Chemistry Of Chromium Research Articles

ChemInform Abstract: Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides

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A study on the uptake of trivalent and hexavalent chromium by paddy ( Oryza sativa): possible chemical modifications in rhizosphere

The uptake and translocation of chromium by paddy ( Oryza sativa L. Arbario) was studied using radiotracers. Chromium was supplied as Cr III and Cr VI through irrigation water and plants were grown in sand and soil culture. The uptake of chromium for both the oxidation states and culture types was found to increase more or less linearly with the increase in chromium concentration though with different slopes. The rate of increase of the uptake when compared to the increase in feed concentration indicates that the uptake by roots is mediated possibly through a passive mechanism. For a given concentration of feed, values of chromium in μgg −1 dry matter in shoot and grain did not vary significantly either for different oxidation states of feed chromium or culture type. The order of uptake for paddy shows a considerably different pattern when compared to onion, maize and groundnut. This has been qualitatively explained on the basis of the different chemistry of chromium in the rhizosphere of paddy. The amount of chromium translocated to grain was also considerably less compared to the other three crops.

Surface Chemistry and Spectroscopy of Chromium in Inorganic Oxides.

Focuses on the surface chemistry and spectroscopy of chromium in inorganic oxides. Characterization of the molecular structures of chromium; Mechanics of hydrogenation-dehydrogenation reactions; Mobility and reactivity on oxidic surfaces.

Biomimetic Chemistry of Chromium. Intramolecular Conversion of Hydroperoxochromium(III) to Oxochromium(V)

Transition metal hydroperoxo complexes, LMOOH (L = ligand system, M = transition metal), are important intermediates in many catalytic oxidations by molecular oxygen and hydrogen peroxide. These complexes typically react as electrophiles and one-electron oxidants and show no tendency to form oxenes, presumably because the nonparticipating ligands do not stabilize the metal in the 4+ or 5+ oxidation state. We now report that a synthetic hydroperoxochromium(III) complex of a non-porphyrin, saturated macrocycle undergoes intramolecular conversion to Cr(V). 30 refs., 1 fig.

Chiral CO-emulating ligands: From arene chromium chemistry to enantioselective catalysis

Abstract

The chemistry underlying chromate toxicity

The fundamental processes involved in the expression of the toxicity of chromate have been particularly well studied in recent years and have attracted a remarkably interdisciplinary group of researchers ranging from traditional toxicologists through molecular biologists to inorganic chemists. There is a clear chemical interest in the problem because chromium in oxidation state six is an established human carcinogen (a) associated with lung cancers. Major reviews of this area have appeared emphasizing its general importance r or specific areas such as studies involving cell or tissue culture r The aim of this short article is not to attempt any comprehensive review but to provide a basically inorganic audience an insight into the problem and an understanding of how inorganic chemistry is important in understanding the expression of the toxicity of chromate. In part the interest in chromate toxicity can be traced to the fact that chromate is an established human carcinogen which is, because of its reactivity, particularly amenable to mechanistic studies (4-7). However, the complicated redox chemistry of chromium in aqueous solution has so far prevented a clear picture as to which mechanism(s) are important in the formation of DNA lesions both in vitro and in vivo. In terms of understanding the in vivo effects one particularly important study has shown that chromate is carcinogenic in rats by inhalation, for some reason this important Study, in which chromate was administered by the route most appropriate to model the suggested effects in humans is frequently overlooked r We shall endeavour to explain why the physiological response of chromate may be very sensitive to the mode of administration as we proceed.

Factors controlling electron transfer reactions and stabilisation of uncommon oxidation states of chromium

Some experimental approaches to seek semi-quantitative understanding of factors controlling outer sphere electron transfer reactions of some transition metal complexes have been made. The relative importance of nuclear and electronic factors to outer sphere processes has been examined. By the manipulation of Franck-Condon or nuclear factors, it has now been possible to gain access into the chemistry of chromium in unusual oxidation states. An example of a reorganisation controlled electron transfer reaction involving Cr(IV)-Cr(III) system has been demonstrated. The bimolecular rate of reduction of diperoxoaquaethylenediamine chromium(IV) and diperoxodiethylenetriamine chromium(IV) is independent of the nature of th reductant employed viz. Fe2+ or VO2+ indicating that the generation of6 coordinate Cr(IV) species from7 coordinate of diperoxochromium(IV) reactant may be rate limiting. Similarly by increasing the barrier for the6 coordinate to4 coordinate structures through equatorial coordination of macrocyclic ligands, it has now been possible to detect through cyclic voltommogram the formation of relatively stable Cr(IV) species in the electrochemical oxidation of Cr(Me4[14] tetraene)(H2O) 2 3+ in aqueous sulphuric acid media. The kinetics and mechanism of the cerium(IV) and iodosyl benzene oxidation of Cr(salen)(H2O) 2 + and Cr(salprn)(H2O) 2 + have been investigated and kinetic and spectroscopic evidence for the formation of Cr(IV) transients and stable Cr(V) products has been presented. The relative importance of Franck-Condon factors in the oxidation of Cr(III) to Cr(IV) and Cr(V) states in different macrocyclic and multidentate ligand environments has been discussed.

Bis(arylimido) complexes of chromium(VI)

A synthetic entry into bis(arylimido) chromium(VI) chemistry is described. Cr(NAr) 2(NHBu t)Cl ( 1; Ar = 2,6-Pr i 2C 6H 3 may be obtained (1) by treatment of Cr(NBu t) 2Cl 2 with two equivalents of 2,6-diisopropylaniline in pentane, or (2) via the reaction of Cr(NBu t) 2Cl 2 with two equivalents of LiNHAr in Et 2O. An X-ray structure determination on 1 reveals a pseudo-tetrahedral coordination geometry with CrN distances of 1.643(3) and 1.654(4) Å for the imido groups and 1.813(3) Å for the tertbutylamido ligand ; the substituents of the amide are orientated perpendicular to the ClCrN(amido) plane. Compound 1 is readily converted to Cr(NAr) 2Cl 2( 2) by treatment with excess BCl 3 in dichloromethane.

The biological and environmental chemistry of chromium

Chemical and physical properties of chromium current recommendations on chromium exposure and intake determination of chromium in environmental and biological materials (sample collection) manufacture, use and distribution of chromium and its compounds the environmental fate of chromium and its compounds toxicology of chromium the role of chromium as an essential micronutrient (importance of chemical speciation).

Chemical Kinetic Mechanism for Atmospheric Chromium

A chemical kinetic mechanism for the atmospheric chemistry of chromium has been developed. The chemistry of chromium is of particular interest because its oxidation state is believed to affect its toxicity. Chromium can be reduced from the hexavalent form to the trivalent form by several chemicals including trivalent arsenic, divalent iron, vanadium, and sulfur dioxide. Trivalent chromium can be oxidized to hexavalent chromium by reactions with manganese. Computer simulations were conducted for several scenarios using typical atmospheric concentrations. The simulation results suggestthattypical conditions favor the reduction of Cr(VI) to Cr(III). Under some extreme conditions, Cr(III) could be oxidized slowly to Cr(VI)

Chromium in the Canadian environment

The widening use of chromium and its compounds by local industries has led to a growing concern about the effects of chromium contamination on the Canadian environment. This report summarizes the data on Canadian sources and the concentrations of chromium in air, water, sediments, soil, terrestrial wildlife and aquatic biota. It reviews what little is currently known about the cycling of chromium in Canadian ecoystems, and the need for measuring Cr(III) and Cr(VI) rather than total Cr in the environmental media is emphasized. The potential effects of elevated levels of chromium on plants, soil microorganisms, wildlife, and aquatic biota are discussed. The human health effects are not covered. The conclusion is reached that chromium pollution has become a threat to Canadian ecosystems, especially at the local scale where the ambient chromium concentrations in some surface waters, sediments, and soils are now close to or above the toxicity threshold for a number of the more sensistive organisms.Key words: chromium pollution, chromium toxicity, chromium chemistry, chromium emission, bioaccumulation of chromium.

Redox chemistry of chromium (IV) complexes

A short introduction summarizes scattered preparations of chromium(V) and chromium(IV) derivatives before 1975 and places these unusual states in perspective with respect to the more usual oxidation states of this element. Preparations of peroxo complexes of Cr(IV), known since the turn of the century, are described and their redox reactions, as investigated since 1988, are treated. Systems in which Cr(IV) has been detected in the course of reductions of water-soluble Cr(V) complexes are enumerated and the role of Cr(IV) in some autocatalytic systems and “clock reactions” is considered. The stabilization of Cr(IV) via chelation with the anions of α-hydroxy acids is described and the reactions of this type of chelate are discussed. The recent preparations of aqua-substituted Cr(IV) are mentioned and the role of the 2e − change Cr(IV)→Cr(II) in the oxidations of organic substrates by Cr(VI) is indicated. Finally, experimental values for the standard potential and electron self-exchange rate for the Cr(III/IV) couple are compared with the corresponding parameters for Cr(IV/V) in a similar medium and structural implications are drawn from these rates.

Synthesis and chemistry of chromium in CrAPO-5 molecular sieves

CrAPO-5 molecular sieves were synthesized hydrothermally starting with different Cr precursors and Cr and template contents. The behavior of Cr was investigated spectroscopically by diffuse reflectance spectroscopy (d.r.s.) and electron spin resonance (e.s.r.). In the gels, Cr 3+ and Cr 6+ are present as, respectively, octahedral Cr(H 2O) 6 3+ and mainly dichromate. Both ions are in the liquid phase of the gels. In all as-synthesized CrAPO-5′s, Cr 3+ is octahedrally coordinated to six oxygen ligands. The corresponding e.s.r. spectra show a broad isotropic signal centered around 1.97, following the Curie-Weiss law. This behavior is comparable with Cr 3+ impregnated on AIPO-5 and assigned to hexa-aquo Cr 3+ ions. Upon calcination, this signal decreases in intensity, a new sharp signal of Cr 5+ is formed, and, in d.r.s., Cr 6+ is formed at the expense of Cr 3+. However, the broad e.s.r. signal of Cr 3+ does not follow the Curie-Weiss law and is due to Cr 3+ clusters. Cr 6+ and Cr 5+ are easily reduced with CO to octahedral Cr 3+ and octahedral and tetrahedral Cr 2+, whereas recalcination reestablishes the original Cr 6+ and Cr 5+ species. All these results support the idea that Cr 3+ is not incorporated into the framework, but is present as octahedral ions at the surface of the AIPO-5 crystals.

Approaches to the aqueous chemistry of chromium (IV) and chromium (V) in macrocyclic and Schiff base complexes

The stabilization of chromium(IV) and chromium(V) in aqueous systems has now been made through designed alterations in Franck-Condon barriers for the interconversion of various oxidation states of chromium. Equatorially coordinated macrocyclic and Schiff base ligands influence the reorganizational barriers for the conversion of Cr(III)-Cr(IV)-Cr(V)-Cr(VI) couples such that Cr(IV) and Cr(V) transients of sufficiently long life time can be generated. A series of macrocyclic and Schiff base complexes of Cr(III) has been synthesised and subjected to non-complementary redox reaction with Ce(IV) or electrochemical oxidation. A Cr(IV) macrocyclic complex, Cr(Me4[14]tetraene)(H2O) 2 4+ , 1 has been generated in aqueous acidic medium, through electrochemical oxidation. The Ce(IV) oxidation of Cr(III) Schiff base complexes, diaqua[N,N′-ethylenebis(salicylideneiminato) chromium(III)], and diaqua[N,N′-propylenebis(salicylideneiminato) chromium(III)] proceed in two stages with 1 Cr:l Ce(IV) redox stoichiometry. Suffīciently long-lived Cr(IV) intermediates and Cr(V) Schiff base products have been generated. ESR evidence for the formation of Cr(V) Schiff base products and the mechanistic outcome of the oxidation of Cr(III) Schiff bases are discussed.

Synthesis of lithium chromium oxide from a hydroxide precursor

The room temperature reaction of Cr(OH) 3·3H 2O with LiOH·H 2O results in an X-ray amorphous solid that appears to form through condensation of the hydrogen-bonded Cr(OH) 3·3H 2O octahedra into edge and corner sharing octahedra. The reaction is explained in terms of analogous sol-gel chemistry of chromium(III). The X-ray amorphous product can be decomposed to yield LiCrO 2 with surface area higher than conventional syntheses.

X‐Ray Absorption Study of Electrochemically Grown Oxide Films on AlCr Sputtered Alloys: II . In Situ Studies

The chemistry of chromium in the passive film on sputter‐deposited chromium and thin films has been investigatedin situ in an electrochemical cell under potential control by studying x‐ray absorption near edge structure. At high potentials, Cr in the alloys was oxidized to the 6‐valent state. Depending on the rate of potential increase, 6‐valent chromium either dissolved from the alloy or was trapped in the passive film where it was electroactive, i.e., the valence state could be reversibly switched between the 3‐ and 6‐valent states by changing the applied potential. The kinetics of these processes were investigated. Ex situ x‐ray photoelectron spectroscopy measurements indicated that, during slow scanning at low potentials, the composition of both the surface oxide and underlying metallic layers changed. These changes resulted in a structure that was susceptible to transpassive dissolution of Cr at potentials above 0.2 V(MSE).

Chemistry of chromium in lakes

The inputs of industrial wastes have changed the levels and forms of chromium in many lakes, and there is growing concern about the accumulation of Cr(VI), a known carcinogen, in drinking water supplies. This report provides a critical review of the effects of speciation, transformations, and solubility on the cycling of chromium in lacustrine environments. Unique features of chromium dynamics associated with the decoupling of redox-driven chromium cycle from those of iron and manganese at the sediment–water and air–water interfaces are highlighted. Reductive precipitation is important in the immobilization of pollutant chromium in sediments. Photooxidative dissolution of Cr(III) especially at the air–water interface is identified as a key area for further research. The available database is inadequate for establishing any linkages between the chromium and biological cycles in lakes.Key words: chromium, lakes, cycling, speciation, redox, geochemistry.

Chromium Redox Chemistry in a Lower Mississippi Valley Bottomland Hardwood Wetland

Experiments were conducted to identify and quantify biogeochemical processes controlling chromium redox chemistry in a seasonally flooded lower Mississippi Valley forested wetland. Chromium speciation, transformations, and solubility were studied in the overlying floodwater column and in the wetland soil. In the floodwater column, chromium chemistry was regulated by Cr(VI) diffusion into the soil, Cr(III) precipitation, complexation by organic ligands, sorption on suspended solids, and Cr(III) to Cr(VI) oxidation.

Synthesis, spectroscopic and magnetic studies of chromium(II) complexes isolated from in vitro reduction of the chromium(VI) ion with the main cellular reductants ascorbic acid, cysteine and glutathione*

The redox chemistry of chromium(VI) is presented with respect to the final products of the interaction of K 2CrO 4 with main cellular reductants. The isolation of a family of chromium(III) complexes obtained from the reduction of the chromate ion with ascorbic acid, crysteine and glutathione is described. The compounds have been characterized by elemental, IR and electronic spectroscopy and magnetic measurements. All complexes exhibit a tetragonal deformation around chromium(III) with the MO 4XX′ chromophore (X,X′—O′,S,N).

Production and Characterization of Extremely Corrosion Resistant Chromium‐Metalloid Alloys

A series of extremely corrosion resistant chromium‐metalloid alloys have been investigated. Glassy , ,, and mixed‐phase glassy‐nanocrystalline were fabricated by thin‐film processing. These binary alloys exhibit extraordinary corrosion resistance when compared with pure crystalline chromium. In chromium dissolves rapidly, ca. 1 g/cm2/day, while negligible corrosion of the chromium‐metalloid alloys occurred, . X‐ray photoelectron spectroscopic analysis of the exposed specimens revealed a strong enrichment of phosphate in the passive film and phosphorus at the metal‐film interface. The interaction of the redox chemistry of chromium and the metalloids is responsible for the superior corrosion resistance.