Rate equation of gelation of chromium(Ⅲ)-polyacrylamide sol

During gelation, the time dependence of the apparent viscosity has been used to analyze the kinetics of the gelation of polyacrylamide (PA) sol with chromium (Ⅲ) ions. The investigations have stablished the following facts: Before gelation the relation between the viscosity,η , of Cr(Ⅲ)-PA sol and the PA concentration isη= f([PA]) = 2.36X1015 [PA]3.15 and the rate equation of gelation in the steady-state stage during gelation is expressed as Vη, = ks f([PA]) [PA]2 [Cr(Ⅲ)]2.     

Study of the behavior of chromium(III) in potassium nitrate solutions at 25°C

Solubility in the KNO₃-Cr(NO₃)₃-H₂O system at 25°C was studied by an isothermal method. The existence of solid phases of potassium nitrate and chromium(III) nitrate nonahydrate was confirmed by constructing a phase diagram, chemical analysis, and IR spectroscopy. Crystallization of potassium nitrate from aqueous solutions was studied. Potassium nitrate does not interact with chromium(III) nitrate within the range of micro and macro concentrations. The capture of chromium(III) by KNO₃ crystals is due to adsorption and occlusion of a mother solution.     

Determination of reaction heat on the hydrolytic polymerization of chromium(Ⅲ) ion by microcalorimetric method

Thermochemistry is the study of the energy changes which occur during a chemi-cal reaction. For example, in the hydrolytic polymerization of Cr~(3+), Cr[Cr(OH)_2]~(4+),Cr[Cr(OH)_2]_2~(5+) and Cr[Cr(OH)_2]_3~(6+) were formed at concentration range of 0.005-0.04     

Ferrocene-containing ligands in the self-assembly of trinuclear μ3-oxocentered carboxylate complexes of chromium(III, III, III)carboxylate complexes of chromium(III, III, III)

New trinuclear μ₃-oxocentered chromium(III, III, III) complexes were obtained by the self-assembly of ferrocenecarboxylate ligands and the Cr₃O fragment. The complexes were investigated by fast-atom bombardment (FAB) mass spectrometry, cyclic voltammetry, and electronic and IR spectroscopy. V. I. Vernadskii Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, 32/34 Prospekt Akademika Vernadskogo, Kiev 03142, Ukraine. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 36, No. 4, pp. 233–237, July–August, 2000.     

Analysis of the Rydberg structure in the vapor-phase electronic absorption spectrum of (η7-cycloheptatrienyl)(η5-cyclopentadienyl) chromium

Parameters of the Rydberg transitions in the vapor-phase absorption spectrum of (η⁷-C₇H₇)(η⁵-C₅H₅)Cr were analyzed in detail. A correspondence between the three Rydberg series in the short-wavelength region of the spectrum and low-frequency Rydberg bands was established. Vibronic structure of the observed transitions to the lowest Rydberg s, p _x,y , p _z , and d _xz,yz levels was interpreted. The long-wavelength and short-wavelength series, respectively characterized by quantum defect (σ) values of 1.26 and 0.82, were unambiguously assigned to the Rydberg p _x,y and d _xz,yz excitations, respectively. Transitions from the 3d _z 2 orbital to theR(n−1)f,Rnd _z 2, andRnp _z levels can contribute to the series characterized by a σ value of 1.04. The assignment was made of Rydberg bands in the spectral region corresponding to the principal quantum number (n) values of 5, 6, and 7 (in this region, interpretation of the spectral pattern is complicated because of the band shifts and broadening). Atn>5, changes in the σ values of the Rydberg excitations with increase in then value are due to configuration interaction. The electronic-excited states, which can be responsible for the observed changes in the Rydberg parameters, were determined. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1677–1684, September, 1999.     

Mechanism of dehydrogenation of cyclohexane on the crystalline α-oxide of chromium

Russian Chemical Bulletin - Dehydrogenation of cyclohexane on crystalline α-Cr2O3 proceeds according to a sextet-type mechanism. This conclusion agrees with the structural principles of...     

Liquid-phase oxidation of dibenzyl ether in the presence of homo- and heteronuclear chromium(III) μ3-oxotrimetalhexapivalates

On the basis of a comparison of the rates of formation of the hydroperoxide, benzaldehyde, and benzoic acid—the basic products of the liquid-phase oxidation of dibenzyl ether in the presence of complexes with the composition Cr₂MOPiv₆·3L (M=Cr(III), Fe(III), Ni(II), Co(II); L=PPy, γ-Pic, H₂O)—it was concluded that, when the sequence of reactions dibenzyl ether→hydroperoxide→benzaldehyde has occurred to a small extent, activation of the substrates occurs at the same catalytic center. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 35, No. 3, pp. 183–189, May–June 1999.     

Kinetics and mechanism of the chromium(III)–picolinato chelate ring opening in some chromium(III)–oxalato–picolinato complexes in acidic aqueous solutions

Two Cr^III–picolinato complexes were obtained and characterized in solution. The [Cr(C₂O₄)(pyac)₂]^– and [Cr(C₂O₄)₂(pyac)]^2– ions (pyac = picolinic acid anion) in acidic solutions undergo a reversible one-end Cr^III–picolinato chelate ring opening via Cr^III—N bond breaking. The reaction rate was determined spectrophotometrically in the 0.1–1.0 M HClO₄ range at I = 1.0 M. The observed pseudo-first order rate constant depends on [H⁺] according to the equation: k _obs = a + b[H⁺] + c/[H⁺]. A reaction mechanism, which assumes participation of the protonated and unprotonated forms of the reactants, has been proposed. The kinetic parameters a, b, c have been defined as a = k ₁, b = k ₂ Q ₁, c = k _–1/Q ₂, where k ₁, k _–1,k ₂ are rate constants for the forward and reverse processes and Q ₁, Q ₂ are the protolytic equilibrium constants in the term of the proposed mechanism. The activation parameters have been determined and discussed.     

Synthesis and characterization of the first transition-metal fullerene complexes containing bis(η-benzene)chromium moieties

While photochemical reaction of C₆₀ with an equimolar amount of Mo(CO)₄(η⁶-Ph₂PC₆H₅)₂Cr (1) in toluene at room temperature produced bimetallic Mo/Cr fullerene complex fac/mer-(η²-C₆₀)Mo(CO)₃[(η⁶-Ph₂PC₆H₅)₂Cr] (2) in 87% yield, the thermal reaction of an equimolar mixture of C₆₀, M(dba)₂ (M = Pd, Pt; dba = dibenzylideneacetone) and (η⁶-Ph₂PC₆H₅)₂Cr (3) in toluene at room temperature afforded bimetallic M/Cr fullerene complexes (η²-C₆₀)M[(η⁶-Ph₂PC₆H₅)₂Cr] (4, M = Pd; 5, M = Pt) in 88% and 92% yields, respectively. Products 2, 4 and 5 are the first transition-metal fullerene complexes containing bis(η⁶-benzene)chromium moieties. While 2, 4 and 5 were characterized by elemental analysis and spectroscopy, the crystal molecular structures of 4 along with the starting materials 1 and 3 have been determined by X-ray diffraction techniques.     

Molecular organization and aggregation of mucin at air–water interface in the presence of chromium(III) complexes

The interfacial organization of mucin (glycoprotein) in the presence of chromium(III) complexes has been assessed from the surface pressure–molecular area (π–A) isotherms in Langmuir films at air–water interface and the surface energy of their LB films through contact angle measurements. At pH 7.0, the electrostatic interaction of [Cr(en)₃]Cl₃ with mucin was found to bring about changes in the average surface area from 3.26 to 1.47 nm²; suggesting the possible formation of large aggregates of mucin. Adsorption experiments using surface potential measurements reveal that [Cr(en)₃]Cl₃ binds at a much faster rate to the available binding sites in mucin when compared to [Cr(salen)(H₂O)₂](ClO₄) which binds coordinatively to mucin.     

Kinetic Investigation of the Electrochemical Oxidation of Bis(benzene)chromium(0) in Diethyl ketone/N,N-Dimethylformamide

The cyclic voltametric technique utilizing a platinum working electrode was applied for the investigation of the electrochemical oxidation of bis(benzene)chromium(0), (C₆H₆)₂Cr to bis(benzene)chromium(I), (C₆H₆)₂Cr⁺ in diethyl ketone (DEK), N,N-dimethylformamide (DMF), and DEK/DMF binary mixtures containing n-tetrabutylammonium hexafluorophosphate (TBAPF₆) as the supporting electrolyte at T=298.15 K. The half-wave potentials (E _1/2) of the (C₆H₆)₂Cr^+/0 redox couple in DEK, DMF and DEK/DMF binary mixtures, were determined. The variation of E _1/2 with the solvent composition was found to be almost linear. The E _1/2 results were analyzed in terms of the electron-donating power of the solvent medium. The diffusion coefficients, D, were calculated using the Randles-Sevcik equation. The kinetics of the electrode reaction were investigated through the determination of the heterogeneous electron-transfer rate constants, k _s, according to the electrochemical rate equation proposed by Nicholson. Furthermore, the activation Gibbs energies for the electron-transfer process (ΔG ^≠) were also calculated. The results indicate that the redox couple (C₆H₆)₂Cr^+/0 exhibits an electrochemically reversible and diffusion-controlled process in all the investigated solvent media.     

Conductance Determination of the Ion Association of Tris-(ethylenediamine)chromium(III) Chloride in Dimethyl Sulfoxide–Water Mixtures at Various Temperatures

The electrical conductances of tris-(ethylenediamine)chromium chloride, ([Cr(en)₃]Cl₃; en = ethylenediamine) were measured as functions of temperature and concentration in 20–80 wt% dimethyl sulfoxide (DMSO)–water mixtures. Conductance data were analyzed by the Kraus–Bray and Shedlovsky models. Also, density and viscosity values for 20–80 wt% DMSO–water mixtures have been determined experimentally at temperatures varying from (288.15 to 318.15) K. The limiting molar conductance, Λ ⁰, and the association constant, K _A, for ([Cr(en)₃]Cl₃ were computed by using Shedlovsky’s equation. The Λ ⁰ values decrease with increase in the percentage of DMSO in the mixed solvent. The K _A values increase with increasing temperature and increasing percentage of DMSO in the mixed solvent at all temperatures. This may be attributed to the increase in association constant with the decrease in the relative permittivity of the mixed solvent. Thermodynamic parameters (Gibbs energy, enthalpy and entropy changes) were determined from the temperature dependence of K _A for ([Cr(en)₃]Cl₃. The results of the study have been interpreted in terms of ion–solvent interactions and solvent properties.     

Coordination polymers of zinc with (η6-benzenecarboxylate) chromium tricarbonyl

Different coordination polymers were obtained by the reaction of (benzoic acid) chromium tricarbonyl with zinc acetate in the presence of various organic dipyridyl linkers. Depending on the nature of the linker either monomeric or polymeric compounds were obtained. Reactions of (benzoic acid) chromium tricarbonyl with zinc acetate and bidentate pyridine based ligands 4,4'-bipyridine (4,4'-bipy), 1,2-bis(4-pyridyl)ethane (bpe), 1,3-bis(4-pyridyl)propane (tmdp), and 2,2'dipyridylamine (DPA) afforded the novel coordination polymers [Zn[{η(6)-C(6)H(5)COO}Cr(CO)(3)](2)(4,4'-bipy)](n), [Zn[{η(6)-C(6)H(5)COO}Cr(CO)(3)](2)(bpe)](n), [Zn[{η(6)-C(6)H(5)COO}Cr(CO)(3)](2)(tmdp)}](n), and the monomeric complex [Zn[{η(6)-C(6)H(5)COO}Cr(CO)(3)](2)(DPA)]. The solid state structures of all compounds were determined by single crystal X-ray diffraction. By using 1,3-bis(4-pyridyl)propane as a linker a chiral infinite helical structure was formed in the solid state. Thermogravimetric analysis (TGA) studies showed that upon heating the carbonyl groups of the {η(6)-C(6)H(5)COO}Cr(CO)(3) anion were lost before the organic ligand sphere was thermally decomposed.     

Liquid secondary ion mass spectrometry of several β-diketonates of cobalt(III) and chromium(III)

The positive, liquid secondary ion (LSI) mass spectra of six cobalt(III) and three chromium(III) (β-diketonates ligand = L^?) were examined in a 3-nitrobenzyl alcohol matrix. The complexes of both metals yield clean, matrix-free mass spectra, but there are important differences between them. The cobalt compounds show prominent peaks assignable to the molecular ion, CoL ₃ ⁺ , of the monomeric chelates, together with abundant dimeric ions, such as Co₂L ₄ ⁺ and Co₂L ₃ ⁺ ; in contrast, chromium complexes show protonated monomers, CrL₃H⁺, in addition to ionized monomers, CrL ₃ ⁺ , and only minor formation of dimeric ions. The collisionally-activated dissociation (CAD) mass spectrum of Co₂L ₄ ⁺ shows fragmentation to CoL ₂ ⁺ and Co₂L ₃ ⁺ . That of Co₂L ₃ ⁺ shows fragmentation only to dimeric ions, including Co₂L ₂ ⁺ and, for thienyl or phenyl substituted ligands, to Co₂L₂Ar⁺ or Co₂LAr⁺ (Ar = thienyl or phenyl). Neither Co₂L ₄ ⁺ nor Co₂L ₃ ⁺ dissociates to the CoL ₃ ⁺ ion. The LSI mass spectrum of a mixture of two different cobalt chelates shows dimeric ions containing both types of ligand, which can be explained by ion-molecule reactions in the selvedge region. The differing behaviors of the cobalt and chromium complexes is attributed to the relatively greater stability of the +2 oxidation state for cobalt than for chromium.     

The effect of complexing agents on the DMF–chromium(VI) reaction. A kinetic study

The kinetics of the reduction of chromium(VI) by dimethylformamide (DMF) in the presence of ethylenediaminetetraacetic acid (EDTA) and 2,2-bipyridyl (bpy) was investigated in aqueous HClO₄ acid solutions spectrophotometrically. The experimental findings, that the reaction has an induction period followed by autoacceleration, is explained. After the reduction, chromium(III) is present as the EDTA- and bpy-complexes, although such complexes form very slowly under the same experimental conditions from chromium(III) and the EDTA and bpy, respectively. Increases in the reaction rate with increasing [EDTA] were observed, while added bpy had negligible effect on the reaction rate. The reaction is first-order each in [Cr^VI] and [H⁺]. The first-order kinetics with respect to EDTA at low concentrations shift to zero-order at higher concentrations. The reaction is considered to proceed through the formation of a very stable chromium(VI)–DMF–EDTA complex. The suggested mechanism refers only to the induction period of the reaction. The net rate of oxidation of DMF, as measured by the consumption of chromium (VI), is given by: –d[Cr^VI]/dt = kK ₁ K _b[H⁺][DMF]_T[EDTA][Cr^VI]_T/(1 + K ₁[EDTA])(1 + K _b[H⁺])     

The effect of the precursor structure on the catalytic properties of the nickel—chromium catalysts of hydrogenation reactions

The influence of the Ni²⁺/Cr³⁺ ratio in the precursor compound on the formation of the catalyst structure and its transformation upon the thermal treatment and reductive activation in hydrogen was studied. The precursors with the cation ratio Ni²⁺/Cr³⁺ = (2.3–3)/1 represent a homogeneous system of the stichtite-type structure. The treatment of the precursors at T ～400 °C in an inert atmosphere forms a nanosized phase of the NiO-type structure with the lattice parameter a = 4.186±0.005 Å. At 600 °C the lattice parameter of this phase decreases to the tabulated value (a = 4.177±0.005 Å). The phase of nickel chromite of the cubic spinel structure with the lattice parameter a = 8.320±0.005 Å is also observed. Hydrogen activation of the catalyst preheated at 300 °C in an inert gas leads to the formation of Ni⁰ crystallites with a size of ～5.5 nm and a specific surface area of ～7.0 m² g^?1. This catalyst exhibits high activity and selectivity in benzene hydrogenation and preferential CO hydrogenation in the presence of CO₂. The catalysts with the ratio Ni²⁺/Cr³⁺ = 1/(2?3) containing nickel and chromium hydroxocarbonates as precursors are less active in the hydrogenation of benzene to cyclohexane.     

Kinetics and mechanism of 2,2′-bipyridine-catalyzed chromium(VI) oxidation of propan-2-ol in the presence and absence of surfactants

Under kinetic conditions, monomeric Cr(VI) has been found kinetically active in the absence of bipy while in the bipy-catalyzed path, Cr(VI)-bipy complex has been suggested as the active oxidant. The uncatalyzed path shows the second-order dependence on [H⁺] while the bipy-catalyzed path shows the first-order dependence on [H⁺]. Both the uncatalyzed and bipy-catalyzed paths show the first-order dependence on [propan-2-ol]_T and [Cr(VI)]_T. The bipy-catalyzed path is the first order in [bipy]_T. Cetylpyridiniumchloride inhibits the reactions while sodium dodecyl sulfate catalyzes the reactions in the presence and in the absence of bipy.     

Solvent extraction—atomic absorption determination of chromium(VI) in soils

The extraction of chromium(VI) from aqueous saline solutions (NaNO₃) using a trioctylamine solution in toluene was studied in order to determine chromium(VI) in soil samples by atomic absorption spectrometry. It was found that the quantitative recovery of chromium(VI) was attained after extraction with the 0.1 M extractant solution (pH 1.5) for 15 min followed by-back extraction with 4 M HNO₃. Chromium(III) was not extracted under these conditions     

A homobimetallic complex of chromium(0) with a σ-borane component

The synthesis, characterization, and reactivity of a chromium(0) complex bearing an amine-borane moiety (η(6)-C(6)H(5)CH(2)NMe(2)·BH(3))Cr(CO)(3) (2) is reported. Photolysis of complex 2 results in the elimination of a CO ligand followed by the formation of an intramolecular σ-borane complex (η(1)-(η(6)-C(6)H(5)CH(2)NMe(2)·BH(2)-H))Cr(CO)(2) (3). This species was characterized in solution by NMR spectroscopy. Reaction of complex 2 with photochemically generated (OC)(5)Cr(THF) affords a novel homobimetallic σ-borane complex (OC)(3)Cr(η(6)-C(6)H(5)CH(2)NMe(2)·BH(2)-H-Cr(CO)(5)) (4), wherein one of the BH moieties is bound to the chromium center in an η(1)-fashion. The σ-borane complex 4 was isolated in moderate to good yield (72%). The BH(3) fragment in the complexes 3 and 4 are highly dynamic involving exchange of the BH hydrogen bound to the metal with the terminal BH hydrogen atoms. The dynamics has been studied using variable-temperature NMR spectroscopy. Complexes 2 and 4 have been characterized by X-ray crystallography.