Redox-active Moieties Research Articles

First synthesis and electronic properties of diphenothiazine dumbbells bridged by heterocycles

According to cyclic voltammetry, symmetrical dumbbell-shaped phenothiazine dyads bridged by heterocycles show intense electronic coupling between the redox-active phenothiazine moieties. Furthermore, the fluorescence of the pyridyl-bridged derivatives can be controlled by pH change giving reversibly switchable redox-active biselectrophore dyads.

Redox stability of neptunium(V) and neptunium(IV) in the presence of humic substances of varying functionality

Summary. The reducing properties of humic substances (humic acid (HA) and fulvic acid (FA)) of varying functionality towards Np(V) have been studied under anaerobic conditions between pH 3.5 and pH 9.0 in batch experiments. For Np redox speciation in solution solvent extraction, NIR absorption spectroscopy and ultrafiltration were applied. The reduction rate varied with type of humic substances, solution pH, HA to Np concentration ratio, and equilibration time. In comparison to natural humic substances, synthetic HA with designed redox properties lead to a stronger reduction of Np(V) to Np(IV). The reducing capacities of humic substances towards Np(V) could be correlated to their phenolic/acidic OH group content, which includes both hydroquinone-like moieties and non-quinoid phenols. By applying a synthetic HA with blocked phenolic/acidic OH groups, the dominance of phenolic/acidic OH groups as the redox-active moieties of humic substances was verified. The Np(IV) formed in the course of the experiments is predominantly humic colloid-bound. Np(IV) oxo/hydroxide colloids, that might be formed in addition, are stabilized by adsorbed humic substances. The remaining Np(V) occurs as NpO2 + ion or Np(V) humate depending on pH. The ability of synthetic HA to effectively maintain Np in the tetravalent state during humate complexation experiments could be shown.

Redox-Active Cellulose Langmuir−Blodgett Films Containing β-Carotene as a Molecular Wire

Redox-active Langmuir-Blodgett (LB) films containing dihydrophytyl ferrocenoate (DFc) and beta-carotene (betaC) were fabricated by use of 6-O-dihydrophytylcellulose (DHPC) as a matrix. A mixture of DFc-DHPC formed a stable monolayer. Atomic force microscopy images revealed that the DFc molecules were dispersed uniformly throughout the surface in the ratio DFc:DHPC = 2:8 at 30 mN m-1. The DFc-DHPC monolayer was transferred successfully onto a substrate, yielding Y-type LB films. Cyclic voltammograms for the DFc-DHPC LB films on an indium tin oxide (ITO) electrode exhibited a well-defined surface wave. The voltammograms of the DFc-DHPC LB films exhibited 60-40% redox-active ferrocene moieties, whereas those of the DFc-DHPC-betaC LB films exhibited 90-70%. X-ray diffraction patterns indicated that the distance between layers was independent of betaC molecules incorporated into the LB films. Consequently, these results suggested that betaC can function as a molecular wire.

Oxyhomologation of the Amide Bond Potentiates Neuroprotective Effects of the EndolipidN-Palmitoylethanolamine

The endolipid N-palmitoylethanolamine (PEA) shows a pleiotropic pattern of bioactivities, whose mechanistic characterization is still unclear and whose pharmacological potential is substantially limited by rapid metabolization by the amido hydrolyzing enzymes fatty acid amide hydrolases and N-acylethanolamine-hydrolyzing acid amidase. To overcome this problem, we have synthesized a new series of PEA homologs and characterized their activity on two in vitro models of neurodegeneration (oxidative stress, excitotoxicity). PEA partially prevented tert-butylhydroperoxide (t-BOOH; 100 microM; 3 h)-induced cell death (maximal effect, 26.3 +/- 7.5% in comparison with t-BOOH-untreated cells at 30 microM), whereas it was ineffective against the L-glutamate (1 mM; 24 h)-induced excitotoxicity at all concentrations tested (0.01-30 microM). Oxyhomologation of the amide bond, although leading to an increased enzymatic stability, also potentiated neuroprotective activity, especially for N-palmitoyl-N-(2-hydroxyethyl)hydroxylamine (EC(50) = 2.1 microM). These effects were not mediated by cannabinoid/vanilloid-dependent mechanisms but rather linked to a decreased t-BOOH-induced lipoperoxidation and reactive oxygen species formation and L-glutamate-induced intracellular Ca(2+) overload. The presence of the hydroxamic group and the absence of either redox active or radical scavenger moieties suggest that the improved neuroprotection is the result of increased metal-chelating properties that boost the antioxidant activity of these compounds.

Aqueous Oxidation of Substituted Dihydroxybenzenes by Substituted Benzoquinones

Strict anaerobic techniques, HPLC, and spectrophotometry are employed to explore rates of reaction between a series of substituted benzoquinone oxidants and substituted dihydroxybenzene reductants, which represent important redox-active moieties within natural organic matter (NOM). Benzoquinones and dihydroxybenzenes that lack electron-withdrawing substituents exhibit reversible reactions within the acidic range of natural waters. Initial rates for reversible reactions are proportional to [H+]-1, attributable to the greater reactivity of monoprotonated versus diprotonated dihydroxybenzene molecules. Reversible reactions are generally faster for pairs having higher thermodynamic driving force. Concentrations in reversible reactions eventually reach plateau values, which coincide with expected values calculated using standard reduction potentials. If a reactant benzoquinone possesses an electron-withdrawing substituent, reaction progress falls short of expected values. If a product benzoquinone possesses an electron-withdrawing substituent, reaction progress extends beyond what is thermodynamically predicted. Electron-withdrawing substituents raise the susceptibility of benzoquinones to side reactions such as the Michael addition reaction.

Toward the Development of Molecular Wires: Ruthenium(II) Terpyridine Complexes Containing Polyferrocenyl as a Spacer

The preparations of multinuclear supramolecules assembled from 1,1‘-bis(terpyridyl)ferrocene, 1,1‘-bis(terpyridyl)biferrocene, and 1,1‘-bis(terpyridyl)triferrocene (tpy-(fc)n-tpy, n = 1−3) redox-active moieties with Ru2+ metal centers are described. The electrochemical measurements of the Ru2+ complexes of tpy-(fc)n-tpy (1a (n = 1); 1b (n = 2); 1c (n = 3)) are dominated by the Ru2+/Ru3+ redox couple (E1/2 from 1.35 to 1.38 V), Fe2+/Fe3+ redox couples (E1/2 from ∼0.4 to ∼1.0 V), and tpy/tpy-/tpy2- redox couples (E1/2 from −1.3 to −1.5 V). The appreciable variations detected in the Fe2+/Fe3+ oxidation potentials indicate that there is an interaction between the spacer and the Ru2+ metal centers. Coordination of Ru2+ metal centers to tpy-(fc)n-tpy results in a red-shifted and more intense 1[(d(π)Fe)6] → 1[(d(π)Fe)5(π*tpyRu)1] transition in the visible region. The observed red-shifted absorption from 526 nm in the monomeric [Ru(fctpy)2]2+ complex to ∼560 nm in 1b and 1c reveals that there is a qualitative electronic coupling within the ferrocenyl array. The Fe−Fe interactions result in a red characteristic of the 1[(d(π)Fe)6] → 1[(d(π)Fe)5(π*tpyRu)1] MMLCT transition.

Supramolecular Control of Oligothienylenevinylene−Fullerene Interactions: Evidence for a Ground-State EDA Complex

Complementary hydrogen-bonding interactions between a barbituric acid-substituted fullerene derivative (1) and corresponding receptor (2) bearing thienylenevinylene units are used to assemble a 1:1 supramolecular complex (K = 5500 M(-1)). Due to the close proximity of the redox-active moieties within the assembly, strong ground-state electron-donor-acceptor interactions are observed. Photoinduced electron transfer from electron-rich thienylenevinylene subunits to the fullerene is very fast (k(et) = 5.5 x 10(12) s(-1)), as determined by fs-time-resolved transient absorption spectroscopy. [reaction: see text]

Comparison of Thermal and Optical Electron-Transfer Barriers in Ruthenium Redox Polyether Melts

Combining redox active moieties with polyether oligomers produces amorphous, highly viscous, room-temperature molten materials notable for their high concentrations of redox sites that in mixed valent form exhibit facile electron-transfer chemistry. This paper describes experimental determinations and comparison of thermal and optical energy barriers for electron transfers in a specific redox polyether melt, namely the metal complex Ru(bpy350)2(CN)2 where bpy350 is 4,4‘-dimethyl-2,2‘-dipyridyl alkyl-bonded to 350 MW polyethylene glycol monomethyl ether. Electrochemical measurements provide apparent electron self-exchange rate constants (kEX) within the Ru(III/II) form of the melt and physical diffusion rates of perchlorate ion (DClO4), along with the associated thermal activation barrier energies. Plasticization by CO2 induced swelling of the melt results in enhanced mass and charge transport rates and decreasing energy barriers for both. Optical spectra of the same, electrochemically generated, mixed valent melt display an absorbance interpreted as an intermolecular Ru(II→III) intervalence charge transfer band. The energy barrier results show that the observed thermal electron transfer energy barrier is only an apparent one, being instead controlled by ion atmosphere relaxation processes, whereas the optical energy barrier corresponds to that of the actual electron-transfer process itself.

Electrochemical Synthesis of Pyridinium-Conjugated Assembly based on Nucleophilic Substitution of Pyrene/Perylene π-Radical Cation

Efficient and selective N-arylation of pyridine derivatives was demonstrated as nucleophilic substitution reaction of electrochemically generated π-radical cations such as pyrene and perylene. 1-Methylimidazole also worked as a nucleophile toward pyrene π-radical cation to give the imidazorium. This reaction provides a widely applicable and powerful method for pyridinium-conjugated assembly in which redox-active pyridinium moieties can be integrated through π-conjugated system. Preparative scale synthesis was demonstrated successfully by using perylene π-radical cation prepared by chemical oxidation with I2/AgClO4. The resulting pyridinium-conjugated assembly shows both one-electron reduction assigned to the pyridium moiety and one-electron oxidation of the pyrene and perylene moieties. UV-visible absorption and fluorescence spectra indicated intramolecular charge transfer character from the pyrenyl and perylenyl moieties to the pyridinium ones of the pyridinium-conjugated assembly.

Influence of surface composition on the kinetics of alachlor reduction by iron pyrite.

Even though naturally occurring iron sulfide minerals have previously been shown capable of promoting reductive dehalogenations, the role of surface composition has not been fully investigated. Some researchers have proposed that sulfur species represent redox-active moieties on iron pyrite surfaces. Results from this study indicate that neitherthe stoichiometric (100) pyrite surface, nor monosulfide defects, play direct roles in the observed reduction of the herbicide alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl)acetanilide]. Pyrite surfaces were initially characterized by X-ray photoelectron spectroscopy (XPS); the samples were then transferred to a liquid cell coupled to the ultrahigh vacuum chamber. Aliquots were periodically removed from the liquid cell to monitor the appearance of the reductive dechlorination product, 2',6'-diethyl-N-(methoxymethyl)acetanilide. In experiments with unaltered pyrite (100) surfaces, rates of reaction decreased over time, even though no change in surface composition could be discerned via XPS. In contrast, ion-bombarded surfaces, which are dominated by monosulfide species, exhibit an initial induction period of low reactivity during which the highly defective surface is oxidized by water. Only after the monosulfide defects undergo oxidation does the rate of alachlor reduction increase, precluding a direct role for these defects as alachlor reductants.

Electrochemical Properties of Natural Organic Matter (NOM), Fractions of NOM, and Model Biogeochemical Electron Shuttles

In this study, cyclic voltammetry was used to characterize the redox properties of natural organic matter (NOM). Using a stationary platinum working electrode, minimal concentrations of electrolyte, and dimethyl sulfoxide (DMSO) as the solvent, we were able to resolve two pairs of oxidation and reduction peaks for a fraction of Georgetown NOM that is enriched in polyphenolic moieties (NOM-PP). Applying our method to other fractions of Georgetown NOM, and to samples of NOM from a wide range of other sources, gave cyclic voltammograms (CVs) that generally contained fewer distinguishing features than those obtained with NOM-PP. For comparison, CVs were also obtained using our method on six quinone model compounds: anthraquinone-2,6-disulfonate (AQDS), lawsone, juglone, menadione, menaquinone-4, and ubiquinone-5. The CVs of these quinones were similar in shape to the CV of NOM-PP, consistent with the notion that quinones are the dominant redox-active moieties associated with NOM. Quantitative analysis of the peaks in these CVs showed that the peak potentials (Ep) were separated by more than 0.059 V and that the peak currents (i(p)) were linearly related to the square root of the scan rate (v0.5) and concentration (C) for both NOM-PP and the model quinones. Equivalent results were obtained with a rotating Pt disk electrode. From this we conclude that NOM-PP and the model quinones undergo similar sequences of two one-electron, quasi-reversible, diffusion controlled, electron transfers at the Pt electrode surface in DMSO. Although it is difficult to relate these results to Nernstian standard potentials vs the standard hydrogen electrode (SHE) under aqueous conditions, it is clear that the apparent formal potential for NOM-PP lies between the corresponding potentials for menadione and juglone and well above that of AQDS. Attempts to derive correlations between Ep and i(p) for the NOMs with quantifiable electrode response and other measurable properties of NOM (including trace metal content and UV-vis absorbance) did not yield any strong relationships.

Redox Chemistry of [Co4(CO)3(μ3-CO)3(μ3-C7H7)(η5-C7H9)] – Reversible Carbon−Carbon Coupling versus Metal Cluster Degradation

Chemical reduction of the tetracobalt cluster complex [Co4(CO)3(μ3-CO)3(μ3-C7H7)(η5-C7H9)] (3), followed by addition of [PPh4]Br, gives the complex [{Co4(CO)3(μ3-CO)3(μ3-C7H7)}2{μ-η4:η4-(C7H9)2}]2− as a mixture of two diastereomers [4A]2− and [4B]2− in high yield. The crystal structure of [4A]2−[PPh4]2·1.5C7H8 has been determined and confirms the reductive coupling of two Co4 cluster coordinated apical cycloheptadienyl rings to form a bridging bicycloheptyl-3,5,3′,5′-tetraene ligand. Reduction of 3 with Li[HBEt3] and subsequent treatment with aqueous [NnBu4]Cl results in the formation of [Co4(CO)3(μ3-CO)3(μ3-C7H7)(C7H10)]− [5]−. Addition of [(η-C6H6)Ru(NCMe)3][BF4]2 after the borohydride reduction gives [Ru(η-C6H6)Co3(CO)3(μ3-CO)3(μ3-C7H7)] (6), a product derived from reductive Co4 cluster degradation. A detailed electrochemical and spectro-electrochemical study of the redox behaviour of 3 and [4]2− has been carried out. The complex potential current response of 3 is rationalized in terms of the formation of the radical anion [3]− as the primary intermediate, which may be reversibly reduced further to give the much more stable [3]2− and then [3]3−. Dimerization of [3]− to give [4]2− occurs by formation of a new carbon−carbon bond between the apical C7H9 ligands. The two redox-active moieties in [4]2− behave as independent, non-interacting redox centres. The oxidized form 4 is unstable and dissociates back to 3 almost quantitatively, thus completing a redox cycle characteristic of a “molecular battery”. The homogeneous rate constant for dimerization has been evaluated as kDIM (2 [3]− [4]2−) = 0.30 ± 0.05 mM−1 s−1.

Mixed Ferrocene–Cobaltocenium Dendrimers: The Most Stable Organometallic Redox Systems Combined in a Dendritic Molecule

Novel peripherally heterogeneous dendrimers in which both ferrocene and cobaltocenium units are attached to the surface of poly(propylene imine) dendritic frameworks (see picture) were prepared. The presence of the two redox-active moieties allows the heterometallic dendrimers to be used as novel types of mediator in amperometric biosensors.

Use of the cyclodextrin unit for the preparation of Langmuir-Blodgett films of tetrathiafulvalene derivatives

The cyclodextrin fragment was persubstituted with pendant redox-active tetrathiafulvalene moieties. These molecular assemblies allow an efficient access to Langmuir-Blodgett films of tetrathiafulvalene derivatives, independently of their hydrophobic character.

Synthesis and electrochemical polymerization of calix[4]arenes containing N-substituted pyrrole moieties

Attaching a redox active moiety, such as ferrocene or quinone, in close proximity to the receptor site of a macrocyclic molecule has been shown to be an efficient way to couple complexation and redox reactions [1,2]. These coupled systems have been utilized to recognize or sense electrochemically various guest species [3-5]. They have the ability to control the stability of a complex formed between the receptor and a charged guest species through the oxidation or reduction of the redox centre via electrochemical means [6,7]. While these successes have been achieved for redox-active macrocyclic molecules in solution, it is a challenge to repeat these results for such molecules immobilized on an electrode surface, which will allow better control and wider applications in the field of chemical sensor technology. For instance, chemically bonding redox-active macrocyclic molecules onto an inert electrode surface or incorporating them into a conducting polymer matrix can provide great convenience in fabricating an electrochemical sensor [8]. In this article we have appended redox-active pyrrole moieties to a calix[4]arene in an effort to produce a novel polypyrrole-calix[4]arene modified electrode via electropolymerization techniques.

Synthesis and properties of viologen functionalized poly(3‐alkylthienylenes)

Viologen functionalized conducting polymers synthesized from the substituted monomers whon in the figure are interesting materials with possible applications in redox or electrocatalysis, sensors, and electrochromic devices. Monomers and polymers with various numbers of methylene groups separating the thiophene units and the redoxactive viologen moieties have been synthesized and characterized.magnified image

Synthesis of unsymmetrical 4,4′-disubstituted 2,2′-bipyridines containing benzo crown ether and ferrocene moieties

A simple one-pot synthesis of unsymmetrical 4,4′-disubstituted 2,2′-bipyridine ligands containing benzo crown ether and redox-active ferrocene moieties is described.