Salen-type Schiff Base Research Articles

Biocompatible alkyne arms containing Schiff base fluorescence indicator for dual detection of CdII and PbII at physiological pH and its application to live cell imaging.

An alkyne arms containing salen-type Schiff base ligand, 6,6'-((1E,1'E)-(1,2-phenylenebis(azanylylidene))bis(ethan-1-yl-1-ylidene))bis(3-(prop-2-yn-1-yloxy)phenol) (H2L), is reported here as a dual chemosensor for CdII and PbII ions. The ligand H2L was characterized by various spectral methods. The ligand H2L acts as a dual sensor for CdII and PbII in methanol/HEPES buffer (5 mM, pH 7.3; 1 : 9 v/v) at room temperature with well-separated excitation and emission wavelengths. The emission intensity at 553 nm of H2L is shifted and a new band appeared at 578 nm with the increase in the presence of CdII when it is excited at 420 nm. With excitation at 410 nm, the emission intensity of H2L at 485 nm shifted and a new band appeared at 505 nm with the increase in the presence of PbII. The quantum yield of H2L increases considerably in the presence of CdII and PbII. The compound H2L is non-fluorescent; however, in the presence of CdII and PbII, the compound H2L is highly fluorescent with well-separated excitation and emission wavelengths, indicating that the metal ion is coordinated through phenolic oxygen and imine nitrogen of the Schiff base blocking the PET (Photoinduced Electron Transfer) process and stimulating the CHEF (Chelation Enhanced Fluorescence) process, to increase the fluorescence intensity of H2L. The detection limit values of H2L are in a nano-molar range for both metal ions, confirming very high sensitivity of H2L. The L·MII binding mode and the recognition mechanism of the sensor were explored by Job's plot, 1H NMR, FT-IR, pH and DFT calculations. Besides, H2L was successfully utilized in cell imaging studies for both metal ions in MCF 7 cells.

Модификация пористого углеродного материала полимерным комплексом кобальта с основанием Шиффа саленового типа для электродов электрохимических суперконденсаторов

Effect of the modification of a porous carbon material used in supercapacitors by a polymeric complex of cobalt with a Salen-type Schiff base ligand on the material capacity have been studied. Conjunction of multielectron redox processes in the polymer with the proposed effective modification technology makes it possible to increase the electrode capacity by 2.4 times in comparison with unmodified carbon electrodes. Specific capacity of the polymer poly-[Co(CH3OSaltmen)] at a polymer deposition density of 1.3 mg•cm-2 was 170 mA•h•g-1.

Tin(IV) Complexes with Salen Type Schiff base: Synthesis, Spectroscopic Characterization, Crystal Structure, Antibacterial Screening and Cytotoxicity

A series of six tin(IV) complexes [SnCl2(L) (I), Me2Sn(L) (II), Bu2Sn(L) (III), Ph2Sn(L) (IV), Oct2Sn(L) (V), BuSnCl(L) (VI)] derived from N,N'-bis(2-hydroxy-1-naphthylidene)-1,2-diaminobenzene [LH2] have been synthesized. The obtained compounds were characterized by elemental analysis, mass spectrometry, FT-IR and NMR (1H, 13C) spectroscopy. The crystal structures of compounds (IV) and (VI) have also been determined by single crystal X-ray analysis (CIF files CCDC nos. 856596 (IV) and 856595 (VI)). The study revealed that the complexes exist as discrete monomeric species and the tin atom is hexa-coordinated in a distorted octahedral geometry. The two phenyl groups in compound (IV) are at trans-positions. Similarly, in complex (VI) the butyl and chloro groups also adopt trans-orientation. The in vitro antibacterial screening and cytotoxicity investigations revealed that the biological activities significantly depend upon the alkyl or others groups present on tin atom. Most of the tin(IV) complexes are active against Escherichia coli and highest activity is shown by complex (IV) against Bacillus subtilis. Furthermore, complex (IV) has also demonstrated the highest cytotoxicity against brine shrimp with LD50 value 0.858 μg/mL.

The Nature of Charge Carriers in Polymeric Complexes of Nickel with Schiff Bases Containing Electron-Withdrawing Substituents

The data on polymeric complexes of nickel with salen-type Schiff bases containing electron-withdrawing nitro groups in ligand’s aromatic fragments are acquired by an in situ technique based on collecting electron paramagnetic resonance spectra simultaneously with absorption spectra in the UV, visible, and near IR regions in the course of oxidation-reduction of a polymer film on the surface of an optically transparent electrode under conditions of cyclic voltammetry. Based on the combined analysis of spectroscopic and electrochemical data, it is shown that as the potential of the polymer-modified electrode shifts in the positive region, the different types of charge carriers successively appear in the system (phenoxyl radical cations, radical pairs, and dications). The results obtained suggest that the positive charge can be delocalized either between aromatic rings in monomer fragments or between fragments of neighboring polymer chains.

A rare flattened tetrahedral Mn(II) salen type complex: Synthesis, crystal structure, biomimetic catalysis and DFT study

A new flattened tetrahedral high spin Mn(II) complex (1) has been synthesized using N2O4 donor Schiff base ligand. Complex 1 was characterized by X-ray diffraction and various spectroscopic techniques. For further understanding of electronic structure of the complex, DFT calculations and electrochemical studies have been performed. This is a rare example of a flattened tetrahedral Mn(II) salen type Schiff base complex. High-spin d5 configuration of the metal center provides no crystal-field stabilization energy to the system and that is the main reason behind the significant deviation of this salen-type ligand from planarity. Notably, the propylenic linker in the ligand provides adequate flexibility so that such an uncommon binding mode of the salen type Schiff base ligand becomes possible. Complex 1 exhibits excellent catalytic property towards oxidation of o-aminophenols in aerobic condition. Detailed kinetic investigations together with the mass spectrometry studies reveal several important information relating to biomimetic catalytic activity of the present complex.

Orientation of Chiral Schiff Base Metal Complexes Involving Azo-Groups for Induced CD on Gold Nanoparticles by Polarized UV Light Irradiation

In this study, we report the synthesis, characterization, and chiroptical properties of azo-group-containing chiral salen type Schiff base Ni(II), Cu(II), and Zn(II) complexes absorbed on gold nanoparticles (AuNPs) of 10 nm diameters. Induced circular dichroism (CD) around the plasmon region from the chiral species weakly adsorbed on the surface of AuNP were observed when there were appropriate dipole–dipole interactions at the initial states. Spectral changes were also observed by not only cis-trans photoisomerization of azo-groups but also changes of orientation due to Weigert effect of azo-dyes after linearly polarized UV light irradiation. Spatial features were discussed based on dipole-dipole interactions mainly within an exciton framework.

Enhanced electrochemical performance of salen-type transition metal polymer with electron-donating substituents

Salen-type Schiff base transition metal monomers with substituents Ni (CH3-salen), Ni (CH3O-salen), and Ni (Cl-salen) have been synthesized and electro-polymerized onto the indium tin oxide substrate electrodes. The effect of electron-donating groups on the electrochemical performance of the polymer is studied. Electron-donating groups enhance the electrochemical activity of the salen-type Schiff base during the electropolymerization process. SEM images show that the morphology of poly [Ni (CH3O-salen)] is nanobelt with a width of 200–500 nm. The cyclic voltammetry plots indicate that the strong electron-donating methoxy group facilitates the polymerization of the salen-type Schiff base. Thus, Ni (CH3O-salen) shows a higher doping level than other three polymers. XPS measurement is conducted to investigate the polymerization process and the mechanism of energy storage. It is proved that the azomethine nitrogen group (−N=CH−) matters a lot in the polymerization and energy storage process. In brief, the azomethine nitrogen group was affected by the introduction of the electron-donating group so that extra redox peaks appear in the cyclic voltammetry plots. There is no chemical valence change of nickel, and the nickel atom worked as a bridge in the system. The electro-donating substituent group activates the benzene ring of the polymer and facilitates the charge transfer and leads to poly [Ni(CH3O-salen)] that exhibits the highest doping level, charge-transfer ability, and electrochemical capacity characteristics than the polymer with weaker electro-donating or electro-withdrawing substituents (polyNi(Cl-salen)). At the current density of 0.1 mA cm−2, the specific capacitance of poly [Ni(CH3O-salen)] is 270.1 F g−1, higher than that of poly [Ni(salen)](136.7 F g−1), poly [Ni(CH3-salen)](148.1 F g−1), and poly [Ni(Cl-salen)](106.0 F g−1).

A tetranuclear nickel/lead complex with a salen type Schiff base: synthesis, structure and exploration of photosensitive Schottky barrier diode behaviour

A tetranuclear nickel(ii)/lead(ii) complex has been synthesized and characterized. The complex based device behaves as a Schottky diode. The charge transfer kinetics of the complex is enhanced after light soaking.

Bioactive Salen-type Schiff Base Transition Metal Complexes as Possible Anticancer Agents.

Although metal-based anticancer drugs have been recognized as the most effective agents over the organic compounds, non-selectivity and high toxic effects have limited their applications in a way that only three Pt-analogues have progressed into clinical use. These problems have spurred chemists to develop different strategies based on alternative targets. This work focuses on predicting potency and mode of interactions of a series of salen type Schiff base transition metal complexes derived from meso-1,2-diphenyl-1,2-ethylenediamine, over some proteins (HDAC7, HDAC2, CatB, B-RAF kinase, TopII, RNR, TS, and rHA) using computational docking method, to be later considered as possible anticancer agents. The obtained results showed that all complexes exhibited higher affinity for HDAC7 than the other targets. Moreover, the bromo-derivatives of the copper compounds were more active on HDAC7 than the other derivatives. Such bromo compounds showed considerable interactions with Kinase, RNR, TS, and CatB. Contrary to Histone deacetylase (HAD)C7; HDAC2 was predicted to be relatively poor target. As expected, formation of the hydrophobic interactions between the metal complexes and the protein targets were essential for activity of the metal compounds. This study provides some more information for further optimizations and development of new metallodrugs as enzyme inhibitors for potential therapeutic agents.

Novel highly conductive cathode material based on stable-radical organic framework and polymerized nickel complex for electrochemical energy storage devices

Redox polymers bearing stable nitroxyl radical groups, such as poly-TEMPO-methacrylate (PTMA), are attractive candidates for application in power sources of novel kind, which combine the high power output of supercapacitors and high energy of rechargeable batteries. An important advantage of PTMA is the availability and low cost of the starting materials combined with high charging/discharging voltage, fast electron transfer kinetics and good mechanical properties of the polymer. However, low electron conductivity and high solubility in common organic electrolytes hamper the broad application of TEMPO-based compounds as cathode materials. In the present work, we report a simple strategy to overcome these limitations by the use of a practical and polymer-rich electrode based on redox-conducting polymer blends of polymerized nickel complexes with salen-type Schiff bases and PTMA. Electrochemical properties of the material are tested both in thin film systems and in asymmetric supercapacitor prototypes with aqueous electrolyte, which demonstrate high specific capacity (83 mAh g−1 at 1C, 53 mAh g−1 at 5C and 47 mAh g−1 at 10C), high rate capability as well as cycling stability (55% capacity retention after 1000 cycles).

Syntheses, crystal structures and photo physical aspects of azido-bridged tetranuclear cadmium (II) complexes: DFT/TD-DFT, thermal, antibacterial and anti-biofilm properties

In this work we have reported two novel tetranuclear Cd(II) complexes viz. [Cd4 (LOMe)2 (μ1,1-N3)3 (μ1,3-N3)]n (1) and [Cd4 (LOEt)2 (μ1,1-N3)3(OAc)]2 (2) where (H2LOMe) and (H2LOEt) are two important less explored salen-type Schiff base ligands. Both of the complexes have been characterized by using routine spectroscopic techniques, elemental analyses (C, H and N), X-ray powder diffraction pattern (PXRD) and thermal analysis by TGA along with single x-ray crystallography. The complete structural study discloses that in both cases the fully deprotonated ligand [LOMe]2- or [LOEt]2- utilized all potential coordination sites to accommodate four Cd(II) ions. Complex 1 is a one-dimensional polymer with azide (N3) linkage having both (μ1,1 end on) and (μ1,3 end-to-end) azido bridging but complex 2 is a discrete octanuclear ensamble where two [Cd4(O)4(N)2]2+ units bridged to each other showing μ1,1 end on end on azide bridging. Exploration of photo physical properties in DMSO solvent reveals that Cd(II) complexes enhance appreciably the fluorescence behavior over free Schiff base ligands (H2LOMe) and (H2LOEt). DFT calculations performed at B3LYP/def2-TZVP level of theory reveal both the energetics and composition of FMOs in these complexes and also show electrophilic and nucleophilic areas via molecular electrostatic maps [ESP] concept. The antibacterial, membrane damage assay and anti-biofilm properties of complexes 1 and 2 were investigated very carefully against some important Gram-positive and Gram-negative bacterial strains.

Salen Type Ligand as a Selective and Sensitive Nickel(II) ion Chemosensor: A Combined Investigation with Experimental and Theoretical Modelling

A novel tetradentate (N,O)-donor salen type Schiff base was designed, prepared and structurally characterized by different analytical techniques. This Schiff base ligand, (L = 2,2'-((1,2 phenylenebis(methanylylidene))bis(azanylylidene))diphenol) behaved as an effective chemosensor towards Ni(II) ion with specific selectivity. The Schiff base (L) displayed a remarkable ratiometric fluorescence enhancement upon binding with Ni(II) ion in acetonitrile (ACN) medium. The L-Ni(II) solution was distinguished indisputably with the naked eye colour change and the crystal was found to be red colour. L selectively detected Ni(II) ion in presence of other competitive cations like Mn(II), Fe(II),Co(II), Cu(II), Zn(II), Cd(II) and Hg(II) ions. This simple but novel Schiff base might be used as a potential pH independent colorimetric and naked-eye chemosensor for Ni(II) ion. Most significantly, L exhibited geometry preferred selectivity towards Ni(II) ion and formed highly stable square planar Ni(II)-Schiff base complex. Single crystal X-ray diffraction study further authenticated that the nickel complex crystallized in orthorhombic system with P212121 space group in solid state. Density functional theory (DFT) calculations attested the experimental results very well. This sensing method could be applied in a wider concentration range and might be used as an alternative qualitative and quantitative test for the detection of Ni(II) ion in analytical chemistry.

A Salen-based covalent organic polymer as highly selective and sensitive fluorescent sensor for detection of Al3+, Fe3+ and Cu2+ ions

A novel covalent organic polymer (Salen-COP) based on Salen-type Schiff base was synthesized by one-pot protocol involving the self-condensation of 3-formyl-4-hydroxyphenylboronic acid and the subsequent aldehyde–amine condensation with benzene-1,2-diamine. The Salen-COP was characterized by 13C CP/MAS NMR, Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and X-ray powder diffraction spectrometry analysis. The morphological features of Salen-COP were also analyzed using scanning electron microscopy. The Salen-COP exhibited the significant fluorescence enhancing for the sensing of Al3+, while the obtained polymer also exhibited the dramatic fluorescence quenching for the sensing of Fe3+ and Cu2+. The detection limits were calculated to be 0.248 µM for Salen-COP@Al3+, 0.545 µM for Salen-COP@Fe3+ and 0.140 µM for Salen-COP@Cu2+, respectively. Furthermore, the Salen-COP was founded to be a reversible chemosensor for Al3+, Fe3+ and Cu2+ ions, which provided a unique opportunity in the development of multi-response polymer-based sensors.

A low-overpotential nature-inspired molecular chromium water oxidation catalyst

The function of tyrosine Yz in photosystem II (PSII) has been considered as a model for establishing a new homogeneous water oxidation catalyst (WOC). In this work, we designed and synthesized a diphenoxy-based salen-type Schiff base ligand (L) in order to overcome the problems that so far have prevented the use of chromium complexes as water oxidation catalysts (WOCs). Cyclic voltammetry measurements performed in the alkaline solution of [Cr(L-2H)Cl] (1) displayed a large irreversible oxidation waves, corresponding to the catalytic water oxidation. The main species in the catalytically active solutions is found to be [Cr(L-2H)ClOH]-∙ The proposed catalytic system generates a current density (J) of 1.0 mA cm−2 at the relatively low overpotential (ƞ) of 0.426 V with a high turnover frequency (TOF) of 49.7 s−1. A comprehensive mechanistic study was carried out based on the results of experiments, calculations and according to previous works. The computational calculations supported the importance of the uncoordinated OH groups present in the ligand moiety of the complex for the oxidation of Cr3+ to Cr4+.

Interaction of DNA with water soluble complex of Nickle and formation of DNA cross-links

Interaction of double stranded DNA with bulky and hydrophobic Salen type Schiff base complex: [N, N′ Bis [3- tert-butyl-5-[triphenyl-phosphonium – methyl] - salicylidene] 1,2 ethylene-diamine nickel(III) acetate (refer to Ni Salen complex) was extensively investigated using the spectroscopic techniques and gel electrophoresis. Absorption titration experiment showed the hypochromic effect and the significant red shift of the complex absorption. In competition experiments with ethidium bromide (EB), Ni Salen complex exhibited non-competitive binding at high concentrations. UV-vis absorption and fluorescence emission data agreed on a binding constant of (1.64 ± 0.01) μM−1, thereby showing the strong interaction of the complex with DNA; also, a binding site size of 2.33 ± 0.01 base pairs per complex was achieved.Thermal denaturation experiment showed that Tm of calf thymus-DNA was increased by approximately 10 °C at a molar ratio of the dye/base of 0.2. The CD spectra of DNA exhibited an increase in both positive and negative peaks without any shift in the position of bands upon addition of the complex. The amplitude of the LD spectra of DNA was decreased in the presence of the complex. Reduced linear dichroism (LDred) revealed that the transition moment of complex was parallel to the DNA helix axis. Gel electrophoresis experiments confirmed that Ni Salen complex had no nuclease/DNA cleaving activity; also, DNA-DNA cross links were formed at high concentrations of complex, leading to the aggregation of DNA.

Synthesis and crystal structures of salen-type Cu(ii) and Ni(ii) Schiff base complexes: application in [3+2]-cycloaddition and A3-coupling reactions

The synthesis and characterisation of two new salen-type Schiff base complexes of the type [Cu(L)]·0.5H2O and [Ni(L)], and their application in the synthesis of 5-substituted 1H-tetrazoles and propargylamines are described.

Synthesis of salen-type Schiff base metal complexes, electropolymerization on graphite electrode surface and investigation of electrocatalytic effects

2-Aminobenzylamine-based unsymmetrical salen-type Schiff base and its Cu (II), Ni (II), Co (II) and Fe (III) metal complexes were synthesized and their structures were investigated by various spectroscopic methods. The electropolymerization of the synthesized metal complexes were achieved on the graphite electrode. The electropolymerization process was conducted by cyclic voltammetry (CV) in a 0.15M LiClO4/Acetonitrile as supporting electrolyte medium. Characterization of the polymer coated electrodes were carried out by CV, SEM, EDX and UV–Vis techniques. The electrocatalytic activities of graphite electrodes modified with Schiff-base metal complex were investigated in the determination of compounds such as Dopamine (DA), Catechol (CC), Cysteine (Cys) and Uric Acid (UA). These studies showed that the salen-type Schiff base-Ni(II)-modified graphite electrode gave the best result in the determination of catechol and dopamine. Square wave voltammetry showed a linear response in the range of 0.09–0.35μM and 0.22–0.58μM with LoD of 0.03μM and 0.06μM for CC and DA, respectively.

Experimental and theoretical studies on photoluminescent Zn(II) host complex with an open book structure: Implication on potential bioactivity and comparison with its ligand and Zn(II), Pd(II) siblings

A novel inclusion Zn(II) complex with water guest molecule is synthesized from 6,6′-diethoxy-2,2′-[2,2-dimethylpropane-1,3-diylbis(nitrilomethylidyne)]-diphenol salen-type Schiff base (H2L) and is characterized through single crystal XRD studies, IR, UV–Vis methods. Thermal profile is studied using TG-DTG–DSC method. Single crystal XRD analysis reveals a rare open book structure for the complex and an interesting star-like supramolecular assembly. The emission properties of the complex is compared with that of the ligand, pseudo-inclusion and inclusion compounds from Zn(II) and Pd(II) ions prepared from the same ligand. Hirshfeld surface for the compounds were mapped with various functions and further quantification of intermolecular interactions was made through the generation of 2D finger print plots. To dissect the nucleophilic and electrophilic regions on the molecular surface, ESP plots of the compounds are generated. Further DFT calculations are done at B3LYP/def2-TZVP level of theory to optimize the geometry of the compounds and to compare frontier orbitals and charge distribution among the compounds. Molecular docking of the compounds with B-DNA dodecamer and Human Serum Albumin (HSA) is performed to gain an insight on the structure–activity relationship of these compounds.

Synthesis, experimental and DFT characterization of the 2-((E)-(2-[(E)-2,3-Dihydroxybenzylideneamino]-5-methylphenyl)iminiomethyl)-6-hydroxyphenolate and its Ni(II) and Cu(II) complexes

Herein, the 2-((E)-(2-[(E)-2,3-Dihydroxybenzylideneamino]-5-methylphenyl)iminiomethyl)-6-hydroxyphenolate H2L Schiff base and its Ni(II) and Cu(II) complexes were synthesized. In addition to experimental characterization, their optimized geometries, tautomerism of the H2L, assignment of the IR bands and NMR chemical shifts, Fukui functions and the Natural Bond Orbital (NBO) analysis were computed by employing the density functional theory (DFT) methods. The H2L salen-type Schiff base can exist as mixture of four possible tautomers, tautomerization of which has low barrier energy in methanol solution. For complexation, the H2L loses two phenolic protons to produce the L2− species, which acts as a dianionic tetradentate ligand. Based on the Fukui functions, the two deprotonated phenolic oxygens and two azomethine nitrogens are the four donating atoms and occupy the four coordination positions in the square-planar structure of the Ni(II) and Cu(II) complexes. In comparison with the its free form, the coordinated ligand shows a more planar structure in the optimized geometries of the complexes. The high-energy gaps of the frontier orbitals confirm that all three investigated compounds are stable.

Preparing Mn(iii) salen-type Schiff base complexes using 1,3-oxazines obtained by Mannich condensation: towards removing ortho-hydroxyaldehydes

Synthesis of Mn(iii)-salen type Schiff base complexes from 1,3-oxazines obtained from Mannich condensation. This method eliminates the restrictions on the use ofortho-hydroxyaldehydes.