One-electron Oxidation Potentials Research Articles

Aqueous reactions of organic triplet excited states with atmospheric alkenes

Abstract. Triplet excited states of organic matter are formed when colored organic matter (i.e., brown carbon) absorbs light. While these “triplets” can be important photooxidants in atmospheric drops and particles (e.g., they rapidly oxidize phenols), very little is known about their reactivity toward many classes of organic compounds in the atmosphere. Here we measure the bimolecular rate constants of the triplet excited state of benzophenone (3BP∗), a model species, with 17 water-soluble C3–C6 alkenes that have either been found in the atmosphere or are reasonable surrogates for identified species. Measured rate constants (kALK+3BP∗) vary by a factor of 30 and are in the range of (0.24–7.5) ×109 M−1 s−1. Biogenic alkenes found in the atmosphere – e.g., cis-3-hexen-1-ol, cis-3-hexenyl acetate, and methyl jasmonate – react rapidly, with rate constants above 1×109 M−1 s−1. Rate constants depend on alkene characteristics such as the location of the double bond, stereochemistry, and alkyl substitution on the double bond. There is a reasonable correlation between kALK+3BP∗ and the calculated one-electron oxidation potential (OP) of the alkenes (R2=0.58); in contrast, rate constants are not correlated with bond dissociation enthalpies, bond dissociation free energies, or computed energy barriers for hydrogen abstraction. Using the OP relationship, we estimate aqueous rate constants for a number of unsaturated isoprene and limonene oxidation products with 3BP∗: values are in the range of (0.080–1.7) ×109 M−1 s−1, with generally faster values for limonene products. Rate constants with less reactive triplets, which are probably more environmentally relevant, are likely roughly 25 times slower. Using our predicted rate constants, along with values for other reactions from the literature, we conclude that triplets are probably minor oxidants for isoprene- and limonene-related compounds in cloudy or foggy atmospheres, except in cases in which the triplets are very reactive.

Quantum chemical calculations of the one-electron oxidation potential of nitroxide spin labels in biologically active compounds

Density functional theory and continuum solvent model have been used to determine standard redox potentials of oxidation of cyclic nitroxide radicals and the nitroxide derivatives of chitosan and fullerene C60 in water. Inclusion of structural relaxation of solute in water improves correlation between the measured and calculated oxidation potentials. The underlying limitations of the thermodynamic cycle model have been discussed. The computed oxidation potential of the piperidine–N–oxide derivative of chitosan is several tens mV higher than of the separate radical while the opposite trend was found for the pyrroline–N–oxide derivative of chitosan. The calculations of fullerene C60–TEMPO have shown that the 6,6–methanofullerene is more stable that the 5,6–fulleroid structure. The determined redox potential of oxidation of fullerene C60–TEMPO is significantly higher than of TEMPO in water.

Synthesis and Electrochemical and Photophysical Properties of Azaterrylene Derivatives.

A series of terrylene derivatives, such as monoazaterrylene (MATerry), 1,6-diazaterrylene (DiATerry) and pristine terrylene (Terry), were synthesized by changing the number of nitrogen atoms at the bay region (1 and 6 positions of the Terry core). The electrochemical measurements suggested that the first one-electron reduction and oxidation potentials became positively shifted with increasing numbers of nitrogen atoms. This agreed with the energies of the corresponding highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states estimated by DFT methods. In contrast, the HOMO-LUMO gaps approximately remained constant. This trend is quite similar to the spectroscopic behaviors observed by absorption and fluorescence spectra. The solvent polarity-dependent spectroscopic trends of DiATerry suggested the intramolecular charge-transfer (ICT) characters. The evaluation of the excited-state dynamics in various solvents indicated the electronic configurational changes of the excited states relative to the ground state via the ICT. This was supported by the Lippert-Mataga plots. Finally, the reversible protonation and deprotonation processes were also observed.

A Mononuclear Non-heme Manganese(III)–Aqua Complex as a New Active Oxidant in Hydrogen Atom Transfer Reactions

A mononuclear non-heme Mn(III)-aqua complex, [(dpaq)MnIII(OH2)]2+ (1, dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate), is capable of conducting hydrogen atom transfer (HAT) reactions much more efficiently than the corresponding Mn(III)-hydroxo complex, [(dpaq)MnIII(OH)]+ (2); the high reactivity of 1 results from the positive one-electron reduction potential of 1 ( Ered vs SCE = 1.03 V), compared to that of 2 ( Ered vs SCE = -0.1 V). The HAT mechanism of 1 varies between electron transfer followed by proton transfer and one-step concerted proton-coupled electron transfer, depending on the one-electron oxidation potentials of substrates. To the best of our knowledge, this is the first example showing that metal(III)-aqua complex can be an effective H-atom abstraction reagent.

Enhanced Electron-Transfer Reactivity of a Long-Lived Photoexcited State of a Cobalt-Oxygen Complex.

Photodynamics and electron-transfer reactivity of an excited state derived from an earth-abundant mononuclear cobalt-oxygen complex ground state, [(TAML)CoIV(O)]2- (1; H4TAML = 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1 H-1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6 H, 11 H)tetrone), prepared by electron-transfer oxidation of Li[(TAML)CoIII]·3(H2O) (2) in a 1:1 acetonitrile/acetone solvent mixture at 5 °C, were investigated using a combination of femtosecond and nanosecond laser absorption spectroscopy. Visible light photoexcitation of 1 (λexc = 393 nm) resulted in generation of the excited state S2* (lifetime: 1.4(4) ps), detected 2 ps after laser irradiation by femtosecond laser spectroscopy. The initially formed excited state S2* converted to a lower-lying excited state, S1* (λmax = 580 nm), with rate constant kc = 7(2) × 1011 s-1 (S2* → S1*). S1* exhibited a 0.6(1) ns lifetime and converted to the initial ground state 1 with rate constant kd = 1.7(3) × 109 s-1 (S1* → 1). The same excited state dynamics was observed when 1 was generated by electron-transfer oxidation of 2 using different one-electron oxidants such as Cu(OTf)2 (OTf- = triflate anion), [Fe(bpy)3]3+ (bpy = 2,2'-bipyridine), and tris(4-bromophenyl)ammoniumyl radical cation (TBPA•+). The electron-transfer reactivity of S1* was probed by nanosecond laser photoexcitation of 1 in the presence of a series of electron donors with different one-electron oxidation potentials ( Eox vs SCE): benzene (2.35 V), toluene (2.20 V), m-xylene (2.02 V), and anisole (1.67 V). The excited state S1* engaged in electron-transfer reactions with m-xylene and anisole to generate π-dimer radical cations of m-xylene and anisole, respectively, observed by nanosecond laser transient absorption spectroscopy, whereas no reactivity was observed toward benzene and toluene. Such differential electron-transfer reactivity depending on the Eox values of electron donors allowed the estimation of the one-electron reduction potential of S1* ( Ered*) as 2.1(1) V vs SCE, which is much higher than that of the ground state ( Ered = 0.86 V vs SCE).

Photooxygenation of Hydrocarbons with Molecular Oxygen By Electron Transfer

Phenol is an very important bulk organic compound, having a variety of applications in industry. Phenol is commercially produced through three-step cumene process via cumene hydroperoxide, which is the key intermediate. In addition acetone is produced as a co-product and a-methylstyrene is produced as a byproduct together with phenol. Thus, phenol synthesis from benzene in a one-step reaction with high benzene conversion and high phenol selectivity is highly desired from the viewpoints of an environmentally benign green process and economical efficiency. Extensive efforts have so far been devoted to develop the one-step direct hydroxylation of benzene using various oxidants. we would like to focus on the catalytic mechanisms of selective hydroxylation of benzene to phenol. First, catalytic mechanisms of liquid phase hydroxylation of benzene with H2O2 using metal complexes are discussed by identifying the reactive intermediates, which act as strong electrophiles for the benzene hydroxylation. Next, photochemical hydroxylation of benzene with an organic oxidant via benzene radical cation produced in photoinduced electron transfer from benzene to the excited state of the organic oxidant is discussed to clarify the reason of the selective hydroxylation to phenol. The reason why phenol is not further oxidized is discussed based on the Marcus theory of electron transfer. Finally, the photocatalytic hydroxylation of benzene with O2 to phenol is made possible by using an appropriate organic photocatalyst. The photochemical hydroxylation of benzene with an organic oxidant to phenol is also combined with the catalytic reoxidation of the reduced organic oxidant by O2 to make the selective hydroxylation of benzene with O2 to phenol catalytic. How overoxidation of phenol can be avoided is discussed in both the thermal and photochemical catalytic hydroxylation of benzene. Photochemical hydroxylation of benzene with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and water occurs under visible light irradiation to yield phenol and DDQH2. The yield of phenol was 99% with 99% conversion (>99% selectivity) [1]. The mechanism of the photochemical hydroxylation of benzene with DDQ was clarified by time-resolved transient absorption spectroscopy. The photochemical reaction was initiated by the photoinduced electron transfer from benzene to 3DDQ* to produce benzene p-dimer radical cation and DDQ•−. The free energy change for electron transfer from benzene to 3DDQ* is largely negative (DG et = −0.70 eV), as determined from the one-electron oxidation potential of benzene (E ox = 2.48 V vs. SCE) and the one-electron reduction potential of 3DDQ* (E red = 3.18 V vs. SCE). Then, benzene radical cation reacts with H2O to produce an OH-adduct radical of benzene. Finally, phenol was produced by hydrogen atom transfer from the OH-adduct radical to DDQ•–. Photoinduced oxygenation of neat cyclohexane in the presence of O2 also occurred under visible light irradiation of DDQ which acts as a super photooxidant. The products detected by GC and NMR analyses were cyclohexanol, cyclohexanone and cyclohexane hydroperoxide with 70, 40, 210% yields, respectively. When cyclohexane was replaced by n-butane, n-pentane and 3-methylpentane, the oxygenation reactions also took place to form the corresponding oxygenated products under the otherwise same photochemical reaction conditions [2-5]. [1] Ohkubo, K.; Fujimoto, A.; Fukuzumi, S. J. Am. Chem. Soc. 2013, 135, 5368. [2] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Chem.–Eur. J. 2015, 21, 2855. [3] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Photochem. Photobiol. Sci. 2016, 15, 731. [4] Ohkubo, K.; Hirose, K.; Fukuzumi, S. Chem. Asia J. 2016, 11, 2218. [5] Fukuzumi, S.; Ohkubo, K. Asian J. Org. Chem. 2015, 4, 836 (Focus Review).

Optical and electrochemical characteristics of Ir(III) complexes with metalated 4-(4-bromophenyl)-2-methyl-1,3-thiazole and isocyanide, ethylenediamine, and diethyldithiocarbamate ligands

The influence of donor–acceptor properties of tert-butyl-, 2.6-dimethylphenyl-, and 4-bromophenyl-isocyanides (BuNC, XylNC, BpNC), ethylenediamine (En), and diethyldithiocarbamate ions (Dtc–) on the 1H and 13C NMR, IR, optical, and electrochemical characteristics of Ir(III) complexes with metalated 4-(4-bromophenyl)-2-methyl-1,3-thiazole is studied. Enhancement of the donor properties of BpNC, XylNC, BuNC, En, and Dtc– ligands leads to a bathochromic shift of metal-to-ligand charge transfer (MLCT) bands and to a decrease in the difference between the one-electron oxidation and reduction potentials of complexes. The bathochromic shift of the low-temperature phosphorescence of complexes in frozen (77 K) solutions with increasing donor properties of BpNC, XylNC, BuNC, En, and Dtc–ligands is caused by a decrease in the admixture of MLCT to the intraligand excited state of {Ir(bptz)2}. Quenching of the phosphorescence of complexes in liquid solutions is attributed to the thermally-induced population of excited d–d* states with subsequent nonradiative deactivation.

Structure-Activity Relationships for Rates of Aromatic Amine Oxidation by Manganese Dioxide.

New energetic compounds are designed to minimize their potential environmental impacts, which includes their transformation and the fate and effects of their transformation products. The nitro groups of energetic compounds are readily reduced to amines, and the resulting aromatic amines are subject to oxidation and coupling reactions. Manganese dioxide (MnO2) is a common environmental oxidant and model system for kinetic studies of aromatic amine oxidation. In this study, a training set of new and previously reported kinetic data for the oxidation of model and energetic-derived aromatic amines was assembled and subjected to correlation analysis against descriptor variables that ranged from general purpose [Hammett σ constants (σ(-)), pKas of the amines, and energies of the highest occupied molecular orbital (EHOMO)] to specific for the likely rate-limiting step [one-electron oxidation potentials (Eox)]. The selection of calculated descriptors (pKa, EHOMO, and Eox) was based on validation with experimental data. All of the correlations gave satisfactory quantitative structure-activity relationships (QSARs), but they improved with the specificity of the descriptor. The scope of correlation analysis was extended beyond MnO2 to include literature data on aromatic amine oxidation by other environmentally relevant oxidants (ozone, chlorine dioxide, and phosphate and carbonate radicals) by correlating relative rate constants (normalized to 4-chloroaniline) to EHOMO (calculated with a modest level of theory).

Ionization Energies and Aqueous Redox Potentials of Organic Molecules: Comparison of DFT, Correlated ab Initio Theory and Pair Natural Orbital Approaches.

The calculation of redox potentials involves large energetic terms arising from gas phase ionization energies, thermodynamic contributions, and solvation energies of the reduced and oxidized species. In this work we study the performance of a wide range of wave function and density functional theory methods for the prediction of ionization energies and aqueous one-electron oxidation potentials of a set of 19 organic molecules. Emphasis is placed on evaluating methods that employ the computationally efficient local pair natural orbital (LPNO) approach, as well as several implementations of coupled cluster theory and explicitly correlated F12 methods. The electronic energies are combined with implicit solvation models for the solvation energies. With the exception of MP2 and its variants, which suffer from enormous errors arising at least partially from the poor Hartree-Fock reference, ionization energies can be systematically predicted with average errors below 0.1 eV for most of the correlated wave function based methods studies here, provided basis set extrapolation is performed. LPNO methods are the most efficient way to achieve this type of accuracy. DFT methods show in general larger errors and suffer from inconsistent behavior. The only exception is the M06-2X functional which is found to be competitive with the best LPNO-based approaches for ionization energies. Importantly, the limiting factor for the calculation of accurate redox potentials is the solvation energy. The errors in the predicted solvation energies by all continuum solvation models tested in this work dominate the final computed reduction potential, resulting in average errors typically in excess of 0.3 V and hence obscuring the gains that arise from choosing a more accurate electronic structure method.

Tuning the Redox Properties of a Nonheme Iron(III)-Peroxo Complex Binding Redox-Inactive Zinc Ions by Water Molecules.

Redox-inactive metal ions play important roles in tuning chemical properties of metal-oxygen intermediates. Herein we report the effect of water molecules on the redox properties of a nonheme iron(III)-peroxo complex binding redox-inactive metal ions. The coordination of two water molecules to a Zn(2+) ion in (TMC)Fe(III) -(O2 )-Zn(CF3 SO3 )2 (1-Zn(2+) ) decreases the Lewis acidity of the Zn(2+) ion, resulting in the decrease of the one-electron oxidation and reduction potentials of 1-Zn(2+) . This further changes the reactivities of 1-Zn(2+) in oxidation and reduction reactions; no reaction occurred upon addition of an oxidant (e.g., cerium(IV) ammonium nitrate (CAN)) to 1-Zn(2+) , whereas 1-Zn(2+) coordinating two water molecules, (TMC)Fe(III) -(O2 )-Zn(CF3 SO3 )2 -(OH2 )2 [1-Zn(2+) -(OH2 )2 ], releases the O2 unit in the oxidation reaction. In the reduction reactions, 1-Zn(2+) was converted to its corresponding iron(IV)-oxo species upon addition of a reductant (e.g., a ferrocene derivative), whereas such a reaction occurred at a much slower rate in the case of 1-Zn(2+) -(OH2 )2 . The present results provide the first biomimetic example showing that water molecules at the active sites of metalloenzymes may participate in tuning the redox properties of metal-oxygen intermediates.

Highly Efficient and Selective Two-Electron Reduction of Dioxygen Catalyzed By Cobalt Chlorin Complexes to Produce Hydrogen Peroxide

Hydrogen peroxide (H2O2) is an ideal energy carrier alternative to hydrogen, because H2O2 can be produced by photocatalytic oxidation of water by O2 using solar energy,1 and H2O2 can be used as a fuel in a one-compartment H2O2 fuel cell with low cost material.2-4 H2O2 can also be produced by electrochemical two-electron reduction of O2 using solar cells.5 Thus, it is highly desired to develop efficient catalysts for selective two-electron reduction of O2 to produce H2O2. We report herein that cobalt chlorin complexes in Chart 1 act as highly efficient and selective catalysts for two-electron reduction of O2 to H2O2. The detailed study based on the kinetics and thermodynamics have provided valuable insight into the development of the more efficient electrocatalyst for two-electron re-duction of O2 to H2O2, which is essential to realize H2O2-energy society. of O2 by 1,1’-dimethylferrocene (Me2Fc) efficiently and selectively to produce H2O2 in the presence of perchloric acid (HClO4) in benzonitrile (PhCN) at 298 K (eq 1). The rate of formation of Me2Fc+was 2Me2Fc + O2 + 2H+ --> 2Me2Fc+ + H2O2 (1) CoII(Ch n ) proportinal to concentrations of CoII(Ch n ) and O2, whereas the rate was independent of concentraton of Me2Fc. On the other hand, the catalytic rate increased with increasing concentraiton of HClO4 to reach a constant value. Such kinetic results indicate that the rate-determining step in the catalytic cycle (Scheme 1) is the proton-coupled electron transfer reduction of O2 by CoII(Ch n ), followed by fast electron transfer from Me2Fc to [CoII(Ch n )]+. The one-electron reduction potential of [CoIII(Ch 1 )]+ was changed from 0.37 V (vs SCE) to 0.48 V by the addition of HClO4 due to the protonation of [CoIII(Ch 1 )]+.6 The introduction of electron-withdrawing groups on the chlorin ligand in CoII(Ch n ) (n = 2, 3) resulted in the positive shift of the redox potential for Co(III/II) from 0.37 V to 0.45 and 0.40 V, respectively. In the presence of HClO4, however, no positive shifts of the redox potential for [CoIII(Ch n )]+/CoII(Ch n ) (n = 2, 3) were observed. As a result, CoII(Ch 3 ), which has the lowest one-electron oxidation potential in the presence of HClO4 among the cobalt chlorin complexes in Chart 1, exhibited the highest catalytic reactivity for the two-electron recution of O2 by Me2Fc in the presence of HClO4in PhCN. The electrocatalytic two-electron reduction of O2 was also examined by using a CoIICh 1 -modified carbon paper electrode with multiwalled carbon nanotubes (MWCNTs) in an aqueous solution (pH = 1). A catalytic current of two-electron reduction of O2 was observed at a small overpotential. The electrocatalytic two-electron reduction of O2 was combined with the photocatalytic oxidation of water using semiconductor photocatalysts to produce H2O2 from H2O and O2using solar energy (eq 2). hν 2H2O + O2 --> 2H2O2 (2) catalysts References [1] Kato, S.; Jung, J.; Suenobu, T.; Fukuzumi, S. Energy Environ. Sci. 2013, 6, 3756. [2] Yamada, Y.; Yoshida, S.; Honda, T.; Fukuzumi, S. Energy Environ. Sci. 2011, 4, 2822. [3] Yamada, Y.; Yoneda, M.; Fukuzumi, S. Chem.–Eur. J. 2013, 19, 11733. [4] Yamada, Y.; Yoneda, M.; Fukuzumi, S. Inorg. Chem. 2014, 53, 1272. [5] Yamada, Y.; Fukunishi, Y.; Yamazaki, S.; Fukuzumi, S. Chem. Commun. 2010, 46, 7334. [6] Mase, K.; Ohkubo, K.; Fukuzumi, S. J. Am. Chem. Soc. 2013, 135, 2800. Figure 1

Platinum(ii)–porphyrin as a sensitizer for visible-light driven water oxidation in neutral phosphate buffer

A water-soluble Pt(ii)–porphyrin with a high potential for one-electron oxidation (∼1.42 V vs. NHE) proves very suitable for visible-light driven water oxidation in neutral phosphate buffer solution in combination with a variety of WOCs.

Synthesis and electronic properties of acetylene- and butadiyne-linked 3,3′-porphycene dimers

The butadiyne- and acetylene-linked 3,3′-porphycene dimers 1 and 2 were synthesized from the common intermediate, 3-ethynyl-2,7,12,17-tetrahexylporphycene 4. The butadiyne-linked dimer 1 was prepared from 4 by copper mediated Glaser-type homo-coupling. The acetylene-linked dimer 2 was synthesized by Sonogashira coupling of 4 and 2,7,12,17-tetrahexyl-3-iodoporphycene 5. These porphycene dimers were characterized by 1 H and 13 C NMR spectroscopies, mass spectroscopy, X-ray diffraction analysis, UV-vis absorption and fluorescence spectra, cyclic voltammetry and differential pulse voltammetry. Crystal structure of 1showed a coplanar structure of two porphycene units. The absorption spectra in CH 2 Cl 2 indicated the small interaction between the porphycene units through the linkage at 3,3′-positions. The electrochemical measurement showed two one-electron oxidation potentials and four one-electron reduction potentials indicating the electric interaction between the porphycene units.

Theoretical studies on the structure and property of alkylated dipenylamine antioxidants

Diarylamines ( Ar 2 NH ) are generally used as antioxidants to inhibit or retard the auto-oxidation degradation of lubricating oil by trapping ROO• radicals. In the present study, 20 kinds of 4,4′-disubstituted diphenylamine compounds were investigated through density functional theory (DFT) calculations. The results indicate that the N – H bond dissociation enthalpy (BDE) linearly correlates its one-electron oxidation potential, the difference in Mulliken atomic charge on the two atoms of N – H bond, the reaction rate constant of hydrogen transfer from Ar 2 NH to peroxy radical, and the chemical hardness of the resulted Ar 2 N • radical, respectively. The substitution of alkyl groups (electron-donating groups) decreases the N – H BDE, one-electron oxidation potential and the reaction rate constant, while that of significant electron-withdrawing groups such as - NO 2 and - COOCH 3 increases these three parameters. The electron-donating groups such as alkyls could improve the antioxidation performance of 4,4′-disubstitued diphenylamines whereas electron-withdrawing groups have the contrary effect. In addition, the frontier molecular orbital of Ar 2 NH has been also analyzed.

Theoretical Study of One-Electron Reduction And Oxidation Potentials of N-Heterocyclic Compounds

Computational protocols that successfully predict standard reduction potentials of N-heterocyclic compounds in dimethyl formamide and their standard oxidation potentials in acetonitrile were developed. Different solvation models were verified in conjunction with the MPWB1K/6-31 + G(d) level of density functional theory. For reduction potentials calculations, the PCM(UA0) and SMD(Bondi) models were used to compute solvation energies of neutral forms and anion-radical forms, respectively. For oxidation potential calculations, the best results were obtained by a combination of SMD(UAHF) and PCM(Bondi) models to compute solvation energies of neutral forms and cation-radical forms, respectively. The mean absolute deviations (MAD) and root mean square errors (RMSE) of the current theoretical models for reduction potentials were found to be 0.09 V and 0.10, respectively, and for oxidation potentials MAD = 0.12 V and RMSE = 0.16.

One electron oxidation potential as a predictor of rate constants of N-containing compounds with carbonate radical and triplet excited state organic matter

Photo-generated transient species, such as the carbonate radical and triplet excited state natural organic matter, mediate the oxidation of pollutants in various sunlit or artificially irradiated systems. In this work, one-electron oxidation potentials for 70 nitrogen-containing compounds were computed, and literature data were used to develop quantitative structure-activity relationships (QSARs) for prediction of the second order reaction rate constants with these two oxidants. For carbonate radical, separate QSARs were necessary for compounds with and without resonance stabilization of the resulting radical, and predicted rate constants were, on average, within a factor of three of experimental values. With the limited data set available, results suggest that one-electron oxidation potential is also a viable descriptor variable for predictions of rate constants with triplet excited states.

The Marriage of Peripherally Metallated and Directly Linked Porphyrins: Bromidobis(phosphine)platinum(II) as a Cation‐Stabilizing Substituent on Directly Linked and Fused Triply Linked Diporphyrins

A meso-bromidoplatiniobis(triphenylphosphine) η(1)-organometallic porphyrin monomer was prepared by the oxidative addition of meso-bromoZnDPP (DPP=dianion of 5,15-diphenylporphyrin) to a platinum(0) species. The meso-meso directly linked dimeric porphyrin (5) was prepared from this monomer by silver(I)-promoted oxidative coupling and planarized to give a triply linked dizinc(II) porphyrin dimer (8). Acidic demetallation of 8 afforded the bis(free base) 9. Dimer 5 was demetallated then remetallated with nickel(II) to give the dinickel(II) analogue 10, the X-ray crystal structure of which showed a twisted molecule with ruffled, orthogonal NiDPP rings, terminated by square-planar trans-[Pt(PPh3)2Br] units. New compounds were fully characterized spectroscopically, and the fused diporphyrin exhibited a broad, low-energy, near-IR electronic absorption band near 1100 nm. Electrochemical measurements of this series indicate that the organometallic fragment is a strong electron donor towards the porphyrin ring. The triply linked organometallic diporphyrin has a substantially lowered first one-electron oxidation potential (-0.35 V versus the ferrocene/ferrocenium couple (Fc/Fc(+))) and a narrow HOMO-LUMO gap of 0.96 V. Solutions prepared for NMR spectroscopy slowly decompose with degradation of the signals, which is attributed to partial oxidation to the cation radical. This paramagnetic species can be reduced in situ by hydrazine to restore the NMR spectrum to its former appearance. The combined influence of the two [Pt(PPh3)2Br] electron-donating substituents is sufficient to make dimer too aerobically unstable to allow further elaboration.

Brønsted Acid-Promoted C–H Bond Cleavage via Electron Transfer from Toluene Derivatives to a Protonated Nonheme Iron(IV)-Oxo Complex with No Kinetic Isotope Effect

The reactivity of a nonheme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), was markedly enhanced by perchloric acid (70% HClO4) in the oxidation of toluene derivatives. Toluene, which has a high one-electron oxidation potential (Eox = 2.20 V vs SCE), was oxidized by [(N4Py)Fe(IV)(O)](2+) in the presence of HClO4 in acetonitrile (MeCN) to yield a stoichiometric amount of benzyl alcohol, in which [(N4Py)Fe(IV)(O)](2+) was reduced to [(N4Py)Fe(III)(OH2)](3+). The second-order rate constant (kobs) of the oxidation of toluene derivatives by [(N4Py)Fe(IV)(O)](2+) increased with increasing concentration of HClO4, showing the first-order dependence on [HClO4]. A significant kinetic isotope effect (KIE) was observed when mesitylene was replaced by mesitylene-d12 in the oxidation with [(N4Py)Fe(IV)(O)](2+) in the absence of HClO4 in MeCN at 298 K. The KIE value drastically decreased from KIE = 31 in the absence of HClO4 to KIE = 1.0 with increasing concentration of HClO4, accompanied by the large acceleration of the oxidation rate. The absence of KIE suggests that electron transfer from a toluene derivative to the protonated iron(IV)-oxo complex ([(N4Py)Fe(IV)(OH)](3+)) is the rate-determining step in the acid-promoted oxidation reaction. The detailed kinetic analysis in light of the Marcus theory of electron transfer has revealed that the acid-promoted C-H bond cleavage proceeds via the rate-determining electron transfer from toluene derivatives to [(N4Py)Fe(IV)(OH)](3+) through formation of strong precursor complexes between toluene derivatives and [(N4Py)Fe(IV)(OH)](3+).

Effects of Proton Acceptors on Formation of a Non-Heme Iron(IV)–Oxo Complex via Proton-Coupled Electron Transfer

Rates of formation of a non-heme iron(IV)-oxo complex, [Fe(IV)(O)(N4Py)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), via electron-transfer oxidation of [Fe(III)(OH)(N4Py)](2+) in acetonitrile (MeCN) containing H2O (0.56 M) were accelerated as much as 390-fold by addition of proton acceptors such as CF3COO(-), TsO(-) (p-MeC6H4SO3(-)), NsO(-) (o-NO2C6H4SO3(-)), DNsO(-) (2,4-(NO2)2C6H3SO3(-)), and TfO(-) (CF3SO3(-)). The acceleration effect of proton acceptors increases with increasing basicity of the proton acceptors. The one-electron oxidation potential of [Fe(III)(OH)(N4Py)](2+) was shifted from 1.24 to 0.96 V vs SCE in the presence of TsO(-) (10 mM). The electron-transfer oxidation of Fe(III)-OH complex was coupled with the deprotonation process by proton acceptors in which deuterium kinetic isotope effects were observed when H2O was replaced by D2O.

Mechanistic Borderline of One-Step Hydrogen Atom Transfer versus Stepwise Sc3+-Coupled Electron Transfer from Benzyl Alcohol Derivatives to a Non-Heme Iron(IV)-Oxo Complex

The rate of oxidation of 2,5-dimethoxybenzyl alcohol (2,5-(MeO)(2)C(6)H(3)CH(2)OH) by [Fe(IV)(O)(N4Py)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) was enhanced significantly in the presence of Sc(OTf)(3) (OTf(-) = trifluoromethanesulfonate) in acetonitrile (e.g., 120-fold acceleration in the presence of Sc(3+)). Such a remarkable enhancement of the reactivity of [Fe(IV)(O)(N4Py)](2+) in the presence of Sc(3+) was accompanied by the disappearance of a kinetic deuterium isotope effect. The radical cation of 2,5-(MeO)(2)C(6)H(3)CH(2)OH was detected in the course of the reaction in the presence of Sc(3+). The dimerized alcohol and aldehyde were also produced in addition to the monomer aldehyde in the presence of Sc(3+). These results indicate that the reaction mechanism is changed from one-step hydrogen atom transfer (HAT) from 2,5-(MeO)(2)C(6)H(3)CH(2)OH to [Fe(IV)(O)(N4Py)](2+) in the absence of Sc(3+) to stepwise Sc(3+)-coupled electron transfer, followed by proton transfer in the presence of Sc(3+). In contrast, neither acceleration of the rate nor the disappearance of the kinetic deuterium isotope effect was observed in the oxidation of benzyl alcohol (C(6)H(5)CH(2)OH) by [Fe(IV)(O)(N4Py)](2+) in the presence of Sc(OTf)(3). Moreover, the rate constants determined in the oxidation of various benzyl alcohol derivatives by [Fe(IV)(O)(N4Py)](2+) in the presence of Sc(OTf)(3) (10 mM) were compared with those of Sc(3+)-coupled electron transfer from one-electron reductants to [Fe(IV)(O)(N4Py)](2+) at the same driving force of electron transfer. This comparison revealed that the borderline of the change in the mechanism from HAT to stepwise Sc(3+)-coupled electron transfer and proton transfer is dependent on the one-electron oxidation potential of benzyl alcohol derivatives (ca. 1.7 V vs SCE).