Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity

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.

Dioxygen Activation by Internally Aromatic Metallacycle: Crystallographic Structure and Mechanistic Investigations

Mononuclear metal-peroxo species are invoked as the key intermediates in metalloenzymatic or synthetic catalysis. However, either transience or sluggishness reactivity of synthetic analogs of metal-peroxo species impedes our understanding of oxygen activation mechanism. Herein, we designed and characterized a dioxygen-derived mononuclear osmium-peroxo complex, in which the peroxo ligand is stabilized by internally aromatic metallacycle. We demonstrate that the osmium-peroxo species shows catalytic activity toward promoterless alcohol dehydrogenations. Furthermore, computational studies provide a new mechanism for the osmium-peroxo-mediated alcohol oxidation, starting with the concerted double-hydrogen transfer and followed by the generation of osmium-oxo species. Interestingly, the internally aromatic metallacycle also plays a vital role in catalysis, which mediates the hydrogen transfer from osmium center to the distal oxygen atom of Os–OOH moiety, thus facilitating the Os–OOH→Os=O conversion. We expect that these insights will advance the development of aromatic metallacycle toward aerobic oxidation catalysis.

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A dioxygen-derived mononuclear osmium-peroxo complex was characterized

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The peroxo ligand is stabilized by internally aromatic metallacycle

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O₂ activation involves the reversible aromatization-dearomatization

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A concerted double-hydrogen transfer mechanism for alcohol dehydrogenation

Structure, Spectroscopy, and Reactivity of a Mononuclear Copper Hydroxide Complex in Three Molecular Oxidation States

Structural, spectroscopic, and reactivity studies are presented for an electron transfer series of copper hydroxide complexes supported by a tridentate redox-active ligand. Single crystal X-ray crystallography shows that the mononuclear [CuOH]¹⁺ core is stabilized via intramolecular H-bonds between the H-donors of the ligand and the hydroxide anion when the ligand is in its trianionic form. This complex undergoes two reversible oxidation processes that produce two metastable "high-valent" CuOH species, which can be generated by addition of stoichiometric amounts of 1e^- oxidants. These CuOH species are characterized by an array of spectroscopic techniques including UV-vis absorption, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopies (XAS), which together indicate that all redox couples are ligand-localized. The reactivity of the complexes in their higher oxidation states toward substrates with modest O-H bond dissociation energies (e.g., 4-substitued-2,6-di-tert-butylphenols) indicates that these complexes act as 2H⁺/2e^- oxidants, differing from the 1H⁺/1e^- reactivity of well-studied [CuOH]²⁺ systems.

Tuning Inner-Sphere Electron Transfer in a Series of Copper/Nitrosoarene Adducts

A series of copper/nitrosoarene complexes was created that mimics several steps in biomimetic O₂ activation by copper(I). The reaction of the copper(I) complex of N,N,N',N'-tetramethypropylenediamine with a series of para-substituted nitrosobenzene derivatives leads to adducts in which the nitrosoarene (ArNO) is reduced by zero, one, or two electrons, akin to the isovalent species dioxygen, superoxide, and peroxide, respectively. The geometric and electronic structures of these adducts were characterized by means of X-ray diffraction, vibrational analysis, ultraviolet-visible spectroscopy, NMR, electrochemistry, and density functional theory (DFT) calculations. The bonding mode of the NO moiety depends on the oxidation state of the ArNO moiety: κN for ArNO, mononuclear η²-NO and dinuclear μ-η²:η¹ for ArNO^•-, and dinuclear μ-η²:η² for ArNO^2-. ¹⁵N isotopic labeling confirms the reduction state by measuring the NO stretching frequency (1392 cm^-1 for κN-ArNO, 1226 cm^-1 for η²-ArNO^•-, 1133 cm^-1 for dinuclear μ-η²:η¹-ArNO^•-, and 875 cm^-1 for dinuclear μ-η²:η² for ArNO^2-). The ¹⁵N NMR signal disappears for the ArNO^•- species, establishing a unique diagnostic for the radical state. Electrochemical studies indicate reduction waves that are consistent with one-electron reduction of the adducts and are compared with studies performed on Cu-O₂ analogues. DFT calculations were undertaken to confirm our experimental findings, notably to establish the nature of the charge-transfer transitions responsible for the intense green color of the complexes. In fine, this family of complexes is unique in that it walks through three redox states of the ArNO moiety while keeping the metal and its supporting ligand the same. This work provides snapshots of the reactivity of the toxic nitrosoarene molecules with the biologically relevant Cu(I) ion.

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.

Exceptional Substrate Diversity in Oxygenation Reactions Catalyzed by a Bis(μ-oxo) Copper Complex

The enzyme tyrosinase contains a reactive side-on peroxo dicopper(II) center as catalytically active species in C-H oxygenation reactions. The tyrosinase activity of the isomeric bis(μ-oxo) dicopper(III) form has been discussed controversially. The synthesis of bis(μ-oxo) dicopper(III) species [Cu2 (μ-O)2 (L1)2 ](X)2 ([O1](X)2 , X=PF6 - , BF4 - , OTf- , ClO4 - ), stabilized by the new hybrid guanidine ligand 2-{2-((dimethylamino)methyl)phenyl}-1,1,3,3-tetramethylguanidine (L1), and its characterization by UV/Vis, Raman, and XAS spectroscopy, as well as cryo-UHR-ESI mass spectrometry, is described. We highlight selective oxygenation of a plethora of phenolic substrates mediated by [O1](PF6 )2 , which results in mono- and bicyclic quinones and provides an attractive strategy for designing new phenazines. The selectivity is predicted by using the Fukui function, which is hereby introduced into tyrosinase model chemistry. Our bioinspired catalysis harnesses molecular dioxygen for organic transformations and achieves a substrate diversity reaching far beyond the scope of the enzyme.

A Manganese(IV)-Hydroperoxo Intermediate Generated by Protonation of the Corresponding Manganese(III)-Superoxo Complex

Earlier work revealed that metal-superoxo species primarily function as radicals and/or electrophiles. Herein, we present ambiphilicity of a Mn^III-superoxo complex revealed by its proton- and metal-coupled electron-transfer processes. Specifically, a Mn^IV-hydroperoxo intermediate, [Mn(BDP^BrP)(OOH)]⁺ (1, H₂BDP^BrP = 2,6-bis((2-(S)-di(4-bromo)phenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) was generated by treatment of a Mn^III-superoxo complex, Mn(BDP^BrP)(O₂^•) (2) with trifluoroacetic acid at -120 °C. Detailed insights into the electronic structure of 1 are obtained using resonance Raman and multi-frequency electron paramagnetic resonance spectroscopies coupled with density functional theory calculations. Similarly, the reaction of 2 with scandium(III) triflate was shown to give a Mn(IV)/Sc(III) bridging peroxo species, [Mn(BDP^BrP)(OO)Sc(OTf)_n]^(3-n)+ (4). Furthermore, it is found that deprotonation of 1 quantitatively regenerates 2, and that one-electron oxidation of the corresponding Mn^III-hydroperoxo species, Mn(BDP^BrP)(OOH) (3), also yields 1.

Theoretical rationalization for the equilibrium between (μ-η2:η2-peroxido)CuIICuII and bis(μ-oxido)CuIIICuIII complexes: perturbational effects from ligand frameworks

DFT calculations are carried out to investigate the geometric effects of the supporting ligands in the relative energies of the (μ-η2:η2-peroxido)CuIICuII complex 1 and the bis(μ-oxido)CuIIICuIII complex 2. The N3-tridentate ligand bearing acyclic propane diamine framework La preferentially provided 1, whereas the N3-tridentate ligand with cyclic diamine framework such as 1,4-diazacycloheptane Lb gave 2 after the oxygenation of the corresponding CuI complexes as reported previously [S. Itoh, et al., Inorg. Chem., 2014, 53, 8786-8794]. Calculations at the B3LYP*-D3 level of theory can reasonably explain the experimental results in relative energies, structures and harmonic frequencies of 1 and 2. Perturbational effects of the diamine chelates of La and Lb especially on the equilibrium of 1 and 2 are investigated in detail. In the range from 2.30 Å to 3.40 Å of the N-N distance in the diamine moiety, 1 is more stable than 2 by 8.4 kcal mol-1 at the distance of 3.40 Å. Calculated potential energies indicate that the decrease in the N-N distance is associated with a decrease in energy of 2, leading that 2 can be most stabilized at the N-N distance of 2.60 Å. Furthermore, molecular orbitals analyses are performed to explain that the energy gaps between the σ* orbital of the O-O bond and the dx2-y2 orbitals of the CuII ions of 1 get small as the diamine moiety is shrunk, leading to facilitate the O-O bond cleavage from 1 to 2.

Phenoxazinone synthase-like catalytic activity of novel mono- and tetranuclear copper(ii) complexes with 2-benzylaminoethanol

Three novel coordination compounds, [Cu(ca)2(Hbae)2] (1), [Cu(va)2(Hbae)2] (2) and [Cu4(va)4(bae)4]·H2O (3), have been prepared by self-assembly reactions of copper(ii) chloride (1 and 2) or tetrafluoroborate (3) and CH3OH (1 and 3) or CH3CN (2) solution of 2-benzylaminoethanol (Hbae) and cinnamic (Hca, 1) or valeric (Hva, 2 and 3) acid. Crystallographic analysis revealed that both 1 and 2 have mononuclear crystal structures, wherein the complex molecules are H-bonded forming extended supramolecular chains. The tetranuclear structure of 3 is based on the {Cu4(μ3-O)4} core, wherein the metal atoms are bound together by μ3 oxygen bridges from 2-benzylaminoethanol forming an overall cubane-like configuration. The strong hydrogen bonding in 1-3 leads to the joining of the neighbouring molecules into 1D chains. Concentration-dependent ESI-MS studies disclosed the equilibria between di-, tri- and tetranuclear species in solutions of 1-3. All three compounds act as catalysts for the aerobic oxidation of o-aminophenol to the phenoxazinone chromophore (phenoxazinone synthase-like activity), with the maximum reaction rates of 4.0 × 10-7, 2.5 × 10-7 and 2.1 × 10-7 M s-1 for 1, 2 and 3, respectively, supported by the quantitative yield of the product after 24 h. The dependence of the reaction rates on catalyst concentrations is evidence of reaction orders higher than one relative to the catalyst. Kinetic and ESI-MS data allowed us to assume that the tetranuclear species, originating from 1, 2 and 3 in solution, possess considerably higher activity than the species of lower nuclearity. Mechanistic and isotopic 18O-labelling experiments suggested that o-aminophenol coordinates to CuII species with the formation of reactive intermediates, while the oxygen from 18O2 is not incorporated into the phenoxazinone chromophore.

An Adaptable N-Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States

A neutral hybrid macrocycle with two trans-positioned N-heterocyclic carbenes (NHCs) and two pyridine donors hosts copper in three oxidation states (+I-+III) in a series of structurally characterized complexes (1-3). Redox interconversion of [LCu]+/2+/3+ is electrochemically (quasi)reversible and occurs at moderate potentials (E1/2 =-0.45 V and +0.82 V (vs. Fc/Fc+ )). A linear CNHC -Cu-CNHC arrangement and hemilability of the two pyridine donors allows the ligand to adapt to the different stereoelectronic and coordination requirements of CuI versus CuII /CuIII . Analytical methods such as NMR, UV/Vis, IR, electron paramagnetic resonance, and Cu Kβ high-energy-resolution fluorescence detection X-ray absorption spectroscopies, as well as DFT calculations, give insight into the geometric and electronic structures of the complexes. The XAS signatures of 1-3 are textbook examples for CuI , CuII , and CuIII species. Facile 2-electron interconversion combined with the exposure of two basic pyridine N sites in the reduced CuI form suggest that [LCu]+/2+/3+ may operate in catalysis via coupled 2 e- /2 H+ transfer.

Mechanistic approaches for chemically modifying the coordination sphere of copper-amyloid-β complexes

Neurotoxic implications of the interactions between Cu(I/II) and amyloid-β (Aβ) indicate a connection between amyloid cascade hypothesis and metal ion hypothesis with respect to the neurodegeneration associated with Alzheimer's disease (AD). Herein, we report a mechanistic strategy for modifying the first coordination sphere of Cu(II) bound to Aβ utilizing a rationally designed peptide modifier, L1. Upon reacting with L1, a metal-binding histidine (His) residue, His14, in Cu(II)-Aβ was modified through either covalent adduct formation, oxidation, or both. Consequently, the reactivity of L1 with Cu(II)-Aβ was able to disrupt binding of Cu(II) to Aβ and result in chemically modified Aβ with altered aggregation and toxicity profiles. Our molecular-level mechanistic studies revealed that such L1-mediated modifications toward Cu(II)-Aβ could stem from the molecule's ability to 1) interact with Cu(II)-Aβ and 2) foster copper-O₂ chemistry. Collectively, our work demonstrates the development of an effective approach to modify Cu(II)-Aβ at a metal-binding amino acid residue and consequently alter Aβ's coordination to copper, aggregation, and toxicity, supplemented with an in-depth mechanistic perspective regarding such reactivity.

Isomerism and dynamic behavior of bridging phosphaalkynes bound to a dicopper complex

A dicopper complex featuring a symmetrically bridging nitrile ligand and supported by a binucleating naphthyridine-based ligand, [Cu₂(μ-η ¹ :η ¹ -MeCN)DPFN](NTf₂)₂, was treated with phosphaalkynes (RC[triple bond, length as m-dash]P, isoelectronic analogues of nitriles) to yield dicopper complexes that exhibit phosphaalkynes in rare μ-η ²:η ² binding coordination modes. X-ray crystallography revealed that these unusual "tilted" structures exist in two isomeric forms (R "up" vs. R "sideways"), depending on the steric profile of the phosphaalkyne's alkyl group (R = Me, Ad, or ^t Bu). Only one isomer is observed in both solution and the solid state for R = Me (sideways) and ^t Bu (up). With intermediate steric bulk (R = Ad), the energy difference between the two geometries is small enough that both are observed in solution, and NMR spectroscopy and computations indicate that the solid-state structure corresponds to the minor isomer observed in solution. Meanwhile, treatment of [Cu₂(μ-η ¹:η ¹-MeCN)DPFN](NTf₂)₂ with 2-butyne affords [Cu₂(μ-η ²:η ²-(MeC[triple bond, length as m-dash]CMe))DPFN](NTf₂)₂: its similar ligand geometry demonstrates that the tilted μ-η ²:η ² binding mode is not limited to phosphaalkynes but reflects a more general trend, which can be rationalized via an NBO analysis showing maximization of π-backbonding.

Bioinspired chemistry at MOF secondary building units

This perspective describes recent developments and future directions in bioinorganic chemistry and biomimetic catalysis centered at metal–organic framework secondary building units.

The secondary building units (SBUs) in metal–organic frameworks (MOFs) support metal ions in well-defined and site-isolated coordination environments with ligand fields similar to those found in metalloenzymes. This burgeoning class of materials has accordingly been recognized as an attractive platform for metalloenzyme active site mimicry and biomimetic catalysis. Early progress in this area was slowed by challenges such as a limited range of hydrolytic stability and a relatively poor diversity of redox-active metals that could be incorporated into SBUs. However, recent progress with water-stable MOFs and the development of more sophisticated synthetic routes such as postsynthetic cation exchange have largely addressed these challenges. MOF SBUs are being leveraged to interrogate traditionally unstable intermediates and catalytic processes involving small gaseous molecules. This perspective describes recent advances in the use of metal centers within SBUs for biomimetic chemistry and discusses key future developments in this area.

Linear free energy relationships for transition metal chemistry: case study of CH activation with copper–oxygen complexes

We propose a computational framework for developing Taft-like linear free energy relationships to characterize steric effects on the catalytic activity of transition metal complexes.

Binuclear O2 activation and hydrogen transfer mechanism for aerobic oxidation of alcohols

Density functional theory calculations reveal a binuclear O₂ activation and hydrogen transfer mechanism with spin-crossovers for aerobic oxidation of alcohols.

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.

Aerobic oxidation of 2-aminophenol catalysed by a series of mononuclear copper(ii) complexes: phenoxazinone synthase-like activity and mechanistic study

Biomimetic catalytic oxidation of 2-aminophenol by three mononuclear copper(ii) complexes and the mechanistic aspects are presented.

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.

Sodalite-type Cu-based Three-dimensional Metal-Organic Framework for Efficient Oxygen Reduction Reaction

Inspired by copper-based oxygen reduction biocatalysts, we have studied the electrocatalytic behavior of a Cu-based MOF (Cu-BTT) for oxygen reduction reaction (ORR) in alkaline medium. This catalyst reduces the oxygen at the onset (E_onset ) and half-wave potential (E^1/2 ) of 0. 940 V and 0.778 V, respectively. The high halfway potential supports the good activity of Cu-BTT MOF. The high ORR catalytic activity can be interpreted by the presence of nitrogen-rich ligand (tetrazole) and the generation of nascent copper(I) during the reaction. In addition to the excellent activity, Cu-BTT MOF showed exceptional stability too, which was confirmed through chronoamperometry study, where current was unchanged up to 12 h. Further, the 4-electrons transfer of ORR kinetics was confirmed by hydrodynamic voltammetry. The oxygen active center namely copper(I) generation during ORR has been understood by the reduction peak in cyclic voltammetry as well in the XPS analysis.

Formation of the CuII -Phenoxyl Radical by Reaction of O2 with a CuII -Phenolate Complex via the CuI -Phenoxyl Radical

Reaction of Cu(ClO₄ )₂ ⋅6 H₂ O with a tripodal 2N2O ligand, H₂ Me₂ NL, having a p-(dimethylamino)phenol moiety, in CH₂ Cl₂ /MeOH (1:1 v/v) under basic conditions under an inert gas atmosphere gave [Cu(Me₂ NL)(H₂ O)] (1). The same reaction carried out under aerobic conditions gave [Cu(Me₂ NL)(MeOH)]ClO₄ (2), which could be obtained also from the isolated complex 1 by reaction with O₂ in CH₂ Cl₂ /MeOH. The X-ray crystal structures of 1 and 2 revealed similar square-pyramidal structures, but 2 showed the (dimethylamino)phenoxyl radical features. Complex 1 exhibits characteristic Cu^II EPR signals of the d x 2 - y 2 ground state in CH₂ Cl₂ /MeOH at 77 K, whereas 2 is EPR-silent. The EPR and X-ray absorption fine structure (XAFS) results suggest that 2 is assigned to the Cu^II -(dimethylamino)phenoxyl radical. However, complex 1 showed different features in the absence of MeOH. The EPR spectrum of the CH₂ Cl₂ solution of 1 exhibits distortion from the d x 2 - y 2 ground state and a temperature-dependent equilibrium between the Cu^II -(dimethylamino)phenolate and the Cu^I -(dimethylamino)phenoxyl radical. From these results, Cu^II -phenoxyl radical complex 2 is concluded to be formed by the reaction of 1 with O₂ via the Cu^I -phenoxyl radical species.

Detection and Characterization of a Novel Copper-Dependent Intermediate in a Lytic Polysaccharide Monooxygenase

Lytic polysaccharide monooxygenases (LPMOs) are copper-containing enzymes capable of oxidizing crystalline cellulose which have large practical application in the process of refining biomass. The catalytic mechanism of LPMOs still remains debated despite several proposed reaction mechanisms. Here, we report a long-lived intermediate (t_1/2 =6-8 minutes) observed in an LPMO from Thermoascus aurantiacus (TaLPMO9A). The intermediate with a strong absorption around 420 nm is formed when reduced LPMO-Cu^I reacts with sub-equimolar amounts of H₂ O₂ . UV/Vis absorption spectroscopy, electron paramagnetic resonance, resonance Raman and stopped-flow spectroscopy suggest that the observed long-lived intermediate involves the copper center and a nearby tyrosine (Tyr175). Additionally, activity assays in the presence of sub-equimolar amounts of H₂ O₂ showed an increase in the LPMO oxidation of phosphoric acid swollen cellulose. Accordingly, this suggests that the long-lived copper-dependent intermediate could be part of the catalytic mechanism for LPMOs. The observed intermediate offers a new perspective into the oxidative reaction mechanism of TaLPMO9A and hence for the biomass oxidation and the reactivity of copper in biological systems.

Facile access to evodiakine enabled by aerobic copper-catalyzed oxidative rearrangement

Oxidation as a fundamentally important method for the synthesis of complex structures is difficult to achieve in a selective manner. Evodiakine, a complex natural product possessing an unprecedented ring system (6/5/5/7/6), has a high oxidation state without a practical solution. Herein, we report the first synthesis of evodiakine via aerobic copper-catalyzed late-stage functionalization of evodiamine.

Theoretical study of aromatic hydroxylation of the [Cu2(H-XYL)O2]2+ complex mediated by a side-on peroxo dicopper core and Cu-ligand effects

In this work, the aromatic hydroxylation mechanism of the [Cu₂(H-XYL)O₂]²⁺ complex mediated by a peroxo dicopper core and Cu-ligand effects are investigated by using hybrid density functional theory (DFT) and the broken symmetry B3LYP method. Based on the calculated free-energy profiles, we proposed two available mechanisms. The first reaction steps of both mechanisms involve concerted O-O bond cleavage and C-O bond formation and the second step involves the Wagner-Meerwein rearrangement of the substrate by a [1,2] H shift (H_A shift from C_A to C_C) or (H_A shift from C_A to O_A) across the phenyl ring to form stable dienone intermediates, and this is followed by the protonation of bridging oxygen atoms to produce the final hydroxylated dicopper(ii) product. The H_A shift from C_A to C_C mechanism is the energetically most favorable, in which the first reaction step is the rate-limiting reaction, with a calculated free-energy barrier of 19.0 kcal mol^-1 and a deuterium kinetic isotope effect of 1.0, in agreement with experimental observations. The calculation also shows that the reaction started from the P-type species of [Cu₂(H-XYL)O₂]²⁺ which is capable of mediating the direct hydroxylation of aromatic substrates without the intermediacy of an O-type species. Finally, we designed some new complexes with different Cu-ligands and found the complex that computationally possesses a higher activity in mediating the hydroxylation of the ligand based aromatic substrate; here, Cu loses a pyridyl ligand donor by dissociation, compared to the [Cu₂(H-XYL)O₂]²⁺ complex.

Theory Demonstrated a "Coupled" Mechanism for O2 Activation and Substrate Hydroxylation by Binuclear Copper Monooxygenases

Multiscale simulations have been performed to address the longstanding issue of "dioxygen activation" by the binuclear copper monooxygenases (PHM and DβM), which have been traditionally classified as "noncoupled" binuclear copper enzymes. Our QM/MM calculations rule out that Cu_M(II)-O₂^• is an active species for H-abstraction from the substrate. In contrast, Cu_M(II)-O₂^• would abstract an H atom from the cosubstrate ascorbate to form a Cu_M(II)-OOH intermediate in PHM and DβM. Consistent with the recently reported structural features of DβM, the umbrella sampling shows that the "open" conformation of the Cu_M(II)-OOH intermediate could readily transform into the "closed" conformation in PHM, in which we located a mixed-valent μ-hydroperoxodicopper(I,II) intermediate, (μ-OOH)Cu(I)Cu(II). The subsequent O-O cleavage and OH moiety migration to Cu_H generate the unexpected species (μ-O^•)(μ-OH)Cu(II)Cu(II), which is revealed to be the reactive intermediate responsible for substrate hydroxylation. We also demonstrate that the flexible Met ligand is favorable for O-O cleavage reactions, while the replacement of Met with the strongly bound His ligand would inhibit the O-O cleavage reactivity. As such, the study not only demonstrates a "coupled" mechanism for O₂ activation by binuclear copper monooxygenases but also deciphers the full catalytic cycle of PHM and DβM in accord with the available experimental data. These findings of O₂ activation and substrate hydroxylation by binuclear copper monooxygenases could expand our understanding of the reactivities of the synthetic monocopper complexes.

High-Valent d7 NiIII versus d8 CuIII Oxidants in PCET

Oxygenases have been postulated to utilize d⁴ Fe^IV and d⁸ Cu^III oxidants in proton-coupled electron transfer (PCET) hydrocarbon oxidation. In order to explore the influence the metal ion and d-electron count can hold over the PCET reactivity, two metastable high-valent metal-oxygen adducts, [Ni^III(OAc)(L)] (1b) and [Cu^III(OAc)(L)] (2b), L = N,N'-(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamidate, were prepared from their low-valent precursors [Ni^II(OAc)(L)]^- (1a) and [Cu^II(OAc)(L)]^- (2a). The complexes 1a/b-2a/b were characterized using nuclear magnetic resonance, Fourier transform infrared, electron paramagnetic resonance, X-ray diffraction, and absorption spectroscopies and mass spectrometry. Both complexes were capable of activating substrates through a concerted PCET mechanism (hydrogen atom transfer, HAT, or concerted proton and electron transfer, CPET). The reactivity of 1b and 2b toward a series of para-substituted 2,6-di-tert-butylphenols (p-X-2,6-DTBP; X = OCH₃, C(CH₃)₃, CH₃, H, Br, CN, NO₂) was studied, showing similar rates of reaction for both complexes. In the oxidation of xanthene, the d⁸ Cu^III oxidant displayed a small increase in the rate constant compared to that of the d⁷ Ni^III oxidant. The d⁸ Cu^III oxidant was capable of oxidizing a large family of hydrocarbon substrates with bond dissociation enthalpy (BDE_C-H) values up to 90 kcal/mol. It was previously observed that exchanging the ancillary anionic donor ligand in such complexes resulted in a 20-fold enhancement in the rate constant, an observation that is further enforced by comparison of 1b and 2b to the literature precedents. In contrast, we observed only minor differences in the rate constants upon comparing 1b to 2b. It was thus concluded that in this case the metal ion has a minor impact, while the ancillary donor ligand yields more kinetic control over HAT/CPET oxidation.

Carboxylate Structural Effects on the Properties and Proton-Coupled Electron Transfer Reactivity of [CuO2CR]2+ Cores

A series of complexes {[NBu₄][LCu^II(O₂CR)] (R = -C₆F₅, -C₆H₄(NO₂), -C₆H₅, -C₆H₄(OMe), -CH₃, and -C₆H₂(ⁱPr)₃)} were characterized (with the complex R = -C₆H₄(m-Cl) having been published elsewhere ( Mandal et al. J. Am. Chem. Soc. 2019 , 141 , 17236 )). All feature N,N',N″-coordination of the supporting L^2- ligand, except for the complex with R = -C₆H₂(ⁱPr)₃, which exhibits N,N',O-coordination. For the N,N',N″-bound complexes, redox properties, UV-vis ligand-to-metal charge transfer (LMCT) features, and rates of hydrogen atom abstraction from 2,4,6,-tri-t-butylphenol using the oxidized, formally Cu(III) compounds LCu^III(O₂CR) correlated well with the electron donating nature of R as measured both experimentally and computationally. Specifically, the greater the electron donation, the lower is the energy for LMCT and the slower is the reaction rate. The results are interpreted to support an oxidatively asynchronous proton-coupled electron transfer mechanism that is sensitive to the oxidative power of the [Cu^III(O₂CR)]²⁺ core.