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.

UV-visible spectroscopy of macrocyclic alkyl, nitrosyl and halide complexes of cobalt and rhodium. Experiment and calculation.

Transition metal complexes (NH3)5CoX(2+) (X = CH3, Cl) and L(H2O)MX(2+), where M = Rh or Co, X = CH3, NO, or Cl, and L is a macrocyclic N4 ligand are examined by both experiment and computation to better understand their electronic spectra and associated photochemistry. Specifically, irradiation into weak visible bands of nitrosyl and alkyl complexes (NH3)5CoCH3(2+) and L(H2O)M(III)X(2+) (X = CH3 or NO) leads to photohomolysis that generates the divalent metal complex and ˙CH3 or ˙NO, respectively. On the other hand, when X = halide or NO2, visible light photolysis leads to dissociation of X(-) and/or cis/trans isomerization. Computations show that visible bands for alkyl and nitrosyl complexes involve transitions from M-X bonding orbitals and/or metal d orbitals to M-X antibonding orbitals. In contrast, complexes with X = Cl or NO2 exhibit only d-d bands in the visible, so that homolytic cleavage of the M-X bond requires UV photolysis. UV-Vis spectra are not significantly dependent on the structure of the equatorial ligands, as shown by similar spectral features for (NH3)5CoCH3(2+) and L(1)(H2O)CoCH3(2+).

Unusual structural motif in a zwitterionic Fe(II) complex of a tetradentate phosphine.

The reaction of meso-DPPEPM (DPPEPM = bis(diphenylphosphino-ethylphenylphosphino)methane) with one equivalent of FeBr(2) in tetrahydrofuran generates a zwitterionic compound [FeBr(κ(2)-DPPEPM)(κ(3)-DPPEPM-FeBr(3))] (1). Compound 1 exhibits an unusual bonding arrangement with two meso-DPPEPM ligands and one bromide coordinated to a single Fe(II) center. One of the DPPEPM ligands binds to iron in a κ(2) mode via two central phosphorus atoms, leaving the terminal phosphines dangling. The second DPPEPM binds through three phosphines, whereas the fourth one coordinates to the iron center of an external FeBr(3)(-) unit. A 1 : 2 reaction of FeBr(2) and meso-DPPEPM in tetrahydrofuran generates [FeBr(κ(2)-DPPEPM)(κ(3)-DPPEPM)]Br ([2]Br) in which the positive charge on the pseudo-octahedral unit is balanced by free Br(-) as opposed to the phosphine-bound FeBr(3)(-) in 1. The compound [2]PF(6) was obtained from [2]Br and TlPF(6). Solution structures for 1, [2]Br and [2]PF(6) were assigned on the basis of (31)P NMR. For all three compounds the data are consistent with five phosphorus atoms bound to the metal.

Transition metal ion-assisted photochemical generation of alkyl halides and hydrocarbons from carboxylic acids.

Near-UV photolysis of aqueous solutions of propionic acid and aqueous Fe(3+) in the absence of oxygen generates a mixture of hydrocarbons (ethane, ethylene and butane), carbon dioxide, and Fe(2+). The reaction becomes mildly catalytic (about five turnovers) in the presence of oxygen which converts a portion of alkyl radicals to oxidizing intermediates that reoxidize Fe(2+). The photochemistry in the presence of halide ions (X(-) = Cl(-), Br(-)) generates ethyl halides via halogen atom abstraction from FeX(n)(3-n) by ethyl radicals. Near-quantitative yields of C(2)H(5)X are obtained at ≥0.05 M X(-). Competition experiments with Co(NH(3))(5)Br(2+) provided kinetic data for the reaction of ethyl radicals with FeCl(2+) (k = (4.0 ± 0.5) × 10(6) M(-1) s(-1)) and with FeBr(2+) (k = (3.0 ± 0.5) × 10(7) M(-1) s(-1)). Photochemical decarboxylation of propionic acid in the presence of Cu(2+) generates ethylene and Cu(+). Longer-chain acids also yield alpha olefins as exclusive products. These reactions become catalytic under constant purge with oxygen which plays a dual role. It reoxidizes Cu(+) to Cu(2+), and removes gaseous olefins to prevent accumulation of Cu(+)(olefin) complexes and depletion of Cu(2+). The results underscore the profound effect that the choice of metal ions, the medium, and reaction conditions exert on the photochemistry of carboxylic acids.

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.

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.

Preparation and characterization of manganese(IV) in aqueous acetic acid.

Mn(IV) acetate was generated in acetic acid solutions and characterized by UV-vis spectroscopy, magnetic susceptibility, and chemical reactivity. All of the data are consistent with a mononuclear manganese(IV) species. Oxidation of several substrates was studied in glacial acetic acid (HOAc) and in 95:5 HOAc-H(2)O. The reaction with excess Mn(OAc)(2) produces Mn(OAc)(3) quantitatively with mixed second-order kinetics, k (25.0 °C) = 110 ± 4 M(-1) s(-1) in glacial acetic acid, and 149 ± 3 M(-1) s(-1) in 95% AcOH, ΔH(‡) = 55.0 ± 1.2 kJ mol(-1), ΔS(‡) = -18.9 ± 4.1 J mol(-1) K(-1). Sodium bromide is oxidized to bromine with mixed second order kinetics in glacial acetic acid, k = 220 ± 3 M(-1) s(-1) at 25 °C. In 95% HOAc, saturation kinetics were observed.

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(+).

Evidence for a tetraoxo intermediate in a reaction between a superoxometal complex and acylperoxyl radicals.

The superoxo complex Cr aq( (18)O (18)O) (2+) reacts with (CH 3) 3C(O) (16)O (16)O (*) to generate quantitative yields of mixed-label dioxygen, (18)O (16)O, demonstrating that this cross-reaction involves head-to-head interaction between the metal-activated and alkyl-activated dioxygen.

Kinetics of dissociation of molecular oxygen from a superoxorhodium(III) complex and reactivity of a macrocyclic rhodium(II) ion.

The kinetics of disappearance of the superoxorhodium complex L2(H2O)RhOO2+ (L2 = meso-hexamethylcyclam) were determined in the presence of several oxidants (H2O2, (NH3)5CoBr2+, and IrCl62-) in both air-free and air-saturated aqueous solutions. Under air-free conditions, the reaction obeyed first-order kinetics. After the correction for the appropriate stoichiometric factors, the value of the rate constant kh was the same irrespective of the oxidant, kh = 2.18 (+/-0.37) x 10(-4) s(-1) at 25.0 degrees C in acidic solutions. The disappearance of L2(H2O)RhOO2+ was slower in the presence of O2. All the data suggest a sequence of reactions beginning with homolytic dissociation of O2 from L2(H2O)RhOO2+, followed by capture of the newly generated L2(H2O)Rh2+ by added oxidants in competition with O2. The equilibrium constant for O2 binding by L2(H2O)Rh2+ is 109-fold greater than that for the cobalt analogue. This difference is attributed to the lower reduction potential of the rhodium complex.

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).

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).

Reactivity of aqueous Fe(IV) in hydride and hydrogen atom transfer reactions.

Oxidation of cyclobutanol by aqueous Fe(IV) generates cyclobutanone in approximately 70% yield. In addition to this two-electron process, a smaller fraction of the reaction takes place by a one-electron process, believed to yield ring-opened products. A series of aliphatic alcohols, aldehydes, and ethers also react in parallel hydrogen atom and hydride transfer reactions, but acetone and acetonitrile react by hydrogen atom transfer only. Precise rate constants for each pathway for a number of substrates were obtained from a combination of detailed kinetics and product studies and kinetic simulations. Solvent kinetic isotope effect for the self-decay of Fe(IV), kH2O/kD2O = 2.8, is consistent with hydrogen atom abstraction from water.

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).

Direct kinetic studies of atom transfer and electron transfer reactions of hydroperoxo and high-valent oxo complexes of chromium.

A macrocyclic hydroperoxo chromium complex reacts with triphenyl phosphine in an acid-catalyzed oxygen-atom transfer reaction, k = 850 M-2 s-1. In a competing process, the hydroperoxo complex is converted to a dioxo chromium(V) species, which reacts with PPh3 by electron transfer, k = 4.4 x 105 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).

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).

Nitrous acid as a source of NO and NO(2) in the reaction with a macrocyclic superoxorhodium(III) complex.

The title reaction takes place according to the stoichiometry 2L(2)RhOO(2+) + 3HNO(2) + H(2)O --> 2L(2)Rh(OH(2))(3+) + 3NO(3)(-) + H(+) (L(2) = meso-Me(6)-[14]ane-N(4)). The kinetics are second order in HNO(2) and independent of the concentration of L(2)RhOO(2+), rate = (k(1) + k(2)[H(+)])[HNO(2)](2), where k(1) = 10.9 M(-1) s(-1) and k(2) = 175 M(-2) s(-1) at 25 degrees C and 0.10 M ionic strength. The reaction produces two observable intermediates, the nitrato (L(2)RhONO(2)(2+)) and hydroperoxo (L(2)RhOOH(2+)) complexes. The product analysis and kinetics are indicative of the initial rate-controlling formation of NO and NO(2), both of which react rapidly with L(2)RhOO(2+) in subsequent steps. The reaction with NO produces mainly L(2)RhONO(2)(2+), which hydrolyzes to L(2)Rh(OH(2))(3+) and NO(3)(-). Another minor pathway generates the hydroperoxo complex, which was detected by its known reaction with Fe(aq)(2+). The reaction of NO(2) with L(2)RhOO(2+) requires an additional equivalent of HNO(2) and produces L(2)Rh(OH(2))(3+) and NO(3)(-) via a proposed peroxynitrato complex L(2)RhOONO(2)(2+). This work provides strong evidence for the long-debated reaction between HNO(2) and H(2)NO(2)(+) to generate N(2)O(3).

Reaction of nitrogen monoxide with a macrocyclic superoxorhodium(III) complex produces an observable nitratorhodium intermediate.

The rapid (k > or = 10(6) M(-1) s(-1)) reaction between NO and L(2)(H(2)O)RhOO(2+) (L(2) = meso-Me(6)-[14]ane-N(4)) generates two strongly oxidizing, scavengeable intermediates, believed to be NO(2) and L(2)(H(2)O)RhO(2+). A mechanism is proposed whereby a peroxynitrito complex L(2)(H(2)O)RhOONO(2+) is formed first. The homolysis of O-O bond produces NO(2) and L(2)(H(2)O)RhO(2+) which were trapped with ABTS(2)(-) and Ni([14]aneN(4))(2+). In the absence of scavengers, the decomposition of L(2)(H(2)O)RhOONO(2+) produces both free NO(3)(-) and a rhodium nitrato complex L(2)(H(2)O)RhONO(2)(2+), which releases NO(3)(-) in an inverse acid-dependent process. The total yield of L(2)(H(2)O)RhONO(2)(2+) is 70%. In a minor, parallel path, NO and L(2)(H(2)O)RhOO(2+) react to give nitrite and the hydroperoxo complex L(2)(H(2)O)RhOOH(2+).