Chromium possible reductants

When a solution of chromic and sulfuric acids in water is added at 0-20° to an alcohol or formate dissolved in acetone, a rapid oxidation takes place with the separation of the green chromium III reduction product as a separate layer. This system is commonly known as Jones reagent. The rate of oxidation is so fast that it is often possible to run the reaction as a titration to an... 

It is of interest that identical products to those derived from the metal atom technique are produced by the reduction of the metal halide (Co, Fe, Cr, V, and Ti) with PrjMgBr in the presence of cycloheptatriene (27, 85, 137). There are tentative reports of the formation of an unstable complex with chromium, possibly Cr(Tj -C7Hg)2, but these have not yet been confirmed (115, 140). 

In equation 1, the Grignard reagent, C H MgBr, plays a dual role as reducing agent and the source of the arene compound (see Grignard reaction). The Cr(CO)g is recovered from an apparent phenyl chromium intermediate by the addition of water (19,20). Other routes to chromium hexacarbonyl are possible, and an excellent summary of chromium carbonyl and derivatives can be found in reference 2. The only access to the less stable Cr(—II) and Cr(—I) oxidation states is by reduction of Cr(CO)g. 

Ghromium(II) Compounds. The Cr(II) salts of nonoxidizing mineral acids are prepared by the dissolution of pure electrolytic chromium metal ia a deoxygenated solution of the acid. It is also possible to prepare the simple hydrated salts by reduction of oxygen-free, aqueous Cr(III) solutions using Zn or Zn amalgam, or electrolyticaHy (2,7,12). These methods yield a solution of the blue Cr(H2 0)g cation. The isolated salts are hydrates that are isomorphous with and compounds. Examples are chromous sulfate heptahydrate [7789-05-17, CrSO 7H20, chromous chloride hexahydrate... 

The key to hexavalent chromium s mutagenicity and possible carcinogenicity is the abiHty of this oxidation state to penetrate the cell membrane. The Cr(VI) Species promotes DNA strand breaks and initiates DNA—DNA and DNA-protein cross-links both in cell cultures and in vivo (105,112,128—130). The mechanism of this genotoxic interaction may be the intercellular reduction of Cr(VI) in close proximity to the nuclear membrane. When in vitro reductions of hexavalent chromium are performed by glutathione, the formation of Cr(V) and glutathione thiyl radicals are observed, and these are beHeved to be responsible for the formation of the DNA cross-links (112). 

It is possible to titrate two substances by the same titrant provided that the standard potentials of the substances being titrated, and their oxidation or reduction products, differ by about 0.2 V. Stepwise titration curves are obtained in the titration of mixtures or of substances having several oxidation states. Thus the titration of a solution containing Cr(VI), Fe(III) and V(V) by an acid titanium(III) chloride solution is an example of such a mixture in the first step Cr(VI) is reduced to Cr(III) and V(V) to V(IV) in the second step Fe(III) is reduced to Fe(II) in the third step V(IV) is reduced to V(III) chromium is evaluated by difference of the volumes of titrant used in the first and third steps. Another example is the titration of a mixture of Fe(II) and V(IV) sulphates with Ce(IV) sulphate in dilute sulphuric acid in the first step Fe(II) is oxidised to Fe(III) and in the second jump V(IV) is oxidised to V(V) the latter change is accelerated by heating the solution after oxidation of the Fe(II) ion is complete. The titration of a substance having several oxidation states is exemplified by the stepwise reduction by acid chromium(II) chloride of Cu(II) ion to the Cu(I) state and then to the metal. 

The first steps involve coordination and cycloaddition to the metal. Insertion of a third molecule of ethene leads to a more instable intermediate, a seven-membered ring, that eliminates the product, 1-hexene. This last reaction can be a (3-hydrogen elimination giving chromium hydride and alkene, followed by a reductive elimination. Alternatively, one alkyl anion can abstract a (3-hydrogen from the other alkyl-chromium bond, giving 1-hexene in one step. We prefer the latter pathway as this offers no possibilities to initiate a classic chain growth mechanism, as was also proposed for titanium [8]. The byproduct observed is a mixture of decenes ( ) and not octenes. The latter would be expected if one more molecule of ethene would insert into the metallocycloheptane intermediate. Decene is formed via insertion of the product hexene into the metallo-cyclopentane intermediate followed by elimination. 

The most stable oxidation states of chromium in the subsurface environment are Cr(III) and Cr(VI), the latter being more toxic and more mobile. The oxidation of Cr(III) in subsurface aqueous solutions is possible in a medium characterized by the presence of Mn(IV) oxides. Eary and Rai (1987), however, state that the extent of Cr(III) oxidation may be limited by the adsorption of anionic Cr(VI) in acidic solutions and the adsorption and precipitation of various forms of Cr(OH). These authors also report a rapid quantitative stoichiometric reduction of aqueous Cr(VI) by aqueous Fe(ll), in a pH range covering the acidity variability in the subsurface even in oxygenated solutions. 

The femtosecond fluorescence up-conversion setup has been described elsewhere [13,14]. Briefly, a second harmonic (SH) of a home-made chromium-forsterite femtosecond laser tunable from 610 to 660 nm was used to excite the sample (Fig.2) [14]. The pulse duration of the SH pulses was about 50 fs at the full width at half maximum (FWHM). We were successful in the cavity-dumping operation of this laser [14] and kept the repetition rate as low as 4 MHz. Reduction of the repetition rate was necessary to avoid multiple hits of the same location of the sample as small as possible. The excitation intensity, controlled by a neutral density filter before the sample cell, was (0.5-l)xl012 photons/cm2/pulse. Special care was taken to work at the lowest excitation light intensity so that the effect of the exciton-exciton annihilation process was negligible. 

The more cationic halogen containing compounds produced other products. Cobalt bis-allyliodide produced cis-polybutadiene and the even more cationic chromium, produced cyclododecatriene. Only with the more cationic system which introduced trans-structures, was cyclization and reduction of the metal able to intercept the polymerization reaction. Cyclization was not possible in the less cationic cobalt which produces all cis-polybutadiene nor was the hydride transfer possible with the less anionic chromium tris-allyl compound. 

Chromium. The chemical properties of the two possible oxidation states Cr(VI) and Cr(III) are very different. Cr(VI) occurs as an anion, whereas Cr(III) is a strongly hydrolyzing cation with a strong tendency to bind to the surfaces of oxides and other particles (77). According to the thermodynamic sequence, the reduction of Cr(VI) to Cr(III) occurs in a pe range similar to that for the reduction of Mn(III,IV) to Mn(II) (Figure 2). 

Thus a drier, less sintered support is obtained by calcining in CO rather than air. Unfortunately the chromium will not tolerate such a severe reduction treatment, and unless Cr is added secondarily [as in the above (Section V,B) examples] a dead catalyst results. This is shown in Fig. 5, where it is apparent that reduction at 350°C improves activity, whereas higher temperatures destroy it. This does not indicate overreduction, as some have claimed. Rather, it indicates a rearrangement of the Cr(II) into a less coordinatively unsaturated form—possibly aggregates. 

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