff base formation
The name Schiff base is given to these classes of compounds after the German chemist Hugo Schiff (1864). Schiff bases are very popular in coordination chemistry due to their potential chelating ability to the metal ions through azomethine moiety. A variety of methods including direct synthesis, template methods, microwave assisted synthesis etc were developed by scientists for the synthesis of Schiff bases [2].
The amazing capacity of several Schiff base molecules in participating chelation process is due to the easiness of providing the lone pair of electron on nitrogen atom which in turn arises due to the low electronegative nature of nitrogen atom. Metal chelates having five or six membered ring system acquires high stability. The stability of metal chelates depends on the strength of the azomethine linkage, basicity of the imino group and sterric factors due to the other groups present in the ligand. Because of the relative easiness of preparation, synthetic flexibility and the peculiar binding nature of the azomethine linkage make the Schiff bases excellent chelating molecules [3]. Schiff bases and their metal chelates can be synthesized by different ways. Important methods are discussed below.
Direct synthesis
This method involves the condensation reaction between carbonyl compound and amino compound in alcohol or a mixture of alcohol and water. Azeotropic distillation followed by the treatment with molecular sieves ensure the complete removal of water molecules from the reaction mixture [4,5]. Dehydrating solvents such as tetramethyl orthosilicate or trimethyl orthoformate can also be used for the removal of water [6,7]. The synthesized Schiff base can be separated and purified by suitable techniques. The purified Schiff base is then allow to react with the metal salts in aqueous or alcoholic medium to obtain chelates. The main advantage of this method is that the minimization of impurities during the metal chelate synthesis.
It is clear from the mechanism of formation of Schiff bases that the efficiency of direct condensation involves the presence of highly electrophilic carbonyls and strongly nucleophilic amino compounds, which can be accelerated by the use of compounds that act as Br??nsted-Lowry or Lewis acids, to activate the carbonyl group, accelerating the nucleophilic attack by amines and dehydrate the system by removing water. Br??nsted-Lowry or Lewis acids used for the synthesis of Schiff bases include ZnCl2, TiCl4, Ti(OR)4, alumina, H2SO4, NaHCO3, MgSO4, Mg(ClO4)2, CH3COOH, Er(OTf)3, P2O5/Al2O3 and HCl [8-16].
2) In situ method
This method is employed only if the recovery of the Schiff base from the reaction mixture is tedious. At first the parent aldehydes or ketone is allow to react. After the completion of the reaction, metal salts in aqueous or alcoholic solution are added and reflux the mixture for a particular period.
Template synthesis:
In situ one-pot template condensation reactions lie at the heart of macrocyclic chemistry. Therefore, template reactions have been widely used for the synthesis of macrocyclic complexes, in which transition metal ions are generally used as the template agent [17-19]. For instance, D. Singh et al have synthesized a novel series of complexes of the type M(C28H24N4)X2], where M = Co(II), Ni(II), Cu(II), Zn(II) and Cd(II), X = Cl’, NO3′, CH3COO’ and C28H24N4 corresponds to the tetradentate macrocyclic ligand, were synthesized by template condensation of 1,8-diaminonaphthalene and diacetyl in the presence of divalent metal salts in methanolic medium [20]. The reaction pathway is depicted in Figure 1.2.
Fig. 1.2 Example for template synthesis
Applications of Schiff Bases and Their Metal Chelates
Many Schiff base and its metal chelates possess wide variety of application in the pharmaceutical field, catalysis reactions, analytical field and anti corrosion compounds. The subsequent paragraphs describe few examples.
Schiff base ligands containing various donor atoms (like N, O, S, etc.) show broad biological activities and are of special interest due to variety of ways in which they can bond to metal ions. It is known that the existence of metal ions bonded to biologically active compounds may enhance their activities [21]. Schiff bases derived from sulfane thiadiazole and salicyladehyde and thiophene-2-aldehyde and their metal chelates exhibit toxicities against insects [22-23]. Zhu et al have reported that fluorination of the aldehyde part of the Schiff base enhance activity against insects [24]. Many thiazole, benzothiazole, pyran and quinazole derived Schiff bases posses effective antifungal activity and the efficiency is enhanced when groups such as methoxy, halogen and naphthyl are attached [25-27].
Several Schiff bases derived from furan and their transition metal chelates found to be very efficient against fungi such as A. niger, A. solani etc [28,29]. Large number of heterocyclic and non heterocyclic Schiff bases and their metal complexes display antibacterial activity. Schiff bases derived from furfural, pyridine aldehydes, salicylaldehyde, thiazole, amino acids, taurine, glucosamine, aminopyridine, aminothiazole, pyrazolone, indole and benzaldehyde and their metal chelates are found to be efficient inhibitors towards the growth of different bacteria [30-41]. A lot of Schiff bases acquire anti-inflammatory, allergic inhibitors reducing activity, radical scavenging and anti-oxidative action. Thiazole and furan based Schiff base and their metal chelates display analgesic activity [42-45]. Several Schiff bases exhibit anti-cancer activity and sometimes the activity enhanced upon complexation with transition metal ions. It is reported that Schiff bases derived from quinoline, pyridine, nitrophenols, vanillin and benzene sulfonanilide, anthracene carboxaldehyde and their metal complexes show significant anti-cancer activity [46-50].
Many Schiff bases containing aromatic ring system and their metal chelates are found to catalyze various reactions such as oxygenation, hydrolysis, reduction and decomposition reactions [51,52]. It was reported by S. Forster et al [53] that some cobalt Schiff base complexes can catalyze the oxidation of anilines with tert-butyl hydroperoxide to give nitrobenzenes. A selective chromogenic chemosensor was designed by M. X. Liu et al using a novel 5-mercapto triazole derived Schiff base. The sensing of Cu2+ by this sensor was found to be reversible, with the Cu2+-induced color being lost upon addition of EDTA [54].
Copper(II) and iron(III) chelates were synthesized from 4-formyl-3-hydroxy benzamidine or 3-formyl-4-hydroxy benzamidine and various L- or D-amino acids and their inhibitory activities for bovine alpha-thrombin were explored by E. Toyota et al [55]. Amine terminated liquid natural rubber(ATNR) on reaction with glyoxal yield poly Schiff base [56], which improves aging resistance of rubber. Organocobalt complexes with tridentate Schiff base act as initiator of emulsion polymerization and co-polymerization of dienyl and vinyl monomers [57]. It has been reported that Zinc(II) complexes with Schiff bases type chelating ligands can be used as an effective emitting layer [58]. Amino acid Schiff base complexes derived from 2-hydroxy-1-naphthaldehydes are important due to their use as radiotracers in nuclear medicine [59]. Many organic molecules containing hetero atoms and compounds containing azomethine linkage (C=N) were reported to act as good corrosion inhibitors for carbon steel, aluminum, copper and zinc in acidic media. A detailed survey regarding the corrosion inhibition capacity of the Schiff bases is given in Part II.
Schiff Bases Derived From Pyridine and Their Metal Chelates- A Review
Novel potential Schiff bases derived from acetylpyridine and their transition metal chelates have been synthesized by various researchers and characterized. Some of them have got properties including pharmaceutical agents. A series of Schiff bases derived from 2-acetylpyridne and 4-(2-aminoethyl)morpholine, and 4-(2-aminoethyl)piperazine and their transition metal complexes were synthesized and characterized by N. S. Gwaram et al using elemental analysis, NMR, FT-IR and UV-Vis spectral studies. Zn (II) complex displayed square pyramidal geometry while Cd(II) complexes exhibited polymeric structure. Ni(II) complexes possessed an octahedral geometry [60].
R.H. Prince and D. A. Stotter [61] reported of a series of metal(II) complexes of a quinquidentate ligand produced in situ or by complexation with the Schiff-base, condensation-product of two moles of 2-acetylpyridine with 3,3′-iminobispropylamine. From the in situ synthesis of the Ni(II) compound only a quadridentate, mono-Schiff-base complex with coordinated acetyl-pyridine is isolable. Analytical data, infra-red studies, magnetic moments and solution-spectra of the complexes were described, and the interconversion of the two types of Ni(II) complex investigated. Binuclear Schiff base complexes derived from glycine (Gly) and 3-acetylpyridine (3-APy) in the presence of M(OAc)2 [M = Co(II), Ni(II), Cu(II), Zn(II) and Cd(II)] have been synthesized by N. A. Nawar [62]. The role of pH in promoting the condensation of glycine and 3-acetylpyridine, as well as the substitution of acetates by hydroxide ion, has been discussed. The reaction of glycine with 3-acetylpyridine in the presence of MCl2 [M = Co(II) and Ni(II)] and MCl3 [M = Fe(III) and Cr(III)] yields mono- and/or binuclear complexes containing both of glycine and 3-acetylpyridine without condensation. Both types of complex were isolated and characterized by chemical analysis, conductance, spectral, magnetic and thermal measurements.
Recently, N. M. Hosny et al [63] synthesized metal chelates of Cu(II), Co(II), Ni(II), Cr(III) and Fe(III) chlorides with a Schiff base ligand derived from 2-acetylpyridine and leucine. The IR spectra show that the Schiff base can act as a neutral tridentate ligand to Cu(II), Co(II) and Ni(II) through the pyridyl nitrogen, azomethine nitrogen and carbonyl oxygen. Another mode of chelation has been established that the Schiff base can act as mononegative tridentate ligand to Fe(III) and Cr(III) through pyridyl nitrogen, azomethine nitrogen and the carbonyl oxygen after the displacement of hydrogen from hydroxyl group. The synthesized metal chelates were subjected to elemental, spectral, thermal, magnetic and molar conductance studies. The results suggest that Co(II) and Ni(II) metal chelates posses tetrahedral geometry while Fe(III) and Cr(III) acquire octahedral geometry. A square planar geometry was assigned to Cu(II) complex. Semi empirical calculation of the complexes was also performed.
In 2007, N. M. Hosny [64] synthesized Schiff-base complexes [ML(H2O)2(Ac)]nH2O (M=Co(II), Ni(II) and Zn(II); L= novel heterocyclic Schiff-base ligand derived from 2-acetylpyridine and alanine and n= 1’3/2) were synthesized and characterized by elemental analysis, spectral (FTIR, UV/Vis, MS, 1Hnmr), thermal (TGA), conductance and magnetic moment measurements. The results suggest octahedral geometry for all the isolated complexes. IR spectra show that the ligand coordinates to the metal ions as mononegative tridentate through pyridyl nitrogen, azomethine nitrogen and carboxylate oxygen after deprotonation of the hydroxyl group. Semi-empirical calculations PM3 and AM1 have been used to study the molecular geometry and the harmonic vibrational spectra to assist the experimental assignments of the complexes.
A new Cd(II) complex with a tridentate Schiff base derivative of gallic hydrazid with 2-acetylpyridine has been prepared by A. A. Alhadi [65]. The structure of the ligand 3,4,5-trihydroxybenzoic acid[1-(pyridyl)-ethylidene]hydrazone (GAPy) was confirmed using the X-ray structure analysis. The elemental analysis, FTIR, UV-Vis, 1Hnmr spectral studies and thermal analysis indicate that the Schiff base ligand GAPy is a tridentate ligand which is coordinated with the Cd(II) complex through N, N and O atoms. They confirm that acetate ion is a bidentate ligand which is coordinated with the metal ion through two O atoms. A series of new Zn(II) complexes of 2-acetylpyridine thiosemicarbazone/semicarbazone Schiff base complexes have been synthesized and characterized by elemental analysis, IR, electronic and 1H NMR spectral studies by R. Manikandan et al [66]. The thiosemicarbazone/semicarbazone ligand coordinates to zinc as tridentate N, N and S/O donors. Based on the analytical and spectral results, tetrahedral geometry has been tentatively proposed by them for all the complexes.
V. R. Souza [67] reported the synthesis and spectroscopic/electrochemical properties of iron(II) complexes of polydentate Schiff bases generated from 2-acetylpyridine and 1,3-diaminopropane, 2-acetylpyrazine and 1,3-diaminopropane, and from 2-acetylpyridine and L-histidine. The complexes exhibit bis(diimine) iron(II) chromophores in association with pyrazine, pyridine or imidazole groups displaying contrasting pi-acceptor properties. In spite of their open geometry, their properties are much closer to those of macrocyclic tetraimineiron(II) complexes. An electrochemical/spectroscopic correlation between E degrees (FeIII/II) and the energies of the lowest MLCT band has been observed, reflecting the stabilization of the HOMO levels as a consequence of the increasing backbonding effects in the series of compounds. They also reported the M??ssbauer data which confirm the similarities in their electronic structure, as deduced from the spectroscopic and theoretical data.
[Cu(2AcPh)Cl]2H2O, [Cu(2AcpClPh)Cl]2H2O, [Cu(2AcpNO2Ph)Cl], [Cu(2BzPh)Cl], [Cu(2BzpClPh)Cl] and [Cu(2BzpNO2Ph)Cl] complexes were synthesized and characterized by A. Angel et al [68], with 2-acetylpyridine-phenylhydrazone (H2AcPh), 2-acetylpyridine-para-chloro-phenylhydrazone (H2AcpClPh), 2-acetylpyridine -para-nitro-phenylhydrazone (H2AcpNO2Ph), 2-benzoylpyridine-phenylhydrazone(H2BzPh), 2-benzoylpyridineparachloro-phenyl hydrazone (H2BzpClPh) and 2-benzoylpyridine-para-nitro-phenylhydrazone (H2BzpNO2Ph) .
Schiff Bases Derived From Furan-2-Aldehyde and Thiophen-2-Aldehyde and Their Metal Chelates – A Review
Many researchers have synthesized and characterized Schiff bases derived from furan-2-aldehyde and thiophene-2-aldehyde. The chelating ability of the newly synthesized Schiff bases was exploited and different transition chelates were prepared and characterized.
Synthesis and characterization of some thiocarbohydrazone Schiff bases derived from pyrole, thiophene and furan carbaldehyde and their complexes with Cu(II), Ni(II), Zn(II), Co(II) and Fe(II) were done by F. Esmadi et al [69]. The prepared Schiff bases are bis(pyrrole-2-carboxaldehyde)thiocarbohydrazone (Pytch), bis(thiophene-2-carboxaldehyde)thiocarbohydrazone (Thtch) and bis(furfuralthiocarbohydrazone (Futch). They found that Futch ligand produced tetracoordinate complexes of the general formual [MLCl2] where they act as neutral bidentates bonding through the two imine nitrogens. Thtch ligand acted as neutral bidentate producing a tetracoordinate complex of the formula [Fe(Thtch)Cl2] or as monobasic tridentate producing tetracoordinate [Zn(Thtch)Cl] complex or as monobasic bidentate forming tetracoordinate [M(Thtch)2] complexes where M = Co, Ni and Cu.
Complexes of Co(II), Ni(II), Cu(II), Zn(II) and Mn(II) of a Schiff base derived from o-phenyldiamine and furfural were synthesized and characterized by F. Dianzhong et al [70] using various physical and chemical methods. Electronic spectra, magnetic moment studies, EPR and XPS studies revealed that these metal chelates have octahedral geometry. The non-electrolytic nature of the complexes was verified by the molar conductance measurements.
In 2009, P. Mittal et al [71] have synthesized and characterized novel Schiff bases derived from furan-2-aldehyde and thiophene-2-aldehyde with vinyl aniline. The chelating ability of these Schiff bases were screened by preparing the transition metal chelates of metal ions such as Co(II), Ni(II), Cu(II) and Mn(II). They verified the octahedral geometry of these complexes with various physico-chemical methods. The complexes were also screened for their antimicrobial activity.
New two nickel(II) and copper(II) complexes of two Schiff base ligands formed by condensation of furfural and benzil with S-benzyldithiocarbazate have been synthesized and characterized by elemental analysis, magnetic and spectroscopic measurements by M. A. Ali et al [72]. The geometries of nickel(II) complexes, were square planar and octahedral, respectively. Cu(II) complexes acquired dimeric/polymeric structure due to low magnetic moment values.
A new series of transition metal complexes of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) were synthesized from the Schiff base ligand derived from 4-aminoantipyrine, furfural and o-phenylenediamine by M. S. Suresh et al [73]. The structural features were derived from their elemental analyses, infrared, UV-visible spectroscopy, NMR spectroscopy, thermogravimetric analyses, ESR spectral analyses and conductivity measurements. The data suggested square planar geometry for the complexes having metal ions with primary valency two.
In 2013, Y. Harinath et al [74] have synthesized a new Schiff base bidentate ligand (L), 5-methyl thiophene-2-carboxaldehyde carbohydrazone and its metal [Cu(II), Cd(II), Ni(II) and Zn(II)] complexes with general stoichiometry [M(L)2X2] (where X=Cl). The ligand and its metal complexes were characterized by elemental analyses, IR, 1Hnmr, ESR spectral analyses, and molar conductance studies. The molar conductance data revealed that all the metal chelates are non-electrolytes. IR spectra showed that ligand (L) is coordinated to the metal ions in a bidentate manner with N and O donor sites of the azomethine-N, and carbonyl-O. ESR and UV-Vis spectral data showed that the geometrical structure of the complexes are orthorhombic.
Scope and Objectives of the Present Investigation
The quest for novel Schiff bases and their metal chelates is the perpetual phenomenon for the chemists and scientists. By the exploration of the chelating ability of the Schiff bases, a novel class of metal complexes may be opened, which may possess potential applications in industrial, pharmaceutical and catalytic fields. By elucidating the structures of the metal chelates and ligands using advanced tools and techniques, a proper correlation can be made with the structure and activity. Even though large number of Schiff bases was prepared and chelating efficacy of the compounds was exploited, still remain wide scope for the synthesis of novel Schiff bases which possesses hetero atoms and their metal chelates. Literature survey showed that Schiff bases which are primarily derived from heterocyclic compounds such as acetylpyridines, furan-aldehydes and thiophene-aldehydes and their metal chelates were not much explored and reported.
In the present course of investigation it is proposed to synthesize and characterize novel heterocyclic Schiff bases derived from 3-acetylpyridine, furan-2-aldehyde and thiophene-2-aldehyde using various spectroscopic techniques such as IR, UV-vis, NMR and mass. It is also proposed to synthesize new transition metal complexes of these Schiff bases by exploiting their chelating ability. The geometry and structure of the metal chelates are to be recognized by analytical tools like as spectral, magnetic, electrical and elemental analyses.
The slow destruction of metal under environmental conditions such as acidic gases, humidity etc is termed as metallic corrosion. Metallic corrosion takes place naturally and usually the metals will be converted into their most stable oxides. Some metals such as silver, copper etc slowly changes into their sulphides and basic carbonates respectively. The slow rusting of the iron that takes place naturally is a well known example for the corrosion in the world. In the chemical point of view, corrosion is an electrochemical phenomenon. The rate of corrosion enhances rapidly in the presence of acidic environment and electrolytes.
Metallic corrosion has got a great attention among scientists and technologists, primarily due to its economic impact and secondly due to its safety consequences. Corrosion will lead to the lowering of the efficiency of plants, increasing the maintenance cost, contamination or loss of the products and may lead to the catastrophic damages. India has been losing around 1.52 lakh crore annually due to corrosion in various sectors such as infrastructure, production and manufacturing, defense, petrochemicals, railway, metal industries and nuclear power plants. In developed countries such as U.S. and Japan, the estimated loss due to corrosion is approximately 3% of their respective gross domestic product (GDP) which is about half of the loss estimated in India.[1]
It is estimated that about 10-15% of the globally extracted iron from its ores will turn back to the nature per annum in the form of rust as a result of natural as well as accelerated corrosion. Apart from the natural and unavoidable corrosions, manmade activities such as acid pickling, de-scaling, oil-well acidizing will escalate the rate of corrosion considerably and these activities are considered as the major reasons for the corrosion problems in the metal industries and oil industries.
De-scaling and Acid Pickling
De-scaling and pickling are the metal surface cleaning techniques which consumes enormous quantities of hydrochloric acid and sulphuric acid. The thin oxide film (e.g., hydrated ferric oxide or rust), organic and inorganic stains and other impurities on the metal surfaces can be eliminated by treating the metal specimens for a stipulated time in acidic solutions and thus the original metallic appearance can be reinstated. Similarly the basic carbonate formed on the surface of copper and brass can be easily removed by treating the surface with mild acids. Treating aluminium with mild acids regains its original metallic luster. To minimize the corrosion during surface cleaning process, it is customary to add certain inhibitors into the aggressive solutions [2-5].
Petrochemical Industry and Corrosion
The economic losses in oil industry due to corrosion mainly occur by the direct contact between the metallic oil pipelines and equipment with the aggressive media. The prolonged interaction between the aggressive media and pipelines is unavoidable during the production of oil, refining and transportation. The addition of corrosion inhibitors during these processes will help to decrease the corrosion rate considerably [6].
The acidic environment in the oil industry arises mainly due to two major reasons. a) Dissolution of the corrosive gases such as H2S and CO2 and b) hydrolysis of the acidic salts present in the aqueous phase to produce hydrochloric acid.
Enormous amount of concentrated acids have been used for stimulating the oil wells and to obtain the unrecovered hydrocarbons. The underground rocks which are basic in nature (e.g., limestone) can be destroyed by the treatment with concentrated hydrochloric acid or acetic acid on injection. The hydrocarbons trapped between the rocks will be easily ejected by this treatment. Hydrofluoric acid is commonly employed for silica or sand stone based rocks. These acidizing process will cause to shoot up the corrosion rate of the metallic pipes inside the oil wells. In the presence of hydrogen sulphide, the dissolved metal in acidic medium will cause to precipitate the iron oxide and iron sulphide. These precipitates will negatively affect the quality of crude oil and oil production equipments. The addition of corrosion inhibitors is very essential to reduce the rate of the corrosion considerably.
Prevention of Corrosion
It is a fact that corrosion can’t be prevented completely, but the most economical solution is to adopt more practical techniques for controlling the rate of corrosion. There are number of corrosion controlling techniques available depending upon the type and nature of corrosion. Surface coating is the most widely accepted method for controlling natural corrosion. Galvanizing, anodizing etc are used for decreasing the rate of galvanic corrosion. Accelerated corrosions such as acid pickling, de-scaling, oil well corrosion etc are chiefly controlled by the addition of certain corrosion inhibitors into the acidic solutions.
Corrosion Inhibitors
The use of corrosion inhibitors is the most practical way for decreasing the rate of corrosion especially in acidic media. Since corrosion is an electrochemical phenomenon, oxidation and reduction are the two major processes taking place during the corrosion. Metal atoms which undergo oxidation will act as the anodic regions and the electrons released by the same atoms will be accepted by the protons, which are at the immediate vicinity of the metal surface and will get reduced to hydrogen atoms. This region of the metal is behaving as cathode. Corrosion inhibitors are classified into three according to their inhibitive mechanism. They are a) anodic inhibitors b) cathodic inhibitors and c) mixed type inhibitors. The role of a corrosion inhibitor is to protect the metallic surface by interacting with metal atoms directly or reacting the environment by which the surface is exposed. The inhibitive action of a corrosion inhibitor on the metal surface in a homogeneous liquid corrosive medium may be due to
Increasing the anodic or cathodic polarization
Reducing the diffusion of H+ ions from the bulk to the metal surface or
Increasing the electrical resistance and thus by reducing the corrosion current density on the metallic surface.
Anodic corrosion inhibitors
The action of these inhibitors is to control the rate of anodic oxidation and thus prevent corrosion [7]. They can make large anodic shift of the corrosion potential. These types of inhibitors can passivate the steel by making passive oxide layers on the metal surface. They may be oxidizing (e.g., chromates, nitrites and nitrates) or non oxidizing type (e.g., ortho phosphate, tungstate and molybdates). The first type make protective layer in the absence of oxygen, while the second type require oxygen for making the passive layer [8,9].
These inhibitors are sometimes referred as ‘dangerous inhibitors’ since, small pores and defects on the oxide layer of these inhibitors, may lead to the accelerated corrosion of the metals [10].
Cathodic corrosion inhibitors
These inhibitors will decrease the rate of reduction process taking place on the cathodic sites, by shifting the potential more towards negative direction (cathodic side). The localized precipitation of species on cathodic site will enhance the corrosion resistance and thus reduce the migration of ions towards cathodic region considerably. The reduction of oxygen will be difficult in this scenario. Some cathodic inhibitors can act as oxygen scavengers and thus help to control the corrosion by preventing the cathodic depolarization caused by oxygen. Examples for cathodic inhibitors are metal ions (calcium, zinc etc), bicarbonates, polyphosphates, sulphites etc. Organic compounds such as imidazole and benzamide are usually used as cathodic inhibitors in boilers, which will help to prevent the deposition of calcium and magnesium [11].
Generally cathodic corrosion inhibitors are termed as ‘safe inhibitors’ since they can reduce the rate of cathodic process even at low concentrations.
Mixed inhibitors
These inhibitors influence both anodic and cathodic processes of corrosion. Many organic molecules come under this category. Various amines, triazoles, thiourea, quinolines can act as mixed corrosion inhibitors especially for steel and copper in acidic media [12,13].
Organic inhibitors
There are several natural as well as synthetic organic molecules, which act as corrosion inhibitors for different metals such as iron, copper, zinc etc in acidic, basic and neutral media. Majority of the organic inhibitors are acting as mixed type corrosion inhibitor. At sufficient concentrations they can make good protective film via adsorption on the surface of the corroding metal. Adsorption may be physical or chemical depending upon the molecular structure of the compound. In general, inhibition efficiency of the organic inhibitors is found to increase with the concentration. Even though most of the organic molecules are acting as the mixed type inhibitors, some of them may affect more at anodic or cathodic site. It was well established that the organic molecules containing hetero atoms such as O, S, N etc and compounds possess azomethine linkage (>C=N-) i.e., Schiff bases act as good corrosion inhibitors for various grades of steels, zinc and copper in acidic as well as NaCl solution. The efficiency of these compounds depend on the number of active probes on the molecule, charge density, molecular size, concentration, nature of adsorption and ability to form metallic complexes. The unshared pair and ?? electrons on the molecule can interact well with the empty orbitals of the metal atoms and cause to the firm adsorption of aromatic Schiff bases on the metal surface. In addition to this processes, the back donation of the electrons from the filled metal orbitals to the unoccupied ??*-orbitals of the Schiff base also come into play during the interaction, which will help the molecule to make good protective layer on the metal surface.
Schiff Bases as Corrosion Inhibitors- A Review
Large numbers of Schiff bases were screened for their corrosion inhibition capacity in acidic media. Many of them were effective against the corrosion of mild or carbon steel in acidic media. Few of them were acted as efficient inhibitor against the corrosion of copper and zinc in aggressive medium. The subsequent paragraphs explore the ability of certain newly synthesized Schiff bases to act as good inhibitors against the metallic corrosion in acidic media, which was reported by the previous researchers.
Two newly synthesised Schiff bases N,N’-ortho-phenylene(salicylaldimine-acetylacetone imine) and N,N’-ortho-phenylene(salicylaldimine-2-hydroxy-1-naphthaldimine) were studied as inhibitors for the corrosion of mild steel in 0.5 M sulphuric acid by M. Hosseini et al [14]. They confirmed by weight loss studies, electrochemical impedance and Tafel polarization measurements that both compounds act as good inhibitors, with efficiencies of around 95% at a concentration of 400 ppm. The nature of inhibition in both cases was mixed type (anodic and cathodic). Temkin isotherm is found to provide an accurate description of the adsorption behaviour of the investigated Schiff bases.
N. Saxena et al investigated the corrosion performance of mild steel in nitric acid solution containing various concentrations of Schiff bases derived from anisaldehyde such as N-(4-nitro phenyl) p-anisalidine, as N-(4-chloro phenyl) p-anisalidine, as N-(4-phenyl) p-anisalidine, as N-(4-methoxy phenyl) p-anisalidine, as N-(4-hydroxy phenyl) p-anisalidine using mass loss, thermometric and potentiostatic polarization studies [15]. All compounds exhibited appreciable corrosion inhibition efficiencies. The inhibition efficiency was found larger than their parent amines and a maximum of 98.32% of efficiency was obtained.
2-alkyl-N-benzylidenehydrazinecarbothioamide of fatty acid hydrazides from nontraditional oils (neem, rice bran and karanja) have been synthesized and evaluated as corrosion inhibitors for mild steel in hydrochloric acid solution using weight loss method by Toliwal et al. Adsorption of all Schiff bases on mild steel surface in acid solution obeyed Temkin adsorption isotherm. Inhibition efficiency of these compounds was increased with the concentration of the compound, and varies with solution temperature, immersion time and concentration of acid solution. Various thermodynamic parameters were also calculated to investigate the mechanism of corrosion inhibition [16].
The inhibiting effect of (NE)-4-phenoxy-N-(3-phenylallylidene) aniline (PAC) on the corrosion of mild steel in 1.0 M HCl has been studied by H. Keles et al, very recently, by electrochemical impedance spectroscopy and Tafel polarization measurements. They determined the corrosion rate theoretically in terms of mm per year, using current density values of mild steel in 1.0 M HCl medium. It was found that PAC has remarkable inhibition efficiency on the corrosion of mild steel especially at high temperatures. By thermometric studies they proved that transformation of physical adsorption into chemical adsorption took place as the temperature of the system increased. The thermodynamic functions of adsorption processes were also evaluated. Scanning electron microscope observations of the electrode surface confirmed the existence of a protective adsorbed film of the inhibitor on the electrode surface [17].
D. Gopi et al has reported the corrosion inhibition efficiency of of 3,5-diamino-1,2,4-triazole Schiff base derivatives, based on the effect of changing functional groups. An attempt has been done to establish a relationship between inhibitor efficiency and molecular structure using weight loss method, electrochemical and Fourier transform infrared spectral techniques. They found that the molecules containing more electron donating groups have higher inhibition efficiency than the corresponding compounds with low electron donating groups. The results indicated that the order of inhibition efficiency of the triazole and its Schiff bases in solution and the extent of their tendency to adsorb on mild steel surfaces were as follows: vanilidine 3,5-diamino-1,2,4-triazole > furfuraldine 3,5-diamino-1,2,4-triazole > anisalidine 3,5-diamino-1,2,4-triazole > 3,5-diamino-1,2,4-triazole [18].
A novel Schiff base 2,2′-[bis-N(4-choloro benzaldimin)]-1,1′-dithio has been synthesized by S. M. A. Hosseini et al and its inhibiting action on the corrosion of mild steel in 0.5 M sulfuric acid was investigated by various corrosion monitoring techniques, such as weight loss and potentiodynamic polarization techniques. They showed that this compound acted as a good corrosion inhibitor for mild steel and the inhibition efficiency increased with the inhibitor concentration. This organic compound behaved as mixed type inhibitor in the acid solution, and its adsorption on the mild steel surface was found to obey the Langmuir adsorption isotherm [19].
Recently A. J. A. Nasser et al have investigated the influence of N-[morpholin-4-yl(phenyl)methyl]benzamide (MPB) on corrosion inhibition of mild steel in 1.0 M HCl by weight loss, effect of temperature, potentiodynamic polarization and electrochemical impedance spectroscopic studies. They found that the adsorption of MPB on the mild steel surface obeyed the Temkin adsorption isotherm. Potentiodynamic polarization curves showed that MPB act as a cathodic inhibition predominantly in hydrochloric acid [20].
Schiff base N-[(2-chloroquinolin-3-yl) methylidene ]-2-methylaniline (CQM) was synthesized by S. Jauhari et al and its inhibitive effect on mild steel in 1.0 M HCl solution was investigated by weight loss measurement and electrochemical tests. From the studies, they observed that the inhibition efficiency increased with the Schiff base concentration and reached a maximum at the optimum concentration. This was further confirmed by the decrease in corrosion rate of mild steel with the inhibitor concentration. They also proved that the system follows Langmuir adsorption isotherm [21].
A. S. Fouda et al [22] have investigated the corrosion behavior of carbon steel in 0.5 M HCl solution in the absence and presence of new five Schiff bases of indole derivatives by electrochemical impedance spectroscopy (EIS), electrochemical frequency modulation (EFM) and potentiodynamic polarization techniques. All the experimental results showed that these Schiff bases have excellent corrosion inhibition performance. The polarization curves showed that these compounds act as mixed type inhibitors. The adsorption of these Schiff bases on carbon steel surface is consistent with Langmuir adsorption isotherm. The effect of temperature on the rate of corrosion in the absence and presence of these compounds were also studied.
The inhibiting action of 4-amino-antipyrine (AAP) and its Schiff bases 4-[(benzylidene)-amino]-antipyrine (BAAP), 4-[(4-hydroxy benzylidene)-amino]-antipyrine (SAAP) and 4-[(4-methoxy benzylidene)-amino]-antipyrine (AAAP) which are derived from 4-amino-antipyrine with benzaldehyde, salicylaldehyde and anisaldehyde, towards the corrosion behavior of mild steel in 1.0 M HCl solution was investigated by K. M. Govindaraju et al [23] using weight loss, potentiodynamic polarization, electrochemical impedance and FT-IR spectroscopic techniques. They found that all the synthesized Schiff base compounds were behaved well to retard the corrosion rate very effectively. The inhibitor efficiencies calculated from all the applied methods were in good agreement and were found to be in the order: AAAP > SAAP > BAAP > AAP.
R. K. Upadhyay et al have reported the corrosion inhibition capacity of Schiff bases N-(furfurlidine)-4-methoxy aniline, N-(furfurlidine)-4-methylaniline, N-(salicylidine)-4-methoxy aniline, N-(cinnamalidine)-4-methoxy aniline, and N-(cinnamalidine)-2-methylaniline. They adopted mass loss and thermometric studies to evaluate the inhibition of corrosion of mild steel in hydrochloric acid. Results of inhibition efficiency yielded by the two methods were in good agreement and depend on the inhibitor and acid concentration. Maximum inhibition efficiency of 98% was reported by them [24].
Two series of long chained Schiff base amphiphiles were prepared by condensation of benzaldehyde or anisaldehyde with three different alkyl chain length fatty amines namely: dodecyl, hexadecyl and octadecyl amine by I. A. Aiad et al. The synthesized Schiff bases were evaluated as corrosion inhibitors for low carbon steel in various acidic media (HCl and H2SO4) using weight loss technique. The corrosion inhibition measurements of these inhibitors showed high protection against corrosion process in the tested acidic media at different doses. Attempts to correlate the inhibition efficiency of these compounds with their chemical structures have also been done [25].
Recently S. Issaadi et al have reported the corrosion inhibition studies of novel thiophene based Schiff bases. The Schiff bases, 4,4′-bis(3-carboxaldehyde thiophene) diphenyl diimino ether and 4,4′-bis(3-carboxaldehyde thiophene) diphenyl diimino ethane, were obtained by the condensation of 3-carboxaldehydethiophene and its corresponding amine. Polarization curves revealed that both compounds were mixed type (cathodic/anodic) inhibitors and inhibition efficiency (%IE) increases with increasing concentration of compounds. They suggested that corrosion inhibitive response of the compounds depend on their concentrations and the molecular structures. Adsorption of compounds on mild steel surface was spontaneous and obeyed Langmuir isotherm [26].
The behavior of the Schiff base N,N’-bis(salicylidene)-1,2-ethylenediamine (Salen), and a mixture of its parent molecules, ethylenediamine and salicylaldehyde, as carbon steel corrosion inhibitors in 1.0 M HCl solution was studied by A. B. da Silva et al [27] using corrosion potential measurements, potentiodynamic polarization curves, electrochemical impedance spectroscopy and spectrophotometry measurements. They reported that results obtained in the presence of Salen were similar to those obtained in the presence of the salicylaldehyde and ethylenediamine mixture, showing that in acid medium the Salen molecule undergoes hydrolysis, regenerating its precursor molecules.
Corrosion inhibition investigations of pyridine based Schiff bases were reported by A. Yurt et al [28] on carbon steel in HCl medium using potentiodynamic and ac impedance studies. The Schiff bases under examination were synthesized by the condensation between pyridine-2-carboxaldehyde and respective amines. All compounds were found to act as good corrosion inhibitors.
Scope and Objectives of the Present Investigation
Vigorous research on corrosion and corrosion prevention techniques are undergoing globally by various scientists and surface engineers to minimize the rate of corrosion, since it is a potential threat which may affect directly or indirectly the economy and safety measures. The need for novel corrosion prevention techniques and corrosion inhibitors are increasing day by day. To reduce rate of corrosion of a metal in an aggressive medium with the aid of corrosion inhibitors is a challenging and interesting area of research. Synthesizing novel molecules and monitoring their corrosion inhibition capacities on various metals, especially mild and carbon steels and to implement these molecules as useful corrosion inhibitors are of keen interest for researchers and corrosion/ surface engineers related to metal and petroleum based industries. Even though a large number of organic molecules especially Schiff bases were screened for their corrosion inhibition capacity on metals in acidic media, still remains unanswered questions about the corrosion behavior of various heterocyclic Schiff bases. A very few of the articles have been reported by the previous researchers on the corrosion inhibition behavior of pyridine, thiophene and furfural based Schiff bases which was confirmed by thorough literature survey.
In the present course of investigation it is proposed to determine the corrosion inhibition properties of eight different heterocyclic Schiff bases derived from 3-acetyl pyridine, furan-2-aldehyde and thiophene-2-aldehyde on carbon steel in hydrochloric acid and sulphuric acid by the conventional mass loss studies, electrochemical studies such as Tafel polarization and ac impedance measurements. It is also proposed to investigate the mechanism of corrosion inhibition by plotting various adsorption isotherms. Thermodynamic parameters such as adsorption equilibrium constant and free energy of adsorptions are also proposed to evaluate from adsorption isotherms.
Temperature effect on corrosion was investigated in order to determine thermodynamic parameters such as activation energy, enthalpy and entropy. Present investigation also aims to improve the corrosion inhibition capacities of certain organic molecules by utilizing the synergistic properties of iodide ions. In the present study, an attempt was also made to correlate the corrosion inhibition capacity of these molecules with their structural interactions on carbon steel surface.
It is not a tedious job to create a natural corrosive environment in the experimental settings of a laboratory. At the same time, since natural corrosion is a slow phenomenon and the monitoring of the rate of decay of a metal is very time consuming process, it is customary to adopt accelerated corrosion techniques which will mimic the corrosive environment. Accelerated corrosion tests of various metals are mainly performed in acidic (aggressive) solutions.
To investigate the rate of corrosion and the behavior of corrosion inhibitors, conventionally accepted acceleration tests are mass loss or gravimetric studies and electrochemical studies. Electrochemical studies are mainly subdivided into Tafel polarization studies and electrochemical impedance spectroscopy (EIS). This chapter describes the preparation of metal specimens used for corrosion studies, its composition, aggressive solutions, details of corrosion monitoring techniques employed for the investigation and the electrochemical instrumental set up used for corrosion measurement.
Metal Specimens
Carbon steel (composition: 0.58 %; Mn, 0.07 %; P, 0.02 %; S, 0.015 %; Si, 0.02 % and the rest Fe, determined by EDAX method) were cut in the dimension 1.5x 1.5x 0.114 cm and abraded with various grades of silicon carbide papers (120, 400, 600, 800, 1000 and 1200) to obtain well polished surfaces as per ASTM standards. The total surface area of the metal specimens was accurately determined using vernier calipers and screw gage. Metal specimens were degreased with acetone, washed with detergent and distilled water, dried and finally weighed. Specimens were immersed in aggressive solutions with and without the inhibitor in different concentrations using hooks and fishing lines.
Aggressive Solutions
HCl and H2SO4 (Merck samples) were diluted to 1.0 M and 0.5 M concentrations respectively using distilled water. A stock solution of the inhibitor was first prepared and diluted with respective acidic solutions to obtain inhibitor solutions having concentrations 0.2 mM ‘1.0 mM for performing the corrosion studies of Schiff bases derived from 3-acetylpyridine and solutions in the concentration 0.1 mM- 0.5 mM for Schiff bases derived from furan-2-aldehyde and thiophene-2-aldehyde. The total volume of the medium was 50ml for gravimetric studies but 100ml was maintained for all electrochemical investigations.
Gravimetric Corrosion Studies
Gravimetric corrosion inhibition studies were performed by immersing the well polished carbon steel (CS) specimens in aggressive solutions having different concentrations of the inhibitor for 24 hours. A blank experiment was also conducted without adding the inhibitor. The weight loss occurred for metal specimens were measured after 24 h. For good reproducibility, all experiments were carried out in duplicate and the average values were reported. The corrosion rates and percentage of inhibition efficiencies were calculated by the following equations. The corrosion rates were expressed in mm/y and the inhibition efficiencies were obtained from corrosion rates.
Rate of corrosion W= (K??wt.loss in grams)/(Area in sq.cm ??time in Hrs ??Density) (1)
where ‘K’ =87600 (This is a factor used for the conversion of cm/hour into mm/year)
Density of CS specimen= 7.88g/cc
Percentage of inhibition or the inhibition efficiency (??) was calculated by
??=(W-W’)/W??100 (2)
where W & W’ are the corrosion rate of the CS specimen in the absence and presence of the inhibitor respectively.
Corrosion Inhibition Studies of Parent Compounds
To compare the corrosion inhibition efficiency of Schiff base and its parent aldehyde/ketone and amine, gravimetric corrosion studies of the parent compounds were performed in aggressive solutions for 24 h. This study has considerable significance in two aspects. At first, one can validate the higher inhibition efficiency of the Schiff base when compared to the corrosion inhibition efficiency of parent compounds. Sometimes Schiff base molecules undergo hydrolysis in the acidic media into their parent compounds and an appreciable change in the inhibition efficiency occur with time. In such cases the corrosion inhibition efficiency of the mixture of parent compounds were performed and compared with the inhibition efficiency of Schiff bases. The information regarding the hydrolysis and inhibition efficiency of the hydrolyzed product is the second aspect of this study.
Synergistic Effect Studies
Synergistic effect study was conducted with aggressive solutions (sulphuric acid) together with 0.2 mM KI solutions. Gravimetric studies and electrochemical studies were performed separately to check the synergistic effect of iodide ions with the Schiff base molecules on carbon steel surface. If synergism plays, the addition of KI (1ml, 0.2 mM KI for gravimetric and 2ml for electrochemical studies) into the aggressive solution will raise the corrosion inhibition efficiency drastically.
Adsorption Isotherms
The mechanism of inhibition of various organic molecules on the surface of a corroding metal can be well explained by adsorption. To verify the nature of interaction between the metal surface and inhibitor molecules, adsorption isotherms were plotted by calculating the surface coverage from the inhibition efficiency. The different models of adsorption isotherms proposed was Langmiur, Freundlich, Temkin and Frumkin and the recently formulated thermodynamic/kinetic model, El-Awady isotherm. Among the isotherms mentioned above, the most suitable one was chosen with the help of correlation coefficient. The important thermodynamic parameters such as adsorption equilibrium constant (Kads) and free energy of adsorption (‘Goads) were calculated from the adsorption isotherms. These parameters are of key important in predicting the spontaneity of the process and the nature of adsorption i.e., physisorption or chemisorption or a combination of both. The important models of adsorption isotherms considered and the equation for the free energy of adsorption isotherm are given as follows [29-32].
Langmiur adsorption isotherm C/?? = 1/K_ads + C (3)
Freundlich adsorption isotherm ??=K_ads C (4)
Temkin adsorption isotherm ef?? = Kads C (5)
Frumkin adsorption isotherm ??/(1-??) exp'(f??)=K_(ads )C (6)
El-Awady adsorption isotherm log ??/(1-??)=’logK’_ + y logC (7)
In the above equations C represents the concentration of the inhibitor, ?? is the surface coverage and Kads is the adsorption equilibrium constant. In El-Awady isotherm, Kads = K1/y, where y= number of active sites. If 1/y is less than 1, it implies multilayer adsorption and if 1/y is greater than 1, suggests that a given inhibitor molecule occupies more than one active site. Free energy of adsorption is related to adsorption equilibrium constant by the following equation.
‘G0ads = -RTln(55.5 Kads) (8)
Surface Analysis Using SEM
Surface morphological studies give insight to the mechanism of inhibition by which an organic molecules decrease the rate of corrosion. This was done by taking the scanning electron micrographs (SEM) of the metal surfaces at different conditions. SEM images of well polished bare metal specimen, metal specimen in acid solution (blank, treated for 48 h) and specimens in the inhibitor solution (treated for 48 h) were taken in the resolution 2.00x and compared. Hitachi SU6600 model scanning electron microscope was used for performing the surface morphological studies.
Temperature Studies
Gravimetric corrosion inhibition studies were performed in the temperature range 30-600C for investigating various thermodynamic parameters of corrosion such as enthalpy of corrosion (‘H*), entropy of corrosion (‘S*), activation energy (Ea) and Arrhenius parameter (A). The rate of corrosion is related to the energy of activation by the well known Arrhenius equation
K=A exp'(-E_a/RT ) (9)
where K is the rate constant, A is pre exponential or Arrhenius factor, Ea is the activation energy, R is the universal gas constant and T is the temperature in Kelvin scale. From the above equation it is evident that a plot of logK Vs 1000/T will be a straight line having slope -Ea/2.303R and intercept log A.
The enthalpy and entropy of activation (‘H*, ‘S*) were calculated from the transition state theory [33]
K= (RT/Nh ) exp ((‘S*)/R) exp ((-‘H*)/RT) (10) Here, N is the Avogadro number and h is the Planks constant. The equation can be rewritten in the form y= mx +c to obtain
log K/T=log R/Nh+’S/(2.303 R)-‘H/(2.303 R T) (11)
The slope of the above equation is -‘H/2.303R, from which enthalpy of activation can be calculated. Entropy of activation can be calculated from the intercept of the above equation i.e., log R/Nh+’S/(2.303 R) (12)
Electrochemical Investigations
It is well known that corrosion is an electrochemical phenomenon. The measurable electrochemical parameters such as corrosion current density, corrosion potential, charge transfer resistance, cathodic and anodic slope values (from current-potential response) etc will quantify the corrosion and help one to predict the rate of corrosion and to determine the mechanism of the corrosion. In the presence of corrosion inhibitors, the electrochemical parameters change considerably and hence affect the rate of corrosion. Exploiting these responses of corroding metals in the presence and absence of the inhibitor with the help of sophisticated electrochemical systems, is the major strategy adopted by the corrosion researchers to predict the inhibitive capacity of the various organic molecules in acidic media. Widely practiced electrochemical corrosion measurement techniques are Electrochemical Impedance Spectroscopy (EIS) and polarization studies. Polarization techniques are further classified into Tafel polarization analysis and linear polarization resistance analysis. Applications of the electrochemical methods are widely accepted for the investigation of corrosion [34-39].
The main advantage of the electrochemical investigation than the conventional gravimetric studies is that the former one require short span of time. All electrochemical measurements are computer assisted, and most modern softwares and electrochemical systems are using for the corrosion analysis. More refined and accurate values for the electrochemical analyses is thus possible than the conventional time consuming gravimetric investigations.
In the present corrosion investigation, Ivium compactstat-e (made in Netherlands) electrochemical system was used. Latest version of the software ‘IviumSoft’ was powered the electrochemical analysis. Various analytical procedures like selection of proper equivalent circuit, simulation of curves obtained by the analysis, calculation of resistance and current densities etc can be easily performed with the software.
For all electrochemical measurements a cell with three electrode assembly was used. Platinum electrode having area 1cm2 was used as the counter or auxiliary electrode and saturated calomel electrode (SCE) was the reference electrode. Well polished metal surface having an exposed area of 1cm2 towards the corroding medium acted as the working electrode.
The three electrode system eliminates the limitations of the conventional two electrode system. The conventional two electrode set up consists of a working electrode and a reference electrode only. A desired potential is applied in a controlled way on the working electrode to facilitate the charger transfer process during the electrochemical experiments. A second electrode having a fixed potential must be used in conjunction with the working electrode to gauge the exact potential of working electrode by balancing the charge added or removed by the working electrode. This setup has serious shortcomings since practically it is very difficult to maintain the constant potential of the reference electrode during the passage of current to the working electrode. These limitations can be overcome with the help of three electrode assembly (Figure 2.1)
Fig. 2.1 Three electrode circuitry
In the above figure, CE, RE and WE represent counter, reference and working electrodes respectively. Additional electrodes namely counter or auxiliary electrode is inserted into the cell assembly. Now the role of the reference electrode is to control and measure the electrode potential of the working electrode only and practically no current is passed through the it. The auxiliary electrode allows the passage of whole current required for balancing the current of the working electrode.
Polarization studies
Polarization studies are performed by changing the applied potential of the working electrode in a controlled manner and scanned at constant rate (potentiodynamic). Mainly, Polarization studies can be divided into two a) Tafel extrapolation technique (Stern method) and b) Polarization resistance studies (Stern and Geary method) [40-43].
Tafel extrapolation technique
The basis of the polarization techniques is mainly derived from mixed potential theory proposed by Wagner and Traud [44,45]. According to this theory, an overall electrochemical reaction can be algebraically divided into half cell reactions i.e., oxidation and reduction half cell reactions. In the cases of iron/copper based metal specimens, the reaction usually takes place at anodic areas is M ‘M2+ + 2e-, where ‘M’ represents iron/copper. The main cathodic reaction takes place during corrosion is the reduction of H+ ions to H2. Since the cathodic reaction is slower than the anodic process, the rate is usually controlled by the cathodic reaction. If the rate of anodic and cathodic processes is equal, there will be no charge accumulation. The mixed potential at this moment is called the open circuit potential (OCP) and commonly designated as corrosion potential or Ecorr. This corrosion potential is distinctly different from the reversible potential of the corroding metal or the species in the solution that is reduced at the cathode. The current at this mixed potential is designated as corrosion current density and denoted by icorr.
An electrode can be polarized by the application of external voltage. The magnitude of polarization can be measured in terms of overvoltage i.e., the difference between the equilibrium potential and the external potential. Polarization can be either anodic direction (noble) or cathodic direction (active). To get iapp (measured or applied current density) as a function of E (applied potential), applied potential between the reference electrode and the working electrode is controlled and scanned at constant rate. The important types of polarization that occur during electrochemical measurements are activation polarization and concentration. Since the main steps in the electrochemical corrosion are controlled by activation or charge transfer, the effect due to concentration polarization can be neglected. For the reversible electrodes which are controlled by activation process, the polarization can be best described by the equation similar to Butler-Volmer equation [46]
i_app= i_corr {exp[ (??_(a ) )/RT zF(E-E_corr ) ]-exp[-(??_(c ) )/RT zF(E-E_corr ) ] } (13)
iapp is applied or measured current density; icorr is corrosion current density; ??a and ??c are the charge transfer coefficients for anodic and cathodic reactions, respectively. E-Ecorr is the polarization or the over potential obtained by the difference between applied and corrosion potential; z is metal valence; F is Faraday constant; R, the gas constant and T is the absolute temperature.
When the polarization is in the anodic direction (positive), E>> Ecorr and the second term in the above equation can be neglected. Now equation 13 can more simply expressed as
i_app= i_corr {exp[ (??_(a ) )/RT zF(E-E_corr ) ] } (14)
The Tafel slope for the anodic process from the above equation is
b_a=(2.303 RT)/(??_a zF)
Similarly, for cathodic process, Ecorr>> E, then the first term in equation 13 can be neglected. The simplified equation for the cathodic reaction can be expressed as
i_app= i_corr {exp[- (??_(c ) )/RT zF(E-E_corr ) ] } (15)
Tafel slope for cathodic reaction can be expressed as
b_c=(2.303 RT)/(??_c zF)
The mixed potential diagram and Tafel extrapolation are described in the Figure 2.2. From the obtained current densities, the percentage of inhibition can be calculated by the following equation
‘ ‘??_(pol )%=(I_corr-‘I^”_corr)/I_corr X100 (16)
where icorr and i’corr are uninhibited and inhibited corrosion current densities respectively.
Fig. 2.2 Tafel extrapolation method Fig. 2.3 Linear polarization method
The slopes of Tafel lines have a significant role in interpreting the mechanism of inhibitor. By comparing the Tafel slopes of the uninhibited and the inhibited solutions, one will get the idea about the nature of inhibition. If the anodic slope (ba) of the inhibited solution only deviates considerably from the anodic slope of the uninhibited solution, it can be assumed that inhibitor molecule affect on anodic process of corrosion. Similarly, the change of cathodic slope (bc) alone is an indication of the adsorption of the inhibited molecules on the cathodic sites. An appreciable change in both ba and bc assures that the inhibitor molecule affect both anodic and cathodic process of corrosion. Tafel extrapolation as well as other polarization techniques badly affect by several factors such as ohmic resistance (arises from the resistivity of the solution) cell geometry, location of the reference electrode, magnitude of the applied current etc, which are few among this. Ohmic resistance may also contribute to overvoltage error sometimes [35,36,47,48].
Linear polarization resistance method
For small overpotential reactions with respect to Ecorr, Stern and Geary modified the kinetic equation [43]. For the activation controlled process the equation can be linearized as
i_corr=((‘i_app)/(2.303 ‘E))((b_a b_c)/(b_a+b_c )) (17)
Rearranging the above equation
i_corr=(1/(2.303 R_p ))((b_a b_c)/(b_a+b_c ))=B/R_p (18)
where Rp is ‘E/’i, called the polarization resistance and ‘B’ is a constant obtained by all the constant terms in the above equation. Most often iapp shows approximate linearity with potential. When one determine the slope of this plot(Figure 2.3) at Ecorr, it may call as polarization resistance and it will be inversely proportional to the corrosion rate [49-50].
Linear polarization technique can be applied for the corrosion monitoring studies in the sense that the polarization resistance (Rp) increases with the inhibitor concentration. The rate of charge transfer process will be decreased considerably as the inhibitor molecules ‘work’ on the surface of the corroding metal by adsorption. In this scenario the rate of corrosion decreases appreciably. From the slope analysis of the linear polarization curves at the corrosion potential, the polarization resistances were obtained.
The corrosion inhibition efficiency can be calculated using the equation
??_(R_p )%=(‘R^”_p-R_p)/’R^”_p X100 (19)
where R’p and Rp are the polarization resistance in the presence and absence of inhibitor respectively [51]
Electrochemical impedance spectroscopy (EIS)
Impedance measurements are of key importance in predicting the corrosion rate of the metal in the aggressive solutions. Studies like corrosion behavior of metals which are coated with protective layers and of the inhibitive role of corrosion inhibitors on the metal surface etc can be easily performed with EIS measurements. In EIS technique, the working electrode is subjected to a small amplitude sinusoidal potential at a number of discrete frequencies ??. The resulting currents at each frequency will display sinusoidal response that is out of phase with applied potential signal by an amount ??. The amplitude of the current is now inversely proportional to the impedance of the interface Z(??). Electrochemical impedance is thus related to excitation voltage and the current response of the system by the following equation
Z(??) = V(??)/i(??) (20)
where V(??) is the excitation voltage and can be expressed as a function of time i.e., V = V0 sin(??t). Here V0 is the amplitude of the signal. In equation 20, ‘i’ is the time varying current density and is given by i = i0 sin(??t + ??), where i0 is the amplitude of the current signal.
Impedance Z(??) can be expressed as a complex function with the real and imaginary components whose values are frequency dependent
Z(??) = Z'(??) + jZ”(??) (21)
where Z'(??) is the real component of the impedance and given by Z'(??) = Z0cos??. Z”(??) is the imaginary part of the impedance i.e., Z”(??) = Z0sin??, j is the imaginary number ‘(-1) and Z0 is the magnitude of the impedance.
The impedance response of a corroding system enables us to determine the equivalent circuit that exactly fit to the electrochemical system under examination. In many cases the simple equivalent circuit which mimic the corroding system consists of two resistances and one capacitance which are aligned in the following manner [52]
Fig. 2.4 Equivalent circuit fitting
where Rs is the solution resistance, Rct is the charge transfer resistance and Cdl is the double layer capacitance. According to the above model circuit, at very low frequencies impedance will be equal to the sum of charge transfer resistance Rct and solution resistance Rs while at higher frequencies impedance will be very close to the solution resistance Rs.
Representation of Impedance Data
Impedance data can be well represented by the Nyquist (Cole-Cole) plot, Bode plot and impedance plot.
Nyquist plot
In this method, the real part of the impedance (x-axis) is plotted against the imaginary component of the impedance (y-axis). The obtained curves will have semicircle shapes. At high frequencies, the impedance of the system is almost equal to the ohmic resistance Rs (or solution resistance). The frequency reaches its high limit at the leftmost end of the semicircle, where the semicircle touches the x- axis. The frequency reaches its low limit at the rightmost end of the semicircle where the impedance of the system will be Rs+Rct (Figure 2.5).
Each point on the Nyquist plots is the impedance at one frequency. It is estimated that if a compound has a corrosion inhibition capacity, the charge transfer resistance of the corroding metal in the presence of that compound will be higher than that of charge transfer resistance of the metal in the absence of the inhibitor in a corroding medium. From the values of the charge transfer resistances the inhibition efficiency of a particular compound can be calculated by the following equation.
??_(EIS )%=(R_ct-‘R^”_ct)/R_ct X100 (22)
where Rct and R’ct are the charge transfer resistances of working electrode with and without inhibitor, respectively [51]. Nyquist plots have one serious disadvantage that one can’t obtain the frequency at a particular point from the curve.
Bode plot and impedance plot
For a simple equivalent circuit represented in Figure 2.4, Bode plot is obtained by drawing the phase angle as a function of frequency. In this plot the absolute impedance and the phase angle, ??, of the resultant wave form is plotted as a function of frequency. The curve representing log|Z| versus log frequency is called the impedance plot. The absolute value of impedance Z is calculated from the equation |Z| = ‘(‘Z”^2+’Z”’^2 ) . Analysis of the impedance plot will provide the values of Rs and Rct. The break point of the impedance curve should lie on a straight line (at intermediate frequency) whose slope will be -1. Extrapolation of this straight line to the y-axis at f=1 or logf = 0 gives the value of Cdl.
|Z|= 1/C_dl (23)
The combined Bode and impedance model plot is given in the Figure 2.6.
A chemist is always curious to check the drug ability of the newly synthesized molecules in the laboratory. Many drugs are discovered accidently by the blind screening of the synthesized molecules on various diseases. Though the modern pharmaceutical chemistry is mainly interested in the development of drugs or modifying the already existing drugs by quantitative structure activity relation approaches (QSAR models), the scope for searching the drug ability of the newly synthesized molecules by random screening approach is still extant. In the present course of study it is proposed to monitor the drug ability of the newly synthesized heterocyclic azomethine class of compounds and their metal chelates against the growth of various pathogens.
Metal Chelates in Pharmacology
The modern organic and inorganic chemistry has contributed many compounds to the pharmaceutical field which have potential activities against various diseases. Medicinal inorganic chemistry can exploit the unique properties of metal ions for the design of new drugs. Many metal chelates are found to have drug abilities on various diseases. Metals are the integral part of many structural and functional components in the body, and the critical role of metals in physiological and pathological fields has always been a subject for researchers. Transition metals exhibit different oxidation states and can interact with a number of negatively charged or electron rich molecules. This ability of transition metals to coordinate with the molecules led into the development of metal based drugs with promising pharmacological activities. This has, for instance, led to the clinical application of chemotherapeutic agents for cancer treatment, such as cisplatin (cis-dichlorodiammine platinum(II)). The development of modern medicinal inorganic chemistry is inspired very much by the discovery of cisplatin (Figure 3.1)
On administration, one of the chloride ligands is slowly substituted by water molecule and the process is termed as aquation. The aqua ligand in the resulting [PtCl(H2O)(NH3)2]+ is itself easily displaced, allowing the platinum atom to bind to bases of DNA. Of the bases on DNA, guanine/purine is primarily coordinate with cis-platin, leading to the formation of adduct [PtCl(base-DNA)(NH3)2]+ (Figure 3.2). After the displacement of next chloride ligand by the water molecule, cross linking can occur with another base. Many types of cisplatin’DNA coordination complexes, or adducts, can also be formed and these complexes are referred to 1,2-intrastrand adducts.
Fig. 3.1 Structure of Cisplatin Fig. 3.2 DNA-cis platin adduct
Chelation therapy is the preferred medical treatment for reducing the toxic effects of metals in the body. Chelating agents are capable of binding with toxic metal ions to form complex structures which are easily excreted from the body and removing them from intracellular or extracellular spaces. CaNa2EDTA etc has long been the support of chelation therapy (Figure 3.3) for lead and cadmium poisoning [1]. Dimercaprol (Figure 3.4) is used for the treatment of arsenic, antimony, lead, mercury and other toxic metal poisoning. It acts as a very strong chelating agent for various toxic metal ions. Dimercapto succinic acid [2] (Figure 3.5) is indicated for the treatment of lead poisoning in children with blood level measured above 45 ??g/dL.
Fig. 3.3 Structure of metal-EDTA complex
Fig. 3.4 Dimercapol chelation
Fig. 3.5 Structure of dimercapto succinic acid
Zinc pyrithione (Figure 3.6) is best known for its use in treating dandruff and seborrhoeic dermatitis [3]. It also has antibacterial properties and iseffective..against..many pathogens from..the..Streptococcus and Staphylococcus genera..Its..other..medical..applications..include..treatments.of psoriasis, ec-zema, ringworm, fungus, athletes foot, dry skin, atopic dermatitis, tinea, and vitiligo.
Fig. 3.6 Structure of zinc pyrithione
It was shown that Zinc pyrithione inhibits fungal growth through increased cellular levels of copper, damaging iron-sulphur clusters of proteins essential for fungal metabolism [4].
Gold salt complexes (sodium aurothiomalate) (Figure 3.7) have been used to treat rheumatoid arthritis. Even though the mechanism of action of this drug is not completely studied, it is believed to interact with albumin and eventually be taken up by immune cells, triggering anti-mitochondrial effects and eventually cell apoptosis. It was established that reactive aldehydes such as malondialdehyde, glycoaldehydes and presumably acrolein and 3-aminopropanal [5, 6] may be the ultimate mediators of cell destruction in rheumatoid arthritis joints. P. L Wood et al [7] demonstrated that thiol-containing disease-modifying antiarthritic agents both directly hide reactive aldehydes and augment intracellular thiol pools, which also can buffer increased aldehyde load and oxidative stress. These data are consistent with clinical data that penicillamine lowers synovial aldehyde levels and augments plasma thiols. In the case of D-penicillamine (Figure 3.8) and sodium aurothiomalate, the key structural feature appears to be a free thiol group.
Fig. 3.7 Structure of sodium aurothiomalate Fig. 3.8 Structure of D-penicillamine
An antibiotic is an agent that kills or inhibits the growth of bacteria. There are many classes of antibiotics, with the classical ones being sulfonamides, penicillins, cephalosporins and aminoglycosides. Many antibiotics are produced naturally by microorganisms (e.g. produced by fungi in the genus Penicillium), while some of these natural compounds provide a building block for the manufacture of synthetic antibacterials for example, the sulfonamides, the quinolones and the oxazolidinones are produced solely by chemical synthesis. Most antibiotics from natural origin have fewer side effects than synthetic compounds and this can be largely attributed to their increased target specificity. Side-effects range from mild to very serious depending on the antibiotics used, the microbial organisms targeted, and the individual patient.
Penicillins and cephalosporins are ??-lactam antibiotics [8-10] which act on the cell wall of bacteria. They share the structural feature of a ??-lactam ring and inhibit the formation of peptidoglycan cross-links within the cell wall, which weakens the wall osmotically and causes cell death. Ampicillin (Figure 3.9), Penicillin-G (Figure 3.10) are the examples for ??-lactam antibiotics
Fig. 3.9 Structure of Penicillin-G Fig. 3.10 Structure of Ampicillin
Streptomycin (Figure 3.11) is the first of a class of drugs called aminoglycosides to be discovered [11,12]. It was the first antimicrobial agent developed after penicillin and the first antibiotic effective in treating tuberculosis. It is derived from the actinobacterium Streptomyces griseus. Streptomycin was discovered by American biochemists Selman Waksman, Albert Schatz, and Elizabeth Bugie in 1943. The drug acts by interfering with the ability of a microorganism to synthesize certain vital proteins. It is used in combination with penicillin for treating infections of heart valves (endocarditic) and with tetracyclines in the treatment of plague, tularaemia, and brucellosis. Adverse effects of this medicine are ototoxicity, nephrotoxicity, fetal auditory toxicity, and neuromuscular paralysis.
Fig. 3.11 Structure of Streptomycin Fig. 3.12 Structure of Gentamicin
Gentamicin (Figure 3.12) is an aminoglycoside antibacterial consists of a linked ring system composed of aminosugars and an aminosubstituted cyclic polyalcohol [13]. They bind to proteins in the ribosome of the bacteria and prevent DNA replication. Aminoglycosides are poorly absorbed when given orally and so are administered intravenously. They also exhibit high toxicity, affecting the ear and kidney. Gentamicin is used to treat many types of bacterial infections, particularly those caused by Gram-negative organisms.
Gentamicin is also ototoxic and nephrotoxic, with this toxicity remaining a major problem in clinical use. It is synthesized by Micromonospora, a genus of Gram-positive bacteria widely present in the environment (water and soil). It was discovered in 1963 by Weinstein, Wagman et al [14-16].
Erythromycin (Figure 3.13) is an antibiotic useful for the treatment of a number of bacterial infections. Erythromycin is a macrolide antibiotic produced by Streptomyces erythreus and has an antimicrobial spectrum similar to or slightly wider than that of penicillin, and is often prescribed for people who have an allergy to penicillins. It inhibits bacterial protein synthesis by binding to bacterial 50S ribosomal subunits; binding inhibits peptidyl transferase activity and interferes with translocation of amino acids during translation and assembly of protein [17,18]. Cefotaxime (Figure 3.14) is a ??-lactam antibiotic (which refers to the structural components of the drug molecule itself). As a class, ??-lactams inhibit bacterial cell wall synthesis by binding to one or more of the penicillin-binding proteins (PBPs). This inhibits the final transpeptidation step of peptidoglycan synthesis in bacterial cell walls, thus inhibiting cell wall biosynthesis [19].
Fig. 3.13 Structure of Erythromycin Fig. 3.14 Structure Cefotaxime
Over many years, some bacterial strains have developed resistance to commercial antibacterial compounds. Resistance usually arises either after long periods of exposure to the drug or in conditions that may support the gradual stepwise development of bacteria. Resistance can arise through either a modification of the target site or enzyme, prevention of access for the antibiotics or production of enzymes that destroy or inactivate the antibiotic.
A combination of mutations and alterations can lead to the occurrence of resistant. The problem of bacterial resistance to antibiotics has preoccupied many scientists for years. Aside from bacteria that have undergone mutations, making them resistant to antibiotics, another kind of bacterium exists as well, which is inherently unaffected by antibiotic treatment, called ‘persistent bacteria’.
Important Pathogens
Bacteria are prokaryotic cells that are essential for life on earth. They are present in every inhabitant on the planet, from soil to hot springs and even deep within the Earth’s crust [20,21]. They have various roles in both the environment and in the bodies of humans and animals. They decompose matter from dead organism and return vital nutrients to the earth. Bacteria have a vital role in balancing several constructive and destructive processes in the environment. But some bacteria are harmful to life of humans, animals and plants by causing diseases. Based on the structural characteristics of the cell walls of bacteria, Hans Christian Gram divided the bacteria into two types using gram stain test. Crystal violet was used for the experiment and he divided the bacteria as Gram-positive and Gram-negative. Gram-positive organisms are able to retain the crystal violet stain because of their thick peptidoglycan layer, which is superficial to the cell membrane. This is in contrast to Gram-negative bacteria, which may have a thick or thin peptidoglycan layer that is located between two cell membranes [22].
Escherichia coli
Escherichia coli (abbreviated E. coli), is a Gram-negative rod shaped bacterium (Figure 3.15) that is commonly found in the lower intestine of humans and warm-blooded animals. They are part of the normal flora of the gastrointestinal tract and are the dominant species in the aerobic faecal flora of humans. In 1885, a German paediatrician, Theodor Escherich, first discovered this species in the feces of healthy individuals and called it Bacterium coli commune due to the fact that it was found in the colon and early classifications of prokaryotes placed these in a handful of genera based on their shape and motility. They are actively mobile due to the presence of flagella.
Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans and are occasionally responsible for product recalls due to food contamination. The harmless bacteria are part of the normal flora of the gut and can benefit their hosts by producing vitamin K [23] and by hindering the establishment of pathogenic bacteria within the intestine [24-25].
The bacterium can also be grown effortlessly and inexpensively in a laboratory setting and has been intensively investigated for over 60 years. E. coli is the most widely studied prokaryotic model organism and an important species in the areas of biotechnology and microbiology, where it has served as the host organism for the majority of work with recombinant DNA.
Transmission of E. coli is by faecal-oral contact (usually by ingestion of food and water) and the presence of E. coli in water or soil is an indicator of faecal contamination. The bacterium produces toxins, known as Shiga toxins, which damage the lining of the intestines and other target organs such as the kidneys. They can cause diarrhoea, urinary tract infections, meningitis, wound infections and pneumonia. Strains of E. coli resistant to many broad spectrum antibiotics have come out over the last number of years.
Staphylococcus aureus
Staphylococcus aureus is a Gram-positive spherical bacterium, a member of the Firmicutes and is commonly found in the human respiratory tract [23] and on our skin (Figure 3.16). Methicillin-resistant Staphylococcus (MRSA) is a strain of S. aureus that is resistant to the ??-lactam class of antibiotics, including the penicillins and cephalosporins. Emerging of antibiotic-resistant forms of .S. aureus (e.g. MRSA) is a worldwide problem in clinical medicine [27].
It can be transmitted by touch alone, and due to this, hospitals and nursing homes, containing patients which are more susceptible to infection, are ideal breeding grounds for it. MRSA becomes particularly problematic if it enters the body and treatment is generally by administration of glycopeptide antibiotics like vancomycin and teicoplanin. These antibiotics inhibit the growth of bacterial cells by binding to the amino acids in the cell walls and preventing peptidoglycan synthesis. S. aureus was discovered in Scotland in 1880 by Sir Alexander Ogston [28].
S. aureus can cause a range of illnesses, from minor skin infections, suchas pimples,impetigo, boils (furuncles), cellulitis..folliculitis, carbuncles, scalded.skin…syndrome…and.. abscesses,.to…life..threatening…diseases.such.as… pneumonia,.. meningitis,….osteomyelitis,.. endocarditis, …toxic……shock..syndrome (TSS), bacteremia and sepsis. It is still one of the five most common causes of nosocomial infections and is often the cause of postsurgical wound infections.
Bacillus subtilis
Bacillus subtilis is a Gram-positive, rod shaped bacteria, commonly found in soil. It was originally named ‘Vibrio subtilis’ when it was discovered in 1835 by Christian Gottfried Ehrenberg. It was renamed as ‘Bacillus subtilis’ in 1872 by Ferdinand Cohn. This bacterium is besides known by the names hay bacillus, grass bacillus or Bacillus globigii. Unlike several other well known species, B. subtilis has been historically classified as an obligate aerobe, though recent research has demonstrated that this is not strictly correct [29,30].
Bacillus subtilis (Figure 3.17) is an endospore forming bacteria, and the endospore that it forms allows it to withstand extreme temperatures as well as dry environments. B. subtilis is considered as obligate aerobe, but can also function anaerobically in the presence of nitrates or glucose. It is not considered pathogenic or toxic and is not a disease causing agent. Bacillus subtilis has a flagellum which makes motility faster. Since this bacterium is resistant to extreme temperatures, it can withstand high cooking temperatures. This is not to cause alarm, as it does not cause sickness, if ingested. This bacterium can cause a stringy consistency in spoiled bread dough, if dough is exposed.
Although this species is commonly found in soil, more evidence suggests that B. subtilis is a normal gut commensal in humans. A study in 2009 compared the density of spores found in soil (~106 spores per gram) to that found in human feces (~104 spores per gram). Bacillus subtilis is readily present everywhere; the air, soil and in plant compost. Along with enzymes, B. subtilis also produces a toxin called subtilisin. Subtilisin can cause allergic reactions if there is repeated exposure in high concentrations. This only poses a risk to fermentation plants that use high quantities of subtilisin.
Bacillus thuringiensis
B. thuringiensis (Bt) is a Gram-positive, soil-dwelling bacterium, commonly used as a biological pesticide; alternatively, the Cry toxin may be extracted and used as a pesticide. B. thuringiensis (Bt) is a gram-positive, soil-dwelling, spore-forming, rod-shaped bacteria. It is approximately 1 ??m in width and 5 ??m in length. It grows at body temperature and produces a diamond-shaped crystal from its crystal proteins and uses it to fend off insects, predators, and other pathogens [31,32].
B. thuringiensis (Figure 3.18) occurs naturally in the gut of caterpillars of various types of moths and butterflies, as well as on leaf surfaces, aquatic environments, animal feces, insect rich environments, flour mills and grain storage facilities [33]. B. thuringiensis was first discovered in 1901 by Japanese biologist Ishiwata Shigetane. In 1911, B. thuringiensis was rediscovered in Germany by Ernst Berliner, who isolated it as the cause of a disease called Schlaffsucht in flour moth caterpillars. During sporulation, many bacterial strains produce crystal proteins (proteinaceous inclusions), called ??-endotoxins, that have insecticidal action. This has led to their use as insecticides and more recently to genetically modified crops using Bt genes. Many crystal-producing Bt strains, though, do not have insecticidal properties [34].
It was first used as a commercial insecticide in France 1938, and then in USA in 1950s. However, these early products were replaced by more effective ones in the 1960s, when various highly pathogenic strains were discovered with particular activity against different types of insect. B. thuringiensis subspecies are neither toxic nor pathogenic to mammals, including humans [35,36]. Animal experimentation, however, has shown that intraperitoneal injection of B. thuringiensis can cause death in guinea pigs and that pulmonary infection can result in the deaths of immune compromised.
Proteus vulgaris
P.’.vulgaris ‘is a Gram-negative rod-shaped bacterium (Figure 3.19) inhabits in the intestinal tract of humans. It can be found in soil, water and fecal matter. It belongs to the family entero bacteriaceae and is an opportunistic pathogen of humans. Urinary tract infections and wound infections are mainly caused by this bacterium.
The first use of the term ‘Proteus’ (meaning: God of rivers in Greek) in bacteriological nomenclature was made by Gustav Hauser (1885) who described under this term three types of organisms which he isolated from putrefied meat. One of the three species, Hauser identified was P. vulgaris and this microbe has a long history in microbiology and pathology.
Major taxonomic revisions have been undergone over the past two decades for the genus Proteus. On the basis of production of indole from tryptotphan, these bacteria were classified into three biogroups in 1982. Biogroup one was indole negative bacteria and represented a new species P. penneri; while biogroup two and three clubbed together as P. vulgaris which satisfactorily answered the indole test [37].
Enterobacter aerogenes
E. aerogen- is a nosocomial and pathogenic bacterium, that causes opportunistic infections including most types of infections…E..aerogenes are (Figure 3.20) rod shaped Gram-.negative bacteria which do not answer indole test.. E. aerogenes are commonly found in the human gastrointestinal tract and does not generally cause disease in healthy individuals. It has been found to live in various wastes, hygienic chemicals and soil. Majority of E..aerogenes are sensitive to most antibiotics designed for this bacteria group, but this is entangled by their inducible opposition mechanisms, particularly lactamase which means that they rapidly become resistant to standard antibiotics during treatment. This needs change of antibiotics to avoid worsening of the disease like sepsis [38].
Some of the infections caused by E. aerogenes result from specific antibiotic treatments, venous catheter insertions and/or surgical procedures. This bacterium has a definite role commercially, in producing hydrogen gas during the fermentation of molasses.
Metal Chelates as Antibacterial Agents- A Review
Many of the metal chelates have been screened for their antimicrobial activity by various scientists and researchers. Among these active metal complexes found, many Schiff base metal chelates have got amazing antimicrobial activity against various bacteria and fungi. Furthermore it has been generally observed that metal chelates were demonstrated higher activity than their respective Schiff bases. The probable mechanism of inhibition of the growth of the microbes by the metal chelates suggested by various researchers is discussed in the third chapter. Reviews of the literature containing the antimicrobial activity of metal chelates of Schiff base are given below.
A series of Schiff bases derived from 2-acetylpyridne and 4-(2-aminoethyl)morpholine, and 4-(2-aminoethyl)piperazine and their metal complexes were synthesized and characterized by N. S. Gwaram et al [39]. The complexes were screened for anti-bacterial activity against Methicillin-resistant Staphylococcus aureus (MRSA), Acinetobacter baumanni (AC), Klebsiella pneumonie (KB) and Pseudomonas aeruginosa (PA) using the disc diffusion and micro broth dilution assays. Based on the overall results, the complexes showed the highest activities against MRSA while a weak antibacterial activity was observed against A. baumanii and P. aeruginosa.
Metal complexes of Ni(II), Co(II), Cu(II), Mn(II), Zn(II) and VO(IV) with a Schiff base derived from 3-ethoxy salicylaldehyde and 2-(2-amino-phenyl) 1-H-benzimidazol(2-[(Z)-{(2-(1H-benzimidazole-2yl)phenyl]imino}methyl]-6-ethoxy phenol-BMEP) were synthesized successfully by M. Sunitha et al [40]. Antimicrobial activity of the ligand and its metal complexes were studied against two Gram-negative bacteria: Escherichia coli, Pseudomonas fluorescence and two Gram-positive bacteria: Bacillus subtilis, Staphylococcus aureus. The activity data show that the metal complexes are more potent than the free ligand. Among the studied complexes, Cu(II) complex displayed higher inhibitory effect on the growth of E. coli. Zn(II) chelate also demonstrated elevated inhibitory action against the growth of all pathogens.
Recently E. Yousif et al [41] have synthesized five new metal complex derivatives of 2N-salicylidene-5-(p-nitro phenyl)-1,3,4-thiadiazole, HL with the metal ions VO(II), Co(II), Rh(III), Pd(II) and Au(III) in alcoholic medium. The preliminary in vitro antibacterial screening activity revealed that all complexes showed moderate activity against tested bacterial strains such as S. aureus, S. typhi and E. coli bacterial than the ligand. Agar diffusion method was employed for the determination of inhibition zone. Rhodium complex showed poor antibacterial activity against the growth of S. aureus. The activity of the gold complex was comparatively higher than all other complexes against the growth of S. typhi. All metal chelates equally exhibited good antimicrobial activity against E. Coli. Authors claim that it is due to the large size of the metal chelates they exhibit moderate antibacterial capacity since the cell penetration power decreases with increase in the molecular size.
Novel transition metal [Co(II), Cu(II), Ni(II) and Zn(II)] complexes of substituted pyridine Schiff-bases (derived from substituted aminopyridine and salicylaldehyde) have been prepared and characterized by physical, spectral and analytical data by Z. H. Chohan et al [42]. In order to evaluate the effect of metal ions upon chelation, the Schiff bases and their complexes have been screened for antibacterial activity against the strains such as E. coli, S. aureus, and P. aeruginosa. The complexed Schiff bases have shown to be more antibacterial against one more bacterial species as compared to uncomplexed Schiff-bases. Paper disc diffusion method was used for the antibacterial screening. All metal ions have varying antibacterial influence on bacterial species. The Co(II) complex of hydroxyl substituted Schiff base was more antibacterial against one species and less against the other as compared to the Co(II) complex of the other Schiff bases (bromo, nitro and methoxy substituted). Same results were found for other metal complexes. They assure that metal ions do play a significant role in enhancing the antibacterial activity of antibacterial agents on chelation. They suggest that in the chelated complex, the positive charge of the metal ion is partially shared with the donor atoms and there is electron delocalization over the whole chelate ring. This increases the lipophilic character of the metal chelate and favours its permeation through lipid layers of the bacterial membranes.
N. Raman et al [43] have synthesized novel tetradentate N2O2 type Schiff base, from 1-phenyl-2,3-dimethyl-4-aminopyrazol-5-one(4-aminoantipyrine) and 3-salicylidene-acetylacetone and the stable complexes with transition metal ions such as Cu(II), Ni(II), Co(II) and Zn(II) in ethanol. The in vitro antimicrobial activities of the investigated compounds were tested against bacteria such as K. pneumoniae, S. aureus, B. subtilis and E. coli and fungi like A. niger and R. bataicola. All the metal chelates showed higher antimicrobial activity for the above microorganisms than that of the free ligand.
The synthesis, characterization, spectroscopic and biological properties of trans-[CoIII(L1)(Py)2]ClO4 and trans-[CoIII(L2)(Py)2]ClO4 complexes, where H2L1 = N,N’-bis(5-chloro-2-hydroxybenzylidene)-1,3-propylenediamine and H2L2 = N,N’-bis(5-bromo-2-hydroxybenzylidene)-1,3-propylenediamine, have been done by M. Salehi et al [44]. The in vitro antimicrobial activity of the Schiff base ligands and their corresponding complexes have been tested against human pathogenic bacteria such as S. aureus, B. subtilis, P. aeruginosa, and E. coli. The cobalt(III) complexes showed lower antimicrobial activity than the free Schiff base ligands.
Mixed ligand complexes of type ML’B (M(II)=Mn(II), Co(II), Ni(II), Cu(II) and Zn(II); HL’=o-vanillidene-2-aminobenzothiazole; B=1,10-phenanthroline) and Schiff base metal complexes of types (ML2″) and (M2L”) (HL”= o-vanillidene-2-amino-N-(2-pyridyl)-benzene sulfonamide) were synthesized and characterized by M. A. Neelakantan et al [45]. The newly synthesized ligands and their metal complexes were screened in vitro for their antibacterial activity against bacteria: E. coli, P. aeruginosa, S. Typhi and V. parahaemolyticus by well diffusion method using agar nutrient. The antifungal activities were tested against fungus: A. Niger, Penicillium, T. virida and yeast: S. cerevisiae by well diffusion method using potato dextrose agar as the medium. Ampicillin and nystatin are used as control for bacteria and fungi, respectively. They found that majority of all complexes are found to be active against various microorganism. Some of them showed the inhibitive action very near to the standard antibiotic. But the metal chelates of the Schiff base o-vanillidene-2-amino-2-N(2-pyridyl)-benzene sulfonamide was totally inactive against the growth of E. coli and P. aeruginosa whereas the nickel complex of Schiff base o-vanillidene-2-aminobenzothiazole exhibited escalated antibacterial activity against the growth of E. coli and P. aeruginosa than the standard antibiotic. Also the antifungal acivity of the metal chelates were appreciable when compared to the activity of standard drug.
The coordination complexes of VO(II), Co(II), Ni(II) and Cu(II) with the Schiff bases derived from isatin with 3-chloro-4-fluoroaniline and 2-pyridinecarboxaldehyde with 4-aminoantipyrine have been synthesized by conventional as well as microwave methods by A. P. Mishra et al [46]. The Schiff base and metal complexes show a good activity against the bacteria; Staphylococcus aureus, Escherichia coli and S. fecalis and fungi A. niger, T. polysporum, C. albicans and A. flavus. The antimicrobial results also indicate that the metal complexes are better antimicrobial agents as compared to the Schiff bases. The minimum inhibitory concentrations of the metal complexes were found in the range 10~40 ??g/mL. Streptomycin and nystatin were used as the standard antibiotics and miconazole was taken as the standard antifungal.
Mechanism of Growth Inhibition
The complete mechanism of the inhibitory action of the metal chelates against the growth of several micro organisms has not yet widely studied. In the present chapter an attempt was made to explain the mechanism of inhibition in tandem with the mechanistic pathways of the growth inhibition of the standard antibiotics. By learning the structural features of microorganisms, microbiologists could reach into possible conclusions regarding the obstruction of the growth of microorganism by antibiotics. Antimicrobial therapy is based on the selective toxicity of the agents on the microbial cells. Antibacterial drugs may be either bacteriostatic or bactericidal. Bacteriostatic agents will inhibit the growth of bacteria while bactericidal drugs will kill bacteria. The activity of penicillins, streptomycin and gentamicin are bacteriocidal in nature, but antibiotics erythromycin, cindamycin, chloramphenicol etc act as bacteriostatic agents.
Based on the cellular structure of bacteria or the function of the affecting antibiotics, the role of the antibiotics may be generally one among the following.
They may 1) inhibit nucleic acid synthesis
2) inhibit protein synthesis
3) inhibit cell wall biosynthesis
4) alter or inhibit cell membrane permeability or transport
5) inhibit folate metabolism
6) antimetabolites
Pictorial representation of sites of the action of antibiotics inside and outside the cell of bacteria is given Figure 3.21.
Fig 3.21 Mechanism of action of antibiotics in microbial cells
The first class of antimicrobial drugs that interfere with cell wall synthesis is the beta-lactam antibiotics). This includes penicillin derivatives. The second class of antimicrobial drugs that interfere with cell wall synthesis are the glycopeptide antibiotics. Significant glycopeptide antibiotics include vancomycin, teicoplanin etc.
Antifolate drugs will inhibit the synthesis of folic acid (vitamin B9). A well known example is Trimethoprim. It is a bacteriostatic antibiotic used mainly in the prophylaxis and treatment of urinary tract infections [47].
The production of the nucleic acid (DNA and RNA) is inhibited by particular antibiotics. (e.g. metronidiazole). These act by generating metabolites that are incorporated into DNA strands, which then are more prone to breakage. These drugs are selectively toxic to anaerobic organisms, but can affect human cells [48]. Protein synthesis inhibitors in general work at different stages of prokaryotic mRNA translation into proteins like initiation, elongation and termination.
Tetracyclines block the A site on the ribosome, preventing the binding of aminoacyl tRNAs. Aminoglycosides interfere with the proofreading process, causing an increased rate of error in synthesis with premature termination. Chloramphenicol blocks the peptidyl transfer step of elongation on the 50S ribosomal subunit in both bacteria and mitochondria. Antimetabolites are structural analogs of normal metabolites that inhibit the action of specific enzymes. They include bacteriostatic (sulfonamide, trimethoprim, para-aminosalicylic acid) and bactericidal (isoniazid) drugs [49].
Microbial organisms attain resistance to certain drugs. They gain the resistance through different means but primarily based on the chemical structure of the antimicrobial agent and the mechanisms through which the agents acted [50].
Resistance can be described in two ways:
Non genetical pathways: arises due to the alteration or loss of the specific receptors, e.g. cell wall membrane.
Genetical reason: results by the mutation of the bacterial cell. This mechanism may be either chromosomal or extra chromosomal resistance. A chromosomal mutation alters the structure of the receptor of the drug or the permeability of the drug. In extra chromosomal mechanism, plasmid enzymes degrade or modify the drug. The different ways in which a bacterium achieves resistance to a drug was effectively studied by Fluit et al [51]. The mechanism is summarised in Figure 3.22.
Fig. 3.22 Illustration of how some antimicrobial agents are rendered ineffective (Fluit et al., 2001)
Scope and Objectives of the Present Investigation
Previous researchers have established that many Schiff base and their metal chelates show antimicrobial activities and other pharmaceutical abilities. But thorough investigations are going on in this area to find out effective antimicrobial agents which can yield prolonged action without affecting the normal cells of living beings adversely.
Contemporary medicines that are widely used as antibiotics are seriously suffering from the potential threat of resistance by various bacteria. Any use of antibiotics can increase selective pressure in a population of bacteria to allow the resistant bacteria to thrive and the susceptible bacteria to die off. The overuse and misuse of antibiotics are considered as the two important reasons for resistance to the microbes. Since the resistance towards antibiotics becomes more common, a serious need for alternative antibiotics arises.
The present investigation which explores the potential activity of the newly synthesized Schiff bases and their metal chelates is very relevant in this scenario. By examining the in vitro drug abilities of these newly synthesized molecules, a class of compounds with high potential antimicrobial activities may emerge.
Cyclic voltammetry (CV) is an important electrochemical technique by which the redox behaviour of molecules can be determined. The rate of oxidation/reduction and the mechanism of probe coupled chemical reaction can be well established using CV studies. Redox behaviour of the chemical species helps the analyst to perform qualitative and quantitative analysis. CV analysis is accomplished with a three electrode arrangement whereby the potential relative to some reference electrode is scanned at a working electrode, while the resulting current flowing through a counter (or auxiliary) electrode is monitored in a quiescent solution. This technique is ideally suited for a quick search of redox couples present in a system. Discouraging of CV for widespread quantitative analysis is due to two important difficulties a) changing current at high scan rates and b) data analysis from the asymmetric shaped peaks.
In CV experiment (which is a simple extension of linear sweep technique), the potential of a small stationary electrode is altered linearly with time. Potential gradually changes from a region where no electrode reaction occurs, to potentials where oxidation or reduction of an analyte occurs. The direction of sweep is reversed after travelling the potential regime in which one or more electrode reaction occurs and the electrode reactions are monitored for the products or intermediates which are formed during the forward scan [1-3]. The total potential traversed and the sweep rate of the analysis determine the time of the experiment.
Similar to the polarographic analysis, the major form of mass transport is the diffusion of analyte to the electrode surface in a quiescent solution. CV is characterized by several parameters such as cathodic (Epc) and anodic (Epa) peak potentials, the cathodic (ipc) and anodic (ipa) peak currents, the cathodic half peak potential (Ep/2) and half wave potential (E1/2). The definition of E1/2 has been adopted from the conventional polarography, according to equation 1.
E_(1/2)= E^0+ RT/nF ln[D_(R/) D_O ]^(1/2) (1)
where E0 is the formal potential pertaining to the ionic strength of the solution used, D0 and DR are the diffusion coefficients of the oxidized and reduced states and n is the number of electrons in the half reaction. Since D0′ DR, E1/2=E0 usually within a few mV of E0.
Cyclic voltammetric systems can be divided into three major divisions, based on the behaviour of analyte towards the applied potential. These are a) reversible system b) irreversible system and c) quasi reversible system.
Cyclic Voltammetry of a Reversible System
When the electrode kinetics is much faster than the rate of diffusion, the system is said to be reversible. Figure 4.1 illustrates the cyclic voltammogram of are reversible system. In such systems, the electron transfer reaction at the electrode surface is so rapid that equilibrium conditions are maintained even with substantial net current and a rapidly changing potential. By the analysis of this figure, the equation for the peak current in the linear sweep voltammetry at 298K can be verified by Randles and Sevick equation (eqn 2) [4,5].
i_p=(2.69′??10’^(5 ))n^(3/2) AD_0^(1/2) C_0 ??^(1/2) (2)
where A is the area of the electrode in cm2, D0 is the diffusion coefficient in cm2s-1, C0 is the concentration of the analyte in mol/cm3, ?? is the scan rate in mV/s and ip is the peak current in amperes. For reversible systems, the peak potential Ep is given by the equation 3.
E_P=E^0-1.109 RT/nF (3)
The peak potential may be difficult to analyze, as the peak is broad and therefore it is easy to calculate the half peak potential E1/2 at ip/2, which is given by equation 4.
E_(1/2)=E^0+1.09 RT/nF (4)
From equation 3 and 4,
|E_P-E_(p/2) |=2.20 RT/nF=56.5/n mV (5)
Equation 5 can be regarded as the first criterion for the reversibility. The separation of peak potentials i.e., Epa-Epc is another significant parameter in CV which can be used to ascertain the reversibility of a process.
??E_p=E_pa-E_pc=2.218RT/(nF )’57/n mV (6)
Thus value of peak potential separation (??Ep) at 298K and |Ep/2-Epc| are independent of scan rate and concentration. The ratio of peak currents ipa/ipc is independent of the scan rate for reversible systems, and this is regarded as the third significant parameter.
Cyclic Voltammetry of Irreversible Systems
When the electrode kinetics is slower than the rate of diffusion, the system is said to be irreversible. For one electron, one’step irreversible process (Ox+e’Red), inverse peak will not emerge in the cyclic voltammogram on changing the direction of scan.
Fig. 4.1 Cyclic voltammogram of a reversible system
Since irreversible reactions give relatively very small current function (about 50%) compared to that of reversible process, a bigger overpotential is required to lead the reduction and therefore peak potential appears at higher values beyond E0. The peak current and the peak potential for irreversible system at 298K is given by equations 7 and 8 respectively.
i_p=(2.99′??10’^(5 ))??^(1/2) AD_0^(1/2) C_0 ??^(1/2) (7)
E_p= E^0- RT/??F [‘0.78+ [D’_0^(1/2) ‘/k’_0 ]^ +ln”[??F??/RT]^(1/2) ‘ (8)
From the above equations,
|E_P-E_(p/2) |=2.20 RT/??F (9)
where k0 is the standard rate constant and ?? is the transfer coefficient. From equation 8, it is clear that for a totally irreversible system (kf>>kb for the cathodic peak and kb>>kf for the anodic peak), Ep depends on the scan rate [6,7]. Irreversibility manifests through ??Ep i.e., Epa-Epc>(57/n) mV and ??Ep increases with increasing scan rate. Figure 4.2 represents the cyclic voltammogram of an electrically irreversible system.
Fig. 4.2 Cyclic voltammogram of an irreversible system
Cyclic Voltammetry of Quasi-reversible Systems
Quasi-reversible systems are intermediate between reversible and irreversible systems. For such systems ip is not sufficiently proportional to ??1/2. The peak potential is represented by an integral equation which is solved by numerical methods. For quasi reversible system, (with 10-1>k0>10-5cm/s), the current is controlled by both the charge transfer and mass transport. The shape of cyclic voltammogram is a function the ratio k_0/([ D_0^(1/2) (‘F/RT)’^(1/2) ??^(1/2)]). As the ratio increases, the process approaches to the reversible case [8,9]. The voltammogram of a quasi reversible system are more drawn out and exhibit larger separation in peak potential compared to reversible system. To calculate the rate constant for a quasi-reversible reaction (??Ep up to 200mV), following equation can be used.
??=k_s ‘[D_o/D_R ]’^(??/2)/'[D_0 ??(nF/RT)]’^(1/2) (10)
where ?? is a function of quasi reversible system whose values bring by numerical approach. Figure 4.3 depicts the cyclic voltammogram of a quasi reversible system.
Fig. 4.3 Cyclic voltammogram of a quasi reversible system
Cyclic Voltammetric Studies on Semicarbazones and Thiosemicarbazones and Their Metal Complexes- A Review
Many organic molecules [10-15] and their metal complexes have been screened for cyclic voltammetric studies to understand the electrochemical response of the molecules. Subsequent paragraphs illustrate the summary of CV investigations on various organic molecules and complexes reported by previous researchers.
B. Lotf et al [16] have reported the anodic oxidation of 4-methoxybenzaldehyde semicarbazone derivatives examined in acetonitrile containing lithium perchlorate (LiClO4) as a supporting electrolyte (20g/l). The electrode system contained a carbon paste working electrode, a platinum wire as counter electrode and saturated calomel as reference electrode. The voltammetric curve obtained for the oxidation of 4-methoxybenzaldehyde semicarbazone shows two oxidation peak potential at 0.9V and the second at 1.35V and no reduction peak potential.
Reduction behaviour of 2-acetylpyridine semicarbazone was studied on glassy carbon electrode in CH3OH-Britton Robinson buffer at pH 5, 7 & 9 using cyclic voltammetry by R. Sangtyani et al [17]. Single irreversible reduction wave is observed due to the reduction of semicarbazone moiety. The effect of change in pH and sweep rate was evaluated. The electrode process was found to be irreversible and diffusion controlled. Kinetic parameters were calculated from cyclic voltammetric measurements. The electrochemical behaviour of isatin-3-hydrazone, 7-methylisatin-3-hydrazone, isatin-3-semicarbazone, isatin-3-thiosemicarbazone, 5-bromoisatin-3-thiosemicarbazone and 5-nitroisatin, at a glassy carbon electrode, using cyclic voltammetry over a wide pH range, was investigated and compared with isatin by B. S. C. Oliveirra et al [18]. Two consecutive irreversible peaks were observed by isatin-semicarbazone. Two reversible cathodic peaks were observed on the first negative going reversible cycle and peak potential separation suggests that two electron process is involved.
Recently, R. K Christian et al [19], studied the electrochemical properties of alpha-N-heterocyclic chalcogen semicarbazones (HL), namely, thiosemicarbazones, selenosemicarbazones, and semicarbazones, and their Ga(III), Fe(III), and Ru(III) complexes [ML2][Y] (M = Ga, Fe or Ru; Y = PF6, NO3- or FeCl4-) by cyclic voltammetric technique. The cyclic voltammogram of the Ga complexes displayed at least two consecutive reversible one-electron reduction waves. These reductions are shifted by approx. 0.6 V to lower potentials in the corresponding Fe and Ru complexes. The electrochemical, chemical and spectroscopic data indicate that the ligand centered reduction takes place at the C=N double bond.
A new bimetallic copper(II) complex has been synthesized with ligand, obtained by the condensation of salicylaldehyde and the amine derived from reduction of nitration product of benzyl by B. Sarma et al [20]. Cyclic voltammetry of Cu(II)L’2H2O in CH3CN was determined using platinum disc as working electrode and Ag’AgCl electrode as the reference. The cyclic voltammetric profile was of quasi reversible one with the redox potential value +0.105 V ??0.005 V. This redox potential is due to Cu(II)/Cu(I) redox couple. The ratio of cathodic to anodic current was found to be 0.949.
S. Datta et al [21] demonstrated that reaction of salicylaldehyde semicarbazone (L1), 2-hydroxyacetophenone semicarbazone (L2), and 2-hydroxynaphthaldehyde semicarbazone (L3) with [Pd(PPh3)2Cl2] in ethanol in the presence of a base (NEt3) affords a family of yellow complexes. In these complexes, the semicarbazone ligands are coordinated to palladium in a rather unusual tridentate ONN-mode, and a PPh3 also remains coordinated to the metal centre. Cyclic voltammetry on all the complexes showed an irreversible oxidation of the coordinated semicarbazone within 0.86’0.93V vs SCE, and an irreversible reduction of the same ligand within ‘0.96 to ‘1.14 V vs SCE.
The coordination behaviour of ferrocenylthiosemicarbazone was investigated in a trinuclear [Ni(Fctsc)2] complex by R. Prabhakaran et al [22]. The redox behaviour of the trinuclear complex in DMF has been studied using cyclic voltammetry at 100 mV scan rate by using a platinum wire counter electrode and a platinum disc working electrode. All the potentials are referenced to Ag/AgCl electrode. The complex exhibits a pair of redox waves on both the positive and the negative potential. They confirmed that the first reduction is due to a single electron Ni(II)’Ni(I) reduction, whereas the second step, which is double in intensity, compared to the first, may be due to the concomitant reduction of both ferrocenyl subunits in two one-electron reductions. The first reduction is observed at -0.35 V for Ni(II)’Ni(I) and the second reduction is at -1.4 V corresponding to Fe(II)’Fe(I). Reversible oxidation potentials are observed for Ni(II)’Ni(III) and Fe(II)’Fe(III) at 0.2 V and 0.8 V, respectively.
Scope and Objectives of the Present Investigation
The electrochemical behaviour of many organic molecules and metal complexes has been investigated by various researchers by voltammetric studies. Majority of the investigations which are primarily done in the area of organic and inorganic fields of research, were helpful to supplement the structural features of the molecules and check the affinity of the metal ion to organic molecules which are acting as ligands. By investigating the electrochemical response of newly synthesized compounds, the probable regions in the molecule which are susceptible to oxidation or reduction process can be evaluated. This idea will be beneficial when the molecules are serving as agents especially in the electrical and analytical fields of applications. In the present course of investigation the newly synthesized Schiff bases and their complexes were screened for their electrochemical response using cyclic voltammetry. By evaluating the voltammetric parameters, an attempt was made to categorize the molecular systems as irreversible or quasi irreversible. To check the factors which govern the rate determining step, adsorption of the molecules on the electrode surface, response towards the higher scan rates etc were the other fields of interest. An attempt was also done to propose the mechanism for the electrochemical behaviour of the synthesized molecules in DMSO.

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