12125-80-3

Cobalt porphine (CoP) dissolved in the organic phase of a biphasic system is used to catalyze O2 reduction by an electron donor, ferrocene (Fc). Using voltammetry at the interface between two immiscible electrolyte solutions (ITIES), it is poss

Chemical oxidation of ferrocene and related derivatives by dibenzoyl peroxide in acetonitrile solution produces ferrocenium and benzoic acid after acidification. The rate law is first order in oxidant and in reductant. Steric effects and activation parameters are consistent with a rate-controlling outer-sphere single-electron transfer (ET) step, and reorganization energies are obtained using Marcus theory with B3LYP calculations. Energetics, optimized structures, and solvent effects indicate that rate is affected more by anion than cation solvation and that oxidation of decamethylferrocene by 3-chloroperoxybenzoic acid does not occur by ET.

Evidence for a single-electron-transfer (SET) activation in the cleavage of cobalt-carbon bonds of alkylcobalt(III) complexes with iodine is shown by the identification of products that could arise only via an SET pathway as well as by the kinetic comparison between the cleavage reaction of alkylcobalt(III) complexes with iodine and the electron-transfer reaction of ferrocene derivatives with iodine in acetonitrile.

Selective two-electron plus two-proton (2e-/2H+) reduction of O2 to hydrogen peroxide by ferrocene (Fc) or 1,1′-dimethylferrocene (Me2Fc) in the presence of perchloric acid is catalyzed efficiently by a mononuclear copper(II) complex, [Cu II(tepa)]2+ (1; tepa = tris[2-(2-pyridyl)ethyl]amine) in acetone. The E1/2 value for [CuII(tepa)]2+ as measured by cyclic voltammetry is 0.07 V vs Fc/Fc+ in acetone, being significantly positive, which makes it possible to use relatively weak one-electron reductants such as Fc and Me2Fc for the overall two-electron reduction of O2. Fast electron transfer from Fc or Me2Fc to 1 affords the corresponding CuI complex [Cu I(tepa)]+ (2), which reacts at low temperature (193 K) with O2, however only in the presence of HClO4, to afford the hydroperoxo complex [CuII(tepa)(OOH)]+ (3). A detailed kinetic study on the homogeneous catalytic system reveals the rate-determining step to be the O2-binding process in the presence of HClO4 at lower temperature as well as at room temperature. The O2-binding kinetics in the presence of HClO4 were studied, demonstrating that the rate of formation of the hydroperoxo complex 3 as well as the overall catalytic reaction remained virtually the same with changing temperature. The apparent lack of activation energy for the catalytic two-electron reduction of O2 is shown to result from the existence of a pre-equilibrium between 2 and O2 prior to the formation of the hydroperoxo complex 3. No further reduction of [CuII(tepa)(OOH)]+ (3) by Fc or Me2Fc occurred, and instead 3 is protonated by HClO4 to yield H2O2 accompanied by regeneration of 1, thus completing the catalytic cycle for the two-electron reduction of O2 by Fc or Me2Fc.

The selective two-electron reduction of dioxygen occurs in the case of a monocobalt porphyrin [Co(OEP)], whereas the selective four-electron reduction of dioxygen occurs in the case of a cofacial dicobalt porphyrin [Co 2(DPX)]. The other cofacial dicobalt porphyrins [Co2(DPA), Co2(DPB), and Co2(DPD)] also catalyze the two-electron reduction of dioxygen, but the four-electron reduction is not as efficient as in the case of Co2(DPX). The μ-superoxo species of cofacial dicobalt porphyrins were produced by the reactions of cofacial dicobalt(II) porphyrins with dioxygen in the presence of a bulky base and the subsequent one-electron oxidation of the resulting μ-peroxo species by iodine. The superhyperfine structure due to two equivalent cobalt nuclei was observed at room temperature in the ESR spectra of the μ-superoxo species. The superhyperfine coupling constant of the μ-superoxo species of Co2(DPX) is the largest among those of cofacial dicobalt porphyrins. This indicates that the efficient catalysis by Co2(DPX) for the four-electron reduction of dioxygen by Fe(C5H4Me)2 results from the strong binding of the reduced oxygen with Co2(DPX) which has a subtle distance between two cobalt nuclei for the oxygen binding. Mechanisms of the catalytic two-electron and four-electron reduction of dioxygen by ferrocene derivatives will be discussed on the basis of detailed kinetics studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The turnover-determining step in the Co(OEP)-catalyzed two-electron reduction of dioxygen is an electron transfer from ferrocene derivatives to Co(OEP) +, whereas the turnover-determining step in the Co 2(DPX)-catalyzed four-electron reduction of dioxygen changes from the electron transfer to the O-O bond cleavage of the peroxo species of Co 2(DPX), depending on the electron donor ability of ferrocene derivatives.

The catalytic effect of tetraphenylporphyrin on the oxygen reduction with ferrocene in 1,2-dichloroethane can be finely tuned by varying the molar ratio of the acid to the catalyst present in the solution. The mechanism involves binding of molecular oxygen to the protonated free porphyrin base, in competition with ion pairing between the protonated base and the acid anion present.

Results of an experimental study using the selection-ion flow tube technique are reported for reactions of bare Fe(+) and iron containing FeX(+) cations (X=C6H6 (or B), c-C5H5 (or Cp), O, (Cp)2, B2) with dinitrogen at 294+/-3 K and at a helium buffer-gas pressure of 0.35+/-0.01 Torr. Fe(+), B2Fe(+) and Cp2Fe(+) do not react with dinitrogen. A very slow sequential addition of two N2 molecules was observed with FeO(+). CpFe(+) and BFe(+) reacted without the subsequent addition of a second N2 molecule. These results provide insight into the bonding of N2 as a ligand with Fe as the coordination centre in the gas-phase, and into intrinsic kinetic aspects of dinitrogen addition.

A ferrocene-quinone dyad (Fc-Q) with a rigid amide spacer and Fc-(Me)Q dyad, in which the amide proton acting as a hydrogen-bonding acceptor is replaced by the methyl group, are employed to examine the effects of hydrogen bonding on both the thermal and the photoinduced electron-transfer reactions. The hydrogen bonding of the semiquinone radical anion with the amide proton in Fc-Q.- produced by the electron-transfer reduction of Fc-Q is indicated by the significant positive shift of the one-electron reduction potential of Fc-Q. The hyperfine coupling constants of Fc-Q.- also indicate the existence of hydrogen bonding, agreeing with those predicted by the density functional calculation. The hydrogen-bonding dynamics in the photoinduced electron transfer from the ferrocene (Fc) to the quinone moiety (Q) in Fc-Q have been successfully detected in the femtosecond laser flash photolysis experiments. Thermal intramolecular electron transfer from Fc to Q in Fc-Q and Fc-(Me)Q also occurs efficiently in the presence of metal ions in acetonitrile at 298 K. The hydrogen bond formed between the semiquinone radical anion and the amide proton in Fc-Q results in remarkable acceleration of the rate of metal ion-promoted electron transfer as compared to the rate of Fc-(Me)Q in which hydrogen bonding is prohibited. The metal ion-promoted electron-transfer rates are well correlated with the binding energies of superoxide ion-metal ion complexes, which are derived from the gzz values of the ESR spectra.

Rates of hydrogen-transfer reactions from a series of para-substituted N,N-dimethylanilines to cumylperoxyl radical and oxygen-transfer reactions from cumylperoxyl radical to a series of sulfides and phosphines have been determined in propionitrile (EtCN) and pentane at low temperatures by use of ESR. The observed rate constants exhibit first-order and second-order dependence with respect to concentrations of N,N-dimethylanilines. This indicates that the hydrogen- and oxygen-transfer reactions proceed via 1:1 charge-transfer (CT) complexes formed between the substrates and cumylperoxyl radical. The primary kinetic isotope effects are determined by comparing the rates of N,N-dimethylanilines and the corresponding N,N-bis(trideuteriomethyl)anilines. The isotope effect profiles are quite different from those reported for the P-450 model oxidation of the same series of substrates. Rates of electron-transfer reactions from ferrocene derivatives to cumylperoxyl radical have also been determined by use of ESR. The catalytic effects of Sc(OTf)3 (OTf = triflate) on the electron-transfer reactions are compared with those of Sc(OTf)3 on the hydrogen- and oxygen-transfer reactions. Such comparison provides strong evidence that the hydrogen- and oxygen- transfer reactions of cumylperoxyl radical proceed via a one-step hydrogen atom and oxygen atom transfer rather than via an electron transfer from substrates to cumylperoxyl radical.

The electron self-exchange rate constant for the Cu(dmbp)22+/+ couple (dmbp = 6,6′-dimethyl-2,2′-bipyridine) was measured in acetonitrile by the NMR method (kex = 5.5 × 103 kg mol-1 s-1, ΔH

A series of [M(diphosphine)2]X2, [HM(diphosphine)2]X, and M(diphosphine)2 complexes have been prepared for the purpose of determining the relative thermodynamic hydricities of the [HM(diphosphine)2]X complexes (M = Ni, Pt; X = BF4, PF6; diphosphine = bis(diphenylphosphino)ethane (dppe), bis(diethylphosphino)ethane (depe), bis(dimethylphosphino)ethane (dmpe), bis(dimethylphosphino)propane (dmpp)). Measurements of the half-wave potentials (E1/2) for the M(II) and M(0) complexes and pKa measurements for the metal hydride complexes have been used in a thermochemical cycle to obtain quantitative thermodynamic information on the relative hydride donor abilities of the metal - hydride complexes. The hydride donor strengths vary by 23 kcal/mol and are influenced by the metal, the ligand substituents, and the size of the chelate bite of the diphosphine ligand. The best hydride donor of the complexes prepared is [HPt(dmpe)2](PF6), a third-row transition metal with basic substituents and a diphosphine ligand with a small chelate bite. The best hydride acceptors have the opposite characteristics. X-ray diffraction studies were carried out on eight complexes: [Ni(dmpe)2](BF4)2, [Ni(depe)2](BF4)2, [Ni(dmpp)2](BF4)2, [Pt(dmpp)2](PF6)2, [Ni(dmpe)2(CH3CN)](BF4)2, [Ni(dmpp)2(CH3CN)](BF4)2, Ni(dmpp)2, and Pt(dmpp)2. The cations [Ni(dmpp)2]2+ and [Pt(dmpp)2]2+ exhibit significant tetrahedral distortions from a square-planar geometry arising from the larger chelate bite of dmpp compared to that of dmpe. This tetrahedral distortion produces a decrease in the energy of the lowest unoccupied molecular orbital of the [M(dmpp)2]2+ complexes, stabilizes the +1 oxidation state, and makes the [HM(dmpp)2]+ complexes poorer hydride donors than their dmpe analogues. Another interesting structural feature is the shortening of the M-P bond upon reduction from M(II) to M(0).

A possibility is established of direct preparation of an organoiron derivative by oxidation of the metal with the system hydrogen chloride-cyclopentadiene-p-xylene at room temperature.

The proton affinity (PA) and site of protonation of 2(μ-CO)(μ-C=H2) (2), as well as the decomposition processes for 2(μ-CO)(μ-C=CH2)H(+) (7), are studied in the gas phase by using Fourier transform mass spectrometry (FTMS).The

The kinetics of quenching of *Cr(bpy)33+ by d6 metallocenes and by ferrocenium ions were evaluated by laser flash photolysis. The quenching by ferrocenium ions proceeds by energy transfer and is dependent on the donor-acceptor distance, as expected for an electron-exchange mechanism. The rate constants for quenching with d6 metallocenes are at or near the diffusion-controlled limit. The reactions partition themselves between electron transfer and energy transfer. The Cr(bpy)32+ and the ferrocenium ions, formed by electron-transfer quenching, undergo rapid back electron transfer, k = (3-9) × 109 M-1 s-1.

Reduction of dioxygen with ferrocene derivatives (Fc) is catalyzed by metalloporphyrins (MTPP+: M = Co, Fe, Mn; TPP = tetraphenylporphyrin) or Co(TIM)3+ (TIM: a tetraaza macrocyclic ligand) in the presence of HClO4 in acetonitrile (MeCN). Electron transfer from Fc to MTPP+ is the rate-determining step for the MTPP+-catalyzed oxidation of Fc by dioxygen, when the rate is independent of the concentration of dioxygen or HClO4. On the other hand, the rate of electron transfer from Fc to Co(TIM)3+ is accelerated by the presence of HClO4 and dioxygen. The rates of these electron-transfer reactions are discussed in light of the Marcus theory of electron transfer to distinguish between outer-sphere and inner-sphere electron-transfer processes. The strong inner-sphere nature of metalloporphyrins in the electron-transfer reactions with dioxygen in the presence of HClO4 plays an essential role in the catalytic reduction of dioxygen.