Synthesis, characterization, and reactivity of cobalt(III)-oxygen complexes bearing a macrocyclic N-tetramethylated cyclam ligand

C-H bond activation by high-valent iron/cobalt-oxo complexes: a quantum chemical modeling approach

High-valent metal-oxo species serve as key intermediates in the activation of inert C-H bonds. Here, we present a comprehensive DFT analysis of the parameters that have been proposed as influencing factors in modeled high-valent metal-oxo mediated C-H activation reactions. Our approach involves utilizing DFT calculations to explore the electronic structures of modeled FeIVO (species 1) and CoIVO ↔ CoIII-O˙ (species 2), scrutinizing their capacity to predict improved catalytic activity. DFT and DLPNO-CCSD(T) calculations predict that the iron-oxo species possesses a triplet as the ground state, while the cobalt-oxo has a doublet as the ground state. Furthermore, we have investigated the mechanistic pathways for the first C-H bond activation, as well as the desaturation of the alkanes. The mechanism was determined to be a two-step process, wherein the first hydrogen atom abstraction (HAA) represents the rate-limiting step, involving the proton-coupled electron transfer (PCET) process. However, we found that the second HAA step is highly exothermic for both species. Our calculations suggest that the iron-oxo species (Fe-O = 1.672 Å) exhibit relatively sluggish behavior compared to the cobalt-oxo species (Co-O = 1.854 Å) in C-H bond activation, attributed to a weak metal-oxygen bond. MO, NBO, and deformation energy analysis reveal the importance of weakening the M-O bond in the cobalt species, thereby reducing the overall barrier to the reaction. This catalyst was found to have a C-H activation barrier relatively smaller than that previously reported in the literature.

Benzylic C(sp3 )-H Bond Oxidation with Ketone Selectivity by a Cobalt(IV)-Oxo Embedded in a β-Barrel Protein

Artificial metalloenzymes have emerged as biohybrid catalysts that allow to combine the reactivity of a metal catalyst with the flexibility of protein scaffolds. This work reports the artificial metalloenzymes based on the β-barrel protein nitrobindin NB4, in which a cofactor [CoII X(Me3 TACD-Mal)]+ X- (X=Cl, Br; Me3 TACD=N,N' ,N''-trimethyl-1,4,7,10-tetraazacyclododecane, Mal=CH2 CH2 CH2 NC4 H2 O2 ) was covalently anchored via a Michael addition reaction. These biohybrid catalysts showed higher efficiency than the free cobalt complexes for the oxidation of benzylic C(sp3 )-H bonds in aqueous media. Using commercially available oxone (2KHSO5  ⋅ KHSO4  ⋅ K2 SO4 ) as oxidant, a total turnover number of up to 220 and 97 % ketone selectivity were achieved for tetralin. As catalytically active intermediate, a mononuclear terminal cobalt(IV)-oxo species [Co(IV)=O]2+ was generated by reacting the cobalt(II) cofactor with oxone in aqueous solution and characterized by ESI-TOF MS.

Mechanistic Insights into Nitrile Activation by Cobalt(III)-Hydroperoxo Intermediates: The Influence of Ligand Basicity

The versatile applications of nitrile have led to the widespread use of nitrile activation in the synthesis of pharmacologically and industrially valuable compounds. We reported the activation of nitriles using mononuclear cobalt(III)-hydroperoxo complexes, [CoIII(Me3-TPADP)(O2H)(RCN)]2+ [R = Me (2) and Ph (2Ph)], to form cobalt(III)-peroxyimidato complexes, [CoIII(Me3-TPADP)(R-C(=NH)O2)]2+ [R = Me (3) and Ph (3Ph)]. The independence of the rate on the nitrile concentration and the positive Hammett value of 3.2(2) indicated that the reactions occur via an intramolecular nucleophilic attack of the hydroperoxide ligand to the coordinated nitrile carbon atom. In contrast, the previously reported cobalt(III)-hydroperoxo complex, [CoIII(TBDAP)(O2H)(CH3CN)]2+ (2TBDAP), exhibited the deficiency of reactivity toward nitrile. The comparison of pKa values and redox potentials of 2 and 2TBDAP showed that Me3-TPADP had a stronger ligand field strength than that of TBDAP. The density functional theory calculations for 2 and 2TBDAP support that the strengthened ligand field in 2 is mainly due to the replacement of two tert-butyl amine donors in TBDAP with methyl groups in Me3-TPADP, resulting in the compression of the Co-Nax bond lengths. These results provide mechanistic evidence of nitrile activation by the cobalt(III)-hydroperoxo complex and indicate that the basicity dependent on the ligand framework contributes to the ability of nitrile activation.

Crown-hydroxylamines are pH-dependent chelating N,O-ligands with a potential for aerobic oxidation catalysis

Despite the rich coordination chemistry, hydroxylamines are rarely used as ligands for transition metal coordination compounds. This is partially because of the instability of these complexes that undergo decomposition, disproportionation and oxidation processes involving the hydroxylamine motif. Here, we design macrocyclic poly-N-hydroxylamines (crown-hydroxylamines) that form complexes containing a d-metal ion (Cu(II), Ni(II), Mn(II), and Zn(II)) coordinated by multiple (up to six) hydroxylamine fragments. The stability of these complexes is likely to be due to a macrocycle effect and strong intramolecular H-bonding interactions between the N-OH groups. Crown-hydroxylamine complexes exhibit interesting pH-dependent behavior where the efficiency of metal binding increases upon deprotonation of the hydroxylamine groups. Copper complexes exhibit catalytic activity in aerobic oxidation reactions under ambient conditions, whereas the corresponding complexes with macrocyclic polyamines show poor or no activity. Our results show that crown-hydroxylamines display anomalous structural features and chemical behavior with respect to both organic hydroxylamines and polyaza-crowns.

Reaction of a Co(III)-peroxo complex with nitric oxide: putative formation of a peroxynitrite intermediate

A Co(II) complex, [CoII(L)2(H2O)2](ClO4)2, 1, having a bidentate ligand L [L = bis(3,5-dimethylpyrazolyl)methane] has been synthesized. Complex 1 in acetonitrile solution at -40 °C, in the presence of H2O2 and NEt3, afforded the corresponding Co(III)-peroxo species, [CoIII(L)2(O22-)]+, as the transient intermediate 1a. Thermal instability precluded its isolation and further characterization. The addition of nitric oxide (NO) gas into the freshly prepared [CoIII(L)2(O22-)]+ in acetonitrile at -40 °C resulted in the corresponding Co(II)-nitrato complex, [CoII(L)2(NO3)](ClO4) (2). The reaction is proposed to proceed through a putative Co(II)-peroxynitrite intermediate 1b. It was evidenced by the characteristic phenol ring nitration reaction.

Oxygen transfer reactivity mediated by nickel perfluoroalkyl complexes using molecular oxygen as a terminal oxidant

Nickel perfluoroethyl and perfluoropropyl complexes supported by naphthyridine-type ligands show drastically different aerobic reactivity from their trifluoromethyl analogs resulting in facile oxygen transfer to perfluoroalkyl groups or oxygenation of external organic substrates (phosphines, sulfides, alkenes and alcohols) using O₂ or air as a terminal oxidant. Such mild aerobic oxygenation occurs through the formation of spectroscopically detected transient high-valent Ni^III and structurally characterized mixed-valent Ni^II-Ni^IV intermediates and radical intermediates, resembling O₂ activation reported for some Pd dialkyl complexes. This reactivity is in contrast with the aerobic oxidation of naphthyridine-based Ni(CF₃)₂ complexes resulting in the formation of a stable Ni^III product, which is attributed to the effect of greater steric congestion imposed by longer perfluoroalkyl chains.

C-H bond activations by the HO˙/(Salophent-Bu)Co(II) radical pair generated via homolysis of a terminal Co(III)-OH bond

The reactive HO˙/(Salophen^t-Bu)Co(II) radical pair was observed to be generated via homolysis of the terminal Co(III)-OH bond in transient (Salophen^t-Bu)(L)Co(III)(OH) (L = Py, MeOH) complexes as indicated by UV-Vis and EPR measurements. Based on this elementary process, C-H bond activations in acetone, 2-butanone, acetonitrile and benzene were achieved under ambient conditions. For the reactions of the first three substrates, the alkylcobalt(III) complexes were formed as the products.

Oxygen versus Sulfur Coordination in Cobalt Superoxo Complexes: Spectroscopic Properties, O2 Binding, and H-Atom Abstraction Reactivity

A five-coordinate, disiloxide-ligated cobalt(II) (S = 3/2) complex (1) was prepared as an oxygen-ligated analogue to the previously reported silanedithiolate-ligated CoII(Me3TACN)(S2SiMe2) (J. Am. Chem. Soc., 2019, 141, 3641-3653). The structural and spectroscopic properties of 1 were analyzed by single-crystal X-ray diffraction, electron paramagnetic resonance (EPR), and NMR spectroscopies. The reactivity of 1 with dioxygen was examined, and it was shown to bind O2 reversibly in a range of solvents at low temperatures. A cobalt(III)-superoxo complex, CoIII(O2·-)(Me3TACN)((OSi2Ph)2O) (2), was generated, and was analyzed by UV-vis, EPR, and resonance Raman spectroscopies. Unlike its sulfur-ligated analogue, complex 2 can thermally release O2 to regenerate 1. Vibrational assignments for selective 18O isotopic labeling of both O2 and disiloxide ligands in 2 are consistent with a 6-coordinate, Co(η1-O2·-)("end-on") complex. Complex 2 reacts with the O-H bond of 4-methoxy-2,2,6,6-tetramethylpiperidin-1-ol (4-MeO-TEMPOH) via H-atom abstraction with a rate of 0.58(2) M-1 s-1 at -105 °C, but it is unable to oxidize phenol substrates. This bracketed reactivity suggests that the O-H bond being formed in the putative CoIII(OOH) product has a relatively weak O-H bond strength (BDFE ∼66-74 kcal mol-1). These thermodynamic and kinetic parameters are similar to those seen for the sulfur-ligated Co(O2)(Me3TACN)(S2SiMe2), indicating that the differences in the electronic structure for O versus S ligation do not have a large impact on H-atom abstraction reactivity.

Effect of the ring size of TMC ligands in controlling C-H bond activation by metal-superoxo species

Metal-superoxo species play a very important role in many metal-mediated catalytic transformation reactions. Their catalytic reactivity is affected by many factors such as the nature of metal ions and ring size of ligands. Herein, for the first time, we report DFT calculations on the electronic structures of a series of metal-superoxo species (M = V, Cr, Mn, Fe, and Co) with two ring size ligands, i.e., 13-TMC/14-TMC, and a detailed mechanistic study on the C-H bond activation of cyclohexa-1,4-diene followed by the effect of the ring size of ligands. Our DFT results showed that the electron density at the distal oxygen plays an important role in C-H bond activation. By computing the energetics of C-H bond activation and mapping the potential energy surface, it was found that the initial hydrogen abstraction is the rate-determining step with both TMC rings and all the studied metal-superoxo species. The significant electron density at the cyclohex-1,4-diene carbon indicates that the reaction proceeds via the proton-coupled electron transfer mechanism. By mapping the potential energy surfaces, we found that the 13-TMC ligated superoxo with the anti-isomer are more reactive than the 14-TMC superoxo species except for the iron-superoxo species where the 14-TMC ligated superoxo species is more reactive i.e. smaller ring size TMC is more reactive towards C-H bond activation. This is also supported by the structural correlation, i.e., the greater contraction in the smaller ring results in the metal being pushed out of plane along the z-axis, which reduces the steric hindrance. Thus, the ring size can help in designing catalysts with better efficiency for catalytic reactions.

The chameleon-like nature of elusive cobalt-oxygen intermediates in C-H bond activation reactions

High-valence metal-oxo (M-O, M = Fe, Mn, etc.) species are well-known reaction intermediates that are responsible for a wide range of pivotal oxygenation reactions and water oxidation reactions in metalloenzymes. Although extensive efforts have been devoted to synthesizing and identifying such complexes in biomimetic studies, the structure-function relationship and related reaction mechanisms of these reaction intermediates remain elusive, especially for the cobalt-oxygen species. In the present manuscript, the calculated results demonstrate that the tetraamido macrocycle ligated cobalt complex, Co(O)(TAML) (1), behaves like a chameleon: the electronic structure varies from a cobalt(III)-oxyl species to a cobalt(IV)-oxo species when a Lewis acid Sc³⁺ salt coordinates or an acidic hydrocarbon attacks 1. The dichotomous correlation between the reaction rates of C-H bond activation by 1 and the bond dissociation energy (BDE) vs. the acidity (pK_a) was rationalized for the first time by different reaction mechanisms: for normal C-H bond activation, the Co(III)-oxyl species directly activates the C-H bond via a hydrogen atom transfer (HAT) mechanism, whereas for acidic C-H bond activation, the Co(III)-oxyl species evolves to a Co(IV)-oxo species to increase the basicity of the oxygen to activate the acidic C-H bond, via a novel PCET(PT) mechanism (proton-coupled electron transfer with a PT(proton-transfer)-like transition state). These theoretical findings will enrich the knowledge of biomimetic metal-oxygen chemistry.

Can you break the oxo-wall? A multiconfigurational perspective

The concept of the "oxo-wall" was conceived about 60 years ago by Harry B. Gray, and has been found to be related to the non-existence of high-valent M-oxo species in the +IV oxidation state in a tetragonal geometry beyond group 8 in the periodic table. Several efforts have been made in the past decades to test and find examples that violate this theory. Several claims of violation in the past were attributed to the difference in the geometries/coordination number and, therefore, these are not examples of true violation. In recent years, substantial efforts have been undertaken to synthesise a true Co^IVO species with various ligand architectures. Co^IVO and Co^III-O˙ are electromers and, while they are interchangeably used in the literature; the former violates the oxo-wall while the latter does not. The possibility that these two species could exist in various proportions similar to resonating structures has not been considered in detail in this area. Furthermore, there have been no attempts to quantify such mixing. In this direction, we have employed density functional theory (DFT) and ab initio CASSCF/NEVPT2 methods to quantify the co-existence of Co^IVO and Co^III-O˙ isomeric species. By thoroughly studying six different metal-oxo species, we affirm that the nature of such electromeric mixing is minimal/negligible for Fe^IVO and Mn^IVO species - both are pre-oxo-wall examples. By studying four different ligand architectures with Co-oxo species, our results unveil that the mixing of Co^IVO ↔ Co^III-O˙ is substantial in all geometries, with dominant Co^IVO species favourable for the S = 3/2 intermediate spin state. The percentage of the Co^III-O˙ species is enhanced substantially for the S = 1/2 low-spin state. We have attempted to develop a tool to estimate the percentage of the Co^III-O˙ species using various structural parameters. Among those tested, a linear relationship between % of Co^III-O˙ and a bond length based ratio is found (, where d_(Co-O) and d_(Co-Nax) are the axial Co-O and Co-N_ax bond lengths in Å, respectively). It is found that the higher the R_d, the greater the Co^III-O˙ character will be and the geometrically portable correlation developed offers a way to qualitatively compute the % of Co^III-O˙ character for unknown geometries.

Selective oxygenation of C-H and CC bonds with H2O2 by high-spin cobalt(II)-carboxylate complexes

Four cobalt(II)-carboxylate complexes [(6-Me₃-TPA)Co^II(benzoate)](BPh₄) (1), [(6-Me₃-TPA)Co^II(benzilate)](ClO₄) (2), [(6-Me₃-TPA)Co^II(mandelate)](BPh₄) (3), and [(6-Me₃-TPA)Co^II(MPA)](BPh₄) (4) (HMPA = 2-methoxy-2-phenylacetic acid) of the 6-Me₃-TPA (tris((6-methylpyridin-2-yl)methyl)amine) ligand were isolated to investigate their ability in H₂O₂-dependent selective oxygenation of C-H and CC bonds. All six-coordinate complexes contain a high-spin cobalt(II) center. While the cobalt(II) complexes are inert toward dioxygen, each of these complexes reacts readily with hydrogen peroxide to form a diamagnetic cobalt(III) species, which decays with time leading to the oxidation of the methyl groups on the pyridine rings of the supporting ligand. Intramolecular ligand oxidation by the cobalt-based oxidant is partially inhibited in the presence of external substrates, and the substrates are converted to their corresponding oxidized products. Kinetic studies and labelling experiments indicate the involvement of a metal-based oxidant in affecting the chemo- and stereo-selective catalytic oxygenation of aliphatic C-H bonds and epoxidation of alkenes. An electrophilic cobalt-oxygen species that exhibits a kinetic isotope effect (KIE) value of 5.3 in toluene oxidation by 1 is proposed as the active oxidant. Among the complexes, the cobalt(II)-benzoate (1) and cobalt(II)-MPA (4) complexes display better catalytic activity compared to their α-hydroxy analogues (2 and 3). Catalytic studies with the cobalt(II)-acetonitrile complex [(6-Me₃-TPA)Co^II(CH₃CN)₂](ClO₄)₂ (5) in the presence and absence of externally added benzoate support the role of the carboxylate co-ligand in oxidation reactions. The proposed catalytic reaction involves a carboxylate-bridged dicobalt complex in the activation of H₂O₂ followed by the oxidation of substrates by a metal-based oxidant.

Generation and Reactivity of a NiIII2(μ-1,2-peroxo) Complex

High-valent transition metal-oxo, -peroxo, and -superoxo complexes are crucial intermediates in both biological and synthetic oxidation of organic substrates, water oxidation, and oxygen reduction. While high-valent oxygenated complexes of Mn, Fe, Co, and Cu are increasingly well-known, high-valent oxygenated Ni complexes are comparatively rarer. Herein we report the isolation of such an unusual high-valent species in a thermally unstable Ni^III₂(μ-1,2-peroxo) complex, which has been characterized using single-crystal X-ray diffraction and X-ray absorption, NMR, and UV-vis spectroscopies. Reactivity studies show that this complex is stable toward dissociation of oxygen but reacts with simple nucleophiles and electrophiles.

Coligand modulated oxidative O-demethylation of a methyl ether appended tetradentate N-ligand in Co(ii) complexes

Two Co(ii) complexes of the formula CoL^OMeX₂ (X = Cl^- (1a); X = I^- (1b)), where L^OMe is 2-methoxy-N,N-bis(pyridin-2-ylmethyl) aniline, were synthesized and their structure, spectra and reactivity were studied. Upon oxidation of 1a and 1b, the ligand L^OMe undergoes demethylation at the metal centre resulting in the formation of Co(iii) complexes with modified phenoxide ligands. This is the very first example of oxidative O-demethylation reported at a Co(ii) centre. The oxidative behaviour exhibits a striking dependence on the nature of coligands coordinated to the metal centre. The Co(ii) complex 1a with stronger chloro coligands requires a strong oxidising agent like t-BuOOH for oxidative demethylation and the subsequent formation of a mononuclear Co(iii) complex with a demethylated ligand, CoL^O-Cl₂ (2). On the other hand, complex 1b with weaker iodo coligands undergoes oxidation in the presence of the weak oxidant O₂ to form a dihydroxo bridged binuclear Co(iii) complex [Co₂(L^O-)₂(OH)₂]²⁺ (3) with modified phenoxide ligands. The oxidation of 1b to 3 is monitored and the intermediate Co(ii) iodo aqua complex [CoL^OMeI(H₂O)]⁺ and Co(ii) diaqua complex [CoL^OMe(H₂O)₂]²⁺ are isolated and characterised.

Single-step benzene hydroxylation by cobalt(ii) catalysts via a cobalt(iii)-hydroperoxo intermediate

Cobalt(ii) complexes reported as efficient and selective catalysts for single-step phenol formation from benzene using H₂O₂. The catalysis proceeds likely via cobalt(iii)-hydroperoxo species.

Computational Mechanistic Insights on the NO Oxidation Reaction Catalyzed by Non-Heme Biomimetic Cr-N-Tetramethylated Cyclam Complexes

The conversion reaction of NO to NO₃⁻ ion catalyzed by the end-on [Cr(III)(n-TMC)(O₂)(Cl)]⁺ superoxo and side-on [Cr(IV)(n-TMC)(O₂)(Cl)]⁺ peroxo non-heme complexes (n = 12, 13, 14 and 15), which are biomimetic systems of nitric oxide dioxygenases (NODs), has been explored using a computational protocol in the framework of density functional theory. Results show that the potential energy profiles for the studied reactions lie above the reagent energies, regardless of the used catalyst. Both the O-O bond breaking in the biomimetics and the NO₃⁻ ion formation require low energy barriers suggesting an efficient catalytic power of the studied systems. The rate-determining step depends on ligand size.

A Mn(iv)-peroxo complex in the reactions with proton donors

Protons play an important role in promoting O-O or M-O bond cleavage of metal-peroxo complexes. Treatment of side-on O2-bound [PPN][MnIV(TMSPS3)(O2)] (1, PPN = bis(triphenylphosphine)iminium and TMSPS3H3 = 2,2',2''-trimercapto-3,3',3''-tris(trimethylsilyl)triphenylphosphine) with perchloric acid (HClO4) in the presence of PR3 (R = phenyl or p-tolyl) results in the formation of neutral five-coordinate MnIII(OPR3)(TMSPS3) complexes (R = phenyl, 2a; p-tolyl, 2b), which are confirmed by X-ray crystallography. Isotope labelling experiments demonstrate that the oxygen atom in the phosphine oxide product derives from the peroxo ligand of 1. Reactions of 1 with weak proton donors, such as phenylthiol, phenol, substituted phenol and methanol, are also investigated to explore the reactivity of the MnIV-peroxo complex, leading to the isolation of a series of five-coordinate [MnIII(L)(TMSPS3)]- complexes (L = phenylthiolate, phenolate or methoxide). Mechanistic aspects of the reactions of the MnIV-peroxo complex with proton donors are discussed as well.

A Nonheme Thiolate-Ligated Cobalt Superoxo Complex: Synthesis and Spectroscopic Characterization, Computational Studies, and Hydrogen Atom Abstraction Reactivity

The synthesis and characterization of a Co(II) dithiolato complex Co(Me₃TACN)(S₂SiMe₂) (1) are reported. Reaction of 1 with O₂ generates a rare thiolate-ligated cobalt-superoxo species Co(O₂)(Me₃TACN)(S₂SiMe₂) (2) that was characterized spectroscopically and structurally by resonance Raman, EPR, and X-ray absorption spectroscopies as well as density functional theory. Metal-superoxo species are proposed to S-oxygenate metal-bound thiolate donors in nonheme thiol dioxygenases, but 2 does not lead to S-oxygenation of the intramolecular thiolate donors and does not react with exogenous sulfur donors. However, complex 2 is capable of oxidizing the O-H bonds of 2,2,6,6-tetramethylpiperidin-1-ol derivatives via H atom abstraction. Complementary proton-coupled electron-transfer reactivity is seen for 2 with separated proton/reductant pairs. The reactivity studies indicate that 2 can abstract H atoms from weak X-H bonds with bond dissociation free energy (BDFE) ≤ 70 kcal mol^-1. DFT calculations predict that the putative Co(OOH) product has an O-H BDFE = 67 kcal mol^-1, which matches the observed pattern of reactivity seen for 2. These data provide new information regarding the selectivity of S-oxygenation versus H atom abstraction in thiolate-ligated nonheme metalloenzymes that react with O₂.

A theoretical study on the oxidation of alkenes to aldehydes catalyzed by ruthenium porphyrins using O2 as the sole oxidant

Density functional theory (DFT) calculations were used to study the ruthenium porphyrin-catalyzed oxidation of styrene to generate an aldehyde. The results indicate that two reactive oxidants, dioxoruthenium and monooxoruthenium-superoxo porphyrins, participate in the catalytic oxidation. In the mechanism, the resultant monooxoruthenium porphyrin acts in the tandem epoxide isomerization (E-I) to selectively yield an aldehyde and generate a dioxoruthenium porphyrin, thereby triggering new oxidation reaction cycles. In this calculation, several key elements responsible for the observed oxidative ability have been established by using Frontier molecular orbital (FMO) theory, natural bond orbital (NBO) analysis, etc., which include the reaction energy, the spin exchange effect, the spin-state conversion process, and the energy level of the lowest unoccupied molecular orbitals (LUMOs) of the reactive oxidants. The comparative oxidative abilities of the ruthenium-oxo/superoxo compounds with different axial ligands are also investigated. The results suggest that the ruthenium-oxo/superoxo species featuring a chlorine axial ligand is more reactive than that substituted with oxygen. This tuneable reactivity can be understood when considering the different electronic characters of the two ligands and the effective atomic number rule (EAN).

A Multifunctional Pincer Ligand for Cobalt-Promoted Oxidation by N2 O

The divalent cobalt complex of the diprotic pincer ligand bis-pyrazolylpyridine, (H₂ L)CoCl₂ , is dehydrohalogenated twice by LiN(SiMe₃ )₂ in the presence of PEt₃ to give monomeric S=1/2 LCo(PEt₃ )₂ (1), fully characterized in the solid-state and solution as a square pyramidal monomer with a long axial Co-P bond. This 17-electron species reacts in time of mixing with N₂ O to form L₂ Co₂ (μ-OPEt₃ ) (2)+3 OPEt₃ , the former the first example of phosphine oxide bridging two transition metals. The same products are formed from O₂ , and divalent cobalt persists even in the presence of excess oxidant. Species (2) catalyzes oxygen atom transfer (OAT) for generation of O=PEt₃ from PEt₃ from either N₂ O or O₂ . Bridging and terminal cobalt oxo intermediates are suggested, and the electron donor power, and potential redox activity of the dianionic pincer ligand is emphasized.

Oxygen activation by mononuclear Mn, Co, and Ni centers in biology and synthetic complexes

The active sites of metalloenzymes that catalyze O₂-dependent reactions generally contain iron or copper ions. However, several enzymes are capable of activating O₂ at manganese or nickel centers instead, and a handful of dioxygenases exhibit activity when substituted with cobalt. This minireview summarizes the catalytic properties of oxygenases and oxidases with mononuclear Mn, Co, or Ni active sites, including oxalate-degrading oxidases, catechol dioxygenases, and quercetin dioxygenase. In addition, recent developments in the O₂ reactivity of synthetic Mn, Co, or Ni complexes are described, with an emphasis on the nature of reactive intermediates featuring superoxo-, peroxo-, or oxo-ligands. Collectively, the biochemical and synthetic studies discussed herein reveal the possibilities and limitations of O₂ activation at these three "overlooked" metals.

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

The reactivity of mononuclear metal-hydroperoxo adducts has fascinated researchers in many areas due to their diverse biological and catalytic processes. In this study, a mononuclear cobalt(III)-peroxo complex bearing a tetradentate macrocyclic ligand, [Co^III(Me₃-TPADP)(O₂)]⁺ (Me₃-TPADP = 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane), was prepared by reacting [Co^II(Me₃-TPADP)(CH₃CN)₂]²⁺ with H₂O₂ in the presence of triethylamine. Upon protonation, the cobalt(III)-peroxo intermediate was converted into a cobalt(III)-hydroperoxo complex, [Co^III(Me₃-TPADP)(O₂H)(CH₃CN)]²⁺. The mononuclear cobalt(III)-peroxo and -hydroperoxo intermediates were characterized by a variety of physicochemical methods. Results of electrospray ionization mass spectrometry clearly show the transformation of the intermediates: the peak at m/z 339.2 assignable to the cobalt(III)-peroxo species disappears with concomitant growth of the peak at m/z 190.7 corresponding to the cobalt(III)-hydroperoxo complex (with bound CH₃CN). Isotope labeling experiments further support the existence of the cobalt(III)-peroxo and -hydroperoxo complexes. In particular, the O-O bond stretching frequency of the cobalt(III)-hydroperoxo complex was determined to be 851 cm^-1 for ¹⁶O₂H samples (803 cm^-1 for ¹⁸O₂H samples), and its Co-O vibrational energy was observed at 571 cm^-1 for ¹⁶O₂H samples (551 cm^-1 for ¹⁸O₂H samples; 568 cm^-1 for ¹⁶O₂²H samples) by resonance Raman spectroscopy. Reactivity studies performed with the cobalt(III)-peroxo and -hydroperoxo complexes in organic functionalizations reveal that the latter is capable of conducting oxygen atom transfer with an electrophilic character, whereas the former exhibits no oxygen atom transfer reactivity under the same reaction conditions. Alternatively, the cobalt(III)-hydroperoxo complex does not perform hydrogen atom transfer reactions, while analogous low-spin Fe(III)-hydroperoxo complexes are capable of this reactivity. Density functional theory calculations indicate that this lack of reactivity is due to the high free energy cost of O-O bond homolysis that would be required to produce the hypothetical Co(IV)-oxo product.

Upside Down! Crystallographic and Spectroscopic Characterization of an [Fe IV(O syn)(TMC)]2+ Complex

Fe(II)(TMC)(OTf)2 reacts with 2-(t)BuSO2-C6H4IO to afford an oxoiron(IV) product, 2, distinct from the previously reported [Fe(IV)(Oanti)(TMC)(NCMe)](2+). In MeCN, 2 has a blue-shifted near-IR band, a higher ν(Fe═O), a larger Mössbauer quadrupole splitting, and quite a distinct (1)H NMR spectrum. Structural analysis of crystals grown from CH2Cl2 reveals a complex with the formulation of [Fe(IV)(Osyn)(TMC)(OTf)](OTf) and the shortest Fe(IV)═O bond [1.625(4) Å] found to date.

Amine oxidative N-dealkylation via cupric hydroperoxide Cu-OOH homolytic cleavage followed by site-specific fenton chemistry

Copper(II) hydroperoxide species are significant intermediates in processes such as fuel cells and (bio)chemical oxidations, all involving stepwise reduction of molecular oxygen. We previously reported a Cu(II)-OOH species that performs oxidative N-dealkylation on a dibenzylamino group that is appended to the 6-position of a pyridyl donor of a tripodal tetradentate ligand. To obtain insights into the mechanism of this process, reaction kinetics and products were determined employing ligand substrates with various para-substituent dibenzyl pairs (-H,-H; -H,-Cl; -H,-OMe, and -Cl,-OMe), or with partially or fully deuterated dibenzyl N-(CH2Ph)2 moieties. A series of ligand-copper(II) bis-perchlorate complexes were synthesized, characterized, and the X-ray structures of the -H,-OMe analogue were determined. The corresponding metastable Cu(II)-OOH species were generated by addition of H2O2/base in acetone at -90 °C. These convert (t1/2 ≈ 53 s) to oxidatively N-dealkylated products, producing para-substituted benzaldehydes. Based on the experimental observations and supporting DFT calculations, a reaction mechanism involving dibenzylamine H-atom abstraction or electron-transfer oxidation by the Cu(II)-OOH entity could be ruled out. It is concluded that the chemistry proceeds by rate limiting Cu-O homolytic cleavage of the Cu(II)-(OOH) species, followed by site-specific copper Fenton chemistry. As a process of broad interest in copper as well as iron oxidative (bio)chemistries, a detailed computational analysis was performed, indicating that a Cu(I)OOH species undergoes O-O homolytic cleavage to yield a hydroxyl radical and Cu(II)OH rather than heterolytic cleavage to yield water and a Cu(II)-O(•-) species.