Complexation of Pu(IV) with the natural siderophore desferrioxamine B and the redox properties of Pu(IV)(siderophore) complexes

The bioavailability and mobility of Pu species can be profoundly affected by siderophores and other oxygen-rich organic ligands. Pu(IV)(siderophore) complexes are generally soluble and may constitute with other soluble organo-Pu(IV) complexes the main fraction of soluble Pu(IV) in the environment. In order to understand the impact of siderophores on the behavior of Pu species, it is important to characterize the formation and redox behavior of Pu(siderophore) complexes. In this work, desferrioxamine B (DFO-B) was investigated for its capacity to bind Pu(IV) as a model siderophore and the properties of the complexes formed were characterized by optical spectroscopy measurements. In a 1:1 Pu(IV)/DFO-B ratio, the complexes Pu(IV)(H2DFO-B)4+, Pu(IV)(H1DFO-B)3+, Pu(IV)(DFO-B)2+, and Pu(IV)(DFO-B)(OH)+ form with corresponding thermodynamic stability constants log beta1,1,2 = 35.48, log beta1,1,1 = 34.87, log beta1,1,0 = 33.98, and log beta1,1,-1 = 27.33, respectively. In the presence of excess DFO-B, the complex Pu(IV)H2(DFO-B)22+ forms with the formation constant log beta2,1,2 = 62.30. The redox potential of the complex Pu(IV)H2(DFO-B)22+ was determined by cyclic voltammetry to be E1/2 = -0.509 V, and the redox potential of the complex Pu(IV)(DFO-B)2+ was estimated to be E1/2 = -0.269 V. The redox properties of Pu(IV)(DFO-B)2+ complexes indicate that Pu(III)(siderophore) complexes are more than 20 orders of magnitude less stable than their Pu(IV) analogues. This indicates that under reducing conditions, stable Pu(siderophore) complexes are unlikely to persist.     

Merging cobalt catalysis and electrochemistry in organic synthesis

There is an increasing demand of using the low-cost and sustainable cobalt to replace its noble congeners(rhodium and iridium) as reflected by the recent upsurge of cobalt catalysis in the diverse organic transformations.Since all the redox reactivity of cobalt catalysis highly relies on the capability of the interconversion between their oxidation states(most frequently+1,+2 and+3),electrochemistry perfectly meets such a require ment owing to its outstanding perfo rmance in the redox manipulation.In this review,we highlight the recent advances in the merger of cobalt catalysis and electrochemistry in organic synthesis.     

Synthesis of 1,3‐Bis(tetracyano‐2‐azulenyl‐3‐butadienyl)azulenes by the [2+2] Cycloaddition–Retroelectrocyclization of 1,3‐Bis(azulenylethynyl)azulenes with Tetracyanoethylene

1,3‐Bis(azulenylethynyl)azulene derivatives 9–14 have been prepared by palladium‐catalyzed alkynylation of 1‐ethynylazulene 8 with 1,3‐diiodoazulene 1 or 1,3‐diethynylazulene 2 with the corresponding haloazulenes 3–7 under Sonogashira–Hagihara conditions. Bis(alkynes) 9–14 reacted with tetracyanoethylene (TCNE) in a formal [2+2] cycloaddition–retroelectrocyclization reaction to afford the corresponding new bis(tetracyanobutadiene)s (bis(TCBDs)) 15–20 in excellent yields. The redox behavior of bis(TCBD)s 15–20 was examined by using CV and differential pulse voltammetry (DPV), which revealed their reversible multistage reduction properties under the electrochemical conditions. Moreover, a significant color change of alkynes 9–14 and TCBDs 15–20 was observed by visible spectroscopy under the electrochemical reduction conditions.     

Near-Infrared Electrochromic Behavior of Dibenzothiepin Derivatives Attached with Two Michler's Hydrol Blue Units

10,11-Bis[bis(4-dimethylaminophenyl)methylene]dibenzo[bf]thiepin ( 1 ) and -oxepin ( 2 ) were prepared as stable yellow crystalline compounds, which are the cyclic analogues of electron-donating hexaarylbutadienes. Upon two-electron oxidation, they are reversibly transformed into the title dications ( 1 ²⁺ and 2 ²⁺) exhibiting near-infrared (NIR) absorptions, which were also isolated as stable salts. These redox pairs can serve as new entries into less well-explored organic NIR-electrochromic systems, and the separation of redox peaks (electrochemical bistability) was attained for 1 / 1 ²⁺ and 2 / 2 ²⁺, thanks to drastic geometrical changes between neutral and dicationic states, as revealed by a series of X-ray analyses. Thiepin-S,S-dioxide analogue ( 3 / 3 ²⁺) exhibits quite similar dynamic redox behavior due to nonaromatic nature of the dibenzothiepin and -oxepin unit in 1 ²⁺ and 2 ²⁺, whereas the thiepin-S-oxide derivative ( 4 / 4 ²⁺) does not exhibit bistability due to the smaller change in geometry upon electron transfer, showing that a subtle change of a bridging atom in the central seven-membered ring can modify the redox properties.     

Polymeric Electron Carriers

Polymers containing 1,4-dihydronicotinamide (P-NAH) alloxan (P-A), and viologen (P-V²⁺) moieties were synthesized and characterized. P-NAH reduced various organic substances such as lipoic acid, alloxan, and viologens and also immobilized quinone mediated by alloxan. P-A was reduced to the polymer-bearing alloxan radical and the dialuric acid structure without crosslinks by one- and two-electron reduction, respectively, and P-A also mediated the redox reaction occurring between aqueous and organic (water-immiscible) layers. P-V²⁺ was converted to the stable viologen radical reversibly by one-electron reduction. Electric potentials and currents on photo-reduction of P-V²⁺ and catalytic behavior of P-V²⁺ in the reduction of carbonyl compounds were examined.     

Quantum-chemical predictions of absolute standard redox potentials of diverse organic molecules and free radicals in acetonitrile

A calibrated B3LYP/6-311++G(2df,2p)//B3LYP/6-31+G(d) method was found to be able to predict the gas-phase adiabatic ionization potentials of 160 structurally unrelated organic molecules with a precision of 0.14 eV. A PCM solvation model was benchmarked that could predict the pK(a)'s of 15 organic acids in acetonitrile with a precision of 1.0 pK(a) unit. Combining the above two methods, we developed a generally applicable protocol that could successfully predict the standard redox potentials of 270 structurally unrelated organic molecules in acetonitrile. The standard deviation of the predictions was 0.17 V. The study demonstrated that computational electrochemistry could become a powerful tool for the organic chemical community. It also confirmed that the continuum solvation theory could correctly predict the solvation energies of organic radicals. Finally, with the help of the newly developed protocol we were able to establish a scale of standard redox potentials for diverse types of organic free radicals for the first time. Knowledge about these redox potentials should be of great value for understanding the numerous electron-transfer reactions in organic and bioorganic chemistry.     

Synthesis and Cleavage Studies of a 1,2-Dioxolane-type Peroxide

A [1,2]dioxolane-type peroxide was synthesized and tested for its cleavage behavior with Fe^2 -cysteinate as a simple model of biological redox species. No S-alkylation product was observed.     

Redox-Reversible 2D Metal–Organic Framework Nanosheets (MONs) Based on the Hydroquinone/Quinone Couple

2D metal–organic nanosheets (MONs), akin to graphene, have aroused immense contemporary interest. In our quest to develop functional 2D MONs based on organic linkers designed de novo, we reasoned that benzene-tetrabenzoic acid, which has been exploited tremendously in the construction of pillared metal–organic frameworks (MOFs), could be maneuvered readily to access redox-active MONs based on the benzoquinone/hydroquinone redox couple. Herein, we show that the self-assembly of 2,3,5,6-tetrakis(p-carboxyphenyl)hydroquinone H₄BTA with Zn(NO₃)₂ does lead to 2D metal–organic nanosheets that stack down the y axis, affording a layered Zn MOF. Although the crystals of the latter do not exhibit a discernible chemically induced redox switching behavior, the 2D MONs accessed by ultrasound-induced liquid-phase exfoliation (UILPE) lend themselves to a facile redox switching behavior. Treatment of a dispersion of the 2D MONs in methanol with phenyliodine(III) diacetate (PIDA) results in the oxidation of the hydroquinone core to benzoquinone. Remarkably, the latter can be reverted to the former by treatment with ascorbic acid as a reducing agent; indeed, the redox process can be made out by the naked eye. The results constitute the first example of chemically induced redox switching of 2D MONs. In view of emergent applications of 2D materials in general and MONs in particular, for example, improvement of the performance of membranes in separations by doping with MONs, the redox-switchable property may lead to the development of unique materials with heretofore unexplored potential.     

苄基紫精在玻碳电极上的电化学行为

本文用循环伏安法研究了苄基紫精(BV~(2+))在玻碳(GC)电极上的电化学行为。BV~(2+)在未活化的GC电极上的电极过程强烈地依赖于BV~(2+)浓度、扫描速度和电位范围。BV~(2+)在活化的GC电极上的吸附能力大为增强,当其浓度低时只表现出吸附BV~(2+)的电极过程,在高浓度下吸附BV~(2+)与溶液中的BV~(2+)同时参与电极过程。讨论了GC电极活化的可能影响。     

Is trace metal release in wetland soils controlled by organic matter mobility or Fe-oxyhydroxides reduction?

Aerobic and anaerobic incubation experiments on a wetland soil samples were used to assess the respective roles of organic matter (OM) release, Fe-oxyhydroxides reduction and redox/speciation changes on trace metal mobility during soil reduction. Significant amounts of Cu, Cr, Co, Ni, Pb, U, Th and Rare Earth Elements (REE) were released during anaerobic incubation, and were accompanied by strong Fe(II) and dissolved organic matter (DOM) release. Aerobic incubation at pH 7 also resulted in significant trace metal and DOM release, suggesting that Fe-oxyhydroxide reduction is not the sole mechanism controlling trace metal mobility during soil reduction. Using these results and redox/speciation modeling, four types of trace metal behavior were identified: (i) metals bound to organic matter (OM) and released by DOM release (REE); (ii) metals bound to both OM and Fe-oxyhydroxides, and released by the combined effect of DOM release and Fe(III) reduction (Pb and Ni); (iii) metals bound solely to soil Fe-oxyhydroxides and released by its reductive dissolution (Co); and (iv) metals for which release mechanisms are unclear because their behavior upon reduction is affected by changes in redox state and/or solution speciation (Cu, Cr, U and Th). Even though the process of soil Fe-oxyhydroxide reduction is important in controlling metal mobility in wetland soils, the present study showed that the dominant mechanism for this process is OM release. Thus, OM should be systematically monitored in experimental studies dedicated to understand trace metal mobility in wetland soils. Due to the fact that the process of OM release is mainly controlled by pH variations, the pH is a more crucial parameter than Eh for metal mobility in wetland soils.     

Anion-Dependent Redox Chemistry of p-Type Poly(vinyldimethylphenazine) Cathode Materials

Metal-free organic electrode materials have attracted vast research attention owing to their designable structures and tunable electrochemical properties. Although n-type cathode materials could be used in various metal-ion batteries, p-type ones with high potential can deliver high energy density. Herein, we report a new p-type polymeric cathode material, poly(2-vinyl-5,10-dimethyl-dihydrophenazine) (PVDMP), with a theoretical capacity of 227 mAh g⁻¹. PVDMP featuring two-step redox reaction will be doped by two anions to maintain electroneutrality during oxidation, which resulted in an anion-dependent electrochemical behavior of PVDMP-based cathode. The suitable dopant anion for PVDMP was selected and the doping mechanism was confirmed. Under the optimized condition, PVDMP cathode can deliver a high initial capacity of 220 mAh g⁻¹ at 5 C and even remains 150 mAh g⁻¹ after 3900 cycles. This work not only provides a new kind of p-type organic cathode materials but also deepens the understanding of its anion-dependent redox chemistry.     

Comparative study of the thermodynamics and kinetics of the ion transfer across the liquid/liquid interface by means of three-phase electrodes

A comparative study of the behavior of different sorts of three-phase electrodes applied for assessing the thermodynamics and kinetics of the ion transfer across the liquid/liquid (L/L) interface is presented. Two types of three-phase electrodes are compared, that is, a paraffin-impregnated graphite electrode at the surface of which a macroscopic droplet of an organic solvent is attached and an edge pyrolytic graphite electrode partly covered with a very thin film of the organic solvent. The organic solvent contains either decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato) as a redox probe. The role of the redox probe, the type of the electrode material, the mass transfer regime, and the effect of the uncompensated resistance are discussed. The overall electrochemical process at both three-phase electrodes proceeds as a coupled electron-ion transfer reaction. The ion transfer across the L/L interface, driven by the electrode reaction of the redox compound at the electrode/organic solvent interface, is independent of the type of redox probe. The ion transfer proceeds without involving any chemical coupling between the transferring ion and the redox probe. Both types of three-phase electrodes provide consistent results when applied for measuring the energy of the ion transfer. Under conditions of square-wave voltammetry, the coupled electron-ion transfer at the three-phase electrode is a quasireversible process, exhibiting the property known as "quasireversible maximum". The overall electron-ion transfer process at the three-phase electrode is controlled by the rate of the ion transfer. It is demonstrated for the first time that the three-phase electrode in combination with the quasireversible maximum is a new tool for assessing the kinetics of the ion transfer across the L/L interface.     

Light- and redox-dependent thermal stability of the reaction center of the photosynthetic Bacterium rhodobacter sphaeroides

Irreversible loss of the photochemical activity and damage of the pigments (bacteriochlorophyll [Bchl] monomer, Bchl dimer [P] and bacteriopheophytin) by combined treatment with intense and continuous visible light and elevated temperature have been studied in a deoxygenated solution of reaction center (RC) protein from the nonsulfur purple photosynthetic bacterium Rhodobacter sphaeroides. Both the fraction of RC in the charge-separated redox state (P+Q-, where Q is a quinone electron acceptor) and the degradation of the pigments showed saturation as a function of increasing light intensity up to 400 mW cm(-2) (488/515 nm) or 1100 microE m(-2) s(-1) (white light). The thermal denaturation curves of the RC in the P+Q- redox state demonstrated broadening and 10-20 degrees C shift to lower temperature (after 30-90 min heat treatment) compared with those in the PQ redox state. Similar but less striking behavior was seen for RC of other redox states (P+Q and PQ-) generated either by light or by electrochemical treatment in the dark. These experiments suggest that it is not the intense light per se but the changes in the redox state of the protein that are responsible for the increased sensitivity to photo- and heat damage. The RC with a charge pair (P+Q-) is more vulnerable to elevated temperature than the RC with (P+Q or PQ-) or without (PQ) a single charge. To reveal both the thermodynamic and kinetic aspects of the denaturation, a simple three-state model of coupled reversible thermal and irreversible kinetic transitions is presented. These effects may have relevance to the heat stability of other redox proteins in bioenergetics.     

Bis(Iminophosphorano)-Substituted Pyridinium Ions and their Corresponding Bispyridinylidene Organic Electron Donors

Optimized synthetic procedures for pyridinium ions featuring iminophosphorano (−N=PR₃; R=Ph, Cy) π-donor substituents in the 2- and 4- positions are described. Crystallographic and theoretical studies reveal that the strongly donating substituents severely polarize the π-electrons of the pyridyl ring at the expense of aromaticity. Moreover, the pyridinium ions are readily deprotonated to generate powerful bispyridinylidene (BPY) organic electron donors. Electrochemical studies show exceptionally low redox potentials for the two-electron BPY/BPY²⁺ couples, ranging from −1.71 V vs the saturated calomel electrode for 3PhPh (with four Ph₃P=N− groups) to −1.85 V for 3CyCy (with four Cy₃P=N− groups). These new compounds represent the most reducing neutral organic electron donors (OEDs) currently known. Some preliminary reductions involving 3CyCy showed enhanced capability owing to its low redox potential, such as the thermally activated reduction of an aryl chloride, but purification challenges were often encountered.     

A Dual Threat: Redox-Activity and Electronic Structures of Well-Defined Donor–Acceptor Fulleretic Covalent-Organic Materials

The effect of donor (D)–acceptor (A) alignment on the materials electronic structure was probed for the first time using novel purely organic porous crystalline materials with covalently bound two- and three-dimensional acceptors. The first studies towards estimation of charge transfer rates as a function of acceptor stacking are in line with the experimentally observed drastic, eight-fold conductivity enhancement. The first evaluation of redox behavior of buckyball- or tetracyanoquinodimethane-integrated crystalline was conducted. In parallel with tailoring the D-A alignment responsible for “static” changes in materials properties, an external stimulus was applied for “dynamic” control of the electronic profiles. Overall, the presented D–A strategic design, with stimuli-controlled electronic behavior, redox activity, and modularity could be used as a blueprint for the development of electroactive and conductive multidimensional and multifunctional crystalline porous materials.     

Cover Feature: Redox-Active Guanidines in Proton-Coupled Electron-Transfer Reactions: Real Alternatives to Benzoquinones? (Chem. Eur. J. 70/2019)

Guanidino-functionalized aromatics (GFAs) are readily available, stable organic redox-active compounds. In this work we apply one particular GFA compound, 1,2,4,5-tetrakis(tetramethylguanidino)benzene, in its oxidized form in a variety of oxidation/oxidative coupling reactions to demonstrate the scope of its proton-coupled electron transfer (PCET) reactivity. Addition of an excess of acid boosts its oxidation power, enabling the oxidative coupling of substrates with redox potentials of at least +0.77 V vs. Fc⁺/Fc. The green recyclability by catalytic re-oxidation with dioxygen is also shown. Finally, a direct comparison indicates that GFAs are real alternatives to toxic halo- or cyano-substituted benzoquinones.     

Electrochemical Properties of an Osmium(II) Copolymer Film and Its Electrocatalytic Ability Towards the Oxidation of Ascorbic Acid in Acidic and Neutral pH

Copolymerization of an osmium(II) functionalized pyrrole moiety, osmium‐bis‐N,N'‐(2,2′‐bipyridyl)‐N‐(pyridine‐4‐ylmethyl‐(8‐pyrrole‐1yl–octyl)‐amine)chloride ( I ) with 3‐methylthiophene was carried out. The resulting conducting polymer film exhibited a clear redox couple associated with the Os^3+/2+ response and the familiar conducting polymer backbone signature. The effect of film thickness upon the redox properties of the copolymer was investigated in organic electrolyte solutions. Scanning electron micrographs (SEM) along with energy dispersive X‐ray (EDX) spectra of the copolymerized films were undertaken, both after formation and redox cycling in neutral buffer solution. These clearly show that electrolyte is incorporated into the polymer film upon redox cycling through the Os^3+/2+ redox system. The Os^3+/2+ response associated with the copolymer was seen to be significantly altered in the presence of ascorbic acid both in acidic and neutral pH buffer solutions. This pointed to an electrocatalytic reaction between the ascorbic acid and the Os³⁺ form of the copolymer. Under acidic conditions the copolymer film exhibited a sensitivity of 1.76 (±0.05) μA/mM with a limit of detection (LOD) of 1.45 μM for ascorbic acid. Under neutral pH conditions the copolymer exhibited a sensitivity of 19.26 (±1.05) μA/mM with a limit of detection (LOD) of 1.28 μM for ascorbic acid.     

A Dual Threat: Redox‐Activity and Electronic Structures of Well‐Defined Donor–Acceptor Fulleretic Covalent‐Organic Materials

The effect of donor (D)–acceptor (A) alignment on the materials electronic structure was probed for the first time using novel purely organic porous crystalline materials with covalently bound two‐ and three‐dimensional acceptors. The first studies towards estimation of charge transfer rates as a function of acceptor stacking are in line with the experimentally observed drastic, eight‐fold conductivity enhancement. The first evaluation of redox behavior of buckyball‐ or tetracyanoquinodimethane‐integrated crystalline was conducted. In parallel with tailoring the D‐A alignment responsible for “static” changes in materials properties, an external stimulus was applied for “dynamic” control of the electronic profiles. Overall, the presented D–A strategic design, with stimuli‐controlled electronic behavior, redox activity, and modularity could be used as a blueprint for the development of electroactive and conductive multidimensional and multifunctional crystalline porous materials.     

腐殖质与铀和超铀元素相互作用的研究进展

腐殖质是一类广泛存在于自然界中的高分子有机化合物,能与放射性核素相互作用,从而影响其在自然环境中的化学形态、迁移沉降、氧化还原行为等。本文介绍了目前国内外关于腐殖质与铀和超铀元素相互作用的研究现状和进展,并对存在的问题及今后的发展方向进行了初步讨论。     

Elongated Thrombin Binding Aptamer: A G‐Quadruplex Cation‐Sensitive Conformational Switch

Aptamer‐based biosensors offer promising perspectives for high performance, specific detection of proteins. The thrombin binding aptamer (TBA) is a G‐quadruplex‐forming DNA sequence, which is frequently elongated at one end to increase its analytical performances in a biosensor configuration. Herein, we investigate how the elongation of TBA at its 5′ end affects its structure and stability. Circular dichroism spectroscopy shows that TBA folds in an antiparallel G‐quadruplex conformation with all studied cations (Ba²⁺, Ca²⁺, K⁺, Mg²⁺, Na⁺, NH₄⁺, Sr²⁺ and the [Ru(NH₃)₆]^2+/3+ redox marker) whereas other structures are adopted by the elongated aptamers in the presence of some of these cations. The stability of each structure is evaluated on the basis of UV spectroscopy melting curves. Thermal difference spectra confirm the quadruplex character of all conformations. The elongated sequences can adopt a parallel or an antiparallel structure, depending on the nature of the cation; this can potentially confer an ion‐sensitive switch behavior. This switch property is demonstrated with the frequently employed redox complex [Ru(NH₃)₆]³⁺, which induces the parallel conformation at very low concentrations (10 equiv per strand). The addition of large amounts of K⁺ reverts the conformation to the antiparallel form, and opens interesting perspectives for electrochemical biosensing or redox‐active responsive devices.