Phenol-imine Form Research Articles

Novel Schiff Base Derived from Amino Pyrene: Synthesis, Characterization, Crystal Structure Determination, and Anticancer Applications of the Ligand and Its Metal Complexes.

In this study, we report the cytotoxicity of a newly synthesized Schiff base HL ((E)-2-ethoxy-6((pyren-1-ylimino)methyl)phenol) and its derived metal complexes (Zn(II), Cu(II), Co(II), Cr(III), and Fe(III)) along with their structural characterizations by means of elemental analysis, magnetic moment, molar conductance, IR, UV-Vis, ESR, and mass spectrometry. The single X-ray diffraction of the HL shows that it exists in the phenol-imine form in its solid state. The NMR and IR data indicate that the bidentate binding of the Schiff base ligand with the metal center occurs during complexation through the azomethine nitrogen atom and the hydroxyl group oxygen atom of the 3-ethoxy salicylaldehyde. The electronic spectra and magnetic measurements indicate that the Co(II) complex has a tetrahedral geometry and that the Cr(III) and Fe(III) complexes have a distorted octahedral geometry. The ESR and electronic spectra suggest that the Cu(II) complex has a distorted tetrahedral geometry. The cytotoxic effects of the HL and all of the metal complexes were studied using human breast cancer (MCF-7) cells. The Cu(II) and Zn(II) complexes exhibited the highest activity against the tested cell line, with IC50 values of 5.66 and 12.74 μg/mL, respectively, and their activity was higher than that of the fluorouracil cancer drug against the MCF-7 cells (18.05 μg/mL).

Synthesis, characterizations, crystal structure, inhibition effects and theoretical study of novel Schiff base on the corrosion of carbon steel in 1 M HCl

A novel Schiff base 2, 4- Bis (2‑hydroxy naphthaldehyde) diiminotholuene (L), was prepared and characterized by the analysis techniques: FT-IR, UV-Vis,1H and 13C NMR. X-Ray Diffraction (XRD), showing that the L was crystallized in phenol-imine form. The corrosion inhibition of carbon steel in 1 M hydrochloric acid solutions of ligand L was studied using chemical (weight loss) and electrochemical (potentiodynamic and electrochemical impedance spectroscopy (EIS)) measurements, these measurements show that the inhibition efficiency obtained by this compound increased by increasing the concentration of L. The polarization studies show that this L acts as mixed type inhibitor. This inhibitor functions through adsorption following Langmuir isotherm. The UV–Vis absorption spectroscopy was used to investigate the forming of complex between inhibitor L and Fe. The scanning electron microscopy (SEM) studies the metal surfaces confirming the formation of the adsorbed layer of inhibitor, which slows down the corrosion process in the corrosive solution. The Density functional theory (DFT) and Simulations of the Molecular Dynamics (SMD) were used to determine the relationship between quantum chemical parameters and their inhibition efficiencies, and the correlation between theoretical and experimental results was discussed.

Structural, spectroscopic, and theoretical investigations of (E)–methyl–3–(3,5–di–tert–butyl–2–hydroxybenzylideneamino)–4–methylbenzoate

AbstractA novel Schiff base ((E)–methyl–3–(3,5–di–tert–butyl–2–hydroxybenzylideneamino)–4–methylbenzoate (MDM), C24H31NO3) at phenol–imine form with the OH⋯N intra–molecular hydrogen bond was synthesized. Its structural and spectroscopic characterizations were performed by using experimental (single crystal X–ray diffraction, Fourier transform infrared [FT–IR], NMR chemical shift, and UV–vis spectroscopies) and theoretical (DFT/B3LYP/6‐311+G[2d,p] computational level) methods. X–ray diffraction investigation shows that the MDM crystallized in phenol–imine form with OH⋯N intra–molecular hydrogen bond. Detailed investigations of the presence and nature of the inter–molecular contacts within solid state crystal packing form of MDM were examined by Hirshfeld surface analysis. The intra–molecular electronic transitions in the compound were investigated and determined by UV–vis electronic absorption wavelengths and FMOs analyses. The structural, vibrational, and NMR chemical shift analyses of azomethine (CHN) and phenolic hydroxyl (OH) groups in MDM supported its phenol–imine structural formation.

Molecular Structure and Supramolecular Architecture of (E)-2-Bromo-6-[(2,4,6-Tribromophenylimino)methyl]-4-Chlorophenol

In this study, the molecular structure and supramolecular architecture of a new compound have been studied in depth using single crystal X-ray diffraction (XRD) technique. Crystallographic results show that the compound exists in phenol-imine form that is twisted with the dihedral angle of 51.77° between the aromatic rings of the molecule. Various types of non-covalent interactions (C⋅⋅⋅O, C–H⋅⋅⋅O, Br⋅⋅⋅Br, C–H⋅⋅⋅Cl, π⋅⋅⋅π, O⋅⋅⋅Br, and Br⋅⋅⋅π) take part effectively in the construction of 3D network in the compound. The C⋅⋅⋅O and C–H⋅⋅⋅O interactions have the role of forming the 1D structure along the direction [010] with the support of π⋅⋅⋅π and π⋅⋅⋅Br interactions. The 2D structure of the compound is reached by the inter-connection of 1D chains in the (101) plane through the C–H⋅⋅⋅Cl interactions. The 3D supramolecular structure of the compound is completed by the non-covalent Br⋅⋅⋅Br and Br⋅⋅⋅O interactions responsible for the connection of 2D sheets.

A Close Look at the Molecular Structure, Prototropic Behavior and Supramolecular Architecture of (E)-4-Bromo-2-[(phenylimino)methyl]-5-methoxyphenol by Spectroscopic, Crystallographic, and Computational Methods

In this study, the molecular structure, prototropic behavior and supramolecular architecture of a new Schiff base have been studied in depth using spectroscopic (UV-vis), crystallographic (XRD) and computational (HOMA and DFT) methods. Regarding the molecular structure and prototropy, the XRD, DFT and HOMA results show that the compound exists in phenol-imine form. The HOMA indices for the structural evaluation show that the compound has a pure aromatic structure in solid state. UV-vis spectra of the compound dissolved in four different solvents were investigated in order to determine which tautomeric form of the compound is dominant in the solution. It is found that the compound prefers only phenol-imine form in apolar solvents while both phenol-imine and keto amine forms appear in polar solvents. This is because polar solvent decrease the activation energy, and thus, it becomes possible to observe both forms in the solution. For an energetic approach, the tautomeric conversion between two forms of the compound was investigated by a PES scan process and close energy values were obtained for two tautomers. Investigation of non-covalent interactions underlines the active roles of C–H···O H-bonds and C···Br interactions in constructing the supramolecular network of the compound.

Investigation of the Molecular Structure of (E)-2-Bromo-6-[(4-bromo-2-methylphenylimino)methyl]-4-chlorophenol

In this study, the molecular structure of (E)-2-bromo-6-[(4-bromo-2-methylphenylimino)methyl]-4-chlorophenol has been investigated using crystallographic (XRD), spectroscopic (UV-vis, NMR), and computational (DFT, HOMA) methods. The refinement parameters in XRD study supports the preference of phenol-imine form by the compound in solid state. The analysis of HOMA indices indicates that C1/C6 ring (tautomeric ring) deviates slightly from the aromaticity while the C8/C13 ring preserves its aromaticity in solid state. For the solvent-media dependence of the tautomerism, UV-vis and NMR spectra of the compound were investigated. It is found that the compound prefers only phenol-imine form in solvent media as in the case of solid state.

The effect of the change in substituents’ positions on the formation of supramolecular networks and the solvent type/substituent dependence of prototropic behavior in three new o-hydroxy Schiff bases

The effect of substituent position on the formation of supramolecular topology, intra-molecular geometry and tautomeric behavior, and the dependence of prototropy on the solvent type have been investigated applying the XRD, UV–Vis, NMR, DFT, PES and HOMA methods on three new o-hydroxy Schiff bases. Based on key geometric parameters, total molecular energy and aromaticity calculations, the XRD, DFT and HOMA studies propose the phenol-imine structure as the most consistent tautomeric form for the compounds. PES and HOMA studies give results that relate the preference of the phenol-imine form to its high π-electron delocalization. For the solvent type dependence of tautomerism, the investigation of UV–Vis and NMR spectra and the PES studies for different solvent media reveals that the co-existence of two tautomeric structures becomes easier by increasing solvent polarity. As to the effect of substituent position on the supramolecular topology, XRD studies show that different substituent positions cause different types of non-covalent inter-molecular interactions in the crystal packings of the compounds. This is resulted in a significant change in the formation and topology of the supramolecular architecture, emphasizing the importance of a substituent position as a regulator in crystal engineering.

Investigation of molecular structure and solvent/temperature effect on tautomerism in (E)-4,6-dibromo-3-methoxy-2-[(p-tolylimino)methyl]phenol, a new thermochromic Schiff base, by using XRD, FT-IR, UV-vis, NMR and DFT methods.

The molecular structure and the solvent/temperature effect on the tautomerism in a new Schiff base, (E)-4,6-dibromo-3-methoxy-2-[(p-tolylimino)methyl]phenol, were investigated using spectroscopic (NMR, UV-vis, FT-IR), crystallographic (XRD), computational (DFT and TD-DFT) methods and harmonic oscillator model of aromaticity (HOMA). The XRD, DFT and FT-IR results show that the compound exists in the phenol-imine form in the solid state. HOMA indices support the aromatic structure of the compound. DFT calculations were performed to understand proton transfer process and relatively close values were obtained for the energies of tautomers. UV-vis studies prove the solvent dependence of the tautomerism in the compound by revealing the existence of both phenol-imine and keto-amine forms in polar solvents and only the phenol-imine form in apolar solvent. The TD-DFT results for the electronic transitions lead to the same conclusion as the absorption spectra. 1H NMR and 13C NMR studies at room and low (-60 °C) temperatures indicate that the tautomeric equilibrium occurs rapidly in the compound. Therefore, it is difficult to observe two tautomers. However, the presence of tautomeric structures is clearly seen in acetone‑d6, alternatively underlying the solvent and temperature dependence of tautomerism in the title compound.

Dependence of tautomerism on substituent type in o-hydroxy Schiff bases

Quantum computational methods were used to elucidate the structures of the o-hydroxy Schiff bases with different substituents. It is possible for a Schiff base to have different tautomeric structures depending on intramolecular proton transfer from the phenolic oxygen atom to the nitrogen atom. Proton transfer results in two tautomeric structures known as the phenol-imine and keto-amine forms. To explain the substituent effect on the proton transfer process in five o-hydroxy-Schiff bases, possible geometric structures in gas phase were optimized using density functional theory (DFT) at the B3LYP/6-311G(d,p) level. To describe tautomerism including intramolecular proton transfer, potential energy surface (PES) scans were performed starting from the optimized geometry of the phenol-imine form. HOMA indices were calculated in order to estimate p-electron delocalization. In addition, the substituent effect on the tautomerization rate was examined using Hammett substituent constants and calculating the activation energies.

Scrutinizing the two new o-hydroxy Schiff bases from the point of tautomeric behavior and non-covalent interactions (H-bond, Br⋯Br, π⋯π and C[sbnd]H⋯π) in their supramolecular architectures

Two new Schiff bases, (E)-4,6-dibromo-2-[(5-chloro-2-methylphenylimino)methyl]-3-methoxyphenol (1) and (E)-4-bromo-2-[(4-bromophenylimino)methyl]-5-methoxyphenol (2), have been investigated by focusing on the prototropy-related-changes in the geometric parameters, the molecular planarity and the way of crystal packing. X-ray diffraction (XRD), density functional theory (DFT), Hartree-Fock theory (HF) and Møller–Plesset perturbation theory (MP2) and harmonic oscillator model of aromaticity (HOMA) studies show the preference of phenol-imine form by the compounds. The crystal packings of the compounds have been studied in detail by noting the importance of the substituent type and position in regulating the non-covalent interactions, thus, the formation of supramolecular networks. The results underline the fact that the halogen atoms of high polarizability and their presence in the appropriate position dominate the construction of supramolecular structure in Schiff bases.

DFT calculation, molecular docking, biological activity and crystal structure of (E)-2-((4-tert-butylbenzylimino)methyl)-4-methoxyphenol

(E)-2-((4-tert-butylbenzylimino)methyl)-4-methoxyphenol is synthesized by the reaction of 4-tert-buthylbenzylamine with 5-methoxysalicylaldehyde. The structure of the Schiff base is investigated by FT-IR, UV-visible, and 1H NMR, 13C NMR, and X-ray data. Moreover, the molecular structure, molecular electrostatic potential surfaces (MEP), frontier molecular orbitals, and nonlinear optical properties (NLO) are also investigated by density functional theory (DFT) calculations. The experimental and calculated results show that the phenol-imine form of the compound is more favoured than the keto-amine form. The most energy favourable docked structures are obtained from the rigid molecular docking of the compound with DNA. The compound binds at the active site of DNA proteins by weak non-covalent interactions. UV—Vis spectroscopy studies of the interactions between the compounds and calf thymus DNA (CT-DNA) show that the compounds interact with CT-DNA via the electrostatic and intercalative binding. The compound inhibits the frameshift and base pair mutations in the absence of S9 mix with a high inhibition rate. Also, the molecular docking is made to identify the interaction between the ligand and DNA.

DFT Calculation, Molecular Docking, Biological Activity, and Crystal Structure of (E)-2-((4-tert-Butylbenzylimino)Methyl)-4-Methoxy-Phenol

(E)-2-((4-tert-butylbenzylimino)methyl)-4-methoxyphenol is synthesized by the reaction of 4-tert-buthylbenzylamine with 5-methoxysalicylaldehyde. The structure of the Schiff base is investigated by FT-IR, UV-visible, and 1H NMR, 13C NMR, and X-ray data. Moreover, the molecular structure, molecular electrostatic potential surfaces (MEP), frontier molecular orbitals, and nonlinear optical properties (NLO) are also investigated by density functional theory (DFT) calculations. The experimental and calculated results show that the phenol-imine form of the compound is more favoured than the keto-amine form. The most energy favourable docked structures are obtained from the rigid molecular docking of the compound with DNA. The compound binds at the active site of DNA proteins by weak non-covalent interactions. UV-Vis spectroscopy studies of the interactions between the compounds and calf thymus DNA (CT-DNA) show that the compounds interact with CT-DNA via the electrostatic and intercalative binding. The compound inhibits the frameshift and base pair mutations in the absence of S9 mix with a high inhibition rate. Also, the molecular docking is made to identify the interaction between the ligand and DNA.

Structural, spectroscopic, radical scavenging activity, molecular docking and DFT studies of a synthesized Schiff base compound

A synthesized Schiff base, (E)-4-nitro-2-[(o-tolylimino)methyl]phenol (L), was prepared and characterized by FT-IR, single crystal X-Ray diffraction, NMR chemical shift and UV–Vis. spectroscopic techniques. Theoretical calculations of L were performed by using density functional theory (DFT) calculations with the B3LYP/6-311++G (d,p) level in the ground state. The obtained calculation geometry was found to be in well consistent with the experimental geometry. X-ray diffraction investigation shows that the L crystallized in phenol-imine form with OH⋯N intramolecular hydrogen bond. X-ray diffraction and Hirshfeld surface analysis were done to examine the contribution of intermolecular contacts in crystal packing of L. Frontier molecular orbitals, electronic absorption wavelengths and non-linear optical features of the L were investigated with molecular modeling methods. It worthy of note that, the radical scavenging activities of L were examined by using ABTS•+, DMPD•+ and DPPH• assays. According to acquired results, L shows effective DPPH• (SC50 10.99 ± 0.12 μg/mL), DMPD•+ (SC50 11.51 ± 0.16 μg/mL), and ABTS•+ (SC50 8.55 ± 0.17 μg/mL) scavenging activities compared with standard antioxidant compounds such as Trolox (TRO), rutin (RUT) and butylated hydroxy anisole (BHA). Molecular docking study was carried out to predict the potency of inhibition of L against mitochondrial Ubiquinol-Cytochrome C Reductase binding (UQCRB) protein. The molecular docking results showed that the L exhibited a good activity with −7.80 kcaL/mol value of binding affinity energy.

Crystal structure and Hirshfeld surface analysis of N,N'-bis-(3-tert-butyl-2-hy-droxy-5-methyl-benzyl-idene)ethane-1,2-di-amine.

The title compound, C26H36N2O2, crystallizes in the phenol-imine form. In the mol-ecule, there are intra-molecular O-H⋯N hydrogen bonds forming S(6) ring motifs, and the two aromatic rings are inclined to each other by 37.9 (7)°. In the crystal, mol-ecules are linked by pairs of weak C-H⋯O hydrogen bonds, forming inversion dimers. Hirshfeld surface analysis and two-dimensional fingerprint plots indicate that the most important contributions to the crystal packing are from H⋯H (77.5%), H⋯C/C⋯H (16%), H⋯O/O⋯H (3.1%) and H⋯N/N⋯H (1.7%) inter-actions.

Experimental and theoretical studies on tautomeric structures of a newly synthesized 2,2′(hydrazine-1,2-diylidenebis(propan-1-yl-1-ylidene))diphenol

In the present study, a single crystal of a Schiff base, 2,2′(hydrazine-1,2-diylidenebis(propan-1-yl-1-ylidene))diphenol, was synthesized. The structure of the synthesized crystal was confirmed by 1H and 13C NMR spectroscopic and X-ray diffraction analysis techniques. Experimental and theoretical studies were carried out on two tautomeric structures. It has been observed that the title compound studied can be in two different tautomeric forms, phenol-imine and keto-amine. Theoretical calculations have been performed to support experimental results. Accordingly, the geometric parameters of the compound were optimized by the density functional theory (DFT) method using the Gaussian 09 and Quantum Espresso (QE) packet program was used for periodic boundary conditions (PBC) studies. Furthermore, the compound was also tested for in vitro antifungal activity against Sclerotinia sclerotiorum, Alternaria solani, Fusarium oxysporumf. sp.lycopersici and Monilinia fructigena plant pathogens. Promising inhibition profiles were observed especially towards A. solani. Finally, molecular docking studies and post-docking procedure based on Molecular Mechanics‐Generalized Born Surface Area (MM‐GBSA) were also carried out to get insight into the compound’s binding interactions with the potential. Although theoretical calculations showed that the phenol-imine form was more stable, keto-amine form was predicted to have better binding affinity which was concluded to result from loss of rotational entropy in phenol-imine upon binding. The results obtained here from both experimental and computational methods might serve as a potential lead in the development of novel anti-fungal agents.

Effect of molar ratios of reagents and solvent on the complexation process of nickel(II) ions by the N2O3-donor Schiff base

The new N2O3-donor pentadentate Schiff base (C29H26N2O3) was synthesized and characterized by different techniques. The single crystal X-ray analysis confirmed that in the solid state one half of the molecule contains the phenol-imine form while in the other one the proton of phenol group is moved towards the azomethine group creating the zwitterionic form. In solution, the studied Schiff base exists mainly in the phenol-imine tautomer but in more polar solvents (MeOH or EtOH) stabilization of the more polar keto-amine form is also observed. The effect of polar protic solvents (methanol and ethanol) on synthesis of NiII complexes with the Schiff base was studied. The synthesis of compounds was performed at the stoichiometric metal:ligand 1:1 and 2:1 ratio. This allowed to obtain four different nickel(II) complexes. The products were characterized using FITR spectroscopy, X-ray diffraction analysis and thermogravimetric methods. When equimolar amounts of reagents were used in both solvents, the mononuclear complexes were formed, [Ni(C29H24N2O3)]·(CH3OH)(H2O)0.25 (1a) and [Ni(C29H24N2O3)]·H2O (2a). In both compounds NiII ions have distorted square planar geometry formed by N2O2-core of Schiff base ligand. Increasing the concentration of metal ions in the synthesis, it was possible to obtain tetranuclear ([Ni4(C29H24N2O3)2(CH3COO)2(CH3OH)3.65(H2O)0.35]·(CH3OH)7 (1b)) and hexanuclear ([Ni6(C29H24N2O3)2(C16H17N2O2)2(CH3COO)2]·(C2H5OH) (2b)) complexes. In crystal 1b the NiII ions have octahedral geometry whereas in complex 2b the metal centres are four- and six-coordinate.

Molecular structure and density functional modelling studies of 2-[(E)-2-(4-hydroxyphenyl)ethyliminomethyl]phenol

In the present work, the tautomerism in 2-[(E)-2-(4-hydroxyphenyl)ethyliminomethyl]phenol was investigated by experimental (MS, FT-IR, NMR and X-ray diffraction) and computational method [density functional theory (DFT)]. The optimized geometrical structures, atomic charges, molecular electrostatic potential (MEP), natural bond orbital (NBO) and nonlinear optical (NLO) effects of the compound have been investigated by using DFT calculations. The potential energy surface (PES) scans about three important torsion angles are performed by using B3LYP/6-311++G (d,p) level of theoretical approximation for the compound. The experimental (FT-IR) and calculated vibrational frequencies (using DFT) of the title compound have been compared. The 1H and 13C NMR chemical shift assignments have been performed using DFT. To investigate the tautomeric stability, some properties such as total energy, HOMO and LUMO energies of the compound were obtained at B3LYP and B1B95/6-311++G(d,p) level in the gas phase. The calculated results showed that the phenol-imine form of the compound was more favorite than keto-amine form. Moreover, a good correlation between experimental and theoretical data for phenol-imine form of the compound was found.

Analysis of tautomeric equilibrium in (E)-4,6-dibromo-2-[(4-fluorophenylimino)methyl]-3-methoxyphenol compound

In this study, the tautomeric equilibrium between the phenol-imine and keto-amine structural forms of (E)-4,6-dibromo-2-[(4-fluorophenylimino)methyl]-3-methoxyphenol compound has been investigated with experimental (XRD, UV–vis and NMR) and theoretical (DFT and TD-DFT) methods. The results clearly show that structural preference of the compound is definitely depended on its state. Namely, the compound exists in phenol-imine form in the solid state while one or both of these forms can be seen in solvent media. For example, the compound prefers phenol-imine form in benzene while both forms exist in EtOH and DMSO solvents. Coexistence of two forms has been quantified with NMR studies, giving a ratio of 11:9 for phenol and keto structures of the compound in acetone-d6 solvent.

Crystal structure, DFT and HF calculations and radical scavenging activities of (E)-4,6-dibromo-3-methoxy-2-[(3-methoxyphenylimino)methyl]phenol

In this study, (E)-4,6-dibromo-3-methoxy-2-[(3-methoxyphenylimino)methyl]phenol has been synthesized and characterized by using X-ray technique and FT-IR experimentally and using B3LYP/6-31G(d,p) and HF/6-31G(d,p) methods theoretically. The intermolecular and intramolecular interactions of the title compound have been determined according to X-ray results. The molecular geometry, vibrational frequencies of the title compound in the ground state have been calculated using the density functional B3LYP and HF method with the 6-31G(d,p) basis set and calculated bond parameters and vibrational frequencies values show good agreement with experimental values. Theoretical and experimental results show that tautomeric form of the structure is phenol-imine form. Besides HOMO–LUMO energy gap, molecular electrostatic potential map were performed at B3LYP/6-31G(d,p) level. It is worthy note of that, the free radical scavenging activities of the title compound were assessed using DPPH, DMPD+, and ABTS+ assays. The obtained results show that the title compound has effective DPPH (SC50 2.61±0.09μg/mL), DMPD+ (SC50 2.82±0.14μg/mL), and ABTS+ (SC50 4.91±0.18μg/mL) radical scavenging activities when compared with standard antioxidants (BHA, rutin, and trolox).

Structural evidence of an intramolecular proton transfer leading to keto-amine tautomer in the crystals of Schiff bases derived from tyrosine and histidine esters

Three Schiff bases derived from of 2,4-dihydroxybenzaldehyde or 2,4-dihydroxyacetophenone and esters of tyrosine and histidine have been synthesized and the crystal and molecular structures determined by single crystal X-ray diffraction. The molecular structures of the three compounds are dominated by short intramolecular hydrogen bonds with distances N⋯O ranging from 2.536(2) to 2.588(2)Å and the hydrogen atom is bonded to the nitrogen. In the solid state, the structures are characterized by the keto-amine tautomer, whereas in the solution the phenol-imine form was detected by 1H NMR spectroscopy. Intermolecular interactions influencing crystal packing are discussed.