Thiol Tautomer Research Articles

Preparation, spectral characterization, cytotoxic and thermal studies of platinum (IV) thiohydrazone complexes

Platinum (IV) complexes of type [Pt(L) 2Cl 2] [where, L=benzaldehyde- N, N-diphenyl- N-thiohydrazone (L 1), salicylaldehyde- N, N-diphenyl- N-thiohydrazone (L 2), acetaphenone- N, N-diphenyl- N-thiohydrazone (L 3) and cyclohexanone- N, N-diphenyl- N-thiohydrazone (L 4)] have been synthesized. The thiohydrazones can exist as thione–thiol tautomer and coordinates as a bidentate N–S ligand. The ligands found to act in monobasic bidentate fashion. Analytical data reveals that metal to ligand stoichiometry is 1:2. The complexes have been characterized by elemental analysis, IR, mass, electronic, 1H NMR spectroscopic studies, and TG/DTA study. Various kinetic and thermodynamic parameters like order of reaction ( n), activation energy ( E a), apparent activation entropy ( S #) and heat of reaction (Δ H) have also been carried out for one complex. Cytotoxic study has also been carried out for one complex.

Platinum (IV) thiohydrazide, thiodiamine and thiohydrazone complexes: A spectral, antibacterial and cytotoxic study

Some platinum (IV) complexes [Pt(L) 2Cl 2] [where, L = 2-aminopyridine- N-thiohydrazide (L 1), (2-aminopyridine- N-thio)-1,3-propanediamine (L 2), benzaldehyde-2-aminopyridine- N-thiohydrazone (L 3) and salicylaldehyde-2-aminopyrtidine- N-thiohydrazone (L 4)] have been synthesized. The thiohydrazides, thiodiamine and thiohydrazones can exist as thione–thiol tautomer and coordinate as a bidentate N–S ligand. The ligands found to act in monobasic bidentate fashion. Analytical data reveals that metal to ligand stoichiometry is 1:2. The complexes have been characterized by elemental analysis, IR, mass, electronic and 1H NMR spectroscopic studies. In vitro antibacterial and cytotoxic study have also been carried out for some complexes.

Preparation, characterization, two-photon absorption and optical limiting properties of a novel metal complex containing carbazole

Abstract The title complex, NiL2 · C6H6, has been successfully synthesized in good yield from reaction of a novel Schiff base ligand and nickel acetate in ethanol. The ligand (hereafter abbreviated as HL), derived from S-methyldithiocarbazate and N-ethyl-3-formylcarbazole, loses a proton from its tautomeric thiol form and coordinates to Ni(II) via the mercapto sulphur and β-nitrogen atoms upon complex formation. The structural, optical and physicochemical properties of the complex were characterized by X-ray diffraction, infrared spectroscopy, electronic spectra, thermal analyse and photoluminescence measurement. Single crystal X-ray diffraction analysis established that the geometry of Ni(II) is in a distorted square planar cis-configuration with two carbazole moieties on the same side and the two carbazole rings are almost parallel, forming a conjugated and electronic delocalization system. Two-photon absorption (TPA) coefficient β and cross-section of the complex were obtained by using the open aperture Z-scan technique. The optical limiting (OL) response was also measured.

Adsorption of 6‐mercaptopurine and 6‐mercaptopurine‐ribosideon silver colloid: A pH‐dependent surface‐enhanced Raman spectroscopy and density functional theory study. II. 6‐mercaptopurine‐riboside

Surface-enhanced Raman spectroscopy (SERS) has been applied to characterize the interaction of 6-mercaptopurine-ribose (6MPR), an active drug used in chemotherapy of acute lymphoblastic leukemia, with a model biological substrate at therapeutic concentrations and as function of the pH value. Therefore, a detailed vibrational analysis of crystalline and solvated (6MPR) based on Density Functional Theory (DFT) calculations of the thion and thiol tautomers has been performed. 6MPR adopts the thion tautomeric form in the polycrystalline state. The SERS spectra of 6MPR and 6-mercaptopurine (6MP) recorded on silver colloid provided evidence that the ribose derivative shows different adsorption behavior compared with the free base. Under acidic conditions, the adsorption of 6MPR on the metal surface via the N7 and possibly S atoms was proposed to have a perpendicular orientation, while 6MP is probably adsorbed through the N9 and N3 atoms. Under basic conditions both molecules are adsorbed through the N1 and possibly S atoms, but 6MP has a more tilted orientation on the silver colloidal surface while 6MPR adopts a perpendicular orientation. The reorientation of the 6MPR molecule on the surface starts at pH 8 while in the case of 6MP the reorientation starts around pH 6. Under basic conditions, the presence of the anionic molecular species for both molecules is suggested. The deprotonation of 6MP is completed at pH 8 while the deprotonation of the riboside is finished at pH 10. For low drug concentrations under neutral conditions and for pH values 8 and 9, 6MPR interacts with the substrate through both N7 and N1 atoms, possibly forming two differently adsorbed species, while for 6MP only one species adsorbed via N1 was evidenced.

Quantum mechanical study of tautomerism of methimazole and the stability of methimazole–I 2 complexes

DFT and ab initio theoretical methods were used to calculate the relative stability of tautomers in the methimazole (MMI). The calculations show that the thione form of MMI 1 is more stable than the thiol tautomer in good agreement with the experimental results. The DFT and ab initio calculations were also used to determine the stability of MMI–I 2 complexes. All methods suggest that the methimazole in the MMI–I 2 complex exists almost exclusively as the thione tautomer. The Gibbs free energy difference between planar and perpendicular forms of thione tautomer of MMI–I 2 complex indicates that the planar form is the predominant complex. The counterpoise corrected Gibbs free energy also shows that the MMI–I 2(plan.) complex is more stable than the MMI–I 2(perp.) complex. These predictions are in good agreement with the experimental results. By using the natural bond orbital (NBO) approach, the effects of charge transfer interactions on the stability of MMI–I 2 complexes were investigated. The LP 3(S)→σ*(I–I) and LP 3(I)→σ*(N–H) charge transfer interactions may be very important in the stability of the planar form. The results show that the LP 3(S)→σ*(I–I) charge transfer interaction causes a greater increase in the σ*(I–I) antibond occupation number, and concomitantly, a greater increase in the corresponding I–I bond length in the planar complex with respect to the perpendicular complex. The LP 3(S)→σ*(I–I) charge transfer interaction is assisted by NH⋯I intermolecular hydrogen bonding. The atom in molecule (AIM) analysis shows that the charge density and its Laplacian at the S⋯I bond critical point of the planar complex is greater than the perpendicular complex.

The preparation and characterization of tin(IV) complexes of 2-quinolinecarboxaldehyde Schiff bases of S-methyl- and S-benzyldithiocarbazates and the X-ray crystal and molecular structures of the 2-quinolinecarboxaldehyde Schiff base of S-benzyldithiocarbazate (Hqaldsbz) and its tin(IV) complex [Sn(qaldsbz)I 3

New tin(IV) complexes of empirical formula, Sn(NNS)I 3 (NNS = anionic forms of the 2-quinolinecarboxaldehyde Schiff bases of S-methyl- and S-benzyldithiocarbazate) have been prepared and characterized by a variety of physico-chemical techniques. In the solid state, the Schiff bases exist as the thione tautomer but in solution and in the presence of tin(IV) iodide they convert to the thiol tautomer and coordinate to the tin atom in their deprotonated thiolate forms. The structures of the free ligand, Hqaldsbz and its triiodotin(IV) complex, [Sn(qaldsbz)I 3] have been determined by X-ray diffraction. The complex, [Sn(qaldsbz)I 3] has a distorted octahedral structure with the Schiff base coordinated to the tin atom as a uninegatively charged tridentate chelating agent via the quinoline nitrogen atom, the azomethine nitrogen atom and the thiolate sulfur atom. The three iodo ligands are coordinated meridionally to the tin atom. The distortion from an ideal octahedral geometry of [Sn(qaldsbz)I 3] is attributed to the restricted bite size of the tridentate Schiff base ligand.

Solid state NMR study of 1,3imidazolidine‐2‐thione, 1,3‐imidazolidine‐2‐selenone and some of their N‐substituted derivatives

Solid‒state NMR spectra were recorded for 1,3‐imidazolidine‐2‐thione, 1,3‐imidazolidine‐2‐selenone and some of their N‐substituted derivatives. Spinning side‐bands of thione and selenone carbons were analysed to yield chemical shift anisotropies for these carbons. The NMR spectrum of imidazolidine‐2‐thione (Imt) showed some evidence for the presence of thiol tautomer. Molecular computations were carried out for Imt and its N‐methyl derivative to yield relative energies of various tautomers.

Adsorption of 2-Mercaptopyrimidine on Silver Nanoparticles in Water

Adsorption of 2-mercaptopyrimidine (2-MP), a prototype of biologically important N-heterocyclic aromatic thiols, has been examined on 100−120 nm silver particles in aqueous solution. Density functional calculations (UBPW91/6-31+G**) on various thione and thiol tautomers of the molecule have been performed to augment experimental data on reference vibrational frequencies. Criteria for determining the structural form of the molecule on the metal−water interface and its binding site and strength have been developed. The frequencies of the ring stretching Wilson modes 8a and 8b and their separation (Δv8a-8b) are good indicators of the protonation state of the molecule. The experimental Δv8a-8b for the Ag-bound 2-MP (20(±2) cm-1 in the pH range 4−11) identifies the structural form with the neutral thiol (C4H3N2S−), rather than thiolate (C4H3N2S-−). The molecule bonds with the Ag surface at its S site. A noticeable change in the SERS intensity pattern occurs around pH 6.5 due to the change in the surface enviro...

The Usefulness of Density Functional Theory To Describe the Tautomeric Equilibrium of 4,6-Dimethyl-2-mercaptopyrimidine in Solution

The tautomeric forms of 4,6-dimethyl-2-mercaptopyrimidine in solution were examined. Experimental support for the existence of the tautomeric forms of this molecule was provided by infrared (FTIR), ultraviolet (UV), and nuclear magnetic resonance (NMR) techniques. Density functional theory (DFT) calculations allowed us to elucidate these molecular structures. The vibrational and electronic spectra, as well as the 1H and 13C chemical shifts, were satisfactorily described by DFT theoretical calculations. The potential of the DFT methodology was confirmed by prediction of a band at 2706 cm-1, associated with the stretching vibration of −SH group, which did not appear in the experimental spectrum because it was overshadowed by the symmetric and antisymmetric vibration modes of −CH3 groups. The existence of the tautomeric thiol−thione equilibrium of 4,6-dimethyl-2-mercaptopyrimidine with the thione structure dominating in polar solvents (CH3OH and DMSO-d6) and the thiol structure in apolar media (cyclohexane) ...

Proton-Transfer Processes in Thiourea: UV Induced Thione → Thiol Reaction and Ground State Thiol → Thione Tunneling

Thiourea monomers isolated in a low-temperature argon matrix adopt exclusively the thione tautomeric form. Upon UV (λ > 300 nm) irradiation a photoreaction converting the initial isomer of the compound into its thiol tautomer occurred. Two structures of the photoproduct (with syn and anti orientation of the imino CN−H group with respect to the C−S bond) were identified in the matrix. Subsequently, a back reaction transforming the thiol form into the initial thione tautomer was observed for the matrix kept at 10 K and in darkness. The molecules with the anti conformation underwent this process, whereas those in the syn form remained unchanged. The only possible mechanism of the ground-state thiol → thione transformation at low temperature is proton-tunneling through the very high energy barrier of 9030 cm-1 (108 kJ mol-1) (calculated at the MP2/6-31++G(d,p) level). The experimentally obtained time constant of this process was 52 h. The structure of the photoproduced species was identified by comparing the ...

UV induced proton transfer in thioacetamide: first observation of thiol form of simple thioamideElectronic supplementary information (ESI): Tables S1 and S2 provide internal coordinates used in the normal mode analysis for the thione and thiol tautomers of thioacetamide. Atom numbering is given in Scheme S1. Infrared spectra of thioacetamide isolated in N2 and Ar matrices are compared in Figs. S1

A photoreaction transforming the thione–amino form of thioacetamide into the thiol–imino tautomer has been observed for the monomers of the compound isolated in argon and nitrogen low temperature matrices. Good agreement between the experimental infrared spectrum recorded after UV irradiation and the spectrum theoretically calculated (at the DFT(B3LYP)/6-31++G(d,p) level) for thioacetamide in the thiol tautomeric form allows reliable identification of the photoproduct. Four conformers of thiol thioacetamide were produced in the photoreaction. The relative populations of these conformers in the matrix were found to depend significantly on the wavelengths of the UV light used for irradiation. Upon λ>300 nm irradiation the photoproducts with NH bond in anti orientation with respect to the sulfur atom are predominantly generated, while upon λ>270 nm irradiation all four possible forms of the thiol tautomer of thioacetamide emerge in comparable amounts. The infrared spectra of the photoproduct(s) resulting from irradiation with UV light of different wavelength were analysed by comparison with theoretically predicted spectra for the four conformers of the compound in its thiol–imino tautomeric form.

Tautomerism of the Thioamide Group in 4‐Methyl‐7‐nitro‐2,3,4,5‐tetrahydro‐1,5‐benzodiazepine‐2‐thione.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Tautomerism of the Thioamide Group in 4-Methyl-7-nitro-2,3,4,5-tetrahydro-1,5-benzodiazepine-2-thione

When 4-methyl-7-nitro-2,3,4,5-tetrahydro-1H-1,5-benzodiazepin-2-one were reacted with phosphorus pentasulfide, the corresponding benzodiazepine-2-thione and its thiol tautomer were formed, which via the 2-methylmercapto derivative were converted to 4-(2-acetylhydrazino)-2-methyl-8-nitro-2,3-dihydro-1H-1,5-benzodiazepine.

Bis{methylNβ-[4-(dipropylamino)benzylidene]dithiocarbazato}nickel(II)

The Schiff base ligand in the title complex, [Ni(C15H22N3S2)2], lost a proton from its tautomeric thiol form and coordinated to Ni(II) via the mercapto S and β-N atoms. The geometry around the Ni atom is square planar with two equivalent Ni—N and Ni—S bonds. The two phenyl rings and the coordination moieties are in one plane forming an extensive electronic delocalization system. The structure is governed by C—H⋯S and C—H⋯N hydrogen bonds, leading to the formation of centrosymmetric dimers.

An organometallic mercaptopyridine complex with unusual bond shift fluxionality: metal-mediated tautomerism of (pentamethylcyclopentadienyl)bis(pyridine-2-thiolato)rhodium(III)

Studies have been carried out on unique molecular non-rigidity of Cp *Rh III(PyS) 2 (PyS=pyridine-2-thiolato). One PyS ligand bonds to the rhodium ion in an S-monodentate mode (RhS(2)=2.3799(9) Å) while the other ligand chelates to the metal in an N,S-bidentate mode (RhS(1)=2.4380(9); RhN(1)=2.089(2) Å). Both PyS ligands possess a significant contribution from the thiol tautomer in the solid state. Even though the conformation and configuration of the molecule are still retained in solution, an unusual metal-mediated tautomeric non-rigidity for the PyS region is observed in the solution around room temperature.

Bis[S-methyl 3-(2-bromobenzylidene)dithiocabazato-N3,S]platinum(II)

The Schiff base ligand in the title complex, [Pt(C9H8BrN2S2)2], is deprotonated from its tautomeric thiol form and coordinated to PtII via the mercapto S and β–N atoms. The configuration about PtII is a perfect square-planar, with two equivalent Pt—N [2.023 (3) Å] and Pt—S [2.293 (1) Å] bonds. The phenyl ring is twisted against the coordination moiety Pt1/N1/N1′/S2′/S2 by 31.8 (2)°, due to the steric hindrance induced by ortho-substituted bulky Br atom.

Selective Photoexcitation of the Thione and Thiol Forms of N-Hydroxypyridine-4(1H)-Thione: A Tautomeric Heteroaromatic System

The strong solvent-dependence of the tautomeric equilibrium exhibited by N-hydroxypyridine-4(1H)-thione (4-NHPT) and the distinct absorption properties of the thione and thiol tautomers offer the r...

SYNTHESIS, CRYSTAL STRUCTURE AND THIRD-ORDER NONLINEAR OPTICAL PROPERTIES OF COPPER(II) COMPLEXES WITH SCHIFF-BASE LIGAND DERIVATIVES FROM DITHIOCARBAZATE

The synthesis and spectroscopic characterization of D-M-D type neutral Cu(II) chelates with Schiff base ligands, R‒C6H4CHNNHCSSCH3 (R = ‒N(CH3)2, ‒CH3O) are reported. Spectroscopic data suggest that the Schiff base ligands act as single negatively charged bidentate ligands forming stable neutral metal complexes. Single crystal X-ray structure determinations of the Cu(II) chelates established that the coordination geometry of Cu(II) is square planar with two equivalent Cu-N and Cu-S bonds. The Schiff base ligands lost a proton from their tautomeric thiol form and coordinated to Cu(II) via the mercapto sulfur and β-nitrogen as expected. The two phenyl rings and the coordination moieties are almost in one plane forming an extensive electronic delocalization system. Magnetic properties and ESR data support the square-planar coordination geometry for both of the Cu(II) chelates. Nonlinear optical properties were measured using z-scan methods in CH3CN solution.

Bis(4-methoxybenzaldehyde thiosemicarbazonato-S,N4)copper(II)

The Schiff base ligand in the title complex, [Cu(C9H10N3OS)2], lost a proton from its tautomeric thiol form and coordinated to CuII via the mercapto S and β-N atoms. The geometry around CuII is square-planar with two equivalent Cu-N and Cu-S bonds. The two phenyl rings and the coordination moieties are in one plane forming an extensive electronic delocalization system.

Crystal structure and third-order nonlinearity of nickel(II) and copper(II) complexes with S-methyl β-N-(4-nitrobenzal)methylenedithiocarbazate

Ni II and Cu II complexes of general formula ML 2 [HL=S-methyl-β-N-(nitrobenzal)methylenedithiocarbazate] have been synthesized and characterized. The Schiff base acts as a single, negatively charged bidentate ligand forming extensive electron delocalization system. Single crystal X-ray diffraction analysis of the copper(II) complex revealed that the ligand lost a proton from its tautomeric thiol form and coordinates to Cu II via mercapto and sulfur and β-nitrogen atoms. The goemetry of the Cu II ions is square-planar with two equivalent Cu S and Cu N bonds. The two nitrobenzal rings and the coordinated moieties are almost in one plane, forming an electronic delocalization system. Third-order response determinations show that the extensive electron delocalization is quite helpful for the complexes exhibit higher second order molecular hyperpolarizability. Crystal structure of the free ligand is also determined for comparison.