Atomic concentration of hydrogen in tetra- and hexaamine heteroligand chromium(III) complexes with cyclic tetraamines

Based on the revealed dependence of the atomic concentration of hydrogen (N_H) on the composition and structure of tetra- and hexaamine heteroligand chromium(III) complexes with cyclic tetraamines, routes to new materials with high N_H for preparing effective composites for protection from neutron radiation were suggested.     

Solid-Phase Transformations in Tetraamine and Hexaamine Mixed-Ligand Complexes of Chromium(III) with Cyclic Tetraamines

We have studied solid-phase transformations in mixed-ligand complexes of chromium(III) with cyclic tetraamines. We have established that tetraamine complexes of chromium(III) with 14-membered tetraaza macrocyclic ligands are relatively thermally stable, and do not undergo isomerization in the solid state. We have observed that solid-phase reactions of ammonia substitution in hexaamine complexes with outer-sphere iodide and boron hydride anions may be accompanied by dehydrogenation processes.     

Investigation of deceleration causes in gold electrowinning from aqueous cyanide solutions on porous carbon electrodes

Deceleration of gold electrowinning from model cyanide solutions on porous cathode of graphitized carbon felt was investigated. It was established that calcium ions present in the solution did not negatively affect the electrolysis rate. The main reason of the termination of gold electrowinning on the graphitized cathode was the corrosion of the stainless steel anode resulting in penetration into the cathode chamber of the electrolyzer of CrO₄²⁻ anions further reduced to Cr(III) and probably Cr(II) compounds. X-ray Photoelectron Spectroscopy investigation of the passivated carbon cathode showed that the film formed thereon consisted of Cr(III) compounds containing cyanide and hydroxy ligands. This film covers the active sites on the surface of the porous carbon cathode preventing the deposition thereon of ad-ions of gold(I). Inert aqua and hydroxo complexes of chromium(III) stronger impede the rate of gold(0) deposition on the porous carbon cathode than chromium(III) cyanide compounds. This fact originates presumably from the requirement of lower cathode potentials for the reduction of the former to labile chromium(II) complexes than those necessary for the thermodynamically stable anions [Cr(CN)₆]³⁻.     

Chromium(III) and indium(III) 3,6-di-<Emphasis Type="Italic">tert</Emphasis>-butyl-<Emphasis Type="Italic">o</Emphasis>-semiquinolate complexes as redox mediators of hydrogen sulfide oxidation in reactions with cycloalkanes

The electrochemical oxidation of the chromium(III) and indium(III) complexes with 3,6-di-tert-butyl-o-semiquinolate leading to the formation of active monocationic species is studied by cyclic voltammetry. The reactions of the latter with hydrogen sulfide generate the radical cation of H₂S, whose fragmentation affords the proton and thiyl radical. These complexes are proposed for the first time as redox mediators for the one-pot thiolation of inert cycloalkanes C₆–C₈, which decreases the activation energy of hydrogen sulfide compared to that for direct electrochemical oxidation. The major products of cycloalkane functionalization involving H₂S are thiols and organic di- and trisulfides. The yield of the synthesized compounds depends on the type of the mediator: the chromium(III) complex exhibits the highest efficiency in the electrocatalytic transformations.     

Binuclear chelates of diethylenetriamine-Pentaacetic acid with chromium(III) and oxovanadium(IV) ions

The reactions of diethylenetriaminepentaacetic acid (DTPA=H₅L) or chromium(III)-DTPA, a ‘chelating agent’, with oxovanadium(IV) were investigated in aqueous solution by potentiometric methods. Homo- and hetero-binuclear species were evidenced as well as mixed complexes with hydrogen or hydroxide ions. The stability constants for these equilibria were calculated in 1.0 mol 1^?1 NaClO₄ solution at 20?C. The mononuclear (VO)H₃L·H₂O and the heterobinuclear (VO)CrL·5H₂O complexes were also obtained as solid compounds and were characterized by IR spectroscopy and thermoanalytical techniques (TG and DSC).     

Solvation of iron and chromium complexes in alcohol–water mixtures

Solubilities are reported for the perchlorates of five iron(II)-diimine complexes in t-BuOH–H₂O and one in MeOH–H₂O mixtures, for three iron(III)-3-hydroxy-4-pyranonate and three iron(III)-3-hydroxy-4-pyridinonate complexes in MeOH–H₂O and t-BuOH–H₂O, and for two chromium(III)-3-hydroxy-4-pyranonate complexes in MeOH–H₂O. Transfer chemical potentials are thence derived for the various iron(II), iron(III) and chromium(III) complexes, for transfer from H₂O into the respective mixed solvents (at 298.2 K). These results are combined with values reported earlier for related complexes, and for other alcohol–H₂O mixtures, to give an overall picture of solvation, expressed in the thermodynamic format of transfer chemical potentials, for iron(II)-diimine, iron(III)-3-hydroxy-4-pyridinonate and chromium(III)-3-hydroxy-4-pyranonate complexes in H₂O-rich aqueous-alcohol mixtures. Some spectroscopic (¹H-n.m.r.; i.r.) and kinetic (aquation rate constants at 298.2 K) data are reported for the chromium(III) complexes.     

Chromium(III) and Chromium(IV) Tetraphenylporphine Complexes

Stable chromium complex (AcO)CrTPP was synthesized through the reaction of meso-tetraphenylporphine with chromium(III) acetate in boiling phenol. Coordination properties of chromium porphyrin in reaction with imidazole and pyridine in o-xylene were studied by electronic absorption spectroscopy and computer modeling. A single-electron oxidation of chromium(III) complex was found to be affected by peroxide compounds. The stability of an extra complex depends on the basic properties of the extra ligand and oxidation number of the central metal atom. The complex stability correlates with the calculated energy of formation of the metal–extra ligand bond. The geometrical structure and energy parameters of hexacoordinated chromium porphyrins were calculated using the quantum-chemical method. The effect of the cis and trans position of ligands in the composition of a macrocyclic compound was established to be significant only in the extra complexes (AcO)CrTPP.     

Thermal behavior of tetradentate Schiff base chromium(III) complexes

Thermal analytical behavior of eight chromium(III) complexes with N,N′-bis(salicylidene)ethylenediamine (salen) Schiff bases, Cr(salen), has been investigated regarding their thermal stability and thermal decomposition pathways. Thus, the ligands and the respective chromium(III) complexes of salen-type Schiff bases derived from salicylaldehyde and its 5-chloro, 5-bromo, 5-methoxy, 5-nitro, 3,5-dicloro, 3,5-dibromo, and 3,5-diiodosalicylaldehyde were synthesized, characterized, and submitted to TG/DTG, DTA, and TG–FTIR evolved gas analysis. The number of steps and, in particular, the starting temperature of decomposition of these complexes was dependent of the ligand nature. The gaseous products evolved during the thermal decomposition of these compounds were identified by TG–FTIR.     

Copper(II), iron(III), and chromium(III) complexes with 5,10-dioxo-4,5,9,10-tetrahydro-4,9-diazapyrene derivatives

Six copper(II), iron(III), and chromium(III) complexes with 5,10-dioxo-4,5,9,10-tetrahydro-4,9-diazapyrene derivatives (H₂L¹-H₂L³) have been synthesized and studied by physical methods (IR and electronic absorption spectroscopy, quantum-chemical calculations). The composition of the complexes has been determined and their stability constants in aqueous dimethylformamide solutions have been calculated. The energy characteristics, electronic structure and geometry of isolated diazapyrenes and their tautomeric forms have been calculated by the PM6 method, and their complexes have been modeled.     

Logistic Model of Ligand Selection for Chromium-Plating Electrolytes Based on Cr(III) Complexes

The effect of the ligand nature in chromium(III) complexes on chromium electrodeposition was studied. The predicting logistic model of ligand selection for chromium-plating electrolytes was constructed from experimental data. It was found that high-quality chromium coatings can be deposited from electrolytes of chromium(III) complexes with a ligand having the following acid–basic characteristics in the protonated form: pK ₁ < 4.0 and 4.0 < pK _n < 18.6. The proposed model was verified experimentally and used to prepare a chromium-plating electrolyte based on a chromium(III) malonate complex, which gives high-quality chromium coatings over a wide current density range.     

The synthesis,spectral and magnetic properties of the complexes of chromium with chrysenequinone and chrysenequinonemonoxime

Reactions of the chromium hexacarbonyl with chrysenequinone and chrysenequinonemonoxime gave the tris(chrysenesemiquinone)chronium(III), Cr(ChrySQ)₃, and tris(chrysenesemiquinonemonoxime)chromium(III), Cr(ChrySQM)₃, complexes. From the stretching frequencies of the carbonyl groups in the IR spectra of the complexes, it was concluded that the ligands bonded to the metal in the semiquinone form. Also, the electronic absorption spectra of the two complexes showed strong UV bands due to metal-to-ligand charge transfer. The magnetic susceptibility determination for the Cr(ChrySQ)₃ complex at 300 K gave a value of 3.261 × 10⁻⁶ e.m.u.g⁻¹ with an effective magnetic moment (μ_eff) of 2.55 μ_B. Investigation of Cr(ChrySQ)₃ and Cr(ChrySQM)₃ by EPR spectroscopy at room temperature showed isotropic g values of 1.997 and 1.991 for the two complexes, respectively. The isotropic g values are greater than those observed previously for various osemiquinone chromium complexes. Chrysenequinonemonoxime reacted with Cr(NO₃)₃ to give the bis(chrysenequinonemonoximato) (chrysenequinonemonoxime)chromium(III)nitrate. Spectroscopic studies of the complex suggested that the chrysenequinonemonoxime ligand is attached to the metal in both nitroso and oxime forms. Examination of the chromium complexes by cyclic voltammetry showed some reversible or quasireversible redox reactions due to tautomeric interconversions of the semiquinone—catechol couples through electron transfer.     

Dinuclear trivalent chromium,manganese, iron and cobalt complexes bridged by aromatic diamines

Summary Trivalent chromium, manganese, iron and cobalt salts react withm-phenylenediamine and acetylacetone to give complexes of the type [M(C₁₆H₁₈N₂O₂)X]: X = Cl, Br, NO₃ or NCS for M = chromium(III) and iron(III); X = Cl, Br, OAc or NCS for manganese(III); and X = OH for cobalt(III). Conductance measurements show the complexes to be nonelectrolytes. Molecular weights determined cryoscopically, show the iron complexes to be dinuclear. Magnetic measurements above 85 K reveal the presence of slight antiferromagnetic interactions. The complexes are dimeric five-coordinate square-pyramidal withm-phenylenediamine residues acting as bridges. The electronic spectra are interpreted in terms of the Normalised Spherical Harmonic Hamiltonian Theory and the DT/DQ values which indicate that chromium complexes are slightly distorted, whereas those of manganese are severely distorted.     

Mass spectrometric investigations of the kinetic stability of chromium and copper complexes with humic substances by isotope-labelling experiments

Isotope-labelling exchange experiments were carried out to investigate the kinetic stability of Cr(III) complexes with humic substances (HS). To compare the results with those of an ion, not expected to form kinetically stable HS complexes with respect to its electron configuration, Cu(II) was investigated under the same conditions. HS solutions of different origin were therefore spiked with ⁵³Cr(III) or ⁶⁵Cu(II) after saturation of HS with chromium and copper of natural isotopic composition. In fractions of metal/HS complexes with different molecular weight, obtained by ultrafiltration and HPLC/ICP-MS using size exclusion chromatography (SEC), respectively, the isotope ratios of chromium and copper were determined by ICP and thermal ionisation mass spectrometry. Distinct differences in the isotopic composition of chromium were found in the permeate of the ultrafiltration compared with the corresponding unseparated solution, which indicates kinetically stable Cr(III)/HS complexes. On the other hand, the copper isotopic composition was identical in the permeate and the unseparated solution, which shows that a total exchange of Cu²⁺ ions took place between free and HS complexed copper ions. The SEC/ ICP-MS experiments also resulted in a different isotopic distribution of chromium in the chromatographically separated complexes whereas the copper complexes, separated by SEC, showed identical isotopic composition. The kinetic stability of Cr(III)/HS complexes could be explained by the d³ electron configuration of Cr³⁺ ions, a fact which is well known from classical Cr(III) complexes, and influences substantially the mobility of this heavy metal in the environment.     

Photochemistry of macromolecular metal complexes. II. Synthesis,thermal, and photochemistry of some polypyridyl complexes of chromium (III) bound to macromolecules

Polymer coordinated chromium(III) complexes [Cr(bpy)₂(PAA)₂]⁺, 1 , [Cr(bpy)₂-(PMA)₂]⁺, 2 , [Cr(phen)₂(PAA)₂]⁺, 3 , and [Cr(phen)₂(PMA)₂]⁺, 4 , [where bpy, phen, PAA and PMA are, respectively, 2,2′-bipyridine, 1,10-phenanthroline, poly(acrylic acid), and poly(methacrylic acid)] were synthesized. The polymer–chromium(III) complexes were characterized by elemental and spectroscopic analyses. Thermal substitution reactions of these macromolecular chromium(III) complexes in basic solutions lead to the replacement of the polypyridyl ligand by hydroxide ion while in strong acidic solutions the polymer complexes precipitate out. The photochemical reactions are qualitatively similar to that of the thermal reactions and the quantum yields are dependant on the pH of the medium. Further, lower quantum yields were observed for the aquation of the polymer complexes in comparison with the monomeric chromium(III) complexes and the results are discussed in terms of the effect of the polymer environment. Flash photolysis of 1 and 3 results in the formation of transients with maxima at 480 nm for 1 and 470 nm, 580 nm for 3 . The decay of the transients were found to obey first order kinetics and the rate constants were determined. The transients were suggested to be the alkyl-chromium complexes. Flash photolysis of 2 and 4 does not produce transients which is interpreted to be due to the presence of a methyl group in the ligand which hinders the formation of the carbonchromium bond.     

The effect of Cr(III)-organic complexes on the determination of inorganic chromium species in groundwater by ammonium pyrrolidinedithiocarbamate–methylisobutylketone extraction procedure

Groundwater samples collected from a tannery contaminated area were analyzed for chromium species with the objective of investigating the interference of Cr(III)-organic complexes in the determination of Cr(VI) using APDC–MIBK extraction procedure. The contribution of Cr(III), Cr(VI) and Cr(III)-organic complexes towards total chromium ranged between 2 and 61%, 27 and 86%, and, 6 and 23%, respectively. The Cr(III)-organic complexes were not extractable by APDC–MIBK, however, HNO₃ digestion released the organic bound Cr(III). Interference of organic bound Cr(III) in Cr(VI) determination due to MIBK soluble Cr(III) was not observed. Significant difference between total dissolved chromium determined after appropriate digestion procedure, and the sum of dissolved Cr(III) and Cr(VI) determined indicates the presence of the Cr(III)-organic complexes. MIBK extraction of samples without APDC is an useful way to check the extractability of organic bound Cr(III). The presence of soluble Cr(III)-organic complexes thus add complexity to chromium speciation analysis by APDC–MIBK procedure.     

Comparative study of analytical methods for the determination of chromium in groundwater samples containing iron

Determination of chromium in groundwater samples containing iron may pose analytical problems due to sorption and fixation of chromium species onto Fe(III) hydroxides. Parks et al. (Water Res. 2004, 38, 2827) hypothesized that chromium species trapped inside Fe(III) hydroxides i.e. “fixed chromium” may not be soluble by HNO₃ digestion (APHA method 3030 B). In such cases, hydroxylamine digestion is required to release “fixed chromium”. To verify the hypothesis, we carried out this study on groundwater samples containing chromium and iron, using different methods of APHA and EPA. The results showed the presence of “fixed chromium”, ranged between 0.1 and 19.2 μg L^{− 1}, contributing 0.2 to 14.1% towards true total chromium. Digestion of samples with HNO₃ released Cr(III) bound to organic complexes, but not the “fixed chromium”. The hydroxylamine digestion released “fixed chromium”, but not the Cr(III) bound to organic complexes. Microwave digestion of samples with HNO₃ + HCl was effective for the release of both “fixed” and “Cr(III)-organic complexes”. Cr(III) was only adsorbed onto suspended matter, whereas Cr(VI), and Cr(III)-organic complexes were not adsorbed onto suspended matter due to their solubility. Sample pH, buffering capacity, and matrix have a significant influence on the adsorption and fixation of chromium species onto Fe(III) hydroxides.     

Regularities of complexation in the series of sorbents on the polystyrene matrix and their complexes with chromium(III) and manganese(II)

The physicochemical properties of polymeric complexing sorbents with the o,o′-dihydroxy-(1-azo-1′)-functional complexing group were studied. Optimal sorption parameters for chromium(III) and manganese(II) (medium acidity, temperature, time), the sorption capacity of the sorbents, and the stability constants (pK _a′)of the complexes were determined. Correlation relations were established between the dissociation constants (pK _a′) of the sorbent functional group and pH₅₀ of the complexation of the considered elements; between and the stability of the formed complexes (logβ); and between pK _a′ and the deprotonation energy (E _depr) of the sorbent hydroxy group located in the para-position to the introduced substituent. The established correlation relations are useful for targeted prediction of the physicochemical parameters of the sorbents, their complexes, and the sorption of manganese(II) and chromium(III) aimed at recovery and pre-concentration of these ions from objects with various chemical composition.     

Prediction and correlations between the stability constants of mono- and polynuclear chromium(III) and iron(III) complexes

Correlations between the experimentally determined stability constants of mono- and polynuclear chromium(III) and iron(III) complexes are discussed. An equation to evaluate the stability constants of mono- and polynuclear chromium(III) complexes is obtained: \(\log \beta [Cr_p^{3 + } (L_i )_{q_i } ] = 0.84\log \beta [Fe_p^{3 + } (L_i )_{q_i } ]\) .     

Mass spectrometric investigations of the kinetic stability of chromium and copper complexes with humic substances by isotope-labelling experiments

Isotope-labelling exchange experiments were carried out to investigate the kinetic stability of Cr(III) complexes with humic substances (HS). To compare the results with those of an ion, not expected to form kinetically stable HS complexes with respect to its electron configuration, Cu(II) was investigated under the same conditions. HS solutions of different origin were therefore spiked with ⁵³Cr(III) or ⁶⁵Cu(II) after saturation of HS with chromium and copper of natural isotopic composition. In fractions of metal/HS complexes with different molecular weight, obtained by ultrafiltration and HPLC/ICP-MS using size exclusion chromatography (SEC), respectively, the isotope ratios of chromium and copper were determined by ICP and thermal ionisation mass spectrometry. Distinct differences in the isotopic composition of chromium were found in the permeate of the ultrafiltration compared with the corresponding unseparated solution, which indicates kinetically stable Cr(III)/HS complexes. On the other hand, the copper isotopic composition was identical in the permeate and the unseparated solution, which shows that a total exchange of Cu²⁺ ions took place between free and HS complexed copper ions. The SEC/ ICP-MS experiments also resulted in a different isotopic distribution of chromium in the chromatographically separated complexes whereas the copper complexes, separated by SEC, showed identical isotopic composition. The kinetic stability of Cr(III)/HS complexes could be explained by the d³ electron configuration of Cr³⁺ ions, a fact which is well known from classical Cr(III) complexes, and influences substantially the mobility of this heavy metal in the environment. Received: 7 December 1998 / Revised: 25 March 1999 / Accepted: 27 March 1999     

Electrode processes and their kinetic parameters in the reduction and oxidation of Cr(III) and Cr(II) in molten CsCl

The reduction of chromium ions in the CsCl-CrCl₃ melt in wide concentration and temperature ranges is reported. It is assumed that the melt contains polynuclear chromium complexes. The reduction of Cr(III) to Cr(II) is a reversible one-electron process. The electroreduction of divalent chromium to chromium metal is an irreversible process involving dinuclear and mononuclear chromium complex ions. The electron transfer rate constant has been calculated for various temperatures and chromium chloride concentrations in the salt melt. The activation energy of electron transfer and its dependence on the CrCl₃ concentration in the electrolyte have been determined for the Cr(III) ↔ Cr(II) process.