Kinetics and mechanism of the base hydrolysis of thecis-chlorobis(ethylenediamme)(benzimidazole)cobalt(III) cation

Summary The hydrolysis ofcis-[CoCl(en)₂(bzmH)]²⁺ (en=ethylenediamine, bzmH=benzimidazole) has been studied over the pH range 8.31–11.58 at I=0.1 mol dm^–3 and 25°. Potentiometric titration of aqueous solutions of the [Co(en)₂(bzmH)OH₂]³⁺ complex obtained by silver(I) catalysed aquation of the chloro-complex give pK₁=5.81 and pK₂ = 8.84 for Equilibria (1) and (2) at 25° and I=0.1 mol dm^–3. Spectrophotometric titration of the hydroxy complex also gives a value of pK₂=8.88 for the ionisation of the coordinated benzimidazole. The kinetic data can be interpreted in terms of base hydrolysis ofcis-[CoCl(en)₂(bzmH)]²⁺ (k_OH=220 dm³ mol^–1s^–1) andcis-[CoCl(en)₂(bzm)]⁺ (k_OH=14.9 dm³ mol^–1s^–1). Comparisons with the corresponding imidazole and pyridine complexes are made.     

Kinetics and mechanism of the copper(II) promoted hydrolysis of the methyl ester of ethylenediaminemonoacetate

Summary Base hydrolysis of methyl ethylenediaminemonoacetate has been studied at I=0.1 mol dm^–3 (NaClO₄) over the pH range 7.4–8.8 at 25 °C. The proton equilibria of the ligand can be represented by the equations, where E is the free unprotonated ester species. Values of pK₁ and pK₂ are 4.69 andca. 7.5 at 25° (I=0.1 mol dm^–3). For base hydrolysis of EH⁺, k_OH=1.1×10³ dm³ mol^–1 s^–1 at 25 °C. The species E is shown to undergo lactamisation to give 2-oxopiperazine (k_lact ca. 1×10^–3 s^–1) at 25 °C. Formation of the lactam is indicated both by u.v. measurements and by isolation and characterisation of the compound.Base hydrolysis of the ester ligand in the complex [CuE]²⁺ has been studied over a range of pH and temperature, k _OH ²⁵ =9.3×10⁴ dm³ mol^–1 s^–1 with H=107 kJ mol^–1 and S ₂₉₈ =209 JK^–1 mol^–1. Base hydrolysis of [CuE]²⁺ is estimated to be some 10⁵5 fold faster than that of the free ester ligand. The results suggest that base hydrolysis occursvia a chelate ester species in which the methoxycarbonyl group of the ligand is bonded to copper(II).     

Kinetics of diamminesilver(I)-catalysed oxidation of sulphur(IV) by dioxygen in ammonia buffers

The kinetics of the silver(I)-catalysed autoxidation of SO₃ ^2– into SO₄ ^2– in ammonia–ammonium nitrate buffer obeyed the rate law:R _obs=k₁ k₂ K[Ag^I]_T[SO₃ ^2-}][O₂] / ([NH₃]+K[SO₃ ^2-])(k₁+k₂[O₂])The values of k ₁, k ₂/k _–1 and K were found to be 1.2l mol^–1 s^–1, 5.3 × 10² l mol^–1 and 0.6 respectively at 30 °C. Two alternative free radical mechanisms have been proposed.     

A new route to the asymmetric Schiff base ligands. The ligand exchange in [Mo(CN)<Subscript>3</Subscript>O(salhy)]<Superscript>2−</Superscript> ion. Kinetics and mechanism

The kinetics of the ligand exchange in (PPh₄)₂[Mo(CN)₃O(salhy)]. 6H₂O (Hsalhy = salicylaldehyde hydrazone) by a solvent molecule and by 2,2-bipyridine (bpy) have been studied in EtOH. For the ligand exchange by a solvent molecule the pseudo-first order rate constant equals k _obs = 3.2 (±0.2) × 10^–3 s^–1 (t=25 °C), H =67 (± 7) kJ mol^–1, S =–75 (±23) J mol^–1 K^–1, while for the exchange by a bpy molecule k _obs=3.5 (±0.2) × 10^–3 s^–1 (t=25 °C), H =56 (±7) KJ mol^–1, S = –104 (±8) J mol^–1 K^–1. It was found, that all reactions proceed via the same mechanism which involves the chelate ring opening cis to the Mo=O bond. The mechanism of the reaction was proposed and was proved by the synthesis of (PPh₄)₂[Mo(CN)₃O(N-pic)]. 2.5H₂O (N-pic denotes that the nitrogen of picolinic acid is trans to Mo=O) by ligand exchange in EtOH, while in aqueous solution the O-pic analogue is formed exclusively.     

Kinetics and mechanism of base hydrolysis oftrans-bis(Hmalonato)bis(ethylenediamine)cobalt(III) ion

Summary The kinetics of the first step of base hydrolysis oftrans-bis(Hmalonato)bis(ethylenediamine)cobalt(III) [malH^–=HO₂CCH₂CO ₂ ^– ] has been investigated in the 15–35° C range, I=0.3 mol dm^–3 (NaClO₄) and [OH^–]=0.015–0.29 mol dm^–3. The rate law is given by –d In[complex]_T/dt=k₁[OH^–] and at 30° C, k₁=8.5×10^–3 dm³ mol^–1s^–1, H=117.0±7.0 kJ mol^–1 and S=99.0±24.0 JK^–1mol^–1. The activation parameters data are consistent with the SN₁ cb mechanism.     

Mechanism of the oxidation of L-ascorbic acid by the bis(pyridine-2,6-dicarboxylate)cobaltate(III) ion in aqueous solution

A detailed investigation of the oxidation of L-ascorbic acid (H₂A) by the title complex has been carried out using conventional spectrophotometry at 510 nm, over the ranges: 0.010 [ascorbate] _T 0.045 mol dm^–3, 3.62 pH 5.34, and 12.0 30.0 °C, 0.50 I 1.00 mol dm^–3, and at ionic strength 0.60 mol dm^–3 (NaClO₄). The main reaction products are the bis(pyridine-2,6-dicarboxylate)cobaltate(II) ion and l-dehydroascorbic acid. The reaction rate is dependent on pH and the total ascorbate concentration in a complex manner, i.e., k _obs = (k ₁ K ₁)[ascorbate] _T /(K ₁ + [H⁺]). The second order rate constant, k ₁ [rate constant for the reaction of the cobalt(III) complex and HA^–] at 25.0 °C is 2.31 ± 0.13 mol^–1 dm³ s^–1. H = 30 ± 4 kJ mol^–1 and S = –138 ± 13 J mol^–1 K^–1. K ₁, the dissociation constant for H₂A, was determined as 1.58 × 10^–4 mol dm^–3 at an ionic strength of 0.60 mol dm^–3, while the self exchange rate constant, k ₁₁ for the title complex, was determined as 1.28 × 10^–5 dm³ mol^–1 s^–1. An outer-sphere electron transfer mechanism has been proposed.     

Kinetics and mechanism of the oxidation of a ternary complex involving nitrilotriacetatocobaltate(II) and malonic acid by N-bromosuccinimide

The kinetics of oxidation of [Co^IINM(H₂O)]^3– (N = nitrilotriacetate, M = malonate) by N-bromosuccinimide (NBS) in aqueous solution have been found to obey the equation: d[Co^III]/dt = k ₁ K ₂[NBS][Co^II]_T/{1 + K₂[NBS] + (H⁺/K₁)} where k ₁ is the rate constant for the electron transfer process, K ₁ the equilibrium constant for dissociation of [Co^IINM(H₂O)]^3– to [Co^IINM(OH)]^4– + H⁺, and K ₂ the pre-equilibrium formation constant. Values of k ₁ = 1.07 × 10^–3 s^–1, K ₁ = 4.74 × 10^–8 mol dm^–3 and K ₂ = 472 dm³ mol^–1 have been obtained at 30 °C and I = 0.2 mol dm^–3. The thermodynamic activation parameters have been calculated. The experimental rate law is consistent with a mechanism in which the deprotonated [Co^IINM(OH)]^4– is considered to be the most reactive species compared to its conjugate acid. It is assumed that electron transfer takes place via an inner-sphere mechanism.     

Kinetics of oxidation of pyruvic acid by [ethylenebis(biguanide)]silver(III) in aqueous acidic media

The oxidation of pyruvic acid by the title silver(III) complex in aqueous acidic (pH, 1.1–4.5) media is described. The reaction products are MeCO₂H and CO₂, together with a colourless solution of the Ag⁺ ion. The free ligand, ethylenebis(biguanide) is released in near-quantitative yield upon completion of the reduction. The parent complex, [Ag(H₂L)]³⁺ and one of its conjugate bases, [Ag(HL)]²⁺, participate in the reaction with both pyruvic acid (HPy) and the pyruvate anion (Py^–) as the reactive reducing species. Ag⁺ was found to be catalytically inactive. At 25.0°C, I=1.0moldm^–3, rate constants for the reactions [Ag(H₂L)]³⁺+HPy (k ₁), [Ag(H₂L)]³⁺+Py^– (k ₂), [Ag(HL)]²⁺+HPy (k ₃) and [Ag(HL)]²⁺+Py^– (k ₄) arek ₁=(94±6)×10^–5dm³mol^–1s^–1, (k ₂ K _a+k ₃ K _a1)= (1.3±0.1)×10^–5s^–1 and k ₄=(58±4)×10^–5dm³mol^–1s^–1, respectively, where K _a1is the first acid dissociation constant of the [Ag(H₂L)]³⁺ and K _a is for pyruvic acid. A comparison between the k ₁ and k ₄ values is indicative of the judgement that k ₂k ₃. A one-electron inner-sphere redox mechanism seems more justified than an outer-sphere electron-transfer between the redox partners.     

Ligand substitution equilibrium in the macrocyclic molybdenum(V) complex

The thermodynamics of the reaction of tetraphenylporphyrin(oxo)(hydroxo)molybdenum with imidazole (Im) in toluene was studied spectrophotometrically in a polythermal mode. It was established that the reaction of Im with O=Mo(OH)TPP includes several stages, two of which can be studied by spectrophotometrical titration. The first equilibrium stage is Im coordination to the eighth coordination site (K ₁ = 1.85 × 10³ l/mol), the second stage is the coordination of the second Im molecule with the displacement of OH^? into the outer sphere (K ₂ = 4.80 × 10² l/mol). The enthalpy and entropy contributions to the stability constant of tetrapheny lporphyrin molybdenum(V) complexes with Im were determined.     

Voltammetric studies on the α-tocopherol anion and α-tocopheroxyl (Vitamin E) radical in acetonitrile

The α-tocopheroxyl radical was generated voltammetrically by one-electron oxidation of the α-tocopherol anion (E ^r_1/2=−0.73 V versus Ag|Ag⁺) that was prepared by reacting α-tocopherol with Et₄NOH in acetonitrile (with Bu₄NPF₆ as the supporting electrolyte). Cyclic voltammograms recorded at variable scan rates (0.05–10 V s⁻¹), temperatures (−20 to 20°C) and concentrations (0.5–10 mM) were modelled using digital simulation techniques to determine the rate of bimolecular self-reaction of α-tocopheroxyl radicals. The k values were calculated to be 3×10³ l mol⁻¹ s⁻¹ at 20°C, 2×10³ l mol⁻¹ s⁻¹ at 0°C and 1.2×10³ l mol⁻¹ s⁻¹ at −20°C. In situ electrochemical-EPR experiments performed at a channel electrode confirmed the existence of the α-tocopheroxyl radical.     

Antioxidant Activity of Adamantylphenols

Antioxidant properties of phenols with ortho- and para-adamantyl substituents are studied by microcalorimetry using cumene oxidation as a model reaction. The experimental rate constants for inhibition by adamantylphenols (k ₇) are comparable to the corresponding values for sterically hindered phenols, and the stoichiometric inhibition coefficient is higher than the calculated one. Adamantylphenols with adamantyl and methyl or methylene bridge groups in the ortho positions with respect to the functional group exhibit the maximal activity. The inhibition rate constant takes a maximal value of k ₇ = 3.3 × 10⁴ l mol^–1 s^–1 for 2,4-di-(adamantyl-1)-6-methylphenol and a minimal value of k ₇ = 1.4 × 10³ l mol^–1 s^–1 for 2,4,6-triadamantylphenol. The difference in the rate constants is due to steric hindrances in the reaction of a peroxy radical with the functional group of a phenol surrounded by bulky adamantyl groups.     

Synthetic and kinetic studies on copper(II), nickel(II) and cobalt(III) complexes of 1,4,7,10,13-pentaazacyclohexadecane ([16]aneN5)

Summary The pentadentate macrocycle 1,4,7,10,13-penta-azacyclo-hexadecane [16]aneN₅=(3)=L} has been prepared and a variety of copper(II), nickel(II) and cobalt(III) complexes of the ligand characterised. The copper complex [CuL](ClO₄)₂, on the basis of its d-d spectrum, appears to be square pyramidal, while [NiL(H₂O)](ClO₄)₂ is octahedral. The copper(II) and nickel(II) complexes dissociate readily in acidic solution and these reactions have been studied kinetically. For the copper(II) complex, rate=k_H[complex][H⁺]² with k_H =4.8 dm⁶ mol^–2s^–1 at 25 °C and I=1.0 mol dm^–3 (NaClO₄) with H=43 kJ mol^–1 and S ₂₉₈ =–89 JK^–1 mol^–1. Dissociation rates of the copper(II) complexes increase with ring size in the order: [15]aneN₅ < [16]aneN₅ < [17]aneN₅. For the dissociation of the nickel(II) complex, rate=k_H[Complex][H⁺] with k_H=9.4×10^–3 dm³mol^–1 s^–1 at 25 °C and I =1.0 mol dm^–3 (NaClO₄) with H=71 kJ mol^–1 and S ₂₉₈ =–47 JK^–1mol^–1.The cobalt(III) complexes, [CoLCl](ClO₄)₂, [CoL(H₂O)]-(ClO₄)₃, [CoL(NO₂)](ClO₄)₂, [CoL(DMF)](ClO₄)₃ (DMF=dimethylformamide) and [CoL(O₂CH)](ClO₄)₂ have been characterised. The chloropentamine [CoCl([16]aneN₅)]²⁺ undergoes rapid base hydrolysis with k_OH=1.1× 10⁵dm³ mol^–1s^–1 at 25°C and I=0.1 mol dm^–3 (H=73 kJ mol^–1 and S ₂₉₈ =98 JK^–1 mol^–1). Rapid base hydrolysis of [CoL(NO₂)]²⁺ is also observed and the origins of these effects are considered in detail.     

Kinetics and mechanism of exchange reaction of [Y(APTA)] (APTA=o-aminophenol-N,N, O-triacetic acid) with copper(II)

Summary The kinetics of the exchange reaction between [Y(APTA)] and Cu^II have been investigated over a range of [H⁺] from 2.5×10^–5 to 7.5×10^–4 mol dm^–3 at 30°C and ionic strength 0.2 mol dm^–3 KNO₃. The results show that the exchange reaction proceeds via both self-and proton-catalyzed dissociation of [Y(APTA)] and also by the direct attack of Cu^II on [Y(APTA)]. The corresponding rate constants k_d, k _h and k_Cu have been evaluated as 6.3s^–1, 8.4×10⁴ mol^–1 dm³ s^–1 and 416mol^–3 dm³ s^–1 respectively. The possible intermediates are discussed in terms of the structure of APTA. The complex-formation rate constants of Y^III with APTA^3- and HAPTA^2- were also obtained.     

Kinetics of base hydrolysis of trans-{Cr([15]aneN4)Cl2}+ ([15]aneN4 = 1,4,8,12-tetra-azacyclopentadecane)

Summary Reaction of CrCl₃(DMF)₃ with [15]aneN₄ (L; L = 1,4,8,12-tetra-azacyclopentadecane) gives the green trans-{Cr([15]-aneN ₄)Cl₂}Cl in high yield. The base hydrolysis kinetics of the cations [CrLCl₂]⁺ and [CrLCl(OH)] ⁺ have been investigated over a temperature range. For the dichloro complex, k _OH = 1.03 dm³ mol^–1 s^–1] at 25° C with H =30.4 kJmol^–1 and S _inf298 ^sup = -143 JK^–1 mol^–1. The substantial negative entropy of activation implies more association of water in the loss of Cl^– from the conjugate base in a D_CB mechanism. The kinetic parameters for the chlorohydroxo complex are k _OH = 1.9 × 10^–2dm³mol^–1 s^–1 at 25°C with H = 78.3kJmol^–1 and H _inf298 ^sup = -15 J K^–1 mol ^–1. The chlorohydroxo complex probably has the trans VI configuration with the chloride ligand on the same side of the equatorial plane as the four chiral sec-NH groups. The visible spectra of a variety of complexes trans-[Cr(L)XY] ⁿ⁺ (X = Y = Cl, OH, OH₂; X = Cl, Y = OH) have been determined.     

Metal ion catalysed aquation of carboxylatoamine cobalt(III) complexes. Kinetics of copper(II) catalysed aquation ofcis(diformato)bis(ethylenediamine)cobalt(III) ion

Summary The aquation ofcis-[(en)₂Co(CO₂H)₂]⁺ tocis-[(en)₂Co(OH₂)(CO₂H)]²⁺ is catalysed by Cu²⁺ and the rate equation, –d[complex]_t/dt=(k_Cu[Cu²⁺]+k_H [H⁺]) [complex)_T is valid at [Cu²⁺]_T=0.01–0.1, I=0.5 and [HClO₄]=0.005 mol dm^–3. The rate measurements are reported at 30, 35, 40 and 45°C and the rate and activation parameters for the Cu²⁺ and H⁺-catalysed paths are: k_H(35°C)=(2.44±0.09)×10^–2 dm³ mol^–1 s^–1, H=83±13 kJ mol^–1, S=–8±42 JK^–1 mol^–1, k _Cu (35°C)=(3.30±0.09)×10^–3 dm³ mol^–1 s^–1, H=73.2±6.1 kJ mol^–1, S=–55±20 JK^–1 mol^–1. The formate-bridged innersphere binuclear complex,cis-[(en)₂Co{(O₂CH)₂Cu}]³⁺ may be involved as the catalytically active intermediate in the copper(II)-catalysed path, just as the corresponding H⁺-bridged species presumed to be present in the acidcatalysed path.     

Kinetics and mechanism of complex formation between beryllium(II) and the 3-nitrosalicylatopentaamminecobalt(III) ion

Summary The kinetics of formation and dissociation of the binuclear complex of Be²⁺ with 3-nitrosalicylatopentaamminecobalt(III) have been investigated in the 20–40 and 25–40 °C ranges (I = 0.3 mol dm ^–3), respectively. At 25 °C the rate and activation parameters for the formation of the binuclear species are: k _f = 26.9 × 10² dm³mol^–1s^–1, H = 104 ± 7kJ mol^–1 S = 91 ± 22JK^–1mor^–1.The rate constant, activation enthalpy and activation entropy for the acid-catalysed dissociation of the binuclear species are: 1.25 ± 0.08dm³mol ^–1 at 25 °C, 53 ± 3kJ mol^–1 and - 67 ± 9 J K ^–1 mol^–1, respectively. The formation of the binuclear species is chelation controlled while the dechelation is acid catalysed.     

Kinetics and mechanism of formation of square-planar copper(II) and nickel(II) complexes of ethylenebisbiguanide in aqueous acetate buffer media

Summary The kinetics of formation of square-planar Cu^II and Ni^II complexes of the quadridentate ligand, ethylenebisbiguanide, have been studied spectrophotometrically in aqueous HOAc–NaOAc buffer, at ionic strength 0.2 mol dm^–3, in the 25–35°C temperature range. The observed rate constants for the formation reactions are independent of pH (and of OAc^– concentration) in the pH range used (3.6–4.8 for Cu^II and 5.0–5.8 for Ni^II) where the product complexes form stoichiometrically, but show first-order dependence on the ligand concentration;i.e. k_obs=k_f[L]_total. At 25°C k_f values (dm³ mol^–1s^–1) are 35.2±0.4 for Cu^II and (8.4±0.1)×10^–3 for Ni^II. The mechanism of the reactions is discussed.     

Kinetics and mechanism of the interaction of azide with [(H2O)(tap)2RuORu(tap)2-(H2O)]2+ ion at physiological pH

The title reaction has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand], pH and temperature at constant ionic strength. At the physiological pH (7.4) the interaction with azide shows two distinct consecutive steps, i.e., it shows a non-linear dependence on the concentration of N₃ ^–; both processes are [ligand]-dependent. The rate constant for the processes are: k ₁10^–3 s^–1 and k ₂10^–5 s^–1. The activation parameters calculated from Eyring plots are: H ₁ = 14.8 ± 1 kJ mol^–1, S ₁ = –240 ± 3 J K^–1 mol^–1, H ₂ = 44.0 ± 1.5 kJ mol^–1 and S ₂ = –190 ± 4 J K^–1 mol^–1. Based on the kinetic and activation parameters an associative interchange mechanism is proposed for the interaction process. From the temperature dependence of the outersphere association equilibrium constant, the thermodynamic parameters calculated are: H ₁ ⁰ = 4.4 ± 0.9 kJ mol^–1, S ₁ ⁰ = 64 ± 3 J K^–1 mol^–1 and H ₂ ⁰ = 14.2 ± 2.9 kJ mol^–1, S ₂ ⁰ = 90 ± 9 J K^–1 mol^–1, which gives a negative G ⁰ value at all temperatures studied, supporting the spontaneous formation of an outersphere association complex.     

Step Complex Formation in the Dichloro(5,10,15,20-tetraphenylporphinate)hafnium(IV)—Pyridine—Toluene System

The thermodynamic characteristics and kinetics of step reactions between dichloro(5,10,15,20-tetraphenylporphinate)hafnium(IV) (Cl)₂HfTPP and a “small” organic base, pyridine (Py), in toluene were studied. The coordination of the Py molecule (K ₁ = 1.50 × 10⁴ l/mol) was determined. The slow irreversible displacement of Cl^? in (Cl)₂(Py)HfTPP into the second coordination sphere (k ₁ = 9.74 × 10si?4 s^?1), the substitution of the second Py molecule for Cl^? in (Cl)₂(Py)HfTPP (K ₂ = 14.15 l/mol), subsequent irreversible displacement of the second Cl^? ion into the outer sphere (k ² = 6.05 × 10^?4 s^?1), and the coordination of the third Py molecule as a result of the replacement of Cl^? with the formation of [(Py)₃HfTPP]²⁺ 2Cl^? (K ₃ = 0.23 l/mol) at the first, second, and third stages, respectively, were observed. Prospects for the use of the metalloporphyrin as a receptor of organic N-bases were considered.     

Rate and Equilibrium Constants of Benzoyl Group Transfer between Pyridine N-Oxides

Kinetic characteristics of 19 transfer reactions of benzoyl group from N-benzoyloxypyridinium salts to pyridine N-oxides and 4-dimethylaminopyridine were studied in acetonitrile by the stopped-flow method. The rate of an identical reaction for 4-methoxypyridine was measured by dynamic NMR spectroscopy. For 5 other identical reactions the rates were estimated from Bronsted correlations. Equilibrium constants were estimated with the use of UV spectrophotometry (6), IR spectroscopy (2), from kinetic data (K _ij = k _ij /k _ji ) (2), and in one case as logK _i−j = logK _i−x − logK _j−x . The second order rate constants (k _ij ) varied in the range 10²–10⁵ l mol⁻¹ s⁻¹, the equilibrium constants (K _ij ) in the range 10²–10⁻²; the activation parameters (ΔH ^≠) were within 15–50 kJ mol⁻¹, (−ΔS ^≠) −20–110 J mol⁻¹ K⁻¹. The reactions under study occur in a single stage following the concerted S_N2 mechanism through an early associative transition state. The benzoyl groups exchange rate and equilibrium are well described by simplified Marcus equation (omitting the quadratic term).__________Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 5, 2005, pp. 788–792.Original Russian Text Copyright © 2005 by Rybachenko, Schroeder, Chotii, Lenska, Red’ko, Kovalenko.