Sterically Stabilized End-On Superoxocopper(II) Complexes and Mechanistic Insights into Their Reactivity with O-H, N-H, and C-H Substrates

Unusual Stability of an End-on Superoxido Copper(II) Complex under Ambient Conditions

Superoxido copper complexes play an important role as usually short-lived intermediates in biology and chemistry. The unusual stability of an end-on superoxido copper complex observed in an oxygen-enhanced atom transfer radical polymerization (ATRP) led to a detailed mechanistic investigation of the formation of [CuII(Me6tren)(O2⋅-)]+ (Me6tren=tris(2-dimethyl-aminoethyl)amine) under ambient conditions. The persistence of the superoxido copper complex could be explained by a reaction cycle including the peroxido complex [(Me6tren)2CuII 2(O2)]2+ together with [CuI(Me6tren)(DMSO)]+ and [CuII(Me6tren)(OH)]+ in the overall reaction.

Copper-oxygen adducts: new trends in characterization and properties towards C-H activation

This review summarizes the latest discoveries in the field of C-H activation by copper monoxygenases and more particularly by their bioinspired systems. This work first describes the recent background on copper-containing enzymes along with additional interpretations about the nature of the active copper-oxygen intermediates. It then focuses on relevant examples of bioinorganic synthetic copper-oxygen intermediates according to their nuclearity (mono to polynuclear). This includes a detailed description of the spectroscopic features of these adducts as well as their reactivity towards the oxidation of recalcitrant Csp3 -H bonds. The last part is devoted to the significant expansion of heterogeneous catalytic systems based on copper-oxygen cores (i.e. within zeolite frameworks).

Light-Induced Reactivity Switch at O2-Activating Bioinspired Copper(I) Complexes

Using light to unveil unexplored reactivities of earth-abundant metal-oxygen intermediates is a formidable challenge, given the already remarkable oxidation ability of these species in the ground state. However, the light-induced reactivity of Cu-O2 intermediates still remains unexplored, due to the photoejection of O2 under irradiation. Herein, we describe a photoinduced reactivity switch of bioinspired O2-activating CuI complexes, based on the archetypal tris(2-pyridyl-methyl)amine (TPA) ligand. This report represents a key precedent for light-induced reactivity switch in Cu-O2 chemistry, obtained by positioning C-H substrates in close proximity of the active site. Open and caged CuI complexes displaying an internal aryl ether substrate were evaluated. Under light, a Cu-O2 mediated reaction takes place that induces a selective conversion of the internal aryl ether unit to a phenolate-CH2- moiety with excellent yields. This light-induced transformation displays high selectivity and allows easy postfunctionalization of TPA-based ligands for straightforward preparation of challenging heteroleptic structures. In the absence of light, O2 activation results in the standard oxidative cleavage of the covalently attached substrate. A reaction mechanism that supports a monomeric cupric-superoxide-dependent reactivity promoted by light is proposed on the basis of reactivity studies combined with (TD-) DFT calculations.

Spectroscopy of End-On Copper(II) Superoxido Complexes: A Wave Function-Based Analysis

Wave function methods are employed to analyze the ground and low-lying excited states of bipyramid trigonal copper(II) superoxido complexes, up to their characteristic ligand to metal charge transfer band. Several multireference methods have been combined to provide new insights into the interpretation of their experimental absorption spectra. We show that the intraligand transition on the dioxygen leads to a dark state. Among the results, we shall highlight the finding of doubly excited states in the region of the d-d transitions and the subtle interplay between Cu(I) and Cu(II) in the ground and excited states. Some of these findings could be obtained only with multireference methods.

Exploring the antioxidant activity of Fe(III), Mn(III)Mn(II), and Cu(II) compounds in Saccharomyces cerevisiae and Galleria mellonella models of study

Reactive oxygen species (ROS) are closely related to oxidative stress, aging, and the onset of human diseases. To mitigate ROS-induced damages, extensive research has focused on examining the antioxidative attributes of various synthetic/natural substances. Coordination compounds serving as synthetic antioxidants have emerged as a promising approach to attenuate ROS toxicity. Herein, we investigated the antioxidant potential of a series of Fe(III) (1), Mn(III)Mn(II) (2) and Cu(II) (3) coordination compounds synthesized with the ligand N-(2-hydroxybenzyl)-N-(2-pyridylmethyl)[(3-chloro)(2-hydroxy)]-propylamine in Saccharomyces cerevisiae exposed to oxidative stress. We also assessed the antioxidant potential of these complexes in the alternative model of study, Galleria mellonella. DPPH analysis indicated that these complexes presented moderate antioxidant activity. However, treating Saccharomyces cerevisiae with 1, 2 and 3 increased the tolerance against oxidative stress and extended yeast lifespan. The treatment of yeast cells with these complexes decreased lipid peroxidation and catalase activity in stressed cells, whilst no change in SOD activity was observed. Moreover, these complexes induced the Hsp104 expression. In G. mellonella, complex administration extended larval survival under H2O2 stress and did not affect the insect's life cycle. Our results suggest that the antioxidant potential exhibited by these complexes could be further explored to mitigate various oxidative stress-related disorders.

Ligand-Copper(I) Primary O2-Adducts: Design, Characterization, and Biological Significance of Cupric-Superoxides

In this Account, we overview and highlight synthetic bioinorganic chemistry focused on initial adducts formed from the reaction of reduced ligand-copper(I) coordination complexes with molecular oxygen, reactions that produce ligand-CuII(O2•-) complexes (O2•- ≡ superoxide anion). We provide mostly a historical perspective, starting in the Karlin research group in the 1980s, emphasizing the ligand design and ligand effects, structure, and spectroscopy of these O2 adducts and subsequent further reactivity with substrates, including the interaction with a second ligand-CuI complex to form binuclear species. The Account emphasizes the approach, evolution, and results obtained in the Karlin group, a synthetic bioinorganic research program inspired by the state of knowledge and insights obtained on enzymes possessing copper ion active sites which process molecular oxygen. These constitute an important biochemistry for all levels/types of organisms, bacteria, fungi, insects, and mammals, including humans.Copper is earth abundant, and its redox properties in complexes allow for facile CuII/CuI interconversions. Simple salts or coordination complexes have been well known to serve as oxidants for the stoichiometric or catalytic oxidation or oxygenation (i.e., O-atom insertion) of organic substrates. Thus, copper dioxygen- or peroxide-centered synthetic bioinorganic studies provide strong relevance and potential application to synthesis or even the development of cathodic catalysts for dioxygen reduction to hydrogen peroxide or water, as in fuel cells. The Karlin group's focus however was primarily oriented toward bioinorganic chemistry with the goal to provide fundamental insights into the nature of copper-dioxygen adducts and further reduced and/or protonated derivatives, species likely occurring in enzyme turnover or related in one or more aspects of formation, structure, spectroscopic properties, and scope of reactivity toward organic/biochemical substrates.Prior to this time, the 1980s, O2 adducts of redox-active first-row transition-metal ions focused on iron, such as the porphyrinate-Fe centers occurring in the oxygen carrier proteins myoglobin and hemoglobin and that determined to occur in cytochrome P-450 monooxygenase turnover. Deoxy (i.e., reduced Fe(II)) heme proteins react with O2, giving FeIII-superoxo complexes (preferably referred to by traditional biochemists as ferrous-oxy species). And, it was in the 1970s that great strides were made by synthetic chemists in generating hemes capable of forming O2 adducts, their physiochemical characterization providing critical insights to enzyme (bio)chemistry and providing ideas and important goals leading to countless person years of future research.

Bioinspired complexes confined in well-defined capsules: getting closer to metalloenzyme functionalities

Reproducing the key features offered by metalloprotein binding cavities is an attractive approach to overcome the main bottlenecks of current open artificial models (in terms of stability, efficiency and selectivity). In this context, this featured article brings together selected examples of recent developments in the field of confined bioinspired complexes with an emphasis on the emerging hemicryptophane caged ligands. In particular, we focused on (1) the strategies allowing the insulation and protection of complexes sharing similarities with metalloprotein active sites, (2) the confinement-induced improvement of catalytic efficiencies and selectivities and (3) very recent efforts that have been made toward the development of bioinspired complexes equipped with weakly binding artificial cavities.

Mechanistic Insight into Peptidyl-Cysteine Oxidation by the Copper-Dependent Formylglycine-Generating Enzyme

The copper-dependent formylglycine-generating enzyme (FGE) catalyzes the oxygen-dependent oxidation of specific peptidyl-cysteine residues to formylglycine. Our QM/MM calculations provide a very likely mechanism for this transformation. The reaction starts with dioxygen binding to the tris-thiolate Cu^I center to form a triplet Cu^II -superoxide complex. The rate-determining hydrogen atom abstraction involves a triplet-singlet crossing to form a Cu^II -OOH species that couples with the substrate radical, leading to a Cu^I -alkylperoxo intermediate. This is accompanied by proton transfer from the hydroperoxide to the S atom of the substrate via a nearby water molecule. The subsequent O-O bond cleavage is coupled with the C-S bond breaking that generates the formylglycine and a Cu^II -oxyl complex. Moreover, our results suggest that the aldehyde oxygen of the final product originates from O₂ , which will be useful for future experimental work.

ROS scavenging of SOD/CAT mimics probed by EPR and reduction of lipid peroxidation in S. cerevisiae and mouse liver, under severe hydroxyl radical stress condition

The interaction between Cu^II, Fe^III and Mn^II complexes, derived from the ligands 1-[bis(pyridine-2-ylmethyl)amino]-3-chloropropan-2-ol (hpclnol) and bis(pyridine-2-ylmethyl)amine (bpma), and the free radical 2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl (DPPH) and reactive oxygen species (ROS), was investigated by colorimetric and EPR (Electron Paramagnetic Resonance) techniques. A comparison between these results and those reported to [Mn(salen)Cl] or EUK-8 was also addressed. EPR studies allowed us the identification of intermediates species such as superoxide‑copper(I) and superoxide‑copper(II), a mixed-valence Fe^IIIFe^II species and a 16-line feature attributed to Mn^III-oxo-Mn^IV species. The biomarker malondialdehyde (MDA) was determined by TBARS assay in S. cerevisiae cells, and the determination of the IC₅₀ indicate that the antioxidant activity shown dependence on the metal center (Cu^II ≈ Fe^III > Mn^II ≈ [Mn(salen)Cl]. The lipid peroxidation attenuation was also investigated in liver homogenates obtained from Swiss mice and the IC₅₀ values were in the nanomolar concentrations. We demonstrated here that all the complexes interact with the free radical DPPH and with ROS (H₂O₂, O₂^•- and hydroxyl radical), enhancing the cellular protection against oxidative stress generated by hydroxyl radical, employing two experimental model systems, S. cerevisiae (in vivo) and mouse liver (ex vivo).

Generation and Aerobic Oxidative Catalysis of a Cu(II) Superoxo Complex Supported by a Redox-Active Ligand

Cu systems feature prominently in aerobic oxidative catalysis in both biology and synthetic chemistry. Metal ligand cooperativity is a common theme in both areas as exemplified by galactose oxidase and by aminoxyl radicals in alcohol oxidations. This has motivated investigations into the aerobic chemistry of Cu and specifically the isolation and study of Cu-superoxo species that are invoked as key catalytic intermediates. While several examples of complexes that model biologically relevant Cu(II) superoxo intermediates have been reported, they are not typically competent aerobic catalysts. Here, we report a new Cu complex of the redox-active ligand ^tBu,TolDHP (2,5-bis((2-t-butylhydrazono)(p-tolyl)methyl)-pyrrole) that activates O₂ to generate a catalytically active Cu(II)-superoxo complex via ligand-based electron transfer. Characterization using ultraviolet (UV)-visible spectroscopy, Raman isotope labeling studies, and Cu extended X-ray absorption fine structure (EXAFS) analysis confirms the assignment of an end-on κ¹ superoxo complex. This Cu-O₂ complex engages in a range of aerobic catalytic oxidations with substrates including alcohols and aldehydes. These results demonstrate that bioinspired Cu systems can not only model important bioinorganic intermediates but can also mediate and provide mechanistic insight into aerobic oxidative transformations.

A caged tris(2-pyridylmethyl)amine ligand equipped with a Ctriazole-H hydrogen bonding cavity

A capped bioinspired ligand built from a tris(2-pyridyl-methyl)amine (TPA) unit and surmounted by a triazole-based intramolecular H-bonding secondary sphere, was prepared. The resulting cage structure describes a well-defined cavity combining...