Synthesis and electrochemical studies of disubstituted ferrocene/dipeptide conjugates with sulfur-containing side chains

A series of 1,1′-disubstituted ferrocenoyl peptides incorporating dipeptide sidearms has been synthesized and studied electrochemically. The target peptides include ferrocene as an electrochemical reporter, sulfur-containing amino acids (l-methionine, S-methyl-l-cysteine, S-trityl-l-cysteine, S-benzhydryl-l-cysteine) as metal binding agents, and amino acids with non-polar side chains (l-alanine, l-valine, l-phenylalanine) as spacers between reporter and metal binding groups. Ferrocene/dipeptide conjugates were prepared using solution phase peptide synthesis methods employing a BOC-protecting group strategy and HBTU- (O-(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate) mediated peptide coupling. The electrochemical properties of these 1,1′-substituted ferrocenoyl peptides have been characterized using cyclic voltammetry. All exhibit fully reversible one electron oxidation steps; forward sweep half wave peaks (E_F), reverse sweep half wave peaks (E_R), peak separations (ΔE_P) and half wave potentials (E_1/2) are reported. Finally, towards the goal of utilizing ferrocenoyl peptides to detect heavy metals in solution, the response of these ferrocene/dipeptide conjugates to metal cations (zinc(II), mercury(II), cadmium(II), lead(II), silver(I)) has been examined. Monitoring changes in the potential of the Fe(II)/Fe(III) redox couple to follow peptide/metal interactions, we have probed the influence of the spacer unit between the redox reporter and the metal-binding amino acid, and shown that these systems respond to mercury(II) more strongly than to other heavy metal ions.     

Mercury binding by ferrocenoyl peptides with sulfur-containing side chains: Electrochemical, spectroscopic and structural studies

Ferrocenoyl peptides incorporating amino acids derived from either l-methionine, l-cysteine or dl-homocysteine have been synthesised and investigated as agents for heavy metal binding and detection. Heavy metal-peptide interactions have been characterised using cyclic voltammetry to follow changes in the potential of the Fe(II)/Fe(III) redox couple, revealing that these systems interact with mercury(II) ions more strongly than with other thiophilic heavy metals such as cadmium(II), silver(I) and lead(II). Proton NMR experiments have demonstrated 1:1 peptide:mercury binding and enabled quantitative characterisation of this binding interaction. Crystal structures for two of these ferrocenoyl peptide derivatives have been elucidated, revealing that these compounds adopt a P-1,3′ open solid state conformation in the absence of mercury; this arrangement precludes intramolecular hydrogen bonding between chains, while extensive intermolecular hydrogen bonding is evident. The particular affinity of these systems for mercury(II) opens the possibility of incorporating them in new, biologically inspired sensors for detecting this toxic pollutant.     

Influence of sulfide and cyanide ions on the electrochemical behavior of the Fe(II)/Fe(Hg) system in thiocyanate solutions at mercury electrodes

The reduction and reoxidation processes of the Fe(II)/Fe(Hg) system in thiocyanate solutions at stationary mercury electrodes have been investigated by cyclic voltammetric, anodic stripping and controlled potential electrolysis methods. In 0.1 M NaSCN and 0.4 M NaClO₄ solution containing 1×10^?3M Fe(II), the voltammogram on the first cycle at. 0.05 V s^?1 gives two consecutive cathodic peaks near ?1.2 and ?1.39 V with a hysteresis on the reversal, and an anodic wave with two large peaks near ?0.58 and ?0.05 V and two small peaks near ?0.52 and ?0.43 V, respectively. The multicyclic voltammogram under the same conditions in the potential region between 0.00 and ?1.50 V gives a cathodic wave with a principal peak near ?1.02 V and two small peaks near ?0.02 and ?0.53 V, respectively, and an anodic wave with a principal peak near ?0.72 V, three small peaks near ?0.64, ?0.52 and ?0.40 V, and with a shoulder near ?0.05 V, respectively. The variation of the shape of the voltammogram on the second and subsequent runs is due to the formation of S^2? and CN^? during the process of electroreduction of Fe(II). A mechanism is proposed which involves an initial reduction of Fe(II)?SCN^? produced in an activation step at a mercury electrode, followed by the chemical redox reaction of a part of Fe(0)?SCN^? in the species giving FeS and CN^?, and takes into account the influence of FeS and CN^? on the further reduction and reoxidation of iron. Both FeS and CN^? stimulate further reduction, and reoxidation of iron. The hysteresis of the cathodic wave on the first cycle arises from the fact that Fe(II) is reduced more easily at the mercury electrode covered with FeS than at a pure mercury electrode.     

Determination of stability constants by semi-integral analysis of voltammograms obtained with a computerized electrochemical system

A versatile computerized electrochemical system is used for the determination of stability constants of lead(II) propanoate and 2-hydroxypropanoate complexes by semi-integral linear sweep voltammetry. Semi-integral voltammograms are analogous in characteristics to d.c. polarograms, and the half-wave potentials (E_{$\text{1}\text{2}$}) and limiting currents (i₁) for a depolarizer can be obtained by nonlinear least-squares fitting of semi-integrally transformed voltammograms. An advantage of the proposed method is that E_{$\text{1}\text{2}$} and i₁ values are obtained from a single mercury drop (hanging or dropping mercury electrode); working time is reduced while accuracy and precision are maintained. The computer-controlled system provides automated reagent addition, signal generation, response measurement and sampling, data evaluation, etc. The results agree well with those obtained by the usual d.c. polarographie method.     

Enzymatic synthesis of chiral amino acid sulfoxides by Fe(II)/α-ketoglutarate-dependent dioxygenase

Asymmetric sulfoxidation of sulfur-containing l-amino acids was successfully achieved through bioconversion using IDO, which is an Fe(II)/α-ketoglutarate-dependent dioxygenase previously found in Bacillus thuringiensis strain 2e2. The IDO catalyzed sulfoxidation of l-methionine, l-ethionine, S-methyl-l-cysteine, S-ethyl-l-cysteine, and S-allyl-l-cysteine into the corresponding (S)-configured sulfoxides such as (+)-methiin and (+)-alliin, which are responsible for valuable physiological activities in mammals, and have high stereoselectivity. Herein we have established an effective preparative laboratory scale production method to obtain enantiomerically pure chiral sulfoxides using an IDO biocatalyst.     

Study on the polarographic catalytic wave of the system cobalt(II)-dimethylglyoxime

In the cobalt (II)-dimethylglyoxime-NH₃-NH₄Cl (pH 9) system, tne complex Co(II)A₂ exhibits a sensitive polarographic wave. The mechanism of this catalytic wave has been investigated by linear potential sweep voltammetry, cyclic voltammerty and anedic stripping voltammetry. The experimental evidences showed that a zero-valence “active cobalt” or its complex formed during the irreversible reduction of Co(II)A₂, which is adsorbed on the mercury electrode surface, and simultaneously DMG is catalytically reduced by this “active cobalt”. The mechanism of this system with the conflicting explanations of a catalytic hydrogen wave or only adsorptive complex wave is discussed.     

Thermal decomposition reactions of metal carboxylato complexes in the solid state. I.: Thermographic and differential thermal studies of metal oxalato,malonato and succinato complexes

Thermal investigation of metal carboxylato complexes of the first transition metals, Mn(II), Fe(II), Fe(III), Co(II), Ni(II) and Cu(II) and non-transition metals like Zn(II) and Cd(II) in the solid state has been carried out under non-isothermal conditions in nitrogen atmosphere by simultaneous TG and DTA. TG and DTA curves inferred that the thermal stability of the complex decreased approximately with the increase of the standard potential of the central metal ion. The thermal parameters like activation energy, E_a, enthalpy change, ΔH, and entropy change, ΔS, corresponding to the dehydration and decomposition of the complexes are determined from TG and DTA curves by standard methods. A linear correlation is found between ΔH and ΔS and E_a and ΔS in dehydration and decomposition processes. DTA curves show an irreversible phase transition for Na₂Mn(mal)₂], Na₂[Cu(mal)₂] and Na₂,[Co(suc)₂] complexes. The residual products in these decomposition processes being a mixture of two oxides, of oxide and carbonate or a mixture of two carbonates.     

An efficient approach for monosulfide bridge formation in solid-phase peptide synthesis

An efficient approach for the synthesis of cyclic peptides containing unnatural thioether side-chain bridges, based on the use of (2S)-9-fluorenylmethyl-2-[(tert-butoxycarbonyl)amino]-4-iodobutanoate and its homologue 5-iodopentanoate, derived from Boc-l-Asp-OFm and Boc-l-Glu-OFm, respectively, is reported. The synthesis was performed by a tandem combination of solid-phase peptide synthesis and microwave-assisted cyclization strategy.     

Electrodimerization of the pyridiniumcarboxylic acids homarine and trigonelline

Homarine (1-methyl-2-carboxypyridinium ion) and trigonelline (1-methyl-3-carboxy-pyridinium ion) are reduced at mercury electrodes in alkaline solutions by a one-electron transfer to the pyridinium ring. Analysis of polarographic wave shapes using deviation-pattern recognition, peak widths of cyclic voltammograms, analysis of the products of bulk electrolyses and dependences of E_1/2 and E_p on scan rate, pH and concentration indicated that these reductions proceed by an EC² mechanism, where the second-order chemical reaction is a homogeneous dimerization. The zwitterionic form of the pyridiniumcarboxylic acid, which predominates in alkaline solutions, undergoes reduction to form a pyridinyl anion radical. Two radicals then couple to yield a dimer dianion which is subject to protonation. The dimer can be oxidized at mercury electrodes.     

An octahedral nickel(II) complex of a hexadentate N<Subscript>2</Subscript>O<Subscript>2</Subscript>S<Subscript>2</Subscript> thioether ligand: synthesis,characterization, X-ray and electronic structure

A hexadentate dibasic thioether N₂O₂S₂ donor ligand (H ₂ L) and its octahedral nickel(II) complex, [Ni(L)] have been synthesized and characterized by physicochemical and spectroscopic techniques. The structures of both H ₂ L and its nickel complex were confirmed by single-crystal X-ray diffraction studies. The cyclic voltammogram of the complex shows a quasi-reversible Ni(II)/Ni(III) oxidation couple (E _1/2 = 0.88 V) along with a ligand-based reduction (E _1/2 = ?0.83 V). The electronic structures and electrochemical properties have been interpreted with the help of DFT calculations. The electronic transitions as calculated by TDDFT/CPCM method are used to assign the UV–Vis absorption bands.     

Total synthesis of maremycins A,B, C1/C2, D1, and D2

The total synthesis of maremycins A, B, C₁/C₂, D₁, and D₂ is achieved starting from the natural amino acids l-isoleucine and S-methyl-l-cysteine, in which the total synthesis of maremycins B, C₁/C₂, and D₂ is accomplished for the first time. The synthesis features a position-selective intramolecular bromination process for the synthesis of key chiral building block, a Pd-catalyzed indole synthesis for the preparation of (2S,3S)-β-methyltryptophan and hydroxylation of oxindoles by molecular oxygen. In addition, the protocol for conversion of maremycins A and B to maremycins C and D was improved.     

Thermal behaviors of N-pyrrolidine-N′-(2-chlorobenzoyl)thiourea and its Ni(II), Cu(II), and Co(III) complexes

The N-pyrrolidine-N??-(2-chlorobenzoyl)thiourea, HL, and their Ni(II), Cu(II), and Co(III) complexes (NiL₂, CuL₂, and CoL₃) have been synthesized and characterized. The thermal decomposition reactions of all the compounds have been investigated by DTA/TG combined systems. The mass spectroscopy technique has been used to identify the products during pyrolytic decomposition. The pyrolytic final products have been analyzed by X-ray powder diffraction method. After comparison of thermogravimetric and mass results of HL, NiL₂, CuL₂, and CoL₃, the decomposition mechanism of these compounds have been suggested. The thermal stability of the Ni(II) and Cu(II) complexes according to the thermogravimetric curves follows the sequence: NiL₂?<?CuL₂. The values of the activation energy, E _a, have been obtained using model-free (Kissenger?CAkahira?CSunose, KAS, Flyn?CWall?COzawa, FWO, and Isoconversional) methods for all decomposition stages. The E _a versus the extent of conversion, ??, plots show that the values of E _a varies as ??. Thirteen kinetic model equations have been tested for selecting correct reaction models. The optimized value of E _a and Arrhenius factor, A, have been obtained using the best model equation. The thermodynamic functions (??H*, ??S*, and ??G*) have been calculated using these values.     

Analytical application of triethylenetetraaminehexa- acetic acid: Part 2. Polarographic Investigation of the reactions of TTHA at the dropping mercury electrode

TTHA gives an anodic d.c. wave and s.w. peak corresponding to oxidation of mercury at the d.m.e. surface with formation of a Hg(II)—TTHA complex. Similar processes are known for other ligands, e.g. EDTA and DCTA, but the greater stability of the Hg(II)—TTHA complex gives a significant improvement in the shapes of the anodic wave and s.w. peak. An increase in pH shifts the E_{$\text{1}\text{2}$} and E_p values towards more negative values because the conditional stability constant of the Hg(II)—TTHA complex is increased. Although the half–wave potentials of the reduction wave of Hg(II)—TTHA and of the anodic wave of TTHA, are the same, other criteria for the reversibility of polarographic reactions suggest that the anodic TTHA process at the d.m.e. is not completely reversible. The temperature coefficient of the wave and s.w. peak as well as the dependence of the wave height on the square root of the mercury head prove that the process is diffusion–controlled. Supporting electrolytes are given for which the d.c. wave and the s.w. peak of TTHA are well-shaped, with linear dependence between the wave or peak height and the concentration of TTHA in the ranges 5 × 10^-5–5 × 10^-4 M (d.c.) and 1 0^-5–1 × 10^-4 M (s.w.).     

Thermal decomposition reactions of metal carboxylato complexes in the solid state: II. Thermographic and differential thermal studies of metal oxalato,malonato and succinato complexes

The thermal investigations of metal carboxylato complexes of the first transition metals, Mn(II), Fe(II), Fe(III), Co(II), Ni(II) and Cu(II) and non transition metals like Zn(II) and Cd(II) in solid state were carried out under non-isothermal condition in nitrogen atmopshere by thermogravimetric (TG) and differential thermal analyses (DTA) methods. The results of DTA curves inferred that the thermal stability of the complex decreased approximately with the increase of standard potential of the central metal ion. The thermal parameters like activation energy (E_a¹), enthalpy change (ΔH) and entropy change (ΔS) corresponding to deaquation, deammoniation and decomposition processes occurred simultaneously or separately were determined from TG and DTA curves by the standard methods. A linear correlation has been found in the plots of ΔH vs. ΔS and E_a¹ vs. ΔS in deaquation, deammoniation and decomposition processes. An irreversible phase transition was noticed for H₂[Mn(suc)₂] and H₂[Co(suc)₂] complexes in DTA curves. The residual pyrolysed products were metal carbonates.     

Stereoselective synthesis of cationic heterobidentate C(NHC)/SR rhodium(I) complexes using stereodirecting N,N-dialkylamino groups

The synthesis of two different types of chiral C/S ligands based upon N-(N,N-dialkylamino)-substituted N-heterocyclic carbenes and thioether functionalities, along with their neutral [RhCl(CNH)(COD)] and cationic [Rh(I)(NHC/S)(COD)]⁺ complexes, has been accomplished. (S)-2-[(Phenylthio)methyl]pyrrolidine, carrying the thioether moiety, and (2S,5S)-2,5-diphenylpyrrolidine, combined with a thioether functionalized side chain, were studied as potential stereodirecting groups. Only the latter provided high selectivity in the formation of the neutral complex, leading to a single atropoisomer (de >98%) of the newly formed, configurationally stable C(NHC)–Rh bond. The synthesis of the corresponding cationic [Rh(I)(NHC/S)(COD)]⁺ complexes, however, resulted in the formation of single (R_a,S_S) and (S_a,S_S) diastereomers, respectively, of the four possible complexes in each case [combinations of the (R_a/S_a) C(NHC)–Rh axis and the (S_s/R_s) stereogenic S center formed upon coordination]. For the proline derivative, the resolution of the mixture of (R_a/S_a)-[RhCl(CNH)(COD)] neutral complexes proceeds via dynamic kinetic resolution through coordinatively unsaturated Rh(I) intermediates formed after halide abstraction. The absolute configurations of both types of cationic complexes were unequivocally assigned on the basis of X-ray diffraction analysis.     

On a polarographic double wave of some Cu(II) alkanoates involving metal-metal interaction

Electrode behaviour of some Cu(II) alkanoates, known to exhibit metal-metal exchange interaction to varying degrees, has been studied in various organo-aqueous solutions under varying experimental conditions, at DME. All the Cu(II) alkanoates except Cu(II) monochloroacetate have been observed to undergo diffusion-controlled, two-electron transfer, irreversible electroreduction in sodium perchlorate and gelatin. In 25% alcohol and 25% acetone, all these alkanoates undergo irreversible electroreductions. In 50% alcohol or 50% acetone, all these Cu(II) alkanoates except Cu(II) trichloroacetate undergo a double wave electroreduction and similar are the observations for Cu(II) formate and acetate in 25% dioxane. The first part of this double wave may be attributed to the monomer electroreduction and the second part to the dimer, which is supposed to be in sluggish equilibrium with the monomer. E_1/2 data have been reported.     

Synthesis,spectroscopic and structural investigation of Zn(NCS)2(nicotinamide)2 and [Hg(SCN)2(nicotinamide)]n

Zinc(II) and mercury(II) thiocyanate complexes with nicotinamide, bis(nicotinamide-N)-bis(thiocyanato-N)zinc(II) (1) and catena-[nicotinamide-N-(μ-thiocyanato-S,N)(thiocyanato-S)mercury(II)] (2), have been prepared and characterized by spectroscopic, thermal and X-ray crystallographic methods. The vibrational bands of diagnostic value are compared to the values of the free ligand and the data are in good correlation with the X-ray results. Centrosymmetrical hydrogen bonded dimers are found, R₂²(10) in 1 and R₂²(8) in 2.     

Alkoxy substituted (E,E)-3,6-bis(styryl)pyridazine—a photosensitive mesogen for liquid crystals

(E,E)-3,6-Bis(styryl)pyridazines (3a-t) bearing 2, 4 or 6 alkoxy chains were prepared by applying the Siegrist reaction of 3,6-dimethylpyridazine (13) and the corresponding azomethines 10a-t. The transversal dipole moment of these calamitic compounds effects an extremely high tendency for self-organization in thermotropic LC phases (N, S_A, S_B, S_C, S_E, S_I/F, and Cub). The conjugated core structure represents moreover a chromophore with a high photosensitivity for (E,E)?(E,Z) isomerization reactions: this property makes the compounds interesting for optical imaging and switching techniques.     

Solvent Effects on the Electrochemical Behavior of Iron(III) Schiff Base Complex

Electrochemical reduction of Fe^IIILCl where L is a Schiff base has been investigated in various aprotic solvents. From a plot of the half wave potential E _1/2 for the reduction of these complexes vs. E _1/2 for the oxidation of ferrocene, the solvent–solute interactions were studied: the E _1/2 variation is found to be a function of Lewis-type acceptor–donor interactions. The diffusion coefficients D in the different solvents were also been determined by linear sweep voltammetry. The variation of D is discussed in terms of viscosity and dielectric constant.     

Bis(N-benzyl-N-furfuryldithiocarbamato-S,S′)mercury(II) as a precursor for the preparation of mercury sulfide nanoparticles

Bis(N-benzyl-N-furfuryldithiocarbamato-S,S′)mercury(II), [Hg(bzfdtc)₂] (1) and bis(N,N-difurfuryldithiocarbamato-S,S′)mercury(II), [Hg(dfdtc)₂] (2) were synthesized and characterized by IR, NMR and single-crystal X-ray crystallography. Single-crystal X-ray structures of 1 and 2 indicate that both complexes are dimeric, with each mercury in a distorted [HgS₅] square pyramidal geometry. The thioureide (N¹³CS₂) carbon signals were observed at 206.8 and 206.7 ppm for 1 and 2, respectively, with very weak intensity, characteristic of the quaternary carbon signal. Complex 1 has been used as a precursor for the preparation of HgS nanoparticles. The as-prepared HgS nanoparticles have been characterized by powder XRD, FESEM, EDAX, UV–visible and IR spectroscopies. FESEM images of HgS nanoparticles show that the particles are spherical in shape. The blue shift in the absorption maxima in the UV–visible spectra of HgS1 and HgS2 is a consequence of the quantum confinement effect.