Chromium electrochemical reduction

However, more than stoichiometric use of the chromium salt was problematic, and the toxicity of the salt makes this versatile process inadequate for large-scale synthesis. Truly catalytic use of chromium was achieved in 1996 by using Mn powder as a co-reductant (Equation (20)). In another approach, electrochemical reduction of... 

Thermodynamically, chromium deposition can occur by electrochemical reduction or chemical dissociation of high valent chromium species. Thus, the focus of the debate on the deposition of chromium during the 02 reduction on LSM based... 

The consequences of the electrochemical reduction of high valence chromium species would be the precipitation of Cr203 solid phase at the cathode-electrolyte interface boundary. These led to the hypothesis that the degradation mechanism of LSM cathode is dominated by an electrochemical reduction of high valence vapor species of chromium (Cr03 and C OH O to solid phase Cr203 in competition with the 02 reduction reaction, followed by the chemical reaction with LSM to form (Cr,Mn)304 phases at the TPB, blocking the active sites [174-180], The process is written as follows [174] ... 

The results show that the deposition of Cr species is not dominated by electrochemical reduction of high-valent chromium species in competition with 02 reduction. The driving force for the deposition reaction was suggested to be related to the... 

Scheme 2. Proposed reaction pathways in the indirect electrochemical reduction of geminal dibromocyclopropanes by chromium (II)...

Liquid wastes containing hexavalent chromium require reduction of chromium to the trivalent state prior to metal removal. Commonly used reducing agents are sodium metabisulfite, sulfur dioxide, ferrous sulfide, and other ferrous ions (ferrous sulfate, ferrous chloride, or electrochemically generated ferrous ion). All of these reagents create some form of chromium sludge, which must be separated and dewatered before disposal. 

Electric arcs, in metal vapor synthesis, 1, 224 Electric-field-induced second harmonic generation Group 8 metallocenes, 12, 109 for hyperpolarizability measurement, 12, 107 Electrochemical cell assembly, in cyclic voltammetry, 1, 283 Electrochemical irreversibility, in cyclic voltammetry, 1, 282 Electrochemical oxidation, arene chromium carbonyls, 5, 258 Electrochemical properties, polyferrocenylsilanes, 12, 332 Electrochemical reduction, bis-Cp Zr(III) and (IV) compounds, 4, 745 Electrochemical sensors biomolecule—ferrocene conjugates... 

Newer methods allow the use of catalytic amounts chromium(II), which is regenerated by reduction with manganese or via electrochemical reduction. 

Dialkylhydrazido(2-) complexes have also been shown to be crucial intermediates in the electrochemical reduction of coordinated dinitrogen to substituted hydrazines (273). Finally a chromium(III) hydra-zido(2-) species has been proposed as an intermediate in the chro-mium(II) reduction of nitroamine (192). 

Electrochemical reduction of the salts (4) provides radicals (18) which dimerize or undergo further reduction to anions (32) or dianions (80MI43100). The reduction potentials are not much affected by substituents. Reduction with zinc in aprotic conditions gives bi(l,2-dithiol-3-yls) (59), and 3-chloro-l,2-dithiolylium salts (35a X = Cl) are converted into bi(l,2-dithiol-3-ylidenes) (20) (75TL3473). Divalent chromium converts the 3,5-dimethyl-l,2-dithiolylium cation into a dithioacetylacetonate ligand (72AJC2547). The reaction of 3,5-diamino-l,2-dithiolylium salts (8) or alkyl derivatives with thiols provides dithiomalonamides (60) by electron transfer (63ACS163). 

Chromium hexacarbonyl is extremely photolabile (equation 6) therefore photochemical substitution is an efficient means of preparing derivatives. Oxidation of the Cr center requires nitric or sulfuric acid, or chlorine. Alternatively, some hgands induce complete carbonyl dissociation with concomitant oxidation, for example, acetylacetonate. Chemical reduction with alkali or alkaline-earth metals or electrochemical reduction proceeds in two-electron steps with loss of two CO molecules to first give [Cr2(CO)io]" and then [Cr(CO)s]. Nucleophilic attack at CO generates a number of stable (Nu = R) and unstable (Nu = N3, OH, H, NEt2) products. The stable [(OC)5CrCOR] ion is a carbene precursor. 

An electrochemical reduction procedure using a chromium(II) salt as the catalyst is effective for the dehalogenation of P-hydroxy halides. This procedure is a convenient entry to deoxynucleosides, as shown in Scheme 8. 

On the other hand, when one thinks in terms of electrochemical reductions or oxidations, special attention is devoted to the coreactant, that is, to the electrode that provides or accepts electrons. Thus, in order to discuss or compare electrochemical reactions with their organic analogs, it is of the utmost importance to use more precise terms than the so inaccurate reduction of oxidation notions. A similar problem has been addressed in the inorganic and organometallic fields. Indeed, it was early recognized that oxidation-reduction reactions at metal centers must be classified according to two types outer sphere or inner sphere reactions. A typical example of this dichotomy is given in Eqs. (14) and (15), which relate to chromium (II) oxidations by cobalt (III) complexes. 

Electrochemical reduction of chrome alum to chromium metal proceeds as follows ... 

Electrochemical Reduction of Chromium(VI) Oxide ( Chromic acid )... 

Several new isocyanide complexes of chromium(m) have been prepared by the electrochemical reduction of heavy-metal adducts of chromium(iii) aquocyano-complexes, and characterized by their visible spectra. The chromium 2p- and 3s-orbital ionization potentials in K3[Cr(NCS)6] have been reported, and the lattice parameters for this anion in the salt with diaquotris-(nicotinic acid)holmium(iii) determined. " The absorption spectra of the... 

Heavy metals, such as chromium, that are toxic to certain microbes may increase production of EPS. Fang and colleagues [101] found an increase in the EPS of an SRB-enriched marine culture when chromium (50-100 ppm) was added to seawater. Exposure of mild steel samples to this solution for 20 d under anaerobic conditions resulted in corrosion. A subsequent study by Chan and coworkers [102] suggested that corrosion of mild steel, immersed in synthetic seawater containing extracted and purified EPS (from the same SRB-enriched culture as above) and incubated under anaerobic conditions as before, was aided by oxidation reactions provided by EPS. The electrochemical reductions of EPS were coupled to iron oxidation. Polysaccharides in EPS were electrochemically reduced and converted to hydrocarbons, as shown by changes in the X-ray photoelectron spectra. 

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