Acid-catalyzed oxidation of iodide ions by superoxo complexes of rhodium and chromium

Superoxometal complexes L(H(2)O)MOO(2+) (L = (H(2)O)(4), (NH(3))(4), or N(4)-macrocycle; M = Cr(III), Rh(III)) react with iodide ions according to the stoichiometry L(H(2)O)MOO(2+) + 3I(-) + 3H(+) --> L(H(2)O)MOH(2+) + 1.5I(2) + H(2)O. The rate law is -d[L(H(2)O)MOO(2+)]/dt = k [L(H(2)O)MOO(2+)][I(-)][H(+)], where k = 93.7 M(-2) s(-1) for Cr(aq)OO(2+), 402 for ([14]aneN(4))(H(2)O)CrOO(2+), and 888 for (NH(3))(4)(H(2)O)RhOO(2+) in acidic aqueous solutions at 25 degrees C and 0.50 M ionic strength. The Cr(aq)OO(2+)/I(-) reaction exhibits an inverse solvent kinetic isotope effect, k(H)()2(O)/k(D)2(O) = 0.5. In the proposed mechanism, the protonation of the superoxo complex precedes the reaction with iodide. The related Cr(aq)OOH(2+)/I(-) reaction has k(H)2(O)/k(D)2(O) = 0.6. The oxidation of (NH(3))(5)Rupy(2+) by Cr(aq)OO(2+) exhibits an [H(+)]-dependent pathway, rate = (7.0 x 10(4) + 1.78 x 10(5)[H(+)])[Ru(NH(3))(5)py(2+)][Cr(aq)OO(2+)]. Diiodine radical anions, I(2)(*)(-), reduce Cr(aq)OO(2+) with a rate constant k = 1.7 x 10(9) M(-1) s(-1).     

Oxidative homolysis of a nitrosylchromium complex

Metal(III)-polypyridine complexes [M(NN)(3)](3+) (M = Ru or Fe; NN = bipyridine (bpy), phenanthroline (phen), or 4,7-dimethylphenanthroline (Me(2)-phen)) oxidize the nitrosylpentaaquachromium(III) ion, [Cr(aq)NO](2+), with an overall 4:1 stoichiometry, 4 [Ru(bpy)(3)](3+) + [Cr(aq)NO](2+) + 2 H(2)O --> 4 [Ru(bpy)(3)](2+) + [Cr(aq)](3+) + NO(3)(-) + 4 H(+). The kinetics follow a mixed second-order rate law, -d[[M(NN)(3)](3+)]/dt = nk[[M(NN)(3)](3+)][[Cr(aq)NO](2+)], in which k represents the rate constant for the initial one-electron transfer step, and n = 2-4 depending on reaction conditions and relative rates of the first and subsequent steps. With [Cr(aq)NO](2+) in excess, the values of nk are 283 M(-1) s(-1) ([Ru(bpy)(3)](3+)), 7.4 ([Ru(Me(2)-phen)(3)](3+)), and 5.8 ([Fe(phen)(3)](3+)). In the proposed mechanism, the one-electron oxidation of [Cr(aq)NO](2+) releases NO, which is further oxidized to nitrite, k = 1.04x10(6) M(-1) s(-1), 6.17x10(4), and 1.12x10(4) with the three respective oxidants. Further oxidation yields the observed nitrate. The kinetics of the first step show a strong correlation with thermodynamic driving force. Parallels were drawn with oxidative homolysis of a superoxochromium(III) ion, [Cr(aq)OO](2+), to gain insight into relative oxidizability of coordinated NO and O(2), and to address the question of the "oxidation state" of coordinated NO in [Cr(aq)NO](2+).     

Disproportionation of aquachromyl(IV) ion by hydrogen abstraction from coordinated water

In aqueous solutions, the aquachromyl(IV) ion, Cr(aq)O(2+), disproportionates to Cr(aq)(3+) and HCrO(4)(-). The reaction exhibits second-order kinetics with an inverse [H(+)] dependence, -d[Cr(aq)O(2+)]/dt = 38.8[Cr(aq)O(2+)](2)[H(+)](-1) at 25 degrees C. The combination of the rate law and substantial kinetic isotope effect, k(H)/k(D) = 6.9, suggests a mechanism whereby a hydrogen atom is abstracted from a coordinated molecule of water or hydroxo group within a singly deprotonated transition state. The buildup of chromate is more complicated and somewhat slower than the loss of chromyl, suggesting the involvement of intermediates.     

Activation energy for the disproportionation of HBrO2 and estimated heats of formation of HBrO2 and BrO2

The kinetics of the reaction HBrO(2) + HBrO(2) --> HOBr + BrO(3)(-) + H(+) is investigated in aqueous HClO(4) (0.04-0.9 M) and H(2)SO(4) (0.3-0.9 M) media and at temperatures in the range 15-38 degrees C. The reaction is found to be cleanly second order in [HBrO(2)], with the experimental rate constant having the form k(exp) = k + k'[H(+)]. The half-life of the reaction is on the order of a few tenths of a second in the range 0.01 M < [HBrO(2)](0) < 0.02 M. The detailed mechanism of this reaction is discussed. The activation parameters for kare found to be E(double dagger) = 19.0 +/- 0.9 kJ/mol and DeltaS(double dagger) = -132 +/- 3 J/(K mol) in HClO(4), and E(double dagger) = 23.0 +/- 0.5 kJ/mol and DeltaS(double dagger) = -119 +/- 1 J/(K mol) in H(2)SO(4). The activation parameters for k' are found to be E(double dagger) = 25.8 +/- 0.5 kJ/mol and DeltaS(double dagger) = -106 +/- 1 J/(K mol) in HClO(4), and E(double dagger) = 18 +/- 3 kJ/mol and DeltaS(double dagger) = -130 +/- 11 J/(K mol) in H(2)SO(4). The values Delta(f)H(29)(8)(0)[BrO(2)(aq)] = 157 kJ/mol and Delta(f)H(29)(8)(0)[HBrO(2)(aq)] = -33 kJ/mol are estimated using a trend analysis (bond strengths) based on the assumption Delta(f)H(29)(8)(0)[HBrO(2)(aq)] lies between Delta(f)H(29)(8)(0)[HOBr(aq)] and Delta(f)H(29)(8)(0)[HBrO(3)(aq)] as Delta(f)H(29)(8)(0)[HClO(2)(aq)] lies between Delta(f)H(29)(8)(0)[HOCl(aq)] and Delta(f)H(29)(8)(0)[HClO(3)(aq)]. The estimated value of Delta(f)H(29)(8)(0)[BrO(2)(aq)] agrees well with calculated gas-phase values, but the estimated value of Delta(f)H(29)(8)(0)[HBrO(2)(aq)], as well as the tabulated value of Delta(f)H(29)(8)(0)[HClO(2)(aq)], is in substantial disagreement with calculated gas-phase values. Values of Delta(r)H(0) are estimated for various reactions involving BrO(2) or HBrO(2).     

Reaction of a superoxochromium(III) ion with nitrogen monoxide: kinetics and mechanism.

The kinetics of the rapid reaction between Cr(aq)OO(2+) and NO were determined by laser flash photolysis of Cr(aq)NO(2+) in O(2)-saturated acidic aqueous solutions, k = 7 x 10(8) M(-1) s(-1) at 25 degrees C. The reaction produces an intermediate, believed to be NO(2), which was scavenged with ([14]aneN(4))Ni(2+). With limiting NO, the Cr(aq)OO(2+)/NO reaction has a 1:1 stoichiometry and produces both free NO(3)(-) and a chromium nitrato complex, Cr(aq)ONO(2)(2+). In the presence of excess NO, the stoichiometry changes to [NO]/[Cr(aq)OO(2+)] = 3:1, and the reaction produces close to 3 mol of nitrite/mol of Cr(aq)OO(2+). An intermediate, identified as a nitritochromium(III) ion, Cr(aq)ONO(2+), is a precursor to a portion of free NO(2)(-). In the proposed mechanism, the initially produced peroxynitrito complex, Cr(aq)OONO(2+), undergoes O-O bond homolysis followed by some known and some novel chemistry of Cr(aq)O(2+) and NO(2). The reaction between Cr(aq)O(2+) and NO generates Cr(aq)ONO(2+), k > 10(4) M(-1) s(-1). Cr(aq)OO(2+) reacts with NO(2) with k = 2.3 x 10(8) M(-1) s(-1).     

Generation of a peroxynitrato metal complex from nitrogen dioxide and coordinated superoxide

The reaction between photogenerated NO(2) radicals and a superoxochromium(III) complex, Cr(aq)OO(2+), occurs with rate constants k(Cr)(20) = (2.8 +/- 0.2) x 10(8) M(-)(1) s(-)(1) (20 vol % acetonitrile in water) and k(Cr)(40) = (2.6 +/- 0.5) x 10(8) M(-)(1) s(-)(1) (40 vol % acetonitrile) in aerated acidic solutions and ambient temperature. The product was deduced to be a peroxynitrato complex, Cr(aq)OONO(2)(2+), which undergoes homolytic cleavage of an N-O bond to return to the starting materials, the rate constants in the two solvent mixtures being k(H)(20) = 172 +/- 4 s(-)(1) and k(H)(40) = 197 +/- 7 s(-)(1). NO(2) reacts rapidly with 10-methyl-9,10-dihydroacridine, k(A)(20) = 2.2 x 10(7) M(-)(1) s(-)(1), k(A)(40) = (9.4 +/- 0.2) x 10(6) M(-)(1) s(-)(1), and with N,N,N',N'-tetramethylphenylenediamine, k(T)(40) = (1.84 +/- 0.03) x 10(8) M(-)(1) s(-)(1).     

Equilibrium and Redox Kinetics of Copper(II)-Thiourea Complexes

Stopped-flow spectrophotometric measurements identify and determine equilibrium data for thiourea (tu) complexes of copper(II) formed in aqueous solution. In excess Cu(II), the complex ion [Cu(tu)](2+) has a stability constant beta(1) = 2.3 +/- 0.1 M(-)(1) and molar absorptivity at 340 nm of epsilon(1) = (4.0 +/- 0.2) x 10(3) M(-)(1) cm(-)(1) at 25.0 degrees C, 2.48 mM HClO(4), and &mgr; = 464 mM (NaClO(4)). The fast reduction of Cu(II) by excess tu obeys the rate law -d[Cu(II)]/dt = k'[Cu(II)](2)[tu](7) with a value for the ninth-order rate constant k' = (1.60 +/- 0.18) x 10(14) M(-)(8) s(-)(1), which derives from a rate-determining step involving the bimolecular decomposition of two complexed Cu(II) species. Copper(II) catalyzes the reduction of hexachloroiridate(IV) by tu according to the rate law -d[IrCl(6)(2)(-)]/dt = (k(2,unc)[tu](2) + k(1,cat) [tu](5)[Cu(II)])[IrCl(6)(2)(-)]. Least-squares analysis yields values of k(2,unc) and k(1,cat) equaling 385 +/- 4 M(-)(2) s(-)(1) and (3.7 +/- 0.1) x 10(13) M(-)(6) s(-)(1), respectively, at &mgr; = 115 mM (NaClO(4)). The corresponding mechanism has a rate-determining step that involves the oxidation of [Cu(II)(tu)(5)](2+) by [IrCl(6)](2)(-) rather than the bimolecular reaction of two cupric-tu complexes.     

Kinetics and mechanism of chromate reduction with hydrogen peroxide in base

[Cr(VI)O(4)](2)(-) is reduced to [Cr(V)(O(2))(4)](3)(-) by hydrogen peroxide in strongly basic media where the acid dissociation of H(2)O(2) (pK(a) = 11.65) is appreciable. The reaction is first order in chromium(VI) and inhibited by hydroxide. The hydrogen peroxide dependence is defined by the form of the effective pseudo-first-order rate constant: k(eff) = [H(2)O(2)](3)/(K(1) + K(2)[H(2)O(2)] + K(3)[HO(2)(-)]) with K(1) = 175(43) s x M(3), K(2) = 403(18) s x M(2), and K(3) = 1422(34) s x M(2). Hydrogen peroxide anion initially attacks chromate, and subsequent equilibrium steps that exchange oxo groups for three peroxo groups precede a rate-determining, one-electron, intramolecular reduction step.     

Aqueous ferryl(IV) ion: kinetics of oxygen atom transfer to substrates and oxo exchange with solvent water

The aqueous iron(IV) ion, Fe(IV)(aq)O(2+), generated from O(3) and Fe(aq)(2+), reacts rapidly with various oxygen atom acceptors (sulfoxides, a water-soluble triarylphosphine, and a thiolatocobalt complex). In each case, Fe(IV)(aq)O(2+) is reduced to Fe(aq)(2+), and the substrate is oxidized to a product expected for oxygen atom transfer. Competition methods were used to determine the kinetics of these reactions, some of which have rate constants in excess of 10(7) M(-1) s(-1). Oxidation of dimethyl sulfoxide (DMSO) has k = 1.26 x 10(5) M(-1) s(-1) and shows no deuterium kinetic isotope effect, k(DMSO-d(6)) = 1.23 x 10(5) M(-1) s(-1). The Fe(IV)(aq)O(2+)/sulfoxide reaction is the product-forming step in a very efficient Fe(aq)(2+)-catalyzed oxidation of sulfoxides by ozone. This catalytic cycle, combined with labeling experiments in H(2)(18)O, was used to determine the rate constant for the oxo-group exchange between Fe(IV)(aq)O(2+) and solvent water under acidic conditions, k(exch) = 1.4 x 10(3) s(-1).     

Ligand effect on the kinetics of hydroperoxochromium(III)-oxochromium(V) transformation and the lifetime of chromium(V)

A macrocyclic superoxochromium complex L(2)(H(2)O)CrOO(2+)(L(2)=meso-Me(6)-[14]aneN(4)) is generated from L(2)Cr(H(2)O)(2)(2+) and O(2) with k(on)=(2.80 +/- 0.07)x 10(7) M(-1) s(-1). One-electron reduction of L(2)(H(2)O)CrOO(2+) produces a transient hydroperoxo complex that readily undergoes intramolecular conversion to L(2)Cr(v), k(1)= 1.00 +/- 0.01 s(-1) in acidic aqueous solutions, and 0.273 +/- 0.010 s(-1) at pH >7, with an apparent pK(a) of 5.9. The decay of L(2)Cr(v) in the pH range 1.3-6.2 obeys the rate law, -d[L(2)Cr(v)]/dt= (0.0080 (+/- 0.0049)+ 8.19 (+/- 0.13)[H(+)])[L(2)Cr(v)]. Both the kinetics of formation and lifetime of L(2)Cr(v) are significantly different from those for the closely related [14]aneN(4) complex. The X-ray structure of the parent Cr(iii) complex, [L(2)Cr(H(2)O)(2)](ClO(4))(3).4H(2)O, shows that the macrocyclic ligand adopts the most stable, "two up-two down" configuration around the nitrogens.     

Oxidation of thioglycolate by [Os(phen)3]3+: an unusual example of redox-mediated aromatic substitution

The aqueous oxidation of thioglycolic acid (TGA) by [Os(phen)(3)](3+) (phen = 1,10-phenanthroline) is catalyzed by traces of ubiquitous Cu(2+) and inhibited by the product [Os(phen)(3)](2+). In the presence of dipicolinic acid (dipic), which thoroughly masks trace Cu(2+) catalysis, and spin trap PBN, the kinetics under anaerobic conditions have been studied in the pH range 1.82-7.32. The rate law is -d[Os(phen)(3)(3+)]/dt = k[TGA](tot)[Os(phen)(3)(3+)], with k = 2{(k(b)K(a1) + k(c)K(a1)K(i))[H(+)] + k(d)K(a1)K(a2)}/{[H(+)](2) + K(a1)[H(+)] + K(a1)K(a2)}; K(a1) and K(a2) are the successive acid dissociation constants of TGA, and K(i) is the tautomerization constant of two TGA monoanions. k(b) + k(c)K(i) = (5.9 +/- 0.3) x 10(3) M(-)(1) s(-)(1), k(d) = (1.6 +/- 0.1) x 10(9) M(-)(1) s(-)(1) at mu = 0.1 M (NaCF(3)SO(3)) and 25 degrees C. The major products in the absence of spin traps are dithiodiglycolic acid, [Os(phen)(3)](2+), and [Os(phen)(2)(phen-tga)](2+), where phen-tga is phenanthroline with a TGA substituent. A mechanism is proposed in which neutral TGA is unreactive, the (minor) thiolate form of the TGA monoanion undergoes one-electron oxidation by [Os(phen)(3)](3+) (k(c)), and the dianion of TGA likewise undergoes one-electron oxidation by [Os(phen)(3)](3+) (k(d)). The Marcus cross relationship provides a good account for the magnitude of k(d) in this and related reactions of TGA. [Os(phen)(2)(phen-tga)](2+) is suggested to arise from a post-rate-limiting step involving attack of the TGA(*) radical on [Os(phen)(3)](3+).     

Kinetics of Water Exchange on the Dihydroxo-Bridged Rhodium(III) Hydrolytic Dimer

()()Conventional (18)O isotopic labeling techniques have been used to measure the water exchange rates on the Rh(III) hydrolytic dimer [(H(2)O)(4)Rh(&mgr;-OH)(2)Rh(H(2)O)(4)](4+) at I = 1.0 M for 0.08 < [H(+)] < 0.8 M and temperatures between 308.1 and 323.1 K. Two distinct pathways of water exchange into the bulk solvent were observed (k(fast) and k(slow)) which are proposed to correspond to exchange of coordinated water at positions cis and trans to bridging hydroxide groups. This proposal is supported by (17)O NMR measurements which clearly showed that the two types of water ligands exchange at different rates and that the rates of exchange matched those from the (18)O labeling data. No evidence was found for the exchange of label in the bridging OH groups in either experiment. This contrasts with findings for the Cr(III) dimer. The dependence of both k(fast) and k(slow) on [H(+)] satisfied the expression k(obs) = (k(O)[H(+)](tot) +k(OH)K(a1))/([H(+)](tot) + K(a1)) which allows for the involvement of fully protonated and monodeprotonated Rh(III) dimer. The following rates and activation parameters were determined at 298 K. (i) For fully protonated dimer: k(fast) = 1.26 x 10(-)(6) s(-)(1) (DeltaH() = 119 +/- 4 kJ mol(-)(1) and DeltaS() = 41 +/- 12 J K(-)(1) mol(-)(1)) and k(slow) = 4.86 x 10(-)(7) s(-)(1) (DeltaH() = 64 +/- 9 kJ mol(-)(1) and DeltaS() = -150 +/- 30 J K(-)(1) mol(-)(1)). (ii) For monodeprotonated dimer: k(fast) = 3.44 x 10(-)(6) s(-)(1) (DeltaH() = 146 +/- 4 kJ mol(-)(1) and DeltaS() = 140 +/- 11 J K(-)(1) mol(-)(1)) and k(slow) = 2.68 x 10(-)(6) s(-)(1) (DeltaH() = 102 +/- 3 kJ mol(-)(1) and DeltaS() = -9 +/- 11 J K(-)(1) mol(-)(1)). Deprotonation of the Rh(III) dimer was found to labilize the primary coordination sphere of the metal ions and thus increase the rate of water exchange at positions cis and trans to bridging hydroxides but not to the same extent as for the Cr(III) dimer. Activation parameters and mechanisms for ligand substitution processes on the Rh(III) dimer are discussed and compared to those for other trivalent metal ions and in particular the Cr(III) dimer.     

Equilibrium and Kinetics of Bromine Hydrolysis

Equilibrium constants for bromine hydrolysis, K(1) = [HOBr][H(+)][Br(-)]/[Br(2)(aq)], are determined as a function of ionic strength (&mgr;) at 25.0 degrees C and as a function of temperature at &mgr; approximately 0 M. At &mgr; approximately 0 M and 25.0 degrees C, K(1) = (3.5 +/- 0.1) x 10(-)(9) M(2) and DeltaH degrees = 62 +/- 1 kJ mol(-)(1). At &mgr; = 0.50 M and 25.0 degrees C, K(1) = (6.1 +/- 0.1) x 10(-)(9) M(2) and the rate constant (k(-)(1)) for the reverse reaction of HOBr + H(+) + Br(-) equals (1.6 +/- 0.2) x 10(10) M(-)(2) s(-)(1). This reaction is general-acid-assisted with a Br?nsted alpha value of 0.2. The corresponding Br(2)(aq) hydrolysis rate constant, k(1), equals 97 s(-)(1), and the reaction is general-base-assisted (beta = 0.8).     

Encapsulation of cyanometalates by a tris-macrocyclic ligand tricopper(II) complex: syntheses, structural variation, and magnetic exchange coupling pathways

The reaction of [M(CN)(6)](3-) (M = Cr(3+), Mn(3+), Fe(3+), Co(3+)) and [M(CN)(8)](4-/3-) (M = Mo(4+/5+), W(4+/5+)) with the trinuclear copper(II) complex of 1,3,5-triazine-2,4,6-triyltris[3-(1,3,5,8,12-pentaazacyclotetradecane)] ([Cu(3)(L)](6+)) leads to partially encapsulated cyanometalates. With hexacyanometalate(III) complexes, [Cu(3)(L)](6+) forms the isostructural host-guest complexes [[[Cu(3)(L)(OH(2))(2)][M(CN)(6)](2)][M(CN)(6)]][M(CN)(6)]30 H(2)O with one bridging, two partially encapsulated, and one isolated [M(CN)(6)](3-) unit. The octacyanometalates of Mo(4+/5+) and W(4+/5+) are encapsulated by two tris-macrocyclic host units. Due to the stability of the +IV oxidation state of Mo and W, only assemblies with [M(CN)(8)](4-) were obtained. The Mo(4+) and W(4+) complexes were crystallized in two different structural forms: [[Cu(3)(L)(OH(2))](2)[Mo(CN)(8)]](NO(3))(8)15 H(2)O with a structural motif that involves isolated spherical [[Cu(3)(L)(OH(2))](2)[M(CN)(8)]](8+) ions and a "string-of-pearls" type of structure [[[Cu(3)(L)](2)[M(CN)(8)]][M(CN)(8)]](NO(3))(4) 20 H(2)O, with [M(CN)(8)](4-) ions that bridge the encapsulated octacyanometalates in a two-dimensional network. The magnetic exchange coupling between the various paramagnetic centers is characterized by temperature-dependent magnetic susceptibility and field-dependent magnetization data. Exchange between the CuCu pairs in the [Cu(3)(L)](6+) "ligand" is weakly antiferromagnetic. Ferromagnetic interactions are observed in the cyanometalate assemblies with Cr(3+), exchange coupling of Mn(3+) and Fe(3+) is very small, and the octacoordinate Mo(4+) and W(4+) systems have a closed-shell ground state.     

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.     

Kinetics and mechanism of the reduction of a macrocyclic Rh(III) complex by chromium(II) ions: pH-controlled selectivity to rhodium(II) vs. rhodium(III) hydride

Aqueous chromium(II) ions reduce a macrocyclic Rh(III) complex L(1)(H(2)O)(2)Rh(3+) (L(1) = 1,4,8,11-tetraazacyclotetradecane) to the hydride L(1)(H(2)O)RhH(2+) in two discrete, one-electron steps. The first step generates L(1)(H(2)O)Rh(2+) with kinetics that are first order in each rhodium(III) complex and Cr(H(2)O)(6)(2+), and inverse in [H(+)], k/M(-1) s(-1) = 0.065/(0.0031 + [H(+)]). Further reduction of L(1)(H(2)O)Rh(2+) to L(1)(H(2)O)RhH(2+) is kinetically independent of [H(+)], k/M(-1) s(-1) = 0.30. The difference in [H(+)] dependence allows relative rates of the two steps to be manipulated to generate either L(1)(H(2)O)Rh(2+) or L(1)(H(2)O)RhH(2+) as the final product.     

Kinetics and mechanisms of the oxidation of iodide and bromide in aqueous solutions by a trans-dioxoruthenium(VI) complex

The kinetics and mechanisms of the oxidation of I (-) and Br (-) by trans-[Ru (VI)(N 2O 2)(O) 2] (2+) have been investigated in aqueous solutions. The reactions have the following stoichiometry: trans-[Ru (VI)(N 2O 2)(O) 2] (2+) + 3X (-) + 2H (+) --> trans-[Ru (IV)(N 2O 2)(O)(OH 2)] (2+) + X 3 (-) (X = Br, I). In the oxidation of I (-) the I 3 (-)is produced in two distinct phases. The first phase produces 45% of I 3 (-) with the rate law d[I 3 (-)]/dt = ( k a + k b[H (+)])[Ru (VI)][I (-)]. The remaining I 3 (-) is produced in the second phase which is much slower, and it follows first-order kinetics but the rate constant is independent of [I (-)], [H (+)], and ionic strength. In the proposed mechanism the first phase involves formation of a charge-transfer complex between Ru (VI) and I (-), which then undergoes a parallel acid-catalyzed oxygen atom transfer to produce [Ru (IV)(N 2O 2)(O)(OHI)] (2+), and a one electron transfer to give [Ru (V)(N 2O 2)(O)(OH)] (2+) and I (*). [Ru (V)(N 2O 2)(O)(OH)] (2+) is a stronger oxidant than [Ru (VI)(N 2O 2)(O) 2] (2+) and will rapidly oxidize another I (-) to I (*). In the second phase the [Ru (IV)(N 2O 2)(O)(OHI)] (2+) undergoes rate-limiting aquation to produce HOI which reacts rapidly with I (-) to produce I 2. In the oxidation of Br (-) the rate law is -d[Ru (VI)]/d t = {( k a2 + k b2[H (+)]) + ( k a3 + k b3[H (+)]) [Br (-)]}[Ru (VI)][Br (-)]. At 298.0 K and I = 0.1 M, k a2 = (2.03 +/- 0.03) x 10 (-2) M (-1) s (-1), k b2 = (1.50 +/- 0.07) x 10 (-1) M (-2) s (-1), k a3 = (7.22 +/- 2.19) x 10 (-1) M (-2) s (-1) and k b3 = (4.85 +/- 0.04) x 10 (2) M (-3) s (-1). The proposed mechanism involves initial oxygen atom transfer from trans-[Ru (VI)(N 2O 2)(O) 2] (2+) to Br (-) to give trans-[Ru (IV)(N 2O 2)(O)(OBr)] (+), which then undergoes parallel aquation and oxidation of Br (-), and both reactions are acid-catalyzed.     

Oxidation of [Ru(NH3)5isn](BF4)2 by hyochlorous acid and chlorine in aqueous acidic media

UV-vis stopped-flow studies of the reaction of [Ru(NH3)5isn](2+) (isn = isonicotinamide) with excess HOCl at 25 degrees C demonstrate that it proceeds in two time-resolved steps. In the first step [Ru(NH3)5isn](3+) is produced with the rate law -d[Ru(II)]/dt = 2(aK(h)[H(+)] + b[H(+)][Cl(-)] + c[Cl(-)])[HOCl](tot)[Ru(II)]/(K(h) + [H(+)][Cl(-)]). Here, K(h) is 1.3 x 10(-3) M(2) and corresponds to the equilibrium hydrolysis of Cl2, a is (8.34 +/- 0.19) x 10(3) M(-2) s(-1) and represents the acid-assisted reduction of HOCl, b is (4.04 +/- 0.13) x 10(4) M(-1) s(-1) and represents the reduction of Cl2, and c is (6.25 +/- 0.59) x 10(2) s(-1) and represents the Cl(-)-assisted reduction of HOCl. In the second step [Ru(NH3)5isn](3+) undergoes further oxidation to a mixture of products with the rate law -d[Ru(III)]/dt = e[Ru(III)][HOCl]/[H(+)] where e is (1.18 +/- 0.01) x 10(-2) s(-1). This step is assigned a mechanism with Cl(+) transfer from HOCl to [Ru(III)(NH3)4(NH2)isn](2+) occurring in the rate-limiting step. These results underline the resistance of HOCl to act as a simple outer-sphere one-electron oxidant.     

Generation and reactivity of rhodium(IV) complexes in aqueous solutions

At pH = 1 and 25 degrees C, the Fenton-like reactions of Fe(aq)(2+) with hydroperoxorhodium complexes LRh(III)OOH(2+) (L = (H(2)O)(NH(3))(4), k = 30 M(-1) s(-1), and L = L(2) = (H(2)O)(meso-Me(6)-[14]aneN(4)), k = 31 M(-1) s(-1)) generate short-lived, reactive intermediates, believed to be the rhodium(IV) species LRh(IV)O(2+). In the rapid follow-up steps, these transients oxidize Fe(aq)(2+), and the overall reaction has the standard 2:1 [Fe(aq)(2+)]/[LRhOOH(2+)] stoichiometry. Added substrates, such as alcohols, aldehydes, and (NH(3))(4)(H(2)O)RhH(2+), compete with Fe(aq)(2+) for LRh(IV)O(2+), causing the stoichiometry to change to <2:1. Such competition data were used to determine relative reactivities of (NH(3))(4)RhO(2+) toward CH(3)OH (1), CD(3)OH (0.2), C(2)H(5)OH (2.7), 2-C(3)H(7)OH (3.4), 2-C(3)D(7)OH (1.0), CH(2)O (12.5), C(2)H(5)CHO (45), and (NH(3))(4)RhH(2+) (125). The kinetics and products suggest hydrogen atom abstraction for (NH(3))(4)RhO(2+)/alcohol reactions. A short chain reaction observed with C(2)H(5)CHO is consistent with both hydrogen atom and hydride transfer. The rate constant for the reaction between Tl(aq)(III) and L(2)Rh(2+) is 2.25 x 10(5) M(-1) s(-1).