Asymmetric Schiff Base Ligand Research Articles

Synthesis, Crystal Structures, Photophysical Properties and Antibacterial Activities of Copper(II) Complex Derived from 4-Chloro-2-{[(2,6-dimethylphenyl)imino]methyl}phenol

A new asymmetrical Schiff base ligand, (E)-4-chloro-2-{[(2,6-dimethylphenyl)amino] methyl}phenol(HL), and its mononuclear Cu(Ⅱ) complex have been synthesized. The molecular structures and spectroscopic properties of the ligand and its complex were experimentally characterized by single crystal X-ray diffraction, elemental analysis, FT-IR, NMR and UV-Vis spectroscopic techniques. The analysis confirmed that the Cu atom in complex is generally four-coordinated by one imino N and one phenolic O atoms of a Schiff-base ligand adopting a trans-configuration of N2O2 donor atoms around the metal ion, forming a tetrahedral geometry. Density functional calculation is being carried out on both HL and copper(Ⅱ) complex to investigate changes in structural parameters, energies of the HOMO and LUMO. The results demonstrated that the HOMO and LUMO were effectively separated with the benzene ring of 2,6-dimethylbenzenamine as a donor unit and the benzene ring of 5-chlorosalicylaldehyde as an acceptor unit. The effective HOMO-LUMO separation helps to induce intramolecular charge transfer from the HOMO to the LUMO. The HOMO-LUMO energy gap become smaller when the Schiff base ligand coordinated with Cu(II) ion, most likely due to the interaction of the d-orbitals of copper(Ⅱ) with the HOMO and/or LUMO of the ligand. These theoretical calculations supported the experimentally observed results.The biological assay revealed that both Schiff base and the complex have different antimicrobial activity. The complex has the MIC value of 0.0781 mmol·L-1 against Escherichia coli.

Heterotrimetallic Cu(II)Ho(III)Co(III) complex based on asymmetric Schiff base Ligand: Synthesis, structures and magnetic properties

Two new complexes, [Ho(DMF)4(H2O)2{Fe(CN)6}]n⋅nH2O (1) and [{CuL(H2O)N3Ho(H2O)3}2{Co(CN)6}]n·n[Co(CN)6]·7nH2O (2) (HL = N,N’-ethylene(3-methoxysalicylaldiimine) were obtained. They were synthesized from the precursors of asymmetric Schiff-base CuII, lanthanide metal ion of HoIII and hexacyanometalates. The single crystal X-ray diffraction measurement revealed their peculiar interesting chain structures: heterobimetallic neutral HoIII-FeIII chain for 1 and heterotrimetallic CuIIHoIIICoIII cationic chain for 2, respectively. Due to the asymmetric Schiff-base, the coordination environment of CuII and HoIII in 2 is quite different from the traditional 3d-4f CuII -LnIII systems prepared from symmetric Schiff base ligands. Magnetic investigation reveals the antiferromagnetic FeIII -HoIII and ferromagnetic CuII -HoIII coupling for 1 and 2, respectively. The complexes provide new evidence of designing heterobimetallic systems using various asymmetric Schiff-base ligands.

Synthesis, characterization, crystal structure, and theoretical studies of a mixed-ligand copper(I) iodide complex of an asymmetric Schiff base ligand 2-((pyridin-4-yl)methelenamino)-3-aminomaleonitrile and triphenylphosphine co-ligand

Single pot synthesis of a new mixed-ligand iodide bridged dimeric Cu(I) complex, [L2CuI2(μ-I)2(PPh3)2](1) has been carried out by combining copper(I)iodide, an asymmetric Schiff base ligand 2-((pyridin-4-yl)methelenamino)-3-aminomaleonitrile (L) and a co-ligand triphenylphosphine (PPh3). L and 1 have been characterized by 1H NMR, IR and EIMS spectrometry, elemental analysis and DFT studies. The crystal structure of 1 has been determined by single-crystal X-ray analysis and it has been found that it is a dimer with —{CuI—(m-I2)—CuI}— rhomboid core with the bridged iodide ion. 1 crystallized as 1·CH2Cl2. At room temperature in dichloromethane the ligand L is non-emissive whereas the complex 1 is photoluminescent. The E1/2 value of the CuI—CuII couple of 1 in dichloromethane is 0.46 V (vs. Ag/AgCl in 1 M KCl, scan rate 100 mV/s). The Hirshfeld surface analysis of the complex 1·CH2Cl2 showed H⋯H, N⋯H and I⋯H interactions of 52.3 %, 13.1 % and 7.4 %, respectively, which revealed that the main intermolecular interactions were H⋯H intermolecular interactions.

Theoretical study on the mechanism for the excited-state double proton transfer process of an asymmetric Schiff base ligand

Excited-state double proton transfer (ESDPT) in the 1-[(2-hydroxy-3-methoxy-benzylidene)-hydrazonomethyl]-naphthalen-2-ol (HYDRAVH2) ligand was studied by the density functional theory and time-dependent density functional theory method. The analysis of frontier molecular orbitals, infrared spectra, and non-covalent interactions have cross-validated that the asymmetric structure has an influence on the proton transfer, which makes the proton transfer ability of the two hydrogen protons different. The potential energy surfaces in both S0 and S1 states were scanned with varying O–H bond lengths. The results of potential energy surface analysis adequately proved that the HYDRAVH2 can undergo the ESDPT process in the S1 state and the double proton transfer process is a stepwise proton transfer mechanism. Our work can pave the way towards the design and synthesis of new molecules.

Asymmetric Schiff base ligand enables synthesis of fluorescent and near-IR emitting lanthanide compounds

In this paper, the HYDRAVH2 ligand as an asymmetric fluorescence Schiff base ligand was used to produce a series of new lanthanide complexes (HYDRAV-Ln(III), Ln: Yb, Er, Gd and Dy). The structures of the Schiff base complexes are characterized using Fourier transform infrared spectroscopy (FT-IR), MALDI-TOF mass spectrometry, CHN elemental analysis, Ultraviolet-Visible (UV–Vis), fluorescence, near-infrared (NIR) and thermogravimetric analysis (TGA). The photoluminescence properties show that the energy transfer from HYDRAVH2 to Ln(III) ions is effective and the energy levels of HYDRAVH2 and Ln(III) ions match well, therefore the Ln-compounds exhibit emission in the visible region. The result of the photophysical properties of HYDRAV-Ln(III) also indicates that strong and characteristic Near-IR luminescence for HYDRAV-Yb and HYDRAV-Er are observed, which are attributed to the antenna effect of the HYDRAVH2 ligands that harvest the light and transfer it to the metal centers. Based on the above results, the ligand and its complexes (HYDRAV-Ln(III)) have remarkable optical features that could be of great utility in optoelectronic devices.

Palladium(ii) complexes of tetradentate donor–acceptor Schiff base ligands: synthesis and spectral, structural, thermal and NLO properties

Structural and NLO behavior of push–pull palladium(ii) complexes of metallocenyl-containing asymmetric Schiff base ligands.

Designing asymmetric Dy2 single-molecule magnets with two-step relaxation processes by the modification of the coordination environments of Dy(iii) ions.

The reaction of Dy(NO3)3·6H2O and an asymmetric Schiff-base linker 5-chloro-2-(((2-hydroxy-3-methoxybenzyl)imino)methyl) phenol (H2L) afforded a dinuclear compound [Dy2L2(HL)(NO3)(EtOH)]·0.5C2H5OH (1). Complex 1 features two inequivalent Dy(iii) sites, where three ligand sets (one HL- moiety and two L2- groups) are shared by two Dy(iii) ions. The strategic introduction of CH3COOH in the reaction system used for synthesizing 1 induces the replacement of the HL- ligand by the CH3COO- ion, consequently resulting in the generation of [Et3NH][Dy2L2(NO3)(CH3COO)2] (2). In complex 2, two Dy(iii) centers adopt NO7 (D2d geometry) and NO6 (C2v) coordination sphere, respectively. DC magnetic susceptibility studies for the two complexes indicate ferromagnetic interactions. Complexes 1 and 2 exhibit single-molecule magnet behavior with two-step slow relaxation processes due to the possession of inequivalent metal sites. The energy barriers of 69.19 and 45.73 K for 1, and 92.77 and 72.95 K for 2 are determined. Theoretical calculations reveal that the two-step relaxation processes in both 1 and 2 mainly originate from the single-ion magnetism of two distinct Dy(iii) ions with inequivalent coordination environments.

SYNTHESIS, MOLLUSCICIDALAND ANTIMICROBIAL POTENTIALITIES OF IRON TRIAD MONONUCLEAR METAL COMPLEXES INCORPORATING TRIDENTATE ASYMMETRICAL SCHIFF BASE LIGANDS CONTAINING SOFT SULFUR COORDINATING ATOM

Objective: This work aimed at synthesizing tridentates asymmetrical Schiff base ligands containing sulfur atom and using them for preparing metal complexes with the iron triad metals. The prepared compounds were assayed in vitro for antimicrobial potential and in vivo molluscicidal activity.
 Methods: The unsymmetrical tridentate Schiff bases (SL1, SL2, and SL3) were prepared using 2-aminothiophenol as primary amine and condensed with 2-carboxybenzaldehyde, 2-hydroxy-1-naphthaldehyde, and 7-formyl-8-hydroxyquinoline. These ligands were used in preparing metal complexes with iron triad metals. The synthesized Schiff base ligands and their corresponding metal complexes were characterized and their proposed structures were confirmed using different physical and spectroscopic analytical techniques. All ligands and their corresponding metal complexes were assayed against different bacterial and fungal strains using the agar disk-diffusion technique. The molluscicidal activity was performed according to the standard reported methods as cited in the literature and by observing the toxicity and lethal dose according to the WHO guidelines.
 Results: The synthesized ligands behave as tridentate (NOS) ligands and form mononuclear complexes with the general formula [M(SL)2] with an octahedral geometry around the central metal ion. Metal complexes were non-electrolytic in nature. The in vitro antibacterial and antifungal examination results showed weak activity of the ligands, and there was enhanced activity with the complexes. The in vivo molluscicidal activity of the tested compounds showed good activity.
 Conclusion: The targeted compounds were prepared successfully, characterized, and showed some biological activity but lower than the standard reference drugs.

Docking studies on an N4-donor Schiff base ligand and its Cu(II) complex supported by structural, spectral and theoretical studies

A new asymmetrical Schiff base ligand, N1-(3-((pyridin-2-ylmethylene)amino)propyl)ethane-1,2-diamine, and its copper(II) complex, [Cu(PPEA)Br]Br·H2O (1), have been prepared. The copper complex has an elongated square pyramidal geometry with a CuN4Br environment. The thermodynamic stability of the compounds and their charge distribution patterns were studied by Density Functional Theory and natural bond orbital analysis. The ability of the ligand and its copper complex to interact with 10 selected biomacromolecules (BRAF kinase, CatB, DNA gyrase, HDAC7, rHA, RNR, TrxR, TS, Top II and B-DNA) was investigated by docking studies and compared with that of doxorubicin.

Synthesis, characterization, crystal structure and DFT studies of a palladium(II) complex with an asymmetric Schiff base ligand

A Pd(II) complex of tetradentate Schiff base ligand, bis(5-methoxysalicylidene)-4-methyl benzene-1,2-diamine (H2L) was prepared from Pd(OAc)2 in refluxing acetonitrile. The complex was characterized using FT-IR, lH NMR, 13C{1H} NMR spectroscopic techniques, elemental analyses (CHN). Also, its molecular structure was determined by single-crystal X-ray diffraction. The crystal structure data showed a tetradentate mode of coordination for Schiff base, through N2O2 donor sets. In addition, DFT calculations at M06-2X/Def2-TZVP level of theory were conducted to obtain electronic and structural properties of the studied species. NBO-CMO analysis combined with TD-DFT method was done to interpret electronic transitions by determining their included occupied and virtual orbitals. NBO-CMO data show that there are MMCT and LMCT in the studied complex. All theoretical data are in line with the experimental results.

Molecular modeling, spectral investigation and thermal studies of the new asymmetric Schiff base ligand; (E)‐N'‐(1‐(4‐((E)‐2‐hydroxybenzylideneamino)phenyl)ethylidene)morpholine‐4‐carbothiohydrazide and its metal complexes: Evaluation of their antibacterial and anti‐molluscicidal activity

A new tetradentate asymmetric Schiff base ligand; (E)‐N'‐(1‐(4‐((E)‐2‐hydroxybenzylideneamino)phenyl)ethylidene)morpholine‐4‐carbothiohydrazide (H2L) was prepared by consecutive condensation reactions between p‐aminoacetophenone, salicylaldehyde and 4‐morpholinethiosemicarbazide. The structures of the obtained ligand along with its Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Cr(III), Fe(III) and Ru(III) complexes were authenticated by different analytical and spectral techniques. The ligand behaves as a monobasic tetradentate one and chelates with metal ions through the deprotonated phenolic oxygen, imine nitrogen, thione sulfur and azomethine nitrogen atoms (ONNS chromophore). All nonelectrolyte metal complexes possess octahedral geometry, except Ni(II) and Cu(II) complexes (2, 3) which have tetra‐coordinate structures. The thermal stability of metal complexes is higher than that of the free ligand and the general thermal degradation pathways were discussed based on thermogravimetric technique. The activation thermodynamic parameters as well as the order of reaction (n) were estimated from DTG curves by using Piloyan method. Thermogravimetric analyses and mass spectra of metal complexes confirm the number of solvents in their inner and outer spheres. Molecular modeling data were used to demonstrate the thermal decomposition pathways of the ligand and its metal complexes, confirming their chemical structures. The ESR spectra of Cu(II) complexes indicate that the unpaired electron is localized mainly in the d(x2‐y2) orbital with axial symmetry. Powder X‐ray diffraction analyses for Co(II), Cu(II) and Ru(III) complexes (1, 3, 7) show that copper complex has a crystalline nature, whereas, cobalt and ruthenium complexes are amorphous. The ligand and its metal complexes were tested in vitro for their anti‐molluscicidal activity against Eobania vermiculata. The results reveal that the investigated compounds have remarkable anti‐molluscicidal activities against the tested land snail with various sensitivity levels. The synthesized compounds were also screened in vitro for their antibacterial activity against Gram‐positive bacteria; Staphylococcus aureus and Gram‐negative bacteria; Escherichia coli and the results were compared with the activity of ceftriaxone. The data show that all investigated compounds display a high antibacterial activity against the tested bacterial strains with different inhibition degrees and the activity is significantly higher against Staphylococcus aureus than Escherichia coli.

Theoretical investigation on molecular structure of a new mononuclear copper(II) thiocyanato complex with tridentate Schiff base ligand

An asymmetrical Schiff base ligand, 4-bromo-2-(2-pyridylmethyliminomethyl)phenol (HL), and its copper(II) complex, [Cu(L)SCN] (1), have been synthesized. Complex 1 is experimentally characterized by elemental analysis, FT-IR and UV–vis spectroscopic techniques, and cyclic voltammetry. The structure of the complex has been established by single-crystal X-ray diffraction studies, which reveal a square planar geometry of the central copper(II) ion in 1. The neighboring molecules of the complex connect each other by π–π stacking interactions with centroid-to-centroid ring distance 3.653 Å. The ligand can display two possible tautomeric forms; therefore, 1 can have an alternate molecular structure. DFT calculations have been employed to investigate the structure and relative stabilities of the suggested tautomeric forms of the ligand and its corresponding copper(II) complex.

Antibacterial Activity of Co(II), Ni(II) and Cu(II) complexes with asymmetrical Schiff base ligand

In this study, biologically active mononuclear complexes of Cu (II), Ni (II) and Co (II) were prepared with a asymmetric Schiff base ligand (H.L).The synthesis process for asymmetric Schiff base ligand is simple and consists of the following steps: (1) condensation of 1-benzoylacetone (1-phenyl-1,3-butanedione) and 1,3-diaminopropane in 1:1 molar ration yielded the half unit compound (H.U). (2) Preparing the new asymmetric Schiff base ligand (H.L) by treating the (H.U) compound with 2-hydroxy-5-nitrobenzaldehyde. The structure of the ligand was elucidated by analytical and spectroscopic analysis. The structure of asymmetric Schiff base ligand and metal complexes were characterized by various techniques such as elemental analysis, IR, 1 H-NMR, UV-vis, conductivity and magnetic moment measurements. Square-planar and octahedral geometries are suggested for the Cu (II), Co (II) and Ni (II) complexes. These compounds were screened for in vitro antibacterial activity against Gram positive bacteria (Staphylococcus aureus, Bacilus subtilis) and Gram negative bacteria (Escherichia coli, Shigella sonnei and Pseudomonas arioginosa). The Cu (II) and Ni (II) complexes had broad-spectrum activity in the inhibitory zones range of 14-25 and 15-28 mm, respectively. All the new complexes have higher activity than the free ligand.

Metal(II)-Induced Synthesis of Asymmetric Fluorescence Benzimidazoles Complexes and Their Dye-Sensitized Solar Cell Performance as Cosensitizers

Four asymmetric complexes with different conjugate structures based on benzimidazole derivatives with formulas of [ZnL1Cl2] (1) (L1: 1-((quinoline-2-yl) methylene)-2-quinoline-2-yl benzimidazole), [CdL2Cl2]n (2) (L2: N1,N2-bis((quinoline-2-yl) methylene)o-phenylenediamine), [HgL3Cl2] (3) (L3: 2-(quinoline-2-yl)benzimidazole), and (HgCl4)(L4)2(C6H7)2 (4) (L4: 1,2,3-((quinoline-2-yl) methylene) benzimidazoleion) are obtained. Interestingly, L1–L4 are derived from the half-condensed asymmetrical Schiff base ligand [N1-((quinoline-2-yl) methylene) N2-(H-(quinoline-2-yl) methylene-hydroxy)o-phenylenediamine] (L) by means of a metal(II)-induced method. To the best of our knowledge, it is the first example of one Schiff base ligand transforming into three different benzimidazole derivatives from the single quinoline ring (L3), double quinoline rings (L1) to three quinoline rings (L4). Complexes 1, 3, and 4 are new mononuclear complexes, while complex 2 is a one-dimensional (1D) chlorine bridged Cd(II)-bis Schiff...

Deoxyribonucleic Acid and Bovine Serum Albumin Interaction with the Asymmetric Schiff Base Ligand and Its Molybdenum (VI) Complex: Multi Spectroscopic and Molecular Docking Studies

ABSTRACTThe interaction of an asymmetric Schiff base ligand derived from allylamine and 2,3-dihydroxybenzaldehyde and its molybdenum (VI) complex with deoxyribonucleic acid (DNA) and bovine serum albumin (BSA) were studied using spectroscopic and molecular docking methods. The spectroscopic results revealed that the DNA and BSA affinity for binding the Mo(VI) complex is greater than its ligand. Furthermore, the molecular docking calculations showed that H-bond, hydrophobic, π-π and π-cation interactions had the dominant roles in the stability of the compound-BSA complexes. The DNA interaction results suggested that the compounds interacted with DNA by the groove binding mechanism.

A new trinuclear N–N bridged Cu(II) complex with an asymmetric Schiff base ligand derived from hydrazine

In this paper we report the synthesis of an asymmetric Schiff base ligand derived from hydrazine and its Cu(II) trinuclear coordination complex, [Cu3(HYDRAV)2Cl2] (HYDRAV-Cu). The ligand HYDRAVH2 and the Cu(II) complex HYDRAV-Cu have been crystallographically characterized. HYDRAVH2 contains a fluorescent naphthyl group and emission properties of both HYDRAVH2 and HYDRAV-Cu are reported. The magnetic properties of HYDRAV-Cu have been investigated. Hydrazine is an excellent precursor for asymmetric Schiff base ligands that afford antiferromagnetic coupling as shown here for HYDRAV-Cu.

Structural variations in (CuL)2Ln complexes of a series of lanthanide ions with a salen-type unsymmetrical Schiff base(H2L): Dy and Tb derivatives as potential single-molecule magnets.

A new series of heterometallic trinuclear Cu2Ln complexes [lanthanide ions Ln = Gd (1), Tb (2), Dy (3), Ho (4) and Er (5)] has been synthesized using a Cu(ii)-metalloligand derived from a N2O2 donor unsymmetrical Schiff base, H2L (where H2L = N-α-methylsalicylidene-N'-salicylidene-1,3-propanediamine), and structurally characterized. Among these complexes, [(CuL)2Gd(NO3)3(CH3CN)2] (1), [(CuL)2Tb(NO3)3(CH3CN)2] (2) and [(CuL)2Dy(NO3)3(CH3CN)2] (3) are isomorphic and isostructural. In these complexes two metalloligands coordinate to the central Ln(iii) (Ln = Gd, Tb and Dy respectively) ion in a transoid fashion viaμ2-phenoxido oxygen atoms. The Ln(iii) ions are deca-coordinated with a distorted tetradecahedron geometry. The two terminal Cu(ii) ions of the complexes possess a hexa-coordinated distorted octahedral geometry. In contrast, in complexes [(CuL)2Ho(NO3)3(CH3CN)], (4) and [(CuL)2Er(NO3)3(CH3CN)]·0.5(CH3CN) (5), the two metalloligands coordinated to the Ln(iii) ions in a cisoid fashion. The Ho(iii) ion in 4 is nona-coordinated with a distorted tricapped trigonal prismatic geometry and the Er(iii) ion in 5 is octa-coordinated with a distorted square antiprismatic geometry. The two terminal Cu(ii) ions in complexes 4 and 5 are penta-coordinated with a distorted square-pyramidal geometry. The dc magnetic susceptibilities and field dependent magnetization measurement of complex 1 reveal the occurrence of ferromagnetic interactions between Cu(ii) and Gd(iii) ions as well as intermolecular antiferromagnetic interactions. Both complexes 2 and 3 show ferromagnetic interactions between Cu(ii) and Ln(iii) ions. The ac magnetic susceptibilities of all the complexes were also recorded and it was found that only complexes 2 and 3 exhibit slow relaxation of magnetization reorientation below 10 K at 2000 Oe applied dc field, this being characteristic of single molecule magnets.

Double-decker luminescent ytterbium and erbium SMMs with symmetric and asymmetric Schiff base ligands

Multifunctional molecules that respond both to magnetic fields and light are subject of study due to possible applications in fields as diverse as imaging or information processing and storage.

Spectral characterization, thermal and biological activity studies of Schiff base complexes derived from 4,4′-Methylenedianiline, ethanol amine and benzil

Some new metal(II) complexes of asymmetric Schiff base ligand were prepared by template technique. The shaped complexes are in binuclear structures and were explained through elemental analysis, molar conductivity, various spectroscopic methods (IR, U.V–Vis, XRD, ESR), thermal (TG) and magnetic moment measurements. The IR spectra were done demonstrating that the Schiff base ligand acts as neutral tetradentate moiety in all metal complexes. The electronic absorption spectra represented octahedral geometry for all complexes, while, the ESR spectra for Cu(II) complex showed axially symmetric g-tensor parameter with g׀׀ > g⊥ > 2.0023 indicating to 2B1g ground state with (dx2−y2)1 configuration. The nature of the solid residue created from TG estimations was affirmed utilizing IR and XRD spectra. The biological activity of the prepared complexes was studied against Land Snails. Additionally, the in vitro antitumor activity of the synthesized complexes with Hepatocellular Carcinoma cell (Hep-G2) was examined. It was observed that Zn(II) complex (5), exhibits a high inhibition of growth of the cell line with IC50 of 7.09 μg/mL.

Hydrogen evolution catalyzed by a cobalt complex containing an asymmetric Schiff-base ligand.

A cobalt(iii) complex containing an asymmetric Schiff-base ligand has been found to be active for proton reduction. Catalysis occurs at -1.2 V vs. Fc(+)/Fc (0.56 V vs. NHE), resulting in an overpotential of 350 mV. Additionally, the complex is active with a turnover frequency of 420 s(-1). An enhancement in activity is observed upon addition of water.