Aliphatic and Aromatic C-H Bond Oxidation by High-Valent Manganese(IV)-Hydroxo Species

The strong C-H bond activation of hydrocarbons is a difficult reaction in environmental and biological chemistry. Herein, a high-valent manganese(IV)-hydroxo complex, [MnIV(CHDAP-O)(OH)]2+ (2), was synthesized and character-ized by various physicochemical measurements, such as ultra-violet-visible (UV-vis), electrospray ionization-mass spectrome-try (ESI-MS), electron paramagnetic resonance (EPR), and helium-tagging infrared photodissociation (IRPD) methods. The one-electron reduction potential (Ered) of 2 was determined to be 0.93 V vs SCE by redox titration. 2 is formed via a transient green species assigned to a manganese(IV)-bis(hydroxo) complex, [MnIV(CHDAP)(OH)2]2+ (2 '), which performs intramolecular aliphatic C-H bond activation. The kinetic isotope effect (KIE) value of 4.8 in the intramolecular oxidation was observed, which indicates that the C-H bond activation occurs via rate-determining hydrogen atom abstraction. Further, complex 2 can activate the C-H bonds of aromatic compounds, anthracene and its derivatives, under mild conditions. The KIE value of 1.0 was obtained in the oxidation of anthracene. The rate constant (ket) of electron transfer (ET) from N,N '-dimethylaniline derivatives to 2 is fitted by Marcus theory of electron transfer to afford the reorganization energy of ET (lambda = 1.59 eV). The driving force dependence of log ket for oxidation of anthracene derivatives by 2 is well evaluated by Marcus theory of electron transfer. Detailed kinetic studies, including the KIE value and Marcus theory of outer-sphere electron transfer, imply that the mechanism of aromatic C-H bond hydroxylation by 2 proceeds via the rate-determining electron-transfer pathway

Reactivity of a Cobalt(III)-Hydroperoxo Complex in Electrophilic Reactions

The reactivity of mononuclear metal-hydroperoxo adducts has fascinated researchers in many areas due to their diverse biological and catalytic processes. In this study, a mononuclear cobalt(III)-peroxo complex bearing a tetradentate macrocyclic ligand, [Co-III(Me-3-TPADP)(O-2)](+) (Me-3-TPADP = 3,6,9- trimethy1-3,6,9-triaza-1(2,6)-pyridinacydodecaphane), was prepared by reacting [Co-II(Me-3-TPADP)(CH3CN)(2)](2+) with H2O2 in the presence of triethylamine. Upon protonation, the cobalt(III)-peroxo intermediate was converted into a cobalt (III)-hydroperoxo complex, [Co-III(Me-3-TPADP) (O2H) (CH3CN)](2+). The mononuclear cobalt(III)-peroxo and -hydroperoxo intermediates were characterized by a variety of physicochemical methods. Results of electrospray ionization mass spectrometry clearly show the transformation of the intermediates: the peak at m/z 339.2 assignable to the cobalt(III)-peroxo species disappears with concomitant growth of the peak at m/z 190.7 corresponding to the cobalt(III)-hydroperoxo complex (with bound CH3CN). Isotope labeling experiments further support the existence of the cobalt(III)-peroxo and hydroperoxo complexes. In particular, the O-O bond stretching frequency of the cobalt(III)-hydroperoxo complex was determined to be 851 cm(-1) for (O2H)-O-16 samples (803 cm(-1) for (O2H)-O-18 samples), and its Co-O vibrational energy was observed at 571 cm(-1) for (O2H)-O-16 samples (551 cm(-1) for (O2H)-O-18 samples; 568 cm(-1) for (O2H)-O-16-H-2 samples) by resonance Raman spectroscopy. Reactivity studies performed with the cobalt(III)-peroxo and hydroperoxo complexes in organic functionalizations reveal that the latter is capable of conducting oxygen atom transfer with an electrophilic character, whereas the former exhibits no oxygen atom transfer reactivity under the same reaction conditions. Alternatively, the cobalt(III)-hydroperoxo complex does not perform hydrogen atom transfer reactions, while analogous low-spin Fe(III)- hydroperoxo complexes are capable of this reactivity. Density functional theory calculations indicate that this lack of reactivity is due to the high free energy cost of O-O bond homolysis that would be required to produce the hypothetical Co(IV)-oxo product

Spectroscopic Characterization and Reactivity Studies of a Mononuclear Nonheme Mn(III)-Hydroperoxo Complex

We report the first example of a mononuclear nonheme manganese(III)-hydroperoxo complex derived from protonation of an isolated manganese(III)peroxo complex bearing an N-tetramethylated cyclam (TMC) ligand, [Mn-III(TMC)(OOH)(2+). The Mn-III-mhydroperoxo intermediate is characterized with various spectroscopic methods as well as with density functional theory (DFT) calculations, showing the binding of a hydroperoxide ligand in an end-on fashion. The Mn-III-hydroperoxo species is a competent oxidant in oxygen atom transfer (OAT) reactions, such as the oxidation of sulfides. The electrophilic character of the Mn-III-hydroperoxo complex is demonstrated unambiguously in the sulfoxidation of para-substituted thioanisoles

Comparison of High-Spin and Low-Spin Nonheme Fe-III-OOH Complexes in O-O Bond Homolysis and H-Atom Abstraction Reactivities

The geometric and electronic structures and reactivity of an S = 5/2 (HS) mononuclear nonheme (TMC)Fe-III-OOH complex are studied by spectroscopies, calculations, and kinetics and compared with the results of previous studies of S = 1/2 (LS) Fe-III-OOH complexes to understand parallels and differences in mechanisms of O-O bond homolysis and electrophilic H-atom abstraction reactions. The homolysis reaction of the HS [(TMC)Fe-III-OOH](2+) complex is found to involve axial ligand coordination and a crossing to the LS surface for O-O bond homolysis. Both HS and LS Fe-III-OOH complexes are found to perform direct H-atom abstraction reactions but with very different reaction coordinates. For the LS Fe-III-OOH, the transition state is late in O-O and early in C-H coordinates. However, for the HS Fe-III-OOH, the transition state is early in O-O and further along in the C-H coordinate. In addition, there is a significant amount of electron transfer from the substrate to the HS Fe-III-OOH at transition state, but that does not occur in the LS transition state. Thus, in contrast to the behavior of LS Fe-III-OOH, the H-atom abstraction reactivity of HS Fe-III-OOH is found to be highly dependent on both the ionization potential and the C-H bond strength of the substrate. LS Fe-III-OOH is found to be more effective in H-atom abstraction for strong C-H bonds, while the higher reduction potential of HS Fe-III-OOH allows it to be active in electrophilic reactions without the requirement of O-O bond cleavage. This is relevant to the Rieske dioxygenases, which are proposed to use a HS Fe-III-OOH to catalyze cis-dihydroxylation of a wide range of aromatic compounds

A Chromium(III)-Superoxo Complex in Oxygen Atom Transfer Reactions as a Chemical Model of Cysteine Dioxygenase

Metal???superoxo species are believed to play key roles in oxygenation reactions by metalloenzymes. One example is cysteine dioxygenase (CDO) that catalyzes the oxidation of cysteine with O2, and an iron(III)???superoxo species is proposed as an intermediate that effects the sulfoxidation reaction. We now report the first biomimetic example showing that a chromium(III)???superoxo complex bearing a macrocyclic TMC ligand, [CrIII(O2)(TMC)(Cl)]+, is an active oxidant in oxygen atom transfer (OAT) reactions, such as the oxidation of phosphine and sulfides. The electrophilic character of the Cr(III)???superoxo complex is demonstrated unambiguously in the sulfoxidation of para-substituted thioanisoles. A Cr(IV)???oxo complex, [CrIV(O)(TMC)(Cl)]+, formed in the OAT reactions by the chromium(III)???superoxo complex, is characterized by X-ray crystallography and various spectroscopic methods. The present results support the proposed oxidant and mechanism in CDO, such as an iron(III)???superoxo species is an active oxidant that attacks the sulfur atom of the cysteine ligand by the terminal oxygen atom of the superoxo group, followed by the formation of a sulfoxide and an iron(IV)???oxo species via an O???O bond cleavage

Investigating Superoxide Transfer through a mu-1,2-O-2 Bridge between Nonheme Ni-III-Peroxo and Mn-II Species by DFT Methods to Bridge Theoretical and Experimental Views

Previously, a fast unprecedented O-2(center dot-) transfer reaction has been observed experimentally when adding a Mn-II complex into a solution containing a Ni-III-peroxo complex. Due to the fast reaction rate, no intermediates were observed. We have investigated this reaction with density functional theory (DFT) and show that DFT is unusually problematic in reproducing the correct spin state for the investigated Ni-III-peroxo complex, something which calls for examination of all previous Ni-dioxygen studies. Surprisingly, the BP86 functional is shown to yield energies more in agreement with known experiments than B3LYP. The calculations reveal for the first time an intermediate structure in a complete O-2(center dot-) transfer reaction, shown here to be a short-lived bridging Ni-(mu-1,2-O-2)-Mn structure

Synthesis, Structural, and Spectroscopic Characterization and Reactivities of Mononuclear Cobalt(III)-Peroxo Complexes

Metal???dioxygen adducts are key intermediates detected in the catalytic cycles of dioxygen activation by metalloenzymes and biomimetic compounds. In this study, mononuclear cobalt(III)???peroxo complexes bearing tetraazamacrocyclic ligands, [Co(12-TMC)(O2)]+ and [Co(13-TMC)(O2)]+, were synthesized by reacting [Co(12-TMC)(CH3CN)]2+ and [Co(13-TMC)(CH3CN)]2+, respectively, with H2O2 in the presence of triethylamine. The mononuclear cobalt(III)???peroxo intermediates were isolated and characterized by various spectroscopic techniques and X-ray crystallography, and the structural and spectroscopic characterization demonstrated unambiguously that the peroxo ligand is bound in a side-on ??2 fashion. The O???O bond stretching frequency of [Co(12-TMC)(O2)]+ and [Co(13-TMC)(O2)]+ was determined to be 902 cm???1 by resonance Raman spectroscopy. The structural properties of the CoO2 core in both complexes are nearly identical; the O???O bond distances of [Co(12-TMC)(O2)]+ and [Co(13-TMC)(O2)]+ were 1.4389(17) ?? and 1.438(6) ??, respectively. The cobalt(III)???peroxo complexes showed reactivities in the oxidation of aldehydes and O2-transfer reactions. In the aldehyde oxidation reactions, the nucleophilic reactivity of the cobalt???peroxo complexes was significantly dependent on the ring size of the macrocyclic ligands, with the reactivity of [Co(13-TMC)(O2)]+ > [Co(12-TMC)(O2)]+. In the O2-transfer reactions, the cobalt(III)???peroxo complexes transferred the bound peroxo group to a manganese(II) complex, affording the corresponding cobalt(II) and manganese(III)???peroxo complexes. The reactivity of the cobalt???peroxo complexes in O2-transfer was also significantly dependent on the ring size of tetraazamacrocycles, and the reactivity order in the O2-transfer reactions was the same as that observed in the aldehyde oxidation reactions

Chromium(IV)-Peroxo Complex Formation and Its Nitric Oxide Dioxygenase Reactivity

The O-2 and NO reactivity of a Cr(II) complex bearing a 12-membered tetraazamacrocyclic N-tetramethylated cyclam (TMC) ligand, [Cr-II(12-TMC)-(Cl)](+) (1), and the NO reactivity of its peroxo derivative, [Cr-IV(12-TMC)(O-2)(Cl)](+) (2), are described. By contrast to the previously reported Cr(III)-superoxo complex, [Cr-III(14-TMC)(O-2)(Cl)](+), the Cr(IV)-peroxo complex 2 is formed in the reaction of 1 and O-2. Full spectroscopic and X-ray analysis revealed that 2 possesses side-on eta(2)-peroxo ligation. The quantitative reaction of 2 with NO affords a reduction in Cr oxidation state, producing a Cr(III)-nitrato complex, [Cr-III(12-TMC)(NO3)(Cl)](+) (3). The latter is suggested to form via a Cr(III)- peroxynitrite intermediate. [Cr-II(12-TMC)(NO)(Cl)](+) (4), a Cr(II)-nitrosyl complex derived from 1 and NO, could also be synthesized; however, it does not react with O-2