Kinetics and mechanism of base hydrolysis of cisα-oxalato(amine)(trien)cobalt(III) and salicylato(amine)(trien)cobalt(III)cations

Oxalato(amine)(trien)cobalt(III) and salicylato(amine)(trien)cobalt(III) perchlorates have been synthesized and tentatively assigned a cis α configuration. The dissociation constants of the complexes have been determined. The base hydrolysis of the complexes have been investigated at 30, 35, 38.6°C and I = 1.0 mol dm⁻³. The rate laws for the oxalato and salicylato complexes are −dln[Complex]_T/dt = k₂[OH⁻] and −dln[Complex]_T = k₁ + k₂[OH⁻] respectively. For the oxalato complex, k₂(30°C) = (2.95 ± 0.05) × 10⁻² dm³ mol⁻¹ s⁻¹, ΔH¹ = (109 ± 4) KJ mol⁻¹, ΔS¹ = (85 ± 13) JK⁻¹ mol⁻¹ and for the salicylato complex, k₁(30°C) = (1.58 ± 0.28) × 10⁻⁴ s⁻¹, ΔH¹ = (166 ± 7) KJ mol⁻¹, ΔS¹ = (229 ± 21) JK⁻¹ mol⁻¹ and k₂(30°C) = (3.56 ± 0.10) × 10⁻³ dm³ mol⁻¹ s⁻¹, ΔH¹ = (101 ± 6) KJ mol⁻¹, ΔS¹ = (42 ± 20) JK⁻¹ mol⁻¹. The base hydrolysis reactions of the complexes were followed in presence of imidazole and ethanolamine, in the presence of added anions and also in D₂O medium. The results are discussed in terms of δ SN₁CB mechanism involving rate limiting CoO bond fission for both k₁ and k₂ paths. However, the possibility of CO bond cleavage in the base hydrolysis of the oxalato complex is not ruled out.     

Kinetics of anation of hexaaquachromium(III) by thiocyanates – specific salt effects

The kinetics of anation of hexaaquachromium(III) by thiocyanates follows the rate law: –d[complex]/dt=k[NCS] (20–40°C, [NCS^–]=0.1–0.6M, I=2.0M, pH=1.0). The specific salt effect has been studied for five media: NaCl, NaBr, NaClO₄, KCl and CsCl. The series of chloride (Na⁺, K⁺ and Cs⁺) salts show a negligible effect on the anation rate. On the contrary, the series of sodium salts (Cl^–, Br^– and ClO ₄ ^– ) reveal a marked difference in the reaction rate. The anation rate decreases sharply with the ionic strength increase (I=0.2–2.0M, NaCl). The results were interpreted within the frame of fast equilibria of ion-pair formation followed by an interchange mechanism step. The difference of reaction rate is a result of competition between anions (thiocyanates and supporting electrolyte anions) to the complex cation at an ion-pair formation process.     

Kinetics and mechanism of decarboxylation of cis-carbonato- and bicarbonato(3,3′,3′′-triaminotripropylamine)cobalt(III) ions

The acid-catalysed hydrolysis of [Co(trpn)(CO3)]+ and [Co(trpn)(HCO3)]2+ ions (trpn = 3,3,3-triaminotripropylamine) have been studied spectrophotometrically in aqueous 1.0m HClO4/NaClO4. For the carbonato complex, [HClO4] = 0.02–0.25m and T = 20–35°C; for the bicarbonato complex, [HClO4] = 0.025–0.30m and T = 25°C. Both complexes hydrolyse to form the same cis-diaqua species. The rate law for the hydrolysis is d(ln[CoIII])/dt = k0+k1[H3O+]. The values of the rate constants (25°C), H (kJmol–1) and S (Jmol–1K–1)are:[Co(trpn)(CO3)]+,k0 = (1.7±0.6)× 10–4s–1, H 0 = 57±21, S 0 = –126±75; k1 = (1.0 ±0.1)×10–2m–1s–1, HDagger;1 = 62±8, S1 = –75 ±21, and for [Co(trpn)(HCO3)]2+, k0 = (2.9±0.7)× 10–4s–1, and k1 = (7.8±1.0)×10–2m–1s–1. The carbonato complex exhibits a deuterium isotope effect with k1D/k1H = 1.9, consistent with a rapid pre-equilibrium protonation, followed by rate-controlling ring-opening. The rate constants k0 and k1 (25°C and = 1.0m) for the ring-opening decarboxylation of the two systems studied lie within the experimental error. The results are compared with o ther related systems and the factors which influence the ring-opening decarboxylation (steric hindrance, ring strain, electron-donor ability of the amines) are discussed. The k1 path is interpreted in terms of concerted ring-opening and bond-making in the highly unstable aquabicarbonato intermediate.     

Nucleotide hydrolysis in the presence of μ-hydroxo-bridged-cobalt(III) complexes

Abstract

Two cobalt(III) complexes with tridentate ligands, the acyclic diethylenetriamine and macrocyclic 1,4,7-triazacyclononane, were examined as potential catalysts for the hydrolysis of adenosine 5′-triphosphate (ATP). Studies were performed primarily at pH 4.5, where the two complexes were in the binuclear di-μ-hydroxo forms. For both complexes, a rapid initial hydrolysis with first order dependence on the concentration of ATP was observed, k_obs of approximately 0.20 min^?1, followed by very slow hydrolysis. Deuterium isotope studies done in D₂O showed a normal isotope effect with k_H/k_D = 1.8. Spectral investigations and ⁵⁹Co NMR studies indicated that the biphasic nature of the hydrolysis reaction may be due to the formation of a complex in which inorganic phosphate is coordinated to the cobalt, effectively poisoning the catalyst.     

Kinetics and mechanism of the oxidation of neutralized α-hydroxy acids by tris(pyridine-2-carboxylato)manganese(III)

The kinetics of oxidation of the neutralized -hydroxy acids: lactic, -hydroxyisobutyric, mandelic, benzilic and atrolactic acids by tris(pyridine-2-carboxylato)manganese(III) have been studied. The reactions were carried out in a Na(pic)-picH [Na(pic) = sodium salt of pyridine-2-carboxylic acid and picH = pyridine-2-carboxylic acid] buffer medium in the 4.89–6.10pH range. The oxidation rate was found to be independent of pH, and rate follows the order: benzilate > mandelate >atrolactate>lactate > -hydroxy isobutyrate. The oxidation products are MeCHO, Me2CO, PhCHO, Ph2CO and PhCOMe for the respective reactions. A mechanism is proposed involving intermediate formation of hepta-coordinated MnIII complexes in a fast step. The complexes then decompose to give free radicals and MnII in the rate determining step. The free radicals subsequently react with another molecule of the MnIII species to give the respective carbonyl compounds in a fast step.     

Kinetic study of the protonations and substitutions of different cobalt(III)–nta complexes

The behaviour of the Cobalt(III)–nta (nta = nitrilotriacetate) system in an acidic medium was investigated. The acid dissociation constant, pK _a1, of [(nta)(H₂O)Co(-OH)Co(H₂O)(nta)]^– was determined as 3.09(3) and the pK _a of the cis-[Co(nta)(H₂O)₂]/[Co(nta)(H₂O)(OH)]^– equilibrium was determined as 6.71(1). cis-[Co(nta)(H₂O)₂] undergoes ring-opening upon acidification below pH = 2.0. The formation of [Co( ³-nta)(H₂O)₃]⁺ was also studied. The substitutions between cis-[Co(nta)(H₂O)₂] and NCS^– ions were investigated in the pH = 2–7 ranges. [Co(nta) (H₂O)(OH)]^– reacts ca. 70 times faster at 24.7 °C with NCS^– ions than cis-[Co(nta)(H₂O)₂], indicating a cis-labilising effect of the OH^– ligand.     

Molecular oxygen oxidation of carbonyl compounds by di-μ-hydroxo homobinuclear copper(II) complexes

Di-μ-hydroxo-bridged di-copper(II) complexes in aprotic media exhibit chemical properties in the oxidation reaction by molecular oxygen. Three oxidation reactions can occur without a base: oxidation of aldehydes into the corresponding acids, oxidative degradation of some carbonyl compounds into their lower homologues, and oxidative dimerisation of phenol. With a base a fourth reaction is observed: the dehydrogenation of alcohols into carbonyl compounds. Ligands, solvent and substrate structure effects are discussed.     

Kinetics and mechanism of the oxidation of L-ascorbic acid by the N,N′-ethylenebis(salicylideneiminato)manganese(III) complex in aqueous solution

Summary The kinetics of oxidation of l-ascorbic acid by the N,N-ethylenebis(salicylideneiminato) manganese(III) complex have been studied over the 4.5–9.3 pH range. An intermediate ascorbate complex was formed which had an inhibiting effect on the rate of the redox reaction. The rapid formation of this intermediate was followed using the stopped-flow technique, whereas its slow decomposition was monitored using a conventional spectrophotometer. The formation of this intermediate was strongly pH dependent. Addition of sodium perchlorate and sodium dodecyl sulphate (anionic surfactant) affected the reaction rate. A probable mechanism comprising both the intermediate formation and the overall redox reaction is discussed.     

Kinetics and mechanism of the reduction of diaquatetrakis (2,2′-bipyridine)-μ-oxo diruthenium(III) by ascorbic acid

Summary The kinetics of the oxidation of ascorbic acid by diaquatetrakis (2,2-bipyridine)--oxo diruthenium(III) in aqueous HClO₄ were investigated. The dependence of the second order rate constantk ₂ on [H⁺] is given by k ₂=a+b[H⁺]^–, indicating that both the undissociated form and the monoanion of ascorbic acid are reactive. Marcus theory was used to estimate the redox potential for the Ru^III-O-Ru^III/Ru^III-O-Ru^II couple and a feasible mechanism has been proposed to explain the results.     

Synthesis,crystal structures and characterization of iron(III) and cobalt(III) complexes bearing 2,2′-bipyridylcarboxylate ligands

Transition Metal Chemistry - The Fe(III) complex [FeIII(bpdc)(Hbpdc)] (1) (bpdc?=?2,2′-bipyridyl-6,6′-dicarboxylate and...     

Medium effects on the reactions of coordination complexes: Base hydrolysisof(αβS)(p-hydroxybenzoato)(tetraethylenepentamine) cobalt(III) in iso-propanol–water,tert-butanol–water and dimethyl sulfoxide–water media

The base hydrolysis of (S)(p-hydroxybenzoato)-(tetraethylenepentamine)cobalt(III) has been investigated in aqueous–organic solvent media using i-PrOH, t-BuOH and dimethyl sulfoxide (DMSO) as cosolvents at 20.0 T (°C) 40.0 (I=0.02 mol dm ^-3) with 80% (v/v) of cosolvents. Only the base-catalysed path (k_obs=k_OH[OH^-]) is observed. The relative second order rate constant k _OH ^os /k _OH ^ow at I=0 increases nonlinearly with increasing mol fraction (x_O.S.) of the cosolvents, the rate acceleration in alcoholic cosolvents being greater than in DMSO. The destabilization of ^-OH in mixed solvent media alone does not explain the observed rate acceleration. The solvent composition dependence, log k _OH ^os = log k _OH ^ow + a_ix _os ⁱ [i=1,2,k _OH ⁰ denotes k_OH at I=0 in mixed solvent(s) and water (w)] indicates specific solute–solvent interactions. The values of the relative transfer free-energy data [_TG(t.s.) - _TG^o (i.s.)](sw)(25 °C)(G), where t.s. and i.s. denote the transition state and initial state of the substrates respectively, are positive for all substrates at all compositions, indicating a greater destabilizing effect of the mixed solvent on the transition state than on the initial state. The G values also correlate with G^E(G = ax_O.S. + cG^E) for all solvents, supporting the fact that solvent structural effects mediate the rates and energetics of the reaction. However, the solvent effects on the solvation components of H and S are mutually compensating, thus indicating that there is no change in the mechanism.     

Kinetics of base hydrolysis of α-amino acid esters catalyzed by palladium(II) piperazine complex

The kinetics of base hydrolysis of glycine, histidine, and methionine methyl esters in the presence of [Pd(pip)(H₂O)₂]²⁺ complex, where pip is piperazine, is studied in aqueous solutions, at T = 25°C, and I = 0.1 mol dm⁻³. The rate of ester hydrolysis for glycine methyl ester is studied at different temperature and dioxane/water solutions of different compositions. The kinetic data are fit under the assumption that the hydrolysis proceeds in one step. The activation parameters for the base hydrolysis of the complexes are evaluated      

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

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

Kinetics and mechanism of the oxidation of tris(2,2′-bipyridine)iron(II) and tris(1,10-phenanthroline)iron(II) complexes by nitropentacyanocobaltate(III) in acidic aqueous medium

The kinetics of the oxidation of tris(2,2′-bipyridyl)iron(II) and tris(1,10-phenanthroline)iron(II) complexes ([Fe(LL)₃]²⁺, LL = bipy, phen) by nitropentacyanocobaltate(III) complex [Co(CN)₅NO₂]^3? was investigated in acidic aqueous solutions at ionic strength of I = 0.1 mol dm^?3 (HCl/NaCl). The reactions were carried out at fixed acid concentration ([H⁺] = 0.01 mol dm^?3) and the temperature maintained at 35.0 ± 0.1 °C. Spectroscopic evidence is presented for the protonated oxidant. Protonation constants of 360.43 and 563.82 dm³ mol^?1 were obtained for the monoprotonated and diprotonated Co(III) complexes respectively. Electron transfer rates were generally faster for [Fe(bipy)₃]²⁺ than [Fe(phen)₃]²⁺. The redox complexes formed ion-pairs with the oxidant with increasing concentration of the oxidant over that of the reductant. Ion-pair constants for these reaction were 160.31 and 131.9 dm³ mol^?1 for [Fe(bipy)₃]²⁺ and [Fe(phen)₃]^2+, respectively. The activation parameters measured for these systems have values as follows: ?H ^≠ (kJ K^?1 mol^?1) = +113.4 ± 0.4 and +119 ± 0.3; ?S ^≠ (J K^?1) = +107.6 ± 1.3 and 125.0 ± 1.6; ?G ^≠ (kJ K^?1) = +81 ± 0.4 and +82.4 ± 0.4; and E _a (kJ mol^?1) = 115.9 ± 0.5 and 122.3 ± 0.6 for LL = bipy and phen, respectively. Effect of added anions (Cl^?, $ {\text{SO}}_{4}^{2 - } $ and $ {\text{ClO}}_{4}^{ - } $ ) on the systems showed decrease in the electron transfer rate constant. An outer-sphere mechanism is proposed for the reaction.     

Kinetics and mechanism of complex formation between O-bonded cis-(diglycinato)bis(ethylenediamine)cobalt(III) and α-cis-(diglycinato) (triethylenetetramine)cobalt(III) with nickel(II) in aqueous medium

Summary The kinetics of reversible complexation of Ni(OH₂) _inf6 ^sup2+ with oxygen-bonded glycinatocobalt(III) substrates N₄-Co(glyH)gly²⁺ [N₄ = (en)₂ or trien; glyH = H₃N⁺CH₂-COO^–] have been investigated by the stopped-flow technique in the 20–35° C range, at pH = 6.08–6.82 and I = 0.3 mol dm^–3. The formation of N₄Co(glyH)glyNi⁴⁺ occurred via the reaction of Ni(OH₂) _inf6 ^sup2+ with the deprotonated form of the cobalt(III) substrates, N₄Co-(glyH)gly²⁺. The rate and activation parameters for the formation and dissociation of the binuclear species are reported. The formation rate constants k _f (at 25° C), activation enthalpy and entropy H , S for N₄Co-(glyH)glyNi⁴⁺ are 320±49, 341 ± 52dm³mol^–1 s^–1, 78 ± 7, 79 ± 5 kJmol^–1 and 64 ± 24, 69 ± 18 JK^–1 mol^–1 for the ethylenediamine and triethylenetetraminecobalt(III) substrates, respectively. This result indicates that the rate and activation parameters are virtually independent of the nature of N₄ moities, which strongly suggests that the formation of mono-bonded species occurs via entry of one of the pendant NH₂ groups into the coordination sphere of nickel(II) via a rate-limiting Ni-OH₂ bond dissociation mechanism (I_d). The binuclear species exist in dynamic equilibrium between the monodentate and chelated forms, with the chelate form predominating. The low values of spontaneous dissociation rate constant for the binuclear species (k _r- 0.095^–1 at 25° C) in comparison with the high values of dissociation rate constants of monodentate nickel(II) complexes reported in the literature also support the chelate nature of the binuclear species.     

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

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

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

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

Kinetics and mechanism of ruthenium(III) catalyzed oxidation of chloromphenicol—An antibiotic drug by diperiodatocuprate(III) in aqueous alkaline medium

The kinetics of ruthenium(III) catalyzed oxidation of chloramphenicol (CHP) by diperiodatocuprate(III) (DPC) in aqueous alkaline medium at a constant ionic strength of 0.1 mol l⁻¹ was studied spectrophotometrically. The reaction between DPC and CHP in alkaline medium exhibits 1: 2 stoichiometry (CHP: DPC). The main oxidation products were identified by spot test, IR, NMR, and GC-MS spectral studies. The reaction is first order with respect to ruthenium(III) and DPC concentrations. The order with respect to chloramphenicol concentration varies from first order to zero order as the chloramphenicol concentration increases. As the alkali concentration increases the reaction rate increases with fractional order dependence on alkali concentration. Increase in periodate concentration decreases the rate. A mechanism adequately describing the observed regularities is proposed. The reaction constants involved in the different steps of the mechanism were calculated. The activation parameters with respect to limiting step of the mechanism are computed and discussed. Thermodynamic quantities are determined.