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

Control of Chemo-Selectivity via Alcohol-Affected Kinetics in Cu-Hydroxylamine Catalyzed Aerobic Oxidation of Mesitol

Chemo-selectivity control is a critical challenge in aerobic C-H oxidations, particularly in preventing overoxidation. In this work, we present an alcohol-tunable strategy to control the oxidation degree of CuCl₂/NH₂OH·HCl-catalyzed mesitol oxidation in alcohols. In tBuOH, the reaction efficiently yields the aldehyde product with high selectivity, whereas in MeOH, the ether intermediate turned out to be the predominant product. Further kinetic analysis and mechanistic studies revealed that the reactivity is driven by the in-situ formation of protonated alkyl nitrate ([RON(O)OH]+), highlighting the critical role of ROH solvents. The differing responses of mesitol and the ether intermediate to the reaction conditions result in distinct kinetics across different alcohols, enabling precise control over the final products. These findings provide mechanistic insights into the origins of alcohol-dependent chemo-selectivity and pave the way for advancing protocols for selectivity control.

Regiospecific arene C-H self-hydroxylation in pentadentate ligand via activation of atmospheric dioxygen utilizing Co(II) precursors under ambient reaction conditions: experimental and DFT optimized studies

The activation of dioxygen on redox-active metal centres is an important area of study in bioinorganic chemistry. One of the key challenges in this field is the activation of atmospheric dioxygen under ambient conditions. In this study, we report the ligand-induced activation of atmospheric dioxygen using cobalt(II) precursors and the oxygenation of arene C-H bonds within a pentadentate ligand under ambient conditions. Herein, a novel pentadentate ligand (BPMAP-H = (E)-2-(bis(pyridin-2-ylmethyl)amino)-N-(2-(phenyldiazenyl)phenyl)acetamide) having carboxamide and azo donor groups was synthesized and utilized for dioxygen activation. A number of cobalt(II) precursors, with BPMAP-H, provide [Co(III)BPMAP-O]ClO4 complex Co-1 with oxidation at the metal centre and phenyl ring oxygenation in the ligand backbone. Both the ligand BPMAP-H and complex Co-1 were well characterized by UV-visible spectroscopy, IR spectroscopy, 1H NMR, 13C NMR, 13C DEPT-135, HRMS, and single-crystal XRD. Several controlled experiments and DFT calculations were performed for mechanistic investigation and the in situ formed cobalt(III)-superoxide was characterized as a key intermediate with the help of EPR studies.

Oxidation-Deformylation Cascade Catalyzed By a Mononuclear Copper Complex

In this study, two copper complexes were synthesized using N3 (arising from two pyridines and one amide group) containing ligands N-(2-picolyl)picolinamide (L1H) and bis(2-pyridylcarbonyl)amine (L2H), forming [(L1)CuII(OH2)(NO3)] (1) and [(L2)CuII(OH2)2](NO3) (2). The reaction of complex 1 with hydrogen peroxide in alcoholic solvents yielded a formate-bound complex. Studies with isotopically labeled 13C ethanol indicated that formate originates from the C1 of ethanol after C-C bond cleavage. Complex 1 was found to catalytically convert primary alcohols into formic acid probably following a two-step process: (i) alcohol oxidation to aldehyde and (ii) aldehyde deformylation. Further experiments with 2-phenylpropionaldehyde (2-PPA) confirm the ability of complex 1 to catalyze aldehyde deformylation. Both steps of the reaction are associated with significant kinetic deuterium isotope effects (KDIE), suggesting that hydrogen atom abstractions (HAA) occur during the rate-determining steps of both conversions. Overall, this system proposes a clean catalytic process for alcohol-to-formic acid conversion, operating under mild conditions, and offering potential synthetic applications.

Mechanical and Covalent Tailoring of Copper Catenanes for Selective Aqueous Nitrate-to-Ammonia Electrocatalysis

Electrocatalytic nitrate reduction reaction (NO3RR) for the selective generation of ammonia (NH3) enables the removal of deleterious nitrate pollutants while simultaneously upcycling them into a value-added fertilizer. The development of nonprecious metal-derived catalysts such as those featuring copper (Cu) as earth-abundant alternatives for the state-of-the-art precious metal catalysts is of urgent need yet suffering from the activity-selectivity-durability trilemma. Rational design of molecular Cu complexes with well-defined coordination structures permitting systematic structure-activity relationship (SAR) investigations is key to addressing the challenge. Here, a series of molecular Cu(I) complexes with [2]catenane ligands are developed as NO3RR electrocatalysts for the first time. By engineering multiple cationic ammoniums on the catenane backbone, acceptance of the anionic nitrate substrate as well as the release of the cationic ammonium product are promoted, thereby facilitating a higher Faradaic efficiency and product selectivity toward ammonia via an 8e- pathway. Of note, the mutual Coulombic repulsion between the multiply charged ligands is overcome by the mechanical interlocking such that the catalyst integrity can be maintained under practical conditions. This report highlights the promise of employing mechanically interlocked ligands as a platform for customizing metal complexes as catalysts for redox processes involving multiple proton-coupled electron transfer steps.

Tuning Reactivity in Cu/TEMPO Catalyzed Alcohol Oxidation Reactions

A dinuclear copper(I) complex Cu2L22 (L2 = 3,3-dimethyl-1-(1-methyl-1H-benzo[d]imidazole-2-yl)-N-(propan-2-ylidene)butan-2-amine) containing benzimidazole and imino donors was previously reported by some of us as an efficient catalyst for the aerobic oxidation of alcohols to aldehydes in presence of TEMPO (2,2,6,6-tetramethylpiperidinyloxyl) and an external base NMI (N-methyl imidazole). Cu(III)2(bis-μ-oxo) and Cu(II)2(bis-μ-hydroxo) cores were trapped as viable intermediates in the reaction, which provided deeper mechanistic insights. Here, we report two new ligand systems L3 (N-isopropyl-3,3-dimethyl-1-(1-methyl-1H-benzol[d]imidazole-2-yl)butane-2-amine) and L4 ((Z)-2,4-di-tert-butyl-6-(((3,3-dimethyl-1-(1-methyl-1H-benzol[d]imidazole-2-yl)butane-2-yl)imino)methyl)phenol), which are designed to perturb the overall electronics of the complexes and the resulting effects on their O2 activation mechanisms. The stronger donation of the secondary amine group stabilizes a mononuclear CuIL3 core, which nevertheless follows a dinuclear O2 activation mechanism as in Cu2L22. Notably, the CuIL3/TEMPO catalyst system performs the aerobic oxidation of alcohols to aldehydes with good yields and turnover numbers, even in the absence of NMI. The dinuclear CuI 2L42 complex involving a non-innocent phenolate group, in contrast, exhibits depleted catalytic activity, because of the instability of the Cu(III)2(bis-μ-oxo) core against intramolecular H-atom abstraction to form an alkoxo bridged dicopper(II) complex.

Controlling outer-sphere solvent reorganization energy to turn on or off the function of artificial metalloenzymes

Metalloenzymes play essential roles in biology. However, unraveling how outer-sphere interactions can be predictably controlled to influence their functions remains a significant challenge. Inspired by Cu enzymes, we demonstrate how variations in the primary, secondary, and outer coordination-sphere interactions of de novo designed artificial copper proteins (ArCuPs) within trimeric (3SCC) and tetrameric (4SCC) self-assemblies-featuring a trigonal Cu(His)3 and a square pyramidal Cu(His)4(OH2) coordination-influence their catalytic and electron transfer properties. While 3SCC electrocatalyzes C-H oxidation, 4SCC does not. CuI-3SCC reacts more rapidly with H2O2 than O2, whereas 4SCC is less active. Electron transfer, reorganization energies, and extended H2O-mediated hydrogen bonding patterns provide insights into the observed reactivity differences. The inactivity of 4SCC is attributed to a significant solvent reorganization energy barrier mediated by a specific His---Glu hydrogen bond. When this hydrogen bond is disrupted, the solvent reorganization energy is reduced, and C-H peroxidation activity is restored.

Bioinspired copper(II) complexes catalyzed oxidative coupling of aminophenols with broader substrate scope

The strategic selection of ligand systems in metal complexes has demonstrated a profound impact on the efficiency and specificity of biomimetic reactions. In this work, we introduce a series of aminoquinoline-based copper(II) complexes (1-4) distinguished by systematic variation in terminal amine substituents: di-n-methyl (L1(H)), di-n-ethyl (L2(H)), di-n-propyl (L3(H)), and di-n-butyl (L4(H)). These complexes are synthesized, characterized, and evaluated as the catalyst for the oxidative coupling of different aminophenol derivatives. Remarkably, complex 1, featuring a methyl substituent, exhibited unparalleled catalytic performance, achieving an 86 % (Kcat - 9.7 × 104 h-1) conversion of o-aminophenol to the desired product, 2-amino-phenoxazin-3-one, alongside water and hydrogen peroxide as byproducts. Notably, complex 1 demonstrated exceptional versatility, extending its catalytic activity to other substrates with remarkable activity. Mechanistic investigations, supported by mass-spectrometric analysis, revealed the formation of a complex-substrate adduct with all substrates, enabling us to propose a detailed reaction pathway. The work highlights the benefits of ligand design in improving catalytic performance and sets a new standard for aminoquinoline-based copper(II) complexes in oxidative coupling reactions. To the best of our knowledge, this work is the first to report a wider substrate scope for PHS activity with copper(II) complexes.

Influence of Hemilabile Arm and Amide Functionality in the Ligand Backbone on Chemical and Electrochemical Dioxygen Reduction Catalyzed by Mononuclear Copper(II) Complexes

Four mononuclear copper(II) complexes, [(DPA-OH)Cu(CH3OH)(ClO4)](ClO4) (1), [(DPA-OMe)Cu(CH3OH)(ClO4)](ClO4) (2), [(6-Amide-DPA-OMe)Cu](ClO4)2 (3) and [(6-Amide2-DPA-OMe)Cu](ClO4)2 (4), of flexidentate ligands bearing hemilabile (hydroxy)methoxyethyl and/or amide group on DPA (di(2-picolyl)amine) backbone were isolated to explore their potency in catalyzing chemical and electrochemical oxygen reduction reaction (ORR). The role of a hemilabile arm, as well as amide functionality on the ligand backbone in affecting the rate-determining step (RDS) of the overall catalytic cycle has been explored and compared with that of the analogous [(tmpa)Cu](ClO4)2 (tmpa = tris(2-pyridylmethyl)amine) and [(PV-tmpa)Cu](ClO4)2 (PV-tmpa = bis(pyrid-2-ylmethyl){[6-(pivalamido)pyrid-2-yl]methyl}-amine) complexes. The hemilabile arm in these complexes results in an overall third-order rate, with kcat values ranging from 103 to 104 M-2s-1 during dioxygen reduction catalysis. All the complexes except complex 4 selectively reduce dioxygen via the 4e-/4H+ reduction pathway to water (H2O) using decamethylferrocene (Fc*) as a sacrificial reductant in acidic acetone at 298 K. In contrast, altering the reaction conditions from a chemical to an electrochemical ambiance in phosphate buffer displays a reverse order of ORR activity of the complexes while maintaining the same product selectivity as observed in chemical ORR catalysis in acetone.

Activation of Molecular Oxygen and Selective Oxidation with Metal Complexes

ConspectusSelective oxidation with molecular oxygen is one of the most appealing approaches to functionalization of organic molecules and, yet at the same time, one of the most challenging reactions facing organic synthesis due to poor selectivity control. Molecular oxygen is a green and inexpensive oxidant, producing water as the only byproduct in oxidation. Not surprisingly, it has been used in the manufacturing of many commodity chemicals in the industry. It is also nature's choice of oxidant and drives a variety of oxidation reactions critical to life and various other biologic processes. While the past decades have witnessed great progress in understanding, both structurally and mechanistically, how nature exploits metalloenzymes, i.e., monooxygenases and dioxygenases, to tackle some of the most challenging oxidation reactions, e.g., methane oxidation to methanol, there are only a small number of well-defined, man-made metal complexes that have been reported to enable selective oxidation with molecular oxygen of compounds more relevant to fine chemical and pharmaceutical synthesis.In the past 10 years or so, our laboratories have developed several transition metal complexes and shown that they are capable of catalyzing selective oxidation under 1 atm of O2. Thus, we have shown that an Fe(II)-bisimidazolidinyl-pyridine complex catalyzes selective oxygenation of C-H bonds in ethers with concomitant release of hydrogen gas instead of water, and when the iron center is replaced with Fe(III), selective oxidative cleavage of C═C bonds of olefins becomes feasible. To address the low activity of the iron complex in oxidizing less active olefins, we have developed a Mn(II)-bipyridine complex, which catalyzes oxidative cleavage of C═C bonds in aliphatic olefins, C-C bonds in diols, and carboxyl units in carboxylic acids under visible light irradiation. Light is necessary in the oxidation to cleave an off-cycle, inactive manganese dimer into a catalytically active Mn═O oxo species. Furthermore, we have found that a binuclear salicylate-bridged Cu(II) complex enables the C-H oxidation of tetrahydroisoquinolines as well as C═C bond cleavage, and when a catalytic vitamin B1 analogue is brought in, oxygenation of tetrahydroisoquinolines to lactams takes place via carbene catalysis. Still further, we have found that a readily accessible binuclear Rh(II)-terpyridine complex catalyzes the oxidation of alcohols, and being water-soluble, the catalyst can be easily separated and reused multiple times. In addition, we recently unearthed a simple protocol that allows waste polystyrene to be depolymerized to isolable, valuable chemicals. A cheap Brønsted acid acts as the catalyst, activating molecular oxygen to a singlet state through complexation with the polymer under light irradiation, thereby depolymerizing the polymer.

High-Valent Cobalt-Difluoride in Oxidative Fluorination of Saturated Hydrocarbons

The heme paradigm where Fe=O acts as the C-H oxidant and Fe-OH rebounds with the formed carbon-centered radical guides the design of the prototypical synthetic hydroxylation catalyst. We are exploring methods to evolve beyond the metal-oxo oxidant and hydroxide rebound, to incorporate a wider array of functional group. We have demonstrated the application of CoII(OTf)2 (10 mol% catalyst; OTf=trimfluoromethanesulfonate) in combination with polydentate N-donor ligands (e. g. BPMEN=N,N'-dimethyl-N,N'-bis(pyrid-2-ylmethyl)ethane-1,2-diamine) and Selectfluor in the oxidative fluorination of saturated hydrocarbons in high yields. The addition of CsF to the reaction mixture induced near-quantitative yields of fluorinated saturated hydrocarbons (>90 % yield of fluorinated product). For 1-hydroxy, 1-acetyl, 1-carboxy-, and 1-acetamido-adamantane, we demonstrated selective fluorination at the 3-position. We propose two mechanisms for the CoII-catalyzed reaction: either (i) an N-radical, derived from Selectfluor, acted as the C-H oxidant followed by radical rebound with CoIII-F; or (ii) a CoIV-(F)2 species was the C-H oxidant followed by radical rebound with CoIII-F. Our combined spectroscopic, kinetic, and chemical trapping evidence suggested that an N-radical was not the active oxidant. We concluded that a CoIV-(F)2 species was the likely active oxidant and CoIII-F was the likely F-atom donor to a carbon centered radical producing a C-F bond.

Activity-based sensing reveals elevated labile copper promotes liver aging via hepatic ALDH1A1 depletion

Oxidative stress plays a key role in aging and related diseases, including neurodegeneration, cancer, and organ failure. Copper (Cu), a redox-active metal ion, generates reactive oxygen species (ROS), and its dysregulation contributes to aging. Here, we develop activity-based imaging probes for the sensitive detection of Cu(I) and show that labile hepatic Cu activity increases with age, paralleling a decline in ALDH1A1 activity, a protective hepatic enzyme. We also observe an age-related decrease in hepatic glutathione (GSH) activity through noninvasive photoacoustic imaging. Using these probes, we perform longitudinal studies in aged mice treated with ATN-224, a Cu chelator, and demonstrate that this treatment improves Cu homeostasis and preserves ALDH1A1 activity. Our findings uncover a direct link between Cu dysregulation and aging, providing insights into its role and offering a therapeutic strategy to mitigate its effects.

Construction of a reaction-based fluorescent sensor for tandem detection of Cu2+ and glutathione in wine

The purpose of this study was to develop a novel reaction-based fluorescent sensor for the detection of Cu2+ and glutathione in real wine samples. The sensor, tris-(2-pyridyl)-methylamine rhodol derivative, was synthesized and validated for the tandem and selective detection of both Cu2+ and glutathione. The sensor exhibited a strong linear correlation between fluorescence intensity and Cu2+ concentration ranging from 100 to 900 nM, while the in situ generated Cu2+ ensemble selectively detected glutathione with a robust linear response from 3 to 30 μM. The detection limits for Cu2+ and glutathione were as low as 28 nM and 0.60 μM, respectively. Additionally, the sensor enabled quantitative detection of Cu2+ and glutathione in real wine samples. This work provides the first reaction-based fluorescence sensor with an "on-off-on" fluorescence response for the tandem detection of Cu2+ and glutathione in wine, offering potential applications in food and beverage quality control.

Copper-Induced Neurodegenerative Disorders and Therapeutic Potential of Curcumin-Loaded Nanoemulsion

Copper accumulation in neurons induces oxidative stress, disrupts mitochondrial activity, and accelerates neuronal death, which is central to the pathophysiology of neurodegenerative diseases like Wilson disease. Standard treatments for copper toxicity, such as D-penicillamine, trientine, and chloroquine, are frequently associated with severe side effects, creating a need for safer therapeutic alternatives. To address this, we developed a curcumin-loaded nanoemulsion (CUR-NE) using the spontaneous emulsification technique, aimed at enhancing the bioavailability and therapeutic efficacy of curcumin. The optimized nanoemulsion displayed a particle size of 76.42 nm, a zeta potential of -20.4 mV, and a high encapsulation efficiency of 93.69%, with a stable and uniform structure. The in vitro tests on SH-SY5Y neuroblastoma cells demonstrated that CUR-NE effectively protected against copper-induced toxicity, promoting significant cellular uptake. Pharmacokinetic studies revealed that CUR-NE exhibited a longer half-life and extended circulation time compared to free curcumin. Additionally, pharmacodynamic evaluations, including biochemical assays and histopathological analysis, confirmed that CUR-NE provided superior neuroprotection in copper overload conditions. These results emphasize the ability of CUR-NE to augment the therapeutic effects of curcumin, presenting a novel approach for managing copper-induced neurodegeneration. The study highlights the effectiveness of nanoemulsion-based delivery platforms in improving chelation treatments for neurological diseases.

Reactivity of a heterobinuclear heme-peroxo-Cu complex with para-substituted catechols shows a pK a-dependent change in mechanism

In biological systems, heme-copper oxidase (HCO) enzymes play a crucial role in the oxygen reduction reaction (ORR), where the pivotal O-O bond cleavage of the (heme)FeIII-peroxo-CuII intermediate is facilitated by active-site (peroxo core) hydrogen bonding followed by proton-coupled electron transfer (PCET) from a nearby (phenolic) tyrosine residue. A useful approach to comprehend the fundamental relationships among H-bonding/proton/H-atom donors and their abilities to induce O-O bond homolysis involves the investigation of synthetic, bioinspired model systems where the exogenous substrate properties (such as pK a and bond dissociation energy (BDE)) can be systematically altered. This report details the reactivity of a heme-peroxo-copper HCO model complex (LS-4DCHIm) toward a series of substituted catechol substrates that span a range of pK a and O-H bond BDE values, exhibiting different reaction mechanisms. Considering their interactions with the bridging peroxo ligand in LS-4DCHIm, the catechol substrates are importantly capable of one or two (i) H-bonds, (ii) proton transfers, and/or (iii) net H-atom transfers, thereby making them attractive, yet complex candidates for studying the redox chemistry of the metal-bound peroxide. A combination of spectroscopic studies and kinetic analysis implies that the suitable modulation of pK a and O-H bond BDE values of catechols result in either double proton transfer with the release of H2O2 or double PCET resulting in reductive O-O bond rupture. The distinguishing role of substrate properties in directing the mechanism and outcome of O2 protonation/reduction reactions is discussed in terms of designing O2-reduction catalysts based on biological inspiration.

Selective hydroxylation of benzene to phenol via CuII(μ-O˙)CuII intermediate using a nonsymmetric dicopper catalyst

The one-step oxidation of benzene to phenol represents a significant and promising advancement in modern industries focused on the production of high-value-added chemical products. Nevertheless, challenges persist in achieving sufficient catalytic selectivity and preventing over-oxidation. Inspired by copper enzymes, we present a nonsymmetric dicopper complex ([CuII2(TPMAN)(μ-OH)(H2O)]3+, 1) for the selective oxidation of benzene to phenol. Utilizing H2O2 as the oxidant, complex 1 demonstrates remarkable catalytic activity (a TON of 14 000 within 29 hours) and selectivity exceeding 97%, comparable to the finest homogeneous catalyst derived from first-row transition metals. It is noteworthy that the significant substituent effect, alongside a negligible kinetic isotope effect (KIE = 1.05), radical trapping experiments, and an inconsistent standard selectivity test of the ˙OH radicals, all contradict the conventional Fenton mechanism and rebound pathway. Theoretical investigations indicate that the active CuII(μ-O˙)CuII-OH species generated through the cleavage of the O-O bond in the CuII(μ-1,1-OOH)CuI intermediate facilitates the hydroxylation of benzene via an electrophilic attack mechanism. The nonsymmetric coordination geometry is crucial in activating H2O2 and in the process of O-O bond cleavage.

Imidazolate-Stabilized Cu(III): Dioxygen to Oxides at Type 3 Copper Sites

Imidazole ligation of metals through histidine is extensive among metalloproteins, yet the role of the imidazolate conjugate base is often neglected, despite its potential accessibility when bonded to an oxidized metal center. Using synthetic models of oxygenated tyrosinase enzymes ligated exclusively by monodentate imidazoles, we find that deprotonation of the μ2-η2:η2-peroxidodicopper(II) species triggers redox isomerization to an imidazolate-ligated bis(μ2-oxido)dicopper(III) species. Formal two-electron oxidation to Cu(III) remains biologically unprecedented, yet is effected readily by addition of base. Spectrophotometric titrations by UV/Visible/near-IR and copper K-edge X-ray absorption spectroscopies are interpreted most simply as two cooperative, 2H+ transformations in which the peroxide O-O is cleaved in the first step. Elaboration from simple imidazoles to a protected histidine extends this isomerization into an amino acid environment. The role of phenolate as a base suggests this four-electron reduction of O2 is energetically viable in a biological context and requires only two copper centers, which act as two-electron shuttles when imidazole deprotonation assists. This existential precedent of viable imidazolate intermediates invites speculation into an alternative mechanism for phenol hydroxylation not previously considered at Type 3 copper sites such as tyrosinases. Structural biological evidence suggests imidazolate ligation of copper may be more widespread than generally understood.

Structural Snapshots, Trajectory and Thermodynamics of the Reversible μ-1,2-Peroxo/μ-1,1-Hydroperoxo Dicopper(II) Interconversion

Hydrogen bonds involving the oxygen atoms of intermediates that result from copper-mediated O2 activation play a key role for controlling the reactivity of Cux/O2 active sites in metalloenzymes and synthetic model complexes. However, structural insight into H-bonding in such transient species as well as thermodynamic information about proton transfer to or from the O2-derived ligands is scarce. Here we present a detailed study of the reversible interconversion of a μ1,2-peroxodicopper(II) complex ([1]+) and its μ1,1-hydroperoxo congener ([2]+) via (de)protonation, including the isolation and structural characterization of several H-bond donor (HBD) adducts of [1]+ and the determination of binding constants. For one of these adducts a temperature-dependent μ1,2-peroxo/μ1,1-hydroperoxo equilibrium associated with reversible H+-translocation is observed, its thermodynamics investigated experimentally and computationally, and effects of H-bonding on spectroscopic parameters of the CuII 2(μ1,2-O2) species are revealed. DFT calculations allowed to fully map and correlate the trajectories of H+-transfer and μ1,2-peroxo→μ1,1-peroxo rearrangement. These findings enhance our understanding of two key intermediates in bioinspired Cu2/O2 chemistry.

How does dopamine convert into norepinephrine? Insights on the key step of the reaction

CONTEXT

Dopamine β -monooxygenase (D β M) is an essential enzyme in the organism that regioselectively converts dopamine into R-norepinephrine, the key step of the reaction, studied in this paper, is a hydrogen atom transfer (HAT) from dopamine to a superoxo complex on D β M, forming a hydroperoxo intermediate and dopamine radical. It was found that the formation of a hydrogen bond between dopamine and the D β M catalyst strengthens the substrate-enzyme interaction and facilitates the HAT which takes place selectively to give the desired enantiomeric form of the product. Six reactions leading to the hydroperoxo intermediate were analyzed in detail using theoretical and computational tools in order to identify the most probable reaction mechanism. The reaction force analysis has been used to demonstrate that the nature of the activation energy is mostly structural and largely due to the initial approach of species in order to get closer to each other to facilitate the hydrogen abstraction. On the other hand, the reaction electronic flux revealed that electronic activity driving the reactions is triggered by polarization effects and, in the most probable reaction among the six studied, it takes place in a concerted and non-spontaneous way. Chemical events driving the reaction have been identified and the energy absorbed or delivered by each one was quantified in detail.

METHODS

The dopamine and a computational model of the copper superoxo complex on D β M were optimized at B3LYP-D3(BJ)/6-311 G(d,p) level theory in the Gaussian 16 software package. Optimization and IRC calculations were performed in the gas phase and through the PCM solvation model to mimic the protein medium. Non-covalent interactions were plotted using the NCI-plot software.

Incidental iron oxide nanoclusters drive confined Fenton-like detoxification of solid wastes towards sustainable resource recovery

The unique properties of nanomaterials offer vast opportunities to advance sustainable processes. Incidental nanoparticles (INPs) represent a significant part of nanomaterials, yet their potential for sustainable applications remains largely untapped. Herein, we developed a simple strategy to harness INPs to upgrade the waste-to-resource paradigm, significantly reducing the energy consumption and greenhouse gas emissions. Using the recycling of fly ash from municipal solid waste incineration (MSWI) as a proof of concept, we reveal that incidental iron oxide nanoclusters confined inside the residual carbon trigger Fenton-like catalysis by contacting H2O2 at circumneutral pH (5.0-7.0). This approach efficiently detoxifies the adsorbed dioxins under ambient conditions, which otherwise relies on energy-intensive thermal methods in the developed recovery paradigms. Collective evidence underlines that the uniform distribution of iron oxide nanoclusters within dioxin-enriched nanopores enhances the collision between the generated active oxidants and dioxins, resulting in a substantially higher detoxification efficiency than the Fe(II)-induced bulk Fenton reaction. Efficient and cost-effective detoxification of MSWI fly ash at 278‒288 K at pilot scale, combined with the satisfactory removal of adsorbed chemicals in other solid wastes unlocks the great potential of incidental nanoparticles in upgrading the process of solid waste utilization and other sustainable applications.

Dimeric Copper(I) Complex with a Disulfonamide-Bridged Core

Pyridine-2-yl-sulfonyl-quinolin-8-yl-amide (psq) has produced the first sulfonamidato-bridged dicopper(I) complex, {Cu[κ4-(μ-κN:κN-psq)]}2 containing the rhombic Cu(I)2N2 core. The single crystal X-ray structure of this complex shows that two anionic psq ligands straddle the metal atoms via bridging sulfonamide N atoms to give a Cu···Cu distance of 2.9593(8) Å. When it is dissolved in chloroform [Cu(psq)]2 activates C-Cl bonds as demonstrated through the rapid formation of [CuCl(psq)]2. While the solid orange compound is stable for weeks under N2. Acetonitrile solutions of [Cu(psq)]2 are rapidly oxidized in air. A 1D-carbonato-bridged coordination polymer, {Cu2[κ4-(μ-κO:κN-psq)]2[μ3-CO3][H2O]}, and the bis-homoleptic complex [Cu(κ3-psq)(κ2-psq)] are concurrently isolated in high yield without evidence of ligand oxidation with H2O2 detected as a side product. This implicit O2 activation was harnessed in the oxidation of phenol substrates. 2,6-Di-tert-butylphenol is catalytically converted to the coupled dione product with 100% yield. If a para-blocked phenol is used, the reaction become stoichiometric and an O2-derived hydroperoxide group is installed into the ortho position. In contrast, nitrophenol is not oxidized and the result is metal-based oxidation and isolation of [Cu(OC6H4NO2)(psq)]2. This is rationalized by this more acidic phenol acting as a proton donor, rather than a H atom donor to a putative O2 adduct.

pH-mediated manipulation of the histidine brace in LPMOs and generation of a tri-anionic variant, investigated by EPR, ENDOR, ESEEM and HYSCORE spectroscopy

Lytic Polysaccharide Monooxygenases (LPMOs) catalyze the oxidative depolymerization of polysaccharides at a monocopper active site, that is coordinated by the so-called histidine brace. In the past, this motif has sparked considerable interest, mostly due to its ability to generate and stabilize highly oxidizing intermediates during catalysis. We used a variety of advanced EPR techniques, including Electron Nuclear Double Resonance (ENDOR), Electron Spin Echo Envelope Modulation (ESEEM) and Hyperfine Sublevel Correlation (HYSCORE) spectroscopy in combination with isotopic labelling (15N, 2H) to characterize the active site of the bacterial LPMO SmAA10A over a wide pH range (pH 4.0-pH 12.5). At elevated pH values, several ligand modifications are observed, including changes in the H x O ligand coordination, but also regarding the protonation state of the histidine brace. At pH > 11.5, the deprotonation of the two remote nitrogen nuclei of the imidazole moieties and of the terminal amine is observed. These deprotonations are associated with major electronic changes, including increased σ-donor capabilities of the imidazolates and an overall reduced interaction of the deprotonated amine function. This observation highlights a potentially more significant role of the imidazole ligands, particularly for the stabilization of potent oxidants during turnover. The presented study demonstrates the application of advanced EPR techniques for a thorough characterization of the active site in LPMOs, which ultimately sets a foundation for and affords an outlook on future applications characterizing reaction intermediates.

Selective oxidation of active site aromatic residues in engineered Cu proteins

Recent studies have revealed critical roles for the local environments surrounding metallocofactors, such as the newly identified CuD site in particulate methane monooxygenases (pMMOs) and the second sphere aromatic residues in lytic polysaccharide monooxygenases (LPMOs), implicated in the protection against oxidative damage. However, these features are subjects of continued debate. Our work utilizes biotin-streptavidin (Sav) technology to develop artificial metalloproteins (ArMs) that mimic the active sites of natural copper metalloenzymes. By engineering ArMs with aromatic residues within their secondary coordination spheres, we systematically investigate the influence of these residues on Cu reactivity and oxidant activation. We demonstrate that the placement and orientation of tyrosine relative to the Cu cofactor critically affect the oxidation outcomes upon exposure to hydrogen peroxide. A key finding is the interplay between the coordination of an active site asparagine and the incorporation of aromatic residues proximal to the artificial Cu cofactor, which are the only variants where oxidation of an engineered residues is observed. These findings underscore the importance of the secondary coordination sphere in modulating Cu center reactivity, suggest a role for amide coordination in C-H bond activation by pMMOs, and potential inactivation pathways in natural copper enzymes like LPMOs.

Quantum effects in CH activation with [Cu2O2]2+ complexes

We investigate the mechanism of primary alkane CH bond activation with dioxo-dicopper ([Cu2O2]2+) complexes, which serve as model catalysts for enzymes capable of activating CH bonds under mild conditions. As large H/D kinetic isotope effects (KIEs) are observed in enzymes and their synthetic mimics, we employ density functional theory along with variational transition-state theory with multidimensional tunneling to estimate reaction rate coefficients. By systematically varying ligand electrophilicity and substrate chain length, we examine trends in rate coefficients and kinetic isotope effects for the two proposed CH activation pathways - one-step oxo-insertion and two-step radical recombination. Although larger tunneling transmission coefficients are obtained for the radical pathway, the oxo-insertion mechanism yields higher rate coefficients on account of lower activation barriers. The question of the preferred CH activation mechanism, however, remains open: excellent agreement is observed between the predicted and known experimental KIE results for the radical pathway, while calculated Hammett slopes for the oxo-insertion pathway closely mirror experiments.

Three Distinct Oxidation States (II/II, II/III, and III/III) of Diorganocopper Complexes

In this report, we present a structurally and spectroscopically characterized diorganocopper system in three distinct oxidation states: [CuIICuII] (1), [CuIICuIII] (2), and [CuIIICuIII] (3). These states are stabilized by a macrocyclic ligand scaffold featuring two square-planar coordination {C2 NHCN2 pyrazole}. We have analyzed the geometric and electronic structures using X-ray diffraction (XRD) and multiple spectroscopic methods including nuclear magnetic resonance (NMR), UV-vis, and electron paramagnetic resonance (EPR) spectroscopies, in combination with density functional theory (DFT) calculations. Remarkably, this study provides a structural determination of mixed-valence diorganocopper(II,III) complex 2, which is at the borderline between valence-trapped or charge-localized class I systems and charge moderately delocalized class II systems in Robin and Day classification. These findings enhance our understanding of the systematic structural and electronic changes that occur in diorganocopper complexes in response to redox transformations.

Evaluation of the Role of [{Cu(PMDETA)}2(O2 2-)]2+ in Open-Air Photo ATRP of Methyl Methacrylate

Herein, we report an open-air, photo accelerated atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) without employing any deoxygenating agent. Under open-air photo ATRP conditions, oxygen reversibly binds with [{Cu (PMDETA)}2(O2 2-)]2+ (1) to form the required activator, which was demonstrated by simple benchtop oxygen/nitrogen purging experiments. The binding mode of oxygen in (1) (μ(η2-η2) peroxo dicopper(II)) was investigated using UV Visible-NIR, FT-Raman and X-ray photoelectron (XPS) spectroscopic techniques. DFT studies and electrochemical measurements further support the catalytic role of (1) in open-air photo ATRP. With the synergistic involvement of Cu (II)Br2, PMDETA ligand and the intensity of light (365 nm, 4.2 mW cm-2), a well-controlled rapid polymerization of MMA under open-air condition was achieved (1.25< Đ < 1.47, 94% conversion in 200 min). The bromo chain end fidelity was exemplified by chain extension experiment, block copolymerization and MALDI-ToF analysis. Other monomers such as methyl acrylate, glycidyl methacrylate, and benzyl methacrylate were also polymerized under open-air condition with reasonable control over molecular weight and Đ. An open-air photo polymerization methodology would be fruitful for applications like photocurable printing, dental, optoelectronics, stereolithography, and protective coatings where simple but rapid photopolymerizations are desirable.

Copper-Catalyzed 1,3-Aminocyclization of Cyclopropanes as a Rapid Entry to γ-Amino Heterocycles

We herein report a copper-catalyzed 1,3-aminocyclization of cyclopropanes as a direct and versatile entry into important heterocycles. This reaction was initiated by a copper-catalyzed, NFSI-promoted ring opening of cyclopropanes, followed by nucleophilic cyclization. A variety of nucleophiles successfully participate in this transformation, including alcohols, carboxylic acids, sulfonamides, and amides, for the construction of diverse cyclic ethers, pyrrolidines, lactones, and iminolactones.

Heteromultimetallic Platform for Enhanced C-H Bond Activation: Aluminum-Incorporated Dicopper Complex Mimicking Cu-ZSM-5 Structure and Oxidative Reactivity

Bimetallic complexes have sparked interest across various chemical disciplines, driving advancements in research. Recent advancements in this field have shed light on complex reactions in metalloenzymes and unveiled new chemical transformations. Two primary types of bimetallic platforms have emerged: (1) systems where both metals actively participate in reactivity, and (2) systems where one metal mediates the reaction while the other regulates reactivity. This study introduces a novel multinucleating ligand platform capable of integrating both types of bimetallic systems. To demonstrate the significance of this platform, we synthesized a unique dicopper complex incorporating aluminum in its coordination environment. This complex serves as the first structural model for the active site in copper-based zeolites, highlighting the role of aluminum in hydrogen atom abstraction reactivity. Comparative studies of oxidative C-H bond activation revealed that the inclusion of aluminum significantly alters the thermodynamic driving force (by -11 kcal mol-1) for bond activation and modifies the proton-coupled electron-transfer reaction mechanism, resulting in a 14-fold rate increase. Both computational and experimental data support the high modularity of this multinucleating ligand platform, offering a new approach to fine-tune the reactivity of bimetallic complexes.

Cu-Promoted ipso-Hydroxylation of sp2 Bonds with Concomitant Aromatic 1,2-Rearrangement Involving a Cu-oxyl-hydroxo Species

Herein, we report the first example of Cu-promoted β ipso-hydroxylation of substituted benzophenones using a bidentate directing group (DG) and H2O2 as an oxidant. In addition to the new C-O bond formed, the ipso-oxidation induces a very unusual 1,2-rearrangement of the iminyl group to the vicinal γ position. This transformation is highly dependent on the substrate utilized (favored for 4-methoxy-substituted benzophenones) and on the DG used (2-picolylamine leads to selective γ-C-H functionalization, while β ipso-oxidation requires 2-(2-aminoethyl)pyridine). An analysis of the oxidation of substrate-ligands derived from 2-(2-aminoethyl)pyridine and unsymmetrical 4-MeO-substituted benzophenones indicates high regioselectivity (up to 89:11 for the MeO-substituted arene ring and up to 92:8 for β ipso- vs γ-C-H hydroxylation). Mechanistic studies (which include spectroscopic characterization of reaction intermediates, kinetics, and calculations) suggest the formation of a mononuclear CuIIOOH species before the rate-determining step (rds) of the reaction. DFT calculations suggest that the γ-C-H hydroxylation pathway involves a one-step concerted O-O cleavage and electrophilic aromatic attack. Conversely, β ipso-hydroxylation occurs in a stepwise fashion, in which O-O bond cleavage produces a CuIII(O·)(OH) before electrophilic aromatic attack. Calculations also shed light on the mechanism of the 1,2-rearrangement step, which involves strain release from a spiro 5-membered to a 6-membered Cu chelate.

Photoredox Oxidation of Alkanes by Monometallic Copper-Oxygen Complexes Using Visible Light Including One Sun Illumination

Oxygenation of hydrocarbons offers versatile catalytic routes to more valuable compounds, such as alcohols, aldehydes, and ketones. Despite the importance of monometallic copper-oxygen species as hydroxylating agents in biology, few synthetic model compounds are known to react with hydrocarbons, owing to high C-H bond dissociation energies. To overcome this challenge, the photoredox chemistry of monometallic copper (pyrazolyl)borate complexes coordinated by chlorate has been explored in the presence of C1-C6 alkanes with BDEs ≥ 93 kcal/mol. Ethane is photooxidized at room temperature under N2 with yields of 15-30%, which increases to 77% for the most oxidizing tris(3,5-trifluoromethyl-pyrazolyl)borate complex (Cu-3). This complex also promotes the photooxidation of methane to methanol in significant yield (38%) when the photoredox reaction is run under aerobic conditions. Ligand modification alters the reaction selectivity by tuning the redox potential. The ability to activate 1° C-H bonds of C1-C6 alkanes using visible light is consistent with the photogeneration of a powerfully oxidizing copper-oxyl, which is supported by photocrystallographic studies of the tris(3,4,5-tribromopyrazolyl)borate chlorate complex. Mechanistic studies are consistent with the hydrogen atom abstraction of the C-H bond by the copper-oxyl intermediate. We demonstrate for Cu-3 with hexane as an exemplar, that the photoredox chemistry may be achieved under solar conditions of one-sun illumination.

Insights into dioxygen binding on metal centers: an ab initio multireference electronic structure analysis

Why does binding of dioxygen (O2) to metal centers, the initial step of O2 storage, transportation, and activation, almost inevitably induce metal-to-O2 single-electron transfer and generate superoxo (O2-˙) species, instead of genuine O02 adducts? To address this question, this study describes highly correlated wavefunction-based ab initio calculations using CASSCF/NEVPT2 (CASSCF = complete active space self-consistent field, and NEVPT2 = N-electron valence state second-order perturbation theory) approaches to explore the electronic-structure evolution of O2 association on Fe(II)(BDPP) (H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Co(II)(BDPP) to produce S = 3 Fe(III)(BDPP)(O2-˙) (1) and Co(III)(BDPP)(O2-˙) (2). CASSCF/NEVPT2 calculations suggest that the processes furnishing 1 and 2 feature an avoided crossing resulting from interactions of two diabatic curves, of which one is characterized as Co(II) and Fe(II) centers interacting with a triplet O2 ligand and the other as Co(III) and Fe(III) centers bound to a superoxo ligand. In both cases, the avoided crossing induces a one-electron transfer from the divalent metal center to the incoming O2 ligand and leads to formation of trivalent metal-O2-˙ complexes. To facilitate the interpretation of complicated multireference wavefunctions, we formulated two-fragment spin eigenfunctions utilizing Clebsch-Gordan coefficients (CGCs) to rationalize computed spin populations on the metal centers and the O2 ligand and compared these results with usual valence bonding (VB) analyses. It turns out that both methods give the same results and are complementary to each other. Finally, the limitation of DFT approaches in describing complex electronic structures involving metal-ligand magnetic couplings is delineated.

Bio-inspired copper complexes with Cu2S cores: (solvent) effects on oxygen reduction reactions

The need for effective alternative energy sources and "green" industrial processes is a more crucial societal topic than ever. In this context, mastering oxygen reduction reactions (ORRs) is a key step to develop fuel cells or to propose alternatives to energy-intensive setups such as the anthraquinone process for hydrogen peroxide production. Achieving this goal using bio-inspired metal complexes based on abundant and non-toxic elements could provide an environmentally friendly option. Given the prevalence of Cu-containing active sites capable of reductive activation of dioxygen in nature, the development of Cu-based catalysts for the ORR thus appears to be a relevant approach. We herein report the preparation, full characterization and (TD)DFT investigation of a new dinuclear mixed-valent copper complex 6 exhibiting a Cu2S core and a bridging triflate anion. Its ORR activity was compared with that of its parent catalyst 1. Two types of solvents were used, acetonitrile and acetone, and various catalyst/Me8Fc (electron source) ratios were tested. Our results highlight a counterintuitive solvent effect for 1 and a drastic drop in the activity for 6 in coordinating acetonitrile together with the modification of its chemical structure.

CO2 activation by copper oxide clusters: size, composition, and charge state dependence

The interaction of CO2 with copper oxide clusters of different size, composition, and charge is investigated via infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations. Laser ablation of a copper target in the presence of an O2/He mixture leads to the preferred formation of oxygen-rich copper oxide cluster cations, CuxOy+ (y > x; x ≤ 8), while the anionic cluster distribution is dominated by stoichiometric (x = y) and oxygen-deficient (y < x; x ≤ 8) species. Subsequent reaction of the clusters with CO2 in a flow tube reactor results in the preferred formation of near-stoichiometric CuxOy(CO2)+/- complexes. IR-MPD spectroscopy of the formed complexes reveals the non-activated binding of CO2 to all cations while CO2 is activated by all anions. The great resemblance of spectra for all sizes investigated demonstrates that CO2 activation is largely independent of cluster size and Cu/O ratio but mainly determined by the cluster charge state. Comparison of the IR-MPD spectra with DFT calculations of the model systems Cu2O4(CO2)- and Cu3O4(CO2)- shows that CO2 activation exclusively results in the formation of a CO3 unit. Subsequent CO2 dissociation to CO appears to be unfavorable due to the instability of CO on the copper oxide clusters indicating that potential hydrogenation reactions will most likely proceed via formate or bicarbonate intermediates.

Heterogenous Copper(0)-Assisted Dopamine Oxidation: A New Pathway to Controllable and Scalable Polydopamine Synthesis

In this study, we introduce an approach for synthesizing polydopamine (PDA) through the controlled oxidation of dopamine using metallic copper. Traditional methods of PDA synthesis often encounter challenges such as scalability, reproducibility, and control over polymerization. Our approach utilizes the catalytic properties of metallic copper in the presence of dissolved oxygen to generate reactive oxygen species (ROS) without additional chemicals. This process allows for precise control over dopamine oxidation, leading to reliable, materials and cost-effective upscalable PDA production. We investigated the reaction kinetics and the role of copper and ROS in dopamine oxidation, using several different experimental techniques. Our results demonstrate that, even at low pH, the copper-assisted method produces PDA with properties comparable to those synthesized through conventional means. We propose a mechanism for PDA synthesis that is initiated by oxygen adsorption onto copper surface, leading to the generation of various ROS which act as oxidizing agents in PDA synthesis. This method presents an advancement in the scalable and controlled production of PDA, with potential applications in various scientific and industrial fields.

A Four-Coordinate End-On Superoxocopper(II) Complex: Probing the Link between Coordination Number and Reactivity

Although the reactivity of five-coordinate end-on superoxocopper(II) complexes, CuII(η1-O2•-), is dominated by hydrogen atom transfer, the majority of four-coordinate CuII(η1-O2•-) complexes published thus far display nucleophilic reactivity. To investigate the origin of this difference, we have developed a four-coordinate end-on superoxocopper(II) complex supported by a sterically encumbered bis(2-pyridylmethyl)amine ligand, dpb2-MeBPA (1), and compared its substrate reactivity with that of a five-coordinate end-on superoxocopper(II) complex ligated by a similarly substituted tris(2-pyridylmethyl)amine, dpb3-TMPA (2). Kinetic isotope effect (KIE) measurements and correlation of second-order rate constants (k2's) versus oxidation potentials (Eox) for a range of phenols indicates that the complex [CuII(η1-O2•-)(1)]+ reacts with phenols via a similar hydrogen atom transfer (HAT) mechanism to [CuII(η1-O2•-)(2)]+. However, [CuII(η1-O2•-)(1)]+ performs HAT much more quickly, with its k2 for reaction with 2,6-di-tert-butyl-4-methoxyphenol (MeO-ArOH) being >100 times greater. Furthermore, [CuII(η1-O2•-)(1)]+ can oxidize C-H bond substrates possessing stronger bonds than [CuII(η1-O2•-)(2)]+ is able to, and it reacts with N-methyl-9,10-dihydroacridine (MeAcrH2) approximately 200 times faster. The much greater facility for substrate oxidation displayed by [CuII(η1-O2•-)(1)]+ is attributed to it possessing higher inherent electrophilicity than [CuII(η1-O2•-)(2)]+, which is a direct consequence of its lower coordination number. These observations are of relevance to enzymes in which four-coordinate end-on superoxocopper(II) intermediates, rather than their five-coordinate congeners, are routinely invoked as the active oxidants responsible for substrate oxidation.

Dioxygenase Chemistry in Nucleophilic Aldehyde Deformylations Utilizing Dicopper O2-Derived Peroxide Complexes

The chemistry of copper-dioxygen complexes is relevant to copper enzymes in biology as well as in (ligand)Cu-O2 (or Cu2-O2) species utilized in oxidative transformations. For overall energy considerations, as applicable in chemical synthesis, it is beneficial to have an appropriate atom economy; both O-atoms of O2(g) are transferred to the product(s). However, examples of such dioxygenase-type chemistry are extremely rare or not well documented. Herein, we report on nucleophilic oxidative aldehyde deformylation reactivity by the peroxo-dicopper(II) species [Cu2II(BPMPO-)(O22-)]1+ {BPMPO-H = 2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol)} and [Cu2II(XYLO-)(O22-)]1+ (XYLO- = a BPMPO- analogue possessing bis(2-{2-pyridyl}ethyl)amine chelating arms). Their dicopper(I) precursors are dioxygenase catalysts. The O2(g)-derived peroxo-dicopper(II) intermediates react rapidly with aldehydes like 2-phenylpropionaldehyde (2-PPA) and cyclohexanecarboxaldehyde (CCA) in 2-methyltetrahydrofuran at -90 °C. Warming to room temperature (RT) followed by workup results in good yields of formate (HC(O)O-) along with ketones (acetophenone or cyclohexanone). Mechanistic investigation shows that [Cu2II(BPMPO-)(O22-)]1+ species initially reacts reversibly with the aldehydes to form detectable dicopper(II) peroxyhemiacetal intermediates, for which optical titrations provide the Keq (at -90 °C) of 73.6 × 102 M-1 (2-PPA) and 10.4 × 102 M-1 (CCA). In the reaction of [Cu2II(XYLO-)(O22-)]1+ with 2-PPA, product complexes characterized by single-crystal X-ray crystallography are the anticipated dicopper(I) complex, [Cu2I(XYLO-)]1+ plus a mixed-valent Cu(I)Cu(II)-formate species. Formate was further identified and confirmed by 1H NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS) analysis. Using 18O2(g)-isotope labeling the reaction produced a high yield of 18-O incorporated acetophenone as well as formate. The overall results signify that true dioxygenase reactions have occurred, supported by a thorough mechanistic investigation.

A Multipurpose Metallophore and Its Copper Complexes with Diverse Catalytic Antioxidant Properties to Deal with Metal and Oxidative Stress Disorders: A Combined Experimental, Theoretical, and In Vitro Study

We report the discovery that the molecule 1-(pyridin-2-ylmethylamino)propan-2-ol (HL) can reduce oxidative stress in neuronal C6 glioma cells exposed to reactive oxygen species (O2-•, H2O2, and •OH) and metal (Cu+) stress conditions. Furthermore, its association with Cu2+ generates [Cu(HL)Cl2] (1) and [Cu(HL)2](ClO4)2 (2) complexes that also exhibit antioxidant properties. Potentiometric titration data show that HL can coordinate to Cu2+ in 1:1 and 1:2 Cu2+:ligand ratios, which was confirmed by monocrystal X-ray studies. The subsequent ultraviolet-visible, electrospray ionization mass spectrometry, and electron paramagnetic resonance experiments show that they can decompose a variety of reactive oxygen species (ROS). Kinetic studies revealed that 1 and 2 mimic the superoxide dismutase and catalase activities. Complex 1 promotes the fastest decomposition of H2O2 (kobs = 2.32 × 107 M-1 s-1), efficiently dismutases the superoxide anion (kcat = 3.08 × 107 M-1 s-1), and scavenges the hydroxyl radical (RSA50 = 25.7 × 10-6 M). Density functional theory calculations support the formation of dinuclear Cu-peroxide and mononuclear Cu-superoxide species in the reactions of [Cu(HL)Cl2] with H2O2 and O2•-, respectively. Furthermore, both 1 and 2 also reduce the oxidative stress of neuronal glioma C6 cells exposed to different ROS, including O2•- and •OH.

Identifying Radical Pathways for Cu(I)/Cu(II) Relay Catalyzed Oxygenation via Online Coupled EPR/UV-Vis/Near-IR Monitoring

Copper-catalyzed C─H oxygenation has drawn considerable attention in mechanistic studies. However, a comprehensive investigation combining radical pathways with a metal-catalytic cycle is challenged by the intricate organic radicals and metallic intermediates. Herein, an online coupled EPR/UV-vis/near-IR detecting method is developed to simultaneously monitor both reactive radical species and copper complex intermediates during the reaction. Focusing on copper-catalyzed phenol oxygenation with cumene hydroperoxide, the short-lived alkylperoxyl radical (EPR signal at g = 2.0143) as well as the unexpected square planar Cu(II)-alkoxyl radical complex (near-IR signal at 833 nm) are unveiled during the reaction, in addition to the observable phenoxyl radical in EPR, quinone product in UV-vis, and Cu(II) center in EPR. With a comprehensive picture of diverse intermediates evolving over the same timeline, a novel Cu(I)/Cu(II) proposed relay-catalyzed sequential radical pathway. In this sequence, Cu(II) activates hydroperoxide through Cu(II)-OOR into the alkylperoxide radical, while the reaction between Cu(I) and hydroperoxide leads to Cu(II)(•OR)OH with high H-atom abstracting activity. These results provide a thorough understanding of the Cu(I)/Cu(II) relay catalysis for phenol oxygenation, setting the stage for mechanistic investigations into intricate radical reactions promoted by metallic complexes.

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

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

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).

Chemical fixation of atmospheric CO2 in tricopper(II)-carbonato complexes with tetradentate N-donor ligands: reactive intermediates, probable mechanisms, and catalytic and magneto-structural studies

In the present era, the fixation of atmospheric CO2 is of significant importance and plays a crucial role in maintaining the balance of carbon and energy flow within ecosystems. Generally, CO2 fixation is carried out by autotrophic organisms; however, the scientific community has paid substantial attention to execute this process in laboratory. In this report, we synthesized two carbonato-bridged trinuclear copper(II) complexes, [Cu3(L1)3(μ3-CO3)](ClO4)3 (1) and [Cu3(L2)3(μ3-CO3)](ClO4)3 (2) via atmospheric fixation of CO2 starting with Cu(ClO4)2·6H2O and easily accessible pyridine/pyrazine-based N4 donor Schiff base ligands L1 and L2, respectively. Under very similar reaction conditions, the ligand framework embedded with the phenolate moiety (HL3) fails to do so because of the reduction of the Lewis acidity of the metal center, inhibiting the formation of a reactive hydroxide bound copper(II) species, which is required for the fixation of atmospheric CO2. X-ray crystal structures display that carbonate-oxygen atoms bridge three copper(II) centers in μ3syn-anti disposition in 1 and 2, whereas [Cu(HL3)(ClO4)] (3) is a mononuclear complex. Interestingly, we also isolated an important intermediate of atmospheric CO2 fixation and structurally characterized it as an anti-anti μ2 carbonato-bridged dinuclear copper(II) complex, [Cu2(L2)2(μ2-CO3)](ClO4)2·MeOH (2-I), providing an in-depth understanding of CO2 fixation in these systems. Variable temperature magnetic susceptibility measurement suggests ferromagnetic interactions between the metal centers in both 1 and 2, and the results have been further supported by DFT calculations. The catalytic efficiency of our synthesized complexes 1-3 was checked by means of catechol oxidase and phenoxazinone synthase-like activities. While complexes 1 and 2 showed oxidase-like activity for aerobic oxidation of o-aminophenol and 3,5-di-tert-butylcatechol, complex 3 was found to be feebly active. ESI mass spectrometry revealed that the oxidation reaction proceeds through the formation of complex-substrate intermediations and was further substantiated by DFT calculations. Moreover, active catalysts 1 and 2 were effectively utilized for the base-free oxidation of benzylic alcohols in the presence of air as a green and sustainable oxidant and catalytic amount of TEMPO in acetonitrile. Various substituted benzylic alcohols smoothly converted to their corresponding aldehydes under very mild conditions and ambient temperature. The present catalytic protocol showcases its environmental sustainability by producing minimal waste.

Cu-Catalyzed Tandem Oxidation-Intramolecular Cannizzaro Reaction of Biorenewables and Bioactive Molecules

A tandem Cu-catalyzed oxidation-intramolecular Cannizzaro reaction leading to bioactive α-hydroxyesters from α-hydroxyketones is reported. The process uses oxygen as a sole oxidant to achieve the formation of glyoxals, which are efficiently converted in situ to important α-hydroxyesters. The mechanistic insights are provided by isotopic labeling and supported by DFT calculations. The transformation proved a robust synthetic tool to achieve the synthesis of human metabolites and hydroxyl esters of various biologically active steroid derivatives.

The influence of thioether-substituted ligands in dicopper(II) complexes: Enhancing oxidation and biological activities

This paper describes the synthesis, structural analysis, as well as the magnetic and spectroscopic characterizations of three new dicopper(II) complexes with dinucleating phenol-based ligands containing different thioether donor substituents: aromatic (1), aliphatic (2) or thiophene (3). Temperature-dependent magnetometry reveals the presence of antiferromagnetic coupling for 1 and 3 (J = -2.27 cm-1 and -5.01 cm-1, respectively, H = -2JS1S2) and ferromagnetic coupling for 2 (J = 5.72 cm-1). Broken symmetry DFT calculations attribute this behavior to a major contribution from the dz2 orbitals for 1 and 3, and from the dx2-y2 orbitals for 2, along with the p orbitals of the oxygens. The bioinspired catalytic activities of these complexes related to catechol oxidase were studied using 3,5-di-tert-butylcatechol as substrate. The order of catalytic rates for the substrate oxidation follows the trend 1 > 2 > 3 with kcat of (90.79 ± 2.90) × 10-3 for 1, (64.21 ± 0.99) × 10-3 for 2 and (14.20 ± 0.32) × 10-3 s-1 for 3. The complexes also cleave DNA through an oxidative mechanism with minor-groove preference, as indicated by experimental and molecular docking assays. Antimicrobial potential of these highly active complexes has shown that 3 inhibits both Staphylococcus aureus bacterium and Epidermophyton floccosum fungus. Notably, the complexes were found to be nontoxic to normal cells but exhibited cytotoxicity against epidermoid carcinoma cells, surpassing the activity of the metallodrug cisplatin. This research shows the multifaceted properties of these complexes, making them promising candidates for various applications in catalysis, nucleic acids research, and antimicrobial activities.

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

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

Computational Assessment of the Mechanistic Journey in Chan-Lam-Based Arylation of Imidazoles

Cu(II)-catalyzed C-N bond formation reactions remain one of most widely practiced and powerful protocols for the synthesis of value-added chemicals, bioactive molecules, and materials. Despite numerous experimental contributions, the overall mechanistic understanding of the C-N coupling reaction based on the Chan-Lam (CL) reaction methodology is still limited and underdeveloped, particularly with respect to the use of different substrates and catalytic species. Herein, we report an in-depth DFT-based study on the mechanism of N-arylation of imidazoles following Collman's experimental setup. Our findings unfold for the first time the ligand-based CL coupling catalyzed by the [Cu(II)(OH)TMEDA]2Cl2 complex. The transmetalation step with an energy span of 26.2 kcal mol-1 is rate-determining, while the subsequent disproportionation and reductive elimination are relatively facile (δE = 16.4 kcal mol-1) in comparison to the CL amination of secondary amines. The final oxidative catalyst regeneration results in the presence of O2, accompanying an energy span of 12.8 kcal mol-1, where hydrogen transfer from the coordinated water allows the reduction of superoxo linkage. Couplings performed in the presence of a combination of bidentate sp3-N ligands with single and double -(CH2)- spacer units afford a kinetically facile transformation (24.5 kcal mol-1). Furthermore, our results agree with the experimental outcomes of regioselective couplings of substituted imidazoles.

Synthetic Copper-(Di)oxygen Complex Generation and Reactivity Relevant to Copper Protein O2-Processing

Synthetic copper-dioxygen complex design, generation and characterization, play a crucial role in elucidating the structure/function of copper-based metalloenzymes, including dopamine β-monooxygenase, lytic polysaccharide monooxygenases, particulate methane monooxygenase, tyrosinase, hemocyanin, and catechol oxidase. Designing suitable ligands to closely mimic the variable active sites found in these enzymes poses a challenging task for synthetic bioinorganic chemists. In this review, we have highlighted a few representative ligand systems capable of stabilizing various copper-dioxygen species such as CuII-(O2 •-)(superoxide), Cu2 II-(μ-η 1:η 1-O2 2-) (trans/cis-peroxide), Cu2 II-(μ-η 2:η 2-O2 2-)(side-on peroxide) and Cun II--OOH (hydroperoxide) species. Here, we discuss the ligand type utilized, syntheses, and spectroscopic characterization of these species. We also delineate reactivity patterns, particularly electrophilic arene hydroxylation by a side-on peroxo species which occurs via a "NIH shift" mechanism and thermodynamic-kinetic relationships among Cu2-(O2 •-)/O2 2-/-OOH moieties.

Intramolecular Phenolic H-Atom Abstraction by a N3ArOH Ligand-Supported (μ-η2:η2-Peroxo)dicopper(II) Species Relevant to the Active Site Function of oxy-Tyrosinase

Synthetic side-on peroxide-bound dicopper(II) (SP) complexes are important for understanding the active site structure/function of many copper-containing enzymes. This work highlights the formation of new {CuII(μ-η2:η2-O22-)CuII} complexes (with electronic absorption and resonance Raman (rR) spectroscopic characterization) using tripodal N3ArOH ligands at -135 °C, which spontaneously participate in intramolecular phenolic H-atom abstraction (HAA). This results in the generation of bis(phenoxyl radical)bis(μ-OH)dicopper(II) intermediates, substantiated by their EPR/UV-vis/rR spectroscopic signatures and crystal structural determination of a diphenoquinone dicopper(I) complex derived from ligand para-C═C coupling. The newly observed chemistry in these ligand-Cu systems is discussed with respect to (a) our Cu-MeAN (tridentate N,N,N',N',N″-pentamethyldipropylenetriamine)-derived model SP species, which was unreactive toward exogenous monophenol addition (J. Am. Chem. Soc. 2012, 134, 8513-8524), emphasizing the impact of intramolecularly tethered ArOH groups, and (b) recent advances in understanding the mechanism of action of the tyrosinase (Ty) enzyme.

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

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

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.

Mimicking the Reactivity of LPMOs with a Mononuclear Cu Complex

Lytic polysaccharide monooxygenases (LPMOs) are Cu-dependent metalloenzymes that catalyze the hydroxylation of strong C-H bonds in polysaccharides using O2 or H2O2 as oxidants (monooxygenase/peroxygenase). In the absence of C-H substrate, LPMOs reduce O2 to H2O2 (oxidase) and H2O2 to H2O (peroxidase) using proton/electron donors. This rich oxidative reactivity is promoted by a mononuclear Cu center in which some of the amino acid residues surrounding the metal might can accept and donate protons and/or electrons during O2 and H2O2 reduction. Herein, we utilize a podal ligand containing H-bond/proton donors (LH2) to analyze the reactivity of mononuclear Cu species towards O2 and H2O2. [(LH2)CuI]1+ (1), [(LH2)CuII]2+ (2), [(LH-)CuII]1+ (3), [(LH2)CuII(OH)]1+ (4), and [(LH2)CuII(OOH)]1+ (5) were synthesized and characterized by structural and spectroscopic means. Complex 1 reacts with O2 to produce 5, which releases H2O2 to generate 3, suggesting that O2 is used by LPMOs to generate H2O2. The reaction of 1 with H2O2 produces 4 and hydroxyl radical, which reacts with C-H substrates in a Fenton-like fashion. Complex 3, which generate 1 via a reversible protonation/reduction, binds H2O and H2O2 to produce 4 and 5, respectively, a mechanism that could be used by LPMOs to control oxidative reactivity.

Copper-Catalyzed Decarboxylation Cross-Coupling Cascade Reaction for Synthesis of Fused Dihydro-benzoxazinones

A protocol for a tandem copper-catalyzed intermolecular decarboxylation cross-coupling cascade between o-bromobenzoic acids and proline or piperic acid has been disclosed. The developed protocol allows access to a variety of synthetically useful fused benzoxazinones scaffolds with high efficiency and good functional group compatibility. A mechanistically sequential approach for the decarboxylation and dehydration coupling process was presented.