Transition metal ions reduction

However, under anhydrous conditions and in the absence of catalytic impurities such as transition metal ions, solutions can be stored for several days with only a few per cent decomposition. Some reductions occur without bond cleavage as in the formation of alkali metal superoxides and peroxide (p. 84). 

Castro CE, WC Kray (1963) The cleavage of bonds by low-valent transition metal ions. The homogeneous reduction of alkyl halides by chromous sulfate. J Am Chem Soc 85 2768-2773. 

It has been shown that on Cu-ZSM-5 and Cu-ZrOj catalysts, reduction of NO and NOj in the presence oflarge excess of Oj proceed at about the same rate [20,21]. This is because over these catalysts, NO2 is rapidly reduced to NO (and not NO being rapidly oxidized to NOj) [20,22,23]. On the other hand, on catalysts that do not contain transition metal ions, such as Na-ZSM-5 [24], GajOj [25], AljO, [26], and H-ZSM-5 [26], NO2 reduction to Nj proceeds much... 

Organic Molecules It can be seen from our earlier discussion that the presence of a transition metal ion is not always required for an electrochromic effect. Indeed, many organic molecules can yield colored products as a result of reversible reduction or oxidation. 4,4 -Bipyridinium salts are the best known example of such compounds. These compounds can be prepared, stored, and purchased in colorless dicationic form (bipm +). One electron reduction of the dication leads to the intensely colored radical cation (bipm+ ). Such radical cations exist in equilibrium with their dimers (bipm ). In the case of methyl viologen, the radical cation is blue and the dimer is red. By varying the substient group in the molecule, different colors can be obtained. 

Late transition metal or 3d-transition metal irons, such as cobalt, nickel, and copper, are important for catalysis, magnetism, and optics. Reduction of 3d-transition metal ions to zero-valent metals is quite difficult because of their lower redox potentials than those of noble metal ions. A production of bimetallic nanoparticles between 3d-transi-tion metal and noble metal, however, is not so difficult. In 1993, we successfully established a new preparation method of PVP-protected CuPd bimetallic nanoparticles [71-73]. In this method, bimetallic hydroxide colloid forms in the first step by adjusting the pH value with a sodium hydroxide solution before the reduction process, which is designed to overcome the problems caused by the difference in redox potentials. Then, the bimetallic species... 

The photolytic reduction of N2 at TiO -suspensions was at first reported by Schrauzer et al. Small amounts of NH3 and N2H4 were obtained as products. The highest activity was found with anatase containing 20-30 % rutile. Very low yields were also obtained with p-GaP electrodes under illumination It is much easier to produce NH3 from NO -solutions at CdS- and Ti02-particles using S -ions as hole scavengers . Efficiencies are not reported yet. Recently the formation of NH3 from NO was observed at p-GaAs electrodes under illumination. In this case NH3-formation was only found in the presence of transition metal ions or their complex with EDTA. 

The presence of transition metal ions has a catalytic effect on reduction of halides and tosylates by LiAlH4.166 Various copper hydride reducing agents are effective for removal of halide and tosylate groups.167 The primary synthetic value of these reductions is for the removal of a hydroxy function after conversion to a halide or tosylate. 

The quinone-hydroquinone system represents a classic example of a fast, reversible redox system. This type of reversible redox reaction is characteristic of many inorganic systems, such as the interchange between oxidation states in transition metal ions, but it is relatively uncommon in organic chemistry. The reduction of benzoquinone to hydroquinone... 

The 14-membered macrocycle 1,4,8,11-tetraazacyclotetradecane (cyclam or [14]aneN4), unlike cyclen, is capable of encircling most transition metal ions and in the case of Co111 the trans configuration is much preferred by comparison with the folded cis isomer. Electrochemical reduction of A,v-[Co(cyclam)(OI I)2]+ in 3M NaOH leads to rapid isomerization to the trans form, and the relative stabilities of the trans and cis isomers of the di- and trivalent complexes were determined from a thermodynamic cycle.702 This preference for trans orientation of the non-macrocyclic donors has enabled the isolation and investigation of many Co complexes without the complications of isomerization. Some novel examples include /r[Pg.61]

Throughout this book a major stress is on catalysis in organisms. Catalysis is confined to non-metals and metal ions of attacking power, either as Lewis acids or in oxidation/reduction and this excludes the simplest ions such as Na+, K+ and Ca2+ (and Cl- among anions). The transition metal ions and zinc are the most available powerful catalysts. The metals in a transition series are known to have selective binding properties, exchange rates and oxidation/reduction states, which can be put to use in catalysis in quite different ways (Table 2.13). It is noticeable that especially the complexes of metal elements... 

Equilibrium considerations other than those of binding are those of oxidation/reduction potentials to which we drew attention in Section 1.14 considering the elements in the sea. Inside cells certain oxidation/reductions also equilibrate rapidly, especially those of transition metal ions with thiols and -S-S- bonds, while most non-metal oxidation/reduction changes between C/H/N/O compounds are slow and kinetically controlled (see Chapter 2). In the case of fast redox reactions oxidation/reduction potentials are fixed constants. 

Enthalpies, Entropies, and Gibb s Energies of Transition Metal Ion Oxidation-Reduction Reactions with Hydrogen Peroxide in Aqueous Solution (T = 298 K) [23]... 

Many organic and inorganic compounds, fibers, and particles are capable of damaging nucleic acids by generating reactive oxygen species via the reduction of dioxygen. These stimuli include different classes of organic compounds, classic prooxidants (anticancer antibiotics, various quinones, asbestos fibers, and so on), and even antioxidants, which can be oxidized in the presence of transition metal ions. 

Another type of spin traps, which have been recommended for the detection of superoxide, are the derivatives of hydroxylamine. In 1982, Rosen et al. [25] showed that superoxide is able to oxidize the hydroxylamine derivative 2-ethyl-1-hydroxy-2,5,5-trimethyl-3-oxazoli-dine (OXANOH) to corresponding free radical 2-ethyl-1-hydroxy-2,5,5-trimethyl-3-oxazolidinoxyl (OXANO). Although this radical is very stable and easily identified by its ESR spectrum, it is also easily reduced by ascorbic acid and other reductants. Furthermore, OXANOH and other hydroxylamines are oxidized by dioxygen in the presence of transition metal ions to form superoxide, and therefore, superoxide detection must be carried out in the presence of chelators. 

SN P spontaneously releases N O both thermally and photochemically [61-65], but is quite stable in the dark and in aqueous in vitro physiological media [66]. This implies that absorption of heat and light energy induces electron transfer from the Fe2+ center to the N 0+ ligand, resulting in weakening of the Fe-N O bond and subsequent release of NO [65]. SNP also decomposes in an aqueous environment in the presence of biological reductants [65, 66] and some transition metal ions to produce nitric oxide. 

An intriguing example of those enzymes that use both tyrosine residues and transition metal ions as partners in affecting redox chemistry is the fungal enzyme GO (GO EC 1.1.3.9 68 kD) (60,61,81), which catalyzes the two-electron oxidation of primary alcohols coupled to the two-electron reduction of O2 to H2O2, Eq. (2). 

Reduction lowers the charge to radius ratio of transition metal ions, promoting higher rates of ligand substitution. Reduced, divalent oxidation states of manganese, iron, cobalt, and nickel are also quite soluble (Table II). 

Preliminary heats of solution of C0CI2 and CuCl2 have been measured up to 300 C by Cobble and Murray (50). Hydrolysis was suppressed by HC1 addition so that when the work is completed and when the extent of Cl complexing (and Cu + reduction) can be allowed for the data will prove extremely valuable. Preliminary concentration cell studies on the Cl complexing of Cd + and Ni + up to 170 C (51) support the conclusions given earlier that such complexing with first row transition metal ions is likely to be significant by 300°C. 

Certain transition metal ions such as Co2, Ti3 are known to form chelates with trimethylphosphate, i.e., dimethoxyphosphato complexes84 8S>. The chelate effect is responsible for the high stabilities of such complexes, which is expressed in the more negative values for the half-wave potentials. All ions producing such complexes are expected to undergo reduction in TMP at more negative potentials than would be expected from interpolation of the curves. 

There are a few features relative to POMs that are necessary for obtaining the best performance. In all cases. Vanadium is present in the structure of the P/Mo Keggin anion, while the cations include different components, that is, protons, divalent transition metal ions (preferably either Fe " " or Cu " "), and alkali metal ions (preferably Cs" "). The role of Cu ions is to catalyze the reduction of molybdenum, thus increasing the activity of the catalyst it also affects the surface acidity. 

Epr is most effective for detecting free radicals that may occur as intermediates in oxidation and reduction reactions involving transition metal ions. Since these transients are invariably quite labile, epr is combined with continuous flow, (more conveniently) stopped-flow, flash photolysis, and pulse radiolysis. 

The aquated iron(III) ion is an oxidant. Reaction with reducing ligands probably proceeds through complexing. Rapid scan spectrophotometry of the Fe(III)-cysteine system shows a transient blue Fe(lII)-cysteine complex and formation of Fe(II) and cystine. The reduction of Fe(lII) by hydroquinone, in concentrated solution has been probed by stopped-flow linked to x-ray absorption spectrometry. The changing charge on the iron is thereby assessed. In the reaction of Fe(III) with a number of reducing transition metal ions M in acid, the rate law... 

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