Laser flash photolysis generation of high-valent transition metal-oxo species: insights from kinetic studies in real time

High-valenttransition metal-oxo species are active oxidizing species in many metal-catalyzed oxidation reactions in both Nature and the laboratory. In homogeneous catalytic oxidations, a transition metal catalyst is oxidized to a metal-oxo species by a sacrificial oxidant, and the activated transition metal-oxo intermediate oxidizes substrates. Mechanistic studies of these oxidizing species can provide insights for understanding commercially important catalytic oxidations and the oxidants in cytochrome P450 enzymes. In many cases, however, the transition metal oxidants are so reactive that they do not accumulate to detectable levels in mixing experiments, which have millisecond mixing times, and successful generation and direct spectroscopic characterization of these highly reactive transients remain a considerable challenge. Our strategy for understanding homogeneous catalysis intermediates employs photochemical generation of the transients with spectroscopic detection on time scales as short as nanoseconds and direct kinetic studies of their reactions with substrates by laser flash photolysis (LFP) methods. This Account describes studies of high-valent manganese- and iron-oxo intermediates. Irradiation of porphyrin-manganese(III) nitrates and chlorates or corrole-manganese(IV) chlorates resulted in homolytic cleavage of the O-X bonds in the ligands, whereas irradiation of porphyrin-manganese(III) perchlorates resulted in heterolytic cleavage of O-Cl bonds to give porphyrin-manganese(V)-oxo cations. Similar reactions of corrole- and porphyrin-iron(IV) complexes gave highly reactive transients that were tentatively identified as macrocyclic ligand-iron(V)-oxo species. Kinetic studies demonstrated high reactivity of the manganese(V)-oxo species, and even higher reactivities of the putative iron(V)-oxo transients. For example, second-order rate constants for oxidations of cis-cyclooctene at room temperature were 6 x 10(3) M(-1) s(-1) for a corrole-iron(V)-oxo species and 1.6 x 10(6) M(-1) s(-1) for the putative tetramesitylporphyrin-iron(V)-oxo perchlorate species. The latter rate constant is 25,000 times larger than that for oxidation of cis-cyclooctene by iron(IV)-oxo perchlorate tetramesitylporphyrin radical cation, which is the thermodynamically favored electronic isomer of the putative iron(V)-oxo species. The LFP-determined rate constants can be used to implicate the transient oxidants in catalytic reactions under turnover conditions where high-valent species are not observable. Similarly, the observed reactivities of the putative porphyrin-iron(V)-oxo species might explain the unusually high reactivity of oxidants produced in the cytochrome P450 enzymes, heme-thiolate enzymes that are capable of oxidizing unactivated carbon-hydrogen bonds in substrates so rapidly that iron-oxo intermediates have not been detected under physiological conditions.     

Reactivity of high-valent iron-oxo species in enzymes and synthetic reagents: a tale of many states

The Account discusses the phenomenon of two-state reactivity (TSR) or multistate reactivity (MSR) in high-valent metal-oxo reagents, projecting its wide-ranging applicability starting from the bare species, through the reagents made by Que, Nam, and collaborators, to the Mn(V)-oxo substituted polyoxometalate, all the way to Compound I species of heme enzymes. The Account shows how the behaviors of all these variegated species derive from a simple set of electronic structure principles. Experimental trends that demonstrate TSR and MSR are discussed. Diagnostic mechanistic probes are proposed for the TSR/MSR scenario, based on kinetic isotope effect, stereochemical studies, and magnetic- and electric-field effects.     

Metal-Oxo Species in P450 Enzymes and Biomimetic Models. Oxo-Hydroxo Tautomerism with Water-Soluble Metalloporphyrins

Oxygenation reactions (hydroxylation, epoxidation, N- or S-oxide formation, etc.) catalyzed by cytochrome P450 enzymes and related biomimetic models involve an electrophilic oxidative species as the active species, namely a high-valent metal-oxo intermediate. Among the different methods to study the oxygenation reactions mediated by high-valent metal-oxo porphyrin complexes, the recent discovery of oxo-hydroxo tautomerism provides a useful tool to investigate the mechanism of O-atom transfer reactions in aqueous media.     

Mononuclear metal-o(2) complexes bearing macrocyclic N-tetramethylated cyclam ligands

Metalloenzymes activate dioxygen to carry out a variety of biological reactions, including the biotransformation of naturally occurring molecules, oxidative metabolism of xenobiotics, and oxidative phosphorylation. The dioxygen activation at the catalytic sites of the enzymes occurs through several steps, such as the binding of O(2) at a reduced metal center, the generation of metal-superoxo and -peroxo species, and the O-O bond cleavage of metal-hydroperoxo complexes to form high-valent metal-oxo oxidants. Because these mononuclear metal-dioxygen (M-O(2)) adducts are implicated as key intermediates in dioxygen activation reactions catalyzed by metalloenzymes, studies of the structural and spectroscopic properties and reactivities of synthetic biomimetic analogues of these species have aided our understanding of their biological chemistry. One particularly versatile class of biomimetic coordination complexes for studying dioxygen activation by metal complexes is M-O(2) complexes bearing the macrocyclic N-tetramethylated cyclam (TMC) ligand. This Account describes the synthesis, structural and spectroscopic characterization, and reactivity studies of M-O(2) complexes bearing tetraazamacrocyclic n-TMC ligands, where M ═ Cr, Mn, Fe, Co, and Ni and n = 12, 13, and 14, based on recent results from our laboratory. We have used various spectroscopic techniques, including resonance Raman and X-ray absorption spectroscopy, and density functional theory (DFT) calculations to characterize several novel metal-O(2) complexes. Notably, X-ray crystal structures had shown that these complexes are end-on metal-superoxo and side-on metal-peroxo species. The metal ions and the ring size of the macrocyclic TMC ligands control the geometric and electronic structures of the metal-O(2) complexes, resulting in the end-on metal-superoxo versus side-on metal-peroxo structures. Reactivity studies performed with the isolated metal-superoxo complexes reveal that they can conduct electrophilic reactions such as oxygen atom transfer and C-H bond activation of organic substrates. The metal-peroxo complexes are active oxidants in nucleophilic reactions, such as aldehyde deformylation. We also demonstrate a complete intermolecular O(2)-transfer from metal(III)-peroxo complexes to a Mn(II) complex. The results presented in this Account show the significance of metal ions and supporting ligands in tuning the geometric and electronic structures and reactivities of the metal-O(2) intermediates that are relevant in biology and in biomimetic reactions.     

Electron deficient manganese(III) corrole catalyzed oxidation of alkanes and alkylbenzenes at room temperature

At room temperature electron deficient manganese (III) corrole complexes (1–3) were successfully employed as catalysts in the oxidation of alkanes and alkylbenzenes using m-chloroperbenzoic acid (m-CPBA) as the terminal oxidant. Adamantane has been selectively hydroxylated to adamantane 1-ol and 2-ol with higher preference for the tertiary position. Cyclohexane has also been oxidized. The present oxidizing system also oxidizes toluene, ethylbenzene and diphenylmethane. High valent oxomanganese(V) species has been proposed to be the active oxidant. The high-valent oxomanganese(V) corrole undergoes hydrogen atom transfer (HAT) reaction with 2,4,6-tri-t-butylphenol (TTBP) resulting in the formation of oxidized phenoxyl radicals. Kinetic studies have led to the determination of second-order rate constants for the hydrogen atom transfer reactions. The kinetic experiments reveal a first order reaction rate dependence on the concentration of catalyst as well as on that of the oxidant.     

High-valent iron(IV)-oxo complexes of heme and non-heme ligands in oxygenation reactions

High-valent iron(IV)-oxo species have been implicated as the key reactive intermediates in the catalytic cycles of dioxygen activation by heme and non-heme iron enzymes. Our understanding of the enzymatic reactions has improved greatly via investigation of spectroscopic and chemical properties of heme and non-heme iron(IV)-oxo complexes. In this Account, reactivities of synthetic iron(IV)-oxo porphyrin pi-cation radicals and mononuclear non-heme iron(IV)-oxo complexes in oxygenation reactions have been discussed as chemical models of cytochrome P450 and non-heme iron enzymes. These results demonstrate how mechanistic developments in biomimetic research can help our understanding of dioxygen activation and oxygen atom transfer reactions in nature.     

Theoretical studies on high-valent manganese porphyrins: Toward a deeper understanding of the energetics,electron distributions,and structural features of the reactive intermediates of enzymatic and synthetic manganese-catalyzed oxidative processes

We present here a relatively comprehensive theoretical study, based on nonlocal density functional theory calculations, of the energetics, electron distributions, and structural features of the low-lying electronic states of various high-valent intermediates of manganese porphyrins. Two classes of molecules have been examined: (a) compounds with the general formula [(P)MnX₂]⁰ (P = porphyrin; X = F, Cl, PF₆) and (b) high-valent manganese-oxo species. For [(P)Mn(PF₆)₂]⁰, the calculations reveal a number of nearly equienergetic quartet and sextet states as the lowest states, consistent with experimental results on a comparable species, [(TMP)Mn(ClO₄)₂]⁰ (TMP = tetramesitylporphyrin). In contrast, [(P)MnCl₂]⁰ and [(P)MnF₂]⁰ have a single well-defined S = 3/2 Mn(IV) ground state, again in agreement with experiment, with the three unpaired spins largely concentrated (>90%) on the manganese atom. Manganese(IV)-oxo porphyrins have an S = 3/2 ground state, with the three unpaired spins distributed approximately 2.3:0.7 between the manganese and oxygen atoms. The metal-to-oxygen spin delocalization, as measured by the oxygen spin population, for Mn^IV = O porphyrins is less than, but still qualitatively similar to, that in analogous iron(IV)-oxo intermediates, indicating that the Mn^IV = O bond is significantly weaker than the Fe^IV = O bond in an analogous molecule. Thus, the optimized metal—oxygen bond distances are 1.654 and 1.674 Å for (P)Fe^IV(O)(Py) and (P)Mn^IV(O)(Py), respectively (Py = pyridine). This is consistent with the experimental observation that Mn^IV = O stretching frequencies are over 10% lower than Fe^IV = O stretching frequencies for analogous compounds. For [(P)Mn(O)(PF₆)]⁰, [(P)Mn(O)(Py)]⁺, and [(P)Mn(O)(F)]⁰, the ground states clearly correspond to a (d_xy)² Mn(V) configuration and the short Mn–O distances of 1.541, 1.546, and 1.561 Å for the three compounds, respectively, reflect the formal triple bond character of the Mn–O interaction. Interestingly, the corresponding Mn(IV)-oxo porphyrin cation radical states are calculated to be a few tenths of an electrovolt higher than the Mn(V) ground states, suggesting that the Mn(IV)-oxo porphyrin cation radicals are not likely to exist as ground-state species.     

Visible-light photolysis of corrole-manganese(IV) nitrites to generate corrole-manganese(V)-oxo complexes

Photolysis of highly photo-labile corrole-manganese(IV) nitrites by visible light was studied in three corrole systems with different electronic environments. As observed in all three systems, homolytic cleavage of ON bond of nitrite ligand resulted in one-electron photo-oxidation to generate manganese(V)-oxo corroles, as determined by their distinct UV–vis spectra and kinetic behaviors. The spectral and kinetic results are rationalized by a multiple oxidation model, where the electron-demand Mn^V-oxo species may serve as direct two-electron oxidant for oxygen atom transfer reactions and less electron-demand systems undergo a disproportionation reaction to form a putative manganese(VI)-oxo corrole as the true oxidant.     

Nitrogen donor-controlled chemoselectivity of reaction in oxidation of sulfides with tetra-n-butylammonium hydrogen monopersulfate catalyzed by a partially β-brominated meso-tetraphenylporphyrinatomanganese(III) acetate: a clue to the nature of active oxidant

Highly efficient and rapid oxidation of different sulfides to the corresponding sulfones with tetra-n- butylammonium hydrogen monopersulfate (TBAO) in the presence of catalytic amounts of Mn(TPPBr₂)OAc at room temperature is reported. Contrary to other nitrogen donors, using 4-cyanopyridine as co-catalyst leads to an increase in the ratio of sulfoxide to sulfone in the products. Comparison of the chemoselectivity of reaction in the presence of different nitrogen donors as co-catalyst shows the involvement of a high-valent Mn-oxo species as well as the six-coordinate Mn(TPPBr₂)(HSO₅)(B) (B = nitrogen donors) complex in sulfide oxidation reactions with TBAO.     

Dioxygen activation at non-heme iron: insights from rapid kinetic studies

One of the common biochemical pathways of binding and activation of dioxygen involves non-heme iron centers. The enzyme cycles usually start with an iron(II) or diiron(II) state and traverse via several intermediates (detected or postulated) such as (di)iron(III)-superoxo, (di)iron(III)-(hydro)peroxo, iron(III)iron(IV)-oxo, and (di)iron(IV)-oxo species, some of which are responsible for substrate oxidation. In this Account, we present results of kinetic and mechanistic studies of dioxygen binding and activation reactions of model inorganic iron compounds. The number of iron centers, their coordination number, and the steric and electronic properties of the ligands were varied in several series of well-characterized complexes that provided reactive manifolds modeling the function of native non-heme iron enzymes. Time-resolved cryogenic stopped-flow spectrophotometry permitted the identification of kinetically competent intermediates in these systems. Inner-sphere mechanisms dominated the chemistry of dioxygen binding, intermediate transformations, and substrate oxidation as most of these processes were controlled by the rates of ligand substitution at the iron centers.     

The Redox Chemistry of Manganese(III) and -(IV) Complexes

The oxidation-reduction thermodynamics for the manganese(III), -(IV), and -(II) ions, and their various complexes, are reviewed for both aqueous and aprotic media. In aqueous solutions the reduction potential for the manganese(III)/(II) couple has values that range from +1.51 V vs. NHE (hydrate at pH 0) to −0.95 V (glucarate complex at pH 13.5). The Mn(IV)/(III) couple has values that range from +1.0 V (solid Mn^IVO₃ at pH 0) to −0.04 V (tris gluconate complex at pH 13.5). With anhydrous media the propensity for the Mn(III) ion to disproportionate to solid Mn^IVO₂ and Mn(II) ion is avoided. For aprotic systems the range of redox potentials for various manganese complexes is from +2.01 V and +1.30 for the Mn(IV)/(III) and Mn(III)/(II) couples (bis terpyridyl tri-N-oxide complex in MeCN), respectively, to −0.96 V for the Mn(IV)/(III) couple (tris 3,5,-di-tert-butylcatecholate complex in Me₂SO). The redox reactions between manganese complexes and dioxygen species (O₂, O₂, and H₂O₂) also are reviewed.     

EPR Spectroscopic Characterization of a Jahn-Teller Distorted (C3v→Cs) Four-Coordinate Chromium(V) Oxo Species

Metal-oxo coordination compounds have garnered significant interest over the years. The reactivity of the metal-oxo bond is governed by the geometry, charge, spin state, and identity of the other ligands. In this report, we characterize a distorted C_3v-symmetric Cr^V-oxo complex that has unique magnetic properties, compared with all other known chromyl species. Continuous wave and pulse electron paramagnetic resonance were used to measure the molecular g-values and ⁵³Cr and ¹⁷O hyperfine interactions. Analysis of density functional theory results and the g and hyperfine tensors, in the context of a crystallographically observed Jahn-Teller distortion, suggests an electronic structure that results from the mixing of two sets of doubly degenerate orbital states. This mixing is only made possible by the approximate three-fold symmetry of the ligand set.     

Partial oxidation reactions on phosphate-based catalysts

In this work emphasis has been placed on phosphate-based catalysts used for partial oxidation reactions, such as vanadyl pyrophosphate, iron phosphates and hydrogen/hydroxy-phosphates, zirconium hydrogen phosphates as layered compounds used to stabilise/entrap Cr and V oxyhydroxy-macrocations. It is shown that, in partial oxidation reactions, the catalyst surface behave in a rather dynamic and labile way, reconstructing under activation and/or catalytic reaction conditions and adapting itself to the stereochemistry of the reactants. The active sites are shown to have a molecular size, to be isolated and to present several catalytic functions, as hydrocarbon activation, H atom abstraction, lattice oxygen incorporation and electron transfer through the solid material to allow the redox process to occur. The metal cations are the active species and the role of the phosphate tetrahedra is not only to bind the MO₆ octahedra together to constitute a dense or layered compound but also to bring some specific redox and acid–base properties. This revised version was published online in August 2006 with corrections to the Cover Date.     

A Dinucleating Ligand System with Varying Terminal Donors to Mimic Diiron Active Sites

To mimic dinuclear active sites of metalloproteins, we have developed a dinucleating ligand system consisting of two tetradentate tripodal ligand compartments with varying terminal donors (carboxylates, phenolates, and pyridines). These ligands provide access to a series of μ-oxo-bridged diferric complexes. The spectroscopic study allows to investigate the molecular structures even in solution, e. g. depending on protonation/deprotonation of coordinated OH⁻ and H₂O ligands or to observe a reversible pH-dependent carboxylate-shift between terminal and bridging binding mode. The electrochemical behavior is strongly influenced by the exogenous ligands, e. g. OH⁻ facilitates oxidation to Fe^IV by 690 mV relative to Cl⁻. Using the terminal carboxylates and a {Fe^III(μ-O)₂Fe^III} core even allows oxidation with O₂ to a high-valent species with Fe^IV (S=2). The implications of this study for further generation of high-valent or peroxo species and their utilization in catalysis is discussed.     

2-甲基咪唑-4,5-二羧酸镍配合物的合成及其晶体结构

以配体2-甲基咪唑-4,5-二羧酸与氯化镍反应,合成出了一个新的配合物[Ni(HMIDC-)]2(H2O)2]n;采用单晶X射线衍射方法测定了配合物的晶体结构。结果表明,所合成的配合物属正交晶系,pbca空间群,晶胞参数a=0.683 74 nm,b=1.388 13 nm,c=1.655 19 nm,V=1 570.97 nm3,Z=4,Dc=1.831 g/cm3,F(000)=1 000.0,GOF=1.304。镍原子与两个不同配体H2MIDC中的咪唑上的氮原子、羧基氧原子以及水分子配位,且形成了六配位八面体构型。配合物由分子间氢键构筑成了三维超分子氢键网络结构。     

The effect of the perturbation of hydrogen bonding on the Zn(II) coordination geometry: synthesis and structure of [Zn(BIP)(H2O)2](ClO4)2 (BIP=1,3-bis[(4-methyl-5-imidazol-1-yl)ethylideneamino]propane)

A polydentate ligand comprising imidazole donor 1,3-bis[(4-methyl-5-imidazol-1-yl)ethylideneamino]propane (BIP) was synthesized, and the crystal structure of its Zn(II) complex [Zn(BIP)(H₂O)₂](ClO₄)₂ has been reported, in which the Zn(II) atom adopts a distorted octahedral coordination geometry with the equatorial positions occupied by four nitrogen atoms of BIP and the axial positions by two aqua molecules. The effect of hydrogen bonding of the aqua ligand on the coordination geometry is discussed.     

Synthesis,aggregation-induced emission and application as chemosensor for explosives of a 1,10-phenanthroline derivative and its rhenium(I) carbonyl complex having triphenylamino and thienyl donors

A C-shaped 1,10-phenanthroline ligand and its rhenium(I) carbonyl complex were designed and synthesized. Single-crystal structure reveals that one chloroform molecule is accommodated on this ligand and the bond lengths of double hydrogen bonding interactions is 2.31 and 2.67 Å, respectively, between two nitrogen atom sites of 1,10-phenanthroline and one hydrogen atom of chloroform. Aggregation-induced fluorescence experiments demonstrate that the ligand presents an aggregation-induced emission enhancement from 60% to 70% water volume fractions, while the complex shows a red-shifted aggregation-caused quenching phenomenon from 461 to 506 nm. Furthermore, fluorescence titration analysis proves that the ligand is one of a potential chemosensors for 2-methyl-1,3-dinitrobenzene in tetrahydrofuran solution. In addition, theoretic computational studies on related ligand and complex have also been carried out.     

New molybdenum(VI) complex with ONS-donor thiosemicarbazone ligand: Preparation,structural characterization,and catalytic applications in olefin epoxidation

New dioxomolybdenum(VI) complex was prepared by reacting 1-(2,4-dihydroxybenzylidene)-N-methyl-N-phenylthiosemicarbazone (H₂L) as ligand and [MoO₂(acac)₂] in acetonitrile solution. The doubly deprotonated ligand is coordinated to molybdenum through sulfur atom, hydrazinic nitrogen atom and phenolic oxygen atom. The resulting complex with the formula [MoO₂L(CH₃CN)] which contains an acetonitrile molecule in sixth site of coordination, was characterized by elemental analyses, ¹H NMR, IR and electronic spectroscopic studies and single crystal X-ray diffraction analysis. This complex was tested as a catalyst for the homogeneous epoxidation of olefins using tert-butyl hydrogen peroxide (TBHP) as an oxidant. The catalyst shows efficient reactivity in the olefins epoxidation reactions giving high yield and selectivity under atmospheric conditions, in most cases.     

Catalytic Activity and Performance of Copper-Based Complexes Mediating Atom Transfer Radical Polymerization

The factors affecting the performance of the atom transfer radical polymerization (ATRP) catalyst, i.e., its activity, ability to control the polymerization, and its propensity to participate in side complexation or redox reactions are summarized in this brief review. The effect of the ligand, transferable halogen atom, and the reaction solvent can be understood and quantified by formally splitting the overall atom transfer equilibrium into contributing reactions, including the homolysis of a carbon halogen bond, reduction of the halogen atom to a halide ion (electron affinity), oxidation of the lower oxidation state metal complex (activator), and formation of the radical deactivator via coordination of a halide anion to the higher oxidation state metal complex (halidophilicity).     

Hydrogen adsorption and hydrogen evolution reaction on a polycrystalline Pt electrode studied by surface-enhanced infrared absorption spectroscopy

Hydrogen evolution reaction (HER) on a polycrystalline Pt electrode has been investigated in Ar-purged acids by surface-enhanced infrared absorption spectroscopy and electrochemical kinetic analysis (Tafel plot). A vibrational mode characteristic to H atom adsorbed at atop sites (terminal H) was observed at 2080-2095 cm⁻¹. This band appears at 0.1 V (RHE) and grows at more negative potentials in parallel to the increase in hydrogen evolution current. Good signal-to-noise ratio of the spectra enabled us to establish the quantitative relation between the band intensity (equivalently, coverage) of terminal H and the kinetics of HER, from which we conclude that terminal H atom is the reaction intermediate in HER and the recombination of two terminal H atoms is the rate-determining step. The quantitative analysis of the infrared data also revealed that the adsorption of terminal H follows the Frumkin isotherm with repulsive interaction.