Kinetic studies of the reduction of ferrioxamine B (Fe(Hdesf)+) by Cr(H2O)62+, V(H2O)62+, and dithionite have been performed. For Cr(H2O)62+ and V(H2O)62+, the rate is ?d[Fe(Hdesf)+]/dt = k[Fe(Hdesf)+][M2+]. For Cr(H2O)62+, k = 1.19 × 104 M?1 sec?1 at 25°C and μ = 0.4 M, and k is independent of pH from 2.6 to 3.5. For V(H2O)62+, k = 6.30 × 102 M?1 sec?1 at 25°C, μ = 1.0 M, and pH = 2.2. The rate is nearly independent of pH from 2.2 to 4.0. For Cr(H2O)62+ and V(H2O)62+, the activation parameters are ΔH≠ = 8.2 kcal mol?1, ΔS≠ ?12 eu and ΔH≠ = 1.7 kcal mol?1, ΔS≠ = ?40 eu (at pH 2.2) respectively. Reduction by Cr(H2O)62+ is inner-sphere, while reduction by V(H2O)62+ is outer-sphere. Reduction by dithionite follows the rate law ?d[Fe(Hdesf)+]/dt =kK[Fe(Hdesf)+][S2O42?] where K is the equilibrium constant for dissociation of S2O42? into SO2? radicals. The value of k at 25°C and μ = 0.5 is 2.7 × 103 M?1 sec?1 at pH 5.8, 3.5 × 103 M?1 sec?1 at pH 6.8, and 4.6 × 103 M?1 sec?1 at pH 7.8, and ΔH≠ = 6.8 kcal mol?1 and ΔS≠ = ?19 eu at pH 7.8. 相似文献
The preparation of the planar yellow [Ni([8]aneN2)2](ClO4)2 is described. The complex dissociates in basic solution, with rate = kOH[NiL][OH?] (L = 1,5-diazacyclo-octane). At 25 °C, kOH = 4.5 x 10?2 M?1 s?1 and the corresponding activation parameters are ΔH≠ = 69.2 kJ mol?1 and ΔS298≠ = ?38.6 J K?1 mol?1. Acid catalysed dissociation in quite slow even in strongly acidic solutions. The kinetic data in this case can be fitted to the expression Kobs = ko + KH[H+], where ko relates to a solvolytic pathway and kH to the acid catalysed pathway. At 60 °C, Ko = 2 x 10?5 s?1 and kH is 2 x 10?5 M?1 s?1. Possible mechanisms for these reactions are considered.The Ni(II)/Ni(III) redox couple for NiLn+ is irreversible on Pt using MeCN as solvent. 相似文献
A kinetic study of the oxidation of (hydroxyethyl)ferrocene (HEF) by [2-pyridylmethylbis(2-ethyl-thioethyl)ainine]copper(II) (Cu(pmas)2+) is reported, with the objective of documenting the influence of the two thioether sulfur ligands on the electron transfer rate. Both reactants exhibit a first-order dependence at pH 6, I = 0.1 M(NaNO3); k(25°C) = 1.3 × 104M−1sec−1, ΔH3 = 10.1 kcal/mole, ΔS3 = −6 eu. The apparent Cu(pmas)2+/+ self-exchange electron transfer rate constant calculated from this reaction on the basis of relative Marcus theory (4.7 × 101M−1 sec−1) agrees well with previous findings on ferrocytochrome c, Fe(CN)64−, and Ru(NH3)5py2+ oxidations. Spectrophotometric titrations of Cu(pmas)2+ and Cu(tmpa)2+ (tmpa = tris(2-pyridylmethyl)amine) with azide ion showed that both Cu(pmas)N3)+ (Kf1 = 3.1 × 103M−1) and Cu(pmas)(N3)2 (Kf2 = 3.5 × 101M−1) but Cu(tmpa)(N3)+ (Kf = 6.6 × 102M−1) are formed up to 0.15 M N3− (25°C, pH 6, I = 0.2 M), suggesting that a thioether sulfur atom is displaced in the uptake of a second N3− ion by Cu(pmas)(N3)+. The effect of thioether sulfur displacement by azide ion on the HEF-Cu(pmas)2+ reaction rate may be understood entirely through the tendency of N3− to shift the position of the redox equilibrium towards the reactant side, without invoking any special role for the sulfur ligand in promoting electron transfer reactivity. 相似文献
The extraction equilibrium of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 complex was carried out in the crown ether1,2-dichloroethaneHCl aqueous solution system at different temperatures. The extraction complex has the overall composition (L)2·(H3O+·χH2O)2·UO2Cl42− (L = dicyclohexano-24-crown-8). The values of the extraction equilibrium constants (Kex) increase steadily with a decrease in temperature: 13.5 (298 K), 7.96 (301 K), 4.20 (303 K) and 2.07 (305 K). A plot of log Kex against 1/T shows a straight line. The value of the enthalpy change, ΔH°, was calculated from the slope and equals −212 kJ mol−1. The value of the entropy change, ΔS°, was calculated from ΔH° and Kex and equals −690 J K−1 mol−1, whereas ΔG° = −6.45 kJ mol−1. Comparing these thermodynamic parameters with those of the dicyclohexano-18-crown-6 isomer A [1] (ΔS° = −314 J K−1 mol−1, ΔH° = −101 kJ mol−1 and ΔG° = −8.37 kJ mol−1), it can be seen that ΔH° and ΔS° are more negative for the former than for the latter, and both are enthalpy-stabilized complexes. The molecular structure of the complex has the feature that there are two H5O2+ ions in it, in contrast to the H3O+ ions in the dicyclohexano-18-crown-6 isomer A complex [1]. Each of the H5O2+ ions is held in the crown ether cavity by four hydrogen bonds. The H5O2+ ion has a central bond. The uranium atom forms UO2Cl42− as a counterion away from the crown ether. The formation of this complex is in good agreement with more negative entropy change and less negative free energy change, as mentioned above. 相似文献
The kinetics of malonate replacement in bis- (malonato)oxovanadate(IV), [VO(mal)2H2O]2−(hereafter water molecule will be omitted), by oxalate has been studied by the stopped-flow method. The reaction was found to consist of two consecutive steps (k1 and k2: first-order rate constants) passing through a mixed ligand complex, [VO(mal)(ox)]2−. The rates for each step depended linearly on the concentrations of free oxalate species, Hox− and ox2−. The second-order rate constants for the replacement by ox2− were much larger in the k1 step than in the k2 step and the activation parameters were determined as follows: ΔH≠= 43.5 ± 5.6 kJ mol−1, ΔS±-53 ± 19 J K−1 mol−1 and ΔH≠= 43.6 ± 0.5 kJ mol−1, δS≠ = -62 ± 2 J K−l mol−1 for the k1 and k2 steps, respectively. The volume of activation was determined to be -0.65 ± 0.75 cm3 mol−1 at 20.2 °C by the high-pressure stopped-flow method for the apparent rate constants. 相似文献
The (NH3)5CoOC(NH2)23+ ion is consumed in water according to the rate law k(obs.) = k1 + k2[OH−], where k1 = 4.0 × 10−5 s−1 and k2 = 14.2 M−1 s−1 (0–0.1 M [OH−];μ = 1.1 M, NaClO4, 25 °C). A hitherto unrecognized intramolecular O- to N- linkage isomerization reaction has been detected. In strongly acid solution only aquation to (NH3)5CoOH23+ is observed, but in 0.1–1.0 M [OH−], 7% of the directly formed products is the urea-N complex (NH3)5CoNHCONH22+ which has been isolated. In the neutral pH region a much greater proportion (25%) of the products is the urea-N species. These results are interpreted in terms of an urea-O to urea-N linkage isomerization reaction competing with hydrolysis for both spontaneous (k1) and base-catalyzed (k2) pathways; the rearrangement is not observed in strongly acidic solution (pH ⩽ 1) because the protonated N-bonded isomer (pK′a ≈ 3) is unstable with respect to the O-bonded form. The appearance of the isomerization pathway as the pH is raised in the 0–6 region is commensurate with a rate increase which cannot be attributed to a contribution from the base catalysis term k2[OH−]. It is argued that this observation establishes, for the spontaneous pathway, that hydrolysis and linkage isomerization are separate reaction pathways — there is no common intermediate. The product distribution and rate data lead to the complete rate law, k(obs.) = k1 + k2[OH−] = (ks + kON) + (kOH + k′ON) [OH−] for the reactions of the O-bonded isomers, where ks, kOH are the specific rates for hydrolysis, and kON, k′ON are the specific rates for O- to N-linkage isomerization, by spontaneous and base-catalyzed pathways respectively; kON = 1.3 × 10−5 s−1 and k′ON = 1.1 M−1 s−1 (μ = 1.0 M, NaClO4, 25 °C). The O- to N- linkage isomerization has been observed also for complexes of N-methylurea, N,N-dimethylurea and N-phenylurea, but not for the N,N′-dimethylurea species. There is an approximately statistical relationship among the data for −NH2 capture (versus H2O), while −NHR and −NR2 do not compete with water as nucleophiles for Co(III) in either the spontaneous or base-catalyzed hydrolysis processes. For each urea-O complex, O- to N-isomerization is a more significant parallel reaction in the spontaneous as opposed to the base-catalyzed pathway. This is interpreted as being indicative of more associative character in the spontaneous route to products, a conclusion supported by other evidence. Some activation parameter data have been recorded and the effect of the N-substitution on the rates of solvolysis (H2O, Me2SO) is discussed. The urea-N complexes have been isolated as their deprotonated forms, [(NH3)5CoNHCONRR′](ClO4)2·xH2O (R,R′ = H, CH3). They are kinetically inert in neutral to basic solution but in acid they protonate (H2O, pK′a 2–3; μ = 1.0 M, 25 °C) and then isomerize rapidly back to their O-bonded forms. Some solvolysis accompanies this N- to O-rearrangement in H2O and Me2SO. Specific rates and activation parameters are reported. The kinetic data follow a rate law of the form kNO(obs.) = (k + kNO)[H+]/(K′a + [H+]) and the active species in the reaction is the protonated form; k, kNO are the specific rates for hydrolysis and isomerization, respectively. Proton NMR data establish that the site of protonation (in Me2SO) is the cobalt-bound nitrogen atom. For the unsubstituted urea species (NH3)5CoNH2CONH23+, diastereotopic exo-NH2 protons arising from restricted rotation about the CN bond are observed. The relevance to the mechanism of the linkage isomerization process is considered. 13C and 1H NMR and electronic absorption spectral data are presented, and distinctions between linkage isomers and the solution structures (electronic and conformational) are discussed. The urea-N/urea-O complex equilibrium is governed by the relation K′NO(obs.) = K′NO[H+]/[H+](K′a), where K′NO is the equilibrium constant = [(NH35Co(urea-O)3+]/[(NH3)5Co(urea-N)3+]. Values for K′NO(=kNO/kON = 260 and pK′a ≈ 3 for the NH2CONH2 system are consistent with the stability of the N-isomer in feebly acidic to basic solution (e.g. pH 6, K′NO(obs.) = 2.6 × 10−2) and instability in acid solution (e.g. pH 1, K′NO(obs.) = 240). The equilibrium data for this and other urea complexes of (NH3)5Co(III) are contrasted with the result for the analogous Rh(III)NH2CONH2 system K′NO ≈ 1). 相似文献
The bis(N-alkylsalicylaldiminato)nickel(II) complexes Ni(R-sal)2 with R = CH(CH2OH)CH(OH)Ph (I), R = CH(CH3)CH(OH)Ph (II) and R = CH2CH2Ph (III; Ph = phenyl) were prepared and characterized. In the solid state I and II are paramagnetic (μ = 3.2 and 3.3 BM at 20 °C, respectively), whereas III is diamagnetic. It follows from the UV-Vis spectra that in acetone solution I is six-coordinate octahedral and III is four-coordinate planar, the spectrum of II showing characteristics of both modes of coordination. Vis spectrophotometry and stopped-flow spectrophotometry were applied to study the kinetics of ligand substitution in I–III by H2salen (= N,N′-disalicylidene-ethylenediamine) in the solvent acetone at different temperatures. The kinetics follow a second-order rate law, rate = k[H2-salen] [complex]. At 20 °C the sequence of rate constants is k(III):k(II):k(I) = 11 850:40.6:1. The activation parameters are ΔH≠(I) = 112, ΔH≠(II) = 40.7, ΔH≠(III) = 35.7 kJ mol−1 and ΔS≠(I) = 92, ΔS≠(II) = −103, ΔS≠(III) = −89 J K−1 mol−1. The enormous difference in rate between complexes I, II and III, which is less pronounced in methanol, is attributed to the existence of a fast equilibrium planar ⇌ octahedral, which is established in the case of I and II by intramolecular octahedral coordination through the hydroxyl groups present in the organic group R. An A-mechanism is suggested to control the substitution in the sense that the entering ligand attacks the four-coordinate planar complex, the octahedral complex being kinetically inert. 相似文献
The complexes CodptX3 and [Codpt(H2O)X2]ClO4 (X = Cl, Br; dpt = dipropylenetriamine = NH(CH2CH2CH2NH2)2) have been prepared and characterized. Rate constants (s−1) for aqueous solution at 25 °C and μ = 0.5 M (NaClO4), for the acid-independent sequential ractions. have been measured spectrophotometrically. For X = Cl: k1 ⋍ 2 × 10−2, k2 = 1.7 × 10−4 and k3 = 4.8 × 10−6, and for X = Br: k1 ⋍ 2 × 10−2, k2 = 5.25 × 10−4 and k3 = 2.5 × 10−5 The primary equation was found to be acid independent, while the secondary and tertiary aquations were acid-inhibited reactions. For the second step, the rate of the reaction was given by the rate equation where Ct is the complex concentration in the aqua-and hydroxodihalo species, k′2 is the rate constant for the acid-dependent pathway and Ka is the equilibrium constant between the hydroxo and aqua complex ions. The activation parameters were evaluated, for X = Cl: ΔH≠2 = 106.3 ± 0.4 kJ mol−1 and ΔS≠2 = 40.2 ± 1.7 J K−1 mol−, and for X = Br: ΔH≠2 = 91.6 ± 0.4 kJ mol−1 and ΔS≠2 = 0.4 ± 1.7 J K−1 mol−1. The results are discussed and detailed comparisons of the reactivities of these complexes with other haloaminecobalt(III) species are presented. 相似文献
cis,cis,trans-[PtIV(NH3)2Cl2(OH)2] reacts reversibly with ascorbic acid to give dehydroascorbic acid and mainly cis-[PtII(NH2Pri)2Cl2]. The parameters for the forward reaction are: kf = 0.584 M− s− at 37.0 °C, ΔH≠f = 108.6 −+ 6.4 kJ mol−1 andΔS≠f = 101 −+ 22 J K−1 mol−1. 相似文献
The magnetic susceptibility of two ACuCl3 salts containing chains of bibridged Cu2Cl62− dimers has been measured. Each copper ion bridges to a chloride in an adjacent dimer, thus assuming a 4 + 1 coordination geometry. Structurally, each dimer is characterized by a bridging CuClCu angle, ø, and a fold angle, σ, at each copper center. The high temperature susceptibility data for (4-benzylpiperidinium)CuCl3 (ø=95.3°, σ=28.9°) obey a Curie-Weiss law which has a positive intercept (θ=13 K), indicating a predominant ferromagnetic interaction. For the low temperature data, the Curie-Weiss plot has a negative intercept (θ′=−4 K), showing that a net antiferromagnetic coupling exists. The data is interpreted quantitatively in terms of a system of ground state triplet dimers, with singlet-triplet energy splitting ΔE(=2J) of ΔE/k=60 K with interdimer interactions, accounted for with a mean field approximation of J′/k=−3 K. The salt (paraquat)Cu2Cl6 (ø=97.5°, σ=31.7°) behaves as an antiferromagnetic alternating chain with J/k=19 K and J′/k=2 K. The magnetic properties of two other salts containing structurally similar chains, (CH3)2NH2CuCl3 and (CH3)2CHNH3CuCl3, have been reinvestigated using pulsed high field magnetization techniques. Comparison of these systems shows that, with ø constant, there is an approximate linear relationship between ΔE and the fold angle, σ. At ø=95.5°±0.3°, the interaction is ferromagnetic for σ>22° and antiferromagnetic for smaller values of σ. These conclusions are confirmed by extended Hückel MO calculations for the intradimer interactions within the Hoffmann framework for exchange coupling. 相似文献
The axial adduct formation of the iron(II) complex of 2,3,9,10-tetraphenyl-l,4,8,11-tetraaza-1,3,8,10-cyclotetradecatetraene (L) with imidazole in dimethyl sulfoxide has been investigated spectrophotometrically at various temperatures and pressures. In the presence of a large excess of imidazole the reaction with the two phases has been observed. The first faster reaction is the formation of the monoimidazole complex of FeL2+, and the second slower reaction corresponds to the formation of the bisimidazole complex. Activation parameters are as follows: for the first step with k1 (25.0°C) = (6.8 ±0.2)×105 mol−1 kg s−1, ΔH31 = 47.5 ± 4.9 kJ mol−1, ΔS31 = 26±16 J K−1 mol−1, and ΔV31 (30.0°C) = 27.2±1.5 cm3 mol−1; for the second step with k2 (25.0°C) = 26.8±0.8 mol−1 kg s−1, ΔH32 = 91.6± 0.8 kJ mol−1, ΔS32 = 90±3 J K−1 mol−1, and ΔV32 (35.0°C) = 21.8±0.9 cm3 mol−1. The large positive activation volumes strongly indicate a dissociative character of the activation process. 相似文献
AbstractThe effect of pH and temperature on the apparent association equilibrium constant (Ka) for the binding of the bovine and porcine pancreatic secretory trypsin inhibitor (Kazal-type inhibitor, PSTI) to human leukocyte elastase has been investigated. At pH8.0, values of the apparent thermodynamic parameters for human leukocyte elastase: Kazal-type inhibitor complex formation are: bovine PSTT – Ka = 6.3 × 104M?1, δ5G° = -26.9kJ/mol, δH° = +11.7kJ/mol, and δS° = +1.3 × 102 entropy units; porcine PSTI –Ka = 7.0 × 103M?1,δG° = -21.5kJ/mol, δH° = +13.0kJ/mol, and δS° = +1.2 × 102 entropy units (values of Ka δG° and δS° were obtained at 21.0°C; values of δH° were temperature independent over the range (between 5.0°C and 45.0°C) explored). On increasing the pH from 4.5 to 9.5, values of Ka for bovine and porcine PSTI binding to human leukocyte elastase increase thus reflecting the acidic pK-shift of the His57 catalytic residue from ?7.0, in the free enzyme, to ?5.1, in the serine proteinase: inhibitor complexes. Thermodynamics of bovine and porcine PSTI binding to human leukocyte elastase has been analyzed in parallel with that of related serine (pro)enzyme/Kazal-type inhibitor systems. Considering the known molecular models, the observed binding behaviour of bovine and porcine PSTI to human leukocyte elastase was related to the inferred stereochemistry of the serine proteinase/inhibitor contact region(s). 相似文献
The reaction of CrCl3 · 6H2O (dehydrated in DMSO) with 1,5,9-triazanonane (3,3-tri) gives mer- CrCl3(3,3-tri), the configuration being established by isomorphism with the corresponding Co(III) complex. This non-electrolyte is hydrolyzed in aqueous acidic solution and mer-[CrCl2(3,3-tri)- (OH2)]ClO4 can be isolated by anation with HCl in the presence of HClO4. Reaction of mer-CrCl3- (3,3-tri) in DMF with diamines produces complexes of the type [CrCl(diamine)(3,3-tri)] Cl2 [diamine= 1,2-diaminoethane (en), 1.2-diaminopropane (pn), 1,3-diaminopropane (tn), 2,2-dimethyl-1,3-diaminopropane (Me2tn) and cyclohexanediamine (chxn, cis plus trans mixture; two isomers A and B)] and these have been characterized as the ZnCl42− salts. The configuration of the triamine ligand in these complexes has been established as mer-(H↓)- by a single crystal X-ray analysis of [CrCl(en)(3,3-tri)]- ZnCl4, monoclinic, P21, a=7.932, b= 14.711, c= 8.312 Å, β=104.6° and Z=2, refined to a conventional R factor of 0.034. The kinetics of the Hg2+- assisted chloride release from [CrCl(diamine)(3,3- tri)]ZnCl4 salts were measured spectrophotometrically (μ=1.0 M HClO4 or HNO3) over 15 K temperature ranges to give, in order, 104kHg (298.2 K) (M−1 s−1), Ea(kJ mol−1), ΔS# (J K−1 mol−1): en- (HClO4): 5.95, 78.1, -53; pn(HClO4); 5.24, 81.2; -44; tn(HClO4): 26.7, 85.6, -15; Me2tn(HClO4): 21.8, 78.6, -40; A-chxn(HNO3): 7.60, 81.0,-41; B-chxn(HNO3): 18.3, 56.8, -115. A ‘non-replaced ligand effect’ on the rate is observed for the first time in this series of homologous Cr(III) complexes. The kinetics of the thermal aquation (kH, 0.1 M HClO4) were measured titrimetrically for CrCl(diamine) (3,3-tri)2+ to give the following kinetic parameters: diamine=en: 107kH (298.2)=5.34 s−1, Ea=99.2 kJ mol−1, ΔS#=-40 J K−1 mol-1; diamine =tn: 107kH (298.2)=5.04 s−1, Ea= 82.8, ΔS#= -96. 相似文献
Complexes of the general formula cis-[MX2(PTA)2] (M = Pd, Pt; X = Cl, Br, I; PTA = 1,3,5-triaza-7-phosphaadamantane) were used to study the catalytic intramolecular hydroamination/cyclization of 4-pentyn-1-amine into 2-methyl-pyrroline in water, methanol, and dimethyl sulfoxide (DMSO). Kinetic data were measured via 1H NMR under homogeneous conditions at 50 °C and showed the following trends in rate: (i) Fastest rates were observed in D2O. (ii) The Pd complexes of this study produced faster rates than the Pt complexes. (iii) The identity of the halide had no effect on the catalytic rate. Cyclization by the catalytic precursor cis-[PdCl2(PTA)2] (4) in D2O was zero-order in substrate and first-order in metal complex with ΔH‡ = 20.0 ± 2.1 kcal/mol, ΔS‡ = −7.4 ± 6.3 cal/mol K, and Ea = 20.6 ± 2.1 kcal/mol. The acetylide complex, trans-[Pt(CC(CH2)3NH2)2(PTA)2] (6) precipitated from a catalytic mixture involving cis-[PtBr2(PTA)2] (2). Spectroscopic and kinetic studies indicated that 6 and its cis analog, 7, were the predominant species in solution and that they were both active catalysts for the cyclization reaction. These data, in conjunction with the rate trends, indicated that the mechanism of the Pd(II) and Pt(II) catalyzed hydroamination of terminal alkynylamines in aqueous solution followed a unique mechanism with cyclization of an acetylenic-amine ligand being rate determining. 相似文献
One isomer of [CrCl(N-Me-tn)(dien)]ZnCl4 has been isolated from the reaction of CrCl3·6H2O, dehydrated in DMF, with the polyamines N-methyl- 1,3-diaminopropane and diethylenetriamine. This complex is isomorphous with δλ-(R,S)usft-[CoCl(N-Me-tn)(dien)] ZnCl4 and thus has the unsym-fac- configuration with the N-Me group trans to the sec-NH group of the coordinated triamine. The Cr(III) complex has been resolved with NH4BCS and the chiroptical parameters of (-)488-[CrCl(N-Me-tn)(dien)]- ZnCl4, derived from the less soluble diastereoisomeride by metathesis, are similar to those obtained for the less soluble (-)534-λ-(S)-a,cb,edf-Co(III) analogue, of known absolute configuration. Kinetic parameters for the rates of thermal aquation (μ= 1.0 M, HClO4) and Hg2+-assisted chloride release (μ= 1.0 M) for usft-[CrCl(N-Me-tn)(dien)]ZnCl4 are kH= 3.7 × 10−6 s−1, Ea=93 ± 8 kJ mol−1, ΔS2984t#= −45 ± 16 J K−1 mol−1 and kHg=2.01 × 10−3 M−1 s−1, Ea=64.2 ± 3.3, ΔS298#=−89.5 ± 7, respectively at 298 K. 相似文献
The reaction of Ru(XTPP)(DMF)2, where XTPP is the dianion of para substituted tetraphenylporphyrins and X is MeO, Me, H, Cl, Br, I, F, with O2 and CO were studied in DMF. The process was found to be first-order in metalloporphyrin, first-order in molecular oxygen and carbon monoxide, and second-order overall. Second-order rate constants for the CO reaction ranged from 0.170 to 0.665 M?1 s?1 at 25°C, those for the O2 reaction from 0.132 to 0.840 M?1 s?1 at 25°C. Similar activation parameters (ΔHCO± = 87 ± 1 kJ mol?1, ΔSCO± = 22 ± 4 JK?1 mol?1; ΔHO2± = 81 ± 1 kJ mol?1, and ΔSO2± = 11 ± 5 JK?1 mol?1) were found within each series. Reactivities of X substituted metalloporphyrins were found to follow different Hammett σ functions. The CO reactions correlated with σ? following normal behavior; the O2 reactions correlated with σ8° indicating O2 is π-bonded in the transition states. A dissociative mechanism is postulated for the process. 相似文献
Reduction of the model platinum(IV) complexes cis-[PtCl4(NH3)2] (1), trans-[PtCl4(NH3)2] (2), trans-[PtCl2(en)2]2+ (3), trans-[PtBr2(NH3)4]2+ (4), [PtCl6]2− (5), and [PtBr6]2− (6) with l-ascorbic acid (H2Asc) in 1.0 M aqueous medium at 25 °C in the region 1.75≤pH≤7.20 has been investigated using stopped-flow spectrophotometry. The redox reactions follow the rate law: −d[Pt(IV]/dt=k[H2Asc]tot[Pt(IV)] where k is a pH-dependent second-order rate constant and [H2Asc]tot, the total concentration of ascorbic acid. The pH-dependence of k is attributed to parallel reduction of Pt(IV) by the protolytic species HAsc− and Asc2−. Analysis of the kinetics data reveals that the ascorbate anion Asc2− is up to seven orders of magnitude more reactive than HAsc− while H2Asc is unreactive. Electron transfer from HAsc−/Asc2− to the Pt(IV) compounds is suggested to take place by a mechanism involving a reductive attack on any one of the mutually trans-halide ligands by Asc2− and/or HAsc− forming a halide-bridged activated complex. The rapid reduction of these complexes supports the assumption that ascorbate Asc2− might be an important reductant at physiological conditions for anticancer active Pt(IV) pro-drugs capable of undergoing reductive trans elimination. The parameters ΔH≠ and ΔS≠ for reduction of Pt(IV) with Asc2− have been determined from the study of the temperature dependence of k. 相似文献