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.     

Effect of molecular association and reactivity on substitution in chromium(III)–salprn complexes

A new chromium(III)–Schiff base complex, [Cr(5-chlorosalprn)(H₂O)₂]ClO₄, where salprn = N,N-propylenebis(salicylideneimine) has been prepared and characterized by electrospray ionization mass spectrometric (ESIMS) analysis and other spectroscopic techniques. Single crystal X-ray data reveal that the complex assumes a trans-diaquo structure, [Cr(C₁₇H₁₈Cl₂N₂O₄)]ClO₄ · H₂O. The effect of phenyl ring substituents on the rate of formation of [O=Cr^V Schiff base]⁺ has been investigated. The bimolecular rate constant for the formation of O=Cr^V species by the [Cr(Schiff base)(H₂O)₂]ClO₄, where the Schiff base = salprn, (1) and 5-chlorosalprn, (2) with PhOI was compared. In the case of (2) the rate was found to be faster by an order of magnitude at pH = 4 compared to (1). The introduction of a chloro-substituent on the phenyl ring not only influences the rate of redox reactivity but also the pKa values of aquo ligands of the complexes, indicating the difference in the electronic environment around the metal ion in both (1) and (2).     

Enantiospecific Synthesis of (–)-Cuspareine and (–)-Galipinine

An enantiospecific route to the synthesis of tetrahydroquinoline alkaloids (–)-cuspareine and (–)-galipinine is reported. Coupling of an iodide derivative of D-serine with aromatic dithianes and Pd-catalyzed intramolecular C–N coupling are the key steps in the synthesis.     

New chromium(III)–nicotinate complexes. Kinetics and mechanism of nicotinate ligand liberation in acidic media

Two new chromium(III)–nicotinate complexes, cis-[Cr(C₂O₄)₂(O-nic)(H₂O)]⁻ and cis-[Cr(C₂O₄)₂(N-nic)(H₂O)]⁻, were obtained and characterized in solution (where O-nic=O-bonded and N-nic=N-bonded nicotinic acid). The kinetics of nicotinate ligand liberation were studied spectrophotometrically in the 0.1–1.0 m HClO₄ range, at I=1.0 m. The rate equations were determined and a mechanism is proposed. The rate of Cr–O bond breaking is [H⁺] dependent: k_obs=k_HQ_H[H⁺], where k_H is the acid-catalyzed rate constant and Q_H is the protonation constant of the nonbonded oxygen atom in the O-coordinated ligand. The Cr–N bond breaking proceeds via two paths: spontaneous and acid-catalyzed; k_obs=k₀ + k_HQ_H[H⁺], where k₀ and k_H are the spontaneous and acid catalyzed rate constants and Q_H is the protonation constant of the carboxylic group in the N-bonded nicotinic acid. The results demonstrate by comparison that Cr–N bond breaking is a much slower process than Cr–O bond fission.     

Binary and ternary chromium(III) complexes with cellular reductants and DNA components isolated from redox type systems. Part 1. Chromium(VI)–cysteine–adenine (adenosine,ATP)

The reduction of CrO₃ with an excess of L-cysteine and its interaction with DNA fragments (adenine, adenosine) and ATP nucleotide was studied by analysis of the isolated solid products. The precipitates were characterised by elemental analyses, FAB mass spectral data, spectroscopic methods (u.v.–vis., f.i.r., i.r.) and magnetic measurements. The Cr^III complexes obtained appear to be various Cr^III cysteinate and adenine or ATP (but not adenosine) ternary species of the addition type bound through the hydrogen bonding network.     

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.     

1,3-Dipolar cycloaddition reaction of nitrone η6-(arene)chromium tricarbonyl complexes with styrene and η6-(styrene)chromium tricarbonyl

The products of 1,3-dipolar cycloaddition of the nitrone-type η⁶-(arene)chromium tricarbonyl complexes (CO)₃CrC₆H₅CH=N⁺(O^?)R, where R = Me, Ph, Bu^t, with styrene and η⁶-(styrene)chromium tricarbonyl were obtained and characterized by a combination of physicochemical methods. This type of reactions proceeded with very high regio- and stereoselectivity to exclusively form cis-2,3,5-tri-substituted isoxazolidines.     

Synthesis and ferromagnetic interaction in oxamido- bridged heterodinuclear copper (Ⅱ)- chromium (Ⅲ) complexes

Four μ-oxamido heterodinuclear complexes, [Cu(oxae)Cr(L)2] (NO3)3,where oxae denotes the N, N'-bis (2-aminoethyl) oxamido dianion and L represents 1, 10-phenanthroline ( phen); 5-nitro-1, 10-phenanthroline (NO2-phen); 5-methyl-1, 10-phenanthroline (Me-phen) and 2,2'-bipyridine (bpy), have been synthesized and characterized by elemental analyses, magnetic moments (at room temperature) and molar conductivity measurements and spectroscopy. It is proposed that these complexes have extended oxamido-bridged structures consisting of a copper (Ⅱ) ion and a chromium (Ⅲ) ion, which have a square planar environment and octahedral environment, respectively. The cryomagnetic properties of the [Cu(oxae)Cr(bpy)2] (NO3)3(1) and [Cu(oxae)Cr(phen)2] (NO3)3(2) complexes have been measured over the range of 4.2-300 K. The least-squares fit of the experimental data based on the spin Hamiltonian, H = - 2JS1·S2, the exchange integrals (J) were evaluated as 36.9 cm-1 for 1 and 35.8 cm-1 for 2. The results have confirmed     

Synthesis and characterization of chromium(I) bis(η-toluene) derivatives containing sterically demanding anions

The reaction of Cr(η⁶-CH₃C₆H₅)₂ with 1-benzoyl-6-hydroxy-6-phenyl fulvene, dbcpH, and with pentakis(methoxycarbonyl)cyclopentadiene, pcmcpH, proceeds with evolution of dihydrogen and the formation of the ionic derivatives [Cr(η⁶-CH₃C₆H₅)₂][X] ([X]⁻ = 1,2-dibenzoylcyclopentadienyl, [dbcp]⁻, pentakis(methoxycarbonyl)cyclopentadienyl, [pcmcp]⁻), which have been characterized by IR and EPR spectroscopies, X-ray diffraction and electrochemical techniques. The sterically demanding anions do not affect the structural and electronic properties of the cations in solution but strongly influence crystal packing. In fact, a rare cis-eclipsed conformation of the toluene rings is found for [Cr(η⁶-CH₃C₆H₅)₂][dbcp] · THF, whereas two independent complexes are observed in the unit cell of [Cr(η⁶-CH₃C₆H₅)₂][pcmcp], one with toluene rings in a cis-eclipsed conformation and the other in a staggered conformation (projections of methyl groups form an angle of 151°).     

Synthesis, characterization and magnetism of μ-chloranilato bi-nuclear chromium (III) complexes

Five new chloranilato-bridged binuclear chromium (III) complexes have been synthesized and identified as [Cr2(CA)L4]-(ClO4)4[L denotes 5-methyl-1,10-phenanthroline (Me-phen); 2,9-dimethyl-1, 10-phenanthroline ( Me2-phen); 5-chloro-1,10-phenanthroline(Cl-phen); diaminoethane (en) or 1,3-diaminopropane (pn)], where CA represents the dianion of chloranilic acid. Based on elemental analyses, molar conductivity and magnetic moment of room-temperature measurements, and IR and electronic spectral studies, it is proposed that these complexes have CA-bridged structures and consist of two chromium (III) ions, each in an octahedral environment. The complexes [Cr2(CA)(Me-phen)4](ClO4)4(1) and [Cr2(CA)(Me2-phen)4](ClO4)4(2) were further characterized by variable temperature (4.2-300 K) magnetic susceptibility measurements and the observed data were successfully simulated by the equation based on the spin Hamiltonian operator, , giving the exchange parameter J = -7.8 cm-1 for (1) and J= -6.5 cm*1 for (2). This result     

Nucleophilic substitution reactions of acetylides on substituted tricarbonyl(η6-fluoroarene)chromium and reactions of tricarbonyl[η6-(2-trimethylsilylethynyl)toluene]chromium and tricarbonyl[η6-(p-ethynyl-phenylethynyl)benzene]chromium with dicobalt octacarbonyl

Nucleophilic substitution reactions of various acetylides on substituted tricarbonyl(η⁶-fluoroarene)chromiums were pursued. The reaction presumably underwent a more complicated mechanism rather than the direct substitution on the fluorine-bearing carbon. The organometallic compounds (η⁶-C₆H₃R¹R²R³)Cr(CO)₃ (R¹: CC–C₆H₄CH₃, R²: o-Me, R³: H (5a), R¹: CC–C₆H₄CH₃, R²: o-OMe, R³: H (6a), R¹: CC–C₆H₄CH₃, R²: m-OMe, R³: H (6b), R¹: CCPh, R²: o-Me, R³: o-OMe (8b), R¹: CCPh, R²: m-Me, R³: m-OMe (8c), R¹: CCSiMe₃, R²: o-Me, R³: H (9a), R¹: CC–C₆H₄CCH, R²: H, R³: H (12), R¹: CC–C₆H₄CCH, R²: o-Me, R³: H (13)) as well as the organometallic dimmer [{(η⁶-o-Me-C₆H₄)Cr(CO)₃(di-ethynyl)] (di-ethynyl: CC–C₆H₄CC (14)) have been synthesized from nucleophilic substitution reactions of tricarbonyl(η⁶-fluoroarene)(chromium) compounds with suitable acetylides. The products have been characterized by spectroscopic means. In addition, (8b) and (8c) were characterized by X-ray diffraction studies. Further reactions of (9a) and (12) with appropriate amount of Co₂(CO)₈ yielded μ-alkyne bridged bimetallic complexes, Co₂(CO)₆{μ-Me₃SiCC–(o-tolueneCr(CO)₃} (10) and (Co₂(CO)₆)₂{μ-HCC–C₆H₄–CC–(benzene)Cr(CO)₃)}(15), respectively. Both (10) and (15) were characterized by spectroscopic means as well as single crystal X-ray crystallography. The core of these molecules is quasi-tetrahedron containing a Co₂C₂ unit. A two-dicobalt-fragments coordinated di-enyls complex, (Co₂(CO)₆)₂{μ-HCC–C₆H₄–CC–H} (17), was synthesized from the reaction of 1,3-diethynylbenzene with Co₂(CO)₈. Crystallographic studies of (17) also show that it exhibits a distorted Co₂C₂ quasi-tetrahedral geometry.     

Solubility of β-diketonate complexes for cobalt(III) and chromium(III) in supercritical carbon dioxide

The solubilities of tris(2,2,6,6-tetramethyl-3,5-heptanedionate) cobalt(III) (Co(thd)₃) and chromium(III) (Cr(thd)₃) in supercritical carbon dioxide (scCO₂) were measured at temperatures ranging from 313 to 343 K. The measurements were carried out using a circulation-type apparatus with a UV–vis spectrometer. The solubilities of both Co(thd)₃ and Cr(thd)₃ increased as both the density of scCO₂ and the temperature increased, which has the same tendency as cobalt(III) acetylacetonate (Co(acac)₃) and chromium(III) acetylacetonate (Cr(acac)₃) had in our previous work. The solubilities of Cr(thd)₃ were higher than that of Co(thd)₃, and the solubilities of Co(thd)₃ and Cr(thd)₃ were about 50- and 70-fold higher than those of Co(acac)₃ and Cr(acac)₃, respectively. The measured solubilities of the metal complexes were correlated with the equation based on Chrastil's equation. The parameters were determined by correlating the experimental data for each metal complex, and the correlated results well reproduced the experimental data, especially Co(thd)₃. Moreover, the charge density distributions on the molecular surface of CO₂ and the metal complexes used in the measurement were estimated by the quantum chemical calculation and the COSMO-RS to clear the effect of the molecular structure of the metal complexes on the affinity for CO₂.     

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.     

Hydrolytic and redox transformations of chromium(III) bis-oxalato complexes with glutaminic acid and glutamine: a kinetic,UV–Vis and EPR,study

The title complexes have been synthesized, chromatographically isolated and characterized by their ligands to metal ratio determinations and spectroscopic analyses. The kinetics of the first aquation stage, i.e., the amino acid chelate ring opening via the Cr–N bond cleavage, has been studied spectrophotometrically in acidic and alkaline media. Hydrogen peroxide oxidizes the complexes in alkaline media to CrO ₄ ^2? anion and a relatively stable Cr(V) complex. Consecutive biphasic kinetics through two first-order steps were observed for the base hydrolysis and the oxidation process, whereas the acid-catalyzed aquation obeys a simple first-order pattern. Based on the kinetic and spectroscopic data, mechanisms of the coordinated amino acid liberation and chromium(III) oxidation are discussed.     

Pyrolysis kinetics of ethylene–propylene (EPM) and ethylene–propylene–diene (EPDM)

The thermal degradation kinetics of several ethylene–propylene copolymers (EPM) and ethylene–propylene–diene terpolymers (EPDM), with different chemical compositions, have been studied by means of the combined kinetic analysis. Until now, attempts to establish the kinetic model for the process have been unsuccessful and previous reports suggest that a model other than a conventional nth order might be responsible. Here, a random scission kinetic model, based on the breakage and evaporation of cleavaged fragments, is found to describe the degradation of all compositions studied. The suitability of the kinetic parameters resulting from the analysis has been asserted by successfully reconstructing the experimental curves. Additionally, it has been shown that the activation energy for the pyrolysis of the EPM copolymers decreases by increasing the propylene content. An explanation for this behavior is given. A low dependence of the EPDM chemical composition on the activation energy for the pyrolysis has been reported, although the thermal stability is influenced by the composition of the diene used.     

Studies on high-temperature thermal transformation and dielectric property of aluminum–chromium phosphates

High-temperature thermal transformation of aluminum–chromium phosphates has been investigated by means of DSC–TG, IR, and XRD analysis. The relative dielectric constant and thermal decomposition were measured and discussed. The results show that crystallization and thermal decomposition started at about 1,273 K, only AlPO₄ and Cr₂O₃ have been found at 1,873 K due to the decomposition of PO ₃ ^? , P₂O ₇ ^2? , and PO ₄ ^3? . The relative dielectric constant is fluctuant.     

Stereoselective hetero Diels–Alder reactions of chiral tricarbonyl (η6-benzaldehyde)chromium complexes

The ZnCl₂-promoted cycloaddition of a series of enantiopure ortho-substituted benzaldehyde–Cr(CO)₃ complexes and Danishefsky's diene gave the corresponding 2-aryl pyranones in good yields and complete enantiospecificity. Some of the mechanistic aspects of the cycloaddition were investigated and the reaction extended to different dienes.     

Synthesis and application in enantioselective hydrogenation of new free and chromium complexed aminophosphine–phosphinite ligands

A straightforward synthesis of new free and Cr(CO)₃ complexed AMPP ligands (4–7) is described starting from (S)-indoline-2-carboxylic acid. The ligands were applied successfully in the asymmetric hydrogenation of α-functionalized ketones i.e. a ketolactone 8, a ketoamide 9 and an aminoketone 10 leading to the corresponding optically active alcohols in 99, 97, and 99% ee respectively.     

Synthesis,characterization and charge-transfer photochemistry of trans-aquo azido-N,N′-trimethylenebis(naphthylideneiminato)chromium(III)

A new Cr^III complex, viz. trans-[Cr(naphprn)(H₂O)₂]ClO₄·2H₂O, (1) {naphprn=N,N-trimethylenebis(naphthylideneimine)} has been synthesized and characterized. Aquo ligand substitution of ( 1 ) by the azide ligand gave rise to trans-[Cr(naphprn)(N₃)(H₂O)], ( 2 ). Irradiation of ( 2) in DMF gave nitrido(naphprn)chromium(V) ( 3 ). Solutions of ( 3 ) showed e.p.r. spectra at room temperature (g _iso=1.9865). The i.r. spectra showed disappearance of a band at 2067cm^–1 showing the breakdown of NN and appearance of a new band at 1074cm^–1which is assigned to the Cr¹⁴N, stretching frequency indicating the formation of a nitrido chromium(V) complex, ( 3). The u.v.–vis. spectrum of complex ( 3 ) exhibited a d–d band maximum at 553nm (=120M^–1cm^–1). The rate of formation of ( 2 ) was found to be 5.0×10^–3M^–1s^–1 in an aqueous acidic medium at [Cr^III]=0.5×10^–3M; [N₃ ^–]=0.01–0.15M; [H⁺]=0.001M and I=0.2M (LiClO₄). The rate of photo-decomposition of ( 2 ) to give rise to ( 3 ) was found to be 0.15×10^–3s^–1 in DMF.