Metal-oxo Species Research Articles

Tuning the Relative Stability and Reactivity of Manganese Dioxygen and Peroxo Intermediates via Systematic Ligand Modification.

Many fundamental processes of life depend on the chemical energy stored in the O–O bond of dioxygen (O2), the majority of which is derived from photosynthetic H2O oxidation. Key steps in these processes involve Mn-, Fe-, or Cu-promoted formation or cleavage of O–O and O–H bonds, the mechanisms of which are not fully understood, especially with Mn. Metal–peroxo and high-valent metal–oxo species are proposed to be involved as intermediates. The metal ion properties that favor O–O and O–H bond formation versus cleavage have yet to be systematically explored. Herein we examine the O2 reactivity of a series of structurally related Mn(II) complexes and show that several metastable intermediates are observed, the relative stabilities of which depend on subtle differences in ligand architecture. We show that in contrast to Fe and Cu complexes, O2 binds irreversibly to Mn(II). By crystallizing an entire series of the first reported examples of Mn(III)–OOR peroxos as well as an O2-derived binuclear trans-μ-1,2-bridged Mn(III)–peroxo with varying degrees of O–O bond activation, we demonstrate that there are distinct correlations between spectroscopic, structural, and reactivity properties. Rate-limiting O–O bond cleavage is shown to afford a reactive species capable of abstracting H atoms from 2,4-tBu2-PhOH or 1,4-cyclohexadiene, depending on the ligand substituents. The weakly coordinated N-heterocycle Mn···Npy,quino distance is shown to correlate with the peroxo O–O bond length and modulate the π overlap between the filled πv*(O–O) and Mn dxz orbitals. We also show that there is a strong correlation between the peroxo → Mn charge transfer (CT) band and the peroxo O–O bond length. The energy difference between the CT bands associated with the peroxos possessing the shortest and longest O–O bonds shows that these distances are spectroscopically distinguishable. We show that we can use this spectroscopic parameter to estimate the O–O bond length, and thus the degree of O–O bond activation, in intermediates for which there is no crystal structure, as long as the ligand environment is approximately the same.

Mn(V)(O) versus Cr(V)(O) Porphyrinoid Complexes: Structural Characterization and Implications for Basicity Controlling H-Atom Abstraction.

Isomorphous crystals of Mn(V)(O) and Cr(V)(O) corrolazines were characterized by single crystal X-ray diffraction. Reactivity studies with H atom donors and separated PCET reagents show a dramatic difference in H atom abstracting abilities for these two complexes. The implied large difference in driving force is opposite the trend in redox potentials, indicating that basicity is a key factor in determining the striking difference in reactivity for two metal-oxo species in identical ligand environments.

Affecting the Catalytic Activity of the Known [Ru(tpy)(bpy)(OH2)]2+ Complex in Water Oxidation by Utilization of a Hangman Ligand

Abstract[Ru(tpy)(bpy)(OH2)]2+ (bpy = 2,2′‐bipyridine, tpy = 2,2′;6′,2″‐terpyridine) is the archetype of many known single‐site ruthenium complexes used for catalytic water oxidation. Its efficiency is likely influenced by installing a proton‐donor/acceptor functionality in proximity to the catalytic site because the reaction mechanism is believed to occur by nucleophilic attack of a water molecule on a high‐valent metal–oxo species assisted by hydrogen‐bonding interactions. We present herein the results of a study of a new metal complex based on the “hangman” motif that possesses a carboxylic functional group close to the ruhenium center. This catalyst was synthesized in very good yield and fully characterized. We discovered that its catalytic activity was in fact hampered by the presence of the functional group. Further investigations revealed a strong dependence of the catalytic performance not only on the solvent, but also on the counter ion and other additives used.

Manganese-Substituted Myoglobin: Characterization and Reactivity of an Oxidizing Intermediate towards a Weak C-H Bond

Metal-substituted hemoproteins have been examined by biochemists for decades, but their potential for diverse functionalities has not been thoroughly investigated. By replacing hemoproteins with non-native metals, heme-containing proteins could be capable of performing a range of chemistries not allowed for in the native protein. The metal within the heme of the oxygen-carrying hemoprotein, myoglobin, can readily be replaced with other first row transition metals such as cobalt, chromium and manganese. Upon oxidation with two-electron oxidants (ex. meta-chloroperbenzoic acid), an oxidizing intermediate is produced in manganese-substituted myoglobin. Electron paramagnetic resonance analyses confirm the oxidation of Mn(III) to Mn(IV). With the addition of weak C-H bonds of 1,4 cyclohexadiene, hydrogen atom abstraction is exhibited by the oxidizing intermediate that displays a second-order rate constant of 2.79 +/− 0.22 M−1 s−1 by the metal-oxo species. The replacement of the iron ion with a manganese ion at the active site of myoglobin displays oxidative capabilities that are not shown in native myoglobin.

Activation of Platinum-Based Centers through Modification with Metal Oxo Species toward Electrocatalytic Oxidation of Dimethyl Ether and Methanol

We have considered nanonstructured metal oxides as promoters for platinum, palladium or bimetallic platinum-ruthenium nanoparticles to fabricate electrocatalytic systems capable of efficient oxidation of dimethyl ether, a potentially new fuel of importance to low temperature fuel cells. The research has been performed in comparison to methanol. We explore the fact that certain inorganic oxides (e.g. WO3, TiO2, ZrO2 and CeO2) and polyoxometallates of molybdenum or tungsten tend to influence supported metal centers in ways other than simple dispersion over electrode area. Among useful characteristics of metal oxides and related systems are the following: they can generate –OH groups at low potentials that induce oxidation of passivating CO adsorbates (e.g. on Pt); they can potentially break C-H bonds (e.g. by hydrogen tungsten oxide bronzes); and they can possibly weaken C-O bonds during dimethyl ether oxidation (e.g. through changes of the electronic properties of Pt or Pd as additional component). Also nanostructured gold in combination with platinum has been demonstrated to enhance the overall electrocatalytic performance during oxidation of such small organic molecules as methanol and dimethyl ether. We pursue a concept of utilization of functionalized titanium dioxide, tungsten oxide and/or zirconium oxide matrices (by admixing them with polyoxometallate-modified gold nanoparticles) for supporting and activating noble metal nanoparticles (Pt, Pt-Ru, Pd, Pt-Pd) toward electrooxidation of dimethyl ether in comaprison to methanol. Remarkable increases of the respective electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account improvement of overall conductivity (due to the presence of nanostructured gold) at the electrocatalytic interface (formed by metal oxide support), as well as and possibility of specific Pt-metal oxide or Pt-Au electronic interactions and existence of active hydroxyl groups on transition metal oxo species.in the vicinity of catalytic Pt sites. Further, the capping layers of Keggin-type phosphomolybdates, which are known to undergo fast stepwise multi-electron redox processes as well as to activate Pt-based electrocatalysts, may also contribute to the overall enhancement effect.

(Invited) Interfacial Functionalization of Carbon Nanotubes: From Effective Charge Propagation and Storage to Enhancement of Electrocatalytic and Bioelectrocatalytic Properties

Of particular interest to the preparation of advanced materials is synthesis and characterization of carbon nanostructures (particularly nanotubes) or noble metal nanoparticles, their stabilization (e.g. through self-assembly), as well as organization into two-dimensional arrays and three-dimensional network films through controlled fabrication (e.g. sequential attraction). Thus functionalized nanosized materials with well-defined composition, structure and thickness can be formed. The interfaces can also be highly functionalized, and they can exhibit specific catalytic or unique electronic, charge storage, optical and sensing properties. We explore here the ability of inorganic structures to stabilize and derivatize carbon nanotubes. Among inorganic systems, polyoxometallates of molybdenum and tungsten are attractive since they can not only adsorb irreversibly on solid surfaces but also exhibit reversible stepwise multi-electron transfer reactions. The concept of the layer-by-layer formation of hybrid assemblies composed of anionic polyoxometallate-protected carbon nanotubes (or metal nanoparticles) and ultra-thin films of positively charged conducting polymers (e.g. PEDOT) or polycationic metal oxo species (e.g. zirconyl or vanadyl) will be described and discussed here. The resulting novel composite materials have been fabricated as thin or moderately thick (µm level) films on electrode surfaces. As evidenced from STM and scanning electron microscopy, their morphology is still granular but the structure is fairly dense. Further, they are characterized by fast dynamics of charge propagation. Obviously, this research is of importance to the construction of effectively operating charge storage devices (capacitors), charge mediators (e.g. in bioelectrochemistry), molecular electronic systems and electrocatalysis. In the latter case, polyoxometallates can also be applied to stabilize and link Pt-Ru, Pt-Sn and various alloyed Pt-based nanoparticles. It is apparent from diagnostic cyclic voltammetric, rotating disk voltammetric and chronoamperometric measurements that such systems exhibit attractive properties towards electroreduction of oxygen or carbon dioxide as well as oxidation of small organic molecules (ethanol, methanol or dimethyl ether). In addition to the fact that introduction of carbon nanotubes improves charge distribution at the electrocatalytic interface, further addition of metal oxide nanospecies can generate –OH groups at low potentials that induce oxidation of passivating CO adsorbates (e.g. on Pt); they can potentially break C-H or C-O bonds, as well as they can possibly weaken C-C bonds during ethanol oxidation (e.g. through changes of the electronic properties of Pt).

Origin of Reactivity Differences Between Heme and Nonheme Iron(III)-Hydroperoxo Complexes

The cytochromes P450 are important enzymes in human physiology involved in biodegradation and biosynthesis pathways in the body.1,2 They generally react with substrates in a catalytic cycle that includes two reduction and two proton transfer steps to form a high-valent iron(IV)-oxo heme cation radical species called Compound I.3 High-valent metal-oxo species are common intermediates in enzymatic processes and have been identified in heme as well as nonheme iron enzymes. Compound I is a versatile and efficient oxidant that is known to react with substrates via aliphatic hydroxylation, epoxidation, aromatic hydroxylation, alkane dehydrogenation, N-dealkylation, sulfoxidation etc. Many of these processes involve the reduction of Compound I; unfortunately, Compound I is short-lived, which makes experimental studies in the field complicated. To overcome this, we have done computational studies on enzymatic heme enzymes as well as synthetic model porphyrins and phthalocyanines with the aim to gain insight into the intricate details of the oxidant to gain insight into the electrochemistry of the active species and the catalyzed reaction mechanism. In the past, these studies assisted experimental efforts in the field and made predictions of reaction mechanisms. It was, for instance, shown that the precursor of Compound I in the catalytic cycle, namely Compound 0, is catalytically inactive for epoxidation, sulfoxidation and aromatic hydroxylation reactions. However, in a recent twist of events, our group showed that nonheme iron(III)-hydroperoxo can be catalytically active.4 To understand the difference in activity between heme and nonheme iron(III)-hydroperoxo, we studied the mechanisms of aromatic hydroxylation and haloperoxidation. These combined experimental and computational studies gave insight into the effects of axial and equatorial ligands on the chemical and catalytic properties of Compound 0 and Compound I. In addition, our studies established the effects of environmental perturbations on the properties and reactivities of Compound I, including hydrogen bonding interactions and secondary sphere amino acid interactions.

Thermal hydrogen-atom transfer from methane: A mechanistic exercise

Hydrogen-atom transfer (HAT) constitutes a key process in a broad range of chemical transformations as it covers heterogeneous, homogeneous, and enzymatic reactions. While open-shell metal oxo species [MO] are no longer regarded as being involved in the heterogeneously catalyzed oxidative coupling of methane (2CH4+<O>→C2H6+H2O), these reagents are rather versatile in bringing about (gas-phase) hydrogen-atom transfer, even from methane at ambient conditions. In this mini-review, various mechanistic scenarios will be presented, and it will be demonstrated how these are affected by the composition of the metal-oxide cluster ions. Examples will be discussed, how ‘doping’ the clusters permits the control of the charge and spin situation at the active site and, thus, the course of the reaction. Also, the interplay between supposedly inert support material and the active site – the so-called ‘aristocratic atoms’ – of the gas-phase catalyst will be addressed. Finally, gas-phase HAT from methane will be analyzed in the broader context of thermal activation of inert CH bonds by metal-oxo species.

Enhanced Reactivity in Hydrogen Atom Transfer from Tertiary Sites of Cyclohexanes and Decalins via Strain Release: Equatorial C-H Activation vs Axial C-H Deactivation.

Absolute rate constants for hydrogen atom transfer (HAT) from cycloalkanes and decalins to the cumyloxyl radical (CumO(•)) were measured by laser flash photolysis. Very similar reactivities were observed for the C-H bonds of cyclopentane and cyclohexane, while the tertiary C-H bond of methylcyclopentane was found to be 6 times more reactive than the tertiary axial C-H bond of methylcyclohexane, pointing toward a certain extent of tertiary axial C-H bond deactivation. Comparison between the cis and trans isomers of 1,2-dimethylcyclohexane, 1,4-dimethylcyclohexane and decalin provides a quantitative evaluation of the role played by strain release in these reactions. kH values for HAT from tertiary equatorial C-H bonds were found to be at least 1 order of magnitude higher than those for HAT from the corresponding tertiary axial C-H bonds (kH(eq)/kH(ax) = 10-14). The higher reactivity of tertiary equatorial C-H bonds was explained in terms of 1,3-diaxial strain release in the HAT transition state. Increase in torsional strain in the HAT transition state accounts instead for tertiary axial C-H bond deactivation. The results are compared with those obtained for the corresponding C-H functionalization reactions by dioxiranes and nonheme metal-oxo species indicating that CumO(•) can represent a convenient model for the reactivity patterns of these oxidants.

Activation of Platinum-Based Centers through Modification with Metal Oxo Species toward Electrocatalytic Oxidation of Dimethyl Ether and Methanol

We studied electrooxidation of dimethyl ether on Pt and PtRu nanoparticles: bare and deposited on ZrO2 film. When compared to the behavior of bare Pt and PtRu nanoparticles (deposited on glassy carbon at the same loading of 100 μg cm-2), the enhancement effect in the presence of ZrO2 was demonstrated in terms of increases of electrocatalytic (voltammetric) currents recorded at room temperature (22oC) in 0.5 mol dm-3 H2SO4 containing 1.05 mol dm-3 DME. Introduction of the metal oxide (zirconia) nanostructures at the electrocatalytic interface should produce high population of reactive –OH groups favoring mobility of protons and inducing the oxidative removal of poisoning (CO-type) intermediates, at platinum and ruthenium sites of the deposited bimetallic PtRu nanoparticles.

Prototypical metal–oxo bonds: the reactions of Cr(PF3)6, Fe(PF3)5, and Ni(PF3)4 with oxygen

Terminally bound oxo complexes of late transition metals have been difficult to synthesize or isolate. Their scarcity is in part due to the electronic repulsion between oxo ligands and metal elements with highly occupied d orbitals. This study attempts to provide extra stability to such metal–oxo species. The prototypical metal–oxo trifluorophosphane complexes [(PF3)5CrO, (PF3)4FeO, and (PF3)3NiO] are studied here. The results are compared with the corresponding carbonyl counterparts [(CO)5CrO, (CO)4FeO, and (CO)3NiO]. Predicted bond dissociation energies are 90 (Cr–O), 83 (Fe–O), and 59 (Ni–O) kcal/mol, higher than those of their carbonyl counterparts by around 10 kcal/mol. Consistent with bonding considerations and population analyses, the metal–oxo bonds are strengthened from the carbonyl to the trifluorophosphine complexes. Although the improvement is modest, it proves clearly that strong electron-withdrawing frameworks help in stabilizing metal–oxo complexes. This general idea may be utilized in further studies to seek or even design elusive terminal metal–oxo species and to keep pushing the limits of this area. In addition, improved viabilities are also found for the dioxygen–metal complexes [(PF3)5CrO2, (PF3)4FeO2, and (PF3)3NiO2], compared with their carbonyl counterparts.

Spectroscopic identification of an Fe(III) center, not Fe(IV), in the crystalline Sc-O-Fe adduct derived from [Fe(IV)(O)(TMC)]²⁺.

The apparent Sc(3+) adduct of [Fe(IV)(O)(TMC)](2+) (1, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) has been synthesized in amounts sufficient to allow its characterization by various spectroscopic techniques. Contrary to the earlier assignment of a +4 oxidation state for the iron center of 1, we establish that 1 has a high-spin iron(III) center based on its Mössbauer and EPR spectra and its quantitative reduction by 1 equiv of ferrocene to [Fe(II)(TMC)](2+). Thus, 1 is best described as a Sc(III)-O-Fe(III) complex, in agreement with previous DFT calculations (Swart, M. Chem. Commun. 2013, 49, 6650.). These results shed light on the interaction of Lewis acids with high-valent metal-oxo species.

ChemInform Abstract: Ruthenium Tetroxide and Perruthenate Chemistry. Recent Advances and Related Transformations Mediated by Other Transition Metal Oxo‐Species

AbstractReview: literature of 2006‐2013; 208 refs.

A DFT Study on the Degradation of Chlorobenzene to p-chlorophenol via Stable Hydroxo Intermediate Promoted by Iron and Manganese Monoxides

The reaction paths for the conversion of chlorobenzene to p-chlorophenol are presented in detail using iron and manganese monoxides via the hydroxo insertion intermediate, HO–M–C6H4Cl (M=FeO, MnO). The molecular geometries and electronic structures for the reactants, intermediates, transition states, and products were optimized and analyzed in detail by density functional methods. The reaction potential surface profiles indicate that the metaloxo species can activate the para C–H bond of the chlorobenzene to lead to the p-chlorophenol via the successive formation and the dissociation of the metal carbon bond, followed by removal of the metal atom (Fe or Mn). The intrinsic reaction co-ordinate (IRC) analyses indicated that no crossover point was searched for between the high-spin and low-spin potential energy surfaces; thus, no spin crossing was found between these two states potential energy surfaces. The low-spin potential energy surface lies above the high-spin one for the entire reaction pathway. Our theoretical study on the possible reaction pathways for the conversion of chlorobenzene to p-chlorophenol will also be useful for analyzing the catalytic functions of C–H bond activation and metal–carbon bond formation by transition metal complexes.

Large-scale one pot synthesis of metal oxide nanoparticles by decomposition of metal carbonates or nitrates

This study identifies bulk metal carbonates and nitrates as cost-effective starting materials for a large-scale, one-pot synthesis of monodisperse metal oxide nanoparticles of MnO, Fe3O4, and CoO phases. The thermal decomposition of bulk metal carbonates or nitrates in hot fatty acid forms monodisperse metal oxide nanoparticles on a large scale. The release of carbon monoxide and nitrogen dioxide gas molecules from these precursors would in situ form molecular metal-oxo species, lightly stabilized by complexation with fatty acid, which would in turn undergo subsequent agglomeration to form metal oxide nanoparticles. The particle size and shape could be controlled simply by varying the reaction conditions, namely the amount and nature of added alkylamine. This general and convenient synthetic methodology would be greatly beneficial to the practical application of thermally synthesized metal oxide nanoparticles.

Vanadyl species-catalyzed complementary β-oxidative carbonylation of styrene derivatives with aldehydes.

A series of oxometallic species and metal acetylacetonates (acac) was examined as catalysts for oxidative carbonylation of styrene with benzaldehyde using t-butylhydroperoxide as the co-oxidant in warm acetonitrile. Among them, VO((acac)2 and vanadyl(IV) chloride were found to be the only catalyst class to achieve cross-coupling processes by judiciously tuning the ligand electronic attributes, leading to β-hydroxylation- and β-peroxidation-carbonylation of styrene, respectively, in a complementary manner. Mechanistic studies indicated that vanadyl-associated acyl radicals generated by t-butoxy radical-assisted, homolytic cleavage of the aldehyde C-H bond were involved in tandem processes with an exclusive syn diastereoselectivity in the case of β-methylstyrene.

Substrate-dependent order of catalytic activity for a series of Fe(III) and Mn(III) porphyrins in the oxidation of organic sulfides and olefins with periodate

The oxidation of organic sulfides and olefins with tetra-n-butylammonium periodate (TBAP) in the presence of Fe(III) and Mn(III) meso-tetraarylporphyrins bearing phenyl, 2-chlorophenyl, 4-chlorophenyl and 4-methoxyphenyl groups at the meso positions and imidazole (ImH) has been studied and compared. With the exception of the metalloporphyrins with 4-methoxyphenyl substituents, the oxidative stability of the iron porphyrins was considerably greater than that of the manganese ones. The stronger π–π interaction between the porphyrin core and the iron center in comparison with the manganese center was suggested to explain the higher oxidative stability of the iron porphyrins. The metalloporphyrins with poor electron-donating substituents, i.e., 2-chlorophenyl and 4-chlorophenyl often provided much lower catalytic activity compared to the other catalysts. The departure of IO3 −, leading to the formation of high valent oxo metal species, seems to be facilitated by the presence of strong electron-donating groups at the meso positions. In this catalytic system, the sulfoxide/sulfone molar ratio is determined by stereoelectronic properties of the substituents attached to the sulfur atom of the organic sulfide and the meso-aryl groups of the metalloporphyrins. Although the iron porphyrins showed significantly higher catalytic activities compared to the manganese counterparts in the oxidation of methyl phenyl sulfide, the catalytic activities were comparable for the oxidation of diallyl sulfide. On the other hand, the oxidation of olefins with TBAP gave different order of catalytic activities for the metalloporphyrins. However, the manganese porphyrins were generally more efficient catalysts compared to the iron one. Also, the iron or manganese porphyrins with more electron-donating meso substituents were found to be more efficient than the other ones. The competitive oxidation of cis- and trans-stilbene provided evidence for the participation of a high valent oxo metal species and a six-coordinate periodato one as the active oxidants, although in the case of MnT(2-Cl)PPOAc and FeT(2-Cl)PPOAc the results are strongly in favor of a high valent oxo metal species as the dominant active oxidant.

Enhancement of Activity of Noble Metal Nanoparticles in Oxidative Electrocatalysis through Modification with Mixed-Metal Oxo-Species

Platinum has been recognized as the most active catalytic metal towards oxidation of ethanol at low and moderate temperatures. But Pt anodes are readily poisoned by the strongly adsorbed intermediates, namely by CO-type species, requiring fairly high overpotentials for their removal. To enhance activity of Pt catalysts towards methanol and ethanol oxidation, additional metals including ruthenium, tin, molybdenum, tungsten or rhodium are usually introduced as the alloying component. More recently it has been demonstrated that catalytic activity of platinum-based nanoparticles towards electrooxidation of ethanol has been significantly enhanced through interfacial modification with ultra-thin monolayer-type films of metal oxo species of tungsten, titanium or zirconium. Also gold in combination with platinum has been demonstrated to produce novel high performance bimetallic catalysts.We pursue a concept of utilization of hybrid mixed metal (tungsten(VI)/zirconium(IV), tungsten(VI)/titanium(IV), tungsten(VI)/cerium(IV)) oxides as matrices for supporting and activating noble metal nanoparticles (e.g. Pt, Pt-Ru, Pd, Ru or Rh) during electrooxidation of methanol, ethanol, dimethyl ether and formic acid. Remarkable increases of electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account possibility of specific interactions of noble metals with transition metal oxide species as well as existence of active hydroxyl groups in the vicinity of catalytic noble metal sites.The concept of utilization of mixed metal oxide supports to active noble metal catalytic sites will be extended to oxidation of such inert reactants as arsenate(III) and nitrate(III) that of importance to analytical chemistry, namely to electroanalytical determinations of toxic oxoanions. Mechanistic considerations will be provided, and comparison will be made to the behavior of small organic molecules mentioned above.

ChemInform Abstract: Quantum Chemical Studies on Dioxygen Activation and Methane Hydroxylation by Diiron and Dicopper Species as well as Related Metal—Oxo Species

AbstractReview: 278 refs.

Ruthenium tetroxide and perruthenate chemistry. Recent advances and related transformations mediated by other transition metal oxo-species.

In the last years ruthenium tetroxide is increasingly being used in organic synthesis. Thanks to the fine tuning of the reaction conditions, including pH control of the medium and the use of a wider range of co-oxidants, this species has proven to be a reagent able to catalyse useful synthetic transformations which are either a valuable alternative to established methods or even, in some cases, the method of choice. Protocols for oxidation of hydrocarbons, oxidative cleavage of C–C double bonds, even stopping the process at the aldehyde stage, oxidative cleavage of terminal and internal alkynes, oxidation of alcohols to carboxylic acids, dihydroxylation of alkenes, oxidative degradation of phenyl and other heteroaromatic nuclei, oxidative cyclization of dienes, have now reached a good level of improvement and are more and more included into complex synthetic sequences. The perruthenate ion is a ruthenium (VII) oxo-species. Since its introduction in the mid-eighties, tetrapropylammonium perruthenate (TPAP) has reached a great popularity among organic chemists and it is mostly employed in catalytic amounts in conjunction with N-methylmorpholine N-oxide (NMO) for the mild oxidation of primary and secondary alcohols to carbonyl compounds. Its use in the oxidation of other functionalities is known and recently, its utility in new synthetic transformations has been demonstrated. New processes, synthetic applications, theoretical studies and unusual transformations, published in the last eight years (2006–2013), in the chemistry of these two oxo-species, will be covered in this review with the aim of offering a clear picture of their reactivity. When appropriate, related oxidative transformations mediated by other metal oxo-species will be presented to highlight similarities and differences. An historical overview of some aspects of the ruthenium tetroxide chemistry will be presented as well.