Showing papers on "Aquation published in 2000"

Slowing of cisplatin aquation in the presence of DNA but not in the presence of phosphate: improved understanding of sequence selectivity and the roles of monoaquated and diaquated species in the binding of cisplatin to DNA.

Abstract: 1H-15N HSQC NMR spectroscopy is used to study the aquation reactions of cisplatin in 9 mM NaClO4 and 9 mM phosphate (pH 6) solutions at 298 K For the first time in a single reaction and, therefore, under a single set of reaction conditions, the amounts of all species formed are followed and the rates of aquation, diaquation, and related anation processes are determined in both media Binding of phosphate to aquated Pt species is observed, but the initial rate of aquation is not affected by the presence of 9 mM phosphate The reaction between cisplatin and the 14-base-pair self-complementary oligonucleotide 5'-d(AATTGGTACCAATT)-3', having a GpG intrastrand binding site, is investigated Various kinetic models for this reaction are evaluated and the most appropriate found to be that with a reversible aquation step and a single binding site for the self-complementary duplex The rate constant for aquation is (162 +/- 002) x 10(-5) s-1, with the anation rate constant fixed at 46 x 10(-3) M-1 s-1, the value obtained from the aquation studies The rate constants for monofunctional binding of cis-[PtCl(15NH3)2-(OH2)]+ to the sequence were 048 +/- 019 and 016 +/- 006 M-1 s-1 for the 3'- and 5'-guanine bases, respectively Closure rate constants for the monofunctional adducts are (255 +/- 007) x 10(-5) and (0171 +/- 0011) x 10(-5) s-1, for the 3'- and 5'-guanines, respectively The presence of DNA slows the aquation of cisplatin by 30-40% compared to that observed in 9 mM NaClO4 or 9 mM phosphate, and there is some evidence that the degree of slowing is sequence dependent The possibility that cis-[Pt(OH)(NH3)2(OH2)]+ contributes to the binding of cisplatin to DNA is investigated, and it is found that about 1% followed this route, the majority of the binding occurring via the monoaquated species cis-[PtCl(NH3)2(OH2)]+ Comparison of the rates of disappearance of cisplatin in reactions at single defined GpG, ApG, GpA, GpTpG and 1,2-interstrand GG binding sites shows that the adduct profile is determined at the level of monofunctional adduct formation

Iridium-based electrocatalytic systems for the determination of insulin.

Abstract: Two electrochemical catalytic systems for the determination of insulin were developed. The homogeneous system was based on the oxidation of insulin by chloro complexes of iridium(IV). Kinetic studies revealed that the aquation of iridium complexes activated them toward the oxidation of insulin in acidic solutions; e.g., the rate constant was equal to 25, 900, and 8400 L mol-1 s-1 for the oxidation of insulin by the IrCl62-, Ir(H2O)Cl5-, and Ir(H2O)2Cl4 complexes, respectively. The inertness of the iridium complexes argued for the outer-sphere mechanism of the homogeneous oxidation reaction. Electroplating of aquated iridium complexes on the glassy carbon electrode resulted in the formation of the iridium oxide (IrOx) surface film, which was used in the heterogeneous detection system for insulin. The catalytic activity of the IrOx film toward insulin oxidation was ascribed to a combination of electron-transfer mediation and oxygen transfer which was related to the acid/base chemistry of the film. The IrOx ...

Equilibrium and kinetic studies of the aquation of the dinuclear platinum complex [[trans-PtCl(NH3)2]2(mu-NH2(CH2)6NH2)]2+: pKa determinations of aqua ligands via [1H,15N] NMR spectroscopy.

Abstract: By the use of [1H,15N] heteronuclear single quantum coherence (HSQC) 2D NMR spectroscopy and electrochemical methods we have determined the hydrolysis profile of the bifunctional dinuclear platinum complex [[trans-PtCl(15NH3)2]2(mu-15NH2(CH2)(6)15NH2)]2+ (1,1/t,t (n = 6), 15N-1), the prototype of a novel class of potential antitumor complexes Reported are estimates for the rate and equilibrium constants for the first and second aquation steps, together with the acid dissociation constant (pKa1 approximately pKa2 approximately pKa3) The equilibrium constants determined by NMR at 25 and 37 degrees C (I = 01 M) were similar, pK1 approximately pK2 = 39 +/- 02, and from a chloride release experiment at 37 degrees C the values were found to be pK1 = 411 +/- 005 and pK2 = 42 +/- 05 The forward and reverse rate constants for aquation determined from this chloride release experiment were k1 = (85 +/- 03) x 10(-5) s-1 and k-1 = 091 +/- 006 M-1 s-1, where the model assumed that all the liberated chloride came from 1 When the second aquation step was also taken into account, the rate constants were k1 = (79 +/- 02) x 10(-5) s-1, k-1 = 118 +/- 006 M-1 s-1, k2 = (106 +/- 30) x 10(-4) s-1, k-2 = 15 +/- 06 M-1 s-1 The rate constants compare favorably with other complexes with the [PtCl(am(m)ine)3]+ moiety and indicate that the equilibrium of all these species favors the chloro form A pKa value of 562 was determined for the diaquated species [[trans-Pt(15NH3)2(H2O)]2(mu-15NH2(CH2)(6)15NH2)]4+ (3) using [1H,15N] HSQC NMR spectroscopy The speciation profile of 1 and its hydrolysis products under physiological conditions is explored

Kinetics and mechanism of formation, acid catalysed aquation, reversible anation and photochemical reaction of trans-(aqua)(sulfito-S)[N,N′-ethylenebis(salicylidiniminato)]cobaltate(III) in aqueous media

Abstract: The reaction of trans-[Co(salen)(OH2)OH] with SO2 yields trans-[Co(salen)(OH2)(SO3-S)]− (S-bonded isomer) for which the rate and activation parameters at 25 °C (I = 0.3 mol dm−3) are kSO2 = (5.9 ± 0.1) × 1010 dm3 mol−1 s−1, ΔH‡ = 66 ± 4 kJ mol−1 and ΔS‡ = 183 ± 14 J K−1 mol−1. One possibility for the SIV substitution is that Co–S bond formation is concerted with Co–O bond breaking. An alternative mechanism, involving a fast equilibrium between SO2 and trans-[Co(salen)(OH2)OH] forming an O-bonded sulfito species which then undergoes sulfite ligand linkage isomerisation, is also possible. An estimated value of the isomerisation rate constant for the trans-[Co(salen)(OH2)(OSO2H)] at 25 °C is ca. 106 s−1. The trans-[Co(salen)(OH2)(SO3-S)]− (pK = 10.1 ± 0.1 at 25 °C, I = 0.3 mol dm−3) undergoes acid catalysed aquation to yield the parent diaqua complex and SIV with kH = 29.5 ± 1.1 dm3 mol−1 s−1, ΔH‡ = 72 ± 3 kJ mol−1, ΔS‡ = 24 ± 9 J K−1 mol−1 at 25 °C (I = 0.3 mol dm−3). Steady state photolysis (254 nm) of trans-[Co(salen)(OH2)(SO3-S)]− resulted in the reduction of CoIII. The redox rate constant and ϕ(Co2+) decreased with increasing pH. Attempts to detect an O-bonded sulfito complex as a transient in the conventional flash photolysis of this aqua-sulfito complex proved unsuccessful. The aqua ligand replacement reactions of trans-[Co(salen)(OH2/OH)(OH2)]+/0 with imidazole and that of the corresponding aqua-sulfito complex with N3−, NCS−, imidazole, and SIV in a large excess of the entering ligands have been studied at 25 °C. A comparison of the rate constants with the analogous data for trans-[Co(AA)2(OH2)(SO3-S)]+ (AA = 1,2-diaminoethane; 1,3-diaminopropane) clearly shows that the kinetic trans-effect of the S-bonded sulfite is substantially attenuated in trans-[Co(salen)(OH2)(SO3-S)]−.

Mechanism of the S-->N isomerization and aquation of the thiocyanato pentaammine cobalt(III) ion.

Abstract: All of the stationary points on the potential energy surface of the S → N isomerization and aquation of the Co(NH3)5SCN2+ ion have been investigated with ab initio quantum chemical methods. Also the corresponding anations of the Co(NH3)5OH23+ ion by the N and S ends of SCN- and the substitution of thiocyanate via the D mechanism have been studied. All calculations have been performed by taking into account hydration. The most favorable reaction of Co(NH3)5SCN2+ is the isomerization. It is concerted, follows the I or Id mechanism, depending on the applied criteria, and proceeds via a T-shaped transition state. The aquations of Co(NH3)5SCN2+ and Co(NH3)5NCS2+ and the corresponding inverse reactions, the anations, all proceed via the Id mechanism. The activation energies, calculated for the isomerization and aquation, agree with experiment, and so does the difference of the activation energies for the anations by the two donors of SCN-. This energy difference reflects the disparate nucleophilicities of the N...

Tris-[3-hydroxy-1,2-dimethyl-pyridin-4(1H)-onato]cobalt(III)

Abstract: The preparation and properties of tris-[3-hydroxy-1,2-dimethyl-pyridin-4(1H)-onato]cobalt(III) are described. The structure of the fac isomer has been established by X-ray diffraction techniques. Solvation characteristics of this complex have been assessed through solubility measurements in methanol–water mixtures. The complex is very inert in neutral solution but undergoes acid-catalysed aquation at lower pHs.

Kinetics and mechanism for reversible chloride transfer between mercury(II) and square-planar platinum(II) chloro ammine, aqua, and sulfoxide complexes. Stabilities, spectra, and reactivities of transient metal-metal bonded platinum-mercury adducts.

Abstract: The Hg2+aq- and HgCl+aq-assisted aquations of [PtCl4]2- (1), [PtCl3(H2O)]- (2), cis-[PtCl2(H2O)2] (3), trans-[PtCl2(H2O)2] (4), [PtCl(H2O)3]+ (5), [PtCl3Me2SO]- (6), trans-[PtCl2(H2O)Me2SO] (7), cis-[PtCl(H2O)2Me2SO]+ (8), trans-[PtCl(H2O)2M32SO]+ (9), trans-[PtCl2(NH3)2] (10), and cis-[PtCl2(NH3)2] (11) have been studied at 25.0 degrees C in a 1.00 M HClO4 medium buffered with chloride, using stopped-flow and conventional spectrophotometry. Saturation kinetics and instantaneous, large UV/vis spectral changes on mixing solutions of platinum complex and mercury are ascribed to formation of transient adducts between Hg2+ and several of the platinum complexes. Depending on the limiting rate constants, these adducts are observed for a few milliseconds to a few minutes. Thermodynamic and kinetics data together with the UV/vis spectral changes and DFT calculations indicate that their structures are characterized by axial coordination of Hg to Pt with remarkably short metal-metal bonds. Stability constants for the Hg2+ adducts with complexes 1-6, 10, and 11 are (2.1 +/- 0.4) x 10(4), (8 +/- 1) x 10(2), 94 +/- 6, 13 +/- 2, 5 +/- 2, 60 +/- 6, 387 +/- 2, and 190 +/- 3 M-1, respectively, whereas adduct formation with the sulfoxide complexes 7-9 is too weak to be observed. For analogous platinum(II) complexes, the stabilities of the Pt-Hg adducts increase in the order sulfoxide << aqua < ammine complex, reflecting a sensitivity to the pi-acid strength of the Pt ligands. Rate constants for chloride transfer from HgCl+ and HgCl2 to complexes 1-11 have been determined. Second-order rate constants for activation by Hg2+ are practically the same as those for activation by HgCl+ for each of the platinum complexes studied, yet resolved contributions for Hg2+ and HgCl+ reveal that the latter does not form dinuclear adducts of any significant stability. The overall experimental evidence is consistent with a mechanism in which the accumulated Pt(II)-Hg2+ adducts are not reactive intermediates along the reaction coordinate. The aquation process occurs via weaker Pt-Cl-Hg or Pt-Cl-HgCl bridged complexes.

Product speciation and aquation after the reaction of 51Cr(VI) with concentrated HCl or H2SO4

Abstract: The present study was undertaken to investigate the reaction of Cr(VI) in concentrated hydrochloric acid or concentrated sulfuric acid, as well as to evaluate the products formed when complexing anions are present during the reaction. The results show that trace level Cr(VI) is rapidly reduced to Cr(III) in both 37% HCI and 98% H2SO4 in the absence of conventional reducing agents and that the initial products are, respectively, CrCl3(H2O)3 and Cr(H2O)2(SO4)2-}, which undergo slow aquation reactions at pH 1, giving Cr(H2O)63+.

Kinetics and mechanism of the base hydrolysis of cis-eq-[Cr(NCS)(S-pdtra)]−, trans-eq-[Cr(NCS)(edtrp)]− and trans-eq-[Cr(NCS)(R-pdtrp)]− complexes. Strong rate enhancement for NCS− ligand release with an increase of [OH−] only for the trans-equatorial isomers

Abstract: The [Cr(NCS)(edtrp)]−, [Cr(NCS)(R-pdtrp)]− and [Cr(NCS)(S-pdtra)]− complexes, that are derivatives of the trans-equatorial isomers of [Cr(edtrp)(H2O)]° and [Cr(R-pdtrp)(H2O)]° and the cis-equatorial isomer of [Cr(S-pdtra)-(H2O)]° (edtrp = ethylenediamine-N,N,N′-tripropionate, R-pdtrp = R-propane-1,2-diamine-N,N,N′-tripropionate, S-pdtra = S-propane-1,2-diamine-N,N,N′-triacetate) undergo aquation in alkaline media with a strong dependence of the rate on [OH−] for the trans-equatorial isomers and a very weak dependence for the cis-equatorial isomer. The thiocyanate ligand release follows a stereoretentive course for all reactants. Based on kinetic data the reaction mechanism has been discussed. Rate differences between the isomers are interpreted in terms of an interchange via a conjugate base (I c.b.) mechanism, assuming an equilibrium between the cis-equatorial-CrIII-S-pdtra complexes with penta- and tetradentate coordination of the edta-like ligand.

Uranyl-catalyzed chemiluminescent reaction of U4+ oxidation by dioxygen in aqueous HClO4 solution

Abstract: Chemiluminescence (CL) accompanying the reaction of U4+ with O2 in 0.0004–0.1M HClO4 was studied. It was found that the electron-excited uranyl ion (UO22+)* is the CL emitter. The fact that the reaction rate and the CL yield increase as the solution acidity decreases was explained by different reactivities of the Uaq4+ aquation and the products of its stepwise hydrolysis, UOH3+ and U(OH)22+, toward O2. Based on the results of analysis of the chain-radical mechanism of the reaction between U4+ and O2, it was concluded that transfer of an electron from the UO2+ ion to the oxidizing agent (a ·OH radical) is the most plausible elementary step of the reaction of (UO22+)* formation. It was found that the reaction rate, as well as the CL yield, increase substantially in the presence of uranyl ion. Catalytic action of UO22+ was explained by the formation of a UO22+·UO2+ complex, which reduces the rate of the UO2+ disproportionation reaction (UO2+ is an intermediate of the reaction and is involved in chain propagation), and by regeneration of the active center, UO2+, in the reaction of UO22+ with U4+.

Aquation of the Chloro Pentaammine Complexes of Cobalt(III) and Chromium(III): Do the Almost Equal Activation Parameters Arise from a Common Mechanism?

Abstract: The activation parameters for the aquation of Co(NH3)5Cl2+ and Cr(NH3)5Cl2+ are almost equal, and the strongly negative volumes of activation, ΔV⧧ (−9.9 and −10.6 cm3 mol-1 for Co and Cr, respectively), might at the first glance suggest a common associative mechanism for both complexes. Computations of the corresponding reaction coordinates indicate that, in agreement with Swaddle and co-workers' studies and as expected, a dissociative interchange mechanism (Id) operates for cobalt(III), but an associative interchange one (Ia) for chromium(III). For these two asymmetric reactions, the bond lengths changes δCo and δCr that are involved in the formation of the transition state and defined with respect to the fully concerted I pathway allow the attribution of the reaction mechanism. The signs of δCo and δCr, being positive and negative, suggest a d and a activation for the above aquations.

Aquation and Base Hydrolysis of trans-Tetraammine(methylamine)sulfatocobalt(III) Complex Ion

Abstract: Abstract The kinetics of aquation and base hydrolysis reactions of fmns-[Co(NH3)4(NH2CH3)(OSO3)]+ have been studied. In acid solution the aquation rate, Raq, follows the equation Raq/[complex] = ks + kc[H+], at constant ionic strenght μ = 1.0Μ. The activation parameters are ΔΗ* = 88.8 kj mol-1, ΔS* = - 50.5 JK-1 mol-1 , ΔΗ* = 96.7 kj mol-1 and ΔS* =-23 JK-1 mol-1. The rate constants at 25°C are ks = 4.15 χ 10-6s-1 and kc = 4.52M -1s-1 . The rate of base hydrolysis, ROH, follows the equation ROH/complex] = k0H [OH-]. The activation parameters are ΔΗ*OH =74.9kJmol-1 and ΔS*OH = 2JK-1 mol-1 and the rate constant is kOH = 0.58M 1 s-1 at 25°C and μ, = 0.15 Μ. The stereochemistry of the hidroxo product has been determined (cis-[Co(NH3)4(NH2CH3)- (OH)]2+ =9%). The results are discussed in the light of the reaction mechanisms proposed so far.