- Nucleation, growth, and repair of a cobalt-based oxygen evolving catalyst
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The mechanism of nucleation, steady-state growth, and repair is investigated for an oxygen evolving catalyst prepared by electrodeposition from Co2+ solutions in weakly basic electrolytes (Co-OEC). Potential step chronoamperometry and atomic force microscopy reveal that nucleation of Co-OEC is progressive and reaches a saturation surface coverage of ca. 70% on highly oriented pyrolytic graphite substrates. Steady-state electrodeposition of Co-OEC exhibits a Tafel slope approximately equal to 2.3 × RT/F. The electrochemical rate law exhibits a first order dependence on Co2+ and inverse orders on proton (third order) and proton acceptor, methylphosphonate (first order for 1.8 mM ≤ [MePi] ≤ 18 mM and second order dependence for 32 mM ≤ [MePi] ≤ 180 mM). These electrokinetic studies, combined with recent XAS studies of catalyst structure, suggest a mechanism for steady state growth at intermediate MePi concentration (1.8-18 mM) involving a rapid solution equilibrium between aquo Co(II) and Co(III) hydroxo species accompanied with a rapid surface equilibrium involving electrolyte dissociation and deprotonation of surface bound water. These equilibria are followed by a chemical rate-limiting step for incorporation of Co(III) into the growing cobaltate clusters comprising Co-OEC. At higher concentrations of MePi ([MePi] ≥ 32 mM), MePO 32- equilibrium binding to Co(II) in solution is suggested by the kinetic data. Consistent with the disparate pH profiles for oxygen evolution electrocatalysis and catalyst formation, NMR-based quantification of catalyst dissolution as a function of pH demonstrates functional stability and repair at pH values >6 whereas catalyst corrosion prevails at lower pH values. These kinetic insights provide a basis for developing and operating functional water oxidation (photo)anodes under benign pH conditions.
- Surendranath, Yogesh,Lutterman, Daniel A.,Liu, Yi,Nocera, Daniel G.
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- Kinetic Investigation of the Oxidation of Bromide Ions by Cobalt(III). Part 2. - The Influence of Pyridine and Hydrocarbon on the Reaction in Acetic Acid Solvent
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The kinetics of cobalt(III) acetate reduction in acetic acid have been studied under nitrogen at 60-93 deg C in the presence of bromide, p-xylene and pyridine.At high p-xylene concentration the initial rate of reaction obeys the rate low with k
- Schlosserova, Julia,Hronec, Milan,Vesely, Vaclav
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- Azetidines as intermediates in polyamine synthesis - Structure and reactions of a quadridentate ligand incorporating an azetidine ring
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Reaction of the tris(benzene sulfonate) of 1,1,1-tris-(hydroxymethyl)ethane with neat 1,2-ethanediamine under relatively mild conditions leads to formation in good yield (60%) of a quadridentate amine incorporating a four-membered, azetidine ring, the nature of the tetramine being established by determination of the crystal structures of two of its cobalt(III) complexes and the reactivity of the azetidine ring being explored in further reactions with 1,2-ethanediamine.
- Harrowfield, Jack M.,Gyu, Hwan Jang,Kim, Yang,Thuery, Pierre,Vicens, Jacques
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- Kinetics of the Iron(II) Reduction of Glycinatobis(malonato)-, trans-Bis(malonato)bis(pyridine)-, Nitrilotriacetato(malonato)-, and Nitrilotriacetato(oxalato)cobaltates(III)
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The kinetics of the iron(II) reduction of glycinatobis(malonato)-, trans-bis(malonato)bis(pyridine), nitrilotriacetato(malonato)-, and nitrilotriacetato(oxalato)cobaltates(III) have been studied in aqueous perchlorate medium at I=1.0 mol dm-3 (LiClO4) and 30 deg C in the +> range 0.01-0.90 mol dm-3.The reductions are found to be second order.The reduction of 2- and - is accelerated by H+, while the reduction of 2- and 2- is independent of +> in the range 0.1+>-3.The reduction of 2- is, however, faster at +>-3.The activation parameters for the reduction of 2-, -, 2-, and 2- are respectively as follows: ΔH=49.8+/-4.8, 51.2+/-2.6, 41.1+/-3.2, and 41.4+3.6 kJ mol-1, ΔS=-98.3+/-8.3, -93.7+/-7.2, -119.2+/-9.7, and -114.6+/-9.6 J K-1 mol-1.The proposed mechanism invokes (L-py2 or gly) formed in a H+-assisted step as the reactive species for the bis(malonato) complexes. 2- is proposed to be present as - while 2- remains unaffected by H+.
- Viswanathan, Ramakrishnan,Vijayaraghavan, Venkatapuram Raman.
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- INTRAMOLECULAR ELECTRON TRANSFER AT METAL SURFACES. 3. INFLUENCE OF BOND CONJUGATION ON REDUCTION KINETICS OF COBALT(III) ANCHORED TO ELECTRODES VIA THIOPHENECARBOXYLATE LIGANDS.
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Unimolecular rate constants, k//e//t, are reported for the one-electron electroreduction of pentaamminecobalt(III) anchored to mercury, gold, or copper electrodes via various thiophenecarboxylate ligands where the thiophene sulfur acts as the surface bind
- Li,Liu,Weaver
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- Kinetics and Mechanism of Oxidation of S2O32? by a Co-Bound μ-Amido-μ-Superoxo Complex
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In acetate buffer media (pH 4.5–5.4) thiosulfate ion (S2O32?) reduces the bridged superoxo complex, [(NH3)4CoIII(μ-NH2,μ-O2)CoIII(NH3)4]4+ (1) to its corresponding μ-peroxo product, [(NH3)4CoIII(μ-NH2,μ-O2)CoIII(NH3)4]3+ (2) and along a parallel reaction path, simultaneously S2O32? reacts with 1 to produce the substituted μ-thiosulfato-μ-superoxo complex, [(NH3)4CoIII(μ-S2O3,μ-O2)CoIII(NH3)4]3+ (3). The formation of μ-thiosulfato-μ-superoxo complex (3) appears as a precipitate which on being subjected to FTIR shows absorption peaks that support the presence of Co(III)-bound S-coordinated S2O32? group. In reaction media, 3 readily dissolves to further react with S2O32? to produce μ-thiosulfato-μ-peroxo product, [(NH3)4CoIII(μ-S2O3,μ-O2)CoIII(NH3)4]2+ (4). The observed rate (k0) increases with an increase in [TThio] ([TThio] is the analytical concentration of S2O32?) and temperature (T), but it decreases with an increase in [H+] and the ionic strength (I). Analysis of the log At versus time data (A is the absorbance of 1 at time t) reveals that overall the reaction follows a biphasic consecutive reaction path with rate constants k1 and k2 and the change of absorbance is equal to {a1 exp(–k1t) + a2 exp(–k2t)), where k1 > k2.
- Singh, Bula,Das, Ranendu Sekhar,Banerjee, Rupendranath,Mukhopadhyay, Subrata
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- Measurement of solvent dynamics effects on the electron transfer reaction of Co(NH3)4ox+ in mixed solvents: A quantitative approach
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For reactions involving electron transfer or nucleophilic attack on the transition state/excited state of metal complex in aquo-organic solvent mixtures, a linear relationship between logarithms of rate constant and solvent empirical parameters can be derived. Fe(CN)64- reduction of Co(NH3)4ox+ and ligand to metal charge transfer (LMCT) excited-state redox reaction of Co(NH3)4ox+ were studied in varying compositions of aqueous mixtures of methanol (MeOH) and 1,4-dioxane (Diox). A quantitative estimation of relative importance of the components was attempted. A number of empirical solvent parameters were used in the multiple regression equations. The correlation analysis showed significant information on the effect of solvent-solvent and solvent-solute interactions on reactivities. The addition of MeOH or Diox to the medium brings about marked structural changes in the prevailing water structure by making progressive desolvation between partners of the transition state/geminate radical pair which in all probability is highly solvated in the water medium. The positive sign of multiparametric coefficients suggested that the solvent mixture strongly solvates the transition state, and the negative sign of the coefficients shows the specific solvation of incipient reactants.
- Anbalagan,Geethalakshmi,Poonkodi
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- Formation of an observable intermediate during the reduction of [Co (III)(NH3)5CN]2+ by ?CR1R2(OH) radicals
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?CR1R2OH, Ri = CH 3 or H, react with the complex [CoIII(NH3) 5CN]2+ to form an observable intermediate probably via bonding to the nitrogen of the cyanide. This intermediate isomerizes to form a second intermediate. The second intermediate decomposes into Co 2+(aq), 5NH4+, CN- and R 1R2CO. The plausible structures of the intermediates are discussed. The radicals ?CH3, ?CH 2CHO, CH(OH)CO2-, CH2C(CH3)2OH,CO2- and CH3O2 are considerably less reactive towards this complex, the formation of intermediates in their presence is not observed.
- Herscu-Kluska, Ronit,Cohen, Haim,Meyerstein, Dan
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- The solid-state electrochemistry of metal octacyanomolybdates, octacyanotungstates, and hexacyanoferrates explained on the basis of dissolution and reprecipitation reactions, lattice structures, and crystallinities
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The electrochemical behavior of solid microparticles of metal (Ag+, Cd2+, Co2+, Cr2+, Cu2+, Fe2+, Mn2+, Ni2+, Pb2+, and Zn2+) octacyanomolybdates, octacyanotungstates, and hexacyanoferrates has been studied by voltammetry, electrochemical quartz crystal microbalance, and microscopic diffuse reflectance spectroelectrochemical measurements. The solid microparticles have been immobilized on the surface of graphite electrodes prior to the electrochemical measurements. A comparative study of the cyclic oxidation and reduction of these compounds in the presence of potassium ions revealed that any interpretation of the electrochemistry requires the solubility equilibria of the reduced compounds to be taken into account, such as in the case of the silver salts {Ag3K[X]} and {Ag4[X]} (with X = Fe(II)(CN)6/4-, M(IV)(CN)8/4- (M = Mo, W)). Because {Ag4[X]} has a lower solubility than {Ag3K[X]}, the electrochemistry is accompanied by a conversion of solid {Ag3K[X]} into solid {Ag4[X]}. Two distinct voltammetric signal systems are generated by these two compounds according to {Ag3K[X]} ? {Ag3-[X]} + K+ + e- and {Ag4[X]} ? {Ag3[X]} + Ag+ + e-. When silver ions are present in the solution adjacent to the microparticles, the silver octacyanometalates and silver hexacyanoferrate show a chemically reversible and very stable voltammetric behavior. Despite the fact that the electrochemistry is based upon a single-electron/single-ion transfer reaction ({Ag4[X]} ? {Ag3[X]} + Ag+ + e-), more than one electrochemical signal is observed because of the simultaneous presence of amorphous and crystalline particles. This study shows that the interplay of solubility equilibria and electrochemical equilibria is generally observed for the other metal octacyanomolybdates, octacyanotungstates, and hexacyanoferrates as well.
- Schr?der, Uwe,Scholz, Fritz
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- Outer-sphere Redox Reactions in Sterically Hindered Pentaam(m)inecobalt(III) Complexes. A Temperature and Pressure Dependence Kinetic Study
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Outer-sphere redox reactions between (3+) and (4-) have been studied as a function of pH, temperature and pressure.The effect of the size of the alkyl substituent on the amine, RNH2, has been investigated for both the aqua- and the hydroxo-species in order to establish possible correlations between size and ion-pair formation constant, electron-transfer rate constant, and thermal and pressure activation parameters.The values obtained (at 45 deg C and ambient pressure) indicate that the ion-pair formation constant decreases with increasing size of R (75, R = H; 40, Me; 23 dm3 mol-1, Et), whereas the electron-transfer rate constant increases in this direction (0.11, R = H; 9.3, Me; 35 s-1, Et).The activation enthalpies do not change, either with decreasing charge on the cobalt complex or with the size of the amine (87, R = H; 79, Me; 84 kJ mol-1, Et).As for the activation volume, although a slight increase is observed on increasing the size of R (26.5, R = H; 29.4, Me; 33.1 cm3 mol-1, Et), it is clear that solvational changes during electron transfer are mainly responsible for the values obtained.
- Martinez, Manuel,Pitarque, Mari-Angel,Eldik, Rudi van
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- Ion pairing effects on the kinetic of the intramolecular electron transfer reaction [FeII(CN)5pzCoIII(NH3)5] → [FeIII(CN)5pzCoII(NH3)5]
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The kinetics of the title reaction was studied in different salt solutions (NaNO3, LiNO3, Ca(NO3)2 and Na2SO4). The results are interpreted as a consequence of ion pairing effects. It has been shown that the anion of the salts is the ion associating with the binuclear complex. This association of the anion produces a less favorable reaction free energy and consequently a diminution of the reaction rate.
- Muriel,Jiménez,Pérez-Tejeda,Sánchez
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- Kinetics and Mechanism of Ce(IV) Oxidation of Free and Coordinated Glyoxylic Acid
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The kinetics and mechanism of Ce(IV) oxidation of glyoxylic acid and pentaammineglyoxalatocobalt(III) perchlorate have been investigated in acidic sulfate media in the temperature range 35 to 50 deg C and at ionic strength 0.95 M.The rate of decrease of was found to be first order in and .The observed second order rate constant, kobsd, for the disappearance of is satisfactorily given by, kobsd = 1 + k2K34->/+>>/34->/+>> (for glyoxylato complex) and kobsd = k1/34->/+>> (for glyoxylic acid).Both Ce(SO4)2 and 4)3)2- appear to be the oxidant species for coordinated glyoxylate while for free glyoxilic acid Ce(SO4)2 seems to be the only oxidant species.The values of k1 and k2 and the associated activation parameters have been computed.A suitable mechanism has been suggested for the reaction.
- Mohanty, Nirod Kumar,Nanda, Rabindra Kumar
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- Kinetic studies on the oxidation of trimeric aquomolybdenum(IV)
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Oxidations of the Mo(IV) aquo trimer, Mo3O44+, with IrCl62- and Fe(phen)33+ have been studied under the conditions [H+] = 0.6-2.0 M and I = 2.0 M (LiClO4). With MoIV3 in large excess, the IrCl62- reaction can be expressed as 2MoIV3 + 6IrIV → 3MoV2 + 6IrIII, and the initial step MoIV3 + IrCl62- is rate determining. Second-order rate constants kIr are dependent on [H+]: kIr = k-1[H+]-1, where at 25°C k-1 = 1.36 s-1, ΔH? = 14.1 kcal mol-1, and ΔS? = -9.6 cal K-1 mol-1. An alternative stoichiometry MoIV3 + 4IrIV → MoV2 + MoVI + 4IrIII cannot be entirely ruled out, and at nearly equivalent amounts of reactants the slower oxidation of MoV2 to MoVI contributes. With Fe(phen)33+ a different reactivity pattern is observed in that oxidation of MoV2 is more rapid than that of MoIV3, and the equation MoIV3 + 6Fe(phen)33+ → 3MoVI + 6Fe(phen)32+ applies. This different behavior is accounted for by the reaction of Fe(phen)33+ occurring exclusively by an outer-sphere mechanism, whereas IrCl62- can react inner sphere. The initial step, MoIV3 + Fe(phen)33+, is again rate determining and at 25°C k-1 for the [H+]-1-dependent path is 0.53 s-1, I = 2.0 M. No evidence was obtained for stable trimeric mixed-oxidation-state species as product(s) of the oxidation of MoIV3. The complex Co(C2O4)33- does not oxidize MoIV3, whereas (as reported elsewhere) oxidation of aquo MoIII and MoIII2 through to MoV2 is rapid. It is concluded that oxidation of monomeric and/or dimeric Mo(IV) occurs more readily than that of the trimer. No oxidation of MoIV3 by the two-electron oxidant PtCl62- is observed.
- Harmer, Mark A.,Richens, David T.,Soares, Allan B.,Thornton, Andrew T.,Sykes, A. Geoffrey
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- Kinetics of the Oxidation of Formic Acid by Aquacobalt(III) Ions in Aqueous Perchlorate Media
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The rate of oxidation of formic acid by Co(III)aq has been measured in aqueous perchlorate media over a wide range of conditions using conventional and stopped-flow spectrophotometry.The order in is unity and the order in lies between zero and unity; the rate decreases with increasing acidity in a complex manner.The mechanism is resolved into two rate determining processes, each involving an intermediate Co(III)-HCOOH complex.These two complexes have an acid-base relationship involving two protons.Values for ΔH* and ΔS* together with the ratio of rates of oxidation of HCOOH and DCOOH are compared for a range of oxidizing ions.
- Varadarajan, R.,Wells, Cecil F.
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- Intramolecular Charge-transfer Decomposition of the μ-Peroxo-bis- Complex in Acidic Solutions. Stabilization by Protonation and the Effect of Chloride and Sulphate
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The kinetics of the decomposition of the μ-peroxo-bis complex, (4+) -> 2Co(2+) + O2 + NH4(1+), generated by fast one-electron reduction of the μ-superoxo-complex, (5+), have been studied using the stopped-flow technique at =0.005-0.100M, l=0.10M (LiClO4).The dependence of first-order rate constants, kobs=kK/( + K), is consistent with the formation of a protonated non-reactive form.At 25 deg C the rate constant for decomposition of the unprotonated (brown) form is k=84 s-1, and ΔH(excit.)=17.5+/-1.0 kcalmol-1, ΔS(excit.)= 8.7+/-3.7 cal K -1mol-1, in excellent agreement with previous data for solutions =4.3-15.0M, l = 2.0M (NH4NO3).The acid dissociation constant K of the protonated (red) complex (25 deg C) is 0.084M, with ΔH=6.0+/-2.5 kcalmol-1, ΔS=15.2+/-9.0 calK-1mol-1.Chloride > 0.6M the effect of and on the rate of decomposition was studied by conventional spectrophotometry, l=2.3M (Cl(1-)/ClO4).Solid samples of the brown and red complexes interconvert rapidly in solution to give identical spectra.The protonated red complex does not react with iodide during the ca. 30 min period required for decomposition.Implications regarding the structure of the protonated complex are considered.
- Ferrer, Miguel,Hand, Timothy D.,Sykes, A. Geoffrey
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- Kinetics and mechanism of the reaction between chromium(II) and 4,4′-bipyridine in acidic aqueous solution
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The reaction between chromium(II) and the diprotonated 4,4′-bipyridine ion was studied at 25°C and ionic strength 1.0 M (LiClO4-HClO4). A highly absorbing, blue intermediate is produced, reaches a maximum concentration, and then fades to yield a solution containing chromium(III) and N,N′-dihydro-4,4′-bipyridine. The intermediate was identified as the 4,4′-bipyridine radical. The kinetics of the formation and disappearance of the radical were studied as a function of the concentrations of 4,4′-bipyridine, chromium(II), and hydrogen ions. Analysis of the observed kinetics indicates a sequence of two pseudo-first-order steps, the radical intermediate being present in transient equilibrium. The first step is the outer-sphere reduction of diprotonated 4,4′-bipyridine (bpyH22+) by chromium(II) according to the rate law kd[Cr2+][bpyH22+], where kd = (2.0 ± 0.1) × 10-2 M-1 s-1 at 25°C and μ = 1.0 M. The second step involves the disappearance of the radical according to the rate law a + b[Cr2+][H+], with a = (4.79 ± 0.62) × 10-2 s-1 and b = 6.46 ± 0.39 M-2 s-1 at 25°C and μ = 1.0 M.
- Miralles, Altony J.,Haim, Albert
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- Highly efficient polyoxometalate-based catalysts for clean-gasoline synthesis
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Poor selectivity for gasoline products is a critical issue for the Fischer-Tropsch synthesis (FTS). Herein, we report that the introduction of a polyoxometalate Cs2.5H0.5PW12O40 (CsPW) into a conventional FTS catalyst (Co/Al2O3) can create a highly efficient bifunctional catalyst, leading to 118% increase in the selectivity of gasoline. Furthermore, it was found that such a significant improvement is due to the effective hydrocracking of heavier hydrocarbon products at CsPW sites.
- Wang, Chunling,Liu, Cheng,Hu, Yunhang,Bu, Xianni,Zhao, Tiejun,Chou, Kuochih,Li, Qian
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p. 21424 - 21427
(2015/11/10)
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- Cobalt(III) complexes of unsaturated carboxylic acids: Synthesis, characterization, and photochemical studies in aqueous medium
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Cobalt(III) ammine complexes coordinated to carboxylic acids are classic examples showing photoredox reactions originating from the ligand to metal charge transfer excited states. Cobalt(III) pentaammine complexes coordinated to unsaturated carboxylic acids as ligands were synthesized and characterized by spectroscopic methods. The photolysis of these complexes produces a carboxylate free radical. This free radical undergoes further transformation to form organic photoproducts. The organic photoproducts were characterized by spectroscopic methods. The nature of the decomposition products from the organic photoproduct was examined in detail. The quantum yields were also determined. The photochemical reactions have the potential to produce novel compounds from the decarboxylation of unsaturated acids, which shows interesting reaction pathways.
- Jothivenkatachalam, Kandasamy,Chandra Mohan, Singaravelu,Natarajan, Paramasivan
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p. 3371 - 3386
(2013/09/23)
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- Kinetics of oxidation of nitrosodisulfonate anion radical with a metallo-superoxide
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The metal bound superoxide in μ-superoxo-bis[pentaamminecobalt(iii)] 5+ (1) oxidizes the nitrosodisulfonate anion radical (NDS 2-) by two electrons. Oxidized NDS2- quickly decomposes to SO42- and NO. 1 is itself reduced to the corresponding hydroperoxo complex which also decomposes fast to Co(ii), NH4 + ions and oxygen. 1.5 moles of volatile products formed per mole of 1 mixed with excess NDS2-. In the absence of superoxide in a bridged complex, e.g. the μ-amido-bis[pentaamminecobalt(iii)]5+ complex fails to oxidize the nitroxyl radicals, NDS2-, TEMPO and 4-oxo TEMPO. With excess NDS2- over 1, the reaction is first-order with respect to [1], [NDS2-] and inverse first order in [H+]. The activation entropy, ΔS≠, is largely negative, increased ionic strength decreased the rate and a Bronsted plot is fairly linear with a negative slope. Oxidant μ-superoxo-bis[(ethylenediamine) (diethylenetriamine)cobalt(iii)]5+ has ligands sterically more crowded though more basic than ammonia in 1. It oxidizes NDS2- much more slowly. No solvent kinetic isotope effect (kH2O/D2O ≈ 1) could be seen; a spin-adduct formation by the conjugate base of 1 followed by electron transfer is postulated. The Royal Society of Chemistry 2012.
- Mandal, Kaustab,Banerjee, Rupendranath
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p. 2714 - 2719
(2012/04/04)
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- Kinetics and mechanism of reduction of a coordinated superoxide with hydroxylamine derivatives
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Rates for the uncatalyzed reactions of the coordinated superoxide in μ-superoxo bis[pentaminecobalt(III)]5+ complex (1) with hydroxylamine-N-monosulfonate, HONH(SO3)- (HMS) and hydroxylamine-N,N′-disulfonate, HON(SO3)22- (HDS) have been determined. Successive replacement of the -NH2 protons with SO3H group (electron withdrawing) increased the reaction rate from HMS to HDS but replacement of the O-H hydrogen halted the reaction. HMS and HDS are oxidized to a mixture of N2O (20%), NO (80%) and SO42-. The reactions are first-order in [1] and [reductant]. Reaction rate increased with pH though neither HMS nor HDS are involved in protic equilibria within the investigated pH range. We propose 1 is in protic equilibrium with its conjugate acid 1H which is kinetically a dead-end species. An electroprotic reaction between 1 and the -OH hydrogen in HMS is proposed whereas a simple electron-transfer mechanism has been proposed for the HDS oxidation.
- Mandal, Kaustab,Mukhopadhyay, Subrata,Banerjee, Rupendranath,Ghosh, Aloke
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p. 2833 - 2836
(2011/01/08)
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- Kinetics and mechanism of uncatalyzed oxidation of hydrazine with superoxide coordinated to cobalt(III)
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Hydrazine, in deoxygenated aqueous solutions at moderate pH 3.8-6.1, quantitatively reduces [(NH3)5CoIII(μ- superoxo)CoIII(NH3)5]5+ (I) to the corresponding peroxo complex, [(NH3)5Co III(μ-peroxo)CoIII(NH3)5] 4+ (II) which, instead of reacting with another molecule of hydrazine quickly decomposes to Co2+ according to the overall stoichiometry: 4(I) + N2H5+ + 35H+ = 8Co 2+ + 40NH4+ + N2 + 4O2 The reaction is dramatically catalyzed by Cu2+, to the degree that impurity levels of Cu2+ are sufficient to mask the direct reaction such that the direct oxidation process is inaccessible, though the reaction is not catalyzed by any of the other usual transition metal centers. Dipicolinic acid is a very effective inhibitor of this trace metal ion catalysis, and with 0.10 mM dipicolinic acid, the direct oxidation can be studied, which reveals an inverse [H+] dependence of reaction rate for the direct reaction. This has been ascribed to the formation of a conjugate base, [(NH 3)4(NH2)CoIII(μ-superoxo)Co III(NH3)5]4+ as the kinetically active species. Change in buffer concentration has no effect on the rate, neither is there any observable kinetic solvent isotope effect Enthalpy of activation has a moderate value while the entropy of activation is moderately negative. A reaction scheme consistent with these kinetic and stoichiometric observations has been proposed.
- Mondal, Amit,Banerjee, Rupendranath
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p. 645 - 649
(2010/01/31)
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- Reactivity of metal oxides: Thermal and photochemical dissolution of MO and MFe2O4 (M=Ni, Co, Zn)
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Dissolution rates of NiO, CoO, ZnO, α-Fe2O3 and the corresponding ferrites in 0.1 mol dm-3 oxalic acid at pH 3.5 were measured at 70 °C. The dissolution of simple oxides proceeds through the formation of surface metal oxalate complexes, followed by the transfer of surface complexes (rate-determining step). At constant pH, oxalate concentration and temperature, the trend in the first-order rate constant for the transfer of the surface complexes (kMe; Me=Ni, Co, Zn, Fe) parallels that of water exchange in the dissolved metal ions (k-w). Thus, the most important factor determining the rates of dissolution of metal oxides is the lability of Me-O bonds, which is in turn defined by the electronic structure of the metal ion and its charge/radius ratio. UV (384 nm) irradiation does not increase significantly the dissolution rates of NiO, CoO and ZnO, whereas hematite is highly sensitive to UV light. For ferrites, the reactivity order is ZnFe2O4>CoFe2O4?NiFe2O4. Dissolution is congruent, with rates intermediate between those of the constituent oxides, Fe2O3 and MO (M=Co, Ni, Zn), reflecting the behavior of very thin leached layers with little Zn and Co, but appreciable amounts of Ni. The more robust Ni2+ labilizes less the corresponding ferrite. The correlation between log kM and log k-w is somewhat blurred and displaced to lower kM values. Fe(II), either photogenerated or added as salt, enhances the rate of Fe(III) phase transfer. A simple reaction mechanism is used to interpret the data.
- Garcia Rodenas, Luis A.,Blesa, Miguel A.,Morando, Pedro J.
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p. 2350 - 2358
(2009/05/30)
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- Dissociation kinetics of cobalt and zinc β-octabromo-meso-tetraphenyl- and β-octaethyl-meso-tetraphenylporphyrin complexes
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Kinetics of solvoprotolytic dissociation of zinc and cobalt porphyrin complexes with a porphyrin macrocycle distorted to different extents (β-octaethyl-meso-tetraphenylporphyrin and β-octabromo-meso- tetraphenylporphyrin) have been studied in binary proton-donating solvents. The main factor decreasing the stability of the porphyrin complexes is the distortion of the planar structure of the macrocycle. The reactivity of the cobalt porphyrins is governed by both structural and electronic effects.
- Pukhovskaya,Guseva,Semeikin,Golubchikov
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p. 190 - 193
(2008/10/09)
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- Reductions by titanium(ii) as catalyzed by titanium(iv)
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The cobalt(iii) complexes, [(NH3)5CoBr]2+ and [(NH3)5CoI]2+ are reduced by Ti(ii) solutions containing Ti(iv), generating nearly linear (zero-order) profiles that become curved only during
- Mukherjee, Ritam,Yang, Zhiyong,Gould, Edwin S.
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p. 772 - 774
(2007/10/03)
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- Kinetics and mechanism of acid assisted reduction of unsymmetrical chelate complex [CoIII(Am) (endibigH) (ClO4)2] (where Am = amino acid and endibigH = ethylenedibiguanide)
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The acid mediated dissociation of three mixed chelate complexes of cobalt(III) containing a common primary quadridentate ligand ethylenedibiguanide and secondary bidentate amino acid anions, viz., glycinate, α-alaninate and valinate have been investigated spectrophotometrically at 490 nm to study the influence of chelate size, ring pattern, bonding nature and basicity of the ligand on the dissociation rates. The kobs values for all these complexes vary linearly with [HCIO4] having a positive slope and no intercept. The reactivity order of the complexes is: DL-valinato a function of ionic strength, dielectric constant of the medium and temperature. The activation parameters have been evaluated and used in support of the proposed mechanistic scheme.
- Majee, Asim Kumar,Chaturvedi,Srivastava, Abhishek,Agarwal, Abhinav,Naik,Nigam
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p. 2105 - 2108
(2007/10/03)
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- Polymerization-induced enhancement of binding and binding-induced polymerization
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We obtained the binding free energy of the complex [Co(C2O 4)3]3- to the peptide H-Lys-Gly-(Lys-Gly) 9-Lys-NH2, and to the monomers (aminoacids) forming the peptide, using the electron transfer reaction between [Ru(NH3) 5pz]2+ and [Co(C2O4) 3]3- as the probe. The polymerization of the monomers increases the negative free energy of binding and changes its character, non-cooperative for the monomers and anti-cooperative for the peptide. This increase in the negative free energy represents a driving force for the polymerization process that produces the peptide from the aminoacids (monomers) in such a way that the polymerization in the presence of the cobalt complex is more favourable, from a thermodynamic point of view, in 16.5 kJ/mol.
- Ortiz,Guichou,Chavanieu,Sánchez,Prado-Gotor
-
p. 266 - 270
(2008/10/09)
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- The kinetics of reduction of several substituted cobaloximes by iron(II) in an acid medium
-
A few cobaloximes such as pyridine-4-carboxylato bis(dimethylglyoximato) cobalt(III), pyridine-2-carboxylato bis(dimethylglyoximato) cobalt(III), benzene-1-amine-4-carboxylato bis(dimethylglyoximato) cobalt(III), benzene-1-amine-2-carboxylato bis(dimethylglyoximato) cobalt(III), and quinoline-2-carboxylato bis(dimethylglyoximato) cobalt(III) were synthesized and the kinetics of reduction of these representative cobaloximes with iron(II) in aqueous dimethyl sulfoxide-perchloric acid medium was carried out. The kinetic orders were first order in complex and first order in iron(II). H+ dependence was determined to be of inverse fractional order in pyridine-4-carboxylato bis(dimethylglyoximato) cobalt(III) and benzene-1-amine-4-carboxylato bis(dimethylglyoximato) cobalt(III) and of inverse first order in benzene-1-amine-2-carboxylato bis(dimethylglyoximato) cobalt(III). From the slopes and intercepts of plots of k1 versus 1/[H+], the acid-dependent and acid-independent rate constants were computed. It was observed that the acid-independent rate constants are relatively higher than the acid-dependent rate constants. This is explained as being due to the better electron mediating nature of the unprotonated form. The H+ dependence also shows that the possible bridging is at the oxime. This is confirmed too by the inert nature of pyridine-2-carboxylato bis(dimethylglyoximato) cobalt(III) with iron(II). Like pyridine-2-carboxylato bis(dimethylglyoximato) cobalt(III), benzene-1-amine-2-carboxylato bis(dimethylglyoximato) cobalt(III) is also inactive toward reduction, but the origin of inactivity is due to peri interaction of the first and eighth positions. The rate equation for the reduction process is (Equation Presented).
- Venkata Nadh,Syama Sundar,Radhakrishnamurti
-
-
- Establishment of the light-absorption ratio variation approach and application to determination of CO(II) in the ng ml-1 level and novel characterization of Co(II) and Zn(II) complexes with 1,5-di(2-hydroxy-5- sulfophenyl)-3-cyanoformazan
-
The light-absorption ratio variation approach (LARVA) has been established and applied to the determination of Co on the ng ml-1 level with 1, 5-di(2-hydroxy-5-sulfophenyl)-3-cyanoformazan (DSPCF) at pH 9.33. Competitive replacement complexatio
- Gao, Hong-Wen,Wang, Hong-Yan,Zhang, Sheng-Yi,Zhao, Jian-Fu
-
p. 1649 - 1656
(2007/10/03)
-
- Synthesis of 2-(nitromethyl)ornithine from ornithine mediated by cobalt(III)
-
Rac.-p-(tris(2-aminoethyl)amine-2-(nitromethyl)ornithine)cobalt(III) trichloride (2d) was obtained by a simple three-step procedure from ornithine using cobalt template chemistry. p-(Tris(2-aminoethyl)amine-ornithine)cobalt(III) trichloride (2a) was obtained from tris(2-aminoethyl)amine (tren) and (S)-ornithine in the presence of cobalt(II), which was oxidised to cobalt(III) during the reaction. Complex 2a was selectively oxidised with thionyl chloride-dimethyl formamide to p-(tris(2-aminoethyl)amine-dehydro-ornithine)cobalt(III) trichloride 2b. Complex 2c, in which reaction of thionyl chloride-dimethyl formamide has also occurred at the δ-amine of ornithine, was obtained at longer reaction times. Complex 2b reacted with nitromethane anion to give rac.-p-(tris(2-aminoethyl)amino-2-(nitromethyl)ornithine)cobalt(III) trichloride (2d). The amino acid rac.-2-(nitromethyl)ornithine (1b) was released by reducing complex 2d with aqueous ammonium sulfide. Complex 2d was expected to release 2-(nitromethyl)ornithine (1b) in hypoxic cells, where the amino acid could act as an inhibitor of ornithine decarboxylase. Preliminary data indicated that complex 2d was weakly cytotoxic in one cell type studied.
- Butler, Philip A.,Crane, Christopher G.,Golding, Bernard T.,Hammershoi, Anders,Hockless, David C.,Petersen, Tue B.,Sargeson, Alan M.,Ware, David C.
-
p. 318 - 321
(2008/10/08)
-
- Preparation and properties of the heterometallic cube [Mo3CdS4(H2O)12]4+ and the reaction with H+
-
The orange-brown cadmium-containing heterometallic cube [Mo3CdS4(H2O)12]4+ has been prepared by reacting [Mo3S4(H2O)9]4+ in 2.0 M HCl with Cdsup
- McLean, Iain J.,Sokolov, Maxim N.,Hernandez-Molina, Rita,Sykes, A. Geoffrey
-
p. 1941 - 1945
(2007/10/03)
-
- Kinetics and mechanism of acid dissociation of cobalt(II) complex of 1,10-phenanthroline tetradentate diamine ligands
-
The dissociation kinetics of cobalt(II) complex of 1,10-phenanthroline tetradentate diamine ligands have been studied using a stopped-flow spectrophotometer in 6.6×10-3~0.5 mol·dm-3 HCl (I=0.5 mol·dm-3 HCl+NaCl) over a range of temperature. The dissociation rate follows the law kobs=kK2[H+]/(1+K2 [H+]). The acid-assisted dissociation can be rationalized in terms of a four-step kinetic process involving two rapid pre-equilibrium protonations and rate-determining cleavage of the Co(II)-N(phenanthroline) bond. The temperature dependence of the rate and equilibrium constants have been determined and the appropriate thermodynamic parameters obtained. Steric effects in the dissociation kinetics have been discussed and the linear relationship between ΔH≠ and ΔS≠ has been found.
- Sun, Hongwei,Lin, Huakuan,Zhou, Zhifen,Zhao, Guanghua,Zhu, Shourong,Su, Xuncheng,Chen, Yunti
-
p. 2514 - 2517
(2007/10/03)
-
- Chromium(II) reduction of cobalt(III) complexes of 3,8-dimethyl-5,6-benzo-4,7-diazadeca-3,7-diene-2,9-dione dioxime: Kinetics and mechanism
-
Kinetics of chromium(II) reduction of complexes of the type trans-[Co(DODOHbzo) (s-py)X]ClO4, where DODOHbzo=3,8-dimethyl-5,6-benzo-4,7-diazadeca-3,7-diene-2,9-dione dioximate; s-py is pyridine(py) or a substituted pyridine like nicotinamide (nic-CONH2), isonicotinamide (isonic-CONH2) nicotinic acid (nic-COOH) or isonicotinic acid (isonic-COOH), and X-=Cl-, Br-or, I-has been studied in DMSO-H2O (1% v/v) at 27 ± 0.1°C and I=0.25mol.dm-3(LiClO4) in the [H+] range 0.001-0.1 mol dm-3under pseudo-first order conditions using an excess of the reductant. The inverse dependence of rate on [H+], trends in the rate constants, viz. Cl---and py? s-py for the axial ligands and product analysis suggest an inner-sphere reaction with bridging by Cr(II) at the oxime oxygen or halide.
- Vijayaraghavan,Dayalan
-
p. 959 - 964
(2007/10/03)
-
- Kinetics of manganese(III) acetate in acetic acid: Generation of Mn(III) with Co(III), Ce(IV), and dibromide radicals; reactions of Mn(III) with Mn(II), Co(II), hydrogen bromide, and alkali bromides
-
The reaction of cobalt(III) acetate with excess manganese(II) acetate in acetic acid occurs in two stages, since the two forms Co(IIIc) and Co(IIIs) are not rapidly equilibrated and thus react independently. The rate constants at 24.5 °C are k(c) = 37.1 ± 0.6 L mol-1 s-1 and k(s) = 6.8 ± 0.2 L mol-1 s-1 at 24.5 °C in glacial acetic acid. The Mn(III) produced forms a dinuclear complex with the excess of Mn(II). This was studied independently and is characterized by the rate constant (3.43 ± 0.01) x 102 L mol-1 s-1 at 24.5 °C. A similar interaction between Mn(III) and Co(II) is substantially slower, with k = (3.73 ± 0.05) x 10-1 L mol-1 s-1 at 24.5 °C. Mn(II) is also oxidized by Ce(IV), according to the rate law -d[Ce(IV)]/dt = k[Mn(II)]2[Ce(IV)], where k = (6.0 ± 0.2) x 104 L2 mol-2 s-1. The reaction between Mn(II) and HBr2·, believed to be involved in the mechanism by which Mn(III) oxidizes HBr, was studied by laser photolysis; the rate constant is (1.48 ± 0.04) x 108 L mol-1 s-1 at ~23 °C in HOAc. Oxidation of Co(II) by HBr2· has the rate constant (3.0 ± 0.1) x 107 L mol-1 s-1. The oxidation of HBr by Mn(III) is second order with respect to [HBr]; k = (4.10 ± 0.08) x 105 L2 mol-2 s-1 at 4.5 °C in 10% aqueous HOAc. Similar reactions with alkali metal bromides were studied; their rate constants are 17-23 times smaller. This noncomplementary reaction is believed to follow that rate law so that HBr2· and not Br· (higher in Gibbs energy by 0.3 V) can serve as the intermediate. The analysis of the reaction steps then requires that the oxidation of HBr2· to Br2 by Mn(III) be diffusion controlled, which is consistent with the driving force and seemingly minor reorganization.
- Jiao, Xiang-Dong,Espenson, James H.
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p. 1549 - 1554
(2008/10/08)
-
- The photodecarboxylation of [N-N-bis-4-pyridylmethyOamino acidato]phenanthrolinecobalt(ni) complexes: - Formation and decomposition of metallacyclic species
-
Photolysis of [N,N-bis(2-pyridylmethyl)amino acidato]phenanthrolinecobalt(in) complexes (amino acidato = glycinato (dpg) or alaninato (dpa) or 2-cyclopropylglycinato (dpc)) with UV light led to elimination of carbon dioxide from the amino acidato chelate and the formation of cobalt(ni) complexes which contain a Co-C-N metallacycle. These photolysis products were characterised by NMR and UV-vis spectroscopy and, in the case of dpg, by X-ray crystallography. The eventual decomposition reactions of the photolysis products were monitored by NMR spectroscopy in both D2O and dilute DC1. The decomposition products included bis(2-pyridylmethyl)amine (bpa) and an aldehyde derived from the carbon atom of the metallacycle and its alkyl substituents. A u-peroxo dinuclear cobalt(in) complex, [(bpa)(phen)(Co(O2)Co(phen)(bpa)]4+, was formed from the decomposition products and its structure determined by X-ray crystallography. Experiments with a cyclopropyl derivative demonstrate that any intermediates with a radical centre on what was the u-carbon of the amino acid must be very short-lived. The Royal Society of Chemistry 2000.
- Hartshorn, Richard M.
-
p. 2801 - 2808
(2007/10/03)
-
- Reactions of polyhomofunctional organic compounds: 2-Kinetics of hydrolysis of four azomethine sites in a trinuclear cobalt(II) complex
-
Kinetic studies on the hydrolysis of a novel trinuclear cobalt(II) complex, (CoBAMC-Sal)2 Co(DMF)2+2 (2) in buffered 90 : 10 aqueous /DMF (v/v) mixtures are reported. Repetitive spectral scans of the reaction mixture exhibit isosbestic points at all pH's. The hydrolysis follows pseudo - first order kinetics with respect to the complex. The rate constants are compared to those reported earlier for the ligand, BAMC - Sal (1) under similar conditions. The difference between the rates of hydrolysis of complex and those of the pure ligand is found to be due to steric factors and stability of the complex. The studies indicate different modes of cleavage of the trinuclear complex at different pH's before the ligand moieties undergo hydrolysis at their azomethine sites. Activation energy, Ea, and thermodynamic parameters such as ΔH≠, ΔG≠ and ΔS≠ are evaluated by Arrhenius and Eyring methods.
- Suryanarayana Raju,Ramachandraiah
-
p. 482 - 488
(2007/10/03)
-
- Kinetics and mechanism of the reaction of trans-(diaqua)N,N' - Ethylene bis-(salicylidineiminato)cobalt(III) with ascorbic acid
-
The kinetics of the reactions of trans-[Co(Salen)(OH2)2]+ (Salen = N,N'-ethylene bis (salicylidineimtnate) with ascorbic acid (H2Asc) have been studied under varying conditions of pH, [ascorbic acid]T and temperature at 0.5 mol dm-3 ionic strength. The initial fast reactions observed in the stopped flow time scale are due to the complex formation between the reactants. This occurs in two phases i.e., the formation of the trans-[(aqua)(ascorbato)CoIII(Salen)] and its transformation to the corresponding ascorbate chelate. The rate constants and the activation parameters for the formation of the monobonded and chelate ascorbate complexes are reported. The low values of ΔH? and negative values of ΔS? for the complexation reaction favour associative interchange mechanism(Ia). The hydroxide in trans-[Co(Salen)(OH)(OH2)] marginally accelerates substitution of the aqua ligand by HAsc- and the trans-[Co(Salen)(OH)(AscH)]- is considered to undergo fast internal proton transfer to generate trans-[Co(Salen)(OH2)(Asc)]- which undergoes chelation of the CoIII centre by the bound ascorbate moiety; the latter reaction is, however, 15 times slower than the corresponding reaction of trans-[Co(Salen)(OH2)(AscH)]. The faster complexation reactions are followed by the slow redox reactions. The rate constant for the internal reduction of CoIII by the coordinated ascorbate in the chelate [Co(Salen)(AscH)] is 5 - 10 times (25°C - 45°C) faster than the same for [Co(Salen)(Asc)]-. This trend in reactivity is due to the low value of ΔH? for the former although the high negative value of ΔS? compensates at least partly the overriding effect of the activation enthalpy. The internal redox occurs via innersphere mechanism. We also have observed a redox path involving trans-[Co(Salen)(OH2)(AscH2)]+ and H2Asc for which electron transfer most likely involves outersphere mechanism.
- Dash, Anadi C.,Das, Arabinda,Bramha, Gouri S.,Mohanty, Prakash
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p. 947 - 960
(2007/10/03)
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- Kinetics of the Acid-mediated Dissociation of Mixed Chelate Complexes : Aquation of Bis(acetylacetonato)ethylenediamine and Acetylacetonatobis(ethylenediamine)cobalt(III) Complexes
-
Mechanisms of the acid-mediated dissociation of two mixed chelate complexes, bis(acetylacetonato)ethylenediamine and acetylacetonatobis(ethylenediamine)cobalt(III) ions have been investigated spectrophotometrically. In the presence of perchloric acid the first complex dissociates to cobaltous ion, free ethylenediamine, free acetylacetone and oxidised products of acetylacetone. The dissociation follows an acid-assisted path. Rate constant (kobs) for the complexes have been evaluated. A mechanism has been proposed for the first complex.
- Majee, Asim Kumar
-
p. 293 - 295
(2007/10/03)
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- Mixed- ligand Complex Formation of Cobalt(II), Nickel(II) and Zinc(II) with N-Benzenesulphonyl-l-cysteine as Primary Ligand and Bipyridine, Ethylenediamine and Glycinate as Secondary Ligands
-
Equilibrium study on the complex formation of M2+ ions (M = Co, Ni and Zn) with N-benzenesulphonyl-l-cysteine (NBSCH2) in the presence of bipyridine, ethylenediamine, glycinate (B) indicates the formation of mixed-ligand complexes of the types M(NBSC)(B), M(H-I NBSC)(B) and M(H-I NBSC)(OH), together with binary M(NBSC) and M(B) complexes. Formation constants of the complexes have been determined by potentiometric method at 25±0.1° in 50% (v/v) ethanol-water medium at a constant ionic strength, I=0.2 M (NaNO3). Stability of the complexes has been correlated with the modes of coordination of NBSC, basicity of the B ligands and π-electron delocalisation in the metal-ligands bonds.
- Mukherjee,Ghosh
-
-
- Outer-sphere redox reactions of [CoIII(NH3)5(HxPyO z)](m-3)- complexes. A temperature- and pressure-dependence kinetic study on the influence of the phosphorus oxoanions
-
Outer-sphere redox reactions between [CoIII(NH3)5(HxPyO z)](m-3)- (HxPyOzm- = H2PO2-, H2PO3-, HPO32-, HP2O73-, P2O74-, γ-H2P3O103-, -HP3O104-, -P3O105-, β-H3P3O102-, -H2P3O103-, -HP3O104- or -P3O105-) and [Fe(CN)6]4- have been studied as a function of pH, HxPyOzm- oxoanion, temperature and pressure. The effect of the oxidation state, size, geometry and extent of protonation of the HxPyOzm- oxoanions on the precursor-complex formation constant, electron-transfer rate constant, and thermal and pressure activation parameters has been investigated. The values obtained indicate that all the precursor-complex formation equilibrium constants, KOS, are the same except for the non-linear β-H3P3O102-, -H2P3O103-, -HP3O104- and -P3O105- oxoanions, where the values are consistently larger, indicating that hydrogen bonding plays a very important role. The electron-transfer rate constant for a series of [Co(NH3)5(HxPyOz)] (m-3)-, with linear oxoanions, increases on decreasing the negative charge on the complex {k308 = 0.73 × 10-3 and (8.5-11) × 10-3 s-1 for the γ-[Co(NH3)5(P3O10)] 2- and γ-[Co(NH3)5(H2P3O 10)], respectively}. For the non-linear β-P3O105- oxoanions a threshold is observed when the external oxo groups are protonated {k308 = 20 × 10-3 for β-[Co(NH3)5(H3P3O 10)]+ species and 0.84 × 10-3 s-1 for β-[Co(NH3)5(H2P3O 10)], -[Co(NH3)5(HP3O10)]- or -[Co(NH3)5(P3O10)]2-}. The ΔH? values are within the range expected, while those of ΔS? and ΔV? vary considerably with the extent of protonation of the phosphorus oxoanionic ligands, being 13 J K-1 mol-1 and +36 cm3 mol-1 and 69 J K-1 mol-1 and +13 cm3 mol-1, respectively for the [Co(NH3)5(HP2O7)]-[Co(NH 3)5(P2O7)]- couple. The ΔV? values depend strongly on the oxo group distribution of the oxophosphorus ligand {+ 13 and + 32 cm3 mol-1 for β- and γ-[Co(NH3)5(P3O10)] 2-, respectively}. Hydrogen bonding and solvent reorganization play a key role in the interpretation of the activation parameters.
- Martinez, Manuel,Pitarque, Mari-Angel,Van Eldik, Rudi
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p. 2665 - 2671
(2007/10/03)
-
- Preparation and properties of the corner-shared double cube [Mo6PbS8(H2O)18]8+ as a derivative of [Mo3S4(H2O)9]4+
-
The heterometallic corner-shared double-cuboidal cluster [Mo6PbS8(H2O)18]8+, with the lead atom common to both cubes, has been prepared for the first time by the reaction of [Mo3S4(H2O)9]4+ with lead shot, which requires up to 1 h to reach completion, and by addition of a solution mix of [Mo3S4(H2O)9]4+ and PbII to an excess of BH4-, when reaction is rapid. The air-sensitive blue-green product has UV/VIS absorbance bands at 387 and 757 nm in 2.0 M HClO4. Inductively coupled plasma atomic emission spectroscopy confirmed the Mo:Pb:S ratio as 6:1:8. It was eluted from a cation-exchange column with 4 M Hpts (toluene-p-sulfonic acid), but not 2 M Hpts (or 4 M HClO4), consistent with a high charge, confirmed as 8 + from the 2:1 stoichiometries for the oxidation reactions with e.g. [Co(dipic)2]- (dipic = pyridine-2,6-dicarboxylate) or [Fe(H2O)6]3+, which yield [Mo3S4(H2O)9]4+. Kinetic studies on the oxidations with [Co(dipic)2]- and [Fe(H2O)6]3+ were also carried out as part of an overall appraisal of the reactivity of the heterometallic clusters.
- Saysell, David M.,Huang,Sykes, A. Geoffrey
-
p. 2623 - 2627
(2007/10/03)
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- Photochemistry of μ-Amido-μ-hyperoxo-dicobalt(III) Complexes with Ammine and Polyamine Ligands in Aqueous Solutions: Effects of Ligand and pH
-
Photolyses of μ-amido-μ-hyperoxo-dicobalt(III) complexes, -2)CoL>4+ (L=4NH3 (1), en+2NH3 (2), 2en (3), tren (4)), (tren: 2,2',2''-triaminotriethylamine, en: ethylenediamine) were investigated.The photolysis of complex 4 in a neutral aqueous solution produced a μ-hydroxo-μ-peroxo-dicobalt(III) complex, 2-2)Co(tren)>3+, together with a mononuclear cobalt(III) complex, 3+.A mononuclear complex and Co2+ were produced in the photolyses of μ-hyperoxo complexes 1 and 2 containing NH3 ligand in neutral aqueous soltuions.In acidic media, only mononuclear complex and Co2+ were produced in the photolysis of all μ-hyperoxo complexes examined.Quantum yields of the complexes with NH3 increased with increasing the number of coordinated NH3 (ψ=0.25, 0.11, 0.023, and 0.028 for complexes 1, 2, 3, and 4, respectively, at 365 nm irradiation in neutral region).The quantum yields obtained in acidic solutions were similar in magnitude to those obtained in neutral solutions.
- Shinohara, Nobuyoshi,Shibukawa, Hirohiko,Shinozaki, Kazuteru,Yoshikai, Mika
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p. 178 - 182
(2007/10/02)
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- Kinetics of the Acid Dissociation of Cyclic and Open-chain Tetramine Complexes of Cobalt(II): General Acid Catalysis with Co-ordinating Phosphate-Citric Acid Buffers
-
The kinetics of dissociation of the cobalt(II) complexes of the quadridentate ligands 1,4,8,11-tetraazacyclotetradecane (cyclam), triethylenetetramine (trien) and 2,2',2 -triaminotriethylamine (tren) was followed spectrophotometrically in the ranges 10 3, NaClO4.The complexes were prepared in situ in preaerated solutions under a nitrogen atmosphere, by the addition of a 10percent excess of the ligand in the form of the free base to a solution of CoCl2*6H2O.The ligand dissociation reaction was then initiated by the addition of perchloric acid or McIlvaine phosphate-citrate buffer.No dissociation was observed for the cyclam complex in perchloric acid media.It was, however, observed in the buffer and obeyed biphasic kinetics comprising two consecutive first-order steps.The action of the phosphate and/or citrate is attributed to their complexing ability and to their association through hydrogen bonding to the axial aqua ligand, thus bringing the proton closer to the dissociating nitrogen and hence catalysing the dissociation.The dissociation kinetics of the open-chain unbranched trien and the tripod tren was observed in perchloric acid as well as the McIlvaine buffer system.Except when too fast to follow by conventional spectrophotometry, the reaction obeyed biphasic kinetics comprising two consecutive first-order steps.The tren complex dissociates at a rate 5-10 times faster than that of trien, the first step being too fast to follow except at 10 deg C in the buffer system.The observed rate dependence is explained on the basis of mechanisms involving solvation, specific acid catalysis and general acid catalysis.The cyclam complex dissociates at a rate 5-30 times slower than those of the two open-chain complexes.This is attributed to the stabilization due to the hindered rotation of the dissociating nitrogen (entropic effect) and to the higher ligand-field stabilization of the macrocycle (enthalpic effect).Mechanisms covering the entire range of pH studied and conforming to the observed rate laws are given.
- Gazzaz, Hanaa A.,El-Guindi, Nabawia M.,El-Awady, Abbas A.
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p. 2313 - 2320
(2007/10/02)
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- Pathways for water oxidation catalyzed by the [(bpy)2Ru(OH2)]2O4+ ion
-
Resonance Raman (RR) spectra of the (μ-oxo)bis[cis-aquabis(2,2′-bipyridine)ruthenium(III)] ion and its congeners in thermodynamically accessible III-IV, IV-V, and V-V higher oxidation states have been obtained in aqueous solutions under a wide range of medium conditions. The RR spectra were dominated by single bands at 370-403 cm-1, assignable to the Ru-O-Ru symmetric stretching mode (vs) on the basis of their 18O-isotope induced shifts. From the magnitudes of the shifts, the Ru-O-Ru angles were calculated to be 155-180° for the various oxidation states. A band appearing at 812 cm-1 in the V-V ion which shifted to ~780 cm-1 in [18O]H2O was assigned to a Ru=O stretching mode of a terminal oxo ligand. Below pH 0.3 the bridging μ-oxo atom became protonated; for the other ions, hydrogen bonding to the bridge was indicated by D2O-induced solvent shifts in vs(Ru-O-Ru). An 18O-isotope labeling study of the water oxidation reaction established the existence of a pathway in which one atom of O2 was obtained from an aqua ligand. These results combined with published rate data suggest that this pathway involves solvent nucleophilic attack of a terminal oxo atom as the critical oxygen-oxygen bond forming step, with the bridging oxygen atom serving the essential role of activating and/or orienting the reactant H2O through formation of a strong hydrogen bond. A second major pathway, for which both O atoms were derived from solvent, was also identified, but no definitive evidence was obtained for a pathway for which both O atoms originated in coordinated H2O.
- Hurst, James K.,Zhou, Jinzhong,Lei, Yabin
-
p. 1010 - 1017
(2008/10/08)
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- Medium effects on the electron-transfer processes: Reduction of aquapentaamminecobalt(III) by hexacyanoferrate(II) in ethyleneglycol-water, ethylene carbonate-water and urea-water media
-
The outer-sphere electron transfer between Co(NH3)5OH23+ and Fe(CN)64- which proceeds via precursor ion-pair formation has been investigated in ethylene glycol-water (0-47.6 wtpercent of EG), ethylene carbonate-water (0-40 wtpercent of EC) at 20-35 deg C and in urea-water (0-19.4 wtpercent of U) at 25 deg C (I = 0.50 mol dm-3 NaNO3).The electron transfer rate constant for the ion pair (k, s-1) follows the order H2O ca.U-H2O less than EG-H2O EC-H2O.The specific solvent effect on the rate constant for EG-H2O and EC-H2O media can be accounted for in the frame work of the Marcus theory by considering the ion solvent dipole interaction in the solvent cospheres of the precursor and the successor ionpairs.Both ΔH(excit.) and ΔS(excit.) decrease linearly with increase in mole fraction of EC(XEC) while for the EG-H2O media a broad miximum is discernible in the plot of ΔH(excit.) (or ΔS(excit.) versus XEG indicating thereby that the solvent structural effects might mediate the activation process.
- Dash, Anadi C.,Dash, Neelamadhab,Pradhan, Jyotsnamayee
-
p. 824 - 831
(2007/10/02)
-
- Kinetics and mechanism of dissociation of di-(2-pyridyl)amine complexes of copper (II), nickel (II) and cobalt (II) in aqueous acid media
-
Kinetics of dissociation of ML2+2 (M=Cu(II), Ni(II) and CoL2+ complexes in acid media forming aqua-metal ions and the protonated ligand as the ultimate products have been studied by stopped-flow spectrophotometry.Dissociation of the bis-complexes occurs in two consecutive steps through formation of the mono complexes in the first step, which is faster than the second.For each of the steps of the different complexes the observed rate constant kx shows acid dependence.Arguments have been put forth in favour of chelate ring-opening as the rate-determining step.These complexes are much more labile to dissociation than the corresponding complexes of 2.2'-bipyridyl.
- Das, Asim K.,Banerjea, D.
-
p. 347 - 349
(2007/10/02)
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- Reaction of 4+ with Cobalt and Mercury. Syntheses and X-Ray Structures of Double Cubane-Type Cluster, *18 H2O and Sandwich Cubane-Type Cluster, (CH3-C6H4-SO3)8*20 H2O
-
Two new clusters, (CH3-C6H4-SO3)8*18 H2O (1) and (CH3-C6H4-SO3)8*20 H2O (2), have been synthesized by the reaction of 4+ (3') with metallic cobalt and mercury, respectively.The X-ray structure analyses revealed that in 1 two Mo3S4Co(H2O)9 moieties are joined through two cobalt-sulfur bonds, and in 2 a mercury atom was sandwiched by two 3's.
- Shibahara, Takashi,Akashi, Haruo,Yamasaki, Mikio,Hashimoto, Kohichi
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p. 689 - 692
(2007/10/02)
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- Kinetic Studies in Aqueous Solutions of Cobalt(II)-Ethylenediamine, -Malonate, and -Glycinate Complexes
-
The dynamics of equilibria in aqueous solutions of cobalt(II)-ethylenediamine (en), -malonate (mal), and -glycinate (glyO) systems have been investigated by measuring the paramagnetic relaxation rate and shift of the CH2 ligand protons over a wide range o
- Micskei, Karoly,Nagypal, Istvan
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p. 2581 - 2586
(2007/10/02)
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- Mechanistic comparison of thermal and photochemical intramolecular electron-transfer reactions: Differentiation on the basis of remarkably different pressure dependencies
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The kinetics of the electron-transfer reaction between Co(NH3)4(pzc)2+, Co(en)2(pzc)2+, and Co(NH3)5pz3+ (where pz = pyrazine and pzc = pyrazinecarboxylate) and Fe(CN)5H2O3- was studied as a function of concentration, temperature, pressure, and light intensity of the analyzing beam. These reactions exhibit two kinetic steps: formation of pz- and pzc-bridged complexes followed by intramolecular electron transfer. The first step is characterized by large positive volumes of activation (between +23 and +28 cm3 mol-1), which are interpreted in terms of a dissociative mechanism involving the breakage of the FeII-H2O bond. The intramolecular electron-transfer reaction also exhibits large positive volumes of activation (between +27 and +37 cm3 mol-1), which disappear almost completely for the photoinduced electron-transfer reaction. These results are interpreted and discussed in reference to related data reported in the literature.
- Guardado, Pilar,Van Eldik, Rudi
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p. 3473 - 3477
(2008/10/08)
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- Unusual Thermodynamic Behaviour on Complexation of Cobalt(II) with Chloride, Bromide and Iodide Ions in Hexamethylphosphoric Triamide
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The complexation of cobalt(II) with chloride, bromide and iodide ions has been studied by spectrophotometry and calorimetry in hexamethylphosphoric triamide (HMPA) containing 0.1 mol dm-3 (n-C4H9)4NClO4 as a constant ionic medium at 25 deg C.The formation of n>(2-n)+ (n = 1-4 with X as Cl, n = 1-3 with X as Br and n = 1 and 2 with X as I) is proposed, and their formation constants, enthalpies and entropies are determined.Electronic spectra of individual cobalt(II) halogeno complexes are also extracted, indicating that the cobalt(II) ion is four-coordinated in HMPA (unlike other oxygen-donor solvents which show six-coordination), and four-coordinate tetrahedral n(HMPA)4-n>(2-n)+ (n = 1-4) are formed stepwise.In spite of the remarkably strong donicity of HMPA compared to other aprotic donor solvents, the ΔH10 values of formation of + are even smaller, or more negative, in HMPA than in the others.Indeed, for + unusual exothermicity in HMPA is observed in contrast to endothermicity in N,N-dimethylformamide or dimethyl sulphoxide.This suggests that, although being sterically allowed, the four-coordination of 2+ is considerably crowded for bulky HMPA molecules, and Co-O(HMPA) bonds are thus elongated to some extent.
- Abe, Yuriko,Ozutsumi, Kazuhiko,Ishiguro, Shin-Ichi
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p. 3747 - 3756
(2007/10/02)
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- Kinetic Salt Effects on the Reaction between the Cobalt(III) Ion, Co(sep)3+, and the Hexa-aquochromium(II) Ion
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Highly specific effects are shown by different salts on the title reaction.Some salts increase the rate by a factor between ca. two and four, whereas for others the increase is between 30 and over a 1000 times.This is attributed to the formation of highly reactive ion pairs (or triplets), whose formation constants have been evaluated, in some cases, from the kinetic data at constant ionic strength.These ion pairs involve essentially the CrII reactant, and their high reactivity is related to the rate of formation of the precursor complex and/or to the rate of electron transfer within the precursor complex.
- Ferranti, Francesco,Indelli, Antonio
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p. 2241 - 2248
(2007/10/02)
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- Temperature, pressure, and viscosity dependence of outer-sphere electron-transfer processes. Reduction of aquapentaamminecobalt(III) by hexacyanoferrate(II)
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The outer-sphere electron-transfer reaction between Co(NH3)5H2O3+ and Fe(CN)64- was studied as a function of temperature (20-35°C), pressure (0.1-100 MPa), and solvent composition (water-glycerol mixtures up to 57 wt % glycerol). The experimental rate data were corrected for changes in viscosity with changes in temperature and pressure, and the resulting activation parameters were interpreted in terms of medium effects on the electron-transfer process. Both ΔH* and ΔS* increase significantly with increasing glycerol concentration, whereas ΔV* remains constant within the experimental error limits. The results point to the importance of solvent rearrangement and collisional contributions in determining the rate of the electron-transfer process.
- Krack,Van Eldik
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p. 851 - 855
(2008/10/08)
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- Photoinduced electron transfer in pentaamminecobalt(III) complexes of α-hydroxy acids
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The rates of photoreduction of pentaamminecobalt(III) complexes of α-hydroxy acids at 254 nm are comparable to the rates of formation of cobalt(II) and the corresponding carbonyl product.This is suggestive of electron transfer to cobalt(III) and C-C bond cleavage in an almost concerted manner.The charge transfer to metal ion has an excited state as t26e* facilitating photoreduction of cobalt(III).The d --> ? back bonding is ligand-centered, favouring C-C bond cleavage.In all these reactions, the quantum yield of Co(2+) is nearly 0.24.
- Subramani, K.,Srinivasan, Vangalur S.
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p. 378 - 381
(2007/10/02)
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- Kinetics of reduction of a series of imidazolium ions and cobalt(III)-imidazole complexes by 1-hydroxy-1-methylethyl radicals
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The rates of electron transfer from ?C(CH3)2OH to a series of imidazolium ions and to a series of (NH3)5Co(X-Im)3+ complexes have been measured. A kinetic competition method based on the homolytic scission of the chromium-carbon bond in (H2O)5CrC(CH3)2OH2+ was used to evaluate rate constants for the reducing agent ?C(CH3)2OH. In both series the rate constants show sensitivity to substituents on the imidazole ring. The rate constants for the free imidazolium ions are correlated by an LFER to the relative electron affinity of the imidazolium ion. The rate constants for the cobalt(III)-imidazole complexes are correlated by the lowest energy d-d absorption band of the complexes.
- Bakac, Andreja,Butkovi?, Vjera,Espenson, James H.,Lovri?, Jasna,Orhanovi?, Matko
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p. 4323 - 4326
(2008/10/08)
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- Properties and Reactivity of Metallocarboxylates. Characterization of Aquobis(ethylenediamine)(hydroxycarbonyl)cobalt(III) (2+) and Its Ethyl Ester, trans-PF6
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The title metallocarboxylate, first described by Vaudo et al. (J.Am.Chem.Soc. 1972, 94, 6655) has been further investigated, and its ethyl ester, prepared.Crystals of the ester (ethoxycarbonyl) PF6 are monoclinic: space group P21/c with a = 9.077(1) Angstroem, b = 16.375(2) Angstroem, c = 12.919(2) Angstroem, β = 92.15(1)deg, V = 1918.9(7) Angstroem3, Z = 4, and refined to a final R value of 0.079.The ethylenediamine ligands are trans to one another (Co-N ave, 1.944(13) Angstroem) with axial ligands CF3CO2(1-)(Co-O,2.046(7) Angstroem) and -C(O)OC2H5(1-)(Co-C, 1.922(9) Angstroem; C=O 1.196(9) Angstroem, νC=O 1645 cm-1).The aquo hydroxycarbonyl complex (UV-vis: λmax, nm -1 cm -1> 410 , 314 sh , 262 ) is a dibasic acid with pK1 = 2.5 +/- 0.5 and pK2 = 3.7 +/- 0.5 at 15 deg C and 0.5 M ionic strength and, at pH > 5, is present as the neutral hydroxy-oxycarbonyl complex Co(en)2(OH)(CO2) (UV-vis: 450 , 357 , 318 , 266 ).The aquo ethoxycarbonyl complex (UV-vis: 424 , 320 sh , 264 ) is a monobasic acid with pKa = 8.7 +/- 0.2 at 25 deg C and 0.5 M ionic strength and, at high pH, is converted to Co(en)2(OH)(CO2Et)(1+) (UV-vis: 430 , 329 sh ).The ethoxy function is hydrolyzed very slowly in the latter complexes (pH, kobsd: 1, 2.5 * 1E-6 s-1; 7, -1; 13, 3 * 1E-6 s-1) to yield ethanol and the hydroxy- or oxycarbonyl, depending upon the pH.At pH > 0 the oxy- or hydroxycarbonyl complex undergoes decomposition through the singly protonated Co(en)2(H2O)(CO2)(1+), exhibiting a rate maximum (kobsd ca. 3 * 1E-3 s-1, 25 deg C) at ca. pH 3.2.
- Katz, Nestor E.,Szalda, David J.,Chou, Mei H.,Creutz, Carol,Sutin, Norman
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p. 6591 - 6601
(2007/10/02)
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- Formation and reactions of an organochromium(III)-acetamido-pentaamminecobalt(III) complex
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The reaction of (NH3)5CoNHC(OH)CH2I3+ with chromium(II) proceeds in a biphasic manner. The first stage yields (H3N)5CoNHC(OH)CH2Cr(OH2) 55+ (II) as the major product, and the next stage produces (H2O)5Cr-OC(NH2)CH2Cr(OH 2)55+ (I) as cobalt(III) is reduced to Co(II). For the first and second stages, respectively, kobsd = (1.16 ± 0.05 M-1 s-1)[Cr2+] and ((8.8 ± 0.25) × 10-5 s-1)[Cr2+][H+]-1, both in 1.0 M LiClO4/HClO4 at 25°C. The reaction of II with Hg2+ has been studied, and the reactivity of the dimer is compared to that of monomer analogues and the Co-NCCH2-Cr dimer. The acidolysis of II is unusual in that it yields two products, which are linkage isomers of II at Co and Cr, respectively. In each case the metal atoms move to the carboxamide oxygen in the products.
- Sharma,Jordan
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p. 168 - 172
(2008/10/08)
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