Redox stability stabilization

Van Cappellen, P. and Ingall, E. D. (1996). Redox stabilization of the atmosphere and oceans by phosphoms-limited marine productivity. Science 271,493-496. 

The dependency of 2 on oxidation and reduction of tris(dithiocarbamato) and bis(dithiocarbamato) complexes is illustrated in Fig. 10. The great stability of the andd compounds viz. Cr(R2 c)3 and Co(R2tiic)3 both to oxidation and reduction is remarkable. A similar trend is present in recently published data 191) of the bipy complexes M(bipy)3, M(bipy)3 and M(bipy)3. Maxima in redox stability are found for M = Fe(II), Cr(0) and V(— 1), respectively, which all have configuration. For the few bis(dithiocarbamato) complexes known, the stability of the Ni(II) complexes is greater than for the d Cu(II) complexes. 

Heterogeneous ET reactions at polarizable liquid-liquid interfaces have been mainly approached from current potential relationships. In this respect, a rather important issue is to minimize the contribution of ion-transfer reactions to the current responses associated with the ET step. This requirement has been recognized by several authors [43,62,67-72]. Firstly, reactants and products should remain in their respective phases within the potential range where the ET process takes place. In addition to redox stability, the supporting electrolytes should also provide an appropriate potential window for the redox reaction. According to Eqs. (2) and (3), the redox potentials of the species involved in the ET should match in a way that the formal electron-transfer potential occurs within the potential window established by the transfer of the ionic species present at the liquid-liquid junction. The results shown in Figs. 1 and 2 provide an example of voltammetric ET responses when the above conditions are fulfilled. A difference of approximately 150 mV is observed between Ao et A" (.+. ... 

The redox stability organic of phase cations is commonly very high in comparison to the anionic phenylborate derivatives. It has been proposed that Fc is reduced to Fc in DCE in the presence of TPB (Ph4B ) [71],... 

DCE interface in the presence of TPBCl [43,82]. The accumulation of products of the redox reactions were followed by spectrophotometry in situ, and quantitative relationships were obtained between the accumulation of products and the charge transfer across the interface. These results confirmed the higher stability of this anion in comparison to TPB . It was also reported that the redox potential of TPBCP is 0.51V more positive than (see Fig. 3). However, the redox stability of the chlorinated derivative of tetra-phenylborate is not sufficient in the presence of highly reactive species such as photoex-cited water-soluble porphyrins. Fermin et al. have shown that TPBCP can be oxidized by adsorbed zinc tetrakis-(carboxyphenyl)porphyrin at the water-DCE interface under illumination [50]. Under these conditions, the fully fluorinated derivative TPFB has proved to be extremely stable and consequently ideal for photoinduced ET studies [49,83]. Another anion which exhibits high redox stability is PFg- however, its solubility in the water phase restricts the positive end of the ideally polarizable window to < —0.2V [85]. 

As a result of their low redox potentials [173], bis(phthalocyaninato) lanthanide complexes are often inadvertently reduced or oxidized, and they are also very sensitive to acids and bases. In order to solve these problems, Veciana et al. achieved certain success on designing a series of novel compounds with characteristics that would give them improved redox stability. Electroactive ligands based on phthalo-cyaninato tetra dicarboximide [175] or perfluorinated phthalocyanine [176] were used to assemble the double-decker lanthanide complexes, with the effect of stabilizing the negative charge of the anionic state of the compounds, which resulted in a strong shift of 0.7 V of their first oxidation potentials. 

Some other reactions of metal nitrosyls LxM(NO) with various nucleophiles (Nuc) are summarized in Table III. The pattern indicated by the studies described above is repeated simple adduct formation occurs when the coordinated nitrosyls are sufficiently electrophilic and the nucleophiles sufficiently basic. The first species formed is probably the N-coordinated nucleophile nitrosyl adduct LrM(N(O)Nuc), e.g. Eq. (27). Subsequent reactions depend on the substitution lability of these species, as well as on the redox stability of the complex and of the ligand. 

Violene is constructed by a conjugated chain with two redox active chromophores. It is classified as two types (see Figure 2). As noted from the position of the end groups, one type is separated by an aromatic core and the other is located inside a cyclic 7c-system. In both cases, the colored species in this system are the radical ionic states. Thus, in general, the violene system does not show high redox stability (2). 

Stabilization of the redox cycle is relatively important in construction of potentially useful electrochromic materials, because the molecules needed for application require high redox-stability. Recently, S. Hiinig et al. proposed the concept of violene-cyanine hybrid to produce stabilized organic electrochromic materials (3). The hybrid is constructed by a violene-type redox system containing delocalized closed-shell polymethine dyes as end groups. The hybrid is expected to exhibit the color of a cyanine dye, by an overall two-electron... 

Fig. 23. The dissociation constants and redox stability of the NO and histamine complexes of the nitrophorins from the saliva of Rhodnius prolixus, and how they aid the insect in obtaining a sufficient blood meal. Modified from Ref (.31).

This mechanism is inconsistent with the following facts First, no effect is observed when Cu+2 is replaced by Zn+2 or Cd+2, which should have the same inductive effect on the electrons of the ligand. Second, in view of the extremely low redox stability of Cu(I) bound to the same ligands (52, 57), it is not likely that Cu(II) should undergo partial reduction by the ligand-i.e., that the electron withdrawal by Cu(II) should exceed that induced by H+ or other cations. Finally, no increased reactivity of the copper bound ligand towards OH or H radicals has been observed (10). These radicals are known to react with aliphatic compounds via hydrogen abstraction. 

The formation of ligated transition metal ions at unstable high states of oxidation, its implications in the mechanisms of metal-catalyzed autoxidation, and the effect of configuration of a metal-ligand system on its redox stability have been pointed out. These considerations may be helpful in interpreting more complex metal-ligand systems including metal-enzyme reactions. 

Phosphate is widely used as a chemical stabilization agent for MSW combustion residues in Japan and North America and is under consideration for use in parts of Europe. The application of this technology to MSW ashes generally parallels its application to contaminated soils. Metal phosphates (notably Cd, Cu, Pb and Zn) frequently have wide pH distribution, pH-pE predominance, and redox stability within complex ash pore water systems. Stabilization mechanisms identified in other contaminated systems (e.g., soils), involving a combination of sorption, heterogeneous nucleation, and surface precipitation, or solution-phase precipitation are generally observed in ash systems. 

Figure 15.2 Redox stability field of liquid water at 25 °C. Solid lines refer to gas partial pressures of 1 bar (standard state), broken lines to 10-6 bar.

Two interlocked macrocyclic ligands as in (15) are topologically related to the catenanes, whence the name catenand derives.34,186 187 These macrocycles complex a variety of metals, presumably in a tetrahedral geometry.34,186 The stabilizing effect of the catenand topology is evident in the observed redox stability of the nickel(I) complex186 as well as the reluctance toward demetallation, observed for the copper(I) complexes.187... 

At the present stage, the in vivo applicability of Eu11 complexes as MRI redox reporters remains questionable, mainly due to the difficulties in controlling the reduced state. Efforts are focused on the synthesis of new water soluble macro-cyclic ligands with matching cavity size for Eu2+ in order to ensure high thermodynamic and redox stability of the Eu11 chelate. 

The latter devices are fuel cells that consist of ceramic components which have to fulfill extremely demanding criteria with regard to thermal, mechanical, chemical, and electrical properties. Just consider the electrolyte It does not only have to be thermally stable but also has to be mechanically and chemically compatible with the electrodes. It does not only have to be chemically stable over a very wide redox window but also has to maintain electrolyte properties within that window (redox stability). Owing to the high mobilities of the electronic carriers and the comparatively steep power law dependencies of their concentrations (see Part I), this requires an extremely high ratio of ionic versus electronic disorder at the reference point of p-n minimum (cf. Part I).2... 

In particular the redox stability of lanthanide cations makes the route superior to, e.g., the usual metathesis reaction involving lanthanide halides. The advantages are evident ... 

The different redox stability of U and Pu is important in nuclear fuel reprocessing, one function of which is to separate unused uranium from 239Pu, which is itself used as a nuclear fuel. Dissolving the spent fuel elements in aqueous HN03 gives Pu(VI) and U(VI). Subsequent separation steps then depend on differences in complexing power and solubility of these ions. 

Convergent and Divergent Syntheses of Large Ferrocenyl Dendrimers with Good Redox Stabilities 421... 

Water samples in As speciation have a place of their own in terms of sample preparation, as they usually feature the two inorganic As species, As(III) and As(V), the major chemical properties of which (e.g., redox stability) have been thoroughly studied in the recent past. The analyst is usually well aware of the importance of preserving the original ratio of inorganic As species. Hence, problems with low chromatographic recovery are rarely faced. 

---