Oxidation of iodide by a series of Fe(III) complexes in acetonitrile

The oxidations of iodide by [Fe(III)(bpy)2(CN)2]NO3, [Fe(III)(dmbpy)2(CN)2]NO3, [Fe(III)(CH3Cp)2]PF6, and [Fe(III)(5-Cl-phen)2(CN)2]NO3 at 25 degrees C, ionic strength of 0.10 M in acetonitrile, are catalyzed by trace levels of copper ions. This copper catalysis can be effectively masked with the addition of 5.0 mM 2,2'-bipyridine (bpy), which permits the rate law of the direct reactions to be determined: -d[Fe(III)]/dt = 2(k1[I-] + k2[I-]2)[Fe(III)]. According to 1H NMR and UV-vis spectra, the products of the reaction are I3- and the corresponding Fe(II) complexes, with the stoichiometric ratio (delta[I3-]/delta[Fe(II)]) of 1:2. Linear free-energy relationships (LFERs) are obtained for both log k1 and log k2 vs E(1/2) with slopes of 16.1 and 13.3 V(-1), respectively. A mechanism is inferred in which k1 corresponds to simple electron transfer to form I* plus Fe(II), while k2 leads directly to I2(-*). From the mild kinetic inhibition of the k1 path by [Fe(II)(bpy)2(CN)2] the standard potential (Eo) of I*/I- is derived: Eo = 0.60 +/- 0.01 V (vs [Fe(Cp)2](+/0)).     

Interference in the kinetic determination of ascorbic acid by sulphide and sulphite

The reduction of 2,6-dichlorophenolindophenol (DCPI) by sulphides and sulphites has been studied kinetically by the stopped-flow technique. The reaction is first-order with respect to each of the reactants. From the distribution diagrams for the species DH(+)(2), DH and D(-) for DCPI and H(2)Q, HQ(-) and Q(2-) for sulphides or sulphites, a mechanism is proposed which suggests partial reactions of all possible combinations of the reacting species at any pH. An equation for calculation of the second-order reaction rate constants k at any pH is derived, which gives k as a function of [H(+)], the partial reaction rate constants and the dissociation constants of DCPI and H(2)S or H(2)SO(3). Values of the overall reaction rate constants over a wide pH-range have been determined, together with values of k for all possible partial reactions. For particular pH-values the second-order reaction rate constant was determined by four different methods. Mean values of k = 251 +/- 1 and 240 +/- 1 l.mole(-1).sec(-1) were obtained for pH 3.15 and 4.17, respectively, for the DCPI-Na(2)S reaction and k = 137 +/- 1, 127 +/- 1 and 136 +/- 1 l.mole(-1).sec(-1) for pH 2.02, 4.25 and 5.10, respectively, for the DCPI-Na(2)SO(3) reaction. From the slopes of the linear Arrhenius plots activation energies of 6.6 +/- 0.2 and 4.0 +/- 0.1 kcal/mole for the DCPI-Na(2)S and DCPI-Na(2)SO(3) reactions, respectively were calculated. The effect of ionic strength on the reactions supports the proposed mechanism.     

Reductions by titanium(II) as catalyzed by titanium(IV)

The cobalt(III) complexes, [(NH3)5CoBr]2+ and [(NH3)5CoI]2+ are reduced by Ti(II) solutions containing Ti(IV), generating nearly linear (zero-order) profiles that become curved only during the last few percent of reaction. Other Co(III)-Ti(II) systems exhibit the usual exponential traces with rates proportional to [Co(III)]. Observed kinetics of the biphasic catalyzed Ti(II)-Co(III)Br and Ti(II)-Co(III)I reactions support the reaction sequence: [Ti(II)(H20)n]2+ + [Ti(IV)F5]- (k1)<==>(k -1) [Ti(II)(H2O)(n-1)]2+ + [(H2O)Ti(IV)F5]-, [Ti(II)(H2O)(n-1)]2+ + Co(III) (k2)--> Ti(III) + Co(II) with rates determined mainly by the slow Ti(IV)-Ti(II) ligand exchange (k1 = 9 x 10(-3) M(-1) s(-1) at 22 degrees C). Computer simulations of the catalyzed Ti(II)-Co(III) reaction in perchlorate-triflate media yield relative rates for reduction by the proposed active [Ti(II)(H2O)(n-1)]2+ intermediate; k(Br)/k(I) = 8.     

Direct oxidation of L-cysteine by [FeIII(bpy)2(CN)2]+ and [FeIII(bpy)(CN)4]-

The oxidation of L-cysteine by the outer-sphere oxidants [Fe(bpy)2(CN)2]+ and [Fe(bpy)(CN)4]- in anaerobic aqueous solution is highly susceptible to catalysis by trace amounts of copper ions. This copper catalysis is effectively inhibited with the addition of 1.0 mM dipicolinic acid for the reduction of [Fe(bpy)2(CN)2]+ and is completely suppressed with the addition of 5.0 mM EDTA (pH<9.00), 10.0 mM EDTA (9.010.0) for the reduction of [Fe(bpy)(CN)4]-. 1H NMR and UV-vis spectra show that the products of the direct (uncatalyzed) reactions are the corresponding Fe(II) complexes and, when no radical scavengers are present, L-cystine, both being formed quantitatively. The two reactions display mild kinetic inhibition by Fe(II), and the inhibition can be suppressed by the free radical scavenger PBN (N-tert-butyl-alpha-phenylnitrone). At 25 degrees C and micro=0.1 M and under conditions where inhibition by Fe(II) is insignificant, the general rate law is -d[Fe(III)]/dt=k[cysteine]tot[Fe(III)], with k={k2Ka1[H+]2+k3Ka1Ka2[H+]+k4Ka1Ka2Ka3{/}[H+]3+Ka1[H+]2+Ka1Ka2[H+]+Ka1Ka2Ka3}, where Ka1, Ka2, and Ka3 are the successive acid dissociation constants of HSCH2CH(NH3+)CO2H. For [Fe(bpy)2(CN)2]+, the kinetics over the pH range of 3-7.9 yields k2=3.4+/-0.6 M(-1) s(-1) and k3=(1.18+/-0.02)x10(6) M(-1) s(-1) (k4 is insignificant in the fitting). For [Fe(bpy)(CN)4]- over the pH range of 6.1-11.9, the rate constants are k3=(2.13+/-0.08)x10(3) M(-1) s(-1) and k4=(1.01+/-0.06)x10(4) M(-1) s(-1) (k2 is insignificant in the fitting). All three terms in the rate law are assigned to rate-limiting electron-transfer reactions in which various thiolate forms of cysteine are reactive. Applying Marcus theory, the self-exchange rate constant of the *SCH2CH(NH2)CO2-/-SCH2CH(NH2)CO2- redox couple was obtained from the oxidation of L-cysteine by [Fe(bpy)(CN)4]-, with k11=4x10(5) M(-1) s(-1). The self-exchange rate constant of the *SCH2CH(NH3+)CO2-/-SCH2CH(NH3+)CO2- redox couple was similarly obtained from the rates with both Fe(III) oxidants, a value of 6x10(6) M(-1) s(-1) for k11 being derived. Both self-exchange rate constants are quite large as is to be expected from the minimal rearrangement that follows conversion of a thiolate to a thiyl radical, and the somewhat lower self-exchange rate constant for the dianionic form of cysteine is ascribed to electrostatic repulsion.     

Catalase-peroxidase activity of iron(III)-TAML activators of hydrogen peroxide

Exceptionally high peroxidase-like and catalase-like activities of iron(III)-TAML activators of H 2O 2 ( 1: Tetra-Amidato-Macrocyclic-Ligand Fe (III) complexes [ F e{1,2-X 2C 6H 2-4,5-( NCOCMe 2 NCO) 2CR 2}(OH 2)] (-)) are reported from pH 6-12.4 and 25-45 degrees C. Oxidation of the cyclometalated 2-phenylpyridine organometallic complex, [Ru (II)( o-C 6H 4py)(phen) 2]PF 6 ( 2) or "ruthenium dye", occurs via the equation [ Ru II ] + 1/2 H 2 O 2 + H +-->(Fe III - TAML) [ Ru III ] + H 2 O, following a simple rate law rate = k obs (per)[ 1][H 2O 2], that is, the rate is independent of the concentration of 2 at all pHs and temperatures studied. The kinetics of the catalase-like activity (H 2 O 2 -->(Fe III - TAML) H 2 O + 1/2 O 2) obeys a similar rate law: rate = k obs (cat)[ 1][H 2O 2]). The rate constants, k obs (per) and k obs (cat), are strongly and similarly pH dependent, with a maximum around pH 10. Both bell-shaped pH profiles are quantitatively accounted for in terms of a common mechanism based on the known speciation of 1 and H 2O 2 in this pH range. Complexes 1 exist as axial diaqua species [FeL(H 2O) 2] (-) ( 1 aqua) which are deprotonated to afford [FeL(OH)(H 2O)] (2-) ( 1 OH) at pH 9-10. The pathways 1 aqua + H 2O 2 ( k 1), 1 OH + H 2O 2 ( k 2), and 1 OH + HO 2 (-) ( k 4) afford one or more oxidized Fe-TAML species that further rapidly oxidize the dye (peroxidase-like activity) or a second H 2O 2 molecule (catalase-like activity). This mechanism is supported by the observations that (i) the catalase-like activity of 1 is controllably retarded by addition of reducing agents into solution and (ii) second order kinetics in H 2O 2 has been observed when the rate of O 2 evolution was monitored in the presence of added reducing agents. The performances of the 1 complexes in catalyzing H 2O 2 oxidations are shown to compare favorably with the peroxidases further establishing Fe (III)-TAML activators as miniaturized enzyme replicas with the potential to greatly expand the technological utility of hydrogen peroxide.     

Ruthenium bisbipyridine complexes of horse heart cytochrome c: characterization and comparative intramolecular electron-transfer rates determined by pulse radiolysis and flash photolysis

The reaction of [Ru(bpy)2L(H2O)]2+ (bpy = 2,2'-bipyridine, L = imidazole, water) with reduced horse heart cytochrome c results in coordination of [RuII(bpy)2L] at the His 33 and His 26 sites. Coordination at the His 33 site gave a diastereomeric [RuII(bpy)2L]-His-cyt c(II) mixture favoring the lambda-Ru form regardless of the substituent on the bipyridine ligands, while substitution at the more buried His 26 site gave an isomeric distribution that varies according to the substituent on the bipyridine ligands. The diastereomeric aquoproteins (L = H2O) are distinguished by their redox potentials and their conversion to the corresponding fluorescent imidazole proteins. Intramolecular electron transfer between the reduced ruthenium bipyridine and cyt c(III) in [RuII(bpy.)(bpy)L]-His33-cyt c(III) was determined by reductive pulse radiolysis using the aqueous electron as a reducing agent, kret = (2.0 +/- 0.3) x 10(5) s-1, and kret is independent of the sixth ligand L = H2O, imidazole. In addition, the rate constant for intramolecular electron transfer from cyt c(II) to the ruthenium(III) center in [RuIII(bpy)2L]-His33-cyt c(II) was determined by oxidative pulse radiolysis using azide and carbonate radicals. This rate is very sensitive to the nature of the sixth ligand. When L = H2O, the intramolecular electron-transfer rate for the major diastereomer lambda-cis-[RuIII (bpy)2(H2O)]-His33-cyt c(II) is k = 1.1 x 10(4) s-1 and is independent of pH between 5.6 and 8.3. The minor delta-cis-[RuIII(bpy)2(H2O)]-His33-cyt c(II) isomer has pH-dependent electrochemistry and a lower rate of intramolecular electron transfer. Complete conversion from L = H2O to L = imidazole is slow, requiring more than 7 days in 1 M imidazole. A lower limit (k > 2 x 10(6) s-1) for the intramolecular electron-transfer rate constant in [RuIII(bpy)2(L)]-His33-cyt c(II), L = imidazole, could be obtained by pulse radiolysis in the absence of the slower reacting aquo species. This observation is in agreement with the value of 3 x 10(6) s-1 measured by flash photolysis. Earlier pulse radiolysis experiments primarily measured the aquoligated ruthenium protein, while the flash photolysis experiments measured the imidazole-ligated fraction because it is the only species oxidatively quenched in the photoinduced reactions. Intramolecular electron-transfer reactions for a new series of ruthenium bipyridine complexes, [Ru(dabpy)2L]-His33-cyt c proteins (dabpy = 4,4'-diamino-2,2'-bipyridine) (L = imidazole, pyridine, isonicotinamide and pyrazine), proceed with lower driving force, resulting in slower rate constants amenable to measurement by oxidative pulse radiolysis. The electron-transfer rate constants for this series spanned a wide range of the Marcus log k vs delta G plot.     

Dissociation kinetics of macrocyclic trivalent lanthanide complexes of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (DO2A)

The [H(+)]-catalyzed dissociation rate constants of several trivalent lanthanide (Ln) complexes of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (LnDO2A(+), Ln = La, Pr, Eu, Er and Lu) have been determined in two pH ranges: 3.73-5.11 and 1.75-2.65 at four different temperatures (19-41.0 °C) in aqueous media at a constant ionic strength of 0.1 mol dm(-3) (LiClO(4)). For the study in the higher pH range, i.e. pH 3.73-5.11, copper(II) ion was used as the scavenger for the free ligand DO2A in acetate/acetic acid buffer medium. The rates of Ln(III) complex dissociation have been found to be independent of [Cu(2+)] and all the Ln(III) complexes studied show [H(+)]-dependence at low acid concentrations but become [H(+)]-independent at high acid concentrations. Influence of the acetate ion content in the buffer on the dissociation rate has also been investigated and all the complexes exhibit a first-order dependence on [Acetate]. The dissociation reactions follow the rate law: k(obs) = k(Ac)[Acetate] + K'k(lim)[H(+)]/(1 + K'[H(+)]) where k(AC) is the dissociation rate constant for the [Acetate]-dependent pathway, k(lim) is the limiting rate constant, and K' is the equilibrium constant for the reaction LnDO2A(+) + H(+) ? LnDO2AH(2+). In the lower pH range, i.e. pH 1.75-2.65, the dye indicator, cresol red, was used to monitor the dissociation rate, and all the Ln(III) complexes also show [H(+)]-dependence dissociation pathways but without the rate saturation observed at higher pH range. The dissociation reactions follow the simple rate law: k(obs) = k(H)[H(+)], where k(H) is the dissociation rate constant for the pathway involving monoprotonated species. The absence of an [H(+)]-independent pathway in both pH ranges indicates that LnDO2A(+) complexes are kinetically rather inert. The obtained k(AC) values follow the order: LaDO2A(+) > PrDO2A(+) > EuDO2A(+) > ErDO2A(+) > LuDO2A(+), whereas the k(lim) and k(H) values follow the order: LaDO2A(+) > PrDO2A(+) > ErDO2A(+) > EuDO2A(+) > LuDO2A(+), mostly consistent with their thermodynamic stability order, i.e. the more thermodynamically stable the more kinetically inert. In both pH ranges, activation parameters, ΔH*, ΔS* and ΔG*, for both acetate-dependent and proton-catalyzed dissociation pathways have been obtained for most of the La(III), Pr(III), Eu(III), Er(III) and Lu(III) complexes, from the temperature dependence measurements of the rate constants in the 19-41 °C range. An isokinetic (linear) relationship is found between ΔH* and ΔS* values, which supports a common reaction mechanism.     

Synthesis, structures, and electrochemistry of gold(III) ethylenediamine complexes and interactions with guanosine 5'-monophosphate

[Au(en)Cl(2)]Cl.2H(2)O, where en = ethylenediamine (1,2-diaminoethane), has been synthesized, and its structure has been solved for the first time by the single-crystal X-ray diffraction method. The complex has square-planar geometry about Au(III), and the anionic Cl- is located in the apical position and at a distance of 3.3033(10) A compared to 2.2811(9) and 2.2836(11) A for the coordinated Cl-. [Au(en)Cl2]Cl.2H2O belongs to the space group Pbca with a = 11.5610(15) A, b = 12.6399(17) A, c = 13.2156(17) A, alpha = beta = gamma = 90 degrees , and Z = 8. Bond lengths of Au-N are 2.03 A. [Au(en)Cl2]Cl.2H2O is less thermally stable than [Au(en)2]Cl3 because of the replacement of two Cl ligands by a second en ligand in the latter. Cyclic voltammetry shows that the formal potential of Au(III)/Au(0) becomes more negative in the series [AuCl4]-, [Au(en)Cl2]+, and [Au(en)2]3+. 1H, 13C, and 31P NMR reveal that in an aqueous solution [Au(en)Cl2]+ bonds to guanosine 5'-monophosphate, 5'-GMP (1:1 mole ratio), via N7, although the stability is not very high. NMR data also indicate that N7-O6 or N7-phosphate 5'-GMP chelation, as found in some gold(III) nucleotide complexes, is not present. The gold(III) complex undergoes hydrolysis at pH >2.5-3.0 and, therefore, N1 coordination to 5'-GMP is not observed. No direct coordination between 5'-GMP and [Au(en)2]Cl3 is observed.     

两种稀土卟啉配合物与大肠杆菌作用的微量热研究

用LKB-2277生物活性检测系统测定了新合成的阳离子型稀土卟啉配合物{[Re(TMP)(H2O)3]Cl, Re=Y、Yb, TMP=5, 10, 15, 20-四(4’-甲氧基苯基)卟啉}在37 ℃时对大肠杆菌作用的产热曲线,根据产热曲线求算了在稀土卟啉配合物作用下,大肠杆菌生长代谢的速率常数k,抑制率I,传代时间tG和半抑制浓度cI,50等热动力学参数.结果表明,稀土卟啉配合物在低浓度下对大肠杆菌有刺激作用,高浓度下为抑制作用,[Yb(TMP)(H2O)3]Cl的半抑制浓度cI,50为143 mg•L-1,其对大肠杆菌的抑制作用优于[Y(TMP)(H2O)3]Cl.     

Stable gold(III) complexes with thiosemicarbazone derivatives

Novel thiosemicarbazonato complexes of gold(III) have been prepared from reactions of [Au(damp-C1,N)Cl2(damp- = 2-(N,N-dimethylaminomethyl)phenyl) or [NBu4][AuCl4] with 2-pyridineformamide thiosemicarbazones (HL). The thiosemicarbazones deprotonate and coordinate as mononegative, tridentate NNS ligands to gold to give [Au(Hdamp-C1)(L)]Cl2 or [AuCl(L)]Cl complexes. The organometallic damp- ligand is protonated during the reactions and the Au-N bond is cleaved. The [AuCl(L)]+ cations represent the first gold(III) complexes with thiourea derivatives which are not stabilised by an additional organometallic ligand. Reactions of [NBu4][AuX4](X = Cl, Br) with diphenylthiocarbazone (dithizone) result in reduction of the metal and the formation of gold(I) complexes of the composition [AuX(SCN4-3,4-Ph2)] where SCN4-3,4-Ph2 is 3,4-diphenyltetrazole thione which is formed from cyclisation of dithizone.     

Kinetics and mechanism of O-O bond cleavage in the reaction of [RuIII(edta)(H2O)]- with hydroperoxides in aqueous solution

The reactions of [Ru(III)(edta)(H(2)O)](-) (1) (edta = ethylenediaminetetraacetate) with tert-butylhydroperoxide ((t)BuOOH) and potassium hydrogenpersulfate (KHSO(5)) were studied kinetically as a function of oxidant concentration and temperature (10-30 degrees C) at a fixed pH of 6.1 using stopped-flow techniques. Kinetic results were analyzed by using global kinetic analysis techniques. The reaction was found to consist of two steps involving the rapid formation of a [Ru(III)(edta)(OOR)](2-) intermediate, which subsequently undergoes heterolytic cleavage to form [(edta)Ru(V)=O](-). Since [(edta)Ru(V)=O](-) was produced almost quantitatively in the reaction of 1 with the hydroperoxides (t)BuOOH and KHSO(5), the common mechanism is one of heterolytic scission of the O-O bond. The water soluble and easy to oxidize substrate 2,2'-azobis(3-ethylbenzithiazoline-6-sulfonate (ABTS), was employed to substantiate the mechanistic proposal. Reactions were carried out under pseudo-first order conditions for [ABTS] > [hydroperoxide] > [1], and were monitored as a function of time for the formation of the one-electron oxidation product ABTS (*+). The detailed suggested mechanism is consistent with the reported rate and activation parameters, and discussed in reference to the results reported for the reaction of [Ru(II)(edta)(H(2)O)](-) with H(2)O(2).     

Gold nano particles catalyzed oxidation of hydrazine by a metallo-superoxide complex: experimental evidences for surface activity of gold nano particles

Gelatin-capped gold nano particles (GNPs) of diameter 23, 28 and 36 nm were prepared and characterized as almost monodispersed, near-spherical solids. In acidic media, these GNPs at their very low concentration level (～10(-13) M) catalyze the oxidation of hydrazine by the metallo-superoxide, [(NH(3))(4)Co(III)(μ-NH(2),μ-O(2))Co(III)(NH(3))(4)](NO(3))(4) (1). In the presence of a large excess of hydrazine over [1], the catalyzed oxidation is first-order in [1], [GNPs] and media alkalinity. The pure first-order dependence implies that the size as well as the nature of the catalyst remained unchanged during the reaction. The catalytic efficacies increased with increased total surface area of the GNPs. Increasing T(Hydrazine) (T(Hydrazine) is the analytical concentration of hydrazine) tends to saturate the first-order rate constant (k(o)) for hydrazine oxidation and a plot of 1/k(o)versus T(Hydrazine) was found to be linear at a particular [GNPs], indicating the GNPs assisted deprotonation of N(2)H(5)(+) to N(2)H(4). The rate constants show a non-linear behavior with temperature studied in the range 288-308 K. At a lower temperature interval, viz. 288-298 K, k(o) increases with increasing temperature whereas at temperature interval, viz. 303-308 K, k(o) decreases with temperature. Such a variation indicates the important process of absorption and desorption of the reactants on and from the surface. A plausible mechanism for the GNPs catalyzed oxidation of hydrazine is suggested.     

Kinetics and Mechanism of the Reaction between Serum Albumin and Auranofin (and Its Isopropyl Analogue) in Vitro

The first detailed kinetic analysis and mechanistic interpretation of the reactions between serum albumin and the second-generation gold drug Auranofin [Et(3)PAuSATg = (triethylphosphine)(2,3,4,6-tetra-O-acetyl-1-beta-D-glucopyranosato-S-) gold(I)] and its triisopropylphosphine analogue, iPr(3)PAuSATg, in vitro are reported. The reactions were investigated using Penefsky spun columns and NMR saturation transfer methods. Under the Penefsky chromatography conditions with 0.4-0.6 mM albumin and a wide range of Et(3)PAuSATg concentrations, the reaction is biphasic. The fast phase is apparently first order in albumin with a rate constant [k(1) = 3.4 +/- 0.3 x 10(-)(2) s(-)(1)] that decreases slightly in magnitude and becomes intermediate in order at low gold concentrations, [Et(3)PAuSATg] < [AlbSH]; it accounts for approximately 95% of the Au(I) that binds. A minor, slower step [k(2) = 2.3 +/- 0.3 x 10(-)(3) s(-)(1)), which accounts for only 5% of the reaction, is also first order with respect to albumin, and zero order with respect to auranofin. For iPr(3)PAuSATg, only the first step was observed, k(1) = (1.4 +/- 0.1) x 10(-)(2) s(-)(1), and is first order in albumin and independent of the iPr(3)PAuSATg concentration. (31)P-NMR saturation transfer experiments utilizing iPr(3)PAuSATg, under equilibrium conditions, yielded second-order rate constants for both the forward (1.2 x 10(2) M(-)(1) s(-)(1)) and the reverse (3.9 x 10(1) M(-)(1) s(-)(1)) directions. A multistep mechanism involving a conformationally altered albumin species was developed. Albumin domain IA opens with concomitant Cys-34 rearrangement, allowing facile gold binding and exchange, and then closes. In conjunction with the steady-state approximation, this mechanism accounts for the different reaction orders observed under the two set of conditions. The rate-determining conformational change of albumin governs the reaction as monitored by the Penefsky columns. Rapid second order exchange of R(3)PAuSATg at the exposed Cys-34 residue is observed under the NMR conditions. The mechanism predicts that under physiological conditions where [Et(3)PAuSATg] is 10-25 &mgr;M, the reaction will be second order and rapid with a rate constant of 8 +/- 2 x 10(2) M(-)(1) s(-)(1). The Penefsky spun columns revealed a previously unreported and novel binding mechanism, association of auranofin in the pocket of albumin-disulfide species, which was confirmed by Hummel-Dreyer gel chromatographic techniques under equilibrium conditions. This albumin-auranofin complex (AlbSSR-Et(3)PAuSATg) is weakly bound and readily dissociates during conventional gel exclusion chromatography.     

Alkene cis-dihydroxylation by [(Me3tacn)(CF3CO2)RuVI O2)]ClO4(Me(3)tacn = 1,4,7-Trimethyl-1,4,7-triazacyclononane): structural characterization of [3 + 2] cycloadducts and kinetic studies

cis-Dioxoruthenium(VI) complex [(Me(3)tacn)(CF(3)CO(2))Ru(VI)O(2)]ClO(4) (1, Me(3)tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) reacted with alkenes in aqueous tert-butyl alcohol to afford cis-1,2-diols in excellent yields under ambient conditions. When the reactions of 1 with alkenes were conducted in acetonitrile, oxidative C=C cleavage reaction prevailed giving carbonyl products in >90% yields without any cis-diol formation. The alkene cis-dihydroxylation and C=C cleavage reactions proceed via the formation of a [3 + 2] cycloadduct between 1 and alkenes, analogous to the related reactions with alkynes [Che et al. J. Am. Chem. Soc. 2000, 122, 11380]. With cyclooctene and trans-beta-methylstyrene as substrates, the Ru(III) cycloadducts (4a) and (4b) [formula; see text] were isolated and structurally characterized by X-ray crystal analyses. The kinetics of the reactions of 1 with a series of p-substituted styrenes has been studied in acetonitrile by stopped-flow spectrophotometry. The second-order rate constants varied by 14-fold despite an overall span of 1.3 V for the one-electron oxidation potentials of alkenes. Secondary kinetic isotope effect (KIE) was observed for the oxidation of beta-d(2)-styrene (k(H)/k(D) = 0.83 +/- 0.04) and alpha-deuteriostyrene (k(H)/k(D) = 0.96 +/- 0.03), which, together with the stereoselectivity of cis-alkene oxidation by 1, is in favor of a concerted mechanism.     

On the mechanism of the initiation reaction in Grubbs-Hoveyda complexes

Grubbs-Hoveyda-type complexes with variable 4-R (complexes 1: 4-R = NEt(2), OiPr, H, F, NO(2)) and 5-R substituents (complexes 2: 5-R = NEt(2), OiPr, Me, F, NO(2)) at the 2-isopropoxy benzylidene ether ligand and with variable 4-R substituents (complexes 3: 4-R = H, NO(2)) at the 2-methoxy benzylidene ether ligand were synthesized and the respective Ru(II/III) redox potentials (ranging from ΔE = +0.46 to +1.04 V), and UV-vis spectra recorded. The initiation kinetics of complexes 1-3 with the olefins diethyl diallyl malonate (DEDAM), butyl vinyl ether (BuVE), 1-hexene, styrene, and 3,3-dimethylbut-1-ene were investigated using UV-vis spectroscopy. Electron-withdrawing groups at the benzylidene ether ligands were found to increase the initiation rates, while electron-donating groups lead to slower precatalyst activation; accordingly with DEDAM, the complex 1(NO(2)) initiates almost 100 times faster than 1(NEt(2)). The 4-R substituents (para to the benzylidene carbon) were found to have a stronger influence on physical and kinetic properties of complexes 1 and 2 than that of 5-R groups para to the ether oxygen. The DEDAM-induced initiation reactions of complexes 1 and 2 are classified as two-step reactions with an element of reversibility. The hyperbolic fit of the k(obs) vs [DEDAM] plots is interpreted according to a dissociative mechanism (D). Kinetic studies employing BuVE showed that the initiation reactions simultaneously follow two different mechanistic pathways, since the k(obs) vs [olefin] plots are best fitted to k(obs) = k(D)·k(4)/k(-D)·[olefin]/(1 + k(4)/k(-D)·[olefin]) + k(I)·[olefin]. The k(I)·[olefin] term dominates the initiation behavior of the sterically less demanding complexes 3 and was shown to correspond to an interchange mechanism with associative mode of activation (I(a)), leading to very fast precatalyst activation at high olefin concentrations. Equilibrium and rate constants for the reactions of complexes 1-3 with the bulky PCy(3) were determined. In general, sterically demanding olefins (DEDAM, styrene) and Grubbs-Hoveyda type complexes 1 and 2 preferentially initiate according to the dissociative pathway; for the less bulky olefins (BuVE, 1-hexene) and complexes 1 and 2 both D and I(a) are important. Activation parameters for BuVE reactions and complexes 1(NEt(2)), 1(H), and 1(NO(2)) were determined, and ΔS(?) was found to be negative (ΔS(?) = -113 to -167 J·K(-1)·mol(-1)) providing additional support for the I(a) catalyst activation.     

Aminolysis of 4-nitrophenyl phenyl carbonate and thionocarbonate: effects of amine nature and modification of electrophilic center from C[double bond]O to C[double bond]S on reactivity and mechanism

A kinetic study is reported for the reactions of 4-nitrophenyl phenyl carbonate (5) and thionocarbonate (6) with a series of alicyclic secondary amines in 80 mol% H(2)O-20 mol% DMSO at 25.0 +/- 0.1 degrees C. The plots of k(obsd) vs. amine concentration are linear for the reactions of 5. On the contrary, the plots for the corresponding reactions of 6 curve upward as a function of increasing amine concentration, indicating that the reactions proceed through two intermediates (i.e., a zwitterionic tetrahedral intermediate T(+/-) and its deprotonated form T(-)). The Br?nsted-type plot for 5 the reactions of with secondary amines exhibits a downward curvature, i.e., the slope decreases from 0.98 to 0.26 as the pK(a) of the conjugate acid of amines increases, implying that the reactions proceed through T(+/-) with a change in the rate-determining step (RDS). The k(N) values are larger for the reactions of with secondary amines than for those with primary amines of similar basicity. Dissection of k(N) values for the reactions of 5 into the microscopic rate constants (i.e., k(1) and k(2)/k(-1) ratio) has revealed that k(1) is larger for the reactions with secondary amines than for those with isobasic primary amines, while the k(2)/k(-1) ratio is nearly identical. On the other hand, for reactions of 6, secondary amines exhibit larger k(1) values but smaller k(2)/k(-1) ratios than primary amines. The current study has shown that the reactivity and reaction mechanism are strongly influenced by the nature of amines (primary vs. secondary amines) and electrophilic centers (C[double bond]O vs. C[double bond]S).     

Self-exchange reaction of [Ni(mnt)2](1-,2-) in nonaqueous solutions

The rate constant, k, for the homogeneous electron transfer (self-exchange) reaction between the diamagnetic bis(maleonitriledithiolato)nickel dianion, [Ni(mnt) 2] (2-), and the paramagnetic monoanion, [Ni(mnt) 2] (1-), has been determined in acetone and nitromethane (CH 3NO 2) using (13)C NMR line widths at 22 degrees C (mnt = 1,2-S 2C 2(CN) 2). The values of k (2.91 x 10 (6) M (-1) s (-1) in acetone, 5.78 x 10 (6) M (-1) s (-1) in CH 3NO 2) are faster than those for the electron transfer reactions of other Ni(III,II) couples; the structures of [Ni(mnt) 2] (1-) and [Ni(mnt) 2] (2-) allow for a favorable overlap that lowers the free energy of activation. The values of k are consistent with the predictions of Marcus theory. In addition to k, the spin-lattice relaxation time, T 1e, of [Ni(mnt) 2] (1-) is obtained from the NMR line width analysis; the values are consistent with those predicted by spin relaxation theory.     

Oxidative behaviour and relative reactivities of α,β-unsaturated compounds towards bis(dihydrogentellurato)cuprate(III) and -argentate(III) in alkaline medium

The kinetics of oxidation of three alpha beta-unsaturated compounds, CH2CHX (X=CN, CONH2 and CO2-) by bis(dihydrogentellurato)metallate(III) ions ([MIII-(H2TeO6)2]5-, M=Cu or Ag) have been studied in an alkaline medium. The reactions take place according to the rate expression: The k3 values are 2.42*10-2, 1.67*10-2 and 1.10*10-2mol6dm-2s-1 for oxidations of the respective substrate by the copper(III) complex and 1.49*10-2, 1.07*10-2 and 0.80*10-2mol6dm-2s-1, respectively, in the oxidations by silver(III), all at 303K. [CuIII(H2TeO6)2]5- reacts faster than the corresponding reactions with [AgIII(H2TeO6)2]5-. The oxidation rates follow the order: CN CONH2CO2-. The oxidations of the substrates by [CuIII(H2TeO6)2]5- take place by an outer-sphere mechanism, unlike [AgIII-(H2TeO6)2]5-, which reacts by an inner-sphere mechanism. The substrates are oxidized to diols under kinetic control and a tentative reaction mechanism leading to the formation of oxidation products is suggested. The activation parameters of the reactions have been evaluated. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Differential Relationships of Thermodynamic Variables for the Equilibrium of Heterogeneous Substances

For the system without adiabatic walls, rigid walls or semi-permeable walls and without chemical reactions or without other restrictions except restrictions of phase equilibrium conditions, if the number of components of the system is k and the number of phases is φ, the degree of freedom of the system at equilibrium is f=k-φ+2. Because the degree of freedom is incapable of being negative, f=k-φ+2≥0, viz.φ≤k+2. For the heterogeneous equilibrium, the number of phases is at least 2, so φ=k+2-f≥2, viz. f≤k. Hence the range of change of φ and f is 2≤φ≤k+2,0≤f≤k, respectitvely. If φ=k+2, there are no independent variables in the system at equilibrium. If φ=k+1, there is one independent variable; if the temperature is selected as the independent variable, the other dependent variables can be expressed as the function of the temperature. If φ=k, there are two independent variables; if the temperature and pressure are selected as the independent variables, the other dependent variables can be expressed as the function of the temperature and pressure. If 2≤φ≤k-1, there are more than two independent variables; if the temperature, pressure and some concentrations are selected as independent variables, the other dependent variables can be expressed as the function of the temperature, pressure and these concentrations. The differential relationships of dependent variables and independent variables are educed out according to the principle of phase equilibriums for 2≤φ≤k-1. In any phase the number of the variables is(k+1), viz. temperature T, pressure p and (k-1) mole fractions x1, x2,…, xk-1. The temperature and pressure are common variables of every phase. The number of independent variables is at best k for the heterogeneous equilibriums of k components. The temperature, pressure and (k-2) concentrations are selected as independent variables. The independent concentration variables are selected entirely from the first phase and the concentration variables of the other phases all act as dependent variables. There is at least one dependent concentration variable in the first phase.     

The role of geometry on regioselectivity and rate of fluorination of fluorene and diphenylmethane with Selectfluor F-TEDA-BF4

Diphenylmethane and fluorene were used as target molecules in an investigation of the effect of the geometry of aromatic molecules on the regioselectivity and rate of fluorination with 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor F-TEDA-BF4). In acetonitrile at 80 degrees C ring fluorination of diphenylmethane was accompanied by oxidation of the saturated carbon atom, while in trifluoroacetic acid only ring fluorination with an ortho-para regioselectivity of 1.8:1 was observed. Fluorene was converted in acetonitrile as well as in trifluoroacetic acid into 2- and 4-fluoro substituted products in the relative ratio of 2:1 and 1.2:1, respectively. The reactions in acetonitrile obey a simple rate equation: v = d[F-TEDA]/dt = k2 x [F-TEDA] x [Substrate] and the second order rate constants for the reactions in acetonitrile at 65 degrees C were determined; values of 0.6 x 10(-4) M-1 s-1 for diphenylmethane and 35.5 x 10(-4) M-1 s-1 for fluorene were obtained. The reaction rates for the various functionalisations of fluorene relative to those for diphenylmethane were found to be considerably influenced by the type of functionalisation. Relative rate factors (k(rel) = k2(fluorene)/k2(diphenylmethane)) with values between 59 for fluorination and 712 for chlorination were determined, while the corresponding data for the biphenyl/diphenylmethane pair were only slightly dependent on the type of functionalisation. A reaction pathway involving electron transfer, thus forming cation radical intermediates, was proposed as the main process in the case of fluorination of fluorene with F-TEDA-BF4.