Quasi-reversible Oxidation Peaks Research Articles

A protonated imine-based water-soluble cadmium complex with N, O donor chelating agent: Synthesis, structure, fluorescence efficiency and catecholase activity

A mononuclear air-stable and water soluble six coordinated cadmium (II) complex [Cd(HL)6](ClO4)2 (1) with bi-dentate phenol based ligand [HL = 2-(benzylimino)methyl)phenol] have been synthesized. The complex has been characterized by various spectroscopic techniques. The solid-state structure, by single crystal X-ray diffraction, revealed that Cd(II) complex crystallizes in trigonal space group R-3. Interestingly, single crystal X-ray structure data show that the Schiff base in the complex is present in a zwitterionic form and coordinated through the phenolic oxygen atom only, providing the first example of more than four monodendate Schiff base in zwitterionic form to be coordinated with the central metal ion. Cyclic voltamogram of the cadmium (II) complex (1) exhibits a quasi-reversible oxidation peak in the positive potential region. The redox process occurs due to the generation of phenoxyl radical from the pheolate group of the attached ligand with the central metal ion. Luminescence spectra show a strong emission band around 500 nm. Additionally, the mononuclear cadmium complex (1) shows catechol oxidase activity in methanol under aerobic conditions. The turnover number of the process is 2.766 × 103 h–1. The oxidation process occurs via formation of ligand oriented phenoxyl radical species.

Mononuclear Mn(III) complex with a terminal azide supported by phenol-based tetradentate N2O2 donor ligand: Synthesis, structure, properties, and catechol oxidase activity

Mixed ligand octahedral Mn(III) complex [Mn(L)(CH3OH)(N3)] (1) with sterically constrained phenol-based tetradentate N2O2 ligand, N,Nʹ-dimethyl-N,Nʹ-bis (2-hydroxy-3,5-dichlorobenzyl)-ethylenediamine (H2L) in association with azide ion and methanol as ancillary ligand has been synthesized. Synthesis, structure, properties, and catecholase activity of the mononuclear manganese complex (1) are described. Complex (1) crystallizes in triclinic space group P-1 with a = 10.5431(5) Å, b = 11.3731(6) Å and c = 22.9638(12) Å. The crystal structure reveals that complex (1) is an Mn (III) complex with flexible phenol-based ligand in association with azide as a terminal monodentate ligand. Electrochemical study of the mononuclear manganese complex (1) gives two quasi reversible oxidation peaks (E11/2≈ 0.18 V and E21/2≈ 0.23 V) in the positive potential range versus Ag/AgCl reference in acetonitrile due to ligand centered phenolate/phenoxyl couple. Complex (1) shows a strong emission band at 440 nm. The complex exhibits catechol oxidase activity in methanol. Oxidation of 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone in methanol has been accelerated in presence of reported manganese complex (1) with a high Kcat value of 1.43 × 104h−1.

Air Stable Chalcogen-Doped Rubicenes with Diradical Character

Air Stable Chalcogen-Doped Rubicenes with Diradical Character

New 2-Acetyl-3-aminophenyl-1,4-naphthoquinones: Synthesis and In Vitro Antiproliferative Activities on Breast and Prostate Human Cancer Cells.

The reaction of 2-acyl-1,4-naphthoquinones with N,N-dimethylaniline and 2,5-dimethoxyaniline, promoted by catalytic amounts of CeCl3·7H2O under “open-flask” conditions, produced a variety of 2-acyl-3-aminophenyl-1,4-naphthoquinones structurally related to the cytotoxic 2-acetyl-3-phenyl-1,4-naphthoquinone, an inhibitor of the heat shock chaperone protein Hsp90. The members of the 2-acyl-3-aminophenyl-1,4-naphthoquinone series were isolated in good yields (63-98%). The cyclic voltammograms of the 2-acyl-3-aminophenyl-1,4-naphthoquinone exhibit two one-electron reduction waves to the corresponding radical-anion and dianion and two quasireversible oxidation peaks. The first and second half-wave potential values (E1/2) of the members of the series are sensitive to the push-pull electronic effects of the substituents in the naphthoquinone scaffold. Furthermore, the in vitro antiproliferative properties of these new quinones were evaluated on two human cancer cells DU-145 (prostate) and MCF-7 (mammary) and a nontumorigenic HEK-293 (kidney) cell line, using the MTT colorimetric method. Two members, within the series, exhibited interesting cytotoxic activities on human prostate and mammary cancer cells.

Half-Wave Potentials and In Vitro Cytotoxic Evaluation of 3-Acylated 2,5-Bis(phenylamino)-1,4-benzoquinones on Cancer Cells.

A broad range of 3-acyl-2,5-bis(phenylamino)-1,4-benzoquinones were synthesized and their voltammetric values, as well as in vitro cancer cell cytotoxicities, were assessed. The members of this series were prepared from acylbenzoquinones and phenylamines, in moderate to good yields (47–74%), through a procedure involving a sequence of two in situ regioselective oxidative amination reactions. The cyclic voltammograms of the aminoquinones exhibit two one-electron reduction waves to the corresponding radical-anion and dianion, and two quasi-reversible oxidation peaks. The first and second half-wave potential values (E1/2) of the members of the series were sensitive to the push-pull electronic effects of the substituents around the benzoquinone nucleus. The in vitro cytotoxic activities of the 3-acyl-2,5-bis(phenylamino)-1,4-benzoquinones against human cancer cells (bladder and prostate) and non-tumor human embryonic kidney cells were measured using the MTT colorimetric method. The substitution of both aniline groups, by either methoxy (electron donating effect) or fluorine (electron withdrawal effect), decreased the cytotoxicity in the aminoquinones. Among the members of the unsubstituted phenylamino series, two of the 18 compounds showed interesting anti-cancer activities. A preliminary assay, looking for changes in the expression of selected genes, was performed. In this context, the two compounds increased TNF gene expression, suggesting an association with an inflammatory-like response.

1,3-Diaza[3]ferrocenophanes functionalized with a nitronyl nitroxide group

An aza-Wittig reaction of 1,1′-bis(triphenylphosphoranylidenamino)-ferrocene with 4-iodo-, 2,5-difluoro-4-iodo-, pentafluoro-, or 4-methoxycarbonylphenylisocyanates is described here. Intermediate betaines, which are products of a nucleophilic attack of the nitrogen atom of the remaining iminophosphorane moiety on the carbon atom of the carbodiimide functional group, were isolated; their structures were proved by monocrystal X-ray diffraction. Hydrolysis of the betaines under mild conditions gave corresponding arylamino-1,3-diaza[3]ferrocenophanes. On the other hand, as demonstrated, under more rigid conditions, the betaines could be converted into ferrocenophanes with the 1,3-diazetidine-2,4-diimine coupler. The iodo- or diiodo-derivatives of 1,3-diaza[3]ferrocenophanes were cross-coupled with a nitronyl nitroxide-2-ide gold(I) complex to obtain corresponding metallocenophanes functionalized with the nitronyl nitroxide group(s). The molecular and crystal structures of the halogenated and spin-labeled ferrocenophanes were solved by X-ray analysis. In arylamino-1,3-diaza[3]ferrocenophanes, according to cyclic voltammetry, the position of the quasi-reversible oxidation peak associated with the Fe(II)/Fe(III) redox couple deviates within 0.09 V depending on substituents in the arylamino group.

Synthesis, half-wave potentials and antiproliferative activity of 1-aryl-substituted aminoisoquinolinequinones.

The synthesis of a variety of 1-aryl-7-phenylaminoisoquinolinequinones from 1,4-benzoquinone and arylaldehydes via the respective 1-arylisoquinolinequinones is reported. The cyclic voltammograms of the new compounds exhibit two one-electron reduction waves to the corresponding radical-anion and dianion and two quasi-reversible oxidation peaks. The half-wave potential values (EI½) of the members of the series have proven sensitive to the electron-donor effect of the aryl group (phenyl, 2-thienyl, 2-furyl) at the 1-position as well as to the phenylamino groups (anilino, p-anisidino) at the 7-position. The antiproliferative activity of the new compounds was evaluated in vitro using the MTT colorimetric method against one normal cell line (MRC-5 lung fibroblasts) and two human cancer cell lines: AGS human gastric adenocarcinoma and HL-60 human promyelocytic leukemia cells in 72-h drug exposure assays. Among the series, compounds 5a, 5b, 5g, 5h, 6a and 6d exhibited interesting antiproliferative activities against human gastric adenocarcinoma. The 1-arylisoquinolinequinone 6a was found to be the most promising active compound against the tested cancer cell lines in terms of IC50 values (1.19; 1.24 µM) and selectivity index (IS: 3.08; 2.96), respect to the anti-cancer agent etoposide used as reference (IS: 0.57; 0.14).

Mixed-valent Complexes of Rhodium with Acetylinic Ligands: Their Electrochemical Studies

Two acetylinic mixed- valent rhodium(II, III) complexes: [cis-{Rh2(II,III)Cl(bpy)2(u-C=C-)}]2, (A) and cis-[Rh(II,III)Cl(bpy)2]-C=C-SiMe3, (B) (where bpy=bipyridine) have been prepared. (A) was accomplished through two pathways. The two complexes were characterized by 1H, 13C NMR, elemental, magnetic, electrochemical analyses and mass spectrophotometry. Cyclic voltametric analyses of the products from 0.00 -1.20 V display two one–electron quasi-reversible oxidation peaks which were attributed to the Rh(II), Rh(III) couple. The redox processes were separated by 528 mV, indicating a significant electronic communication between the two metallic centres. The yields of (A) and (B) were found to be 37 and 32 % respectively.

Unprecedented 1,3-Diaza[3]ferrocenophane Scaffold as Molecular Probe for Anions

The guanidine unit in the guise of 2-aminoimidazole in the new structural motif 2-arylamino-1,3-diaza[3]ferrocenophane 4 acts as a binding site for anions. The electrochemical behavior of this compound has been studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) and was found to exhibit a quasi reversible oxidation peak, associated to the Fe(II)/Fe(III) redox couple (Ep = 440 mV), and a non-reversible oxidation wave (Ep = 817 mV), probably associated to the oxidation of the C═N unit present in the guanidine bridge. Recognition of AcO(-), PhCO(2)(-), F(-), Cl(-), and Br(-) anions by the free receptor and the less basic anions Br(-), Cl(-), and NO(3)(-) by its monoprotonated form takes place by unusual redox-ratiometric measurements and spectroscopic ((1)H NMR and UV-vis) changes.

Coordination polymers containing ferrocene backbone. Synthesis, structure and electrochemistry

The reaction of 1,1'-ferrocenedicarboxylic acid (LH(2)) with bis(triphenyltin) oxide afforded a molecular heterobimetallic compound [(Ph(3)Sn)(2)L]. In the latter the two carboxylate units of [L](2-) are involved in an anisobidentate chelating coordination mode to two triphenyl tin units. The reaction of LH(2) with trimethyltin hydroxide or bis(tri-n-butyltin) oxide afforded 2D-coordination polymers [(Me(3)Sn)(2)L](n) and [(n-Bu(3)Sn)(2)L](n) which are formed as a result of anisobidentate bridging coordination action of the two carboxylate units of [L](2-). Interestingly the 2D-coordination polymers contain 24-membered macrocycles each of which is comprised of four trialkyl tin units. The coordination unsaturation of [(Ph(3)Sn)(2)L] can be utilized to form coordination polymers. Accordingly the reaction of LH(2) with bis(triphenyltin) oxide in the presence of ditopic nitrogen ligands such as 4,4'-bipyridine, 4,4'-trimethylenebipyridine or 4,4'-vinylenebipyridine afforded one-dimensional coordination polymers which contain in their backbone three distinct structural components viz., two triorganotin units, a ferrocenyl unit and a bridging nitrogen ligand unit. The coordination polymers, however, do not retain their structural integrity in solution and fall apart to their monomeric units. Electrochemical studies on these hybrid orgaonotin/ferrocene systems reveal that most of them exhibit a single quasi-reversible oxidation peak.

The electrochemical oxidation of catechol and dopamine on platinum in 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 2mim][NTf 2]) and 1-Butyl-3-methylimidazolium tetrafluoroborate ([C 4mim][BF 4]): Adsorption effects in ionic liquid voltammetry

The electrochemical oxidation of catechol and dopamine has been studied at a platinum micro-electrode (10 μ m diameter) in two room temperature ionic liquids (RTILs): 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 2mim][NTf 2]) and 1-Butyl-3-methylimidazolium tetrafluoroborate ([C 4mim][BF 4]). For catechol in [C 2mim][NTf 2], an electrochemically quasi-reversible oxidation peak was observed at 1.1 V vs. Pt with a back peak at 0.4 V vs. Pt. This is assigned to the two-electron oxidation of catechol to doubly protonated o-benzoquinone. Double-step chronoamperometry gave a diffusion coefficient for the catechol and the oxidised species which is 3.8 × 10 - 11 m 2 s - 1 for both. For catechol in [C 4mim][BF 4], a two-electron oxidation wave was observed at 1.0 V vs. Pt with no back peak. Another peak at less positive potential was also observed at 0.6 V vs. Pt in [C 4mim][BF 4] but not in [C 2mim][NTf 2] which is assigned to the adsorption of electrochemically formed neutral o-benzoquinone on the platinum electrode. The oxidised protonated o-benzoquinone is suggested to be deprotonated by the [ BF 4 ] - anion, but not by the [ NTf 2 ] - anion: hence adsorption of the neutral species at the platinum electrode, not the charged species. For dopamine in both RTILs, two chemically irreversible oxidation peaks were observed at 0.75 V and 1.1 V vs. Pt, and assigned to the oxidation of dopamine to the corresponding semi-quinone and the quinone. Potential-step chronoamperometry was carried out on the oxidation waves of dopamine in [C 2mim][NTf 2] and the diffusion coefficient of species in solution was calculated to be 6.85 × 10 - 12 m 2 s - 1 and confirmed that the waves corresponded to one and two electron processes. A third wave was observed at 1.8 V vs. Pt which is attributed to the oxidation of the amine group to a radical cation with likely subsequent follow up chemistry. In [C 4mim][BF 4] a peak at less positive potential was observed for dopamine, similar to catechol which is assigned to the adsorption of the neutral quinone species on the platinum electrode formed by the reaction of the removal of protons from the oxidised dopamine with the [ BF 4 ] - anion.

Organostannoxane‐Supported Multiferrocenyl Assemblies: Synthesis, Novel Supramolecular Structures, and Electrochemistry

Organostannoxane-based multiredox assemblies containing ferrocenyl peripheries have been readily synthesized by a simple one-pot synthesis, either by a solution method or by room-temperature solid-state synthesis, in nearly quantitative yields. The number of ferrocenyl units in the multiredox assembly is readily varied by stoichiometric control as well as by the choice of the organotin precursors. Thus, the reaction of the diorganotin oxides, R2SnO (R = Ph, nBu and tBu) with ferrocene carboxylic acid affords tetra-, di-, and mononuclear derivatives [{Ph2Sn[OC(O)Fc]2}2] (1), [{[nBu2SnOC(O)Fc]2O}2] (2), [nBu2Sn{OC(O)Fc}2] (3), [{tBu2Sn(OH)OC(O)Fc}2] (4), and [tBu2Sn{OC(O)Fc}2] (5) (Fc = eta(5)C5H4-Fe-eta(5)C5H5). The reaction of triorganotin oxides, R3SnOSnR3 (R = nBu and Ph) with ferrocene carboxylic acid leads to the formation of the mono-nuclear derivatives [Ph3SnOC(O)Fc] (6) and [{nBu3SnOC(O)Fc}(n)] (7). Molecular structures of the compounds 1-4 and 6 have been determined by single-crystal X-ray analysis. The molecular structure of compound 1 is new among organotin carboxylates. In this compound, ferrocenyl carboxylates are involved in both chelating and bridging coordination modes to the tin atoms to form an eight-membered cyclic structure. In all of these compounds, the acidic protons of the cyclopentadienyl groups are hydrogen bonded to the carboxylate oxygens (C-HO) to form rich supramolecular assemblies. In addition to this, pi-pi, T-shaped, L-shaped, and side-to-face stacking interactions involving ferrocenyl groups also occur. Compound 6 shows an interesting and novel intermolecular CO2-pi stacking interaction. Electrochemical analysis of the compounds 1-4, 6, and 7 shows a single, quasi-reversible oxidation peak corresponding to the simultaneous oxidation of four, two, and one ferrocenyl substituents, respectively. Compound 5 shows two quasi-reversible oxidation peaks. This is attributed to the positional difference among the ferrocenyl substituents on the tin atom. Additionally, while compounds 2 and 4 are electrochemically quite robust and do not decompose even after ten continuous CV cycles, compounds 1, and 3, 5-7 start to show decomposition after five cycles.

Synthesis and electrochemical investigations of molecular architectures involving C60 and tetraphenylporphyrin as building blocks

A C60–porphyrin diad 3a has been prepared by connecting a zinc porphyrin and C60via nucleophilic cyclopropanation. The fullerene diad as well as the related precursor porphyrins 1a, 2a have been investigated by cyclic and differential pulse voltammetry in dichloromethane. Whereas no oxidation peaks were observed for C60, all compounds 1a–3a exhibit two successive, quasi-reversible oxidation peaks in terms of an EE mechanism. The peak potentials of all three species almost coincide, due to an oxidation of the porphyrin ring system in all cases. In the cathodic process, on the other hand, the compounds are reduced in quasi-reversible four (C60), two (1a,2a) and five (3a) one-electron transfers. The first two formal potentials of 3a (resulting from the C60 moiety) are shifted towards negative potentials by 40 mV, as compared to C60, which is typical for methanofullerenes. Aside from coulombic effects, there is almost no electronic interaction between the C60 and porphyrin parts in 3a. Furthermore, a mixed hexaadduct 4 of C60 with an octahedral addition pattern incorporating a zinc tetraphenylporphyrin has been synthesized.