Kinetics and mechanism of the aquation of the trinuclear cation, [μ3-oxo-triaqua-hexakis(acetato)tris(iron(III))]in perchloric acid media

The aquation of the title complex cation in aqueous perchloric acid proceeded via two steps, both postulated to be the proton attack on the oxygen atom which binds the acetate ligand to the metal centre, followed by Fe-O bond cleavage. This was followed by rapid decomposition to produce aqueous iron(III) and acetate ions. The first-order rate constants for the first and second steps at 25 °C are: k₁ = (4.16 ± 0.58) × 10⁻² s⁻¹ and k₂ = (2.09 ± 0.42) × 10⁻³ s⁻¹, respectively, and their corresponding activation parameters are . The spontaneous hydrolysis rate constants for the first and second steps were also determined at 25 °C and ionic strength of 1 mol dm⁻³ and they are k₀ = (3.10 ± 0.82) × 10⁻³ s⁻¹ and , respectively. The corresponding activation parameters are .     

Kinetic studies of the reduction of hexaaquacobalt(III) perchlorate by a cobaloxime, [Co(dmgBF2)2(H2O)2]

The reaction of with Co(dmgBF₂)₂(H₂O)₂ in 1.0 M HClO₄/LiClO₄ was found to be first-order in both reactants and the [H⁺] dependence of the second-order rate constant is given by k_2obs = b/[H⁺], b at 25 °C is 9.23 ± 0.14 × 10² s⁻¹. The [H⁺] dependence at lower temperatures shows some saturation effect that allowed an estimate of the hydrolysis constant for as K_a = 9.5 × 10⁻³ M at 10 and 15 °C. Marcus theory and the known self-exchange rate constant for Co(OH₂)₅OH^2+/+ were used to estimate an electron self-exchange rate constant of k₂₂ = 1.7 × 10⁻⁴ M⁻¹ s⁻¹ for .     

Anation kinetics of pentacyanoaquachromate(III) by thiocyanate and azide

The kinetics of the reaction of Cr(CN)₅(H₂O)²⁻ with NCS⁻ and were studied at pH 5.0 and at pH 6.3-7.0, respectively, as a function of the temperature between 25.0 and 55.0 °C, and at various ionic strengths. Anation occurs in competition with aquation of CN⁻, with rate constants that exhibit less-than-first-order dependence on the concentration of the entering anions. The results are interpreted in terms of ligand interchange in a context of association of the two reacting anions mediated by the Na⁺ or Ca²⁺ counterions. The degree of aggregation depends mainly on the total cationic charge rather than on the ionic strength, and is ca. 2-fold larger for than for NCS⁻. Within the associated species, is a better entering ligand than NCS⁻ by a factor of 4.5. The Cr(CN)₅(NCS)³⁻ and Cr(CN)₅(N₃)³⁻ complexes were also synthesized, and the rates of aquation of NCS⁻ and were measured at pH 5.0 and between 55.0 and 80.0 °C, over the same range of ionic strengths. The ionic strength enhances the anation rates but has little effect on the aquation rates. The average activation enthalpies of the interchange step are 80 ± 3 and 76 ± 3 kJ mol⁻¹ for entry of NCS⁻ and , respectively. Those of the corresponding aquation reactions are 94 ± 4 and 107 ± 4 kJ mol⁻¹. Within error limits, all ΔH^‡ values are independent of the ionic strength. The results are consistent with an I_d mechanism for substitution in Cr(CN)₅X^z− complexes.     

Kinetic studies of tris(2,2′-bipyridine)iron(III) perchlorate with cobaloxime, [Co(dmgBF2)2(H2O)2]

The kinetics of the reduction of by Co(dmgBF₂)₂(H₂O)₂ in 0.041 M HNO₃/NaNO₃ was found to be first-order in both the oxidizing and reducing agents and the second-order rate constant is given by k_obs = k₁ + k₂K[Cl⁻], with k₁=1.59 × 10⁶ M⁻¹ s⁻¹and k₂K = 1.83 × 10⁸ M⁻² s⁻¹, at 25 °C. The term that is first-order in [Cl⁻] is attributed to the formation of an ion-pair between and Cl⁻. For k₁, the activation parameters ΔH* and ΔS* are 2.22 ± 0.02 kcal mol⁻¹ and −22.7 ± 0.8 cal mol⁻¹ K⁻¹, respectively. The self-exchange rate constant of k₂₂ ≈ 8.7 × 10⁻³ M⁻¹ s⁻¹ for was estimated using Marcus theory and the known self-exchange rate constant for .     

The aluminium(III)-sialic acid interaction: A potential role in aluminium-induced cellular membrane degeneration

The coordination between Al(III) and sialic acid (N-acetylneuraminic acid, HL, pK_a = 2.58 ± 0.01) was studied by potentiometric titrations at 25 °C in aqueous 0.2 M KCl, by ¹H NMR, and by electrospray ionization mass spectrometry (ESI-MS). The potentiometric measurements gave the following aluminium complex stoichiometries and stability constants: , log β(AlLH₋₂) = −6.34 ± 0.02, and log β(AlL₂H₋₁) = −1.14 ± 0.04. The ¹H NMR spectra yielded structural information on species . The ESI-MS data confirmed the metal-ligand stoichiometry of the complexes.The metal-ligand speciation at micromolar Al(III) concentrations (i.e., under in vivo conditions) at physiological pH values reveals that considerable amount of Al(III) is complexed. This suggests that the toxic effect of Al(III) towards cellular membranes might be due to its coordination by protein-bound sialic acid.     

Peroxynitrite detoxification by ferryl Mycobacterium leprae truncated hemoglobin O

During infection, Mycobacterium leprae is faced with the host macrophagic environment limiting the growth of the bacilli. However, (pseudo-)enzymatic detoxification systems, including truncated hemoglobin O (Ml-trHbO), could allow this mycobacterium to persist in vivo. Here, kinetics of peroxynitrite (ONOOH/ONOO⁻) detoxification by ferryl Ml-trHbO (Ml-trHbOFe(IV)O), obtained by treatment with H₂O₂, is reported. Values of the second-order rate constant for peroxynitrite detoxification by Ml-trHbOFe(IV)O (i.e., of Ml-trHbOFe(III) formation; k_on), at pH 7.2 and 22.0 °C, are 1.5 × 10⁴ M⁻¹ s⁻¹, and 2.2 × 10⁴ M⁻¹ s⁻¹, in the absence of and presence of physiological levels of CO₂ (∼1.2 × 10⁻³ M), respectively. Values of k_on increase on decreasing pH with a pK_a value of 6.7, this suggests that ONOOH reacts preferentially with Ml-trHbOFe(IV)O. In turn, peroxynitrite acts as an antioxidant of Ml-trHbOFe(IV)O, which could be responsible for the oxidative damage of the mycobacterium. As a whole, Ml-trHbO can undertake within the same cycle H₂O₂ and peroxynitrite detoxification.     

Kinetics and mechanism of the oxidation of oxalic acid by tetrachloroaurate(III) ion

The oxidation of oxalic acid by tetrachloroaurate(III) ion in 0.005 ? [HClO₄] ? 0.5 mol dm⁻³ is first order in and a fractional order in [oxalic acid], the reactive entities being AuCl₃(OH)⁻ and ions. The pseudo first-order rate, k_obs, with respect to [Au(III)], is retarded by increasing [H⁺] and [Cl⁻]. The retardation by H⁺ ion is caused by the dissociation equilibrium . A mechanism in which a substitution complex, is formed from AuCl₃(OH)⁻ and ions prior to its rate limiting disproportionation into products is suggested. The rate limiting constant, k, has been evaluated and its activation parameters are reported. The equilibrium constant K₁ for the formation of the substitution complex and its thermodynamic parameters are also reported.     

Mechanistic studies on monodentate-ligand substitution of five-coordinate trigonal-bipyramidal platinum (II) complexes with tris[2-(diphenylphosphino)ethyl]phosphine

Reaction of the five-coordinate trigonal-bipyramidal platinum(II) complex, [Pt(pt)(pp₃)](BF₄) (pt = 1-propanethiolate, pp₃ = tris[2-(diphenylphosphino)ethyl]phosphine), with I⁻ in chloroform gave the five-coordinate square-pyramidal complex with a dissociated terminal phosphino group and an apically coordinated iodide ion in equilibrium. The thermodynamic parameters for the equilibrium between the trigonal-bipyramidal and square-pyramidal geometries, [Pt(pt)(pp₃)]⁺ + I⁻ ? [PtI(pt) (pp₃)], and the kinetic parameters for the chemical exchange were obtained as follows: , ΔH⁰ = − 10 ± 2.4 kJ mol⁻¹, ΔS⁰ = − 36 ± 10 J K⁻¹ mol⁻¹, , ΔH^‡ = 34 ± 4.7 kJ mol⁻¹, ΔS^‡ = − 50 ± 21 J K⁻¹ mol⁻¹. The square-planar trinuclear platinum(II) complex was formed by bridging reaction of one of the terminal phosphino groups of trigonal-bipyramidal [PtCl(pp₃)]Cl with trans-[PtCl₂(NCC₆H₅)₂] in chloroform. From these facts, ligand substitution reactions of [PtX(pp₃)]⁺ (X = monodentate anion) are expected to proceed via an intermediate with a dissociated phosphino group. The rate constants for the chloro-ligand substitution reactions of [PtCl(pp₃)]⁺ with Br⁻ and I⁻ in chloroform approached the respective limiting values as concentrations of the entering halide ions are increased. These kinetic results confirmed the preassociation mechanism in which the square pyramidal intermediate with a dissociated phosphino group and an apically coordinated halide ion is present in the rapid pre-equilibrium.     

Endothelium negatively modulates the vascular relaxation induced by nitric oxide donor, due to uncoupling NO synthase

Nitrosyl ruthenium complexes have been characterized as nitric oxide (NO) donors that induce relaxation in the denuded rat aorta. There are some differences in their vascular relaxation mechanisms compared with sodium nitroprusside. This study investigates whether the endothelium could interfere with the [Ru(terpy)(bdq)NO]³⁺-TERPY-induced vascular relaxation, by analyzing the maximal relaxation (Emax) and potency (pD₂) of TERPY. Vascular reactivity experiments showed that the endothelium negatively modulates (pD₂: 6.17 ± 0.07) the TERPY relaxation in intact rat aortic rings compared with the denuded rat aorta (pD₂: 6.65 ± 0.07). This effect is abolished by a non-selective NO-synthase (NOS) inhibitor L-NAME (pD₂: 6.46 ± 0.10), by the superoxide anion () scavenger TIRON (pD₂: 6.49 ± 0.08), and by an NOS cofactor BH₄ (pD₂: 6.80 ± 0.10). The selective dye for (DHE) shows that TERPY enhances concentration in isolated endothelial cells (intensity of fluorescence (IF):11258.00 ± 317.75) compared with the basal concentration (IF: 7760.67 ± 381.50), and this enhancement is blocked by L-NAME (IF: 8892.33 ± 1074.41). Similar results were observed in vascular smooth muscle cells (concentration of superoxide after TERPY: 2.63 ± 0.17% and after TERPY + L-NAME: −4.63 ± 0.14%). Considering that TERPY could induce uncoupling NOS, thus producing , we have also investigated the involvement of prostanoids in the negative modulation of the endothelium. The non-selective cyclooxygenase (COX) inhibitor indomethacin and the selective tromboxane (TXA₂) receptor antagonist SQ29548 reduce the effect of the endothelium on TERPY relaxation (pD₂ INDO: 6.80 ± 0.17 and SQ29548: 6.85 ± 0.15, respectively). However, a selective prostaglandin F_2α receptor antagonist (AH6809) does not change the endothelium effect. Moreover, TERPY enhances the concentration of TXA₂ stable metabolite (TXB₂), but this effect is blocked by L-NAME and TIRON. The present findings indicate that TERPY induces uncoupling of eNOS, enhancing concentration. This enhancement in concentration induces COX activation, producing TXA₂, which negatively modulates the rat aorta relaxation induced by the NO donor TERPY.     

Applicability of new spin trap agent, 2-diphenylphosphinoyl-2-methyl-3,4-dihydro-2H-pyrrole N-oxide, in biological system

Electron spin resonance using spin-trapping is a useful technique for detecting direct reactive oxygen species, such as superoxide (). However, the widely used spin trap 2,2-dimethyl-3,4-dihydro-2H-pyrrole N-oxide (DMPO) has several fundamental limitations in terms of half-life and stability. Recently, the new spin trap 2-diphenylphosphinoyl-2-methyl-3,4-dihydro-2H-pyrrole N-oxide (DPhPMPO) was developed by us. We evaluated the biological applicability of DPhPMPO to analyze in both cell-free and cellular systems. DPhPMPO had a larger rate constant for and formed more stable spin adducts for than DMPO in the xanthine/xanthine oxidase (X/XO) system. In the phorbol myristate acetate-activated neutrophil system, the detection potential of DPhPMPO for was significantly higher than that of DMPO (k_DMPO = 13.95 M⁻¹ s⁻¹, k_DPhPMPO = 42.4 M⁻¹ s⁻¹). These results indicated that DPhPMPO is a potentially good candidate for trapping in a biological system.     

Kinetic studies on the reversible ring opening-closure reaction of the triazine ligand and structural properties of palladium(II) complexes

A Pd(II) complex containing didentate triazine ligand L¹ (2-(2-methylphenyl)-3-(2-pyridyl)-2,3-dihydronaphtho[2,1-e][1,2,4]triazine) [PdCl₂(L¹)] (1) was prepared, and the structure was determined by X-ray crystallography. The absorption spectrum of complex 1 in dichloromethane changed gradually with isosbestic points when methanol was added to the solution, and [PdCl(L²)] (2) (L² = N-[methoxy(2-pyridyl)methyl]-1-(2-methylphenylazo)-2-naphthylamide) was obtained from the resulting solution after the reaction was completed. Addition of hydrogen chloride to the solution of complex 2 led to the recovery of complex 1. Thus, a reversible ring opening and closure reaction of the triazine ligand was observed. The progress of the ring opening reaction was monitored by observing the absorbance changes at several wavelengths of the visible spectra as a function of the concentration of methanol. The absorbance plots were fitted successfully with a mechanism that includes the consumption of two methanol molecules and the release of HCl, whose concentration is equivalent to that of the produced Pd complex . In dichloromethane with a low dielectric constant, the polar HCl molecule will be stabilized by forming an adduct with methanol. The equilibrium constant was determined as at T = 25.0 °C. The kinetics of the reaction of [PdCl₂(L¹)] with methanol was investigated by monitoring the absorbance change of the reacting solution with time. We obtained rate constant values of k₁ = (2.40 ± 0.07) × 10⁻³ s⁻¹ and k₂ = (5.8 ± 0.1) × 10⁻³ M⁻¹ s⁻¹ at T = 25.0 °C on the observed pseudo-first-order rate constant of the forward reaction, k_f = k₁ + k₂ [CH₃OH].     

Electron transfer. Part 166 . Reactions of Ti(II)(aq) with oxidants having strongly positive potentials

Titanium(II) solutions, prepared by dissolving titanium metal in triflic acid+HF, react readily with chelated complexes of Ag(III), nickel(IV) and copper(III). Reactions with excess Ti(II) yield Ti(III) and are strongly catalyzed by added Ti(IV), but stoichiometry is unaffected. Rapid reactions of Ti(II) with nonchelated oxidants, VO²⁺, and do not exhibit catalysis by Ti(IV). Reductions by Ti(III) are unaffected by Ti(IV). The Ag(III)-Ti(II) reaction, as catalyzed by Ti(IV), is subject to kinetic saturation with an association quotient 4 × 10² M⁻¹ for the Ti(IV)-activated species. It is proposed that the catalyzed reductions of the Ag(III) and Ni(IV) oxidants are initiated by 1e⁻ steps, but that the initially formed cation pairs undergo geminate follow-up reactions to give the observed stable products.     

Reaction of hydrogen peroxide with hexaaquacobalt(III) perchlorate

The reaction of with H₂O₂ in 1.0 M HClO₄/LiClO₄ was found to be first-order in both reactants and the [H⁺] dependence of the second-order rate constant is given by k_2obs = b/[H⁺], b at 25 °C is 26.4 ± 0.5 s⁻¹. The rate law shows a simple inverse dependence on [H⁺] that is consistent with a rapidly maintained equilibrium between and its hydrolyzed form Co(H₂O)₅(OH)²⁺, followed by the rate controlling step, i.e. oxidation of H₂O₂ by Co(H₂O)₅(OH)²⁺.