Salen-type Schiff-base Ligand Research Articles

Rapid and Efficient Dual Detection Of Zn2+ Ions and Oxytetracycline Hydrochloride Using a Responsive Fluorescent "On-Off" Sensor Based on Simple Salen-Type Schiff Base Ligand.

This research has progressed to an effective detection chemosensor of zinc, aluminum ions and oxytetracycline hydrochloride antibiotic based on the fluorescence technique. A straightforward method utilizing microwave irradiation was employed to synthesize the salen-type Schiff base ligand N,N'-bis(salicylaldehyde)4,5-dichloro-1,2-phenylenediamine (H2I), providing a good 70 % yield. In ethanol, the H2I sensor demonstrated remarkable rapidity, selectivity, and sensitivity in detecting zinc ions. The fluorescence spectrum exhibited a 44-fold substantial enhancement at 522 nm and achieved a low limit of detection (LOD) of 1.47 μM. Furthermore, the H2I probe's emission intensity increased by 124 times when compared to the ligand's ability to detect Al3+ ions at 494 nm with a LOD value of 7.4 μM. Additional research was done using the H2I probe's effective Zn2+ detection capability. The ability to recognize zinc ions in different real water samples demonstrated a recovery rate of 98.67 % to 103.31 %. Interestingly, a naked-eye visible fluorescence color of H2I solution impregnated filter papers turned colorless into yell ow under UV irradiation by adding Zn2+ ions, renders it suitable for developing a practical zinc ion detection kit test. In particular, the I-Zn2+ complex effectively quenched the fluorescence toward oxytetracycline hydrochloride (OTC) with a LOD value of 1.49×10-2 μM in DMSO:H2O (6 : 4, v/v). This is a novel and effective procedure for sensing OTC antibiotic by the I-Zn2+ complex. These findings hold immense potential for the development of dual fluorescent probes, thereby enhancing sensitivity and specificity in identifying metal ions and antibiotics in a wide range of applications.

Synthesis, crystal structures, and alcohol vapor adsorption properties of Cd(II) and Cu(II) complexes based on a salen-type Schiff base

A salen-type Schiff base ligand (H4L) was designed and synthesized from the condensation reaction. Four complexes, formulated as {[Cd(H5L+)Cl2(μ2 − Cl)]·2CH3OH}n (1), {[Cd(H4L)Br2]·H2O}n (2), {[Cd2(H4L)2I4]·CH3OH}n (3), and [Cu3(L4-)(μ3 − Ac)2]·CH3OH·H2O (4) were synthesized and characterized by single-crystal X-ray diffraction. Complex 1 exists as a 2D sheet structure and each Cd(II) ion in the complex is six-coordinated with an octahedral geometry. Both complexes 2 and 3 exist as 1D zig-zag structures and each Cd(II) ion in the two complexes is five-coordinated with a trigonal bipyramidal geometry. Complex 4 exists as a centrosymmetric trinuclear structure, in which two Cu(II) ions are five-coordinated with a square pyramidal geometry and the other Cu(II) ion is six-coordinated with an octahedral geometry. The structural differences are attributed to coordination ability of the coordination sites and coordination configurations of the metal ions. Furthermore, alcohol vapor adsorption measurements showed that all the synthesized complexes exhibited good adsorption capacities toward methanol vapor.

In vitro anti-microbial, DNA-binding, In silico pharmacokinetics and molecular docking studies of Schiff-based Cu(II), Zn(II) and Pd(II) complexes

Synthesis of salen type Schiff base ligand by the condensation of butane 1,4-diamine with 5-bromosalicyaldehyde has been done. Further, three new mononuclear Schiff based complexes of Cu(II), Zn(II) and Pd(II) have been prepared by the reaction of the Schiff base ligand with appropriate metal salts. The developed Schiff base ligand and metal complexes of Cu(II), Zn(II) and Pd(II) have been well characterized by various spectroscopic techniques like UV–Vis, FT-IR, 1H-NMR, 13C-NMR, EPR, Mass and powder XRD. All the synthesized Schiff base compounds have been tested for antibacterial, antifungal, hemolysis assay and DNA binding activities. The zone inhibition measurements revealed the supremacy and enhanced efficiency of metal complexes as compared to free ligand. The molecular modelling studies were carried to analyze the bonding affinity and inhibition probability of the synthesized complexes with the S. aureus nucleoside diphosphate kinase receptor. The in-silico pharmacokinetics studies such as Lipinski's rule, Veber's rule, drug-likeness score and physiochemical parameters analysis predicted the drug-likeness of the synthesized compounds.

Chiral 4f and 3d-4f Complexes from Enantiopure Salen-Type Schiff Base Ligands

This review summarizes the structural characteristics and physicochemical properties of chiral 4f and 3d-4f complexes based on enantiopure salen-type Schiff base ligands. The chirality originates from the enantiopure diamines and is imparted to the Schiff base ligands and complexes and finally to the crystal structures. The reported enantiopure Schiff base ligands derive from the condensation of aromatic aldehydes, such as salicylaldehyde and its various derivatives, and the enantiopure diamines, (1R,2R) or (1S,2S)-1,2-diamino-cyclohexane, (1R,2R) or (1S,2S)-1,2-diamino-1,2-diphenylethane, (R) or (S)-2,2′-diamino-1,1′-binaphthalene, and 1,2-diaminopropane.

Rapid and reliable ratiometric fluorescence detection of isoquercitrin based on a high-nuclearity Zn(II)-Nd(III) nanomolecular sensor

Rapid and quantitative detection of isoquercitrin (Isq) has been attracting much attention due to its outstanding pharmacological and physiological activities. Herein, an interesting 48-metal Zn(II)-Nd(III) nanocluster (1, molecular sizes 1.3 × 2.8 × 3.1 nm) with salen-type Schiff base ligand was constructed as molecular sensor for the luminescence detection of Isq. 1 exhibits visible ligand-centered emission and NIR luminescence of Nd(III), and shows ratiometric fluorescence response to Isq with high sensitivity even in the presence of other interferences. The fluorescence sensing behavior can be expressed by a second-order equation I1060nm/I480nm = A*[Isq]2 + B*[Isq] + C, which is used to quantitatively analyze the Isq concentrations in DMF and FCS. The LODs to Isq for the ligand-centered and lanthanide emissions of 1 in DMF are 0.21 μM and 0.11 nM, respectively. The quenching of the ligand-centered emission of 1 caused by Isq is attributed to the competitive absorption of light energy and “inner effect”, while, the luminescence enhancement is due to the “antenna effect”.

Insight into the conformational selectivity of cobalt(iii) complexes with a tetradentate salen-type Schiff base ligand and their biorelevant catalysis: a combined experimental and theoretical study

A series of mononuclear CoIII complexes showing conformational selectivity (stepped vs. umbrella) and biomimetic catalytic activities are reported.

An in silico design of antivirus nickel (II) complexes as therapeutic drug candidates

A series of novel nickel (II) complexes [Ni(L1)](1), [Ni(L2)](2), [Ni(L3)](3) and [Ni(L4)](4) of salen-type Schiff base ligands (L1H2-L4H2), have been designed and synthesized, and their interaction with SARS-CoV-2 for COVID-19 and HIV virus were studied by molecular docking methods for possible therapeutic drug candidates as anti-COVID-19 and anti-HIV agents. The salen-type Schiff base ligands were condensation products of ethylene diamine with related aldehydes (3,5-Dichlorosalicylaldehyde, 5-Bromo-3-methoxy-2-hydroxy-benzaldehyde, 3,5-Diiodosalicylaldehyde, 3,5-Dinitrosalicylaldehyde). They were coordinated to metal ions through the tetradentate-N2O2 donor atoms. The newly synthesized complexes were fully characterized by different spectroscopic and physicochemical methods. The molecular and electronic structures of the complexes are studied by DFT based quantum chemical calculations. Additionally, inspired from recent developments to find inhibitors of the SARS-CoV-2 main protease, molecular docking studies are performed on the complexes to predict the binding mode and interactions between the ligands and the main protease of the SARS-CoV-2 (PDB ID: 7O46) for COVID-19. Also the binding potential of the nickel(II) complexes with HIV virus (PDB ID: 1UUI) are studied using in-silico molecular docking approach. The X-ray crystallographic structure of the main protease of the SARS-CoV-2 and HIV virus are retrieved from the protein data bank and used as receptor proteins. The molecular docking calculations of the nickel (II) complexes (1)-(4) with SARS-CoV-2 (PDB ID: 7O46) virus revealed the higher binding energy (-9.6 to -6.9 kcal/mol) than that of HIV virus (-9.3 to -6.7 kcal/mol) as well as docking results of chloroquine (-6.293 kcal/mol), hydroxychloroquine (-5.573 kcal/mol) and remdesivir (-6.352 kcal/mol) as anti-SARS-CoV-2 drugs. Overall, in-silico molecular docking study offers the potential role of the nickel (II) complexes as anti-COVID-19 and anti-HIV agents.

Design and development of salen-type Schiff bases as potential antivirus agents: Experimental and theoretical approach

Five new salen-type Schiff base compounds (LH2-L4H2) have been designed and synthesized and their interaction with SARS-CoV-2, HIV virus and DNA polymerase IV have been studied by in silico approaches. The newly synthesized salen-type Schiff base ligands have been derived from the condensation of substituted salicylaldehydes and ethylenediamine in MeOH. 1H and 13C NMR, FT-IR and UV-Vis spectral techniques have been applied in order to confirm the structural elucidation of the desired products. The crystal structure of the compound LH2 is determined by the single crystal X-ray diffraction. Density functional theory (DFT) calculations have been employed to evaluate the optimized electronic structure, HOMO-LUMO, energy gap, and global parameters. Extensive classical molecular dynamics simulations have been performed to investigate the consequences of docking of the synthesized ligand, LH2 on a protein moiety (PDB id: 7O46). Molecular docking studies have been performed on compounds (LH2-L4H2) to predict the binding mode and interactions between the ligands and the main protease of the SARS-CoV-2 (PDB ID: 7O46) for COVID-19 and HIV virus (PDB ID: 1UUI). The molecular docking results show that compounds (LH2-L4H2) with SARS-CoV-2 and HIV virus exhibit good binding affinity at binding site of receptor protein. As potential drug candidates, Swiss-ADME and target predictions (pharmacokinetics and drug-likeness prediction) analyses have also been studied and the results are compared with Chloroquine (CQ) and Hydroxychloroquine (HCQ) as anti-SARS-CoV-2 drugs. Salen-type ligands have also been docked into the DNA polymerase IV (PDB ID: 5YUX) for surface binding and intercalation. Overall, the present study offers the therapeutic potential for a series of compounds (LH2-L4H2) for biomedical applications with reference to coronavirus (SARS-CoV-2) and HIV virus.

A heptanuclear Cd(II)-Nd(III) nanocluster with NIR luminescence response to nitrofuran antibiotics

A 7-metal Cd(II)-Nd(III) nanocluster 1 (molecular size 0.7×1.5×2.5 nm) was synthesized from a new salen-type Schiff base ligand. 1 exhibits NIR luminescence response to nitrofuran antibiotics (NFAs) with high sensitivity and selectivity, which is not affected by other antibiotics. The quenching constants (Ksv) and limits of detection (LOD) of NFAs range from 3.24×104 M−1 to 0.94×104 M−1 and from 4.2×10−7 M to 7.9×10−6 M, respectively. It also can be used to detect NFAs in real antimicrobial drugs sold in pharmacy.

Synthesis, Spectral Characterization, and Biological Activities of Some Metal Complexes Bearing an Unsymmetrical Salen-Type Ligand, (Z)-1-(((2-((E)-(2-Hydroxy-6-methoxybenzylidene)amino)phenyl)amino) methylene) Naphthalen-2(1H)-one

An unsymmetrical salen-type Schiff base ligand, (Z)-1-(((2-((E)-(2-hydroxy-6-methoxybenzylidene)amino)phenyl)amino)methylene)naphthalen-2(1H)-one, and its Zn(II), Cu(II), Co(II), Mn(II), and Fe(III) complexes were synthesized and characterized by mass (MS), nuclear magnetic resonance (NMR), infrared (IR), ultraviolet-visible (UV-Vis) spectra, and effective magnetic moments. The thermal analyses of the obtained ligand and metal complexes were conducted by thermogravimetric analysis (TGA). Antimicrobial activity of the unsymmetrical Schiff base ligand and its metal complexes were examined for Staphylococcus aureus as Gram-positive bacteria and Escherichia coli as Gram-negative bacteria. In vitro anticancer property of synthetic compounds was estimated against human cancer cell lines, a subline of Hela tumor cell line (KB), and a human liver cancer cell line (HepG-2) as well.

Role of macrocyclic salen-type Schiff base ligands in one-dimensional Co(II) complexes for superior activities toward oxygen reduction/evolution reactions

Developing high activities, stable, non-precious metal based bi-functional electrocatalysts oxygen evolution/reduction reactions (OER/ORR) in rechargeable metal-air batteries and regenerative fuel cell technologies is essential for future energy conversion and storage. In this work, the potential of utilizing the synthesized one-dimensional transition metal salen-type complexes (TM-SCs) as bi-functional electrocatalysts of ORR and OER is systematically explored by computational screening approach. The results demonstrate that different types of macrocyclic ligands, including N2O2, N3O and N4 as donor groups around the active sites, govern the OER/ORR catalytic performances. Co–SCs with N2O2 ligands exhibit the highest bifunctional catalytic activities. In particular, low limiting overpotentials of 0.22 V for OER and 0.33 V for ORR can be observed on Co sites, which are even superior to those of noble metal catalysts. Analyzing the linear relationships between the adsorption strength of intermediates and the overpotentials shows that the origin of excellent electrocatalytic performance is the smaller slope (0.86) for OOH∗ vs OH∗ on TM-SCs compared to metal surfaces, resulting in strengthened binding of the OOH∗ intermediate. Besides, the adsorption energies of the intermediates bound on Co–N2O2 are close to the ideal values, while too strong on the Co–N3O and Co–N4 catalysts. By applying external strains, the adsorption strengths of reaction intermediates can be further modulated due to the tunable d-band centers, and the resulting ORR/OER activities are further boosted. Considering that the Co salen-based chain has been synthesized experimentally, this work highlights the excellent electrocatalytic performances of this new material and devises novel strategy by straining for catalyst optimizations.

A NIR luminescent “tetra-decker” Nd(III) salen nanocluster for rapid ratiometric fluorescence detection of quercetin

Two NIR luminescent “bi-decker” and “tetra-decker” Cd(II)-Yb(III) and Nd(III) clusters 1 and 2 were synthesized from a new salen-type Schiff base ligand. 2 exhibits rapid ratiometric fluorescence response to Que with high sensitivity. The quenching constants of Que to the ligand-centered and lanthanide emissions of 2 are 2.9 × 104 M−1 and 3.8 × 104 M−1, respectively. The detection limits to Que are 6.75 μM for the ligand-centered emission and 2.03 μM for the lanthanide luminescence. The response of 2 to Que is not influenced by some potential interferents such as biomolecules and ions.

Construction of Cr(III)-Ln(III)-Salen (Ln = Nd, Yb, Er or Gd) hetero-binuclear complexes with high-purity near-infrared (NIR) luminescence

Based on the treatments of the Salen-type Schiff-base ligand H2L (N,N’-bis(3-methoxy-salicylidene)cyclohexane-1,2-diamine) with CrCl2 and the subsequent Ln(OAc)3⋅6H2O (Ln = Nd, Yb, Er or Gd), series of hetero-binuclear Cr(III)-Ln(III)-Salen complexes [Cr(L)Cl(μ1-OAc)Ln(μ2-OAc)Cl(H2O)] (Ln = Nd, 1; Yb, 2; Er, 3 or Gd, 4) were obtained, respectively. Photo-physical property results show that in comparison to the Cr3+-characteristic (2E → 4A2 transition) high-purity NIR-luminescence for the complex [Cr(L)Cl(μ1-OAc)Gd(μ2-OAc)Cl(H2O)] (4), the complexes [Cr(L)Cl(μ1-OAc)Ln(μ2-OAc)Cl(H2O)] (Ln = Nd, 1; Yb, 2 or Er, 3), due to the effective intra-molecular Cr(III)-to-Ln(IIII) energy transfer, exhibit the corresponding Ln(III)-centered (Ln = Nd, Yb or Er) high-purity NIR-emissions, respectively.

Copper(II) Schiff base complex derived from salen ligand: structural investigation, Hirshfeld surface analysis, anticancer and anti-SARS-CoV-2

This work deals with the synthesis and characterization of copper(II) complex [Cu(salen)(H2O)](1) of salen-type Schiff base ligand derived from the condensation of 5-bromo-2-hydroxy-3-methoxybenzaldehyde and ethylenediamine in EtOH. This complex was characterized by different spectroscopic and physicochemical methods. Single crystal X-ray crystallography study revealed that Cu(II) in complex (1) is five-coordinate and adopts a distorted square pyramidal geometry. A DFT calculation was employed to evaluate the optimized electronic structure, HOMO-LUMO, energy gap, and global parameters. A detailed structural and non-covalent interaction on the complex is investigated by single crystal structure analysis and computational approaches. The strength of the interaction and 3D topology of the crystal packing are visualized through an energy framework. Hirshfeld surface and 2D fingerprint plots have been explored in the crystal structure of the complex. The anticancer properties of copper(II) complex was studied against the selected cancerous cell lines of breast cancer, cervical cancer, colon cancer and hepatocellular carcinoma. Additionally, molecular docking and MD simulations was performed on the complex to predict the binding mode and interactions between the ligand and the main protease of the SARS-CoV-2 (PDB ID: 7CBT and 7D1M). The molecular docking calculations of the complex (1) with SARS-CoV-2 virus revealed the binding energy of –8.1 kcal/mol and –7.5 kcal/mol with an inhibition constant of 3.245 µM and 2.318 µM at inhibition binding site of receptor towards 7CBT and 7D1M main protease (Mpro), respectively. Besides this, molecular docking results (–7.6 kcal/mol, 3.196 µM) towards Escherichia coli PBP2 targets (PDB ID: 6G9S) was also studied. Communicated by Ramaswamy H. Sarma

Luminescence of Zn-Yb dinuclear Schiff base complex: Enhanced NIR emission by modification with larger conjugated light-harvesting moieties

The fundamental challenge for the highly efficient NIR-luminescence of lanthanides is to identify the suitable chromophores as sensitizer. Herein, the first Salen-type Schiff base ligand modified by conjugated fluorene has been designed to enhance the near-infrared emission of the ytterbium complex. The photophysical properties reveal the potential of the Schiff base dyes as an efficient and strong visible light sensitizer for the lanthanide-based NIR emitters. The modified rigid Schiff base ligand is noted to encapsulate the Ln3+ ions as it prevents the coordination of the solvent molecules, thus, tending to quench the luminescence phenomenon. Due to the intermolecular π-π interactions, the excited state energy levels of ligands are observed to reduce, thus, leading to the highly efficient energy transfer from the triplet state of ligands to the Yb3+ ions. In addition, the shifting of the absorption wavelength of the complex towards low energy (450 nm) makes it suitable for biomedical applications. Further, the ytterbium complex exhibits the highest intrinsic quantum yields (3.68%) among the Salen-type ytterbium complexes.

Oxidative kinetic resolution of P-chiral phosphines catalyzed by chiral (salen)dioxomolybdenum complexes

Abstract Synthetic LMoO2 compounds have long been of keen interest both as structural and functional models for molybdoenzymes and in their own right as catalysts for a variety of oxygen atom-transfer (OAT) reactions. Investigations of their use as catalysts for stereoselective OAT transformations, however, are little known. In this study chiral diimine-salen molybdenum complexes, L*MoO2, are evaluated for their potential to catalyze oxidative kinetic resolution of racemic monophosphines by pyridine N-oxide. A set of six L*MoO2 complexes incorporating chiral salen-type Schiff base ligands derived from 1,2-diaminocyclohexane (7-10) and 1,1’-diaminobinapthylene (12) has been prepared and characterized. Compounds 7-10 and 12 are active catalysts for the oxidation of racemic PMePhtBu by pyridine N-oxide, affording low to moderate enantioselectivities (0 - 35 % ee) of the phosphine oxide OPMePhtBu. Key structure/reactivity features of the catalysts for these reactions include: the presence of a p-NO2 substituent on the salen-unit increases the catalyst activity; and increasing the steric bulk of the ortho-salen-substituent increases enantioselectivity. DFT computational analysis has identified a viable reaction pathway that features a stereochemically-defining O-transfer transition state involving phosphine attack on the chiral LMoO2 complex, which accounts for the experimental stereoselectivity and catalyst activity.

Eu3+-to-Cr3+ energy transfer for the improved Cr3+- characteristic near-infrared (NIR) phosphorescence in the Cr(III)-Eu(III)-Salen complex

Through the self-assembling of Ln(OAc)3⋅6H2O (Ln = Eu or Tb) with the chromium(III)-complex precursor [Cr(L)Cl(H2O)] formed from the oxidation and the coordination reactions of CrCl2 and the Salen-type Schiff-base ligand H2L (N,N’-bis(3-methoxy-salicylidene)cyclohexane-1,2-diamine), two hetero-binuclear complexes [Cr(L)Cl(μ1-OAc)Ln(μ2-OAc)Cl(H2O)] (Ln = Eu, 1 or Tb, 2) were obtained, respectively. Moreover, attributing to effective Eu3+-to-Cr3+ energy transfer, the [Cr(L)Cl(μ1-OAc)Eu(μ2-OAc)Cl(H2O)] (1) exhibits the significantly improved Cr3+-characteristic near-infrared (NIR) phosphorescence in comparison to that of the Cr(III)-complex precursor [Cr(L)Cl(H2O)] or the [Cr(L)Cl(μ1-OAc)Tb(μ2-OAc)Cl(H2O)] (2).

When the Metal Makes the Difference: Template Syntheses of Tridentate and Tetradentate Salen-Type Schiff Base Ligands and Related Complexes

The reaction of organic molecules mediated by a metal center (template synthesis) can result in a final connectivity that may differ from the one obtained in the absence of the metal. The condensation of carbonyl fragments with primary amines form C=N iminic bonds, the so-called Schiff bases, which can act as ligands for the templating metal center by means of the lone pair on the nitrogen atom. This review focuses on the template methods for the reaction between a carbonyl compound (mainly salicylaldehyde) and a primary aliphatic diamine able to prevent the double condensation on both amine groups and obtain tridentate N2O ligands. These adducts, still having one free amino group, can further react, yielding tetradentate salen-type Schiff base ligands. A screening over the transition metals able to show such a template effect will be presented, with particular attention to copper(II), together with their peculiar reactivity and the available crystal structure of the metal complexes and related coordination geometries.

Synthesis, spectral characterization, crystal structure and antibacterial activity of nickel(II), copper(II) and zinc(II) complexes containing ONNO donor Schiff base ligands

In the present study, a series of mononuclear Ni(II), Cu(II) and Zn(II) complexes with ONNO donor salen-type Schiff base ligands, derived from different 3,5-dihalosalicylaldehyde with polymethylenediamines of varying chain length, have been prepared. All the synthesized compounds were characterized by using various spectroscopic (FT-IR and 1H NMR) and analytical (CHN etc.) techniques. Furthermore, three complexes (ZnL1, NiL3 and NiL4) were successfully characterized by single crystal X-ray diffraction. The data revealed that the geometry around Zn(II) was distorted square pyramidal for ZnL1, while NiL3 and NiL4 complexes adapted slightly distorted octahedral geometries around Ni(II). The antibacterial activities of the synthesized ligands and their respective complexes were elaborated by screening them against two strains of Gram positive (Staphylococcus aureus and Bacillus cereus) and two strains of Gram negative (Escherichia coli and Pseudomonas aeruginosa) bacteria. It was found from the measurements of zones of inhibition that metal chelates are marginally more effective than free ligands.

NIR luminescent detection of quercetin based on an octanuclear Zn(ii)-Nd(iii) salen nanocluster.

A NIR luminescent octanuclear Zn(ii)–Nd(iii) nanocluster 1 was constructed by the use of a salen-type Schiff base ligand. 1 exhibits a lanthanide luminescent response to Que with high sensitivity. The quenching constant of Que to the lanthanide emission is 2.6 × 104 M−1, and the detection limit of 1 to Que is 2.5 μM. The response behavior of 1 to Que is not affected by the existence of some potential interferents such as biomolecules.