Peroxo Complexes Research Articles

Computational and experimental insights into reactive forms of oxygen species on dynamic Ag surfaces under ethylene epoxidation conditions

The partial oxidation of ethylene to ethylene oxide (EO) proceeds over supported Ag particles. Adsorption of molecular oxygen upon Ag forms the reactive oxygen species responsible for both epoxidation and combustion reactions, however, the catalyst surface structure and the molecular structure and identity of the reactive oxygen species present remain debated despite past use of appropriate forms of in situ spectroscopy. Specifically, there is no consensus for the molecular origin of widely reported Raman features that appear on contact with only O2 or in reactant mixtures of C2H4 and O2 at pressures and temperatures relevant for the industrial process, in part, due to the lack of concerted ab initio studies that compute vibrational frequencies for oxygen-containing Ag surfaces. Here, we elucidate the molecular structure of the catalyst surface and reactive oxygen species by coordinating spectral deconvolution of transient and steady-state surface-enhanced Raman spectroscopy of Ag catalysts exposed to oxygen (2 – 101 kPa O2, 523 – 673 K) and mixtures of oxygen and ethylene (2 – 101 kPa O2, 0.5 – 9.8 kPa C2H4, 523 K) with ab initio thermodynamic modeling and vibrational frequency calculations. These comparisons suggest that during EO catalysis reconstructed surface oxides form and partially or completely encase metallic Ag particles. Raman features near 600 cm−1 that persist in O2 or cofed C2H4 represent O–Ag–O structural motifs that form only on surface oxides and high oxygen-coverage reconstructed AgxOy surfaces. We assign features centered near 810 – 840 cm−1 and at higher frequencies, which are ubiquitous throughout the literature, to dioxygen complexes partially embedded within an oxide-like overlayer that forms during exposure either to O2 or during steady-state epoxidations. Taken together our results implicate the presence of a combination of monatomic and diatomic surface oxygen species, which emerge with appreciable quantities of subsurface oxygen at temperatures and O2 pressures representative of EO catalysis.

Spectroscopic Evidence for a Cobalt-Bound Peroxyhemiacetal Intermediate.

Aldehyde deformylation reactions by metal dioxygen adducts have been proposed to involve peroxyhemiacetal species as key intermediates. However, direct evidence of such intermediates has not been obtained to date. We report the spectroscopic characterization of a mononuclear cobalt(III)-peroxyhemiacetal complex, [Co(Me3-TPADP)(O2CH(O)CH(CH3)C6H5)]+ (2), in the reaction of a cobalt(III)-peroxo complex (1) with 2-phenylpropionaldehyde (2-PPA). The formation of 2 is also investigated by isotope labeling experiments and kinetic studies. The conclusion that the peroxyhemiacetalcobalt(III) intermediate is responsible for the aldehyde deformylation is supported by the product analyses. Furthermore, isotopic labeling suggests that the reactivity of the cobalt(III)-peroxo complex depends on the second reactant. The aldehyde inserts between the oxygen atoms of 1, whereas the reaction with acyl chlorides proceeds by a nucleophilic attack. The observation of the peroxyhemiacetal intermediate provides significant insight into the initial step of aldehyde deformylation by metalloenzymes.

Cr(VI)-Mediated Homogeneous Fenton Oxidation for Decolorization of Methylene Blue Dye: Sludge Free and Pertinent to a Wide pH Range.

Fe(II)-mediated Fenton process is commonly employed for oxidative degradation of recalcitrant pollutants in wastewater. However, the method suffers from limitations like narrow working pH range and iron sludge formation. The present work deals with the degradation of Methylene Blue (MB) dye using Fenton-like oxidation by replacing Fe(II) with Cr(VI), which eliminates the limitations of classical Fenton oxidation. The Fenton-like oxidation of MB is brought about by HO• radicals generated by the disproportionation of chromium-coordinated peroxo complexes. It was observed that the working pH range for the Cr(VI)-mediated Fenton oxidation was 3–10, and no sludge formation takes place up to four cycles as the oxidation remains in the pure solution phase. The complete mineralization of dye was confirmed by observing the decay of MB peaks by a spectrophotometer and cyclic voltammetry. The reaction parameters like pH of the solution, temperature, degradation time, concentrations of H2O2, Cr(VI), and MB were studied for optimal performance of the Cr(VI) as the catalyst. Kinetic studies revealed that the Cr(VI)-mediated Fenton reaction follows pseudo-first-order reaction kinetics and depends on the concentration of HO• radicals. The proposed Cr(VI)-mediated Fenton oxidation in the present work is best suited for the degradation of organic dyes by adding H2O2 as a precursor in chromate-contaminated wastewaters.

Metal-doped titanium oxide films for suppressing potential-induced degradation of photovoltaic modules

Titanium oxide films were prepared from peroxotitanium complex aqueous solution containing a peroxo complex of vanadium, molybdenum or tungsten of 2, 5, 10 mol % by heating at 200 °C for the purpose of suppressing the photocatalytic activities that may be developed by crystallization during long-term use as a prevention film against potential-induced degradation (PID) of crystalline silicon (c-Si) photovoltaic (PV) module. The films containing those dopants crystallized to anatase by hydrothermal treatment at 100 °C for 15 h as an acceleration test, but hardly showed any photocatalytic activity. According to the lattice constants, X-ray photoelectron spectroscopy analyses and UV–visible light reflection spectra, it was estimated that vanadium (V), molybdenum (VI) and tungsten (VI) were substituted with titanium (IV) in titanium oxide heated at low temperature of 200 °C. The titanium oxide films containing the dopants of 2 mol % were coated at 200 °C onto the rear side of front cover glass with 200 nm thickness. No significant PID was observed in the c-Si PV modules based on the film-coated glass by a PID test by applying −1000 V at 85 °C for 2 h.

Synthesis and Characterization of Manganese Dithiocarbamate Complexes: New Evidence of Dioxygen Activation.

(1) Background: Metal dithiocarbamate compounds have long been the subject of research due to their ease of formation, excellent properties and potential applications. However, manganese complexes with dithiocarbamates, to our knowledge, have never been used for medical imaging applications. With the aim of developing a new class of mononuclear manganese(II)-based agents for molecular imaging applications, we performed a specific investigation into the synthesis of mononuclear bis-substituted Mn(II) complexes with dithiocarbamate ligands. (2) Methods: Synthesis in either open or inert atmosphere at different Mn(II) to diethyldithiocarbamate molar ratios were performed and the products characterized by IR, EA, ESI-MS and XRD analysis. (3) Results: We found that only under oxygen-free atmospheric conditions the Mn(II) complex MnL2, where L = diethyldithiocarbamate ligand, is obtained, which was further observed to react with dioxygen in the solid state to form the intermediate superoxo Mn(III) complex [MnL2(η2-O2)]. The existence of the superoxo complex was revealed by mass spectroscopy, and this species was interpreted as an intermediate step in the reaction that led the bis-substituted Mn(II) complex, MnL2, to transform into the tris-substituted Mn(III) complex, MnL3. A similar result was found with the ligand L’ (= bis(N-ethoxyethyl)dithiocarbamate). (4) Conclusions: We found that in open atmosphere and in aqueous solution, only manganese(III) diethyldithiocarbamate complexes can be prepared. We report here a new example of a small-molecule Mn(II) complex that efficiently activates dioxygen in the solid state through the formation of an intermediate superoxide adduct.

Kinetics and mechanistic studies for the oxidation of N-Benzylhydroxylamine by a CoIII-bound bridging superoxo complex in perchloric acid medium

In aqueous perchloric acid medium (pH = 0.522 – 1.3) , N–benzylhydroxylamine ( two electron reductant) reduces the one electron oxidant, superoxo ligand in [(dien)(en)CoIII(O2)CoIII(en)(dien)](ClO4)5 (1) to the corresponding hydroperoxo complex, [(en)(dien)CoIII(HO2)CoIII(en)(dien)]5+ (2) and itself gets oxidised to PhCH2NO following both proton coupled electron transfer (PCET) path and an electron – transfer (ET) reaction. The kinetics, stoichiometry and reaction mechanism clearly indicate that oxidation of PhCH2NHOH occurs through the formation of an intermediate, benzyl derivative of aminoxyl radical (PhCH2NHO•). In the presence of excess PhCH2NHOH over 1, the reaction obeys first-order kinetics and rate of the reaction increases with [PhCH2NHOH]. The reaction rate, however, decreases with increase in [H+] and the plot of 1/ko with [H+] is linear with a small but noteworthy intercept. It is also remarked that the reaction rate remarkably decreases with increasing proportion of D2O replacing H2O in the solvent. Therefore, an H-atom transfer (HAT) from PhCH2NHOH to the bridging superoxide in 1 seems reasonable at the rate determining step.

A Mononuclear Non-heme Iron(III)-Peroxo Complex with an Unprecedented High O-O Stretch and Electrophilic Reactivity.

A mononuclear non-heme iron(III)-peroxo complex, [Fe(III)(O2)(13-TMC)]+ (1), was synthesized and characterized spectroscopically; the characterization with electron paramagnetic resonance, Mössbauer, X-ray absorption, and resonance Raman spectroscopies and mass spectrometry supported a high-spin S = 5/2 Fe(III) species binding an O2 unit. A notable observation was an unusually high νO-O at ∼1000 cm-1 for the peroxo ligand. With regard to reactivity, 1 showed electrophilic reactivity in H atom abstraction (HAA) and O atom transfer (OAT) reactions. In the HAT reaction, a kinetic isotope effect (KIE) value of 5.8 was obtained in the oxidation of 9,10-dihydroanthracene. In the OAT reaction, a negative ρ value of -0.61 in the Hammett plot was determined in the oxidation of p-X-substituted thioanisoles. Another interesting observation was the electrophilic reactivity of 1 in the oxidation of benzaldehyde derivatives, such as a negative ρ value of -0.77 in the Hammett plot and a KIE value of 2.2. To the best of our knowledge, the present study reports the first example of a mononuclear non-heme iron(III)-peroxo complex with an unusually high νO-O value and unprecedented electrophilic reactivity in oxidation reactions.

Sensing Mechanism of H2O, NH3, and O2 on the Stability-Improved Cs2Pb(SCN)2Br2 Surface: A Quantum Dynamics Investigation.

Although the perovskite sensing materials have shown high sensitivity and ideal selectivity toward neutral, oxidative, or reductive gases, their structural instability hampers the practical application. To exploit perovskite-based gas-sensing materials with improved stability and decent sensitivity, three adsorption complexes of H2O, NH3, and O2 on the Cs2Pb(SCN)2Br2 surface are built by doping Br– and Cs+ in the parent (CH3NH3)2Pb(SCN)2I2 structure and submitted to quantum dynamics simulations. Changes in the semiconductor material geometric structures during these dynamic processes are analyzed and adsorption ability and charge transfer between Cs2Pb(SCN)2Br2 and the gas molecules are explored so as to further establish a correlation between the geometrical structure variations and the charge transfer. By comparing with the previous CH3NH3PbI3 and (CH3NH3)2Pb(SCN)2I2 adsorption systems, we propose the key factors that enhance the stability of perovskite structures in different atmospheres. The current work is expected to provide clues for developing innovative perovskite sensing materials or for constructing reasonable sensing mechanisms.

Fragmentation of two dimethylaluminum peroxy radical isomers formed by the hypergolic reaction of trimethyl aluminum with O2

We have investigated computationally the kinetics and mechanisms for the fragmentation of the (CH3)2Al(O)O and CH3Al(O)OCH3 radicals formed by different O2-for-CH3 substitution reactions of (CH3)3Al with O2 at the CCSD(T)//6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) level of theory. The fragmentation of these radicals may play a key role in the chain-propagation of hypergolic combustion of trimethyl aluminum in air under ambient conditions. Rate constants for various product channels have been calculated under different pressures between 100 and 7600 Torr at temperatures from 300 to 2000 K. Among the many fragmentation processes of the CH3Al(O)OCH3 radical, produced by the predominant (CH3)3Al + O2 reaction, its exothermic isomerization to CH2Al(OH)OCH3 (−26.7 kcal/mol) with a small barrier (9.7 kcal/mol), to be followed by decomposition giving a reactive product pair CH3AlOH + CH2O with a small endothermicity of 15.8 kcal/mol is believed to be most important. The ensuing exothermic and barrierless reaction of CH3AlOH with O2 gives rise to a ring intermediate, cy_CH3Al(O2)OH (−70.5 kcal/mol), which readily fragments to HOAlO2 + CH3 with an overall exothermicity of 21.7 kcal/mol. The (CH3)2Al(O)O radical isomer, which is effectively the association complex of (CH3)2Al and O2, on the other hand, is stable and has no low-energy exit paths to reactive products including its isomer CH3Al(O)OCH3. Rate constants for the three key reactions leading to the ultimate formation of CH3 + HOAlO2 + CH2O have been calculated for combustion modeling applications.

Structural and Spectroscopic Evidence for a Side-on Fe(III)-Superoxo Complex Featuring Discrete O-O Bond Distances.

The O–O bond length is often used as a structural indicator to determine the valence states of bound O2 ligands in biological metal–dioxygen intermediates and related biomimetic complexes. Here, we report very distinct O–O bond lengths found for three crystallographic forms (1.229(4), 1.330(4), 1.387(2) Å at 100 K) of a side-on iron–dioxygen species. Despite their different O–O bond distances, all forms possess the same electronic structure of Fe(III)–O2•–, as evidenced by their indistinguishable spectroscopic features. Density functional theory and ab initio calculations, which successfully reproduce spectroscopic parameters, predict a flat potential energy surface of an η2-O2 motif binding to the iron center regarding the O–O distance. Therefore, the discrete O–O bond lengths observed likely arise from differential intermolecular interactions in the second coordination sphere. The work suggests that the O–O distance is not a reliable benchmark to unequivocally identify the valence state of O2 ligands for metal–dioxygen species in O2-utilizing metalloproteins and synthetic complexes.

Structure and Reactivity of a High-Spin, Nonheme Iron(III)- Superoxo Complex Supported by Phosphinimide Ligands.

Nonheme iron oxygenases utilize dioxygen to accomplish challenging chemical oxidations. A further understanding of the Fe-O2 intermediates implicated in these processes is challenged by their highly transient nature. To that end, we have developed a ligand platform featuring phosphinimide donors intended to stabilize oxidized, high-spin iron complexes. O2 exposure of single crystals of a three-coordinate Fe(II) complex of this framework allowed for in crystallo trapping of a terminally bound Fe-O2 complex suitable for XRD characterization. Spectroscopic and computational studies of this species support a high-spin Fe(III) center antiferromagnetically coupled to a superoxide ligand, similar to that proposed for numerous nonheme iron oxygenases. In addition to the apparent stability of this synthetic Fe-O2 complex, its ability to engage in a range of stoichiometric and catalytic oxidation processes demonstrates that this iron-phosphinimide system is primed for development in modeling oxidizing bioinorganic intermediates and green oxidation chemistry.

Functional Myoglobin Model Composed of a Strapped Porphyrin/Cyclodextrin Supramolecular Complex with an Overhanging COOH That Increases O2/CO Binding Selectivity in Aqueous Solution.

A water-soluble strapped iron(III)tetraarylporphyrin (FeIIIPor-1) bearing two propylpyridinium groups at the side chains and a carboxylic acid group at the overhanging position of the strap was synthesized to mimic the function of myoglobin with the distal polar functionality in aqueous solution. FeIIIPor-1 forms a stable 1:1 inclusion complex with a per-O-methylated β-cyclodextrin dimer having a pyridine linker (Py3OCD), providing a hydrophobic environment and a proximal fifth ligand to stabilize the O2-complex. The ferrous complex (FeIIPorCD-1) binds both O2 and CO in aqueous solution. The O2 and CO binding affinities (P1/2O2 and P1/2CO) and half-life time (t1/2) of the O2 complex of FeIIPorCD-1 are 6.3 and 0.021 Torr, and 7 h, respectively, at pH 7 and 25 °C. The control compound without the strap structure (FeIIPorCD-2) has similar oxygen binding characteristics (P1/2O2 = 8.0 Torr), but much higher CO binding affinity (P1/2CO = 3.8 × 10-4 Torr), and longer t1/2 (30 h). The O2 and CO kinetics indicate that the strapped structure in FeIIPorCD-1 inhibits the entrance of these gaseous ligands into the iron(II) center, as evidenced by lower konO2 and konCO values. Interestingly, the CO complex of FeIIPorCD-1 is significantly destabilized (relatively larger koffCO), while the koffO2 value is much smaller than that of FeIIPorCD-2, resulting in significantly increased O2/CO selectivity (reduced M value, where M = P1/2O2/P1/2CO = 320) in FeIIPorCD-1 compared to FeIIPorCD-2 (M = 21000).

Structurally Characterized μ-1,2-Peroxo/Superoxo Dicopper(II) Pair.

Superoxo complexes of copper are primary adducts in several O2-activating Cu-containing metalloenzymes as well as in other Cu-mediated oxidation and oxygenation reactions. Because of their intrinsically high reactivity, however, isolation of Cux(O2•-) species is challenging. Recent work (J. Am. Chem. Soc. 2017, 139, 9831; 2019, 141, 12682) established fundamental thermochemical data for the H atom abstraction reactivity of dicopper(II) superoxo complexes, but structural characterization of these important intermediates was so far lacking. Here we report the first crystallographic structure determination of a superoxo dicopper(II) species (3) together with the structure of its 1e- reduced peroxo congener (2; a rare cis-μ-1,2-peroxo dicopper(II) complex). Interconversion of 2 and 3 occurs at low potential (-0.58 V vs Fc/Fc+) and is reversible both chemically and electrochemically. Comparison of metric parameters (d(O-O) = 1.441(2) Å for 2 vs 1.329(7) Å for 3) and of spectroscopic signatures (ν̃(16O-16O) = 793 cm-1 for 2 vs 1073 cm-1 for 3) reflects that the redox process occurs at the bridging O2-derived unit. The CuII-O2•--CuII complex has an S = 1/2 spin ground state according to magnetic and EPR data, in agreement with density functional theory calculations. Computations further show that the potential associated with changes of the Cu-O-O-Cu dihedral angle is shallow for both 2 and 3. These findings provide a structural basis for the low reorganization energy of the kinetically facile 1e- interconversion of μ-1,2-superoxo/peroxo dicopper(II) couples, and they open the door for comprehensive studies of these key intermediates in Cux/O2 chemistry.

Unveiling Oxygen Redox Activity in P2-Type NaxNi0.25Mn0.68O2 High-Energy Cathode for Na-Ion Batteries

Na-ion batteries are emerging as convenient energy-storage devices for large-scale applications. Enhanced energy density and cycling stability are key in the optimization of functional cathode materials such as P2-type layered transition metal oxides. High operating voltage can be achieved by enabling anionic reactions, but irreversibility of O2–/O2n–/O2 evolution still limits this chance, leading to extra capacity at first cycle that is not fully recovered. Here, we dissect this intriguing oxygen redox activity in Mn-deficient NaxNi0.25Mn0.68O2 from first-principles, by analyzing the formation of oxygen vacancies and dioxygen complexes at different stages of sodiation. We identify low-energy intermediates that release molecular O2 at high voltage, and we show how to improve the overall cathode stability by partial substitution of Ni with Fe. These new atomistic insights on O2 formation mechanism set solid scientific foundations for inhibition and control of this process toward the rational design of new anionic redox-active cathode materials.

Isolation of a Ru(IV) side-on peroxo intermediate in the water oxidation reaction.

The electrons that nature uses to reduce CO2 during photosynthesis come from water oxidation at the oxygen-evolving complex of photosystem II. Molecular catalysts have served as models to understand its mechanism, in particular the O-O bond-forming reaction, which is still not fully understood. Here we report a Ru(IV) side-on peroxo complex that serves as a 'missing link' for the species that form after the rate-determining O-O bond-forming step. The Ru(IV) side-on peroxo complex (η2-1iv-OO) is generated from the isolated Ru(IV) oxo complex (1iv=O) in the presence of an excess of oxidant. The oxidation (IV) and spin state (singlet) of η2-1iv-OO were determined by a combination of experimental and theoretical studies. 18O- and 2H-labelling studies evidence the direct evolution of O2 through the nucleophilic attack of a H2O molecule on the highly electrophilic metal-oxo species via the formation of η2-1iv-OO. These studies demonstrate water nucleophilic attack as a viable mechanism for O-O bond formation, as previously proposed based on indirect evidence.

Catalytic Oxygenation of Hydrocarbons by Mono-μ-oxo Dicopper(II) Species Resulting from O-O Cleavage of Tetranuclear CuI /CuII Peroxo Complexes.

One of the challenges of catalysis is the transformation of inert C−H bonds to useful products. Copper‐containing monooxygenases play an important role in this regard. Here we show that low‐temperature oxygenation of dinuclear copper(I) complexes leads to unusual tetranuclear, mixed‐valent μ4‐peroxo [CuI/CuII]2 complexes. These Cu4O2 intermediates promote irreversible and thermally activated O−O bond homolysis, generating Cu2O complexes that catalyze strongly exergonic H‐atom abstraction from hydrocarbons, coupled to O‐transfer. The Cu2O species can also be produced with N2O, demonstrating their capability for small‐molecule activation. The binding and cleavage of O2 leading to the primary Cu4O2 intermediate and the Cu2O complexes, respectively, is elucidated with a range of solution spectroscopic methods and mass spectrometry. The unique reactivities of these species establish an unprecedented, 100 % atom‐economic scenario for the catalytic, copper‐mediated monooxygenation of organic substrates, employing both O‐atoms of O2.

Matrix isolation infrared study of the interaction of dioxygen with chromium(II)tetraphenylporphyrin

The infrared spectra of Cr(II)(TPP) (TPP is meso-tetraphenylporphyrinato dianion) and its adduct with molecular oxygen have been recorded in argon matrices at 10 K, as well as in thin sublimated films. Formation of the Cr(TPP)(O2) adduct have been confirmed using 16O2 and 18O2. The ν(O2) stretching bands are located at 1146 and 1082 cm−1, respectively. These and literature data together with the results of our DFT calculations, are consistent with the end-on dioxygen binding mode in the Cr(III)(TPP)(O2–) superoxo complex.

Oxidation of Organic Substrates in Two-Phase Liquid Systems in the Presence of Tungsten Peroxo Complexes with the Formation of Carboxylic Acids

Oxidation of Organic Substrates in Two-Phase Liquid Systems in the Presence of Tungsten Peroxo Complexes with the Formation of Carboxylic Acids

Sequential Post-Synthetic Modification and Evaluation of Catalytic Activity of Hierarchically Porous Sulfated Geopolymer in the Oxidative Desulfurization of Dibenzothiophene

A novel sulfated hierarchically porous geopolymer catalyst for the oxidation of dibenzothiophene was prepared through sequential treatment with desilication, ion-exchange and sulfation techniques. Desilication and ion-exchange treatments were aimed at the creation of hierarchical porosity and controlling the surface area of the geopolymer, while sulfation treatment was primarily done to enhance the acidity of the hierarchically porous geopolymer. The properties of the catalyst obtained after the modifications was studied by BET, NH3-TPD, SEM, TEM, XRD, FTIR and XRF analyses. As a result of the proposed sequential treatment, the activity of the geopolymer towards the oxidation of dibenzothiophene was profoundly increased and reflected in the enhanced conversion from 14 to 85%. A plausible electronic mechanism proposed for the reaction accentuated that the reaction might take place in two stages: the formation of sulfated geopolymer peroxo complex as the active catalyst and nucleophilic attack by dibenzothiophene over the peroxo complex to form the product, both were elaborated with suitable kinetic model. The interaction between dibenzothiophene and peroxo complex was considered to be of primary concern, based on which, Langmuir–Hinshelwood kinetic model was developed. Effective regeneration upto three cycles emphasized its potential to be exploited as an efficient catalyst for oxidative desulfurization.

Electronic structure of Schiff-base peroxo{2,2'-[1,2-phenyl-enebis(nitrilo-methanylyl-idene)]bis-(6-meth-oxy-phenolato)}titanium(IV) monohydrate: a possible model structure of the reaction center for the theoretical study of hemoglobin.

An extensive characterization of [Ti(C22H18N2O6)]·H2O was performed by topological analysis according to Bader's quantum theory of atoms in molecules (QTAIM) from the experimentally (multipole model) and theoretically (DFT) determined electron density. To the best of our knowledge, this study is the first example of an experimental electronic structure of a coordination compound in which a peroxo anion is bonded to a 3d central atom. The titanium coordination polyhedron could be described as a deformed tetrahedral pyramid if the midpoint of the peroxide O-O bond (side-on mode) is considered to be in the quasi-apical position. According to the multipole model (MM) results, the titanium atom has a positive QTAIM charge of 2.05 e- which does not correspond to the formal Ti (IV) oxidation state. On the other hand, the peroxo oxygen atoms O(1) and O(2) have MM QTAIM charges of -0.27 and -0.12, respectively. This asymmetric charge density distribution on the peroxo oxygens is in agreement with the distorted orientation of the O2 moiety with respect to the titanium atom. Despite the fact that the overall MM charge of the O2 moiety is more remote from the formal -2 charge than from neutral O2, the O-O distance remains close to that in the peroxo O2 2- anion. In the case of DFT results, the titanium atom charge is also found to be close to +2, the O2 x- moiety charge is around -1, the optimized O-O distance is shorter by only ca 0.04 Å than the experimental value of 1.5005 (16) Å, and the DFT d-populations on titanium are found to be lower than the experimental MM value. This study is the first experimental electronic structure of a transition metal peroxo complex.