Peroxo Complexes Research Articles

Rare‐earth compositional screening of high‐entropy diborides for improved oxidation resistance

AbstractCompositional screening is an important strategy to improve the oxidation resistance of high‐entropy diborides (HEB2), yet the related studies are limited. Here, we report a rare‐earth (RE) element compositional screening strategy to improve the oxidation resistance of HEB2. To be specific, the single‐phase (Hf0.28Zr0.28Ta0.28RE0.16)B2 (HEB2‐RE) samples with 12 different RE elements are fabricated via ultrafast ultrahigh‐temperature synthesis and spark plasma sintering techniques, and their oxidation resistance is explored by isothermal oxidation tests. The results show that the as‐obtained HEB2‐Sc samples possess the best oxidation resistance among all the as‐fabricated HEB2‐RE samples. Such outstanding oxidation resistance is ascribed to the enhanced viscosity of the generated B2O3 glass originating from the solid solution of (Zr, Me)0.84Sc0.16O2 complex oxides, as confirmed by transmission electron microscope observations and first‐principles calculations.

A benchmark study of dioxygen complexes based on coupled cluster and density functional theory

A benchmark study of dioxygen complexes based on coupled cluster and density functional theory

Trapping an Elusive Fe(IV)-Superoxo Intermediate Inside a Self-Assembled Nanocage in Water at Room Temperature.

Molecular cavities that mimic natural metalloenzymes have shown the potential to trap elusive reaction intermediates. Here, we demonstrate the formation of a rare yet stable Fe(IV)-superoxo intermediate at room temperature subsequent to dioxygen binding at the Fe(III) site of a (Et4N)2[FeIII(Cl)(bTAML)] complex confined inside the hydrophobic interior of a water-soluble Pd6L412+ nanocage. Using a combination of electron paramagnetic resonance, Mössbauer, Raman/IR vibrational, X-ray absorption, and emission spectroscopies, we demonstrate that the cage-encapsulated complex has a Fe(IV) oxidation state characterized by a stable S = 1/2 spin state and a short Fe-O bond distance of ∼1.70 Å. We find that the O2 reaction in confinement is reversible, while the formed Fe(IV)-superoxo complex readily reacts when presented with substrates having weak C-H bonds, highlighting the lability of the O-O bond. We envision that such optimally trapped high-valent superoxos can show new classes of reactivities catalyzing both oxygen atom transfer and C-H bond activation reactions.

Cobalt(II)-Substituted Cysteamine Dioxygenase Oxygenation Proceeds through a Cobalt(III)-Superoxo Complex.

We investigated the metal-substituted catalytic activity of human cysteamine dioxygenase (ADO), an enzyme pivotal in regulating thiol metabolism and contributing to oxygen homeostasis. Our findings demonstrate the catalytic competence of cobalt(II)- and nickel(II)-substituted ADO in cysteamine oxygenation. Notably, Co(II)-ADO exhibited superiority over Ni(II)-ADO despite remaining significantly less active than the natural enzyme. Structural analyses through X-ray crystallography and cobalt K-edge excitation confirmed successful metal substitution with minimal structural perturbations. This provided a robust structural basis, supporting a conserved catalytic mechanism tailored to distinct metal centers. This finding challenges the proposed high-valent ferryl-based mechanism for thiol dioxygenases, suggesting a non-high-valent catalytic pathway in the native enzyme. Further investigation of the cysteamine-bound or a peptide mimic of N-terminus RGS5 bound Co(II)-ADO binary complex revealed the metal center's high-spin (S = 3/2) state. Upon reaction with O2, a kinetically and spectroscopically detectable intermediate emerged with a ground spin state of S = 1/2. This intermediate exhibits a characteristic 59Co hyperfine splitting (A = 67 MHz) structure in the EPR spectrum alongside UV-vis features, consistent with known low-spin Co(III)-superoxo complexes. This observation, unique for protein-bound thiolate-ligated cobalt centers in a protein, unveils the capacities for O2 activation in such metal environments. These findings provide valuable insights into the non-heme iron-dependent thiol dioxygenase mechanistic landscape, furthering our understanding of thiol metabolism regulation. The exploration of metal-substituted ADO sheds light on the intricate interplay between metal and catalytic activity in this essential enzyme.

The [(NH3)4Cu2O2]2+‐Peroxo Complex as the Key Intermediate for NH3‐SCR Activity and Deactivation of Cu‐CHA Catalysts.

AbstractIn NH3‐SCR over Cu‐CHA catalysts in the low‐temperature range 150–300 °C, the activation of oxygen occurs via oxidation of a pair of mobile (NH3)2CuI‐complexes located in the cages of the zeolite. In this step, a reactive [(NH3)4Cu2O2]2+‐peroxo complex (μ‐η2,η2‐peroxo diamino dicopper(II)‐complex) is formed. The chemistry of this complex determines several catalytic properties of the Cu‐CHA catalyst. The reaction of NO with the [(NH3)4Cu2O2]2+‐peroxo complex governs the NH3‐SCR activity. A reaction of the [(NH3)4Cu2O2]2+‐peroxo complex with ammonia hinders the reaction of NO with the complex, thus leading to an inhibition of the NH3‐SCR reaction. Finally, the deactivation in presence of SO2 is due to a reaction of SO2 with the [(NH3)4Cu2O2]2+‐peroxo complex, leading to the formation of Cu−S compounds in the catalyst. In this review, the characterization and the reactions of the [(NH3)4Cu2O2]2+‐peroxo complex with NO, NH3, and SO2, and mean‐field kinetic models based on first principles calculations for NH3‐SCR activity and SO2 poisoning are discussed.

A Mechanistic Spectrum of O-H Bond Cleavage Observed for Reactions of Phenols with a Manganese Superoxo Complex.

Reaction of a rare and well-characterized MnIII-superoxo species, Mn(BDPBrP)(O2⋅) (1, H2BDPBrP=2,6-bis((2-(S)-di(4-bromo)phenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine), with 4-dimethylaminophenol at -80 °C proceeds via concerted proton electron transfer (CPET) to produce a MnIII-hydroperoxo complex, Mn(BDPBrP)(OOH) (2), alongside 4-dimethylaminophenoxy radical; whereas, upon treatment with 4-nitrophenol, complex 1 undergoes a proton transfer process to afford a MnIV-hydroperoxo complex, [Mn(BDPBrP)(OOH)]+ (3). Intriguingly, the reactions of 1 with 4-chlorophenol and 4-methoxyphenol follow two routes of CPET and sequential proton and electron transfer to furnish complex 2 in the end. UV-vis and EPR spectroscopic studies coupled with DFT calculations provided support for this wide mechanistic spectrum of activating various phenol O-H bonds by a single MnIII-superoxo complex, 1.

Hydrogen peroxide activation and liquid‐phase selective oxidations over Ti,Zr‐based metal–organic framework PCN‐415

A mixed Ti,Zr‐based metal–organic framework, PCN‐415 (Porous Coordination Network), and the corresponding Ti,Zr‐oxocluster [Ti8Zr2O12(CH3COO)16] have been synthesized and comprehensively characterized by powder X‐Ray diffraction (PXRD), low‐temperature nitrogen adsorption, scanning electron microscopy, thermogravimetric analysis, Fourier‐transform infrared spectroscopy, and Raman spectroscopy. The catalytic performance of both PCN‐415 and the Ti,Zr‐oxocluster was evaluated in the selective oxidation of representative thioethers, alkenes, and alkylphenols using aqueous hydrogen peroxide and tert‐butyl hydroperoxide as oxidants. The nature of the oxidation products formed indicates that, most likely, both Ti and Zr sites contribute to the catalytic activity of PCN‐415, although the contribution of Ti seems to be more significant. Examination of PCN‐415 after treatment with H2O2 using diffuse reflectance UV–vis spectroscopy indicates the formation of Ti peroxo complexes while Raman spectroscopy shows evidence for the formation of both Ti and Zr peroxo species. The potential formation of missing linker‐type defects and their role in facilitating catalytic activity were addressed. The stability of the PCN‐415 structure under the catalytic conditions was confirmed by PXRD. The absence of metal leaching and the heterogeneous nature of the catalysis were verified. The catalyst could be easily recovered from the reaction mixture and reused several times without loss of the catalytic properties.

A Co(III)-peroxo-arylboronate complex formed by nucleophilic reaction of a Co(III)-peroxo species

In this work, we report the generation and characterization of two new Co(III)-peroxo complexes 2 and 3. 2 is best described as a mononuclear CoIII-(O2) complex that exhibits an 18O-isotope sensitive OO bond stretching vibration at 845(−49) cm−1, indicating a relatively weak peroxo moiety compared to those of other CoIII-(O2) complexes reported previously. Complex 3 is a CoIII-peroxo-arylboronate species having a rare {CoIIIOOBO} five-membered metallocycle, which is structurally characterized using X-ray crystallography. Investigations of the reaction mechanism using density functional theory calculations show that 2 likely undergoes a nucleophilic attack to an arylboronic acid, which is generated by hydrolysis of the BPh4− anion in wet acetonitrile solution, to first form a CoIII-peroxo-arylboronic acid adduct, followed by the loss of one benzene molecule to generate the five-membered metallocycle. The entire reaction is thermodynamically favorable. Taken together, the conversion of 2 to 3 represents the discovery of a novel nucleophilic reactivity that can be carried out by mononuclear Co(III)-peroxo complexes.

The rotamer of the second-sphere histidine in AA9 lytic polysaccharide monooxygenase is pH dependent

Lytic polysaccharide monooxygenases (LPMOs) catalyze a reaction that is crucial for the biological decomposition of various biopolymers and for the industrial conversion of plant biomass. Despite the importance of LPMOs, the exact molecular-level nature of the reaction mechanism is still debated today. Here, we investigated the pH-dependent conformation of a second-sphere histidine (His) that we call the stacking histidine, which is conserved in fungal AA9 LPMOs and is speculated to assist catalysis in several of the LPMO reaction pathways.Using constant-pH and accelerated molecular dynamics simulations, we monitored the dynamics of the stacking His in different protonation states for both the resting Cu(II) and active Cu(I) forms of two fungal LPMOs. Consistent with experimental crystallographic and neutron diffraction data, our calculations suggest that the side chain of the protonated and positively charged form is rotated out of the active site toward the solvent. Importantly, only one of the possible neutral states of histidine (HIE state) is observed in the stacking orientation at neutral pH or when bound to cellulose. Our data predict that, in solution, the stacking His may act as a stabilizer (via hydrogen bonding) of the Cu(II)-superoxo complex after the LPMO-Cu(I) has reacted with O2 in solution, which, in fine, leads to H2O2 formation. Also, our data indicate that the HIE-stacking His is a poor acid/base catalyst when bound to the substrate and, in agreement with the literature, may play an important stabilizing role (via hydrogen bonding) during the peroxygenase catalysis.Our study reveals the pH titration midpoint values of the pH-dependent orientation of the stacking His should be considered when modeling and interpreting LPMO reactions, whether it be for classical LPMO kinetics or in industry-oriented enzymatic cocktails, and for understanding LPMO behavior in slightly acidic natural processes such as fungal wood decay.

Mechanism of formation of p-benzylenephenol peroxide radical (p-PhC(O2•)HPhOH).

The reactions of radicals with O2 play the important role in the biological, medicinal, and industrial processes. The mechanism of this reaction is studied previously for the alkane, alkene, alkyne, phenols, and close-related radicals. According to these studies, the formation of intermediates in these reactions is predicted only for the aromatic radicals. Thus, the Van der Waals complexes of O2 with phenyl or benzyl radicals are predicted, as well as the π-π cluster for benzene. However, the possibility of the formation of such intermediate π-π clusters in the case of bisphenol radicals and the thermochemistry of its formation is not studied. Bisphenols are one of the main components of bio-oil, produced during pyrolysis of lignin-contained biomass. Synthetic bisphenols are used in polycarbonate plastics, epoxy resins, and thermal papers. Their mechanism of oxidation is important for the determination of fire safety of these materials, the possibility of using them as additives for fuels for the decreasing and the description of the ignition delays, as well as for the determination of its health risk assessment in medicine. The five DFT (M06-2X (i = 1), B3LYP (i = 2), wB97XD (i = 3), M08HX (i = 4), MN15 (i = 5)) approaches with 6-311 + + G(d,p) basis set are used for the determination of standard enthalpies of atomization (ΔraHo(Yi)) of considered compounds (molecules, radicals, and transition states). These values of ΔraHo(Yi) are corrected using the empirical linear calibration dependencies, reported previously. The different calibration dependencies are used for the hydrocarbons (including the aromatics and simple oxygenated derivatives) and for the peroxides. The corrected values of ΔraHo(Yi, CORR) are used according to Hess's law for the determination of ΔfHo(Yi, CORR). The most consistent values of ΔfHo(Y, MEAN) are derived from the coordination of the values of ΔfHo(Yi, CORR) using the intersection of their values of standard deviations (3SDi). These values of ΔfHo(Y, MEAN), as well as the B3LYP values of So(Y), which are accounting the frequency correction and internal rotations, as well as their temperature dependencies, are used for the determination of thermochemistry of considered reactions and of the calculation, within transition state theory (TST), of the values of high-pressure limits of the rate constant. The values of Ho(Yi), So(Yi), and Go(Yi) are calculated using the Gaussian 16w program. The temperature dependencies of thermochemical properties and the values of rate constants are determined using the ChemRate program (v.1.5). The optimized structures are visualized using the Chemcraft.

DFT mechanistic insights into the formation of the metal-dioxygen complex [Co(12-TMC)O2]+ using H2O2 as an [O2] unit source.

The reaction of [M(L)]n with H2O2 as an [O2] unit source and NEt3 as a base is a widely used biomimetic transition metal-peroxo and -superoxo complex [M(L)O2]n-1 synthesis method, but the mechanism and accurate stoichiometry of the synthesis remain elusive. In this study, we performed DFT calculations to deeply understand the mechanism, using the synthesis of the cobalt-peroxo complex [CoIII(12-TMC)O2]+ (12-TMC = (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane)) from the reaction of [CoII(12-TMC)]2+ and H2O2 in the presence of NEt3 as an example. The study found that cobalt-peroxo complex formation proceeds via three stages: (Stage I) the conversion of [CoII(12-TMC)]2+ and H2O2 to [CoIII(12-TMC)OH]2+ and OOH˙ radical, (Stage II) the coordination of OOH˙ to [CoII(12-TMC)]2+ to give [CoIII(12-TMC)OOH]2+, followed by deprotonation with NEt3, affording [CoIII(12-TMC)O2]+, and (Stage III) the transformation of [CoIII(12-TMC)OH]2+ which is generated in Stage I to [CoIII(12-TMC)O2]+. The overall stoichiometry of the synthesis is 2*[Co(12-TMC)]2+ + 3*H2O2 + 2*NEt3 → 2*[Co(12-TMC)O2]+ + 2*HNEt3+ + 2*H2O. In addition, compared to its analog [CoIII(TBDAP)O2]+ (TBDAP = N,N-di-tert-butyl-2,11-diaza[3.3](2,6)-pyridinophane) which is synthesized by the same method and has the same Co(III) oxidation state exhibits dioxygenase-like reactivity to nitriles, [CoIII(12-TMC)O2]+ could be inactive towards acetonitrile because the reaction severely deteriorates the coordination of the 12-TMC ligand to the Co center, which results in high reaction barriers.

Increasing the electron donation in a dinucleating ligand family: molecular and electronic structures in a series of CoIICoII complexes.

We have developed a family of dinucleating ligands with varying terminal donors to generate dinuclear peroxo and high-valent complexes and to correlate their stabilities and reactivities with their molecular and electronic structures as a function of the terminal donors. It appears that the electron-donating ability of the terminal donors is an important handle for controlling these stabilities and reactivities. Here, we present the synthesis of a new dinucleating ligand with potentially strong donating terminal imidazole donors. As CoII ions are sensitive to variations in donor strength in terms of coordination number, magnetism, UV-Vis-NIR spectra, redox potentials, we probe the electron donation ability of this new ligand in CoIICoII complexes in comparison to the parent CoIICoII complexes with terminal pyridine donors and we synthesize the analogous CoIICoII complexes with terminal 6-methylpyridines and methoxy-substituted pyridines. The molecular structures show indeed strong variations in coordination numbers and bond lengths. These differences in the molecular structures are reflected in the magnetic properties and in the d-d transitions demonstrating that the molecular structures remain intact upon dissolution. The redox potentials are analyzed with respect to the electron donation ability and are the only handle to observe an effect of the methoxy-substituted pyridines. All data taken together show the following order of electron donating ability for the terminal donors: 6-methylpyridines ≪ pyridines < methoxy-substituted pyridines ≪ imidazoles.

Preparations of trans- and cis-μ-1,2-Peroxodiiron(III) Complexes.

The iron(II) complex with cis,cis-1,3,5-tris(benzylamino)cyclohexane (Bn3CY) (1) has been synthesized and characterized, which reacted with dioxygen to form the peroxo complex 2 in acetone at -60 °C. On the basis of spectroscopic measurements for 2, it was confirmed that the peroxo complex 2 has a trans-μ-1,2 fashion. Additionally, the peroxo complex 2 was reacted with benzoate anion as a bridging agent to give a peroxo complex 3. The results of resonance Raman and 1H-NMR studies supported that the peroxo complex 3 is a cis-μ-1,2-peroxodiiron(III) complex. These spectral features were interpreted by using DFT calculations.

Electronic Structure and Reactivity of Mononuclear Nonheme Iron-Peroxo Complexes as a Biomimetic Model of Rieske Oxygenases: Ring Size Effects of Macrocyclic Ligands.

We report the macrocyclic ring size-electronic structure-electrophilic reactivity correlation of mononuclear nonheme iron(III)-peroxo complexes bearing N-tetramethylated cyclam analogues (n-TMC), [FeIII(O2)(12-TMC)]+ (1), [FeIII(O2)(13-TMC)]+ (2), and [FeIII(O2)(14-TMC)]+ (3), as a model study of Rieske oxygenases. The Fe(III)-peroxo complexes show the same δ and pseudo-σ bonds between iron and the peroxo ligand. However, the strength of these interactions varies depending on the ring size of the n-TMC ligands; the overall Fe-O bond strength and the strength of the Fe-O2 δ bond increase gradually as the ring size of the n-TMC ligands becomes smaller, such as from 14-TMC to 13-TMC to 12-TMC. MCD spectroscopy plays a key role in assigning the characteristic low-energy δ → δ* LMCT band, which provides direct insight into the strength of the Fe-O2 δ bond and which, in turn, is correlated with the superoxo character of the iron-peroxo group. In oxidation reactions, reactivities of 1-3 toward hydrocarbon C-H bond activation are compared, revealing the reactivity order of 1 > 2 > 3; the [FeIII(O2)(n-TMC)]+ complex with a smaller n-TMC ring size, 12-TMC, is much more reactive than that with a larger n-TMC ring size, 14-TMC. DFT analysis shows that the Fe(III)-peroxo complex is not reactive toward C-H bonds, but it is the end-on Fe(II)-superoxo valence tautomer that is responsible for the observed reactivity. The hydrogen atom abstraction (HAA) reactivity of these intermediates is correlated with the overall donicity of the n-TMC ligand, which modulates the energy of the singly occupied π* superoxo frontier orbital that serves as the electron acceptor in the HAA reaction. The implications of these results for the mechanism of Rieske oxygenases are further discussed.

Reaction Pathway for the Aerobic Oxidation of Phosphines Catalyzed by Oxomolybdenum Salen Complexes

AbstractCatalysis of O‐atom transfer (OAT) reactions is a characteristic of both natural (enzymatic) and synthetic molybdenum‐oxo and ‐peroxo complexes. These reactions can employ a variety of terminal oxidants, e. g. DMSO, N‐oxides, and peroxides, etc., but rarely molecular oxygen. Here we demonstrate the ability of a set of Schiff‐base‐MoO2 complexes (cy‐salen)MoO2 (cy‐salen=N,N’‐cyclohexyl‐1,2‐bis‐salicylimine) to catalyze the aerobic oxidation of PPh3. We also report the results of a DFT computational investigation of the catalytic pathway, including the identification of energetically accessible intermediates and transition states, for the aerobic oxidation of PMe3. Starting from the dioxo species, (cy‐salen)Mo(VI)O2 (1), key reaction steps include: 1) associative addition of PMe3 to an oxo‐O to give LMo(IV)(O)(OPMe3) (2); 2) OPMe3 dissociation from 2 to produce mono‐oxo complex (cy‐salen)Mo(IV)O (3); 3) stepwise O2 association with 3 via superoxo species (cy‐salen)Mo(V)(O)(η1‐O2) (4) to form the oxo‐peroxo intermediate (cy‐salen)Mo(VI)(O)(η2‐O2) (5); 4) the O‐transfer reaction of PMe3 with oxo‐peroxo species 5 at the oxo‐group, rather than the peroxo unit leading, after OPMe3 dissociation, to a monoperoxo species, (cy‐salen)Mo(IV)(η2‐O2) (7); and 5) regeneration of the dioxo complex (cy‐salen)Mo(VI)O2 (1) from the monoperoxo triplet 37 or singlet 17 by a concerted, asynchronous electronic isomerization. An alternative pathway for recycling of the oxo‐peroxo species 5 to the dioxo‐Mo 1 via a bimetallic peroxo complex LMo(O)‐O−O‐Mo(O)L 8 is determined to be energetically viable, but is unlikely to be competitive with the primary pathway for aerobic phosphine oxidation catalyzed by 1.

Yellow TiO2 from titanium peroxo complexes: Verification of the visible light activity and a rational enhancement of its photocatalytic efficiency

It is widely believed that colored TiO2 materials (yellow, blue, black, violet, etc.) are visible-light active photocatalysts. In this article, we verify this claim with respect to the yellow TiO2 photocatalyst that has been synthesized using peroxo-titanium complexes formation route. The surface oxygen species, the electronic structure, the photoelectrochemical properties, and the photoactivity of yellow TiO2 have been investigated. FT-IR and XPS analysis confirms the presence of surface peroxo-species that caused the yellow color. Spectroelectrochemical measurements confirmed the absorption of violet light (400–430 nm). The reduced band gap energy (3.05 eV) has resulted from the anodic shift of both conduction and valence band edges by 0.30 and 0.15 eV, respectively. However, the wavelength-dependent photocurrent generation by yellow TiO2 in the presence of oxygen and under deaerated conditions proved that it does not contribute to the photoactivity enhancement in the visible region (λ > 400 nm). The proposed mechanism has shown that although the driving force at the oxidation side is theoretically improved, the reduction counterpart is parallelly weakened and the photoinduced electrons cannot efficiently reduce regular electron acceptors in aqueous systems (e.g., O2, H2O). Adding a low amount of a stronger electron acceptor (e.g., H2O2) into the system provides extra HO• radicals through the reductive mechanism. At higher concentrations of H2O2, the oxidation of H2O2 is more favorable. The hydroxylation of terephthalic acid via the oxidative and reductive pathway along with the bleaching of Congo Red indicates improved photocatalytic activity due to enhanced hydrogen peroxide activation. Although yellow TiO2 is active only upon UV light irradiation, this paper reports how to boost the photocatalytic activity of this material.

Reactivities and Electronic Structures of μ-1,2-Peroxo and μ-1,2-Superoxo CoIIICoIII Complexes: Electrophilic Reactivity and O2 Release Induced by Oxidation.

Peroxo complexes are key intermediates in water oxidation catalysis (WOC). Cobalt plays an important role in WOC, either as oxides CoOx or as {CoIII(μ-1,2-peroxo)CoIII} complexes, which are the oldest peroxo complexes known. The oxidation of {CoIII(μ-1,2-peroxo)CoIII} complexes had usually been described to form {CoIII(μ-1,2-superoxo)CoIII} complexes; however, recently the formation of {CoIV(μ-1,2-peroxo)CoIII} species were suggested. Using a bis(tetradentate) dinucleating ligand, we present here the synthesis and characterization of {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} and {CoIII(μ-OH)2CoIII} complexes. Oxidation of {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} at -40 °C in CH3CN provides the stable {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} species and activates electrophilic reactivity. Moreover, {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} catalyzes water oxidation, not molecularly but rather via CoOx films. While {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} can be reversibly deprotonated with DBU at -40 °C in CH3CN, {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} undergoes irreversible conversions upon reaction with bases to a new intermediate that is also the decay product of {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} in aqueous solution at pH > 2. Based on a combination of experimental methods, the new intermediate is proposed to have a {CoII(μ-OH)CoIII} core formed by the release of O2 from {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} confirmed by a 100% yield of O2 upon photocatalytic oxidation of {CoIII(μ-1,2-peroxo)(μ-OH)CoIII}. This release of O2 by oxidation of a peroxo intermediate corresponds to the last step in molecular WOC.

Dioxygen and glucose force motion of the electron-transfer switch in the iron(III) flavohemoglobin-type nitric oxide dioxygenase

Kinetic and structural investigations of the flavohemoglobin-type NO dioxygenase have suggested critical roles for transient Fe(III)O2 complex formation and O2-forced movements affecting hydride transfer to the FAD cofactor and electron-transfer to the Fe(III)O2 complex. Stark-effect theory together with structural models and dipole and internal electrostatic field determinations provided a semi-quantitative spectroscopic method for investigating the proposed Fe(III)O2 complex and O2-forced movements. Deoxygenation of the enzyme causes Stark effects on the ferric heme Soret and charge-transfer bands revealing the Fe(III)O2 complex. Deoxygenation also elicits Stark effects on the FAD that expose forces and motions that create a more restricted NADH access to FAD for hydride transfer and switch electron-transfer off. Glucose also forces the enzyme toward an off state. Amino acid substitutions at the B10, E7, E11, G8, D5, and F7 positions influence the Stark effects of O2 on resting heme spin states and FAD consistent with the proposed roles of the side chains in the enzyme mechanism. Deoxygenation of ferric myoglobin and hemoglobin A also induces Stark effects on the hemes suggesting a common ‘oxy-met’ state. The ferric myoglobin and hemoglobin heme spectra are also glucose-responsive. A conserved glucose or glucose-6-phosphate binding site is found bridging the BC-corner and G-helix in flavohemoglobin and myoglobin suggesting novel allosteric effector roles for glucose or glucose-6-phosphate in the NO dioxygenase and O2 storage functions. The results support the proposed roles of a ferric O2 intermediate and protein motions in regulating electron-transfer during NO dioxygenase turnover.

Systematic Electronic Tuning on the Property and Reactivity of Cobalt–(Hydro)peroxo Intermediates

A series of cobalt(III)-peroxo complexes, [CoIII(R2-TBDAP)(O2)]+ (1R2; R2 = Cl, H, and OMe), and cobalt(III)-hydroperoxo complexes, [CoIII(R2-TBDAP)(O2H)(CH3CN)]2+ (2R2), bearing electronically tuned tetraazamacrocyclic ligands (R2-TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)-p-R2-pyridinophane) were prepared from their cobalt(II) precursors and characterized by various physicochemical methods. The X-ray diffraction and spectroscopic analyses unambiguously showed that all 1R2 compounds have similar octahedral geometry with a side-on peroxocobalt(III) moiety, but the O-O bond lengths of 1Cl [1.398(3) Å] and 1OMe [1.401(4) Å] were shorter than that of 1H [1.456(3) Å] due to the different spin states. For 2R2, the O-O bond vibration energies of 2Cl and 2OMe were identical at 853 cm-1 (856 cm-1 for 2H), but their Co-O bond vibration frequencies were observed at 572 cm-1 for 2Cl and 550 cm-1 for 2OMe, respectively, by resonance Raman spectroscopy (560 cm-1 for 2H). Interestingly, the redox potentials (E1/2) of 2R2 increased in the order of 2OMe (0.19 V) < 2H (0.24 V) < 2Cl (0.34 V) according to the electron richness of the R2-TBDAP ligands, but the oxygen-atom-transfer reactivities of 2R2 showed a reverse trend (k2: 2Cl < 2H < 2OMe) with a 13-fold rate enhancement at 2OMe over 2Cl in a sulfoxidation reaction with thioanisole. Although the reactivity trend contradicts the general consideration that electron-rich metal-oxygen species with low E1/2 values have sluggish electrophilic reactivity, this could be explained by a weak Co-O bond vibration of 2OMe in the unusual reaction pathway. These results provide considerable insight into the electronic nature-reactivity relationship of metal-oxygen species.

Hydrogenation of CO2 to formate using nanopolyoxomolybdate supported onto dendritic fibrous nanosilica

The creation of green heterogeneous nanocatalysts with chemical modifications to convert CO2 into value-added products has many supporters. Here, we report a green and cost-effective CO2 reduction method for the synthesis of formate salts in the participation of nanosized polyoxomolybdate caprate [Mo368] supported by cellulose inter-dendritic fibrous nanosilica (DFNS). To generate the nanocatalyst as a substrate to support [Mo368], cellulose was utilized to control the metal leaching during the reaction. The products were conveniently isolated from the catalyst, and the green medium was deployed several times without a notable decrease in selectivity and function. The results of Raman spectral data, UV–vis, FT-IR, FESEM, TEM, XRD, and TGA confirmed that Mo (VI) peroxo complex collaborated in hydrogenation of CO2. To prepare the catalyst, the surface of DFNS was activated with cellulose to act as a substrate to support [Mo368] nanoparticles. The anatomy of the final complex changed the catalytic properties of the cellulose catalyst for the production of the final product.