Biological aspects of Schiff base–metal complexes derived from benzaldehydes: an overview

Schiff bases are stable imines containing C=N, where N is bonded to an alkyl or aryl group, but not with hydrogen and are prepared by condensation of aliphatic or aromatic primary amine with carbonyl compounds. They have the general formula R₁R₂C?=?NR₃, where R₃?≠?H. The presence of the basic donor N atom and the stability of the imine function render Schiff bases as the most favored ligands that have the ability to stabilize metal ions in different oxidation states. The chelating environment in a Schiff base profoundly influences the electron distribution in the coordination sphere of metal in a complex and thereby regulates the property of the compounds in a big way. The structural diversity in some of the metal complexes with multidentate Schiff base ligands has triggered a wide range of applications of this class of compounds in sensors, catalysis, biology, medicines, and photonics. This review compiles the synthesis and biological activities (antimicrobial, antioxidant, anticancer, antitubercular, DNA interaction studies) of benzaldehyde-based Schiff bases and their metal complexes.     

Asymmetric cyclopropanation catalyzed by copper–Schiff’s base complexes

The asymmetric cyclopropanation of styrene with alkyl diazoacetate were performed with copper complexes of Schiff bases, derived from substituted salicylaldehydes and a new chiral amino alcohol, as the catalysts. The electronic and steric properties, as well as the position of those substituents show obvious effects on the enantioselectivities, and ee higher than 98% were achieved under optimal conditions.     

Symmetric binuclear complexes with an ‘end-off’ compartmental Schiff base ligand

Four binuclear transition metal(II) complexes: [Co₂L(-Cl)Cl₂] · 2H₂O, [Ni₂L(-Cl)Cl₂(H₂O)₂] · 2H₂O, [Cu₂L(-Cl)Cl₂] · 2H₂O and [Zn₂L(-Cl)Cl₂] · 2H₂O, where LH is the binucleating ligand 2,6-diformyl-4-methylphenol bis(2-hydrazino benzothiazole), were prepared. Based on the i.r. spectra, elemental analysis, conductivity measurements and thermal analysis, we propose that these complexes contain an endogenous phenoxide bridge and an exogenous chloride bridge. Magnetic and spectral data supports the existence of a weak antiferromagnetic interaction between the metal ions, and octahedral for Ni^II and a square pyramidal environment for remaining complexes. The compounds show significant growth inhibitory activity against the fungi, Aspergillus niger and Candida albicans, as compared to antibacterial activity against Bacillus cirroflagellosus and Pseudomonas auregenosa.     

Copolymer of lactide and ε-caprolactone catalyzed by bimetallic Schiff base aluminum complexes

A series of copolymers of lactide(LA) and e-caprolactone(ε-CL) with different monomer feed ratios were achieved using three kinds of bimetallic Schiff aluminum complexes as catalysts. The ratios of LA and ε-CL units in different copolymers and the average segments length were determined by NMR analysis. The comparative kinetic study of L-LA/ε-CL and rac-LA/ε-CL copolymerization systems showed that the polymerization rate of LA was faster than ε-CL, and L-LA showed polymerization rate slightly faster than rac-LA. It was inferred that the copolymers achieved by these complexes were gradient copolymers with gradual change in distribution of LA and e-CL units. The thermal properties of these copolymers were characterized by DSC analysis, which showed that the glass transition temperature(Tg) of these copolymers changed regularly according to the pro-portion change of two structural units.     

Thermodynamic behavior and stability constants of lanthanides–Schiff base complexes in mixed aqueous solvents

The stability constant of complexes of Ln³⁺ ions and butanamide-3-[(2-hydroxyphenyl methylene)amino]ethylimino N-phenyl were determined potentiometrically in 75% (v/v) dioxane?Cwater, 0.1?M (KNO₃) at different temperatures applying the Irving and Rossotti techniques. The nonlinear variation of the thermodynamic parameters (??G, ??H, and ??S) as a function of ionic potential of lanthanide elements was discussed. This behavior was explained in terms of difference in the dehydration of lighter Ln³⁺ from that of heavier ones. In different organic solvents (75% solvent?Cwater media), the stability constant values follow the order: dioxane?>?methanol?>?DMF, depending upon both the dielectric constant and the hydrogen bonding structure of the medium. The thermodynamic parameters at all complexes have been analyzed in terms of their electrostatic (el) and non-electrostatic (non) parts.     

Infrared spectral studies on the stability of some binuclear transition metal—Schiff base complexes

Some di-Schiff bases like N,N′-o-phenylene-bis-salicylaldimine (DSB₁), N,N′-m-phenylene-bis-salicylaldimine (DSB₂) and N,N′-p-phenylene-bis-salicylaldimine (DSB₃) have been synthesized by the condensation of salicylaldehyde with o-, m- and p-phenylenediamines, respectively. The synthesized complexes of Cu(II), Ni(II) and Co(II) with these ligands have been assigned their molecular structure on the basis of their elemental analyses, molecular weight determination, magnetic measurements, electronic spectra, molar volume and infrared spectral studies. The nature of bonding between metal ion and Schiff bases is studied by i.r. spectrophotometry. The shift in the band position of the groups involved in coordination has been utilized to estimate the coordination bond lengths. The value of the coordination bond length of Cu(II) complexes is shorter than the corresponding values for Ni(II) and Co(II) complexes.     

Polymeric and sandwich Schiff’s bases complexes derived from 4,4′-methylenedianiline

Cobalt(II), nickel(II) and copper(II) complexes of Schiff’s bases derived from 4,4′-methylenedianiline and pyridine-2-carboxaldehyde(L¹), furan-2-carboxaldehyde(L²) or thiophene-2-carboxaldehyde(L³), were prepared and characterized by different analytical and spectral methods. The Schiff bases behave as neutral tetradentate ligands. The chloro-complexes of (L²) with (2:3) mole ratio have a polymeric nature. However, that of L¹ and L³ with (1:1) mole ratio showed a sandwich structure. All complexes display an octahedral geometry, except complex (2) which has a tetrahedral one. O _h/T _d equilibrium for chloro-complexes of cobalt with ligand L¹ was established in the solid state. ESR spectra of solid copper(II) complexes showed isotropic and axial type with dx ²-y ² ground state. The thermal study showed that the complexes with different solvents of crystallization exercise different types of interaction. The observed thermochromic phenomenon for cobalt complex was explained along with its thermal behaviour.     

Ugi–Smiles and Ullmann reactions catalyzed by Schiff base derived from Tröger’s base and BINOL

Research on Chemical Intermediates - A Schiff base catalyst (1a) combining a Tröger’s base-NH2 and the (R)-BINOL-(CHO)2 was used to promote the Ugi-Smiles reaction. By using isocyanide,...     

Photoluminescence property of polymer–rare earth complexes containing acetaldehyde/aminophenol type bidentate Schiff base ligand

Acetaldehyde was introduced onto the side chains of polysulfone, and then Schiff base reactions were carried out between the introduced acetaldehyde and ortho-aminophenol (OAP) or meta-aminophenol (MAP). Two bidentate Schiff base (B) ligands of acetaldehyde/aminophenol type, OAPB and MAPB, were bonded on the side chains of polysulfone, and two new bidentate Schiff base ligand functionalized-polysulfones, PSF-OAPB and PSF-MAPB, were obtained. The triplet state energies of OAPB and MAPB are well matched with the resonant level energy of Tb(III), and the Tb(III) complexes emit the strong characteristic fluorescence of Tb(III) (green luminescence). Complexes of Eu(III) have no fluorescence emission because of the mismatching of the energy levels. In comparison, the fluorescence intensity of the binary complex PSF-(MAPB)₃-Tb(III) is stronger than that of the binary complex PSF-(OAPB)₃-Tb(III) because of the structured difference of the chelating ring. The ternary complexes PSF-(MAPB)₃-Tb(III)-(Phen)₁ (Phen represents 1,10-phenanthroline) and PSF-(OAPB)₃-Tb(III)-(Phen)₁ have stronger fluorescence emissions than the corresponding binary complexes. The fluorescence emission intensities of solid films of the complexes are stronger than that of their solutions. The prepared luminescent polymer-Tb(III) complexes containing acetaldehyde/aminophenol type bidentate Schiff base ligands have very high quantum yields (80–86%), reflecting the high intramolecular energy transfer efficiencies from the ligands to Tb(III).     

Synthesis of β-amino alcohols catalyzed by poly(vinyl chloride)-supported Schiff base metal complexes

Abstract  

Eight transition metal complexes of various Schiff bases supported on poly(vinyl chloride) (PVC) were prepared and characterized. These metal complexes were screened as heterogeneous catalysts in the synthesis of β-amino alcohols by ring opening of epoxides with amines. The best catalyst was identified as a Ni(II) complex of PVC-supported 2-[(2-aminoethylimino)methyl]phenol and it was used in the synthesis of a number of different β-amino alcohols. The catalyst was found to be reusable for up to five cycles.     

Assessment of silver(Ⅰ) complexes of salicylaldehyde derivatives—histidine Schiff base as novel a-glucosidase inhibitors

In this study,a novel class of histidine Schiff base silver(Ⅰ) complexes derived from salicylaldehyde,1a-9a,was found to be an effective inhibitor of α-glucosidase.The results of this study showed that the newly synthesized complexes inhibited α-glucosidase through noncompetitive mechanisms;the IC_(50)values were ranging from 0.00431 μmol L ~1 to 0.492 μmol L ~1.The structure-activity relationship was established as well.These results demonstrated that compound 7a,5-nitro salicylaldehyde Schiff base silver complex,is the most promising α-glucosidase inhibitor with the lowest IC_(50) value,which could be exploited as a drug candidate to alleviate postprandial hyperglycemia in the treatment of type II diabetes mellitus.This research provided a catalyst-free,simple,and environmentally benign reaction to synthesize compounds using mechanochemistry.     

Preparation of a novel Mixed-Ligand divalent metal complexes from solvent free Synthesized Schiff base derived from 2,6-Diaminopyridine with cinnamaldehyde and 2,2′‐Bipyridine: Characterization and antibacterial activities

The solvent-free conditions were employed to synthesise symmetrical Schiff base ligand from 2,6-diaminopyridine with cinnamaldehyde in (1 min) with a fair yield utilizing formic acid as a catalyst. Through coordination chemistry, new heteroleptic complexes of Cu(II), Co(II), Ni(II), Pt(II), Pd(II) and Zn(II) were achieved from Schiff base as a primary chelator (L¹) and 2,2′‐bipyridine (2,2′-bipy) as a secondary chelator (L²). The prepared compounds have been characterized by elemental analysis, molar conductivity, magnetic susceptibility, FTIR, ¹H NMR, UV–visible, mass spectrometry, and thermal gravimetric analysis, and screened in vitro for their potential as antibacterial activity by the agar well diffusion method. The metal complexes were formulated as [M (L¹) (L²) (X)] Y⋅nH₂O, L¹ = Schiff base, L² = 2,2′-bipy, (M = Cu(II), Co(II), Zn(II), Y = 2NO₃, n = 1), (M = Ni(II), X = 2H₂O, Y = 2NO_3, n = 0) and (M = Pd(II) Pt(II), Y = 2Cl, n = 0). Both L¹ and L² act as a neutral bidentate ligand and coordinates via nitrogen atoms of imine and 2,2′-bipy to metal ions. The metal complexes were found to be electrolytic, with square-planar heteroleptic Cu(II), Co(II), Pt(II), and Pd(II) chelates and octahedral Ni(II) complex. As well as tetrahedral geometry, has been proposed for the complex of Zn(II). Furthermore, the biological activity study revealed that some metal chelates have excellent activity than Schiff base when tested against Gram-negative and Gram-positive strains of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Finally, it was found that the Zn(II) and Pd(II) complexes were more effective against both types of bacteria tested than the imine and other metal complexes.     

Acidichromism in the LB film of bolaform Schiff base

A bolaform(BNC 10)and single-headed(HNOA)amphiphilic Schiff bases containing naphthyl group were designed and their Langmuir-Blodgett films were investigated.It was found that both the LB films show acidichromism,i.e.a reversible color change upon alternatively exposing the films to HCl and NH_3 gases,respectively.It was further found that the bolaform Schiff bases film could trap NH_3 gas during the acidichromic process.     

Synthesis, spectroscopy and biological investigations of manganese(III) Schiff base complexes derived from heterocyclic β-diketone with various primary amine and 2,2'-bipyridyl

The mixed ligand mononuclear complex [Mn(bipy)(HPMFP)(OAc)]ClO(4) was synthesized by reaction of Mn(OAc)(3)·2H(2)O with HPMFP and 2,2'-bipyridyl. The corresponding Schiff base complexes were prepared by condensation of [Mn(bipy)(HPMFP)(OAc)]ClO(4) with ethylenediamine, ethanolamine and glycine (where HPMFP=1-phenyl-3methyl-4-formyl-2-pyrazolin-5one, bipy=2,2'-bipyridyl). All the compounds have been characterized by elemental analysis, magnetic susceptibility, conductometry measurements and (1)H and (13)C NMR, FT-IR, mass spectrometry. Electronic spectral and magnetic susceptibility measurements indicate square pyramidal geometry around manganese(III) ion. The thermal stabilities, activation energy E*, entropy change ΔS*, enthalpy change ΔH* and heat capacity of thermal degradation for these complexes were determined by TGA and DSC. The in vitro antibacterial and antifungal activity of four coordination compounds and ligand HPMFP were investigated. In vitro activates of Bacillus subtillis (MTCC-619), Staphylococcus aureus (MTCC-96), Escherichia coli (MTCC-722) and Klebsiella pneumonia (MTCC-109) bacteria and the fungus Candida albicans (ATCC-90028) were determined. All the compounds showed good antimicrobial activity. The antimicrobial activities increased as formation of Schiff base.     

Synthesis,characterization and oxidase catalytic activity of vanadium(IV) and oxovanadium(IV) complexes with Schiff base ligands derived from β-diketones

New complexes of vanadium(IV) and oxovanadium(IV) with Schiff base ligands derived from -diketones and ethanolamine or o-aminophenol have been prepared and characterized by elemental analyses, electrical conductance, magnetic moment measurements, and by i.r., u.v.–vis. and e.p.r. spectroscopy. A distorted octahedral environment was proposed for the vanadium(IV) and oxovanadium(IV) complexes. The spectroscopic results were utilized to compute the important ligand field parameters. Three adduct complexes were isolated owing to the interaction of one oxovanadium complex with Lewis-bases in MeOH. Vanadium(IV) complexes exhibit promising catalytic activity towards the aerobic oxidation of p-phenylenediamine (PPD) to the corresponding semi-oxidized form (PPD⁺). A linear correlation exists between the oxidase catalytic activity and the Lewis acidity of the central vanadium(IV) ion created by the donating properties of the parent ligand.     

Open-air N-arylation of N–H heterocycles with arylboronic acids catalyzed by copper(II) Schiff base complexes

Two copper Schiff base complexes, in both homogeneous and heterogeneous forms, were prepared and characterized by using elemental analysis, FTIR, UV–Vis spectroscopy and scanning electron microscopy. The catalytic performances of these complexes were studied in the N-arylation of N–H heterocycles with arylboronic acids in methanol without any added base at 40 °C under open air. The effects of various parameters such as solvent and temperature on the reaction system were studied. The reaction is applicable to a wide variety of N–H heterocycles and arylboronic acids. The heterogeneous catalyst was recovered by simple filtration, and reusability experiments showed that this catalyst can be used five times without much loss in the catalytic activity.