Spectroscopic capture and reactivity of a low-spin cobalt(IV)-oxo complex stabilized by binding redox-inactive metal ions

Do multinuclear 3d metal catalysts achieve O-O bond formation via radical coupling or via water nucleophilic attack? WNA leads the way in [Co4O4]n

Catalytic water oxidation is a required process for clean energy production based on the concept of artificial photosynthesis. Here, we provide in situ spectroscopic and computational analysis for the closest known photosystem II analog, [Co₄O₄]ⁿ⁺ ([Co₄O₄Py₄Ac₄]⁰, Py = pyridine and Ac = CH₃COO^-), which catalyzes electrochemical water oxidation. In situ extended X-ray absorption fine structure detects an ultrashort, Co^IV=O (~1.67 Å) moiety, a crucial intermediate for O-O bond formation. Density function theory analyses show that the intermediate has two Co^IV centers and a Co^IV=O unit of strong radicaloid character sufficient to support a Co^IV=O + H₂O = Co-OOH + H⁺ transition, where the carboxyl ligand accepts the proton and the bridging oxygen stabilizes the peroxide via hydrogen bonding. The proposed water nucleophilic attack mechanism accounts for all prior spectroscopic evidence on the Co₄O₄⁴⁺ core. Our results are important for the design and development of efficient water oxidation catalysts, which contribute to the ultimate goal of clean energy from artificial photosynthesis.

Ligand-Constraint-Induced Peroxide Activation for Electrophilic Reactivity

μ‐1,2‐peroxo‐bridged diiron(III) intermediates P are proposed as reactive intermediates in various biological oxidation reactions. In sMMO, P acts as an electrophile, and performs hydrogen atom and oxygen atom transfers to electron‐rich substrates. In cyanobacterial ADO, however, P is postulated to react by nucleophilic attack on electrophilic carbon atoms. In biomimetic studies, the ability of μ‐1,2‐peroxo‐bridged dimetal complexes of Fe, Co, Ni and Cu to act as nucleophiles that effect deformylation of aldehydes is documented. By performing reactivity and theoretical studies on an end‐on μ‐1,2‐peroxodicobalt(III) complex 1 involving a non‐heme ligand system, L1, supported on a Sn₆O₆ stannoxane core, we now show that a peroxo‐bridged dimetal complex can also be a reactive electrophile. The observed electrophilic chemistry, which is induced by the constraints provided by the Sn₆O₆ core, represents a new domain for metal−peroxide reactivity.

Preparation of organocobalt(iii) complexes via O2 activation

The coupling of selective C-H activation with O₂ activation is an important goal for organic synthesis. New experimental and computational results, along with the results from experimental work accumulated over many decades, now unequivocally link O₂ activation with C-H activation by the classic Co(salen) complexes. A common holistic mechanistic framework can rationalise the formation of ostensibly diverse peroxo, superoxo, organo and alkoxide complexes of Co^III(salen). DFT calculations show that cobalt(iii)superoxo, dicobalt(iii)peroxo and cobalt(iii)hydroperoxo complexes are all viable intermediates as participants in hydrogen atom transfer reactions, whereas a Co(iv)oxo intermediate is unlikely. The reaction conditions will determine the pathway followed and all pathways are initiated through the initial formation of a superoxo complex, Co^III(salen)(O₂˙)(MeOH) (EPR: g = 2.025, A = 19 G). Organo and alkoxide ligands are derived from solvent media and the trends in reactivity reveal that combination of the pK_a and BDE of the C-H of the respective solvent substrates are important. These data explain why landmark, structurally characterized, μ₂-η¹,η²-peroxide and η¹-superoxide Co(salen)-O₂ adducts were predominantly isolated from solvents with high C-H pK_a values (DMSO, DMF, DMA).

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

Transition metal-mediated O–O bond formation and activation in chemistry and biology

O–O bond formation and activation reactions proceed via multi-step reactions in chemistry and biology and involve similar reactive intermediates like metal–oxo/oxyl, metal–superoxo, and/or metal–(hydro)peroxo species.

Opening the CoIII,IV2(μ-O)2 Diamond Core by Lewis Bases Leads to Enhanced C-H Bond Cleaving Reactivity

The high-valent diiron(IV) intermediate Q is the key oxidant that cleaves strong C-H bonds of methane in the catalytic cycle of soluble methane monooxygenase (sMMO). sMMO-Q was previously reported as a bis-μ-oxo Fe^IV₂(μ-O)₂ diamond core but was recently described to have an open core with a long Fe···Fe distance. We recently reported a high-valent Co^III,IV₂(μ-O)₂ diamond core complex (1) that is highly reactive with sp³ C-H bonds. In this work, we demonstrated that the C-H bond cleaving reactivity of 1 can be further enhanced by introducing a Lewis base X, affording faster kinetic rate constants and the ability to cleave stronger C-H bonds compared to 1. We proposed that 1 first reacts with X in a fast equilibrium to form an open core species X-Co^III-O-Co^IV-O (1-X). We were able to characterize 1-X using EPR spectroscopy and DFT calculations. 1-X exhibited an S = 1/2 EPR signal distinct from that of the parent complex 1. DFT calculations showed that 1-X has an open core with the spin density heavily delocalized in the Co^IV-O unit. Moreover, 1-X has a more favorable thermodynamic driving force and a smaller activation barrier than 1 to carry out C-H bond activation reactions. Notably, 1-X is at least 4 orders of magnitude more reactive than its diiron open core analogues. Our findings indicate that the diamond core isomerization is likely a practical enzymatic strategy to unmask the strong oxidizing power of sMMO-Q necessary to attack the highly inert C-H bonds of methane.

Elucidation of Factors That Govern the 2e-/2H+ vs 4e-/4H+ Selectivity of Water Oxidation by a Cobalt Corrole

Considering the importance of water splitting as the best solution for clean and renewable energy, the worldwide efforts for development of increasingly active molecular water oxidation catalysts must be accompanied by studies that focus on elucidating the mode of actions and catalytic pathways. One crucial challenge remains the elucidation of the factors that determine the selectivity of water oxidation by the desired 4e^-/4H⁺ pathway that leads to O₂ rather than by 2e^-/2H⁺ to H₂O₂. We now show that water oxidation with the cobalt-corrole CoBr₈ as electrocatalyst affords H₂O₂ as the main product in homogeneous solutions, while heterogeneous water oxidation by the same catalyst leads exclusively to oxygen. Experimental and computation-based investigations of the species formed during the process uncover the formation of a Co(III)-superoxide intermediate and its preceding high-valent Co-oxyl complex. The competition between the base-catalyzed hydrolysis of Co(III)-hydroperoxide [Co(III)-OOH]^- to release H₂O₂ and the electrochemical oxidation of the same to release O₂ via [Co(III)-O₂^•]^- is identified as the key step determining the selectivity of water oxidation.

Concerted Proton-Electron Transfer Reactivity at a Multimetallic Co4O4 Cubane Cluster

High-valent oxocobalt(IV) species have been invoked as key intermediates in oxidative catalysis, but investigations into the chemistry of proton-coupled redox reactions of such species have been limited. Herein, the reactivity of an established water oxidation catalyst, [Co₄O₄(OAc)₄(py)₄][PF₆], toward H-atom abstraction reactions is described. Mechanistic analyses and density functional theory (DFT) calculations support a concerted proton-electron transfer (CPET) pathway in which the high energy intermediates formed in stepwise pathways are bypassed. Natural bond orbital (NBO) calculations point to cooperative donor-acceptor σ interactions at the transition state, whereby the H-atom of the substrate is transferred to an orbital delocalized over a Co₃(μ₃-O) fragment. The mechanistic insights provide design principles for the development of catalytic C-H activation processes mediated by a multimetallic oxo metal cluster.

Crystal Structure and C-H Bond-Cleaving Reactivity of a Mononuclear CoIV-Dinitrate Complex

High-valent FeIV═O intermediates with a terminal metal-oxo moiety are key oxidants in many enzymatic and synthetic C-H bond oxidation reactions. While generating stable metal-oxo species for late transition metals remains synthetically challenging, notably, a number of high-valent non-oxo-metal species of late transition metals have been recently described as strong oxidants that activate C-H bonds. In this work, we obtained an unprecedented mononuclear CoIV-dinitrate complex (2) upon one-electron oxidation of its Co(III) precursor supported by a tridentate dianionic N3 ligand. 2 was structurally characterized by X-ray crystallography, showing a square pyramidal geometry with two coordinated nitrate anions. Furthermore, characterization of 2 using combined spectroscopic and computational methods revealed that 2 is a low-spin (S = 1/2) Co(IV) species with the unpaired electron located on the cobalt dz2 orbital, which is well positioned for substrate oxidations. Indeed, while having a high thermal stability, 2 is able to cleave sp3 C-H bonds up to 87 kcal/mol to afford rate constants and kinetic isotope effects (KIEs) of 2-6 that are comparable to other high-valent metal oxidants. The ability to oxidize strong C-H bonds has yet to be observed for CoIV-O and CoIII═O species previously reported. Therefore, 2 represents the first high-valent Co(IV) species that is both structurally characterized by X-ray crystallography and capable of activating strong C-H bonds.

Redox-Inactive Metal Ions That Enhance the Nucleophilic Reactivity of an Alkylperoxocopper(II) Complex

The importance of redox-inactive metal ions in modulating the reactivity of redox-active biological systems is a subject of great current interest. In this work, the effect of redox-inactive metal ions (M³⁺ = Sc³⁺, Y³⁺, Yb³⁺, La³⁺) on the nucleophilic reactivity of a mononuclear ligand-based alkylperoxocopper(II) complex, [Cu(ⁱPr₂-tren-C(CH₃)₂O₂)]⁺ (1), was examined. 1 was prepared by the addition of hydrogen peroxide and triethylamine to the solution of [Cu(ⁱPr₃-tren)(CH₃CN)]⁺ (ⁱPr₃-tren = tris[2-(isopropylamino)ethyl]amine) via the formation of [Cu(ⁱPr₃-tren)(O₂H)]⁺ (2) in methanol (CH₃OH) at 30 °C. 1 was characterized using density functional theory (DFT) calculations and spectroscopic methods such as UV-vis, resonance Raman (rR), and electron paramagnetic resonance (EPR). DFT calculations support the electronic structure of 1 with an intermediate geometry between the trigonal-bipyramidal and square-pyramidal geometries, which is consistent with the observed EPR signal exhibiting a signal with g_⊥ = 2.03 (A_⊥ = 16 G) and g_|| = 2.19 (A_|| = 158 G). The Cu-O bond stretching frequency of 1 was observed at 507 cm^-1 for ¹⁶O₂ species (486 cm^-1 for ¹⁸O₂ species), and its O-O vibrational energy was determined to be 799 cm^-1 for ¹⁶O₂ species (759 cm^-1 for ¹⁸O₂ species) by rR spectroscopy. The reactivity of 1 was investigated in oxidative nucleophilic reactions. The positive slope of the Hammett plot (ρ = 2.3(1)) with para-substituted benzaldehydes and the reactivity order with 1°-, 2°-, and 3°-CHO demonstrate well the nucleophilic character of this copper(II) ligand-based alkylperoxo complex. The Lewis acidity of M³⁺ improves the oxidizing ability of 1. The modulated reactivity of 1 with M³⁺ was revealed to be an opposite trend of the Lewis acidity of M³⁺ in aldehyde deformylation.

Iron and manganese oxo complexes, oxo wall and beyond

High-valent metal-oxo species with multiply-bonded M-O groups have been proposed as key intermediates in many biological and abiological catalytic oxidation reactions. These intermediates are implicated as active oxidants in alkane hydroxylation, olefin epoxidation and other oxidation reactions. For example, [Fe^ivO(porphyrinato^•-)]⁺ cofactors bearing π-radical porphyrinato^•- ligands oxidize organic substrates in cytochrome P450 enzymes, which are common to many life forms. Likewise, high-valent Mn-oxo species are active for H₂O oxidation in photosystem II. The chemistry of these native reactive species has inspired chemists to prepare highly oxidized transition-metal complexes as functional mimics. Although many synthetic Fe-O and Mn-O complexes now exist, the analogous oxo complexes of the late transition metals (groups 9-11) are rare. Indeed, late-transition-metal-oxo complexes of tetragonal (fourfold) symmetry should be electronically unstable, a rule commonly referred to as the 'oxo wall'. A few late metal-oxos have been prepared by targeting other symmetries or unusual spin states. These complexes have been studied using spectroscopic and theoretical methods. This Review describes mononuclear non-haem Fe-O and Mn-O species, the nature of the oxo wall and recent advances in the preparation of oxo complexes of Co, Ni and Cu beyond the oxo wall.

Synthesis of a "Masked" Terminal Zinc Sulfide and Its Reactivity with Brønsted and Lewis Acids

The "masked" terminal Zn sulfide, [K(2.2.2-cryptand)][^Me LZn(S)] (2) (^Me L={(2,6-ⁱ Pr₂ C₆ H₃ )NC(Me)}₂ CH), was isolated via reaction of [^Me LZnSCPh₃ ] (1) with 2.3 equivalents of KC₈ in THF, in the presence of 2.2.2-cryptand, at -78 °C. Complex 2 reacts readily with PhCCH and N₂ O to form [K(2.2.2-cryptand)][^Me LZn(SH)(CCPh)] (4) and [K(2.2.2-cryptand)][^Me LZn(SNNO)] (5), respectively, displaying both Brønsted and Lewis basicity. In addition, the electronic structure of 2 was examined computationally and compared with the previously reported Ni congener, [K(2.2.2-cryptand)][^tBu LNi(S)] (^tBu L={(2,6-ⁱ Pr₂ C₆ H₃ )NC(^t Bu)}₂ CH).

Isolation of a Bimetallic Cobalt(III) Nitride and Examination of Its Hydrogen Atom Abstraction Chemistry and Reactivity toward H2

Room temperature photolysis of the bis(azide)cobaltate(II) complex [Na(THF)_x][(^ketguan)Co(N₃)₂] (^ketguan = [(tBu₂CN)C(NDipp)₂]^-, Dipp = 2,6-diisopropylphenyl) (3a) in THF cleanly forms the binuclear cobalt nitride Na(THF)₄{[(^ketguan)Co(N₃)]₂(μ-N)} (1). Compound 1 represents the first example of an isolable, bimetallic cobalt nitride complex, and it has been fully characterized by spectroscopic, magnetic, and computational analyses. Density functional theory supports a Co^III═N═Co^III canonical form with significant π-bonding between the cobalt centers and the nitride atom. Unlike other group 9 bridging nitride complexes, no radical character is detected at the bridging N atom of 1. Indeed, 1 is unreactive toward weak C-H donors and even cocrystallizes with a molecule of cyclohexadiene (CHD) in its crystallographic unit cell to give 1·CHD as a room temperature stable product. Notably, addition of pyridine to 1 or photolyzed solutions of [(^ketguan)Co(N₃)(py)]₂ (4a) leads to destabilization via activation of the nitride unit, resulting in the mixed-valent Co(II)/Co(III) bridged imido species [(^ketguan)Co(py)][(^ketguan)Co](μ-NH)(μ-N₃) (5) formed from intermolecular hydrogen atom abstraction (HAA) of strong C-H bonds (BDE ∼ 100 kcal/mol). Kinetic rate analysis of the formation of 5 in the presence of C₆H₁₂ or C₆D₁₂ gives a KIE = 2.5 ± 0.1, supportive of a HAA formation pathway. The reactivity of our system was further probed by photolyzing benzene/pyridine solutions of 4a under H₂ and D₂ atmospheres (150 psi), which leads to the exclusive formation of the bis(imido) complexes [(^ketguan)Co(μ-NH)]₂ (6) and [(^ketguan)Co(μ-ND)]₂ (6-D), respectively, as a result of dihydrogen activation. These results provide unique insights into the chemistry and electronic structure of late 3d metal nitrides while providing entryway into C-H activation pathways.

Ligand-Based Reactivity of Oxygenation and Alkylation in Cobalt Complexes Binding with (Thiolato)phosphine Derivatives

In our efforts to understand the nature of metal thiolates, we have explored the chemistry of cobalt ion supported by (thiolato)phosphine ligand derivatives. Herein, we synthesized and characterized a square-planar Co^II complex binding with a bidentate (thiolato)phosphine ligand, Co(PS1″)₂ (1) ([PS1″]^- = [P(Ph)₂(C₆H₃-3-SiMe₃-2-S)]^-). The complex activates O₂ to form a ligand-based oxygenation product, Co(OPS1″)₂ (2) ([OPS1″]^- = [PO(Ph)₂(C₆H₃-3-SiMe₃-2-S)]^-). In addition, an octahedral Co^III complex with a tridentate bis(thiolato)phosphine ligand, [NEt₄][Co(PS2*)₂] (3) ([PS2*]^2- = [P(Ph)(C₆H₃-3-Ph-2-S)₂]^2-), was obtained. Compound 3 cleaves the C-Cl bond in dichloromethane via an S-based nucleophilic attack to generate a chloromethyl thioether group. Two isomeric products, [Co(PS2*)(PSS^CH₂Cl*)] (4 and 4') ([PSS^CH₂Cl*]^- = [P(Ph)(C₆H₃-3-Ph-2-S)(C₆H₃-3-Ph-2-SCH₂Cl)]^-), were isolated and fully characterized. Both transformations, oxygenation of a Co^II-bound phosphine donor in 1 and alkylation of a Co^III-bound thiolate in 3, were monitored by spectroscopic methods. These reaction products were isolated and fully characterized. Density functional theory (DFT, the B3LYP functional) calculations were performed to understand the electronic structure of 1 as well as the pathway of its transformation to 2.

Mechanistic dichotomies in redox reactions of mononuclear metal–oxygen intermediates

This review article focuses on various mechanistic dichotomies in redox reactions of metal–oxygen intermediates with the emphasis on understanding and controlling their redox reactivity from experimental and theoretical points of view.

Ligand Redox Noninnocence in [CoIII(TAML)]0/- Complexes Affects Nitrene Formation

The redox noninnocence of the TAML scaffold in cobalt-TAML (tetra-amido macrocyclic ligand) complexes has been under debate since 2006. In this work, we demonstrate with a variety of spectroscopic measurements that the TAML backbone in the anionic complex [Co^III(TAML^red)]^– is truly redox noninnocent and that one-electron oxidation affords [Co^III(TAML^sq)]. Multireference (CASSCF) calculations show that the electronic structure of [Co^III(TAML^sq)] is best described as an intermediate spin (S = 1) cobalt(III) center that is antiferromagnetically coupled to a ligand-centered radical, affording an overall doublet (S = ¹/₂) ground-state. Reaction of the cobalt(III)-TAML complexes with PhINNs as a nitrene precursor leads to TAML-centered oxidation and produces nitrene radical complexes without oxidation of the metal ion. The ligand redox state (TAML^red or TAML^sq) determines whether mono- or bis-nitrene radical complexes are formed. Reaction of [Co^III(TAML^sq)] or [Co^III(TAML^red)]^– with PhINNs results in the formation of [Co^III(TAML^q)(N^•Ns)] and [Co^III(TAML^q)(N^•Ns)₂]^–, respectively. Herein, ligand-to-substrate single-electron transfer results in one-electron-reduced Fischer-type nitrene radicals (N^•Ns^–) that are intermediates in catalytic nitrene transfer to styrene. These nitrene radical species were characterized by EPR, XANES, and UV–vis spectroscopy, high-resolution mass spectrometry, magnetic moment measurements, and supporting CASSCF calculations.

Highly Reactive CoIII,IV2(μ-O)2 Diamond Core Complex That Cleaves C-H Bonds

The selective activation of strong sp³ C-H bonds at mild conditions is a key step in many biological and synthetic transformations and an unsolved challenge for synthetic chemists. In nature, soluble methane monooxygenase (sMMO) is one representative example of nonheme dinuclear iron-dependent enzymes that activate strong sp³ C-H bonds by a high-valent diiron(IV) intermediate Q. To date, synthetic model complexes of sMMO-Q have shown limited abilities to oxidize strong C-H bonds. In this work, we generated a high-valent Co^III,IV₂(μ-O)₂ complex 3 supported by a tetradentate tris(2-pyridylmethyl)amine (TPA) ligand via one-electron oxidation of its Co^III₂(μ-O)₂ precursor 2. Characterization of 2 and 3 using X-ray absorption spectroscopy and DFT calculations showed that both species possess a diamond core structure with a short Co···Co distance of 2.78 Å. Furthermore, 3 is an EPR active species showing an S = 1/2 signal with clearly observable hyperfine splittings originated from the coupling of the ⁵⁹Co nuclear spin with the electronic spin. Importantly, 3 is a highly reactive oxidant for sp³ C-H bonds, and an oxygenation reagent. 3 has the highest rate constant (1.5 M^-1 s^-1 at -60 °C) for oxidizing 9,10-dihydroanthracene (DHA) compared to diamond core complexes of other first-row transition metals including Mn, Fe and Cu reported previously. Specifically, 3 is about 4-5 orders of magnitude more reactive than the diiron analogs Fe^III,IV₂(μ-O)₂ and Fe^IV₂(μ-O)₂ supported by TPA and related ligands. These findings shed light on future development of more reactive approaches for C-H bond activation by bioinspired dicobalt complexes.

A High-Valent Manganese(IV)-Oxo-Cerium(IV) Complex and Its Enhanced Oxidizing Reactivity

A mononuclear nonheme manganese(IV)-oxo complex binding the Ce⁴⁺ ion, [(dpaq)Mn^IV (O)]⁺ -Ce⁴⁺ (1-Ce⁴⁺ ), was synthesized by reacting [(dpaq)Mn^III (OH)]⁺ (2) with cerium ammonium nitrate (CAN). 1-Ce⁴⁺ was characterized using various spectroscopic techniques, such as UV/Vis, EPR, CSI-MS, resonance Raman, XANES, and EXAFS, showing an Mn-O bond distance of 1.69 Å with a resonance Raman band at 675 cm^-1 . Electron-transfer and oxygen atom transfer reactivities of 1-Ce⁴⁺ were found to be greater than those of Mn^IV (O) intermediates binding redox-inactive metal ions (1-Mⁿ⁺ ). This study reports the first example of a redox-active Ce⁴⁺ ion-bound Mn^IV -oxo complex and its spectroscopic characterization and chemical properties.

A Combined Experimental and Theoretical Study of the Versatile Reactivity of an Oxocerium(IV) Complex: Concerted Versus Reductive Addition

A combined experimental and theoretical investigation on the cerium(IV) oxo complex [(L_OEt )₂ Ce(=O)(H₂ O)]⋅MeC(O)NH₂ (1; L_OEt ^- =[Co(η⁵ -C₅ H₅ ){P(O)(OEt)₂ }₃ ]^- ) demonstrates that the intermediate spin-state nature of the ground state of the cerium complex is responsible for the versatility of its reactivity towards small molecules such as CO, CO₂ , SO₂ , and NO. CASSCF calculations together with magnetic susceptibility measurements indicate that the ground state of the cerium complex is of multiconfigurational character and comprised of 74 % of Ce^IV and 26 % of Ce^III . The latter is found to be responsible for its reductive addition behavior towards CO, SO₂ , and NO. This is the first report to date on the influence of the multiconfigurational ground state on the reactivity of a metal-oxo complex.

M-O Bonding Beyond the Oxo Wall: Spectroscopy and Reactivity of Cobalt(III)-Oxyl and Cobalt(III)-Oxo Complexes

Terminal oxo complexes of late transition metals are frequently proposed reactive intermediates. However, they are scarcely known beyond Group 8. Using mass spectrometry, we prepared and characterized two such complexes: [(N4Py)CoIII (O)]+ (1) and [(N4Py)CoIV (O)]2+ (2). Infrared photodissociation spectroscopy revealed that the Co-O bond in 1 is rather strong, in accordance with its lack of chemical reactivity. On the contrary, 2 has a very weak Co-O bond characterized by a stretching frequency of ≤659 cm-1 . Accordingly, 2 can abstract hydrogen atoms from non-activated secondary alkanes. Previously, this reactivity has only been observed in the gas phase for small, coordinatively unsaturated metal complexes. Multireference ab-initio calculations suggest that 2, formally a cobalt(IV)-oxo complex, is best described as cobalt(III)-oxyl. Our results provide important data on changes to metal-oxo bonding behind the oxo wall and show that cobalt-oxo complexes are promising targets for developing highly active C-H oxidation catalysts.

Brønsted Acid Scaling Relationships Enable Control Over Product Selectivity from O2 Reduction with a Mononuclear Cobalt Porphyrin Catalyst

The selective reduction of O₂, typically with the goal of forming H₂O, represents a long-standing challenge in the field of catalysis. Macrocyclic transition-metal complexes, and cobalt porphyrins in particular, have been the focus of extensive study as catalysts for this reaction. Here, we show that the mononuclear Co-tetraarylporphyrin complex, Co(por^OMe) (por^OMe = meso-tetra(4-methoxyphenyl)porphyrin), catalyzes either 2e^-/2H⁺ or 4e^-/4H⁺ reduction of O₂ with high selectivity simply by changing the identity of the Brønsted acid in dimethylformamide (DMF). The thermodynamic potentials for O₂ reduction to H₂O₂ or H₂O in DMF are determined and exhibit a Nernstian dependence on the acid pK _a, while the Co^III/II redox potential is independent of the acid pK _a. The reaction product, H₂O or H₂O₂, is defined by the relationship between the thermodynamic potential for O₂ reduction to H₂O₂ and the Co^III/II redox potential: selective H₂O₂ formation is observed when the Co^III/II potential is below the O₂/H₂O₂ potential, while H₂O formation is observed when the Co^III/II potential is above the O₂/H₂O₂ potential. Mechanistic studies reveal that the reactions generating H₂O₂ and H₂O exhibit different rate laws and catalyst resting states, and these differences are manifested as different slopes in linear free energy correlations between the log(rate) versus pK _a and log(rate) versus effective overpotential for the reactions. This work shows how scaling relationships may be used to control product selectivity, and it provides a mechanistic basis for the pursuit of molecular catalysts that achieve low overpotential reduction of O₂ to H₂O.

Effects of Lewis Acidic Metal Ions (M) on Oxygen-Atom Transfer Reactivity of Heterometallic Mn3MO4 Cubane and Fe3MO(OH) and Mn3MO(OH) Clusters

The modulation of the reactivity of metal oxo species by redox inactive metals has attracted much interest due to the observation of redox inactive metal effects on processes involving electron transfer both in nature (the oxygen-evolving complex of Photosystem II) and in heterogeneous catalysis (mixed-metal oxides). Studies of small-molecule models of these systems have revealed numerous instances of effects of redox inactive metals on electron- and group-transfer reactivity. However, the heterometallic species directly involved in these transformations have rarely been structurally characterized and are often generated in situ. We have previously reported the preparation and structural characterization of multiple series of heterometallic clusters based on Mn₃ and Fe₃ cores and described the effects of Lewis acidity of the heterometal incorporated in these complexes on cluster reduction potential. To determine the effects of Lewis acidity of redox inactive metals on group transfer reactivity in structurally well-defined complexes, we studied [Mn₃MO₄], [Mn₃MO(OH)], and [Fe₃MO(OH)] clusters in oxygen atom transfer (OAT) reactions with phosphine substrates. The qualitative rate of OAT correlates with the Lewis acidity of the redox inactive metal, confirming that Lewis acidic metal centers can affect the chemical reactivity of metal oxo species by modulating cluster electronics.

Kinetics and mechanisms of catalytic water oxidation

The kinetics and mechanisms of thermal and photochemical oxidation of water with homogeneous and heterogeneous catalysts, including conversion from homogeneous to heterogeneous catalysts in the course of water oxidation, are discussed in this review article. Molecular and homogeneous catalysts have the advantage to clarify the catalytic mechanisms by detecting active intermediates in catalytic water oxidation. On the other hand, heterogeneous nanoparticle catalysts have advantages for practical applications due to high catalytic activity, robustness and easier separation of catalysts by filtration as compared with molecular homogeneous precursors. Ligand oxidation of homogeneous catalysts sometimes results in the dissociation of ligands to form nanoparticles, which act as much more efficient catalysts for water oxidation. Since it is quite difficult to identify active intermediates on the heterogeneous catalyst surface, the mechanism of water oxidation has hardly been clarified under heterogeneous catalytic conditions. This review focuses on the kinetics and mechanisms of catalytic water oxidation with homogeneous catalysts, which may be converted to heterogeneous nanoparticle catalysts depending on various reaction conditions.

Redox Reactivity of a Mononuclear Manganese-Oxo Complex Binding Calcium Ion and Other Redox-Inactive Metal Ions

Mononuclear nonheme manganese(IV)-oxo complexes binding calcium ion and other redox-inactive metal ions, [(dpaq)Mn^IV(O)]⁺-M ⁿ⁺ (1-Mⁿ⁺, M ⁿ⁺ = Ca²⁺, Mg²⁺, Zn²⁺, Lu³⁺, Y³⁺, Al³⁺, and Sc³⁺) (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino- N-quinolin-8-yl-acetamidate), were synthesized by reacting a hydroxomanganese(III) complex, [(dpaq)Mn^III(OH)]⁺, with iodosylbenzene (PhIO) in the presence of redox-inactive metal ions (M ⁿ⁺). The Mn(IV)-oxo complexes were characterized using various spectroscopic techniques. In reactivity studies, we observed contrasting effects of M ⁿ⁺ on the reactivity of 1-M ⁿ⁺ in redox reactions such as electron-transfer (ET), oxygen atom transfer (OAT), and hydrogen atom transfer (HAT) reactions. In the OAT and ET reactions, the reactivity order of 1-M ⁿ⁺, such as 1-Sc³⁺ ≈ 1-Al³⁺ > 1-Y³⁺ > 1-Lu³⁺ > 1-Zn²⁺ > 1-Mg²⁺ > 1-Ca²⁺, follows the Lewis acidity of M ⁿ⁺ bound to the Mn-O moiety; that is, the stronger the Lewis acidity of M ⁿ⁺, the higher the reactivity of 1-M ⁿ⁺ becomes. In sharp contrast, the reactivity of 1-M ⁿ⁺ in the HAT reaction was reversed, giving the reactivity order 1-Ca²⁺ > 1-Mg²⁺ > 1-Zn²⁺ > 1-Lu³⁺> 1-Y³⁺> 1-Al³⁺ ≈ 1-Sc³⁺; that is, the higher is Lewis acidity of M ⁿ⁺, the lower the reactivity of 1-M ⁿ⁺ in the HAT reaction. The latter result implies that the Lewis acidity of M ⁿ⁺ bound to the Mn-O moiety can modulate the basicity of the metal-oxo moiety, thus influencing the HAT reactivity of 1-M ⁿ⁺; cytochrome P450 utilizes the axial thiolate ligand to increase the basicity of the iron-oxo moiety, which enhances the reactivity of compound I in C-H bond activation reactions.

Nitric Oxide Dioxygenase Activity of a Nitrosyl Complex of Cobalt(II) Porphyrinate in the Presence of Hydrogen Peroxide via Putative Peroxynitrite Intermediate

The reaction of a cobalt porphyrin complex, [(F₈TPP)Co], 1 {F₈TPP = 5,10,15,20- tetrakis(2,6-difluorophenyl)porphyrinate dianion} in dichloromethane with nitric oxide (NO) led to the nitrosyl complex, [(F₈TPP)Co(NO)], 2. Spectroscopic studies and structural characterization revealed it as a bent nitrosyl of {CoNO}⁸ description. It was stable in the presence of dioxygen. However, it reacts with H₂O₂ in acetonitrile (or THF) solution at -40 °C (or -80 °C) to result in the corresponding Co(III)-nitrate complex, [(F₈TPP)Co(NO₃)], 3. The reaction presumably proceeds via the formation of a Co-peroxynitrite intermediate. X-Band electron paramagnetic resonance and electrospray ionization-mass spectroscopic studies suggest the intermediate formation of the [(porphyrin)Co(III)-O^•] radical, which in turn supports the generation of the corresponding Co(IV)-oxo species during the reaction. This is in accord with the homolytic cleavage of the O-O bond in heme-peroxynitrite proposed in the nitric oxide dioxygenases activity. In addition, the characteristic peroxynitrite-induced phenol ring reaction was also observed.

Molecular and electronic structure of an azidocobalt(iii) complex derived from X-ray crystallography, linear spectroscopy and quantum chemical calculations

X-ray diffraction, UV/Vis electronic spectroscopy and Fourier-transform infrared spectroscopy are used with DFT and CAS calculations to explore the molecular and electronic structure of the cationic complex (cyclam)(diazido)cobalt(iii).

Reactive Cobalt⁻Oxo Complexes of Tetrapyrrolic Macrocycles and N-based Ligand in Oxidative Transformation Reactions

High-valent cobalt–oxo complexes are reactive transient intermediates in a number of oxidative transformation processes e.g., water oxidation and oxygen atom transfer reactions. Studies of cobalt–oxo complexes are very important for understanding the mechanism of the oxygen evolution center in natural photosynthesis, and helpful to replicate enzyme catalysis in artificial systems. This review summarizes the development of identification of high-valent cobalt–oxo species of tetrapyrrolic macrocycles and N-based ligands in oxidation of organic substrates, water oxidation reaction and in the preparation of cobalt–oxo complexes.

Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O₂ to H₂O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O₂ to water via Co(III)-O-O-Co(III) peroxo intermediates.

A cocoon silk chemistry strategy to ultrathin N-doped carbon nanosheet with metal single-site catalysts

Development of single-site catalysts supported by ultrathin two-dimensional (2D) porous matrix with ultrahigh surface area is highly desired but also challenging. Here we report a cocoon silk chemistry strategy to synthesize isolated metal single-site catalysts embedded in ultrathin 2D porous N-doped carbon nanosheets (M-ISA/CNS, M = Fe, Co, Ni). X-ray absorption fine structure analysis and spherical aberration correction electron microscopy demonstrate an atomic dispersion of metal atoms on N-doped carbon matrix. In particular, the Co-ISA/CNS exhibit ultrahigh specific surface area (2105 m² g⁻¹) and high activity for C–H bond activation in the direct catalytic oxidation of benzene to phenol with hydrogen peroxide at room temperature, while the Co species in the form of phthalocyanine and metal nanoparticle show a negligible activity. Density functional theory calculations discover that the generated O = Co = O center intermediates on the single Co sites are responsible for the high activity of benzene oxidation to phenol.

Single-site catalysts supported by ultrathin two-dimensional (2D) porous matrix are desirable for catalytic reactions, yet their synthesis remains a great challenge. Herein the authors report a cocoon silk chemistry strategy to synthesize isolated metal single-site catalysts embedded in ultrathin 2D porous N-doped carbon nanosheets.

Hydrogen Atom Transfer Reactions of Mononuclear Nonheme Metal-Oxygen Intermediates

Molecular oxygen (O₂), the greenest oxidant, is kinetically stable in the oxidation of organic substrates due to its triplet ground state. In nature, O₂ is reduced by two electrons with two protons to produce hydrogen peroxide (H₂O₂) and by four electrons with four protons to produce water (H₂O) by oxidase and oxygenase metalloenzymes. In the process of the two-electron/two-proton and four-electron/four-proton reduction of O₂ by metalloenzymes and their model compounds, metal-oxygen intermediates, such as metal-superoxido, -peroxido, -hydroperoxido, and -oxido species, are generated depending on the numbers of electrons and protons involved in the O₂ activation reactions. The one-electron reduction of metal-oxygen intermediates is coupled with the binding of one proton. Such a hydrogen atom transfer (HAT) is defined as proton-coupled electron transfer (PCET), and there is a mechanistic dichotomy whether HAT occurs via a concerted PCET pathway or stepwise pathways [i.e., electron transfer followed by proton transfer (ET/PT) or proton transfer followed by electron transfer (PT/ET)]. The metal-oxygen intermediates formed are oxidants that can abstract a hydrogen atom (H-atom) from substrate C-H bonds. The H-atom abstraction from substrate C-H bonds by the metal-oxygen intermediates can also occur via a concerted PCET or stepwise PCET pathways. In the PCET reactions, a proton can be provided not only by the substrate itself but also by an acid that is added to a reaction solution. This Account describes the reactivities of metal-oxygen intermediates, such as metal-superoxido, -peroxido, -hydroperoxido, and -oxido complexes, in HAT reactions, focusing on the mechanisms of PCET reactions of metal-oxygen intermediates and on the mechanistic dichotomy of concerted versus stepwise pathways. Recent developments in the reactivity studies of Cr-, Fe-, and Cu-superoxido complexes in H-atom and hydride transfer reactions are discussed. Reactivities of an iron(III)-hydroperoxido complex and an iron(III)-peroxido complex binding redox-inactive metal ions are also summarized briefly. Mononuclear nonheme iron(IV)- and manganese(IV)-oxido complexes have shown high reactivities in HAT reactions, and their chemistry in PCET reactions is discussed intensively. Acid-catalyzed HAT reactions of metal-oxygen intermediates are also discussed to demonstrate a unified driving force dependence of logarithm of the rate constants of acid-catalyzed oxidation of various substrates by an iron(IV)-oxido complex and that of PCET from one-electron donors to the iron(IV)-oxido complex. PCET reactions of metal-oxygen intermediates are shown to proceed via a concerted pathway (one-step HAT) or a stepwise ET/PT pathway depending on the ET and PCET driving forces (-Δ G). The boundary conditions between concerted versus stepwise PCET pathways are clarified to demonstrate a switchover of the mechanisms only by changing the reaction temperature in the boundary conditions. This Account summarizes recent developments in the HAT reactions by synthetic mononuclear nonheme metal-oxygen intermediates over the past 10 years.

Enhanced Electron-Transfer Reactivity of a Long-Lived Photoexcited State of a Cobalt-Oxygen Complex

Photodynamics and electron-transfer reactivity of an excited state derived from an earth-abundant mononuclear cobalt-oxygen complex ground state, [(TAML)Co^IV(O)]^2- (1; H₄TAML = 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1 H-1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6 H, 11 H)tetrone), prepared by electron-transfer oxidation of Li[(TAML)Co^III]·3(H₂O) (2) in a 1:1 acetonitrile/acetone solvent mixture at 5 °C, were investigated using a combination of femtosecond and nanosecond laser absorption spectroscopy. Visible light photoexcitation of 1 (λ_exc = 393 nm) resulted in generation of the excited state S₂* (lifetime: 1.4(4) ps), detected 2 ps after laser irradiation by femtosecond laser spectroscopy. The initially formed excited state S₂* converted to a lower-lying excited state, S₁* (λ_max = 580 nm), with rate constant k_c = 7(2) × 10¹¹ s^-1 (S₂* → S₁*). S₁* exhibited a 0.6(1) ns lifetime and converted to the initial ground state 1 with rate constant k_d = 1.7(3) × 10⁹ s^-1 (S₁* → 1). The same excited state dynamics was observed when 1 was generated by electron-transfer oxidation of 2 using different one-electron oxidants such as Cu(OTf)₂ (OTf^- = triflate anion), [Fe(bpy)₃]³⁺ (bpy = 2,2'-bipyridine), and tris(4-bromophenyl)ammoniumyl radical cation (TBPA^•+). The electron-transfer reactivity of S₁* was probed by nanosecond laser photoexcitation of 1 in the presence of a series of electron donors with different one-electron oxidation potentials ( E_ox vs SCE): benzene (2.35 V), toluene (2.20 V), m-xylene (2.02 V), and anisole (1.67 V). The excited state S₁* engaged in electron-transfer reactions with m-xylene and anisole to generate π-dimer radical cations of m-xylene and anisole, respectively, observed by nanosecond laser transient absorption spectroscopy, whereas no reactivity was observed toward benzene and toluene. Such differential electron-transfer reactivity depending on the E_ox values of electron donors allowed the estimation of the one-electron reduction potential of S₁* ( E_red*) as 2.1(1) V vs SCE, which is much higher than that of the ground state ( E_red = 0.86 V vs SCE).