Oxidation States In Solution Research Articles

Electrochemical Cycling of Redox-Active Boron Cluster-Based Materials in the Solid State.

This work demonstrates the first successful electrochemical cycling of a redox-active boron cluster-based material in the solid state. Specifically, we designed and synthesized an ether-functionalized dodecaborate cluster, B12(OCH3)12, which is the smallest redox-active building block in the B12(OR)12 family. This species can reversibly access four oxidation states in solution, ranging from a dianion to a radical cation. We show that a chemically isolated and characterized neutral [B12(OCH3)12]0 cluster can be utilized as a cathode active material in a PEO-based rechargeable all-solid-state cell with a lithium metal anode. The cell exhibits an impressive active material utilization close to 95% at C/20 rate, a high Coulombic efficiency of 96%, and reversibility, with only 4% capacity fade after 16 days of cycling. This work represents a conceptual departure in the development of redox-active components for electrochemical storage and serves as an entry point to a broader class of borane-based materials.

What is the maximum charge uptake of Lindqvist-type polyoxovanadates in organic-inorganic heterostructures?

One of the striking characteristics of the tris(alkoxo)-ligated Lindqvist-type polyoxovanadates [VV6O13{(OCH2)3CR}2]2- in highest oxidation state in solution is the ease of their chemical post-functionalization via the R group. On surfaces it is their conductivity as a function of individual V(3d) redox states. In both cases, the structural stability of the fully-oxidized dianion is enabled by charge-balancing counterions. In this Article, we explore the charge stability and the charge distibution across the molecular Lindqvist-type hexavanadate structure regarding the R functionality (R = OC2H4N3, CH2N3, and O3C29H36N5) and the different type of countercations (Cat = K+, Li+, NH4+, H+, or Mg2+). We show that the hexavanadate core can accept in its vacant V(3d) orbitals at least four and, in some cases, up to nine additional electrons if the negative charge is offset by the corresponding cation(s), without electron leakage to the covalently attached R groups. Remarkably, the maximum number of accepted electrons strongly depends on the type of cation(s) and is independent on the type of the remote R group exploited herein. The (Cat)n[VV6O13{(OCH2)3CR}2] complexes exibit the structural integrity in all studied charged states. Our study demonstrates the importance of the countercations of multistate polyoxovanadate nanoswitches for the development of multi-charge based molecular memories and/or batteries.

Novel pyrene-BODIPY dyes based on cyclotriphosphazene scaffolds: Synthesis, photophysical and spectroelectrochemical properties

Two novel cyclotriphosphazene compounds (Cpz-2, 3) bearing mono- and distyryl(pyrene) borondipyrromethene (BODIPY) dyes (Bod-4, 5) were synthesized and characterized by using elemental analysis, FT-IR, MALDI-TOF, 1H NMR, 13C NMR and 31P NMR techniques. The photophysical properties of these novel compounds were investigated by UV-Vis, fluorescence spectroscopy, and electrochemical properties were investigated by CV and SWV measurements. Subsequently, electrochromic properties of novel cyclotriphosphazene compounds (Cpz-2, 3) and their precursor dyes (Bod-4, 5) were investigated via in-situ spectroelectrochemical measurements. According to results, they gave electrochemically manufactured types having various colors in their diverse oxidation states in solution, suggesting that they may have a significant role in development of the color exchange matter in electrochromic device technologies.

Exploring the Coordination of Plutonium and Mixed Plutonyl-Uranyl Complexes of Imidodiphosphinates.

The coordination chemistry of plutonium(IV) and plutonium(VI) with the complexing agents tetraphenyl and tetra-isopropyl imidodiphosphinate (TPIP- and TIPIP-) is reported. Treatment of sodium tetraphenylimidodiphosphinate (NaTPIP) and its related counterpart with peripheral isopropyl groups (NaTIPIP) with [NBu4]2[PuIV(NO3)6] yields the respective PuIV complexes [Pu(TPIP)3(NO3)] and [Pu(TIPIP)2(NO3)2] + [PuIV(TIPIP)3(NO3)]. Similarly, the reactions of NaTPIP and NaTIPIP with a Pu(VI) nitrate solution lead to the formation of [PuO2(HTIPIP)2(H2O)][NO3]2, which incorporates a protonated bidentate TIPIP- ligand, and [PuO2(TPIP)(HTPIP)(NO3)], where the protonated HTPIP ligand is bound in a monodentate fashion. Finally, a mixed U(VI)/Pu(VI) compound, [(UO2/PuO2)(TPIP)(HTPIP)(NO3)], is reported. All these actinyl complexes remain in the +VI oxidation state in solution over several weeks. The resultant complexes have been characterized using a combination of X-ray structural studies, NMR, optical, vibrational spectroscopies, and electrospray ionization mass spectrometry. The influence of the R-group (R = phenyl or iPr) on the nature of the complex is discussed with the help of DFT studies.

Chelation and stabilization of berkelium in oxidation state +IV.

Berkelium (Bk) has been predicted to be the only transplutonium element able to exhibit both +III and +IV oxidation states in solution, but evidence of a stable oxidized Bk chelate has so far remained elusive. Here we describe the stabilization of the heaviest 4+ ion of the periodic table, under mild aqueous conditions, using a siderophore derivative. The resulting Bk(IV) complex exhibits luminescence via sensitization through an intramolecular antenna effect. This neutral Bk(IV) coordination compound is not sequestered by the protein siderocalin-a mammalian metal transporter-in contrast to the negatively charged species obtained with neighbouring trivalent actinides americium, curium and californium (Cf). The corresponding Cf(III)-ligand-protein ternary adduct was characterized by X-ray diffraction analysis. Combined with theoretical predictions, these data add significant insight to the field of transplutonium chemistry, and may lead to innovative Bk separation and purification processes.

Spectroscopic Characterization of the 3+ and 2+ Oxidation States of Europium in a Macrocyclic Tetraglycinate Complex.

The 3+ and 2+ oxidation states of europium have drastically different magnetic and spectroscopic properties. Electrochemical measurements are often used to probe EuIII/II oxidation state changes, but a full suite of spectroscopic characterization is necessary to demonstrate conversion between these two oxidation states in solution. Here, we report the facile conversion of an europium(III) tetraglycinate complex into its EuII analogue. We present electrochemical, luminescence, electron paramagnetic resonance, UV-visible, and NMR spectroscopic data demonstrating complete reversibility from the reduction and oxidation of the 3+ and 2+ oxidation states, respectively. The EuII-containing analogue has kinetic stability within the range of clinically approved GdIII-containing complexes using an acid-catalyzed dissociation experiment. Additionally, we demonstrate that the 3+ and 2+ oxidation states provide redox-responsive behavior through chemical-exchange saturation transfer or proton relaxation, respectively. These results will be applicable to a wide range of redox-responsive contrast agents and Eu-containing complexes.

A spectrophotometric study of the reduction of Pu(VIII) and Pu(VII) in alkaline solutions

The reduction of Pu(VIII) with water in ozonized 15 and 6 MNaOH solutions after termination of the ozone bubbling was studied by spectrophotometry. Kinetic curves reflecting the whole set of processes that occur in NaOH solutions of different concentrations were obtained. Calculations performed with the experimental curves made it possible to construct the kinetic curves describing the behavior of plutonium in higher oxidation states in solution and to obtain the individual absorption spectra of Pu(VII) and Pu(VIII). The processes occurring in ozonized solutions of Pu(VI) in 6 and 15 M NaOH after termination of the ozone bubbling were found to occur by different mechanisms, yielding in 4 days Pu(VI) and Pu(V), respectively.

Temperature-dependent phosphorus precipitation and chromium removal from struvite-saturated solutions

The effect of temperature from 25 to 300°C on the precipitation of phosphorus (P) from struvite-saturated (MgNH4PO4⋅6H2O) solutions was explored. Scanning electron microscopy (SEM) revealed reduced particle size and a change in morphology from elongated to rhombohedral crystals with temperature. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) indicated that at 25°C, the precipitate was struvite, while newberyite (MgHPO4⋅3H2O) formed at 100°C, and magnesium pyrophosphate (Mg2P2O7) at 300°C. Increased temperature reduced the association of ammonium and water of crystallization with the solid and increased P polymerization. The behavior of dissolved chromium (Cr) under these conditions was also assessed. Removal of Cr with the solid phase from Cr(III) solutions was observed at all temperatures, whereas removal from Cr(VI) solutions was significant only at 300°C. X-ray absorption fine structure spectroscopy (XAFS) revealed that regardless of initial oxidation state in solution, Cr(III) was associated with the solid, interacting by the adsorption of short-range Cr polymers. Therefore, for struvite-saturated solutions, increasing the temperature changed both the mineralogy of the P phase recovered and enhanced the interaction of otherwise unreactive Cr(VI) with the substrate. These results have implications for the temperature-enhanced recovery of P from wastewater.

Investigation of the hydrolysis of plutonium(IV) by a combination of spectroscopy and redox potential measurements

The hydrolysis of tetravalent plutonium in 0.5 M HCl/NaCl has been investigated by a combination of spectroscopy and redox potential measurements in the concentration range from 10-5to 4×10-4M and at pHc0.3–2.1. Redox potentials were both measured and calculated from the plutonium oxidation state distributions using well-known equilibrium constants for the redox reactions of plutonium. The plutonium oxidation states in solution were quantified by spectroscopy. Thepevalues calculated from the plutonium redox couples Pu(V)/Pu(VI) and PuO2(am,hyd)/Pu(V) were found to be consistent with the measuredpevalues whereas those calculated from the Pu(III)/Pu(IV) couple were found to deviate, suggesting that the formation constants selected in recent reviews for the mononuclear hydroxide complexes of Pu(IV) are overestimated. Combining the redox potentials and the spectroscopically determined Pu(III) and Pu(IV) concentrations, the formation constants of the first, second, and third mononuclear hydroxide complexes Pu(OH)n4-natI=0.5 M and 23 ± 2 °C are calculated and corrected to zero ionic strength using the specific ion interaction theory (SIT): logβ°11= 14.0 ± 0.2, logβ°12= 26.8 ± 0.6, and logβ°13= 38.9 ± 0.9.

How to make big molecules fly out of liquid water: applications, features and physics of laser assisted liquid phase dispersion mass spectrometry

Applications, features, and mechanistic details of laser assisted liquid phase dispersion mass spectrometry are highlighted and discussed. It has been used in the past to directly isolate charged molecular aggregates from the liquid phase and to determine their molecular weight employing sensitive time-of-flight mass spectrometry. The liquid matrix in this MALDI (matrix assisted laser desorption and ionization) type approach consists of a 10 microm diameter free liquid filament in vacuum (or a free droplet) which is excited with a focused infrared laser pulse tuned to match the absorption frequency of the OH-stretch vibration of bulk water near 2.8 microm. Due to these features we will refer to the approach as free liquid matrix assisted laser dispersion of ions or ionic aggregates (IR-FL-MALDI), although also LILBID ("laser induced liquid beam (bead) desorption and ionization") has been proposed early as a descriptive acronym for the technique and may be used alternatively. Low-charge-state macromolecular adducts are isolated in the gas phase from solution via a yet poorly characterized mechanism which sensitively depends upon the laser intensity and wavelength, and after the gentle liquid-to-vacuum transfer the aggregates are analyzed via time-of-flight (TOF) mass spectrometry (MS). Possible mechanisms for the isolation and charging of biomolecules directly from liquid solution are discussed in the present contribution. Recent technical advances such as minimizing the sample consumption, strategies for high throughput mass spectrometry, and coupling of liquid beam MS with HPLC will be highlighted as well. An interesting feature of IR-FL-MALDI is what we call the linear response, i.e., a surprising linearity of the gas phase mass signal on the solution concentration over many orders of magnitude for a large number of biomolecular systems as well as ions. Due to these features the approach may be regarded as a true solution probing spectroscopy, which enables elegant biokinetic studies. Several experiments in which time resolved IR-FL-MALDI-MS has recently been employed successfully are given. A particular highlight is the possibility to quantitatively detect oxidation states in solution, which clearly distinguishes the present approach from other established MS source concepts. Due to the good matrix tolerance also proteins in complex mixtures can be monitored quantitatively.

A XANES study of Cu speciation in high-temperature brines using synthetic fluid inclusions

Cu K-edge X-ray absorption near edge structure (XANES) spectra were recorded from individual synthetic brine fluid inclusions as a function of temperature up to 500 °C. The inclusions serve as sample cells for high-temperature spectroscopic studies of aqueous Cu-Cl speciation. Cu + and Cu 2+ can both be identified from characteristic pre-edge features. Mixed oxidation states can be deconvoluted using linear combinations of Cu + and Cu 2+ spectra. This work illustrates how complex Cu XANES spectra can be interpreted successfully. Cu 2+ is the stable oxidation state in solution at room temperature and Cu + at high temperatures. The change in oxidation state with temperature was completely reversible. Cu + was found to occur exclusively as the linear species [CuCl 2 ] - in solutions containing KCl with Cu:Cl ratios up to 1:6. In the absence of K + , there is evidence for higher order coordination of Cu + , in particular the tetrahedral complex [CuCl 4 ] 3- . The importance of such complexes in natural ore-forming fluids is yet to be determined, but may explain the vapor-phase partitioning of Cu as a Cl complex from a Cl-rich brine.

XANES investigation of the Co oxidation state in solution and in cancer cells treated with Co(III) complexes

XANES spectroscopy has been used to investigate whether it is possible to determine the oxidation state and coordination environment of Co complexes following treatment of cancer cells with Co(III) or Co(II) complexes. Our results show that the variation of the XANES with coordination geometry make it impossible to do this in a completely reliable way which is in contrast to the situation for platinum and chromium. It was established that the XANES spectrum obtained from cells treated with [Co(diNOsar)]Br 3 remained unchanged with respect to its XANES spectrum obtained in solution, demonstrating that the [Co(diNOsar)]Br 3 complex remained intact after 24 h in cellular media (diNOsar = 1,8-dinitro-3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane). In contrast, the XANES spectra obtained from cells treated with Na[Co(acac) 3] and [Co(acac) 3] differed from the XANES spectra of the respective complexes obtained in solution, indicating a change in co-ordination environment for both complexes upon uptake in cells. The similarity of these spectra suggests that appearance of this XANES can be used as an indication of loss of the carrier ligands, a useful indicator in the study of hypoxia selective complexes. The results obtained for Na[Co(acac) 3] and [Co(acac) 3] are consistent with the intracellular coordination of cobalt(III) to sulfur ligands upon cellular uptake.

Lower Oxidation States of f Elements: I. Preparation and Identification of Actinides and Lanthanides in Lower Oxidation States

Studies on the title subject, published mainly in the second half of the XX century and made both with macroamounts of lanthanides (Ln) and actinides (An) and with micro- and ultramicroamounts of radionuclides of these elements, are considered. Procedures for preparing f elements in lower oxidation states in solutions, melts, and solid matrices and methods for identifying these states (electrochemistry, spectrometry, high-temperature extraction, cocrystallization, etc.) are discussed.

Heme Ligation and Conformational Plasticity in the Isolatedc Domain of Cytochrome cd1 Nitrite Reductase

The heme ligation in the isolated c domain of Paracoccus pantotrophus cytochrome cd(1) nitrite reductase has been characterized in both oxidation states in solution by NMR spectroscopy. In the reduced form, the heme ligands are His69-Met106, and the tertiary structure around the c heme is similar to that found in reduced crystals of intact cytochrome cd1 nitrite reductase. In the oxidized state, however, the structure of the isolated c domain is different from the structure seen in oxidized crystals of intact cytochrome cd1, where the c heme ligands are His69-His17. An equilibrium mixture of heme ligands is present in isolated oxidized c domain. Two-dimensional exchange NMR spectroscopy shows that the dominant species has His69-Met106 ligation, similar to reduced c domains. This form is in equilibrium with a high-spin form in which Met106 has left the heme iron. Melting studies show that the midpoint of unfolding of the isolated c domain is 320.9 +/- 1.2 K in the oxidized and 357.7 +/- 0.6 K in the reduced form. The thermally denatured forms are high-spin in both oxidation states. The results reveal how redox changes modulate conformational plasticity around the c heme and show the first key steps in the mechanism that lead to ligand switching in the holoenzyme. This process is not solely a function of the properties of the c domain. The role of the d1 heme in guiding His17 to the c heme in the oxidized holoenzyme is discussed.

Identification of metal cations, metal complexes, and anions by electrospray mass spectrometry in the negative ion mode

Pneumatically assisted electrospray mass spectrometry (ES-MS) is used in the negative ion mode for aqueous metal (M) solutions in an excess of hydrochloric or nitric acid, where the major anion X = Cl- or NO3-. A collision energy of approximately 20 eV removes anion-solvent clusters for most elements and leaves negative complex ions of the general form (Mn+Xn+1)-. Complexation with anions prevents charge reduction reactions at least to n = 3, even in cases where the third ionization energy of M greatly exceeds the first ionization energy of the solvent. These negative ions thus preserve the charge state of the metal cation from the solution and allow identification of both cations and anions in a single set of electrospray conditions. Cations such as Fe3+ or Cu2+ that have a lower oxidation state in solution produce a distribution of negative ions, each with a single negative charge overall; e.g., an Fe3+ solution produces both Fe(III)X4- and Fe(III)X3-. This distribution of FeIII and FeII species is attributed to electrochemical reduction of Fe3+ at the negatively charged ES needle. "Native" anions such as perrhenate or molybdate produce singly charged analogues such as ReO4- or HMoO4-. Metal-EDTA complexes are seen as M(III)Y- or M(II)HY-. The sensitivity for these "native" anions is suppressed by competition with the excess chloride or nitrate used to produce the metal-containing complex ions.

In-situ X-ray Absorption Spectroelectrochemistry for Determination of the Oxidation States and the Local Structure of Metalloprotein Model Compounds.

X-ray absorption spectroscopy combined with an in-situ electrochemical cell has been developed for characterization of unstable, but electrochemically stable metalloprotein model compounds with various oxidation states in solution. The technique has been applied to some dimanganese Schiff-base complexes as a model of Photosystem II and [Ru 2 (μ-O)(μ-CH3COO) 2 (bpy) 2 L 2 ] (L=pyridine and 1-methylimidazole) as a model of hemerythrin. It has been found that X-ray absorption near-edge structure (XANES) patterns correlate well with the formal oxidation states of the mctal ions and aggregation. The analysis of extended X-ray absorption fine structure (EXAFS) data has revealed characteristic metal-ligand and metal-metal distances within the model compounds with different oxidation states.

Investigations into sulfur speciation by electrospray mass spectrometry

Chromium is an element whose toxicity is oxidation state dependent. Chromium(VI) is considered toxic to humans whereas CrIII is considered to be an essential nutrient. It therefore becomes important to be able to distinguish between the two oxidation states in solution. In this study it is shown that electrospray mass spectrometry (ESMS) has the potential to provide useful information on chromium species in solution samples. The various solution forms of both oxidation states may be observed directly and are presented. In addition, other more uncommon species of both oxidation states such as polymeric CrIII and the polyanion trichromate are discussed. Finally, the ability of ESMS to monitor other aspects of the solution chemistry of both chromium oxidation states is explored.

Current efficiency during anodic dissolution of iron to ferrate(vi) in concentrated alkali hydroxide solutions

The dependence of the current efficiency for oxidation of an iron anode to ferrate(vi) ions in 14m NaOH was measured in the region of free convection. The highest current yield of 40% was obtained at a current density of 2.1 mA cm−2 and temperature of 30°C. The iron anode was activated by cathodic prepolarization. The iron concentration in low oxidation states in solution was determined as 0.13 ± 0.1 and 0.29 ± 0.25 g Fe dm−3 at 20 and 30°C, respectively. The steady state anodic polarization curves of iron in the transpassive potential region are shifted to lower potential values with increasing NaOH concentration from 11 to 171 m. At 40°C all the curves show a limiting current density around 660 mV vs Hg/HgO, namely 9 and 23 mA cm−2 at NaOH concentrations of 11 and 17 m, respectively.

Electronic structure of hydrated Ce4+ ions in solution: An x-ray-absorption study.

X-ray-absorption measurements at the Ce ${L}_{3}$ and ${L}_{2}$ edges have been performed for a series of acidic solutions of formally ${\mathrm{Ce}}^{4+}$(4${f}^{0}$) hydrated ions at various concentrations. It is found that the ${L}_{3}$,2-edge x-ray-absorption near-edge structure of these ions exhibit two white lines. This apparent double-white-line feature is observed for the first time for formally ${\mathrm{Ce}}^{4+}$(4${\mathrm{f}}^{0}$) centers (hydrated ions) in liquid solutions. Except for a smaller energy separation, the double-white-line feature resembles the well-known ${L}_{3}$-edge characteristics of many Ce and rare-earth valence-fluctuating (mixed-valence) metallic compounds and the more recently reinterpreted interatomic-intermediate-valence compounds, ${\mathrm{CeO}}_{2}$ and Ce(SO${)}_{4}$\ensuremath{\cdot}4${\mathrm{H}}_{2}$O, which are insulators in the solid state. These results are discussed in terms of the initial-state-configuration interaction of the localized, ionic 4${f}^{0}$ and 4${f}^{1}$L states (L denotes a hole in the ligand). This model has been recently proposed by Bianconi et al. to explain the apparent double-white-line feature observed in ${\mathrm{CeO}}_{2}$ and similar nonmetallic rare-earth systems. The ${L}_{3}$-edge white-line profiles of Ce(IV) compounds and complexes are correlated with ionicity. Analysis of the extended x-ray-absorption fine structure of the ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Ce}}^{4+}$ hydrated ions yields a more accurate Ce-${\mathrm{H}}_{2}$O bond-length difference of 0.080(5) A\r{} between the two different oxidation states in solution. Implication of the results to the [Ce(${\mathrm{H}}_{2}$O${)}_{\mathrm{n}}^{3+}$]/[Ce (${\mathrm{H}}_{2}$O${)}_{\mathrm{n}}^{4+}$] (n\ensuremath{\approxeq}9) electron exchange reaction is also discussed.

Determination of aqueous plutonium oxidation states by solvent extraction

The reliability of two solvent extraction techniques for the determination of Pu oxidation states in solution was tested with low-ionic-strength solutions and with high-Na and high-Mg brines that contained Pu concentrations sufficient for spectrophotometric analysis. One procedure only differentiates between reduced Pu [Pu(III) and Pu(IV)] and oxidized Pu [Pu(V) and Pu(VI)], whereas the second procedure was designed to differentiate between Pu(IV), Pu(V), and Pu(VI) in solution. Both procedures successfully differentiated between oxidized and reduced Pu in both dilute solutions and brines when tested with samples that contained only the Pu(IV), Pu(V), or Pu(VI) oxidation states. However, when the second solvent extraction procedure, which differentiates between Pu(V) and Pu(VI), was employed for solutions that did not contain a strong oxidant to maintain the Pu(VI) oxidation state, significant quantities of Pu(VI) were reduced to Pu(V) during extraction, indicating that accurate quantification of Pu(V) and Pu(VI) is not possible with this procedure.