Chromium reduced species

For the latter reaction very pure chromium is required to prevent formation of Cr34, but the reaction with a zinc amalgam is clean and is a convenient source of the strongly reducing species. Addition ofCr2 solutions to saturated solutions of sodium acetate precipitates chromium(U) acetate ... 

The Phillips catalyst contains hexavalent chromium after calcining, but the early discoverers quickly realized that reduction takes place in the reactor on contact with ethylene, leaving chromium in a lower oxidation state as the active species. A worldwide debate has continued to this day about the valence of this reduced species. Chromium in every valence state from Cr(II) to Cr(VI) has been proposed as the active site, either alone or in combination with another valence. The question has received far more attention than it probably deserves, undoubtedly at the expense of more fundamental issues, like the polymerization mechanism. 

Sheldon and co-workers have circumvented this problem to some extent by three approaches 46 the use of sulfuric acid to reduce the pH, by addition of ammonium fluoride and by addition of ammonia. Ammonia stabilizes monomeric chromium(III) species via the formation of amine complexes, and the fluoride effects dissolution of the silica at near-neutral pH. The three catalysts that were synthesized were evaluated in the oxidative cleavage of styrene with 35% m/m hydrogen peroxide in 1,2-dichloroethane at 70 °C (Table 4.4). The... 

The zinc catalyst probably functions by generating kinetically labile chromium(II) species. The present observation is reminiscent of several others, notably (1) the ready dissolution of anhydrous chromium(III) chloride in water and other solvents only in the presence of chromium(II) ion or reducing agents,... 

Chromium(VI) compounds are reduced to chromium(III) species in molten KNCS with the formation of a variety of products (433). The reaction between NCS " and HCr04 " in solution indicates the formation of an intermediate [CrOg(CNS)] species (562), which is tentatively assumed to be N-bonded (545). 

The chromium(VI) species are excellent oxidizing agents, especially in acidic solution, where chromium(VI) as the dichromate ion (Cr2C>7- ) is reduced to the Cr3+ ion ... 

In the nickel(ll)-catalyzed NHK reaction, the first step is the reduction of Ni " to Ni that inserts into the halogen-carbon bond via an oxidative addition. The organonickel species transmetallates with Cr " to form the organochromium(lll) nucleophile, which then reacts with the carbonyl compound. To make the process environmentally benign, a chromium-catalyzed version was developed where a chlorosilane was used as an additive to silylate the chromium alkoxide species in order to release the metal salt from the product. The released Cr " is reduced to Cr " with manganese powder. 

Examples of supports modifying the properties of transition metal oxides have also appeared in the literature. Recent work points to iron oxide phases as important species in Fischer-Tropsch synthesis (3 ). Iron oxide supported on SiO2 (4 ) and TiO ( ) resist reduction under conditions in which bulk iron oxide easily reduces. Thus supported iron oxide catalysts are potentially interesting Fischer-Tropsch catalysts. The extensive studies on ethylene polymerization catalysts suggests that chromium (VI) species exist on a SiOp surface at temperatures above which bulk chromic anhydride (CrOg) decomposes ( ). 

These experiments were repeated with a catalyst made by depositing dibenzenechromium(O) on AIPO4, and similar results were obtained, except that the poisoning was not as severe. Preexposure of the catalyst to CO reduced the activity by about 40%, and again decreased the size of the low-MW peak. A quick preexposure to dry air reduced the activity very little, but it did diminish the GPC peak associated with phosphate, a result that again suggests selective oxidation to a chromium oxide species. 

Let s balance the redox reaction between dichromate ion and iodide ion to form chromium(III) ion and solid iodine, which occurs in acidic solution (Figure 21.2). The skeleton ionic reaction shows only the oxidized and reduced species ... 

Kinetic measurements on the concentration of complex B revealed that it docs not react with E -p-mcthylstyrcne directly, acting only as a reservoir of the active 0x0 chromium(V) species A [102]. It was found that coordination of DMF reduced the reactivity of complex A significantly (with respect to that in CHtCN) this could be seen from x a values of complexes A in DMF (ti/2(An, DMF) greater than 1 d ii 2(Aij, DMF)= 6 h). The increased enantiosclectivity (+11% ee upon addition of DMF [35]. stoichiometric procedure, catalyst non-deuterated counterpart of 13) could result from reduced reactivity, thus representing the effect of axial ligation. 

The carcinogenicity and mutagenicity of chromium(VI) are well established.The toxicity is usually considered in terms of the uptake/reduction modeP since chromium(VI) readily passes into the cell, via anion channels, and once within the cell it is eventually reduced by cellular components to chromium(III) species. Figure 6 illustrates the likely fate of chromate within a mammalian cell. As can be appreciated from Figure 6 it is complexes trapped within the cell that are the agents responsible for the toxic effects of chromate. The systems which reduce the chromiumfvi) are as yet unknown, as are the final products of the reaction. However, microsomes are capable of reducing chromium(VI) as are various nucleotides and even fulvic acids.In these cases chromium(V) species of considerable stability have been observed using EPR spectroscopy. Within the cell reduction by a sulfide is the most probable reaction. 

The reaction of GSH with chromium(VI) has been studied in acidic solutions under which the complete reduction of chromate to chromium(III) is rapid. Mechanisms involving thiolate esters of chromium(VI), which react with either protons or GSH, were suggested to be important. A detailed study of the reduction of chromate by various biological reducing agents (in neutral, buffered solutions, at low GSH concentrations) indicates that chromium(V) species are not generated and also suggests the importance of thiolate esters. 

Many of the preferred reagents for the oxidation of primary alcohols to aldehydes (secondary alcohols to ketones) contain the transition metal chromium in its highest oxidation state, VI. Upon reaction with an alcohol, the yellow-orange chromium(VI) species is reduced to the blue-green chromium(III) state. Normally the reaction is carried out in aqueous acid solution using the sodium dichromate salt, Na2Cr207, or the oxide, CrOs. A typical reaction is shown here ... 

A chromium-catalyzed version has been developed by Fiirstner which makes this process environmentally benign. The key feature of this process uses chlorotrimethylsilane as an additive for the silylation of the chromium alkoxide species 5 in order to release the metal salt from the product 6. The liberated Cr(III)Cl2X can then be reduced to the active species Cr(II)Cl2 by means of a stoichiometric and nontoxic reducing agent as a manganese(O) metal. 

Oxidation and reduction reactions can be used to immobilize heavy metals in-situ. Oxidation of soluble Fe and Mn to their insoluble hydrous oxides, Fe203 xH20 and Mn02 xH20, respectively, can precipitate these metal ions and coprecipitate other heavy-metal ions. However, subsurface reducing conditions could later result in reformation of soluble reduced species. Reduction can be used in situ to convert soluble, toxic chromate to insoluble chromium(III) compounds. 

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