Retention of chromium(III) and chromium(VI) in rat tissues after different drying procedures

Losses of chromium(III) and chromium(VI) during freeze-drying and ovendrying at different temperatures from various rat tissues and feces containing the radioactive isotopes are assessed. Significant loss of chromium(III) occurs from fur samples (hair with skin) on freeze-drying. Oven-drying at 80°C does not cause losses of either form of chromium but at 105°C there are minor losses from kidney and feces samples. At 120°C, chromium(III) is lost from all samples to varying extents whereas losses of chromium(VI) are less significant.     

Reduction of chromium(VI) by phosphonic acid

The kinetics of electron transfer between chromium(VI) and H3PO3, yielding chromium(III), have been investigated in HClO4 and H2SO4 media by visible spectrophotometry. The rate of reaction increased with increasing [H2SO4] and [HClO4]. A rate law based on ester formation preceding the electron transfer has been established and a possible mechanism has been proposed. The mechanism and the derived rate law are consistent with the observed kinetics. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effects of chromium(III) coordination on the dissociation of acidic peptides

The complexes formed between chromium(III) and synthetic acidic peptides were studied by sustained off-resonance irradiation collision-induced dissociation (SORI-CID) in a Fourier transform ion-cyclotron resonance (FT-ICR) mass spectrometer equipped with electrospray ionization (ESI). Neutral peptides and peptides containing one, two, and multiple acidic residues were studied. Formation of [M + Cr-2H]+ occurred for all peptides. Three noteworthy features were found in the CID spectra of [M + Cr-2H]+. The first is that fewer fragment ions were produced from [M + Cr-2H]+ than from [M + H]+. The reason may be that multiple coordination between chromium(III) and carboxylate or carbonyl groups hinders the production of fragment ions by continuing to bind pieces of the peptide to chromium(III) after cleavage of bonds within the peptide. The second feature is loss of CO from [M + Cr-2H]+ and [y(n) + Cr-H]+. A mechanism involving coordination of chromium(III) with carboxylate groups is proposed to rationalize elimination of CO. The third feature is that chromium(III) is retained in all fragment ions, indicating strong binding of the metal ion to the peptides. The complex [M + 2Cr-5H]+ is formed as the peptide chain length and number of acidic residues increases. Longer peptides have more sites to coordinate with chromium(III) and more conformational flexibility. In addition, formation of [M + Cr-2H]+ from AGGAAAA-OCH(3), which has no carboxylic acid groups, suggests that chromium(III) can coordinate with sites on the peptide backbone, albeit in low abundance. In the negative mode, [M + Cr-4H](-) was only found for peptides containing four or more carboxylic acid groups. This is consistent with deprotonated carboxylic acid groups being involved in chromium(III) coordination and with chromium existing in the 3 + state in the gas-phase ions.     

Pentacarbonyl(2,6-diaminopyridine)chromium(0): synthesis and molecular structure

Photolysis of hexacarbonylchromium(0) in the presence of 2,6-diaminopyridine in toluene solution at 10 °C yields pentacarbonyl(2,6-diaminopyridine)chromium(0), which could be isolated from solution as plate-like crystals and fully characterized by using the single crystal X-ray diffractometry and MS, IR, ¹H and ¹³C NMR spectroscopy. The complex was found to have the 2,6-diaminopyridine ligand bonded to the chromium atom through one of the NH₂ groups. A single crystal X-ray structure of the complex reveals that the coordination sphere around the chromium atom is a slightly distorted octahedron, involving five carbonyls and one 2,6-diaminopyridine ligand. Because of the steric requirement of 2,6-diaminopyridine the four equatorial carbonyl groups are bended away from the N-donor ligand. The pyridine plane makes an angle of 112.9(3)° with the OC-Cr-N bond axis. The Cr-C distances have values between 1.833(7) and 1.935(7) Å. The Cr-N distance is 2.236(5) Å.     

Ion-pair chromatographic determination of chromium(VI)

Ion-pair chromatography (IPC) with conductometric detection was investigated as a precise and selective analytical method for the determination of chromium in electro-plating solutions and waste waters. Chromatographic parameters were optimized for separation of Cr(VI) and SO(2-)(4). The analytical column (100 x 6 mm) was packed with 10 mum silasorb C(18) (Czechoslovakia). Tetrabutylammonium butyrate (TBAB), at pH 7.0 in acetonitrile-water (18:82 v/v) mixture, was used as the eluent. Two samples of solution are taken for the analysis. In the first of them the amount of Cr(VI) is determined, in the second one Cr(III) is oxidized to Cr(VI) with H(2)O(2) in alkaline medium and the total amount of Cr is determined. From the difference of the two obtained results the concentration of Cr(III) is calculated. The detection limit of Cr(VI) is 0.1 mug/ml and the relative standard deviation (at the 1.0 mug/ml) is 4.0%. The IPC results for chromium agreed closely with these obtained by spectrophotometry.     

Solvent extraction of the ion-pairs of chromium(VI) and molybdenum(VI) with trioctylmethylammonium chloride and benzyldimethylcetylammonium chloride

The number of capriquat molecules per chromium(VI) atom in the chromate-capriquat ion-association complex has been found to be between one and two. The distribution ratio in the extraction of chromium(VI) with capriquat is dependent on the dielectric constant of the organic solvent, with a minimum at a dielectric constant of about 8. The absorption spectra of the ion-pair extracted into cyclohexane, carbon tetrachloride, benzene and n-butanol are very similar to that of chromate in aqueous solution. The absorption spectra of the chromium(VI)-capriquat extracts in these organic solvents gradually change to an absorption spectrum similar to that of HCrO(4)(-) in aqueous solution. Chromium(VI)-capriquat extracted into chloroform and 1,2-dichloroethane gives absorption spectra similar to that of HCrO(4)(-)in aqueous medium. The chromium(VI)-capriquat species extracted into 1,2-dichloroethane may be (Q(+))(2).CrO(4)(2-)(H(2)O)(n). In contrast, chromium(VI) is extracted with capriquat into the other organic solvents from ammoniacal medium as a mixture of (Q(+))(2).CrO(4)(2-)(H(2)O)(n) and Q(+).NH(4)(+).CrO(4)(2-)(H(2)O)(n). The spectral change is ascribed to the change of the extracted species from (Q(+))(2).CrO(4)(2-)(H(2)O)(n) and Q(+)NH(4)(+).CrO(4)(-)(H(2)O)(n) to Q(+).HCrO(4)(2-)(H(2)O)(n-1). The chromium(VI)-zephiramine species extracted is formulated as (Q(+), NH(4)(+))(2)CrO(4)(2-)(H(2)O)(n).(Q(+).Cl(-))(m). Molybdenum(VI) is extracted with capriquat into the same organic solvents as a mixture of (Q(+))(2).MoO(4)(2-)(H(2)O)(n) and Q(+).NH(4)(+).MoO(4)(2-).(H(2)O)(n).     

Kinetics and mechanism of oxidation of chromium(III)-l-arginine complex by periodate

Oxidation of the chromium(III)-l-arginine complex [CrIII(L)2(H2O)2]+ by periodate has been investigated. In aqueous solutions, [CrIII(L)2(H2O)2]+ is oxidized by IO−4 according to the rate law: d[CrVI]/dt=k2K5[CrIII]T [IVII]T/1 +([H+]/K1)+K5[IVII]T where k2 is the rate constant for the electron transfer process, K1 the equilibrium constant for the dissociation of [CrIII(L)2- (H2O)2]+ to [CrIII(L)2(H2O)(OH)]+H+, and K5 the pre-equilibrium formation constant. Values of k2= 4.02×10−3s−1, K1=5.60×10−4m and K5=171m−1 were obtained at 30°C and I=0.2m. Thermodynamic activation parameters were calculated. It is proposed that electron transfer proceeds through an inner-sphere mechanism via coordination of IO−4 to chromium(III). This revised version was published online in June 2006 with corrections to the Cover Date.     

Gas-phase reactions of chromium and chromium fluoride cations CrFn+ (n = 0-4) with phosphane

The reactions of chromium and chromium fluoride monocations CrFn+ (n = 0-4) with phosphane are investigated by Fourier-transform ion cyclotron resonance mass spectrometry. Besides condensing slowly with phosphane, Cr+ is unreactive. The ionic products of the chromium fluoride cations are as follows: (i) CrF+ yields CrPH2+ and subsequently CrPH3+; (ii) from CrF2+, the ions PH3+, Cr+, and CrF2H+ are generated; and (iii) both CrF3+ and CrF4+ yield PH3+. The structure and formation of [Cr,P,H3]+ are investigated by collision-induced dissociation and isotopic labeling experiments. For the neutral species [P,H3,F2] formed by reaction of CrF2+ with phosphane, the structures are interrogated by quantum-mechanical calculations at the MP2/6-31++G** level of theory.     

Effect of pH on chromium(VI) species in solution

Published values of equilibrium constants were used to calculate the percentage of each chromium(VI) species (CrO(4)(2-), Cr(2)O(7)(2-), HCrO(4)(-) and H(2)CrO(4)) present in aqueous solution at total chromium(VI) concentrations of 10(-2)-10(-6)M in the pH range 1-8.     

Chromium(II) and chromium(III) complexes supported by tris(2-pyridylmethyl)amine: synthesis, structures, and reactivity

This report describes the synthesis, structural characterization, and polymerization behavior of a series of chromium(II) and chromium(III) complexes ligated by tris(2-pyridylmethyl)amine (TPA), including chromium(III) organometallic derivatives. For instance, the combination of TPA with CrCl(2) yields monomeric (TPA)CrCl(2) (1). A similar reaction of CrCl(2) with TPA, followed by chloride abstraction with NaBPh(4) or NaBAr(F)(4) (Ar(F) = 3,5-(CF(3))(2)C(6)H(3)), provides the weakly associated cationic dimers [(TPA)CrCl](2)[BPh(4)](2) (2A) and [(TPA)CrCl](2)[BAr(F)(4)](2) (2B), respectively. X-ray crystallographic analysis reveals that each chromium(II) center in 1, 2A, and 2B is a tetragonally elongated octahedron; such Jahn-Teller distortions are consistent with the observed high spin (S = 2) electronic configurations for these chromium(II) complexes. Likewise, reaction of CrCl(3)(THF)(3) with TPA, followed by anion metathesis with NaBPh(4) or NaBAr(F)(4), yields the monomeric, cationic chromium(III) complexes [(TPA)CrCl(2)][BPh(4)] (4A) and [(TPA)CrCl(2)][BAr(F)(4)] (4B), respectively. Treatment of 4A with methyl and phenyl Grignard reagents produces the cationic chromium(III) organometallic derivatives [(TPA)Cr(CH(3))(2)][BPh(4)] (5) and [(TPA)CrPh(2)][BPh(4)] (6), respectively. Similar reactions of 4A with organolithium reagents leads to intractable solids, presumably due to overreduction of the chromium(III) center. X-ray crystallographic analysis of 4A, 5, and 6 confirms that each possesses a largely undistorted octahedral chromium center, consistent with the observed S = (3)/(2) electronic ground states. Compounds 1, 2A, 2B, 4A, 4B, 5, and 6 are all active polymerization catalysts in the presence of methylalumoxane, producing low to moderate molecular weight high-density polyethylene.     

Biphasic oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) by N-bromosuccinimide and the formation of chromium(IV) and chromium(V)

The kinetics of oxidation of diaquadichloro(1,10-phenanthroline)chromium(III) complex, [Cr^III(phen)(H₂O)₂Cl₂]⁺, by N-bromosuccinimide (NBS) is biphasic. The first faster step involves the oxidation of Cr(III) to Cr(IV). The second slower step is due to the oxidation of Cr(IV) to Cr(V). The reaction product is isolated and characterized by electron spin resonance (ESR), IR, and elemental analysis. The chromium(V) product is consistent with the formula [Cr^V(phen)Cl₂(O)]Br. The rate constants k_f and k_s, for the faster and the slower steps respectively, were obtained using an Origin 9.0 software program. Values of both k_f and k_s, varied linearly with [NBS] at constant reaction conditions. The effect of pH on the reaction rate is investigated over the pH (4.11–6.01) range at 25.0°C. The rate constants k_f and k_s increased with increasing pH. This is consistent with hydroxo forms of the chromium species being more reactive than the aqua forms. Chromium(III) complexes, more often than not, are inert. The oxidation of the Cr(III) complex to Cr(IV), most likely, proceeds by an outer sphere mechanism. Since chromium(IV) is labile the mechanism of its oxidation to chromium(V) is not certain.     

Two-coordinate, quasi-two-coordinate, and distorted three coordinate, T-shaped chromium(II) amido complexes: unusual effects of coordination geometry on the lowering of ground state magnetic moments

The synthesis and characterization of the mononuclear chromium(II) terphenyl substituted primary amido-complexes Cr{N(H)Ar(Pr(i)(6))}(2) (Ar(Pr(i)(6)) = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-(i)Pr(3))(2) (1), Cr{N(H)Ar(Pr(i)(4))}(2) (Ar(Pr(i)(4)) = C(6)H(3)-2,6-(C(6)H(3)-2,6-(i)Pr(2))(2) (2), Cr{N(H)Ar(Me(6))}(2) (Ar(Me(6)) = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Me(3))(2) (4), and the Lewis base adduct Cr{N(H)Ar(Me(6))}(2)(THF) (3) are described. Reaction of the terphenyl primary amido lithium derivatives Li{N(H)Ar(Pr(i)(6))} and Li{N(H)Ar(Pr(i)(4))} with CrCl(2)(THF)(2) in a 2:1 ratio afforded complexes 1 and 2, which are extremely rare examples of two coordinate chromium and the first stable chromium amides to have linear coordinated high-spin Cr(2+). The reaction of the less crowded terphenyl primary amido lithium salt Li{N(H)Ar(Me(6))} with CrCl(2)(THF)(2) gave the tetrahydrofuran (THF) complex 3, which has a distorted T-shaped metal coordination. Desolvation of 3 at about 70 °C gave 4 which has a formally two-coordinate chromous ion with a very strongly bent core geometry (N-Cr-N= 121.49(13)°) with secondary Cr--C(aryl ring) interactions of 2.338(4) ? to the ligand. Magnetometry studies showed that the two linear chromium species 1 and 2 have ambient temperature magnetic moments of about 4.20 μ(B) and 4.33 μ(B) which are lower than the spin-only value of 4.90 μ(B) typically observed for six coordinate Cr(2+). The bent complex 4 has a similar room temperature magnetic moment of about 4.36 μ(B). These studies suggest that the two-coordinate chromium complexes have significant spin-orbit coupling effects which lead to moments lower than the spin only value of 4.90 μ(B) because λ (the spin orbit coupling parameter) is positive. The three-coordinated complex 3 had a magnetic moment of 3.79 μ(B).     

Sorption of chromium(III) and chromium(VI) on lead sulfide

The sorption of chromium(III) and chromium(VI) on lead sulfide has been investigated in dependence on pH, time of sorption and the concentrations of sorbate and sorbent. The mechanisms of the sorption of Cr³⁺ and CrO ₄ ^2– traces on lead sulfide are discussed; a difference between CrO ₄ ^2– sorption on PbS and -Fe₂O₃ has been found. Sulfates and molybdates affect the removal of chromates from aqueous solutions. Lead sulfide carrier prepared in this work was also used for the preconcentration of chromium(III) and chromium(VI) from tap water.     

Triamidoamine complexes of chromium(III) and chromium(IV)

Treatment of [CrCl3(THF)3] with slightly more than 1 equiv of Li3(N3N) [(N3N)(3-) = ((Me3SiNCH2CH2)3N)(3-)] affords the triamidoamine complex [Cr(N3N)] (1) in 75% yield. 1 is oxidized by PhICl2, CuCl2, or AgCl to give the chromium(IV) complex [Cr(N3N)Cl] (2) in moderate yields. Alternatively, complex 2 is obtained directly from [CrCl3(THF)3] in 50% yield after treatment with 0.5 equiv of Li3(N3N). Both compounds are high-spin complexes bearing three and two unpaired electrons, respectively. Their molecular structures are described revealing a trigonal monopyramidal and trigonal bipyramidal coordination geometry of the chromium center, respectively.     

Kinetics and mechanism of the oxidation of diaqua(nitrilotri- acetato)chromium(III) complex by N-bromosuccinimide

The kinetics of oxidation of the diaqua(nitrilotriacetato)chromium(III) complex [CrIIInta(H2O)2], by N-bromosuccinimide (NBS) to chromium(VI) in aqueous solution obeys the equation: where k1 is the rate constant for the electron-transfer process, K1 the equilibrium constant for the dissociation of [CrIIInta(H2O)2] to [CrIIInta(H2O)(OH)]−, and K2 is the pre-equilibrium formation constant for the precursor complex [CrIIInta(OH)(NBS)]−. The thermodynamic activation parameters were calculated and it is proposed that electron transfer proceeds via an inner-sphere mechanism. This revised version was published online in June 2006 with corrections to the Cover Date.     

Binuclear half-metallocene chromium(III) complexes mediated ethylene polymerization with alkylaluminium as cocatalyst

Binuclear half-metallocene chromium complexes {Cp*[3-(CH==NR)-2-O-C(10)H(5)]CrCl}(2) [Cp* = C(5)Me(5); R = (i)Pr (1), Ph (2), 2,6-(i)Pr(2)C(6)H(3) (3)] based on 1,1'-binaphthyl ligands, as well as their mononuclear analogues Cp*[3-(CH==NR)-2'-R'-2-O-C(20)H(11)]CrCl [R = (i)Pr, R' = (n)BuO (4), R = Ph, R' = (n)BuO (5), R = 2,6-(i)Pr(2)C(6)H(3), R' = (n)BuO (6), R = (i)Pr, R' = H (7)], were synthesized and characterized by mass spectrometry, elemental analysis, magnetic measurement, and UV-vis spectroscopy. The molecular structures of complexes 1, 3, 5 and 6 were further confirmed by single-crystal X-ray crystallographic analysis. When activated with a small amount of AlMe(3), these binuclear complexes exhibited higher activities in catalyzing ethylene polymerization in comparison with their mononuclear analogues, affording high molecular weight polymers with unimodal molecular weight distributions. The highest activity up to 2.87 × 10(6) g PE (mol Cr)(-1) h(-1) was achieved in the catalyst system of complex 3 bearing a bulky 2,6-(i)Pr(2)C(6)H(3) group on the imine nitrogen atom in the presence of 25 equiv. AlMe(3) as activator at 20 °C. (13)C NMR analysis indicates the resultant polymers are linear and no evidence on branch was found.     

Spin-state alteration from sterically enforced ligand rotation in bis(indenyl)chromium(II) complexes

The rotational orientation of cyclopentadienyl rings usually has no effect on d-orbital energy levels and splitting in transition metal complexes. With related but less symmetrical carbocyclic ligands, however, the magnetic properties of the associated complexes can be altered by the alignment of the ligands. Examples of this effect are found in substituted organochromium(II) bis(indenyl) complexes. The monosubstituted compounds (1-RC(9)H(6))(2)Cr (R = t-Bu, SiMe(3)) are prepared from the substituted lithium indenides and CrCl(2) in THF; they are high-spin species with four unpaired electrons. Their spin state likely reflects that in the unknown monomeric (C(9)H(7))(2)Cr, which is calculated to have a high-spin (S = 2) ground state in the staggered configuration (180 degrees rotation angle). However, the analogous bis(indenyl) complexes containing t-Bu or SiMe(3) groups in both the 1 and 3 positions on the indenyl ligands ((1,3-R(2)C(9)H(5))(2)Cr) are low-spin compounds with two unpaired electrons. X-ray diffraction results indicate that [1-(t-Bu)C(9)H(6)](2)Cr exists in a staggered conformation, with Cr-C (av) = 2.32(4) A. In contrast, the average Cr-C distances in [1,3-(t-Bu or SiMe(3))(2)C(9)H(5)](2)Cr are 2.22(2) and 2.20(2) A, respectively, and the rings are in a gauche configuration, with rotation angles of 87 degrees. The indenyl conformations are sterically imposed by the bulk of the t-Bu and SiMe(3) substituents. The change from a staggered to a gauche indenyl orientation lowers the symmetry of a (C(9)H(7))(2)M complex and allows greater mixing of metal and ligand orbitals. Calculations indicate that previously nonbonding pi orbitals of the indenyl anion are able to interact with the chromium d orbitals, producing bonding and antibonding combinations. The latter remain unpopulated, and the resulting increase in the HOMO-LUMO gap forces the complexes to adopt a low-spin configuration. The possibility of using sterically imposed ligand rotation as a means of spin-state manipulation makes indenyl compounds a potentially rich source of magnetically adjustable molecules.     

Investigation of acid sites in a zeotypic giant pores chromium(III) carboxylate

A study of the zeotypic giant pores chromium(III) tricarboxylate Cr(III)3OF(x)(OH)(1-x)(H2O)2 x {C6H3-(CO2)3}2 x nH2O (MIL-100) has been performed. First, its thermal behavior, studied by X-ray thermodiffractometry and infrared spectroscopy, indicates that the departure of water occurs without any pore contraction and no loss in crystallinity, which confirms the robustness of the framework. In a second step, IR spectroscopy has shown the presence of three distinct types of hydroxy groups depending on the outgassing conditions; first, at high temperatures (573 K), only Cr-OH groups with a medium Br?nsted acidity are present; at lower temperatures, two types of Cr-H2O terminal groups are observed; and at room temperature, their relatively high Br?nsted acidity allows them to combine with H-bonded water molecules. Finally, a CO sorption study has revealed that at least three Lewis acid sites are present in MIL-100 and that fluorine atoms are located on a terminal position on the trimers of octahedra. A first result of grafting of methanol molecules acting as basic organic molecules on the chromium sites has also been shown, opening the way for a postsynthesis functionalization of MIL-100.     

Method for the determination of chromium (VI) as chromium (VI)-dibenzyidithiocarbamate

Dibenzyldithiocarbamic acid (DBDC) exhibits the ability to speciate between chromium(VI) and chromium(III), since only the chromium(VI) will form complexes with DBDC. The complex is then extracted into an organic solvent and assayed using an ultraviolet-visible (UV-VIS) spectrophotometer at 498.8 nm. Using 250 ml of aqueous sample detection limits less than 1 ng/ml are possible, while the linear range extends to 500 gmg/ml when working at 498.8 nm. Oxidation of the chromium(III) to chromium (VI) using cerium (IV) enables the determination of total chromium and subsequently the chromium (III) in solution. Evaluation of the method with a standard reference material produced only 4.81 part per thousand error in the determination of chromium(VI).     

Solid-state dinuclear-to-trinuclear conversion in an oxalato-bridged chromium(III)-cobalt(II) complex as a new route toward single-molecule magnets

A novel bis(oxalato)chromium(III) salt of a ferromagnetically coupled, oxalato-bridged dinuclear chromium(III)-cobalt(II) complex of formula [CrL(ox)(2)CoL'(H(2)O)(2)][CrL(ox)(2)]·4H(2)O (1) has been self-assembled in solution using different aromatic α,α'-diimines as blocking ligands, such as 2,2'-bipyridine (L = bpy) and 2,9-dimethyl-1,10-phenanthroline (L' = Me(2)phen). Thermal dehydration of 1 leads to an intriguing solid-state reaction between the S = 3/2 Cr(III) anions and the S = 3 Cr(III)Co(II) cations to give a ferromagnetically coupled, oxalato-bridged trinuclear chromium(III)-cobalt(II) complex of formula {[CrL(ox)(2)](2)CoL'} (2). Complex 2 possesses a moderately anisotropic S = 9/2 Cr(III)(2)Co(II) ground state, and it exhibits slow magnetic relaxation behavior at very low temperatures (T(B) < 2.0 K).