Spectro-thermal characterization of chromium(III) and manganese(II) complexes with carboxyamide

Complexes of Cr(III) and Mn(II) with N′,N″-bis(3-carboxy-1-oxopropanyl) 2-amino-N-arylbenzamidine (H₂L¹) and N′,N″-bis(3-carboxy-1-oxophenelenyl) 2-amino-N-arylbenzamidine (H₂L²) have been synthesized and characterized by various physico-chemical techniques. The vibrational spectral data are in agreement with coordination of amide and carboxylate oxygen of the ligands with the metal ions. The electronic spectra indicate octahedral geometry around the metal ions, supported by magnetic susceptibility measurements. The thermal behavior of chromium(III) complexes shows that uncoordinated nitrate is removed in the first step, followed by two water molecules and then decomposition of the ligand; manganese(II) complexes show two waters removed in the first step, followed by removal of the ligand in subsequent steps. Kinetic and thermodynamic parameters were computed from the thermal data using Coats and Redfern method, which confirm first order kinetics. The thermal stability of metal complexes has been compared. X-ray powder diffraction determines the cell parameters of the complexes.     

Thermal,spectral and magnetic properties of 2-hydroxy-1,4-naphthoquinone monoximates of Ho(III), Er(III) and Yb(III)

Ho(III), Er(III) and Yb(III) complexes of 2-hydroxy-1,4-naphthoquinone-1-oxime derivatives having [ML₃(H₂O)₂] are characterized using spectral and thermal decomposition studies. The thermolytic patterns suggested that they are decomposed in three distinct stages; 1^ststage is related to the loss of two coordinated water molecules while one of the coordinated ligands and remaining two ligands are lost during subsequent 2^nd and 3^rd stages of degradation. After the 2^nd stage, the structure of these complexes is reorganized reflecting that the structural associations through intermolecular hydrogen bonding network is essential for thermal stability. The kinetic parameters computed for 2^nd step using the non-isothermal procedures of Coats-Redfern are applied to the respective differential thermogravimetric plots to ascertain the thermal degradation mechanism in air. The order of thermal decomposition reaction is found to be between 1-2 indicating that more than one intermediate might have simultaneously been formed. It also reveals the intermixing of 1^st and 3^rd stages of decomposition with the predominant 2^nd stage leading to more gradual degradation. Energy of activation for 2^nd stage of decomposition for these complexes is comparatively lower than those observed earlier for similar types of complexes. Other spectral data indicate oximino nitrogen and phenolato oxygen as coordination sites of 2-hydroxy-1,4-naphthoquinone monoximates. This revised version was published online in July 2006 with corrections to the Cover Date.     

Biphasic Oxidation of cis‐Diaquabis(1,10‐phenanthroline)chromium(III) by N‐Bromosuccinimide and the Formation of Chromium(IV) and Chromium(V)

The oxidation of cis‐diaquabis(1,10‐phenanthroline)chromium(III) [cis‐Cr^III(phen)₂(H₂O)₂]³⁺ by ‐bromosuccinimide (NBS) to yield cis‐dioxobis(1,10‐phenanthroline)chromium(V) has been studied spectrophotometrically in the pH 1.57–3.56 and 5.68–6.68 ranges at 25.0°C. The reaction displayed biphasic kinetics at pH < 4.0 and a simple first order at the pH > 5.0. In the low pH range, the reaction proceeds by two successive steps; the first faster step corresponds to the oxidation of Cr(III) to Cr(IV), and the second slower one corresponds to the oxidation of Cr(IV) to Cr(V), the final product of the reaction. The formation of both Cr(IV) and Cr(V) has been detected by electron spin resonance (ESR). The ESR clearly showed the formation and decay of Cr(IV) as well as the formation of Cr(V). Each oxidation process exhibited a first‐order dependence on the initial [Cr(III)]. The pseudo–first‐order rate constants k₃₄ and k₄₅, for the faster and slower steps, respectively, were obtained by a computer program using Origin7.0. Both rate constants showed first‐order dependence on [NBS] and increased with increasing pH.     

Spectroscopic and thermal studies on the decomposition of l,3,5-triamino-2,4,6-trinitrobenzene (TATB)

The kinetics of the thermal decomposition of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) in condensed state has been investigated by high temperature infrared spectroscopy (IR) and thermogravimetry (TG) in conjunction with pyrolysis gas analysis, differential thermal analysis (DTA) and hot stage microscopy. The decomposition proceeds in two main stages under isothermal conditions and the initial stage involving about 24% loss in weight obeys Avrami-Erofe'ev equation (n= 1), and is governed by an activation energy (E) of 150.58 kJ·mol^–1 and log(A in s^–1) 12.06. The second stage corresponding to 24 to 90% loss in weight gave best fit for Avrami-Erofe'ev equation,n=2, withE=239.56 kJ·mol^–1 and log(A in s^–1) 19.88 by isothermal TG. The effect of additives, on the initial thermolysis of TATB has also been studied. Evolved gas analysis by IR showed that NH₃, CO₂, NO₂, HCN and H₂O are produced in the initial stage of decomposition. The decomposition in KBr matrix in the temperature range 272 to 311.5°C shows relative preferential loss in the -NH₂ to -NO₂ band intensity which indicates that the rupture of C-NH₂ bond, weakened also by the interaction of the NH₂ with the neighbouring NO₂ group, appears to be the primary step in the thermolysis of TATB.     

Iron(II) complexes of 2,6-bis(5-alkyl-1,5-dimethyl-4,5-dihydro-1H-imidazol-4-on-2-yl)pyridine ligands. Synthesis, characterisation and solvolytic stability

New 2,6-bis(5-alkyl-1,5-dimethyl-4,5-dihydro-1H-imidazol-4-on-2-yl)pyridines have been prepared and characterized by ¹H-, ¹³C-NMR and elemental analysis. These ligands form complexes with FeCl₂, which were isolated and characterized by elemental analysis and MS. Both in the solid phase and in aqueous and methanolic solutions the complexes have two ligand molecules per one ferrous ion. By means of the kinetic stopped-flow method, the rate constants of acid catalyzed decomposition, which proceeds as a system of two consecutive reactions, were determined. It was found that increasing steric demands of 5-alkyl substituent in the 4,5-dihydro-1H-imidazol-4-one cycle significantly retards the acid catalyzed solvolysis. The methyl derivative is solvolyzed cca two orders faster than the corresponding t-Bu derivative.     

Kinetics and mechanism for the aquation of bismalonatoethylenediaminecobaltate(III) ion in acid perchlorate media

Summary The title complex aquates in acid media, first to [Co(mal)-(H₂O)₂(en)]⁺ (1) (Step 1) and subsequently to [Co(H₂O)₄(en)]³⁺ (2) (Step 2). Complex species (1) has been separated and characterised in solution. While Step 1 involves only a second-order acid catalysed path, Step 2 involves both a first-order acid independent path and a second-order acid catalysed path. The rate constants and activation parameters evaluated for these reaction paths have been compared with those for similar carboxylato-cobalt(III) complexes. This, together with an observed isokinetic relation, indicates that the rate-determining step involves opening of the unprotonated (in the spontaneous acid independent path) or the protonated (for the acid catalysed path) chelate ring of the malonate ligand and insignificant solvation of the central metal ion.     

Radiation effects on the extraction of americium(III) with di(2-ethylhexyl) phosphoric acid

Radiation effects on the extraction of Am(III) with di (2-ethylhexyl) phosphoric acid (DEHPA) was studied by exposing DEHPA to gamma rays under various conditions. Gamma irradiation of undiluted DEHPA causes an enhancement of extraction of Am(III) due to the formation of mono (2-ethylhexyl) phosphoric acid (MEHPA) similarly to that of Nd(III). The presence of diluent during irradiation brought about a slight difference from the results in the absence of a diluent. The marked change occurred in Df when the organic solvent was exposed to γ-ray while being mixed with nitric acid solution. An initial slight increase of Df for Am(III) and Nd(III) was followed by a subsequent decrease beyond an absorbed dose of approximately 200 Wh·1⁻¹. This phenomenon was explained by the enhanced decomposition of DEHPA and the subsequent strong hydrolytic and radiolytic decomposition of MEHPA to H₃PO₄ in the aqueous phase, and the complex forming nature of H₃PO₄ with Am(III) and Nd(III).     

Voltammetric behaviour of dichlorobis(2,2′-bipyridine) iridium(III) and dichlorobis(1,10-phenanthroline) iridium(III) complexes

The electrochemical behaviour of [Ir(bipy)₂Cl₂]⁺ and [Ir(phen)₂Cl₂]⁺ (bipy = 2,2′-bipyridine; phen = 1,10-phenanthroline) has been investigated in N,N-dimethylformamide (DMF). In potential sweep voltammetry [Ir(bipy)₂Cl₂]⁺ exhibits four reduction peaks. The first two processes involve one electron and are reversible in our conditions. The third reduction step is irreversible and has been attributed to the addition of three electrons to [Ir(bipy)₂Cl₂]⁺ followed by fast liberation of ligands. The data obtained for the fourth peak are consistent with a one-electron reversible process. The behaviour of [Ir(phen)₂Cl₂]⁺ is more complicated than that found for the bipy complex. In this case in fact, in addition to the four peaks observed in the case of the bipy complex, two other peaks appear. The latter have been attributed to the reduction of phen molecules liberated by the reduction of the complex. A qualitative MO discussion of the nature of the molecular levels involved in the reduction processes is also reported.     

Studies of the catalytic activity of Fe(III)(salen) complexes as epoxidation catalysts

Two new Fe(III)(salen) complexes, FeL¹ClO₄·2H₂O (1) and FeL²ClO₄ (2) [L¹ = N,N′-ethylenebis(3-formyl-5-methylsalicylaldimine) and L² = N,N′-cyclohexenebis(3-formyl-5-methylsalicylaldimine)], have been synthesized and characterized. The catalytic activity of the complexes for epoxidation of alkenes has been investigated in the presence of two terminal oxidants PhIO and NaOCl, with two solvents CH₃CN and CH₂Cl₂. As alkenes styrene and (E)-stilbene have been chosen for investigation; styrene is a better substrate than electron-rich (E)-stilbene. The study also suggests that unlike their Mn(III) counterparts, 1 and 2 are poor epoxidation catalysts; catalysis proceeds with formation of one intermediate, rather than forming more than one intermediate depending on the terminal oxidant used. Use of exogenous neutral donor ligands such as Py, PyNO and 1-MePy is effective to improve catalytic behavior.     

Thermal behaviour of dimethylgold(III) carboxylates

The thermal properties of dimethylgold(III) carboxylates of general formula [(CH₃)₂Au(OOCR)]₂ (R=methyl (1), tert-buthyl (2), trifluoromethyl (3), or phenyl (4)) in solid state have been investigated by the thermogravimetric analysis. The temperature dependences of saturated vapour pressure of complexes have been studied by the Knudsen effusion method with mass spectrometric indication. The thermodynamic parameters Δ_sub H _T ⁰ and Δ_sub S _T ⁰ of the sublimation processes have been calculated. Thermal decomposition of the vapour of complexes 1 and 2 has been studied by means of high temperature mass spectrometry in vacuum, and by-products of decomposition have been determined.     

Thermal decomposition of lanthanide(III) and Y(III) 3,4,5-trihydroxybenzoates: Preparation, properties

Complexes of lanthanides(III) (La-Lu) and Y(III) with 3,4,5-trihydroxybenzoic acid (gallic acid) were obtained and their thermal decomposition, IR spectra and solubility in water have been investigated. When heated, the complexes with a general formula Ln(C₇H₅O₅)(C₇H₄O₅)·nH₂O (n=2 for La-Ho and Y: n=0 for Er-Lu) lose their crystallization water and decompose to the oxides Ln₂O₃, CeO₂, Pr₆O₁₁, and Tb₄O₇, except of lanthanum and neodymium complexes, which additionally form stable oxocarbonates such as Ln₂O₂CO₃. The complexes are sparingly soluble in water (0.3·10^–5–8.3·10^–4 mol dm^–3).This revised version was published online in November 2005 with corrections to the Cover Date.     

Kinetics and mechanism of the reaction between chromium(III) and picolinic acid in weak acidic aqueous solution

Abstract  

The interaction between chromium(III) and picolinic acid in weak acid aqueous solution was studied, resulting in the formation of a complex upon substitution of water molecules in the chromium(III) coordination sphere. Experimental results show that the reaction takes place in multiple steps. The first step is the formation of an ion pair, the second step (two consecutive steps) is the slow one corresponding to substitution of the first water molecule from the chromium aqueous complex coordination sphere by a picolinic acid molecule via oxygen atom of the carboxylic acid group and substitution of the second water molecule via nitrogen of the pyridine ring forming an 1:1 complex. Both consecutive steps were independent of chromium concentration. The rate constants of the 1st and 2nd consecutive steps were increased by increasing picolinic acid concentration. The corresponding activation parameters are ∆H _1obs ^* = 28.4 ± 4 kJ mol⁻¹, ∆S _1obs ^* = −202 ± 26 J K⁻¹ mol⁻¹, ∆H _2obs ^* = 39.6 ± 5 kJ mol⁻¹, and ∆S _2obs ^* = −175 ± 19 J K⁻¹ mol⁻¹. The third step is fast, corresponding to formation of the final complex [Cr(pic)₃]. The logarithms of the formation constants of 1:1 and 1:3 complexes were found to be 1.724 and 4.274, respectively.     

[Cu(TO)2(H2O)4](PA)2的制备、结构表征和热分解机理研究

[Cu（TO）2（H2O）4]（PA）2 was prepared by the reaction of aqueous 1,2,4-triazol-5-one （TO） solution with the solution of copper picrate Cu（PA）2 and characterized by elemental analysis, FT IR and X-ray powder diffraction analysis. The title complex has been studied by means of TG-DTG and DSC under conditions of linear temperature increase. The thermal decomposition residues were examined by FT IR analysis. Thermal decomposition mechanism of the title complex was proposed. In the temperature range of 30-680 ℃, the thermal decomposition process was composed of four major stages. The first stage was an endothermic process with the loss of four coordination water molecules. Since the dehydration product was unstable, when it was heated, it would be decomposed much more easily. The second stage was composed of an acute endothermic process and a continued strong exothermic process and the main decomposed residues were CuCO3, Cu（NCO）2 and polymers during this stage. The third stage was a sharp exothermic process, which resulted from the decomposition of the polymer. After the forth stage, the final decomposed residues were certainly copper oxide. The Arrhenius parameters have been also studied on the dehydration process and the first-step exothermic decomposition of [Cu（TO）2（H2O）4]（PA）2 using Kissinger＇s method and Ozawa-Doyle＇s method. The results using both methods were consistent with each other. The Arrhenius equation can be expressed as in k=24.0-179.8 × 10^3/RT for the dehydration process and in k= 16.7-206.0 × 10^3/RT for the first-step exothermic decomposition, on the basis of the average of Ea and In A through the two methods.     

TG and DSC studies on Sm(III) and Tb(III) tartrates

The tartrate monohydrates of Sm(III) and Tb(III), Sm₂C₁₂H₁₂O₁₈·H₂O and Tb₂C₁₂H₁₂O₁₈·H₂O, were prepared and characterized on the basis of their elemental analysis and IR spectral studies. The thermal decompositions of these compounds, studied by TG and DSC methods, were found to follow an almost uniform pattern. Decomposition occurs in four steps. The first step involves dehydration, accompanied by partial decomposition to the oxalate, followed by conversion to the carbonate. The ultimate product in each case is the oxide M₂O₃, whereM=Sm or Tb. Reflectance spectra of the terbium compound were recorded at various stages of decomposition. The kinetics of the first decomposition step was studied by the non-isothermal method. TG and DSC data for this step were analysed for the evaluation of various kinetic parameters. Reasonable values ofE, Z, andΔS ⁺ were obtained.α vs. T curves were drawn on the basis of the TG and DSC data. The results suggest that the mechanism involves random nucleation.     

Are Two Metal Ions Better than One? Mono- and Binuclear α-Diimine-Re(CO)3 Complexes with Proton-Responsive Ligands in CO2 Reduction Catalysis

Here, the reduction chemistry of mono- and binuclear α-diimine-Re(CO)₃ complexes with proton responsive ligands and their application in the electrochemically-driven CO₂ reduction catalysis are presented. The work was aimed to investigate the impact of 1) two metal ions in close proximity and 2) an internal proton source on catalysis. Therefore, three different Re complexes, a binuclear one with a central phenol unit, 3 , and two mononuclear, one having a central phenol unit, 1 , and one with a methoxy unit, 2 , were utilised. All complexes are active in the CO₂-to-CO conversion and CO is always the major product. The catalytic rate constant k_cat for all three complexes is much higher and the overpotential is lower in DMF/water mixtures than in pure DMF (DMF=N,N-dimethylformamide). Cyclic voltammetry (CV) studies in the absence of substrate revealed that this is due to an accelerated chloride ion loss after initial reduction in DMF/water mixtures in comparison to pure DMF. Chloride ion loss is necessary for subsequent CO₂ binding and this step is around ten times faster in the presence of water [ 2 : k^Cl(DMF)≈1.7 s⁻¹; k^Cl(DMF/H₂O)≈20 s⁻¹]. The binuclear complex 3 with a proton responsive phenol unit is more active than the mononuclear complexes. In the presence of water, the observed rate constant k_obs for 3 is four times higher than of 2 , in the absence of water even ten times. Thus, the two metal centres are beneficial for catalysis. Lastly, the investigation showed that the phenol unit has no impact on the rate of the catalysis, it even slows down the CO₂-to-CO conversion. This is due to an unproductive, competitive side reaction: After initial reduction, 1 and 3 loose either Cl⁻ or undergo a reductive OH deprotonation forming a phenolate unit. The phenolate could bind to the metal centre blocking the sixth coordination site for CO₂ activation. In DMF, O−H bond breaking and Cl⁻ ion loss have similar rate constants [ 1 : k^Cl(DMF)≈2 s⁻¹, k^OH≈1.5 s⁻¹], in water/DMF Cl⁻ loss is much faster. Thus, the effect on the catalytic rate is more pronounced in DMF. However, the acidic protons lower the overpotential of the catalysis by about 150 mV.     

Synthesis and Thermal Decomposition Kinetics of Two Dy(III) Complexes with Benzoate Derivative and 2,2′‐Bipyridine as Co‐ligands

Two Dy(III) complexes with benzoate derivative and 2,2′‐bipyridine ligands, [Dy(2,4‐DClBA)₃bipy]₂ and [Dy(o‐MOBA)₃bipy]₂·4H₂O (2,4‐DClBA=2,4‐dichlorobenzoate; o‐MOBA=o‐methoxybenzoate; bipy=2,2′‐bipyridine), were prepared and characterized by elemental analysis, infrared spectra, ultraviolet spectra and thermogravimetry and differential thermogravimetry techniques. The thermal decomposition behavior of the two complexes under a static air atmosphere was discussed by thermogravimetry, differential thermogravimetry and infrared spectral techniques. The non‐isothermal kinetics were investigated by using a double equal‐double step method, a non‐linear isoconversional integral method and a Starink method. The mechanism functions of the first decomposition step for [Dy(2,4‐DClBA)₃bipy]₂ and the second decomposition step for [Dy(o‐MOBA)₃bipy]₂·4H₂O were determined. Meanwhile, the thermodynamic parameters (ΔH^ne;, ΔG^ne; and ΔS^ne;) and kinetic parameters (activation energy E and the pre‐exponential factor A) for the two complexes were also calculated.     

Reaction of pentaammine(urea-O)cobalt(III) cation with chlorine and hypochlorous acid in aqueous solution

Summary The Co(NH₃)₅[OC(NH₂)₂]³⁺ cation in aqueous acid reacts with chlorine and hypochlorous acid, with two sequential steps observed in each case. Rate constants for both steps show a first-order dependence on [oxidant], with k₁/k₂ always <20, but varying with the choice of reactant and acid. Rate constants with Cl₂ as reactant are faster than with HOCl, possibly related to preferential attack by Cl⁺ compared with OCl^– on the bound urea. Competition by ions (HSO ₄ ^– , Cl^– or NO ₃ ^– ) measured by product analysis of reactions conducted in 1 M acid produced competition ratios R (R=[CoX]/[CoOH₂][X]) which are similar to values determined with a range of leaving groups previously, indicating a mechanistic constancy. No formation of Co(NH₃)₅Cl²⁺ was observed in reactions conducted in H₂SO₄ or HNO₃, implying that free Cl^– is not generated at the reaction site and captured by the metal ion. Electronic and vibrational spectra of the intermediate formed in the two stage reaction is indicative of a change from an O-bound to an N-bound ligand in forming that intermediate, although it cannot be a simple isomerization due to the dependence on [oxidant]. A possible mechanism is discussed.     

Decomposition of cis-and trans- potassium diaquobis(oxalato)chromate(III)

The decomposition of cis- and trans-K[Cr(C₂O₄)₂](H₂O)₂] has been studied using differential scanning calorimetry. Dehydration occurs as the first step with activation energies being 27.5 and 13.9 mol^?1, respectively, for the cis and trans complexes. After dehydration, continued heating results in loss of CO amd CO₂. For the trans complex, an additional endothermic peak is seen and the mass loss indicated that CO has been lost in a single step. In both cases, the final product indicated by mass loss data is KCrO₂.     

Kinetics and Mechanism of Chromium(III)-Picolinato and Chromium(III)-Dipicolinato Complexes Aquation in HNO3 Solutions

The acid-catalyzed aquation of [Cr(pic)(H₂O)₄]₂ ²⁺ and [Cr(dpic)(H₂O)₃]⁺(pic = picolinic acid anion, dpic = dipicolinic acid dianion) in nitrate(V) media was studied. The reaction is reversible in the case of the pic-complex and practically irreversible in the case of the dpic-complex. It is assumed that the reactive form of the substrate undergoes fast chelate ring-opening followed by protolytic equilibria, followed by the rate of the Cr—O bond breaking of the monodentate bonded ligand which is the rate-determining step. The kinetics of pic/dpic ligand liberation were followed spectrophotometrically in the 0.4–2.0 M HNO₃ range at I= 2.0 M. The following dependences of the pseudo-first order rate constants on [H⁺] have been established:k _obs=a+b[H⁺](where b and a are apparent rate constants for the forward and the reverse reaction of the pic-complex) and k _obs=b[H⁺]+c[H⁺]²(where b and c are apparent rate constants for the dpic liberation). Fast protolytic pre-equilibria, leading to protonation of the carboxylic oxygen atom on the monodentate bonded ligand, preceeds ligand liberation.     

Synthesis,Crystal Structure and Thermal Decomposition Process of Complex [Sm(BA)3phen]2

A binuclear samarium(III) complex with benzoic acid and 1,10‐phenanthroline, [Sm(BA)₃phen]₂ was synthesized and characterized by elemental analysis, UV, IR and TG‐DTG techniques. The structure of the title complex was established by single crystal X‐ray diffraction. The crystal is triclinic, space group P1 with a = 10.8216(11) Å, b = 11.9129(13) Å, c = 12.425(2) Å, α = 105.007(2)°, β = 93.652(2)°, γ = 113.2630(10)°, Z = 1, D_c = 1.650 mg·m^?3, F(000) = 690. The carboxylate groups are bonded to the samarium ion in three modes: bidentate chelating, bidentate bridging, and tridentate chelating‐bridging. Each Sm³⁺ ion is coordinated to one bidentate chelating carboxylate group, two bidentate bridging and two tridentate chelating‐bridging carboxylate groups, as well as one 1,10‐phenanthroline molecule, forming a nine‐coordinate metal ion. Based on thermal analysis, the thermal decomposition process of [Sm(BA)₃phen]₂ has been derived.