Keto-amine Form Research Articles

Synthesis, characterization and biological activities of Ni(II), Cu(II) and UO2(VI) complexes of N'-((2Z,3E)-3-(hydroxyimino)butan-2-ylidene)-2-phenylacetohydrazide

In this study, the mononuclear Ni(II), Cu(II) and UO2(VI) complexes of N'-((2Z,3E)-3-(hydroxyimino)butan-2-ylidene)-2-phenylacetohydrazide ligand (LH2) have been synthesized. All compounds have been characterized by using elemental, spectral, X-ray analyses and magnetic moment measurements. The IR spectra of Ni(II) and UO2(VI) complexes show that LH2 has coordinated to the metal ions in tridentate manner with ON2 donor sites of deprotonated phenolic OH, the imine N atoms of hydrazone (CON) and oxime (CNOH) groups. In the Cu(II) complex, the ligand is ketoamine form and bond to metal ion via carbonyl oxygen, imine nitrogen atoms of oxime and hydrazone groups. The spectral and magnetic studies show that the Ni(II), and Cu(II) complexes exhibit octahedral geometry. UO2(VI) complex has a monomeric structure constructed of a hexagonal bipyramidal uranyl centre with two monodentate ligand in trans-position. Newly complexes have been tested for antimicrobial and antioxidant activity. All of the complexes have active against L. monocytogenes, M. luteus, S. epidermis, B. cereus, S. typhi H, E. coli, Klebsiella pneumonia, Br. abortus and C. albicans. Results of antioxidant activities indicate that the ligand and complexes have multiple antioxidant activities, such as total antioxidant (phosphomolybdenum) assay, reducing power, free radical (DPPH), superoxide anion and ABTS+ scavenging activity tests. The Ni(II) complex exhibited more effective total antioxidant activity and reducing power than BHA and trolox but weak the ABTS·+, free radical (DPPH•), and superoxide anion (O2·-) scavenging activity. The ABTS·+ and DPPH• scavenging activities of Cu(II) were found excellent in comparison with standards. Ni(II), and UO2(VI) acted a powerful inhibitor of superoxide anion. The Cu(II) complex crystallized in the monoclinic space group C2/c, while UO2(VI) complex crystallized in the monoclinic space group P21/c.

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.

Synthesis, crystal structure, thermal, spectroscopic and theoretical studies of N3O2-donor Schiff base and its complex with CuII ions

The new o-hydroxy N3O2-donor Schiff base (H2L) was synthesized and characterized using FITR and UV–Vis spectroscopies, X-ray diffraction analysis, thermogravimetric methods and computational studies. In the solid state intramolecular proton transfer from the phenolic oxygen to the imino nitrogen atom is observed. The resulting compound occurs as a mixture of phenol-imine and zwitterionic forms. The temperature does not affect significantly the shift of equilibrium from the phenol-imine tautomer to the zwitterion form. In solution the H2L exists mainly in the phenol-imine tautomeric form but in more polar solvents stabilization of the more polar keto-amine form is also observed. The reaction of H2L with CuII ion at the stoichiometric metal:ligand 2:1 ratio results in formation of cyclic, tetranuclear complex where the coordination geometry around each metal centres is distorted square pyramid.

Aromatic hydrazones derived from nicotinic acid hydrazide as fluorimetric pH sensing molecules: Structural analysis by computational and spectroscopic methods in solid phase and in solution

Structural analyses of aroylhydrazones were performed by computational and spectroscopic methods (solid state NMR, 1 and 2D NMR spectroscopy, FT-IR (ATR) spectroscopy, Raman spectroscopy, UV–Vis spectrometry and spectrofluorimetry) in solid state and in solution. The studied compounds were N′-(2,3-dihydroxyphenylmethylidene)-3-pyridinecarbohydrazide (1), N′-(2,5-dihydroxyphenylmethylidene)-3-pyridinecarbohydrazide (2), N′-(3-chloro-2-hydroxy-phenylmethylidene)-3-pyridinecarbohydrazide (3), and N′-(2-hydroxy-4-methoxyphenyl-methylidene)-3-pyridinecarbohydrazide (4). Both in solid state and in solution, all compounds were in ketoamine form (form I, CONHNC), stabilized by intramolecular H-bond between hydroxyl proton and nitrogen atom of the CN group. In solid state, the CO group of 1–4 were involved in additional intermolecular H-bond between closely packed molecules. Among hydrazones studied, the chloro- and methoxy-derivatives have shown pH dependent and reversible fluorescence emission connected to deprotonation/protonation of salicylidene part of the molecules. All findings acquired by experimental methods (NMR, IR, Raman, and UV–Vis spectra) were in excellent agreement with those obtained by computational methods.

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.

SbCl3 as effective catalyst for the preparation of 2,3-Dihydroquinazolin-4(1H)-ones, spectroscopic investigation and DFT study

A simple and efficient method has been developed for the synthesis of Quinazolines using SbCl3 as a heterogeneous catalysis at room temperature. This method provides a good pathway for the synthesis of 2,3-Dihydroquinazolin-4(1H)-ones derivatives in the terms of excellent yields and short reaction times. Also we studied theoretically and experimentally on 2-phenyl-2,3-dihydroquinazolin-4(1H)-one (PDQ). Using density functional theory (DFT), the tautomerism of PDQ was also studied. Thermal stability of PDQ was studied by thermo gravimetric analysis (TGA). The spectroscopic results and theoretical calculations indicate that the strength of intramolecular hydrogen bonding (IHB) of PDQ is stronger than that in 2-methyl-4-quinolinol (2MQ). The absorption spectra of the PDQ in solvents with different polarity were obtained and the results show that PDQ exists in both keto-amine and enol-imine forms in THF, while it has keto-amine form in other solvents. Theoretical results show that the conductance of the two tautomers (keto-amine and enol-imine) varies greatly, which offers that the potential usage of this molecule is as a molecular device.

Synthesis, tautomeric stability, spectroscopy and computational study of a potential molecular switch of (Z)-4-(phenylamino)pent-3-en-2-one

ABSTRACTThe (Z)-4-(phenylamino) pent-3-en-2-one (PAPO) was synthesised applying carbon-based solid acid and described by experimental techniques. Calculated results reveal that its keto-amine form is more stable than its enol-imine form. A relaxed potential energy surface scan has been accomplished based on the optimised geometry of NH tautomeric form to depict the potential energy barrier related to intramolecular proton transfer. The spectroscopic results and theoretical calculations demonstrate that the intramolecular hydrogen bonding strength of PAPO is stronger than that in 4-amino-3-penten-2-one)APO(. In addition, molecular electrostatic potential, total and partial density of stats (TDOS, PDOS) and non-linear optical properties of the compound were studied using same theoretical calculations. Our calculations show that the title molecule has the potential to be used as molecular switch.

Tautomerism in N-(2-hydroxy-1-naphthylidene)amino acids and the search for an answer to the difficult question about where the proton belongs

N-(2-hydroxy-1-naphthylidene)-l-valine 1, N-(2-hydroxy-1-naphthylidene)-l-phenylalanine 2, and N-(2-hydroxy-1-naphthylidene)-l-threonine 3 were prepared and characterized with spectroscopic methods, elemental analyses, and values of optical rotation. Compound 1 undergoes a solid state order-disorder phase transition at 231 K. The X-ray structures of the high and low temperature phase of 1 have been determined. Single crystal X-ray structures of 2 and 3 have been determined as well. The tautomerism of N-(2-hydroxy-1-naphthylidene)amino acid derivatives is discussed controversial in the literature. A bond lengths statistical analysis shows that all three compounds exist uniformly in the keto-amine form in the solid state. Quantum chemical calculations, NMR, and UV–Vis spectroscopy were used to obtain further insight into the existence of phenol-imine and keto-amine structures in this class of compounds.

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.

Synthesis, spectroscopy, X-ray crystallography, DFT calculations, DNA binding and molecular docking of a propargyl arms containing Schiff base

A propargyl arms containing Schiff base (L) was synthesized by the condensation of 1-[2-hydroxy-4-(prop-2-yn-1-yloxy)phenyl]ethanone with trans-1,2-diaminocyclohexane. The structure of L was characterized by IR, 1H NMR, 13C NMR and UV–Vis spectroscopy and by single crystal X-ray diffraction analysis. The UV–Visible spectral behavior of L in different solvents exhibits positive solvatochromism. Density functional calculation of the L in gas phase was performed by using DFT (B3LYP) method with 6-31G basis set. The computed vibrational frequencies and NMR signals of L were compared with the experimental data. Tautomeric stability study inferred that the enolimine is more stable than the ketoamine form. The charge delocalization has been analyzed using natural bond orbital (NBO) analysis. Electronic absorption and emission spectral studies were used to study the binding of L with CT-DNA. The molecular docking was done to identify the interaction of L with A-DNA and B-DNA.

Picking Out Logic Operations in a Naphthalene β-Diketone Derivative by Using Molecular Encapsulation, Controlled Protonation, and DNA Binding.

On–off switching and molecular logic in fluorescent molecules are associated with what chemical inputs can do to the structure and dynamics of these molecules. Herein, we report the structure of a naphthalene derivative, the fashion of its binding to β-cyclodextrin and DNA, and the operation of logic possible using protons, cyclodextrin, and DNA as chemical inputs. The compound crystallizes out in a keto-amine form, with intramolecular N−H⋅⋅⋅O bonding. It shows stepwise formation of 1:1 and 1:2 inclusion complexes with β-cyclodextrin. The aminopentenone substituents are encapsulated by β-cyclodextrin, leaving out the naphthalene rings free. The binding constant of the β-cyclodextrin complex is 512 m−1. The pKa value of the guest molecule is not greatly affected by the complexation. Dual input logic operations, based on various chemical inputs, lead to the possibility of several molecular logic gates, namely NOR, XOR, NAND, and Buffer. Such chemical inputs on the naphthalene derivative are examples of how variable signal outputs based on binding can be derived, which, in turn, are dependent on the size and shape of the molecule.

Theoretical analysis (NBO, NPA, Mulliken Population Method) and molecular orbital studies (hardness, chemical potential, electrophilicity and Fukui function analysis) of (E)-2-((4-hydroxy-2-methylphenylimino)methyl)-3-methoxyphenol

The molecular structure and spectroscopic properties of (E)-2-((4-hydroxy-2-methylphenylimino)methyl)-3-methoxyphenol, were characterized by X-ray diffraction, FT-IR and UV–Vis spectroscopy. All of theoretical calculations and optimized geometric parameters have been calculated by using density functional theory (DFT) with hybrid method B3LYP by 6-31G(d,p) basis set. The title compound of C15H15N1O3 have been analyzed according to electronic and energetics behaviors for enol-imine and keto-amine tautomers. Both these tautomers engender six-membered ring due to intramolecular hydrogen bonded interactions. Two types of intramolecular hydrogen bonds (a) strong O–H⋯N interactions in enol-imine form and (b) N–H⋯O interactions in keto-amine form are compared particularly. The theoretical vibrational frequencies have been found in good agreement with the corresponding experimental data. A study on the electronic and optical properties, absorption wavelengths, excitation energy, dipole moment, molecular electrostatic potential (MEP) and frontier molecular orbital energies are performed using DFT method. Additionally, geometry optimizations in solvent media were performed with the same level of theory by the polarizable continuum model (PCM). The effect of solvents on the tautomeric stability has been investigated. Mulliken Population Method and natural population analysis (NPA) have been studied. NBO analysis is carried out to picture the charge transfer between the localized bonds and lone pairs. The local reactivity of the molecule has been studied using the Fukui function. NLO properties related to polarizability and hyperpolarizability are also discussed.

The spectroscopic (FT-IR, UV–vis), Fukui function, NLO, NBO, NPA and tautomerism effect analysis of (E)-2-[(2-hydroxy-6-methoxybenzylidene)amino]benzonitrile

A new o-hydroxy Schiff base, (E)-2-[(2-hydroxy-6-methoxybenzylidene)amino]benzonitrile was isolated and investigated by experimental and theoretical methodologies. The solid state molecular structure was determined by X-ray diffraction method. The vibrational spectral analysis was carried out by using FT-IR spectroscopy in the range of 4000–400cm−1. Theoretical calculations were performed by density functional theory (DFT) method using 6-31G(d,p) basis set. The results of the calculations were applied to simulated spectra of the title compound, which show excellent agreement with observed spectra. The UV–vis spectrum of the compound was recorded in the region 200–800nm in several solvents and electronic properties such as excitation energies, and wavelengths were calculated by TD-DFT/B3LYP method. The most prominent transitions were corresponds to π→π∗.Hybrid density functional theory (DFT) was used to investigate the enol-imine and keto-amine tautomers of titled compound. The titled compound showed the preference of enol form, as supported by X-ray and spectroscopic analysis results. The geometric and molecular properties were compaired for both enol-imine and keto-amine forms. Additionally, geometry optimizations in solvent media were performed with the same level of theory by the integral equation formalism polarizable continuum (IEF-PCM). Stability of the molecule arises from hyperconjugative interactions, charge delocalization and intramolecular hydrogen bond has been analyzed using natural bond orbital (NBO) analysis. Mulliken population method and natural population analysis (NPA) have been studied. Also, condensed Fukui function and relative nucleophilicity indices calculated from charges obtained with orbital charge calculation methods (NPA). Molecular electrostatic potential (MEP) and non linear optical (NLO) properties are also examined.

Theoretical and experimental investigation of (E)-2-([3,4-dimethylphenyl)imino]methyl)-3-methoxyphenol: Enol–keto tautomerism, spectroscopic properties, NLO, NBO and NPA analysis

The molecular structure and spectroscopic properties of (E)-2-([3,4-dimethylphenyl)imino]methyl)-3-methoxyphenol were investigated by X-ray diffraction, FT-IR and UV–vis spectroscopy. The vibrational frequencies calculatedusing DFT/B3LYP/6-31G(d,p) method. Results showed better agreement with the experimental values. The electronic properties was studied and the most prominent transition corresponds to π→π* and n→π*. Two types of intramolecular hydrogen bonds are strong OH⋯N interactions in enol-imine form and NH⋯O interactions in keto-amine form are compared by using density functional theory (DFT) method with B3LYP applying 6-31G(d,p) basis set. Both enol–keto tautomers engender six-membered ring due to intramolecular hydrogen bonded interactions.Geometry optimizations in solvent media were performed with the same level of theory by the polarizable continuum model (PCM). The effect of solvents on the tautomeric stability has been investigated. Stability of the molecule arises from hyperconjugative interactions, charge delocalization and intramolecular hydrogen bond has been analyzed using natural bond orbital (NBO) analysis. Molecular electrostatic potential (MEP) of the titled compound was studied for predicting the reactive sites. Mulliken population method and natural population analysis (NPA) have been studied. Population methods and MEP generally provides information regarding the chemical reactivity regions and charge distributions. Additionally, Frontier Molecular Orbitals analysis hasbeen performed from the optimized geometry. These orbitals also related to ionization potential, electron affinity, kinetic stability and hyperpolarizability of the molecule. The molecule exhibited good nonlinear optical (NLO) activity and first order hyperpolarizability. The predicted nonlinear optical properties of the title compound are 18 times greater than ones of urea.

Crystal structure, spectroscopic properties and density functional theory study of (Z)-1-[(2,4-dimethoxyphenylamino)methylene]naphthalen-2(1H)-one

The Schiff base (Z)-1-[(2,4-dimethoxyphenylamino)methylene]naphthalen-2(1H)-one was synthesized from the reaction of 2-hydroxy-1-naphthaldehyde with 2,4-dimethoxyaniline. The title compound has been characterized by FT-IR, UV-Vis and, X-ray single-crystal techniques. The present X-ray investigation shows that the compound exists in the keto-amine tautomeric form. Molecular geometry and vibrational frequencies of the compound in the ground state have been calculated using the density functional theory (DFT) with 6–311G(d, p) basis set and compared with the experimental data. The calculated results show that the optimized geometry is compatible with the crystal structure and the theoretical vibrational frequencies are in good agreement with the experimental values. Besides, molecular electrostatic potential (MEP), frontier molecular orbital analysis (HOMO-LUMO) and non-linear optical (NLO) properties of the compound were investigated using the same theoretical calculations.

Synthesis, Characterization, and Biological Activity of N (')-[(Z)-(3-Methyl-5-oxo-1-phenyl-1,5-dihydro-4H-pyrazol-4-ylidene)(phenyl)methyl]benzohydrazide and Its Co(II), Ni(II), and Cu(II) Complexes.

Reaction of 1-phenyl-3-methyl-4-benzoyl-pyrazol-5-one and benzoyl hydrazide in refluxing ethanol gave N ′-[(Z)-(3-methyl-5-oxo-1-phenyl-1,5-dihydro-4H-pyrazol-4-ylidene)(phenyl)methyl]benzohydrazide (HL1), which was characterized by NMR spectroscopy and single-crystal X-ray structure study. X-ray diffraction analyses of the crystals revealed a nonplanar molecule, existing in the keto-amine form, with intermolecular hydrogen bonding forming a seven-membered ring system. The reaction of HL1 with Co(II), Ni(II), and Cu(II) halides gave the corresponding complexes, which were characterized by elemental analysis, molar conductance, magnetic measurements, and infrared and electronic spectral studies. The compounds were screened for their in vitro cytotoxic activity against HL-60 human promyelocytic leukemia cells and antimicrobial activity against some bacteria and yeasts. Results showed that the compounds are potent against HL-60 cells with the IC50 value ≤5 μM, while some of the compounds were active against few studied Gram-positive bacteria.

Crystal structure, spectroscopic characterization and density functional studies of (E)-1-((3-methoxyphenylimino)methyl)naphthalen-2-ol

The Schiff base compound (E)-1-((3-methoxyphenylimino)methyl)naphthalen-2-ol was synthesized from the reaction of 2-hydroxy-1-naphthaldehyde with 3-methoxyaniline. The structural properties of the compound has been characterized by using FT-IR, UV–vis and X-ray single-crystal methods. According to X-ray diffraction result, the title compound exists in the phenol-imine tautomeric form. The molecular geometry, vibrational frequencies of the compound in the ground state have been calculated using the density functional theory (DFT/B3LYP) method with the 6-311++G(d,p) basis set, and compared with the experimental data. The obtained results show that the optimized molecular geometry is well reproduce the crystal structure. The theoretical vibrational frequencies are in good agreement with the experimental values. The calculations of electronic absorption spectra of tautomeric forms of the compound were performed by using TD-DFT calculations both in the gas phase and ethanol solvent. To investigate the tautomeric stability, optimization calculations at the B3LYP/6-311++G(d,p) level were performed for the phenol-imine and keto-amine forms of the compound. According to calculated results, the OH form is more stable than NH form. In addition, molecular electrostatic potential (MEP), frontier molecular orbital analysis (HOMO–LUMO), thermodynamic and, non-linear optical (NLO) properties of the compound were investigated using same theoretical calculations.

Existence of a resonance hybrid structure as a result of proton tautomerism in (±)-(E)-4-Bromo-2-[(2,3-dihydroxypropylimino) methyl]phenol racemate

o-Hydroxy Schiff bases have two tautomers known as phenol-imine and keto-amine forms. In the present work, the tautomerism in (E)-4-Bromo-2-[(2,3-dihydroxypropylimino)methyl]phenol compound has been investigated by experimental (XRD, FT-IR and UV–vis) and computational (DFT and TD-DFT) methods. The X-ray diffraction (XRD) study reveals that the title compound favors a resonance hybrid structure of phenol-imine and keto-amine forms in the solid state rather than having these forms separately or jointly. Experimental UV–vis study of proton transfer process in solvent media (Benzene, DMSO and EtOH) shows the preference of phenol-imine form in benzene while both phenol-imine and keto-amine characteristics are present in EtOH and DMSO.

Structural investigations of aroylhydrazones derived from nicotinic acid hydrazide in solid state and in solution

Structural forms of aroylhydrazones derived from nicotinic acid hydrazide have been studied in the solid state by FT-IR spectroscopy and in solution by NMR, UV–Vis and ATR spectroscopy. The studied compounds were N′-benzylidene-3-pyridinecarbohydrazide (1), N′-(2,4-dihydroxyphenylmethylidene)-3-pyridinecarbohydrazide (2), N′-(5-chloro-2-hydroxyphenylmethylidene)-3-pyridinecarbohydrazide (3), and N′-(3,5-dichloro-2-hydroxymethoxyphenylmethylidene)-3-pyridinecarbohydrazide (4). The compound 1 adopted the most stable ketoamine form (form I, CONHNC) in the solid state as well as in various organic solvents. In mixtures of organic solvents with water the UV–Vis and ATR spectra implied intermolecular hydrogen bonding of 1 with water molecules. The presence of both tautomeric forms I and II (form II, COHNNC) was proposed for the solid substance and highly concentrated solutions of 2, whereas form I was detected as the predominant one in diluted solutions. For compounds 3 and 4 a coexistence of forms I and III (form III, CONHNHCCCO) was noticed in the solid state and in polar protic organic solvents. The conversion to form III was induced by increasing the water content in the solvent mixtures. This process was the most pronounced for compound 4. When exposed to daylight, an appearance of a new band was observed during time in the UV–Vis spectrum of 4 in organic solvent/water 1/1 mixtures, which implied that tautomeric interconversion was most likely followed by E/Z isomerisation.

Tautomerism in Quinoxalines Derived from the 1,4-Naphthoquinone Nucleus: Acid Mediated Synthesis, X-ray Molecular Structure of 5-Chlorobenzo[f]quinoxalin-6-ol and Density Functional Theory Calculations

The reaction of tert-butyl 2-(3-chloro-1,4-dioxo-1,4-dihydronaphthalen-2-ylamino) ethylcarbamate with CF3COOH/CH2Cl2 yields 5-chloro-3,4-dihydrobenzo[f]quinoxalin6(2H)-one which undergoes acid-promoted dehydrogenation in the presence of water to give novel 5-chlorobenzo[f]quinoxalin-6-ol. The molecular structure of 5-chlorobenzo[f]quinoxalin6-ol in the solid state, determined by an X-ray diffraction (XRD) study, and the solution data confirm that it exists as the enol-imine tautomer, both in the solid state and in solution, differently from 5-chloro-3,4-dihydrobenzo[f]quinoxalin-6(2H)-one, which exhibits the keto-amine arrangement. Density functional theory (DFT) calculations confirmed the preference of 5-chlorobenzo[f]quinoxalin-6-ol and of the derivatives containing H and CH3 groups in place of the Cl atom for the enol-imine tautomer. It is suggested that the enol-imine structure is preferred for 5-chlorobenzo[f]quinoxalin-6-ol as a consequence of the higher aromatic character of this structure in comparison with the keto-amine form. DFT calculations carried out on the two tautomers of the benzo[a]phenazin-5(7H)-ones analogous to the benzo[f]quinoxalin-6(4H)-ones showed that the relative stabilities are dominated by solvation effects in the first case and the degree of aromaticity, in the latter.