The synthesis,characterization, electrochemical character,catalytic and antimicrobial activity of novel,azo-containing Schiff bases and their metal complexes

Three novel Schiff base ligands containing the azo group, 2-((E)-(4-((E)-phenyldiazenyl)phenylimino)methyl)phenol, 3-((E)-(4-((E)-phenyldiazenyl)phenylimino)methyl)benzene-1,2-diol and 4-((E)-(4-((E)-phenyldiazenyl)phenylimino)methyl)benzene-1,2,3-triol, were synthesized from the reaction of p-aminoazobenzene with salicylaldehyde, 2,4-dihydroxybenzaldehyde and 2,3,4-trihydroxybenzaldehyde, respectively. The mononuclear Co(II) and Cu(II) complexes of the Schiff base ligands were prepared and characterized using elemental analyses, IR, UV–visible spectroscopy, magnetic susceptibility and conductance measurements; ¹H NMR and mass spectra of the ligands were also recorded. The Co(II) and Cu(II) metal complexes are formed by the coordination of the N and O atoms of the ligands. The electrochemical properties of the metal complexes were investigated at 100 mV s^?1 scan rate in DMSO; the oxidative C–C coupling properties of the Co(II) and Cu(II) complexes were investigated on the sterically hindered 2,6-di-tert-butylphenol (DTBP). In addition, the Schiff base ligands and their complexes were evaluated for both their in vitro antibacterial activity using the disc diffusion method.     

Electron-rich salen-type Schiff base complexes of Cu(II) as catalysts for oxidation of cyclooctene and styrene with tert-butylhydroperoxide: A comparison with electron-deficient ones

Two square planar copper(II) complexes of tetradentate Schiff base ligands derived from aromatic aldehydes and 2,2′-dimethylpropandiamine (H₂{salnptn(3-OMe)₂}, H₂{hnaphnptn}) have been prepared and used as catalysts for oxidation of cyclooctene and styrene with tert-butylhydroperoxide (TBHP). Oxidation of cyclooctene with TBHP gave cyclooctene oxide as the sole product, but in the case of styrene a mixture of styrene oxide and benzaldehyde has been obtained in ca. 1:3 molar ratio. It has been shown that the rate and selectivity of reaction depend to the electron-donor ability of substituents at the phenyl groups of the ligand and can be improved by introduction of π-electron-donating groups at the aromatic rings of salen-type Schiff bases. The structure of Cu{salnptn(3-OMe)₂} has been determined by X-ray crystallography at 291 K with results generally in agreement with those previously reported. The results suggest that the symmetrical Schiff base ligands are bivalent anions with tetradentate N₂O₂ donors derived from the phenolic oxygen and azomethine nitrogen atoms.     

Synthesis,crystal structures and NIR luminescence properties of binuclear lanthanide Schiff Base complexes

Three binuclear lanthanide complexes [Nd₂(L¹)₃(MeOH)]·MeOH·H₂O (1) and [Ln₂(H₂L²)₂(OAc)₄]·2(CF₃SO₃)·MeOH·EtOH (Ln = Nd (2) and Ho (3)) were prepared using two Schiff base ligands. Interestingly, 1 has a triple-decker structure with two lanthanide ions enclosed by three rigid conjugated Schiff base ligands (H₂L¹), while 2 and 3 show nanoscale ring structures (8 × 12 × 12 Å) formed by flexible long-chain Schiff base ligands (H₂L², ~ 23 Å) with the lanthanide ions located in the center. Upon excitation of the ligand-centered absorption bands, 1–2 and 3 show typical NIR emission spectra for Nd^3 + and Ho^3 + ions, respectively. In 1, the Nd(III) centers are shielded within the decker-like structure and surrounded by chromogenic Schiff base ligands (energy transfer donors). Luminescence studies show that the NIR emission lifetime of 1 is longer than that of 2 in solution.     

Structural evidence of a ketimine as the product of an amino acid with an aldehyde. An intermediate in the racemization and transamination of amino acids

The aldimine and ketimine forms of two Schiff base complexes formed by the condensation of two isomeric imidazole carboxaldehydes with an amino acid are reported. Reaction of L¹, the Schiff base condensate of 5-methyl-4-imidazolecarboxaldehyde (5Me4Im) and valine, with copper(II) perchlorate results in the isolation of [Cu(L¹)(5Me4Im)(ClO₄)] while the analogous reaction of L², the Schiff base condensate of 1-methyl-2-imidazolecarboxaldehyde (1Me2Im) with alanine, and nickel(II) results in the isolation of [Ni(L²)₂]. L¹ exhibits the expected aldimine form of the amino acid derived Schiff base, 5Me4Im-CH = N-CH(R)CO₂⁻, while L² exhibits the tautomeric ketimine form, 1Me2Im-CH₂–N = C(R’)CO₂⁻. Structural data clearly support the two tautomeric forms. The ketimine form, observed in [Ni(L²)₂], has been proposed as an intermediate in the racemization and transamination of amino acids.     

Synthesis and characterization of calcium complexes: Efficient catalyst for ring-opening polymerization of L-lactide

Four calcium complexes coordinated with N,N,O-tridentated pyrazolonate (1–2) or Schiff base (3–4) ligands have been prepared. These reactions can be monitored by ¹H NMR spectroscopic studies with the chemical shift from 2.20 to 2.36 ppm for 1, and 2.22 to 2.28 ppm for 2 for the NMe₂ group and the proton peak of the imino group shifted from 8.23 ppm to 7.95 ppm for 3, and 8.52 ppm to 8.14 ppm for 4. Single crystal X-ray studies reveal that complex 1 bears a mononuclear feature with a hexa-coordinated calcium metal center bonded to two NNO-tridentate ligands. Variable temperature dependent ¹H NMR spectroscopic studies of 3 indicate that the nitrogen atom of NMe₂ group dissociates slowly in solution ranging from − 20 to 80 °C. In the presence of alcohol, all of these complexes show good catalytic activity toward ring-opening polymerization of L-lactide in a well-controlled manner, yielding polymers with expected molecular weights and narrow polydispersity indexes. The activity of these complexes is affected by the basicity of donor atoms.     

Structure and magnetic properties of A Cu(II) coordination polymer supported by a tetrapodal Schiff base ligand

A Cu(II) Schiff base coordination polymer, {[Cu₃L(μ₂-NO₃)₂·(H₂O)₂].3H₂O}_n, is readily prepared by complexation of the tetrapodal Schiff base, H₄L (H₄L = 1,1,1,1-tetrakis[(salicylaldimino)methyl]methane), with cupric nitrate trihydrate. It has been demonstrated that the coordination polymer consists of linear trinuclear Cu(II) entities, and displays a cooperative coordination mode for nitrate anions, coordinated water molecules, and Schiff base ligands to Cu(II) ions. In addition, variable-temperature magnetic susceptibility measurements reveal a strong antiferromagnetic coupling interaction between adjacent copper(II) ions with large J value of − 289.66 cm^− 1.     

Uranyl adsorption on (001) surfaces of kaolinite: A molecular dynamics study

The aim of the present paper is to investigate the adsorption of uranyl species (UO₂)^2 +(H₂O)₅ onto kaolinite (001) surfaces. To this end we have employed molecular dynamic simulations based on CLAYFF force field potential. Various types of surface model for inner-sphere adsorption complexes and one model for outer-sphere adsorption complexes were optimized. In order to have a neutral structure, the uranyl (UO₂)^2 +(H₂O)₅ or the kaolinite was deprotonated to form the outer-sphere or inner-sphere adsorption complexes. Both singly protonated and partially deprotonated states of the Al(0) kaolinite surface were considered for adsorption in the model of inner-sphere complexes. The first uranyl coordination shell exhibits pentagonal bi-pyramidal symmetry with the pentagonal formed by 5 water molecules. We show that the average U–OW distances are between 2.49 and 2.57 Å for water molecules. The bond of uranyl with deprotonated O⁻ center is always short because of the charge attraction. The obtained results agree well with density functional calculations and EXAFS measurements, and show how and why the adsorption of uranyl appears on the surface of kaolinite.     

Stereo-regulated methyl methacrylate (MMA) polymerization catalyzed by asymmetric Salen-type Schiff-base Cu(II) complexes

A series of mononuclear asymmetric Salen-type Schiff-base complexes [Cu(Lⁿ)] (n = 1–5, 1–5) were obtained by the Cu(II)-templated approach in the presence of one of five salicylaldehyde derivatives, respectively. These complexes could effectively catalyze the metal-coordinative polymerization of MMA for the formation of the syndio-enriched PMMAs (poly(methyl methacrylates)) in the presence of AIBN, where the Cu(II)-to-chelate-plane distances are positively relative to the catalytic activity. Moreover, the introduction of the encumbering substituent ortho to the phenoxide ring of the ligands plays a much more important and influential role on the tacticity and chain-growth of the obtained PMMAs than the variation of reaction conditions under the controllable polymerization.     

Investigation of antitumor mechanism of the chiral ruthenium complex Λ-[Ru(phen)2p-MOPIP]2 + in human gastric cancer MGC-803 cells

There has been a vast increase in telomerase inhibition research over the past several years, which was demonstrated as an attractive anti-tumor strategy. Our previous study found that the chiral ruthenium complex, [Ru(phen)₂p-MOPIP]^2 + (Phen = 1,10-phenanthroline, p-MOPIP = 2-(4-methoxyphenyl)-imidazo[4,5f] Markman (2003), Janaratne et al. (2007) phenanthroline) (dl-OMe) and its enantiomer Δ/Λ-[Ru(phen)₂p-MOPIP]^2 + (Δ/Λ-OMe) could bind to and stabilize G-quadruplex DNA structure in telomeres, and inhibit telomerase activity. In this study, cytotoxic activity of these Ru complexes was studied by MTT assay. The anti-tumor mechanisms of Λ-OMe were investigated using TRAP assay, Western blot analysis, flow cytometry, Hochest staining, and RT-PCR. Results showed that among several Ru complexes, Λ-OMe demonstrated a better anti-tumor activity against gastric cancer cell line (MGC-803), and had less effect on normal gastric epithelial cell. Λ-OMe effectively inhibited the cell growth by inhibiting cellular telomerase activity, triggering cell cycle arrest, and inducing apoptosis of MGC-803 cells. The inhibitory effect on telomerase activity was associated with the altered expression of telomere-related proteins TRF1 and TRF2. Cell-cycle arrest was associated with increased levels of P21 mRNA. Apoptosis of MGC-803 cell was triggered by modulating the expression of apoptosis-related genes Bax, Bcl-2, and caspase-3. Overall, the results suggest that Λ-OMe may be a new promising agent for human gastric cancer therapy.     

Intercalation of Ga3 +-salicylidene-amino acid Schiff base complexes into layered double hydroxides: Synthesis,characterization, acid resistant property,in vitro release kinetics and antimicrobial activity

This study is aimed to evaluate the potential application of a drug delivery system of Ga^3 + complexes in layered double hydroxide, i.e. [Ga(L)₂]⁻-LDH (L represents Schiff base ligands made up of salicylidene with glycine, alanine, tyrosine, phenylalanine, aspartic acid and glutamic acid for complexes 1–6, respectively). The successful intercalation of Ga^3 + complexes into Mg/Al-NO₃-LDH by delaminating/restacking method was proved by elemental analysis, XRD, FTIR spectra, UV–vis DR spectra, TG analysis and SEM measurements. A monolayer intercalation of 5 and 6, bilayer of 1 and 3 and trilayer of 2 and 4 in interlayer region of hybrids which have extended gallery height of 11.86–21.22 Å were deduced. Better acid resistance than the pristine complexes in wide pH range from 7 to 1 was revealed by UV–vis spectra. The cumulative release profile of Ga^3 + complexes from the hybrids in phosphate buffer (pH 7.4) following first-order kinetics via ion-exchange between guest anions and phosphate anions was established by UV–vis spectra. Antibacterial activity of hybrids with MIC values of 65–137 μmol/L against Pseudomonas aeruginosa was demonstrated by the broth twofold dilution method. These results suggest that LDH is a promising host material for carry and controlled delivery of biologically active Ga^3 + complexes.     

Synthesis,characterization and DNA interaction of copper (II) complexes with Schiff base ligands derived from 2-pyridinecarboxaldehyde and polyamines

A series of copper (II) complexes a–d with Schiff bases ligands derived from the condensation reactions between 2-pyridinecarboxaldehyde and different polyamines (ethylenediamine, diethylenetriamine, triethylenetetramine and tetraethylenepentamine) were synthesized and characterized by elemental analysis, FT-IR spectroscopy, HRMS, molar conductance and molecular modeling studies. The interactions of the copper complexes a–d with DNA were investigated by the UV spectra, viscosity measurements and gel electrophoresis under physiological conditions. The experimental results indicated that four complexes could bind to DNA via an intercalative mode and showed a different DNA cleavage activity with the sequence: d > c > a > b.     

Adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin

Series of resin selection experiments were carried out and the KIP210 strong base anion exchange resin was confirmed to have the maximum equilibrium adsorption capacity to remove Cr(VI) from wastewater. The adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin were investigated completely and systematically. The static experiments were performed to study the effects of various parameters, such as shaking speed, resin dosage and pH during the adsorption process. The results indicate that the effect of external diffusion is eliminated at 160 rpm, the best pH value is 3.0 and the removal percentage of Cr(VI) increases with the increase of the resin dosage. The adsorption of Cr(VI) on KIP210 agrees well with the Langmuir isotherm and the adsorption parameters of thermodynamics are ΔH = 26.5 kJ mol⁻¹, ΔS = 126.7 J mol⁻¹ K⁻¹ and ΔG < 0. It demonstrates that the adsorption of Cr(VI) on KIP210 is a spontaneously endothermic physisorption process. Moreover, the adsorption process can be described well by a pseudo-second-order kinetic model and the activation energy is 30.9 kJ mol⁻¹. The kinetic analysis showed that the adsorption rate is controlled by intraparticle diffusion. The resin is successfully regenerated using the NaOH solutions.     

Synthesis,crystal structure and urease inhibitory activities of Schiff base metal complexes

Six new transition metal complexes (1–6) with Schiff base were synthesized and crystallographically characterized. Their urease inhibitory activities were evaluated. Three of them showed potent inhibitions against jack bean urease. Mn(II) complex (2) possessed the best jbU inhibitory activity with IC₅₀ of 8.30 ± 0.93 μM, which was much better than those of the corresponding ligand and control ion. The investigation on the stability constants and the structure-activity relationships of the complexes indicated that the complexes interacted with the enzyme in the whole complex forms rather than the free ions.     

A novel chelate-assisted C−C bond formation on a Cd(II) complex of an asymmetric heptadentate(N7) tripodal Schiff base ligand

Cadmium(II) complexes of some potentially heptadentate tripodal Schiff base ligands with the general formula N{(CH₂)_nNCHC₅H₄N}₂{(CH₂)_mNCHC₅H₄N}) (where n = m = 2, L₂₂₂; n = 2, m = 3, L₃₂₂; n = 3, m = 2, L₃₃₂) have been studied. These ligands are products of the full condensation of a number of tripodal tetraamines with 2-pyridinecarboxaldehyde. An unusual reaction of an additional 2-pyridine-carboxaldehyde with a methylene group adjacent to the imine bond only during template preparation of [Cd(L₃₃₂)]²⁺ was observed. The latter reaction is a novel chelate-assisted C?C bond formation because it is occurred only where a rigid five-membered chelate ring is present.     

Dioxomolybdenum(VI) complexes with amino acid functionalized N-salicylidene hydrazides: Synthesis,structure and catalytic activity

A cis-dioxomolybdenum(VI) complex with an amino acid functionalized hydrazone ligand derived from salicylaldehyde and Boc-protected β-alanine hydrazide (Boc = t-butyloxycarbonyl) has been synthesized. The X-ray crystal structure reveals a distorted octahedral geometry at the molybdenum atom with an O₅N donor set and vast hydrogen bonding interactions including the amino acid side chain of the ligand. The new cis-dioxomolybdenum(VI) complex is an efficient catalyst for the peroxidic oxidation of sulfides, but does not show any activity towards the oxidation of bromide.     

Photocatalytic reduction of Cr(VI) to Cr(III) in solution containing ZnO or ZSM-5 zeolite using oxalate as model organic compound in environment

Photocatalytic reduction of Cr(VI) to Cr(III) in aqueous solution containing ZnO or ZSM-5 zeolite under ambient condition was studied by using oxalate as model organic compound in the natural environment. ZSM-5 zeolite was characterized by X-ray diffraction (XRD), and point of zero net proton charge (PZNPC) titration. The effect of illumination time, mass content of catalyst (m/V), Cr(VI) initial concentrations, pH, ionic strength, and oxalate concentrations on the photocatalytic reduction of Cr(VI) was determined. The results indicate that the PZNPC of ZSM-5 zeolite is at pH 3.6 ± 0.1. At C[Cr(VI)_(initial)] = 2.00 × 10^?4 mol/L, pH 7.5 ± 0.1 and after illumination time of 24 h, the reduction of Cr(VI) were 1.1 × 10^?5 mol/L (no ZSM-5 zeolite, 4.0 × 10^?3 mol/L oxalate) and 1.0 × 10^?5 mol/L (0.4 g/L ZSM-5 zeolite, no oxalate), respectively; whereas the reduction of Cr(VI) achieved 1.0 × 10^?4 mol/L in the presence of 0.4 g/L ZSM-5 zeolite and 4.0 × 10^?3 mol/L oxalate. The removal of Cr(VI) from solution is dependent on pH value. The results are important for the application of zeolites in the treatment of Cr(VI) polluted solution in the natural environment.     

Adsorption of uranyl ions on kaolinite,montmorillonite, humic acid and composite clay material

Adsorption of uranyl ions onto kaolinite, montmorillonite, humic acid and composite clay material (both clays and humic acid) was studied by measuring the system response to clay suspensions (pre-equilibrated with or without uranyl) and to perturbations of the solution chemistry. Adsorption behavior of selected materials under the frame of batch experiments was tested at high uranyl concentrations (6–1170 μg/mL; 2.5 × 10^− 2 to 4.9 μM), whereas that under flow through continuous stirred reactor experiments was tested at low concentrations (1.00 × 10^− 4 to 1.18 × 10^− 4 M). Both experiments were developed at pH 4.5 and ionic strength 0.2 mM. The adsorption experiments follow a Langmuir isotherm model with a good correlation coefficient (R² > 0.97). The calculated amount of adsorbed and desorbed uranyl was carried out by numeric integration of the experimental data, whereas the desorption rates were determined from the breakthrough curve experiments. Kaolinite with highly disordered structure adsorbed less uranyl (3.86 × 10^− 6 mol/g) than well-ordered kaolinite (1.76 × 10^− 5 mol/g). Higher amount of uranyl was adsorbed by montmorillonite (3.60 × 10^− 5 mol/g) and only half of adsorbed amount was desorbed (1.85 × 10^− 5 mol/g). The molecular interactions between kaolinite, montmorillonite, humic acid, composite material and saturated uranyl ion solutions were studied by molecular fluorescence, infrared and X-ray photoelectron spectroscopy. The Stern–Volmer constant obtained for montmorillonite (2.6 × 10³ M^− 1) is higher than for kaolinite (0.3 × 10³ M^− 1). Molecular vibrations of SiO stretching and AlOH bending related to hydroxylated groups (SiOH or AlOH) of kaolinite and montmorillonite show structural changes when uranyl ions are adsorbed. X-ray photoelectron spectroscopy shows that the U 4f_7/2 core level signals occur at 380.5 eV in either kaolinite or montmorillonite that resulted from the interaction of aluminol surface sites with the (UO₂)₃(OH)₅⁺.     

Ligand concentration and reaction time controlled successive substitution of chloro and η6-benzene of di-μ-chlorobis{η6-benzene)dichlororuthenium(II)} with selenated Schiff base: Formation of half sandwich Ru(II) complex highly active catalyst for oxidation of alcohols

Schiff base (L) synthesized by reacting 2-methylthiobenzeldehyde with 2-(phenylseleno)ethylamine on reaction with di-μ-chlorobis{η⁶-benzene)dichloro-ruthenium(II)}(a) forms two type of species: (i) [Ru(L)₂][PF₆]₂ (1) [L:a = 4:1 and reaction time ～8 h] and (ii) [Ru(η⁶-C₆H₆)(L)][PF₆]₂ (2) [L:a = 2:1 and reaction time ～1 h]. This is first example in which chloro as well benzene ring both are successively substituted by controlling metal:ligand ratio and duration of reaction. The geometry around Ru in complex 1 is distorted octahedral. The 2 has a pseudo-octahedral half sandwich “piano-stool” disposition of ligands around Ru. The Ru–Se distances are 2.4683(10)–2.5082(7) ?. The proton and carbon-13 NMR spectra of L and its both complexes 1 and 2 authenticate them. The 2 shows high catalytic activity for oxidation of primary and secondary alcohols both (TON upto 9.6 × 10⁴; TOF upto 4.80 × 10⁴ h^?1).     

Synthesis and magnetic properties of a new hexanuclear iron(III) compound

A hexanuclear Fe(III) compound with new core topology, [Fe₆(OMe)₂(DHMP)₄(AD)₂] (1, H₃DHMP = (E)-2-((1,3-dihydroxy-2-methylpropan-2-ylimino)methyl)phenol, H₂AD = adipic acid), is reported. The hexanuclear Fe(III) core can be viewed as two Fe₃(DHMP)₂ units connected through double μ₂-OMe^? bridges, featuring adjacent Fe···Fe distances of 3.105 and 3.109 Å within unit and 3.111 Å between units, respectively. Variable temperature magnetic studies indicate antiferromagnetic exchanges between adjacent Fe(III) ions and which mediated by μ₂-OMe^? bridges is stronger than those by DHMP alkoxo bridges. Magnetic-structural correlation is also proposed.     

Epoxidation of natural propenylbenzenes catalyzed by [FeIII(Salen)Cl] and [FeIII(TPP)Cl]

Epoxidation of natural propenylbenzenes (safrol, isosafrol, anethol, eugenol and isoeugenol) was carried out using iron complexes as catalysts ([Fe^III(TPP)Cl] and [Fe^III(Salen)Cl] · H₂O, where TPP = 5, 10, 15, 20-meso-tetraphenylporphyrin and Salen = N,N′-(salicylaldehyde)ethylenediamine) in presence of hydrogen peroxide 30% and glacial acetic acid. Reaction products were identified by gas chromatography and satisfactory yields were obtained. Quantitative conversion of isosafrol to the corresponding epoxide was achieved in presence of [Fe^III(TPP)Cl] as catalyst.