Hydrogen Atom Abstraction by Metal−Oxo and Metal−Superoxo Complexes: Kinetics and Thermodynamics

Selective oxygenation of C-H and CC bonds with H2O2 by high-spin cobalt(II)-carboxylate complexes

Four cobalt(II)-carboxylate complexes [(6-Me₃-TPA)Co^II(benzoate)](BPh₄) (1), [(6-Me₃-TPA)Co^II(benzilate)](ClO₄) (2), [(6-Me₃-TPA)Co^II(mandelate)](BPh₄) (3), and [(6-Me₃-TPA)Co^II(MPA)](BPh₄) (4) (HMPA = 2-methoxy-2-phenylacetic acid) of the 6-Me₃-TPA (tris((6-methylpyridin-2-yl)methyl)amine) ligand were isolated to investigate their ability in H₂O₂-dependent selective oxygenation of C-H and CC bonds. All six-coordinate complexes contain a high-spin cobalt(II) center. While the cobalt(II) complexes are inert toward dioxygen, each of these complexes reacts readily with hydrogen peroxide to form a diamagnetic cobalt(III) species, which decays with time leading to the oxidation of the methyl groups on the pyridine rings of the supporting ligand. Intramolecular ligand oxidation by the cobalt-based oxidant is partially inhibited in the presence of external substrates, and the substrates are converted to their corresponding oxidized products. Kinetic studies and labelling experiments indicate the involvement of a metal-based oxidant in affecting the chemo- and stereo-selective catalytic oxygenation of aliphatic C-H bonds and epoxidation of alkenes. An electrophilic cobalt-oxygen species that exhibits a kinetic isotope effect (KIE) value of 5.3 in toluene oxidation by 1 is proposed as the active oxidant. Among the complexes, the cobalt(II)-benzoate (1) and cobalt(II)-MPA (4) complexes display better catalytic activity compared to their α-hydroxy analogues (2 and 3). Catalytic studies with the cobalt(II)-acetonitrile complex [(6-Me₃-TPA)Co^II(CH₃CN)₂](ClO₄)₂ (5) in the presence and absence of externally added benzoate support the role of the carboxylate co-ligand in oxidation reactions. The proposed catalytic reaction involves a carboxylate-bridged dicobalt complex in the activation of H₂O₂ followed by the oxidation of substrates by a metal-based oxidant.

Free Energies of Proton-Coupled Electron Transfer Reagents and Their Applications

We present an update and revision to our 2010 review on the topic of proton-coupled electron transfer (PCET) reagent thermochemistry. Over the past decade, the data and thermochemical formalisms presented in that review have been of value to multiple fields. Concurrently, there have been advances in the thermochemical cycles and experimental methods used to measure these values. This Review (i) summarizes those advancements, (ii) corrects systematic errors in our prior review that shifted many of the absolute values in the tabulated data, (iii) provides updated tables of thermochemical values, and (iv) discusses new conclusions and opportunities from the assembled data and associated techniques. We advocate for updated thermochemical cycles that provide greater clarity and reduce experimental barriers to the calculation and measurement of Gibbs free energies for the conversion of X to XH_n in PCET reactions. In particular, we demonstrate the utility and generality of reporting potentials of hydrogenation, E°(V vs H₂), in almost any solvent and how these values are connected to more widely reported bond dissociation free energies (BDFEs). The tabulated data demonstrate that E°(V vs H₂) and BDFEs are generally insensitive to the nature of the solvent and, in some cases, even to the phase (gas versus solution). This Review also presents introductions to several emerging fields in PCET thermochemistry to give readers windows into the diversity of research being performed. Some of the next frontiers in this rapidly growing field are coordination-induced bond weakening, PCET in novel solvent environments, and reactions at material interfaces.

The interplay of Ag and ferromagnetic MgFe2O4 for optimized oxygen-promoted hydrogen evolution via formaldehyde reforming

The interplay of Ag and ferromagnetic MgFe2O4 for optimized oxygen-promoted hydrogen evolution via formaldehyde reforming.

The Influence of Nucleophilic and Redox Pincer Character as well as Alkali Metals on the Capture of Oxygen Substrates: The Case of Chromium(II)

Dimeric [CrL]₂ , where L is the conjugate base of bis-pyrazolyl pyridine, is evaluated for its ability to undergo inner sphere and outer sphere redox chemistry. It reacts with Cp₂ Fe⁺ to give [Cr₄ (HL)₄ (μ₄ -O)]²⁺ , still containing divalent Cr. Reduction (KC₈ ) of [CrL]₂ by two electrons gives [K₂ (THF)₃ Cr₃ L₃ (μ₃ -O)], and by four electrons gives [K₄ (THF)₁₀ Cr₂ L₂ (μ-O)], each of which has scavenged (hydr)oxide from glass surface because of the electrophilicity of the underligated Cr. [K₄ (THF)₁₀ Cr₂ L₂ (μ-O)], is shown by comprehensive DFT calculations and analysis of intra-ligand bond lengths to contain a pyridyl radical L^3- and Cr^II , illustrating that this pincer is proton-responsive, redox active, and a versatile donor to associated K⁺ ions here. The K⁺ electrophiles interact with electron-rich oxo, but do not significantly (>5 kcal mol^-1 ) alter spin state energies. Inner sphere oxidation of [CrL]₂ with a quinone gives [Cr₂ L₂ (semiquinone)₂ ], while pre-reduced [CrL]₂ ^2- reacts with quinone to give [K₃ (THF)₃ Cr₂ L₂ (catecholate)₂ (μ-OH)], a product of capture of two undercoordinated LCr(catecholate)^1- by hydroxide.

Ultrasmall Silver Clusters Stabilized on MgO for Robust Oxygen-Promoted Hydrogen Production from Formaldehyde Reforming

Efficient molecular hydrogen generation from renewable biomass-derived resources and water is of great importance to the sustainable development of the future society. Herein, ultrasmall Ag nanoclusters supported on a defect-rich MgO matrix (AgUCs/MgO) are synthesized by a facile impregnation/calcination method and are applied to robust oxygen-promoted formaldehyde reforming into H₂ at room temperature. Density functional theory calculations and experimental observations show that the catalyst spatially builds up a channel for directional electron transfer from electron-rich Ag sites to the anti-bonding π orbital of chemisorbed bridged O₂ molecules, leading to the implementation of low-temperature O₂ adsorption and activation. The catalytically active species, ^•OOH, is thus selectively generated via a preferential two-electron reduction of O₂ with a low energy barrier on Ag sites, involving an unusual long-range proton-coupled electron transfer process. The ^•OOH-AgUCs/MgO active center is efficient for the subsequent C-H activation and H₂ generation, leading to a 3-fold improvement of the turnover frequency as compared with its analogous AgNPs/MgO catalyst. Our atomic-level design and synthetic strategy provide a platform that facilitates the construction of an electron-proton transfer channel for catalysis, altered adsorption configurations of activated reactants, and enhancement of catalytic hydrogen generation activity, extending a promising direction for the development of next-generation energy catalysts.

Oxygen activation by mononuclear Mn, Co, and Ni centers in biology and synthetic complexes

The active sites of metalloenzymes that catalyze O₂-dependent reactions generally contain iron or copper ions. However, several enzymes are capable of activating O₂ at manganese or nickel centers instead, and a handful of dioxygenases exhibit activity when substituted with cobalt. This minireview summarizes the catalytic properties of oxygenases and oxidases with mononuclear Mn, Co, or Ni active sites, including oxalate-degrading oxidases, catechol dioxygenases, and quercetin dioxygenase. In addition, recent developments in the O₂ reactivity of synthetic Mn, Co, or Ni complexes are described, with an emphasis on the nature of reactive intermediates featuring superoxo-, peroxo-, or oxo-ligands. Collectively, the biochemical and synthetic studies discussed herein reveal the possibilities and limitations of O₂ activation at these three "overlooked" metals.

Photocatalytic Oxygenation of Substrates by Dioxygen with Protonated Manganese(III) Corrolazine

UV-vis spectral titrations of a manganese(III) corrolazine complex [Mn(III)(TBP8Cz)] with HOTf in benzonitrile (PhCN) indicate mono- and diprotonation of Mn(III)(TBP8Cz) to give Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] with protonation constants of 9.0 × 10(6) and 4.7 × 10(3) M(-1), respectively. The protonated sites of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] were identified by X-ray crystal structures of the mono- and diprotonated complexes. In the presence of HOTf, the monoprotonated manganese(III) corrolazine complex [Mn(III)(OTf)(TBP8Cz(H))] acts as an efficient photocatalytic catalyst for the oxidation of hexamethylbenzene and thioanisole by O2 to the corresponding alcohol and sulfoxide with 563 and 902 TON, respectively. Femtosecond laser flash photolysis measurements of Mn(III)(OTf)(TBP8Cz(H)) and [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] in the presence of O2 revealed the formation of a tripquintet excited state, which was rapidly converted to a tripseptet excited state. The tripseptet excited state of Mn(III)(OTf)(TBP8Cz(H)) reacted with O2 with a diffusion-limited rate constant to produce the putative Mn(IV)(O2(•-))(OTf)(TBP8Cz(H)), whereas the tripseptet excited state of [Mn(III)(OTf)(H2O)(TBP8Cz(H)2)][OTf] exhibited no reactivity toward O2. In the presence of HOTf, Mn(V)(O)(TBP8Cz) can oxidize not only HMB but also mesitylene to the corresponding alcohols, accompanied by regeneration of Mn(III)(OTf)(TBP8Cz(H)). This thermal reaction was examined for a kinetic isotope effect, and essentially no KIE (1.1) was observed for the oxidation of mesitylene-d12, suggesting a proton-coupled electron transfer (PCET) mechanism is operative in this case. Thus, the monoprotonated manganese(III) corrolazine complex, Mn(III)(OTf)(TBP8Cz(H)), acts as an efficient photocatalyst for the oxidation of HMB by O2 to the alcohol.

Catalytic two-electron reduction of dioxygen by ferrocene derivatives with manganese(V) corroles

Electron transfer from octamethylferrocene (Me8Fc) to the manganese(V) imidocorrole complex (tpfc)Mn(V)(NAr) [tpfc = 5,10,15-tris(pentafluorophenyl)corrole; Ar = 2,6-Cl2C6H3] proceeds efficiently to give an octamethylferrocenium ion (Me8Fc(+)) and [(tpfc)Mn(IV)(NAr)](-) in acetonitrile (MeCN) at 298 K. Upon the addition of trifluoroacetic acid (TFA), further reduction of [(tpfc)Mn(IV)(NAr)](-) by Me8Fc gives (tpfc)Mn(III) and ArNH2 in deaerated MeCN. TFA also results in hydrolysis of (tpfc)Mn(V)(NAr) with residual water to produce a protonated manganese(V) oxocorrole complex ([(tpfc)Mn(V)(OH)](+)) in deaerated MeCN. [(tpfc)Mn(V)(OH)](+) is rapidly reduced by 2 equiv of Me8Fc in the presence of TFA to give (tpfc)Mn(III) in deaerated MeCN. In the presence of dioxygen (O2), (tpfc)Mn(III) catalyzes the two-electron reduction of O2 by Me8Fc with TFA in MeCN to produce H2O2 and Me8Fc(+). The rate of formation of Me8Fc(+) in the catalytic reduction of O2 follows zeroth-order kinetics with respect to the concentrations of Me8Fc and TFA, whereas the rate increases linearly with increasing concentrations of (tpfc)Mn(V)(NAr) and O2. These kinetic dependencies are consistent with the rate-determining step being electron transfer from (tpfc)Mn(III) to O2, followed by further proton-coupled electron transfer from Me8Fc to produce H2O2 and [(tpfc)Mn(IV)](+). Rapid electron transfer from Me8Fc to [(tpfc)Mn(IV)](+) regenerates (tpfc)Mn(III), completing the catalytic cycle. Thus, catalytic two-electron reduction of O2 by Me8Fc with (tpfc)Mn(V)(NAr) as a catalyst precursor proceeds via a Mn(III)/Mn(IV) redox cycle.

Moving protons and electrons in biomimetic systems

An enormous variety of biological redox reactions are accompanied by changes in proton content at enzyme active sites, in their associated cofactors, in substrates and/or products, and between protein interfaces. Understanding this breadth of reactivity is an ongoing chemical challenge. A great many workers have developed and investigated biomimetic model complexes to build new ways of thinking about the mechanistic underpinnings of such complex biological proton-coupled electron transfer (PCET) reactions. Of particular importance are those model reactions that involve transfer of one proton (H(+)) and one electron (e(-)), which is equivalent to transfer of a hydrogen atom (H(•)). In this Current Topic, we review key concepts in PCET reactivity and describe important advances in biomimetic PCET chemistry, with a special emphasis on research that has enhanced efforts to understand biological PCET reactions.

Hydrogen atom abstraction from hydrocarbons by a copper(III)-hydroxide complex

With the aim of understanding the basis for the high rate of hydrogen atom abstraction (HAT) from dihydroanthracene (DHA) by the complex LCuOH (1; L = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide), the bond dissociation enthalpy of the reaction product LCu(H2O) (2) was determined through measurement of its pK(a) and E(1/2) in THF solution. In so doing, an equilibrium between 2 and LCu(THF) was characterized by UV-vis and EPR spectroscopy and cyclic voltammetry (CV). A high pK(a) of 18.8 ± 1.8 and a low E(1/2) of -0.074 V vs Fc/Fc(+) in THF combined to yield an O-H BDE for 2 of 90 ± 3 kcal mol(-1) that is large relative to values for most transition metal oxo/hydroxo complexes. By taking advantage of the increased stability of 1 observed in 1,2-difluorobenzene (DFB) solvent, the kinetics of the reactions of 1 with a range of substrates with varying BDE values for their C-H bonds were measured. The oxidizing power of 1 was revealed through the accelerated decay of 1 in the presence of the substrates, including THF (BDE = 92 kcal mol(-1)) and cyclohexane (BDE = 99 kcal mol(-1)). CV experiments in THF solvent showed that 1 reacted with THF via rate-determining attack at the THF C-H(D) bonds with a kinetic isotope effect of 10.2. Analysis of the kinetic and thermodynamic data provides new insights into the basis for the high reactivity of 1 and the possible involvement of species like 1 in oxidation catalysis.

Oxygen activation with transition-metal complexes in aqueous solution

Coordination to transition-metal complexes changes both the thermodynamics and kinetics of oxygen reduction. Some of the intermediates (superoxo, hydroperoxo, and oxo species) are close analogues of organic oxygen-centered radicals and peroxides (ROO(*), ROOH, and RO(*)). Metal-based intermediates are typically less reactive, but more persistent, than organic radicals, which makes the two types of intermediates similarly effective in their reactions with various substrates. The self-exchange rate constant for hydrogen-atom transfer for the couples Cr(aq)OO(2+)/Cr(aq)OOH(2+) and L(1)(H(2)O)RhOO(2+)/L(1)(H(2)O)RhOOH(2+) was estimated to be 10(1+/-1) M(-1) s(-1). The use of this value in the simplified Marcus equation for the Cr(aq)O(2+)/Cr(aq)OOH(2+) cross reaction provided an upper limit k(CrO,CrOH) <or= 10((-2+/-1)) M(-1) s(-1) for Cr(aq)O(2+)/Cr(aq)OH(2+) self-exchange. Even though superoxo complexes react very slowly in bimolecular self-reactions, extremely fast cross reactions with organic counterparts, i.e., acylperoxyl radicals, have been observed. Many of the intermediates generated by the interaction of O(2) with reduced metal complexes can also be accessed by alternative routes, both thermal and photochemical.

Reaction of hydrogen peroxide with coordinated superoxide in [(NH3)5CoIII(micro-O2)CoIII(NH3)5]5+: a mechanistic study

In aqueous acetate buffer media, hydrogen peroxide reduces the bridging superoxide in [(NH3)5CoIII(micro-O2)CoIII(NH3)5]5+ (1) to the corresponding peroxide in the complex, [(NH3)5CoIII(micro-O2H)CoIII(NH2)(NH3)4]4+ (2), itself being oxidized to HO2*. The complex 2 thus produced decomposes rapidly to the final products, CoII, NH3, etc. instead of reacting with a second molecule of hydrogen peroxide. In the presence of excess [H2O2] over (1), the reaction obeyed first-order kinetics and exhibited inverse proton dependence. [(NH3)5CoIII(micro-O2)CoIII((NH2)(NH3)4]4+ (3), a conjugate base of 1, seems to be the kinetically reactive species and the cause for the observed inverse proton dependence. Kinetics is little affected when one of the hydrogen atoms from hydrogen peroxide is replaced with an alkyl group, as in tert-butyl hydroperoxide. But replacement of both the H atoms with alkyl groups halts the reaction as seen with di-tert-butyl peroxides, and peroxodisulfate ion. The reaction rate with hydrogen peroxide significantly decreases with increasing proportion of D2O replacing water in the solvent and the rate-limiting step seems to be an H-atom transfer.

Slow hydrogen atom transfer reactions of oxo- and hydroxo-vanadium compounds: the importance of intrinsic barriers

Reactions are described that interconvert vanadium(IV) oxo-hydroxo complexes [V(IV)O(OH)(R(2)bpy)(2)]BF(4) (1a-c) and vanadium(V) dioxo complexes [V(V)O(2)(R(2)bpy)(2)]BF(4) (2a-c) [R(2)bpy = 4,4'-di-tert-butyl-2,2'-bipyridine ((t)Bu(2)bpy), a; 4,4'-dimethyl-2,2'-bipyridine (Me(2)bpy), b; 2,2'-bipyridine (bpy), c]. These are rare examples of pairs of isolated, sterically unencumbered, first-row metal-oxo/hydroxo complexes that differ by a hydrogen atom (H(+) + e(-)). The V(IV)-(t)Bu(2)bpy derivative 1a has a useful (1)H NMR spectrum, despite being paramagnetic. Complex 2a abstracts H(*) from organic substrates with weak O-H and C-H bonds, converting 2,6-(t)Bu(2)-4-MeO-C(6)H(2)OH (ArOH) and 2,2,6,6-tetramethyl-N-hydroxypiperidine (TEMPOH) to their corresponding radicals ArO(*) and TEMPO, hydroquinone to benzoquinone, and dihydroanthracene to anthracene. The equilibrium constant for 2a + ArOH <==> 1a + ArO(*) is (4 +/- 2) x 10(-3), implying that the VO-H bond dissociation free energy (BDFE) is 70.6 +/- 1.2 kcal mol(-1). Consistent with this value, 1a is oxidized by 2,4,6-(t)Bu(3)C(6)H(2)O(*). All of these reactions are surprisingly slow, typically occurring over hours at ambient temperatures. The net hydrogen-atom pseudo-self-exchange 1a + 2b <==> 2a + 1b, using the (t)Bu- and Me-bpy substituents as labels, also occurs slowly, with k(se) = 1.3 x 10(-2) M(-1) s(-1) at 298 K, DeltaH(double dagger) = 15 +/- 2 kcal mol(-1), and DeltaS(double dagger) = 16 +/- 5 cal mol(-1) K. Using this k(se) and the BDFE, the vanadium reactions are shown to follow the Marcus cross relation moderately well, with calculated rate constants within 10(2) of the observed values. The vanadium self-exchange reaction is ca. 10(6) slower than that for the related Ru(IV)O(py)(bpy)(2)(2+)/Ru(III)OH(py)(bpy)(2)(2+) self-exchange. The origin of this dramatic difference has been probed with DFT calculations on the self-exchange reactions of 1c + 2c and on monocationic ruthenium complexes with pyrrolate or fluoride in place of the py ligands. The calculations reproduce the difference in barrier heights and show that transfer of a hydrogen atom involves more structural reorganization for vanadium than the Ru analogues. The vanadium complexes have larger changes in the metal-oxo and metal-hydroxo bond lengths, which is traced to the difference in d-orbital occupancy in the two systems. This study thus highlights the importance of intrinsic barriers in the transfer of a hydrogen atom, in addition to the thermochemical (bond strength) factors that have been previously emphasized.

Kinetics of oxidation of nitroxyl radicals with superoxometal complexes of chromium and rhodium

In acidic aqueous solutions, nitroxyl radicals (X)TEMPO (X = H, 4-OH, and 4-oxo) and 3-carbamoyl-PROXYL readily reduce CraqOO2+ and Rh(NH3)4(H2O)OO2+ to the corresponding hydroperoxo complexes. The kinetics are largely acid independent for CraqOO2+, but acid catalysis dominates the reactions of the rhodium complex. This emerging trend in oxidations with superoxometal complexes seems to be directly related to the thermodynamics of electron transfer. The weaker the oxidant, the more important the acid-assisted path. The rate constants for the oxidation of (X)TEMPO by CraqOO2+ are 406 M(-1) s(-1) (X = H), 159 (4-OH), and (20. 6 + 77.5 [H+]) (4-oxo). For the rhodium complex, the values are (40 + 2.20 x 10(3) [H+]) (X = H), (25 + 1.10 x 10(3) [H+]) (4-HO), and 2.21 x 10(3) [H+] (4-oxo). An inverse solvent kinetic isotope effect, kH/kD = 0.8, was observed in the reaction between (O)TEMPO and (NH3)4(H(D))2O)RhOO2+ in 0.10 M H(D)ClO4 in H2O and D2O.

Hydrogen-atom transfer from transition metal hydroperoxides, hydrogen peroxide, and alkyl hydroperoxides to superoxo and oxo metal complexes

Superoxochromium(III) complexes L(H2O)CrOO2+ (L = (H2O)4 and 1,4,8,11-tetraazacyclotetradecane) oxidize hydroperoxo complexes of rhodium and cobalt in an apparent hydrogen-atom transfer process, i.e., L(H2O)CrOO2+ + L(H2O)RhOOH2+ --> L(H2O)CrOOH2+ + L(H2O)RhOO2+. All of the measured rate constants fall in a narrow range, 17-135 M-1 s-1. These values are about 2.5-3.0 times smaller in D2O, where the hydroperoxo hydrogen is replaced by deuterium, and coordinated molecules of water by D2O. The failure of the back reaction to take place in the available concentration range places the O-H bond dissociation energy in RhOO-H2+ at <or=320 kJ/mol. The rates of oxidation of L(H2O)RhOOH2+ by CraqOO2+ are comparable to those for the oxidation of the corresponding hydrides despite the great difference (>or=80 kJ/mol) in the driving force for the two types of reactions. A chromyl ion, CrIVaqO2+, oxidizes L(H2O)RhOOH2+ and the cobalt analogs to the corresponding superoxo complexes. The rate constants are approximately 102-fold larger than those for the oxidation by CraqOO2+. The oxidation of tert-BuOOH by CrIVaqO2+ has k = 160 M-1 s-1 and exhibits an isotope effect kBuOOH/kBuOOD = 12. Hydrogen atom transfer from H2O2 to CraqOO2+ is slow, k approximately 10-3 M-1 s-1.

Reduction and Oxidation of Hydroperoxo Rhodium(III) Complexes by Halides and Hypobromous Acid

Oxygen atom transfer from trans-L(H(2)O)RhOOH(2+) [L = [14]aneN(4) (L(1)), meso-Me(6)[14]aneN(4) (L(2)), and (NH(3))(4)] to iodide takes place according to the rate law -d[L(H(2)O)RhOOH(2+)]/dt = k(I)[L(H(2)O)RhOOH(2+)][I(-)][H(+)]. At 0.10 M ionic strength and 25 degrees C, the rate constant k(I)/M(-)(2) s(-)(1) has values of 8.8 x 10(3) [L = (NH(3))(4)], 536 (L(1)), and 530 (L(2)). The final products are LRh(H(2)O)(2)(3+) and I(2)/I(3)(-). The (NH(3))(4)(H(2)O)RhOOH(2+)/Br(-) reaction also exhibits mixed third-order kinetics with k(Br) approximately 1.8 M(-)(2) s(-)(1) at high concentrations of acid (close to 1 M) and bromide (close to 0.1 M) and an ionic strength of 1.0 M. Under these conditions, Br(2)/Br(3)(-) is produced in stoichiometric amounts. As the concentrations of acid and bromide decrease, the reaction begins to generate O(2) at the expense of Br(2), until the limit at which [H(+)] <or= 0.10 M and [Br(-)] <or= 0.010 M, when Br(2)/Br(3)(-) is no longer observed and O(2) is produced quantitatively. At this limit, the loss of (NH(3))(4)(H(2)O)RhOOH(2+) is about twice as fast as it is at the high [H(+)] and [Br(-)] extreme, and the stoichiometry is 2(NH(3))(4)(H(2)O)RhOOH(2+) --> 2(NH(3))(4)(H(2)O)RhOH(2+) + O(2); i.e., the reaction has turned into the bromide-catalyzed disproportionation of coordinated hydroperoxide. In the proposed mechanism, the hydrolysis of the initially formed Br(2) produces HOBr, the active oxidant for the second equivalent of (NH(3))(4)(H(2)O)RhOOH(2+). The rate constant k(HOBr) for the HOBr/(NH(3))(4)(H(2)O)RhOOH(2+) reaction is 2.9 x 10(8) M(-)(1) s(-)(1).

X-ray Absorption Spectroscopic Studies of Chromium(V/IV/III)− 2-Ethyl-2-hydroxybutanoato(2−/1−) Complexes

Structures of the complexes [Cr(V)O(ehba)(2)](-), [Cr(IV)O(ehbaH)(2)](0), and [Cr(III)(ehbaH)(2)(OH(2))(2)](+) (ehbaH(2) = 2-ethyl-2-hydroxybutanoic acid) in frozen aqueous solutions (10 K, [Cr] = 10 mM, 1.0 M ehbaH(2)/ehbaH, pH 3.5) have been determined by single- and multiple-scattering fitting of X-ray absorption fine structure (XAFS) data. An optimal set of fitting parameters has been determined from the XAFS calculations for a compound with known crystal structure, Na[Cr(V)O(ehba)(2)] (solid, 10 K). The structure of the Cr(V) complex [Cr(V)O(ehba)(2)](-) does not change in solution in the presence of excess ligand. Contrary to the earlier suggestions made from the kinetic data (Ghosh, M. C.; Gould, E. S. J. Chem. Soc., Chem. Commun. 1992, 195-196), the structure of the Cr(IV) complex (generated by the Cr(VI) + As(III) + ehbaH(2) reaction) is close to that of the Cr(V) complex (five-coordinate, distorted trigonal bipyramidal) and different from that of the Cr(III) complex (six-coordinate, octahedral). For both Cr(V) and Cr(IV) complexes, some disorder in the position of the oxo group is observed, which is consistent with but not definitive for the presence of geometric isomers. The structure of the Cr(IV) complex differs from that of Cr(V) by protonation of alcoholato groups of the ligands, which leads to significant elongation of the corresponding Cr-O bonds (2.0 vs 1.8 A). This is reflected in the different chemical properties reported previously for the Cr(IV) and Cr(V) complexes, including their reactivities toward DNA and other biomolecules in relation to Cr-induced carcinogenicity.

Evidence that dioxygen and substrate activation are tightly coupled in dopamine beta-monooxygenase. Implications for the reactive oxygen species

Oxygen activation occurs at a wide variety of enzyme active sites. Mechanisms previously proposed for the copper monooxygenase, dopamine beta-monooxygenase (DbetaM), involve the accumulation of an activated oxygen intermediate with the properties of a copper-peroxo or copper-oxo species before substrate activation. These are reminiscent of the mechanism of cytochrome P-450, where a heme iron stabilizes the activated O2 species. Herein, we report two experimental probes of the activated oxygen species in DbetaM. First, we have synthesized the substrate analog, beta,beta-difluorophenethylamine, and examined its capacity to induce reoxidation of the prereduced copper sites of DbetaM upon mixing with O2 under rapid freeze-quench conditions. This experiment fails to give rise to an EPR-detectable copper species, in contrast to a substrate with a C-H active bond. This indicates either that the reoxidation of the enzyme-bound copper sites in the presence of O2 is tightly linked to C-H activation or that a diamagnetic species Cu(II)-O2* has been formed. In the context of the open and fully solvent-accessible active site for the homologous peptidylglycine-alpha-hydroxylating monooxygenase and by analogy to cytochrome P-450, the accumulation of a reduced and activated oxygen species in DbetaM before C-H cleavage would be expected to give some uncoupling of oxygen and substrate consumption. We have, therefore, examined the degree to which O2 and substrate consumption are coupled in DbetaM using both end point and initial rate experimental protocols. With substrates that differ by more than three orders of magnitude in rate, we fail to detect any uncoupling of O2 uptake from product formation. We conclude that there is no accumulation of an activated form of O2 before C-H abstraction in the DbetaM and peptidylglycine-alpha-hydroxylating monooxygenase class of copper monooxygenases, presenting a mechanism in which a diamagnetic Cu(II)-superoxo complex, formed initially at very low levels, abstracts a hydrogen atom from substrate to generate Cu(II)-hydroperoxo and substrate-free radical as intermediates. Subsequent participation of the second copper site per subunit completes the reaction cycle, generating hydroxylated product and water.

Oxidation of C-H bonds by [(bpy)2(py)RuIVO]2+ occurs by hydrogen atom abstraction

Anaerobic oxidations of 9,10-dihydroanthracene (DHA), xanthene, and fluorene by [(bpy)(2)(py)Ru(IV)O](2+) in acetonitrile solution give mixtures of products including oxygenated and non-oxygenated compounds. The products include those formed by organic radical dimerization, such as 9,9'-bixanthene, as well as by oxygen-atom transfer (e.g., xanthone). The kinetics of these reactions have been measured. The kinetic isotope effect for oxidation of DHA vs DHA-d(4) gives k(H)/k(D) > or = 35 +/- 1. The data indicate a mechanism of initial hydrogen-atom abstraction forming radicals that dimerize, disproportionate and are trapped by the oxidant. This mechanism also appears to apply to the oxidations of toluene, ethylbenzene, cumene, indene, and cyclohexene. The rate constants for H-atom abstraction from these substrates correlate well with the strength of the C-H bond that is cleaved. Rate constants for abstraction from DHA and toluene also correlate with those for oxygen radicals and other oxidants. The rate constant for H-atom transfer from toluene to [(bpy)(2)(py)Ru(IV)O](2+) appears to be close to that predicted by the Marcus cross relation, using a tentative rate constant for hydrogen atom self-exchange between [(bpy)(2)(py)Ru(III)OH](2+) and [(bpy)(2)(py)Ru(IV)O](2+).

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).

Water oxidation chemistry of photosystem II

The O(2)-evolving complex of photosystem II catalyses the light-driven four-electron oxidation of water to dioxygen in photosynthesis. In this article, the steps leading to photosynthetic O(2) evolution are discussed. Emphasis is given to the proton-coupled electron-transfer steps involved in oxidation of the manganese cluster by oxidized tyrosine Z (Y(*)(Z)), the function of Ca(2+) and the mechanism by which water is activated for formation of an O-O bond. Based on a consideration of the biophysical studies of photosystem II and inorganic manganese model chemistry, a mechanism for photosynthetic O(2) evolution is presented in which the O-O bond-forming step occurs via nucleophilic attack on an electron-deficient Mn(V)=O species by a calcium-bound water molecule. The proposed mechanism includes specific roles for the tetranuclear manganese cluster, calcium, chloride, Y(Z) and His190 of the D1 polypeptide. Recent studies of the ion selectivity of the calcium site in the O(2)-evolving complex and of a functional inorganic manganese model system that test key aspects of this mechanism are also discussed.

Hydrocarbon oxidation by Bis-mu-oxo manganese dimers: electron transfer, hydride transfer, and hydrogen atom transfer mechanisms

Described here are oxidations of alkylaromatic compounds by dimanganese mu-oxo and mu-hydroxo dimers [(phen)(2)Mn(IV)(mu-O)(2)Mn(IV)(phen)(2)](4+) ([Mn(2)(O)(2)](4+)), [(phen)(2)Mn(IV)(mu-O)(2)Mn(III)(phen)(2)](3+) ([Mn(2)(O)(2)](3+)), and [(phen)(2)Mn(III)(mu-O)(mu-OH)Mn(III)(phen)(2)](3+) ([Mn(2)(O)(OH)](3+)). Dihydroanthracene, xanthene, and fluorene are oxidized by [Mn(2)(O)(2)](3+) to give anthracene, bixanthenyl, and bifluorenyl, respectively. The manganese product is the bis(hydroxide) dimer, [(phen)(2)Mn(III)(mu-OH)(2)Mn(II)(phen)(2)](3+) ([Mn(2)(OH)(2)](3+)). Global analysis of the UV/vis spectral kinetic data shows a consecutive reaction with buildup and decay of [Mn(2)(O)(OH)](3+) as an intermediate. The kinetics and products indicate a mechanism of hydrogen atom transfers from the substrates to oxo groups of [Mn(2)(O)(2)](3+) and [Mn(2)(O)(OH)](3+). [Mn(2)(O)(2)](4+) is a much stronger oxidant, converting toluene to tolyl-phenylmethanes and naphthalene to binaphthyl. Kinetic and mechanistic data indicate a mechanism of initial preequilibrium electron transfer for p-methoxytoluene and naphthalenes because, for instance, the reactions are inhibited by addition of [Mn(2)(O)(2)](3+). The oxidation of toluene by [Mn(2)(O)(2)](4+), however, is not inhibited by [Mn(2)(O)(2)](3+). Oxidation of a mixture of C(6)H(5)CH(3) and C(6)H(5)CD(3) shows a kinetic isotope effect of 4.3 +/- 0.8, consistent with C-H bond cleavage in the rate-determining step. The data indicate a mechanism of initial hydride transfer from toluene to [Mn(2)(O)(2)](4+). Thus, oxidations by manganese oxo dimers occur by three different mechanisms: hydrogen atom transfer, electron transfer, and hydride transfer. The thermodynamics of e(-), H(*), and H(-) transfers have been determined from redox potential and pK(a) measurements. For a particular oxidant and a particular substrate, the choice of mechanism is influenced both by the thermochemistry and by the intrinsic barriers. Rate constants for hydrogen atom abstraction by [Mn(2)(O)(2)](3+) and [Mn(2)(O)(OH)](3+) are consistent with their 79 and 75 kcal mol(-)(1) affinities for H(*). In the oxidation of p-methoxytoluene by [Mn(2)(O)(2)](4+), hydride transfer is thermochemically 24 kcal mol(-)(1) more facile than electron transfer; yet the latter mechanism is preferred. Thus, electron transfer has a substantially smaller intrinsic barrier than does hydride transfer in this system.

Reactions of superoxo and oxo metal complexes with aldehydes. Radical-specific pathways for cross-disproportionation of superoxometal ions and acylperoxyl radicals

The aquachromyl(IV) ion, Cr(aq)O(2+), reacts with acetaldehyde and pivaldehyde by hydrogen atom abstraction and, in the presence of O(2), produces acylperoxyl radicals, RC(O)OO(*). In the next step, the radicals react with Cr(aq)OO(2+), a species accompanying Cr(aq)O(2+) in our preparations. The rate constant for the Cr(aq)OO(2+)/CH(3)C(O)OO(*) cross reaction, k(Cr) = 1.5 x 10(8) M(-1) s(-1), was determined by laser flash photolysis. The evidence points to radical coupling at the remote oxygen of Cr(aq)OO(2+), followed by elimination of O(2) and formation of CH(3)COOH and Cr(V)(aq)O(3+). The latter disproportionates and ultimately yields Cr(aq)(3+) and HCrO(4)(-). No CO(2) was detected. The Cr(aq)OO(2+)/C(CH(3))(3)C(O)OO(*) reaction yielded isobutene, CO(2), and Cr(aq)(3+), in addition to chromate. In the suggested mechanism, the transient Cr(aq)OOOO(O)CC(CH(3))(3)(2+) branches into two sets of products. The path leading to chromate resembles the CH(3)C(O)OO(*) reaction. The other products arise from an unprecedented intramolecular hydrogen transfer from the tert-butyl group to the CrO entity and elimination of CO(2) and O(2). A portion of C(CH(3))(3)C(O)OO(*) was captured by (CH(3))(3)COO(*), which was in turn generated by decarbonylation of acyl radicals and oxygenation of tert-butyl radicals so formed.

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).

Rapid cross-disproportionation between superoxometal ions and acylperoxyl radicals

A superoxochromium complex Cr(aq)OO(2+) reacts with acetylperoxyl radicals, CH(3)C(O)OO(*), with a rate constant of 1.49 x 10(8) M(-1) s(-1). The kinetics were determined by laser flash photolysis, using an organocobalt complex as a radical precursor and ABTS(*-) as a kinetic probe. The initial step is believed to involve radical coupling at the remote oxygen of Cr(aq)OO(2+), followed by elimination of O(2) and formation of CH(3)COOH and Cr(V)(aq)O(3+). The latter disproportionates and ultimately yields Cr(aq)(3+) and HCrO(4)(-).

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).

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.

Kinetics and mechanism of the oxidation of a substituted phenol by a superoxochromium(III) ion

A superoxochromium(III) ion, CraqOO2+, abstracts the hydrogen atom from the hydroxylic group of a substituted, cationic phenol (ArOH), kCrOO = 1.24 M-1 s-1 in acidic aqueous solution at 25 degrees C. The reaction has a large kinetic isotope effect, kArOH/kArOD approximately 12 and produces ArO., which also reacts with CraqOO2+ in a rapid second step, kArO = 1.26 x 10(4) M-1 s-1. The final oxidation product is an o-quinone, which was identified by its behavior on a cation-exchange resin, UV-visible spectrum, and reaction with iodide ions. This work has extended to three the types of element-hydrogen bonds that react with CraqOO2+ about 10(2) times more slowly than with CraqO2+. The mechanistic implications of these findings are discussed.

Mechanism of photosynthetic water oxidation: combining biophysical studies of photosystem II with inorganic model chemistry

A mechanism for photosynthetic water oxidation is proposed based on a structural model of the oxygen-evolving complex (OEC) and its placement into the modeled structure of the D1/D2 core of photosystem II. The structural model of the OEC satisfies many of the geometrical constraints imposed by spectroscopic and biophysical results. The model includes the tetranuclear manganese cluster, calcium, chloride, tyrosine Z, H190, D170, H332 and H337 of the D1 polypeptide and is patterned after the reversible O2-binding diferric site in oxyhemerythrin. The mechanism for water oxidation readily follows from the structural model. Concerted proton-coupled electron transfer in the S2-->S3 and S3-->S4 transitions forms a terminal Mn(V)=O moiety. Nucleophilic attack on this electron-deficient Mn(V)=O by a calcium-bound water molecule results in a Mn(III)-OOH species, similar to the ferric hydroperoxide in oxyhemerythrin. Dioxygen is released in a manner analogous to that in oxyhemerythrin, concomitant with reduction of manganese and protonation of a mu-oxo bridge.