Oleg Pestovsky

pH-induced mechanistic changeover from hydroxyl radicals to iron(IV) in the Fenton reaction

A major pathway in the reaction between Fe(II) and H2O2 at pH 6–7 in non-coordinating buffers exhibits inverse kinetic dependence on [H+] and leads to oxidation of dimethyl sulfoxide (DMSO) to dimethyl sulfone (DMSO2). This step regenerates Fe(II) and makes the oxidation of DMSO catalytic, a finding that strongly supports Fe(IV) as a Fenton intermediate at near-neutral pH. This Fe(IV) is a less efficient oxidant for DMSO at pH 6–7 than is (H2O)5FeO2+, generated by ozone oxidation of Fe(H2O)62+, in acidic solutions. Large concentrations of DMSO are needed to achieve significant turnover numbers at pH ≥ 6 owing to the rapid competing reaction between Fe(II) and Fe(IV) that leads to irreversible loss of the catalyst. At pH 6 and ≤0.02 mM Fe(II), the ratio of apparent rate constants for the reactions of Fe(IV) with DMSO and with Fe(II) is ∼104. The results at pH 6–7 stand in stark contrast with those reported previously in acidic solutions where the Fenton reaction generates hydroxyl radicals. Under those conditions, DMSO is oxidized stoichiometrically to methylsulfinic acid and ethane. This path still plays a role (1–10%) at pH 6–7.

Oxidation of a water-soluble phosphine and some spectroscopic probes with nitric oxide and nitrous acid in aqueous solutions.

In acidic aqueous solutions, nitrogen monoxide oxidizes monosulfonated triphenylphosphine, TPPMS(-), to the corresponding phosphine oxide. The NO-derived product is N(2)O. This chemistry parallels that reported for the reaction of NO with the unsubstituted triphenylphosphine in nonpolar organic solvents, but the rate constant measured in this work, 5.14 x 10(6) M(-2) s(-1), is greater by several orders of magnitude. This makes TPPMS(-) a useful analytical reagent for NO in aqueous solution. The increased rate constant in the present work appears to be a medium effect, and unrelated to the introduction of a single sulfonate group in the phosphine. The reaction between nitrous acid and TPPMS(-) has a 2:1 [TPPMS(-)]/[HNO(2)] stoichiometry and generates NH(2)OH quantitatively. The rate law, rate = 4k(d)[HNO(2)](2)[TPPMS(-)], identifies the second-order self-reaction of HNO(2) as the rate-limiting step that generates the active oxidant(s) for the fast subsequent reaction with TPPMS(-). It appears that the active oxidant is N(2)O(3), although the oxides NO and NO(2) derived from it may be also involved. Bimolecular self-reaction of HNO(2) also precedes the oxidations of ABTS(2-) and TMPD. Competing with this path are the acid-catalyzed oxidations of both reagents via NO(+).

Synthesis of Monomeric Fe(II) and Ru(II) Complexes of Tetradentate Phosphines

rac-Bis[{(diphenylphosphino)ethyl}-phenylphosphino]methane (DPPEPM) reacts with iron(II) and ruthenium(II) halides to generate complexes with folded DPPEPM coordination. The paramagnetic, five-coordinate Fe(DPPEPM)Cl(2) (1) in CD(2)Cl(2) features a tridentate binding mode as established by (31)P{(1)H} NMR spectroscopy. Crystal structure analysis of the analogous bromo complex, Fe(DPPEPM)Br(2) (2) revealed a pseudo-octahedral, cis-α geometry at iron with DPPEPM coordinated in a tetradentate fashion. However, in CD(2)Cl(2) solution, the coordination of DPPEPM in 2 is similar to that of 1 in that one of the external phosphorus atoms is dissociated resulting in a mixture of three tridentate complexes. The chloro ruthenium complex cis-Ru(κ(4)-DPPEPM)Cl(2) (3) is obtained from rac-DPPEPM and either [RuCl(2)(COD)](2) [COD = 1,5-cyclooctadiene] or RuCl(2)(PPh(3))(4). The structure of 3 in both the solid state and in CD(2)Cl(2) solution features a folded κ(4)-DPPEPM. This binding mode was also observed in cis-[Fe(κ(4)-DPPEPM)(CH(3)CN)(2)](CF(3)SO(3))(2) (4). Addition of an excess of CO to a methanolic solution of 1 results in the replacement of one of the chloride ions by CO to yield cis-[Fe(κ(4)-DPPEPM)Cl(CO)](Cl) (5). The same reaction in CH(2)Cl(2) produces a mixture of 5 and [Fe(κ(3)-DPPEPM)Cl(2)(CO)] (6) in which one of the internal phosphines has been substituted by CO. Complexes 2, 3, 4, and 5 appear to be the first structurally characterized monometallic complexes of κ(4)-DPPEPM.

Electron transfer reactivity of the aqueous iron(IV)–oxo complex. Outer-sphere vs proton-coupled electron transfer

The kinetics of oxidation of organic and inorganic reductants by aqueous iron(IV) ions, Fe(IV)(H2O)5O(2+) (hereafter Fe(IV)aqO(2+)), are reported. The substrates examined include several water-soluble ferrocenes, hexachloroiridate(III), polypyridyl complexes M(NN)3(2+) (M = Os, Fe and Ru; NN = phenanthroline, bipyridine and derivatives), HABTS(-)/ABTS(2-), phenothiazines, Co(II)(dmgBF2)2, macrocyclic nickel(II) complexes, and aqueous cerium(III). Most of the reductants were oxidized cleanly to the corresponding one-electron oxidation products, with the exception of phenothiazines which produced the corresponding oxides in a single-step reaction, and polypyridyl complexes of Fe(II) and Ru(II) that generated ligand-modified products. Fe(IV)aqO(2+) oxidizes even Ce(III) (E(0) in 1 M HClO4 = 1.7 V) with a rate constant greater than 10(4) M(-1) s(-1). In 0.10 M aqueous HClO4 at 25 °C, the reactions of Os(phen)3(2+) (k = 2.5 × 10(5) M(-1) s(-1)), IrCl6(3-) (1.6 × 10(6)), ABTS(2-) (4.7 × 10(7)), and Fe(cp)(C5H4CH2OH) (6.4 × 10(7)) appear to take place by outer sphere electron transfer (OSET). The rate constants for the oxidation of Os(phen)3(2+) and of ferrocenes remained unchanged in the acidity range 0.05 < [H(+)] < 0.10 M, ruling out prior protonation of Fe(IV)aqO(2+) and further supporting the OSET assignment. A fit to Marcus cross-relation yielded a composite parameter (log k22 + E(0)Fe/0.059) = 17.2 ± 0.8, where k22 and E(0)Fe are the self-exchange rate constant and reduction potential, respectively, for the Fe(IV)aqO(2+)/Fe(III)aqO(+) couple. Comparison with literature work suggests k22 < 10(-5) M(-1) s(-1) and thus E(0)(Fe(IV)aqO(2+)/Fe(III)aqO(+)) > 1.3 V. For proton-coupled electron transfer, the reduction potential is estimated at E(0) (Fe(IV)aqO(2+), H(+)/Fe(III)aqOH(2+)) ≥ 1.95 V.

Mechanistic study of the co-ordination of hydrogen peroxide to methylrhenium trioxide

The activation parameters (ΔH‡, ΔS‡, ΔV‡) for the co-ordination of hydrogen peroxide to methylrhenium trioxide have been determined. They indicate a mechanism involving nucleophilic attack. The protons lost in converting H2O2 to a co-ordinated η2-O22– group are transferred to one oxide oxygen, which remains on the metal as an aqua ligand. The rate of reaction is not pH dependent, consistent with the deuterium kinetic isotope effect (kH/kD= 2.8). The method used to study the reaction is based on the ability of ReMeO3 to catalyse the reaction between Br– and H2O2. The activation parameters for the uncatalysed reaction of Br– and H2O2 were also determined. The value found for ΔV‡ is consistent with the accepted mechanism, proton-assisted nucleophilic displacement.

Kinetics and Mechanism of Hydrogen‐Atom Abstraction from Rhodium Hydrides by Alkyl Radicals in Aqueous Solutions

The kinetics of the reaction of benzyl radicals with [L(1)(H(2)O)RhH{D}](2+) (L(1)=1,4,8,11-tetraazacyclotetradecane) were studied directly by laser-flash photolysis. The rate constants for the two isotopologues, k=(9.3±0.6) × 10(7) M(-1) s(-1) (H) and (6.2±0.3) × 10(7) M(-1) s(-1) (D), lead to a kinetic isotope effect k(H)/k(D)=1.5±0.1. The same value was obtained from the relative yields of PhCH(3) and PhCH(2)D in a reaction of benzyl radicals with a mixture of rhodium hydride and deuteride. Similarly, the reaction of methyl radicals with {[L(1)(H(2)O)RhH](2+) + [L(1)(H(2)O)RhD](2+)} produced a mixture of CH(4) and CH(3)D that yielded k(H)/k(D)=1.42±0.07. The observed small normal isotope effects in both reactions are consistent with reduced sensitivity to isotopic substitution in very fast hydrogen-atom abstraction reactions. These data disprove a literature report claiming much slower kinetics and an inverse kinetic isotope effect for the reaction of methyl radicals with hydrides of L(1)Rh.

Oxygen activation by a macrocyclic chromium complex. Mechanism of hydroperoxo-chromium(III) to oxo-chromium(V) transformation

The transformation of the hydroperoxo complex L1(H2O)CrOOH2+ (L1 = 1, 4 8, 11-tetraazacyclotetradecane) to an oxo-chromium(v) species is a first-order process throughout the pH range examined, 1.7 < pH < 9.2. The pH dependence of the rate constant (k1) yielded an apparent pKa of 5.6 for L1(H2O)CrOOH2+. In the acidic range, (pH <4), the value of k1 is 0.191 s(-1). At the other extreme, pH >7.5, k(1)= 0.025 s(-1). No [H+]-dependence is observed within the two limiting regimes, clearly ruling out a simple attack by H+ at the hydroperoxo group. The temperature dependence of k1 in 0.020 M HClO4 yielded the activation parameters DeltaH++ = 53.7 kJ mol(-1) and DeltaS++ = -80.5 J mol(-1) K(-1).

Kinetics and thermodynamics of nitric oxide binding to transition metal complexes. Relationship to dioxygen binding

The kinetics and activation parameters for ˙NO dissociation from L(H2O)Co(NO)2+ (L = L1 = cyclam and L2 = meso-Me6-cyclam), L2(H2O)Rh(NO)2+, and Cr(H2O)5NO2+ were determined in aqueous solution in the presence of IrCl62−, IrBr62−, or O2 as scavengers for ˙NO and/or metal(II) complexes. The rate constants k–NO at 25 °C are (5.7 ± 0.1) × 10−3 s−1 for L2(H2O)Co(NO)2+, (1.0 ± 0.1) × 10−4 for L1(H2O)Co(NO)2+, (5.9 ± 2.0) × 10−8 for Cr(H2O)5NO2+ and (3.5 ± 1.8) × 10−9 s−1 for L2(H2O)Rh(NO)2+. The kinetics of the reverse reaction were determined by laser flash photolysis at 25 °C, kNO = (1.7 ± 0.2) × 107 M−1 s−1 for L2Co(H2O)22+ and (1.8 ± 0.1) × 108 M−1 s−1 for L2(H2O)Rh2+. The rate constants kNO and k–NO obtained here and those available in the literature were used to calculate the equilibrium binding constants KNO for a series of nitrosyl metal complexes. A good correlation was found between log KNO and log KO2, the latter corresponding to O2 binding. The correlation includes complexes of four different transition metals as well as hydrogen atom, and covers a range of about 3 V in reduction potentials. The implications of these results are discussed.

Competing Coordination and Oxygen Atom Transfer in a Reaction Between Triarylphosphines and a Chromium Hydroperoxo Complex

There is a remarkable resemblance in the chemistry and intermediates involved in the autoxidation of a macrocyclic Cr(II) complex LCr(H2O)2 (L1 = 1,4,8,11-tetraazacyclotetradecane, or cyclam) and the activation of molecular oxygen by cytochrome P450 enzymes [1,2]. For every major step in the P450 chemistry there is a corresponding step in the chemistry of L1Cr complex, including the hydroperoxo to high-valent oxo conversion of eq 1. The kinetics of this transformation have been studied earlier [2,3].

Superoxometal-catalyzed co-oxidation of alcohols and nitrous acid with molecular oxygen

Abstract A superoxochromium(III) ion, Cr aq OO 2+ , acts as a catalyst for the co-oxidation of alcohols and nitrous acid with molecular oxygen according to the stoichiometry: CH 3 OH+HNO 2 +O 2 →CH 2 O + NO 3 − + H 2 O+H + . The kinetics are second order in [HNO 2 ] and independent of the concentrations of the superoxochromium catalyst, substrate, and O 2 . The proposed mechanism features the disproportionation of HNO 2 to NO and NO 2 , both of which react rapidly with Cr aq OO 2+ . The Cr aq OO 2+ /NO reaction generates another equivalent of NO 2 and a mole of Cr aq O 2+ , the active oxidant. The two-electron oxidation of the alcohol by Cr aq O 2+ produces Cr aq 2+ , which reacts rapidly with O 2 to regenerate the catalyst, Cr aq OO 2+ . The NO 2 /Cr aq OO 2+ reaction yields the peroxynitrato complex, Cr aq OONO 2 2+ , in a dead-end equilibrium process that has no effect on the catalytic reaction. The disproportionation of NO 2 yields the final nitrogen-containing product, NO 3 − , and regenerates an equivalent of HNO 2 . Under a fixed set of conditions, the relative catalytic efficiency (CE) of Cr aq OO 2+ decreases as its concentration increases owing to the competition between O 2 and Cr aq OO 2+ for the intermediate Cr aq 2+ .