Paper type: Review Pages: 8 (1866 words)
Based on the literature review, it is noteworthy that the metal complexes of Schiff bases were playing a significant character in progress of coordination chemistry . Schiff’s bases exhibit chances for persuading substrate chirality, extensively used as homogeneous or heterogeneous catalyst[150-152], potentiometric sensor[153,154], light emitting diode. The Schiff bases were used to synthesize conjugate polymers containing azomethine bond in biological activities [156, 157] . K.Shiva Prasad et al. described the preparation of Cu(II) Schiff base complex (Figure. 4) derived from 4(3H)-quinazolinone and the characterization of the synthesized complex was carried out by physical and spectral methods.
Based on the spectral data the ligand acts as bidentate ligand with nitrogen and oxygen as coordinating sites. Copper, Nickel, and Zinc metal complexes (Figure. 5) derived from phenylazo- and thiazolylazo-2,4-pentanediones derivatives was reported by K. Krishnankutty etal.  The characterization of the ligand and its metal complexes were carried out 1H-NMR, IR and mass spectral data. The data shows that ligand acts as tetradentate ligand.
Figure: 4 Figure: 5 Schiff bases metal complexes exhibit extensive range of biological application. N. Raman et al. stated the synthesis of novel transition metal complexes of Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), VO (IV) and Hg(II) resulting from Schiff base derivative of 3-hydroxy-4-nitrobenzaldehyde, 4-aminoantipyrine and o-phenylenediamine. Based on UV-Vis, IR, mass, 1H NMR and ESR spectral studies the characterization of ligand and its metal complexes were carried out. ESR and UV-Visible studies shows that all the metal complexes exhibit square planar geometry (Figure 6) except VO(IV) which exhibited a square pyramidal geometry. The metal complexes exhibited good antimicrobial activity than compared to a free ligand. Figure: 6 K.R. Joshi et al. reported the synthesis of two novel Schiff bases 2-((3,4-difluorophenylimino)methyl)-6-methoxy-4-nitrophenol (FPM) and 2-((2,4-dimethylphenylimino)methyl)-6-methoxy-4-nitrophenol (MPM) by refluxing 2-hydroxy-3-methoxy-5-nitrobenzaldehyde with 3,4-difluoroaniline or 2,4-dimethylaniline. The metal complexes Cu(II) and Ni(II) were synthesized by the above Schiff base derivatives which were characterized by spectral analysis such as mass, 1H-NMR, UV-Visible, IR and TGA studies (Figure 7). From the spectral studies it is evident that O, N are coordinating with the metal complexes and thus ligand acting as bi-dentate form. Antimicrobial studies were performed by Broth Dilution method showed that the ligand and its complexes exhibited good results. Figure: 7 Copper shows effective positive redox potential and binding ability [162-165]. It is one of the well-known bio-essential element and its complexes comprising of heterocyclic Schiff bases have been explored to show cytotoxic activity and strong DNA interactions. A series of isonicotinoyl hydrazones were synthesized by P. Ramadevi etal. through template method which were further complexed with Cu (II). The ligands with azomethine nitrogen and enolic oxygen are coordinated to Copper (II) ion thus exhibiting a square planar geometry (Figure 8). DNA binding studies and bovine serum albumin binding tendencies of the complex was determined spectrophotometrically. The complexes showed the high binding constant values for DNA interaction studies and good binding propensity to BSA. Cytotoxic studies were performed on A549 human lung cancer cell and by various staining techniques the mode of cell damage was examined, it was found to be apoptotic. Figure: 8 The preparation of neutral tetra-dentate chelate complexes of Manganese (II), Cobalt (II), Nickel (II), Copper (II) and Zinc (II) from Schiff base derivatives of 2-aminothiophenol/2-aminophenol and acetoacetanilido-4-aminoantipyrine was stated by Natarajan et.al . Based on analytical studies, magnetic moment, FT – IR, 1H- NMR, UV Visible and ESR spectral studies structures of the chelates were established. In-vitro antimicrobial studies of the compounds were studied against the S. typhi, S. aureus, K.pneumoniae, Bacillus, Pseudomonas and A. niger. The complexes exhibited good activity than the free Schiff base ligands. Recently in the treatment of cancer cis-platin and its derivatives are extensively used in anticancer treatments, but they cause numerous side effects. Consequently, bioinorganic chemists are concentrating to design and synthesize new Schiff base metal complexes. P. Rabindra Reddy and co-workers. reported the synthesis of [Cu(FDIB)Phen]+2 (Figure 9) resulting from the Schiff base ligand FDIB (fluorine diimino benzene). A distorted square planar geometry has been proposed based on the spectral data and the ligand is acting as N, N donor. Cytotoxic studies of the complex against HeLa cell line showed that the complex exhibited more activity than compared to the drug doxorubicin (standard). Figure: 9 A.D. Tiwari and co-workers , stated Co(II), Ni(II), Cu(II) and Zn(II) metal complexes synthesis and their DNA binding studies. The complexes were prepared by refluxing thiocarbohydrazide with pyridine-2-carbaldehyde. The synthesized ligand and its complexes structural elucidation was done by IR, electronic spectroscopy, 13C-NMR, 1H-NMR and mass spectral studies. Antimicrobial studies of compounds were performed on E. coli. Escherichia coli plasmid DNA was used to perform DNA binding studies by electronic absorption spectroscopy and Kb (intrinsic binding constants) values were calculated. Based on the results obtained, the metal complexes showed an enhanced interaction because of trans-geometrical isomers than ligand which is a cis isomers. DNA is frequently mentioned as the molecule of heredity, as it is the cause for propagation of genetic material for all traits. Considerable interest on DNA binding studies to different types of metal complexes was shown over the past decades. Different transition metal complexes have been used as tools to study the structure of DNA, to know the DNA interaction studies, as DNA cleavage agents and have remained as DNA probes. Based on the above studies, N. Vamsikrishna and co-workers, reported the synthesis, antimicrobial, DNA interactions and cleavage studies of a Co(II), Ni(II) and Cu(II) complexes derived from two novel Schiff base ligands L1=2-(-(benzothiazol-6-ylimino)methyl)-4-nitrophenol and L2=2-(-(benzothiazol-6-ylimino)methyl)-4-chlorophenol(BTEMCP). Based on the analytical and spectral studies square planar geometry has been proposed for all the metal complexes. DNA binding studies revealed the intercalative mode of binding of metal complexes towards DNA. Gel electrophoresis was used for cleavage studies and the complexes can cleave pBR DNA. Biological studies revealed that the metal complexes showed effective antimicrobial activity than the free ligands. DNA interaction studies, synthesis and the characterization of Cefixime metal complexes such as [Fe(cefbenz)2(H2O)Cl3].5H2O, [Ni (cefbenz)2Cl2]6H2O, [Cu(cefbenz)2Cl2], 2H2O and [Zn(cefbenz)2Cl2] 2H2O was described by S. PushpaLatha. IR, ESR, NMR, UV-Visible and XRD spectroscopic techniques were used to characterize the synthesized compounds. From the spectral studies, square planar geometry has been proposed for the metal complexes. DNA binding studies and higher biological activities of the complexes than the parent drug shows that the synthesized complexes might be of interest for upcoming research. In current years, many researches are focusing on DNA interaction studies using small molecules. Since, Small molecule can interrelate with DNA by non-covalent interactions i.e, external electrostatic interaction, groove binding, intercalation etc. Intercalation is one of the most important DNA-binding modes among all these interactions. DNA interaction, synthesis, characterization and pharmacological studies of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes derived from substituted benzophenone Schiff base derivatives was reported by P. Subbaraj et. al.. The synthesized compounds were characterized by analytical spectral and analytical studies. The complexes exhibited octahedral and tetrahedral geometry. Biological, DNA binding and cleavage studies of the synthesized compounds were carried out. Metal complexes of Schiff bases can inhibit rust formation of mild steel in oilfield formation by forming a protective film. M.H. Mahross et al., based on the above concept, described the corrosion inhibition action of Cu(II), Ni(II) and Zn(II) metal complexes derived from Salicylaldehyde thiosemicarbazone (STSC) ligand. In this research article, among all the synthesized complexes Cu (II)- STSC complex showed better effective results compared to the other complexes. Based on the polarization studies corrosion inhibition mechanism was explained. SEM technique was used to explain the surface morphology. The results interpreted by Quantum chemical parameters and the hypothetically calculated AE parameters were found to be closely correlated to the inhibition efficiencies of inhibitors. The theoretical and experimental results were found to be in good agreement. Gold nanoparticles (AuNPs) have shown considerable attention for their widespread applications in drug distribution [177,178], MRI (magnetic resonance imaging) [179, 180], X-ray CT (X-ray computed tomography) , catalytic process , biosensing [183,184], and so on because the Schiff base ligands and their metal complexes can easily control the shape, size, and surface functionalization of AuNPs. Ligand Coated Gold Nanoparticles for the Chemical Sensing of ferric ions was reported by Abiola Azzez Jimoh . The paper explains the synthesis of a simple and delicate nano gold-based Schiff base chemosensor that shows greater selectivity to ferric ions than compared to other cations in a solvent mixture of ethanol or water. The novel Schiff base ligand synthesized was coated with AuNPS (gold nanoparticles). The structural elucidation of the synthesized ligand was carried out by photoluminescence, electronic absorption spectroscopy, TEM and TGA studies. The detection limit for ferric ions was found to be 1.2µM, without interference from other metal ions. From the absorption and emission studies, binding constants were found to be 2.9 x 105 M-1 and 8.5 x 105M-1 respectively, and the mode of bonding was 1:1. Therefore, from sensing point of view the synthesized compound can be used in the biological system with good sensitivity and selectivity for the detection of ferric ion. Ruthenium complexes derived from Schiff base derivatives show excellent pharmacological activity. Research on anti-proliferative studies of ruthenium complexes has increased in the recent years. Based on the above study, C. Shoba Devi and S. Satyanarayana , reported the synthesis, DNA binding studies of [Ru(L4)(PyIP)]+2 and [Ru(L4)(AIP)]+2 complexes of Ru(II) (Figure 10) where L is pyridine and 4-amino pyridine, PyIP=2-(1-pyrenyl)-1H-imdizole[4,5-f][1,10]phenanthroline and AIP= 2-(-9-anthryl)-1H-imidazole[4,5-f][1,10]phenanthroline. The complexes were characterized by spectral and analytical studies. DNA binding studies were performed by UV Visible absorption, fluorescence and viscosity measurements. The order of binding affinities of these complexes is given as [Ru(Py)4(AIP)]+2 < [Ru(4-APy)4(AIP)]+2 < [Ru(Py)4(PyIP)]+2 < [Ru(4-APy)4(PyIP)]+2. Nuclease studies of metal complexes was performed on pBR 322 DNA and the complexes showed effective cleavage studies. The above results showed that the complexes can act as intercalators and can be used as probe of DNA conformation. Figure: 10 P. Nagababu et al. stated the synthesis of [Ru(en)2phen]+2, [Ru(en)2bpy]+2, [Ru(en)2IP]+2 and [Ru(en)2PIP]2+ (Figure 11) (where phen= 1,10-phenantroline, bpy=2,2-bipyridine, IP= imidazol[4,5-f[1,10]phenanthroline and PIP=2-phenylimidazol[4,5-f][1,10]phenanthroline). 1H-NMR, IR and UV-Visible studies was carried out to know the structural elucidation of the metal complexes. DNA interaction studies revealed that the complexes bind to CT- DNA by groove binding. DNA cleavage studies showed that the complexes can effectively cleave plasmid DNA and act as efficient photosensitizers. Figure: 11 P. Nagababu and co-workers  reported the synthesis and characterization of [Ru(en)(aip)]+2, [Ru(en)(pyip)]+2 and [Co(en)(aip)]+3 and [Co(en)(pyip)]+3 (Figure 12) where (aip= 2(-(9-anthryl)-1H-imidazo[4,5-f][1,10]phenanthr-oroline, pyip = (2-(1-pyrenyl)-1H-imidazo[4,5-f][1,10]phenanthroline, en=ethylendiamine). The binding studies were investigated by absorption, emission spectroscopy, viscosity measurements as well as DNA melting and plasmid DNA cleavage studies. The above results suggested that the complexes bind to DNA through intercalation and all the four complexes showed cleavage of plasmid DNA pBR 322. Figure: 12 [Cu2(salophen)2] (1) and [Cu2(salen)2] (2) complexes (Figure 13) derived from ethylenediamine or o-phenylenediamine with salicylaldehyde was reported by A. Shilpa et al. The geometry of the complexes is square planar and ligand acts as N2O2 donor. The complexes exhibited excellent binding propensities with CT-DNA. Complexes showed good DNA-cleavage abilities under oxidative and hydrolytic conditions. DNA cleavage studies showed that complex 1 could effectively cleave DNA than compared to complex 2 which may be due to the existence of extended conjugation in complex 1.
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