Reactivity of Dioxygen−Copper Systems

High phenoxazinone synthase activity of two mononuclear cis-dichloro cobalt(ii) complexes with a rigid pyridyl scaffold

Two mononuclear cis-dichloro cobalt(II) complexes with bidentate pyridyl ligands have been successfully synthesized and employed as active o-aminophenol oxidation catalysts resulting in high turnover numbers under aerobic conditions.

Influence of the spatial distribution of copper sites on the selectivity of the oxygen reduction reaction

Moving towards a hydrogen economy raises the demand for affordable and efficient catalysts for the oxygen reduction reaction. Cu-bmpa (bmpa = bis(2-picolyl)amine) is shown to have moderate activity, but poor selectivity for the 4-electron reduction of oxygen to water. To enhance the selectivity towards water formation, the cooperative effect of three Cu-bmpa binding sites in a single trinuclear complex is investigated. The catalytic currents in the presence of the trinuclear sites are lower, possibly due to the more rigid structure and therefore higher reorganization energies and/or slower diffusion rates of the catalytic species. Although the oxygen reduction activity of the trinuclear complexes is lower than that of mononuclear Cu-bmpa, the selectivity of the copper mediated oxygen reduction was significantly enhanced towards the 4-electron process due to a cooperative effect between three copper centers that have been positioned in close proximity. These results indicate that the cooperativity between metal ions within biomimetic sites can greatly enhance the ORR selectivity.

Activation and catalytic transformation of methane under mild conditions

In the last few decades, worldwide scientists have been motivated by the promising production of chemicals from the widely existing methane (CH₄) under mild conditions for both chemical synthesis with low energy consumption and climate remediation. To achieve this goal, a whole library of catalytic chemistries of transforming CH₄ to various products under mild conditions is required to be developed. Worldwide scientists have made significant efforts to reach this goal. These significant efforts have demonstrated the feasibility of oxidation of CH₄ to value-added intermediate compounds including but not limited to CH₃OH, HCHO, HCOOH, and CH₃COOH under mild conditions. The fundamental understanding of these chemical and catalytic transformations of CH₄ under mild conditions have been achieved to some extent, although currently neither a catalyst nor a catalytic process can be used for chemical production under mild conditions at a large scale. In the academic community, over ten different reactions have been developed for converting CH₄ to different types of oxygenates under mild conditions in terms of a relatively low activation or catalysis temperature. However, there is still a lack of a molecular-level understanding of the activation and catalysis processes performed in extremely complex reaction environments under mild conditions. This article reviewed the fundamental understanding of these activation and catalysis achieved so far. Different oxidative activations of CH₄ or catalytic transformations toward chemical production under mild conditions were reviewed in parallel, by which the trend of developing catalysts for a specific reaction was identified and insights into the design of these catalysts were gained. As a whole, this review focused on discussing profound insights gained through endeavors of scientists in this field. It aimed to present a relatively complete picture for the activation and catalytic transformations of CH₄ to chemicals under mild conditions. Finally, suggestions of potential explorations for the production of chemicals from CH₄ under mild conditions were made. The facing challenges to achieve high yield of ideal products were highlighted and possible solutions to tackle them were briefly proposed.

Design, synthesis, structural, spectral, and redox properties and phenoxazinone synthase activity of tripodal pentacoordinate Mn(II) complexes with impressive turnover numbers

Catechol oxidase (CO) and phenoxazinone synthase (PHS) are two enzymes of immense significance due to their capability to oxidize catechols and o-aminophenols to o-quinones and phenoxazinones, respectively. In this connection two mononuclear manganese complexes with the molecular framework [Mn^II(Ln)Cl]Cl {L1: tris((1H-benzo[d]imidazol-2-yl)methyl)amine; n = 1 and L2: tris(N-methylbenzimidazol-2-ylmethyl)amine; n = 2} have been designed to be potential catalysts for OAPH (o-aminophenol) oxidation. Both the ligands and their corresponding metal complexes have been successfully synthesized and thoroughly characterized by different spectroscopic and analytical techniques such as FT-IR, ¹H NMR, UV-vis spectroscopy, EPR spectroscopy and ESI mass spectroscopy. The molecular structures of [Mn^II(L1)Cl]Cl (1) and [Mn^II(L2)Cl]Cl (2) have been revealed by a single-crystal X-ray diffraction study. The spectral properties and redox behaviour of both the complexes were examined. Under ambient conditions, 1 and 2 show excellent phenoxazinone synthase activity as both are very susceptible to oxidize o-aminophenol to phenoxazinone. The kinetic parameters for both complexes have been determined by analyzing the experimental spectroscopic data. The turnover numbers (k_cat value) of these two complexes are extremely high, 440 h^-1 and 234 h^-1 for 1 and 2, respectively. The present report offers a thorough overview of information involving the role of the metal ions and their extent of phenoxazinone synthase mimicking activity. The oxidation of o-aminophenol to 2-amino-3H-phenoxazine-3-one (APX) by catalytic oxidation of oxygen (O₂) by the reaction with transition metal complexes has been an important study for the last few decades. The current study evidently showed better performance of our synthesized Mn(II) complexes than all the predecessors. The plausible mechanism has been reiterated based on the experimental data via ESI-MS spectra and considering the concepts from the previously reported mechanisms involved in the formation of hydrogen peroxide (H₂O₂) as an intermediate substrate is fairly indicating the involvement of molecular oxygen in the catalytic cycle.

Dioxygenation of unprotected mesoionic N-heterocyclic olefins

We report the dioxygenation of mesoionic N-heterocyclic olefins (mNHOs) using molecular dioxygen. For 1,2,3-triazole-derived mNHOs possessing a vinyl proton and at least one acidic C-H group, they are oxidized into the corresponding triazolium benzoate salts, whereas those without vinyl proton or an acidic C-H group are oxidized into triazolium oxide and ketones/aldehydes.

Rejigging Electron and Proton Transfer to Transition between Dioxygenase, Monooxygenase, Peroxygenase, and Oxygen Reduction Activity: Insights from Bioinspired Constructs of Heme Enzymes

Nature has employed heme proteins to execute a diverse set of vital life processes. Years of research have been devoted to understanding the factors which bias these heme enzymes, with all having a heme cofactor, toward distinct catalytic activity. Among them, axial ligation, distal super structure, and substrate binding pockets are few very vividly recognized ones. Detailed mechanistic investigation of these heme enzymes suggested that several of these enzymes, while functionally divergent, use similar intermediates. Furthermore, the formation and decay of these intermediates depend on proton and electron transfer processes in the enzyme active site. Over the past decade, work in this group, using in situ surface enhanced resonance Raman spectroscopy of synthetic and biosynthetic analogues of heme enzymes, a general idea of how proton and electron transfer rates relate to the lifetime of different O2 derived intermediates has been developed. These findings suggest that the enzymatic activities of all these heme enzymes can be integrated into one general cycle which can be branched out to different catalytic pathways by regulating the lifetime and population of each of these intermediates. This regulation can further be achieved by tuning the electron and proton transfer steps. By strategically populating one of these intermediates during oxygen reduction, one can navigate through different catalytic processes to a desired direction by altering proton and electron transfer steps.

Using a monocopper-superoxo complex to prepare multicopper-peroxo species relevant to proposed enzyme intermediates

With the goal of generating a (peroxo)tricopper species analogous to the Peroxy Intermediate proposed for multicopper oxidases, solutions of the copper-superoxide complex [K(Krypt)][LCuO₂] (L = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Krypt = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) were reacted with the dicopper(I) complex [(TPBN)Cu₂(MeCN)₂][PF₆]₂ at -70 °C (TPBN = N,N,N',N'-tetrakis-(2-pyridylmethyl)-1,4-diaminobutane). A metastable intermediate formed, which on the basis of UV-vis, EPR, and resonance Raman spectroscopy was proposed to derive from reaction of two equivalents of the copper-superoxide with one equivalent of the dicopper(I) complex to yield a complex with two (peroxo)dicopper moieties rather than the desired (peroxo)tricopper PI model. A similar intermediate formed upon reaction of [K(Krypt)][LCuO₂] with [(BPMA)Cu(MeCN)][PF₆] (BPMA = N,N-bis(2-pyridylmethyl)-methyl-amine), which contained the same donor set as provided by TPBN. Comparison of resonance Raman data and consideration of structural preferences for LCuX species led to hypothesis of a μ-η¹:η²-peroxo structure for both intermediates.

Calcium-Ion Binding Mediates the Reversible Interconversion of Cis and Trans Peroxido Dicopper Cores

Coupled dinuclear copper oxygen cores (Cu₂ O₂ ) featured in type III copper proteins (hemocyanin, tyrosinase, catechol oxidase) are vital for O₂ transport and substrate oxidation in many organisms. μ-1,2-cis peroxido dicopper cores (^C P) have been proposed as key structures in the early stages of O₂ binding in these proteins; their reversible isomerization to other Cu₂ O₂ cores are directly relevant to enzyme function. Despite the relevance of such species to type III copper proteins and the broader interest in the properties and reactivity of bimetallic ^C P cores in biological and synthetic systems, the properties and reactivity of ^C P Cu₂ O₂ species remain largely unexplored. Herein, we report the reversible interconversion of μ-1,2-trans peroxido (^T P) and ^C P dicopper cores. Ca^II mediates this process by reversible binding at the Cu₂ O₂ core, highlighting the unique capability for metal-ion binding events to stabilize novel reactive fragments and control O₂ activation in biomimetic systems.

Influence of the amine donor on hybrid guanidine-stabilized Bis(μ-oxido) dicopper(III) complexes and their tyrosinase-like oxygenation activity towards polycyclic aromatic alcohols

The tyrosinase-like activity of hybrid guanidine-stabilized bis(μ-oxido) dicopper(III) complexes [Cu₂(μ-O)₂(L)₂](X)₂ (L = 2-{2-((Diethylamino)methyl)phenyl}-1,1,3,3-tetramethylguanidine (TMGbenzNEt₂, L2) and 2-{2-((Di-isopropylamino)methyl)phenyl}-1,1,3,3-tetramethylguanidine (TMGbenzNiPr₂, L3); X = PF₆^-, BF₄^-, CF₃SO₃^-) is described. New aromatic hybrid guanidine amine ligands were developed with varying amine donor function. Their copper(I) complexes were analyzed towards their ability to activate dioxygen in the presence of different weakly coordinating anions. The resulting bis(μ-oxido) species were characterized at low temperatures by UV/Vis and resonance Raman spectroscopy, cryo-ESI mass spectrometry and density functional theory calculations. Small structural changes in the ligand sphere were found to influence the characteristic ligand-to-metal charge transfer (LMCT) features of the bis(μ-oxido) species, correlating a redshift in the UV/Vis spectrum with weaker N-donor function of the ligand. DFT calculations elucidated the influence of the steric and electronic properties of the bis(μ-oxido) species leading to a higher twist of the Cu₂O₂ plane against the CuN₂ plane and a stretching of the Cu₂O₂ core. Despite their moderate stability at -100 °C, the bis(μ-oxido) complexes exhibited a remarkable activity in catalytic oxygenation reactions of polycyclic aromatic alcohols. Further the selectivity of the catalyst in the hydroxylation reactions of challenging phenolic substrates is not changed despite an increasing shield of the reactive bis(μ-oxido) core. The generated quinones were found to form exclusively bent phenazines, providing a promising strategy to access tailored phenazine derivatives.

Revisiting Alkane Hydroxylation with m-CPBA (mChloroperbenzoic Acid) Catalyzed by Nickel(II) Complexes

Mechanistic studies are performed on the alkane hydroxylation with m -CPBA ( m -chloroperbenzoic acid) catalyzed by nickel(II) complexes, Ni II (L). In the oxidation of cycloalkanes, Ni II (TPA) acts as an efficient catalyst with a high yield and a high alcohol selectivity. In the oxidation of adamantane, the tertiary carbon is predominantly oxidized. The reaction rate shows first-order dependence on [substrate] and [Ni II (L)] but is independent on [ m CPBA]; v obs = k 2 [substrate][ Ni II (L)]. The reaction exhibited a relatively large kinetic deuterium isotope effect ( KIE ) of 6.7, demonstrating that the hydrogen atom abstraction is involved in the rate-limiting step of the catalytic cycle. Furthermore, Ni II (L) supported by related tetradentate ligands exhibit apparently different catalytic activity, suggesting contribution of the Ni II (L) in the catalytic cycle. Based on the kinetic analysis and the significant effects of O 2 and CCl 4 on the product distribution pattern, possible contributions of (L)Ni II -O• and the acyloxyl radical as the reactive oxidants are discussed.

Cu-promoted intramolecular hydroxylation of CH bonds using directing groups with varying denticity

In this research article, we describe the Cu-promoted intramolecular hydroxylation of sp² and sp³ CH bonds using directing groups with varying denticity (bi-, tri- and tetradentate) and natural oxidants (O₂ and H₂O₂). We found that bidentate directing groups, in combination with Cu and H₂O₂, led to high hydroxylation yields. On the other hand, tetradentate directing groups did not form the hydroxylation products. Our mechanistic investigations suggest that bidentate directing groups allow for generating reactive mononuclear copper(II) hydroperoxide intermediates while tetradentate systems form dinuclear Cu₂O₂ species that do not oxidize CH bonds. Our findings might shed light on the reaction mechanism(s) by which Cu-dependent metalloenzymes such as particulate methane monooxygenase or lytic polysaccharide monooxygenase oxidize strong CH bonds.

Transition Metal-Catalyzed C-H Oxidation of Saturated Hydrocarbons with Molecular Oxygen

The C-H oxidation of saturated hydrocarbons to the corresponding alcohols and ketones can be performed efficiently at room temperature with molecular oxygen (1 atm) in the presence of acetaldehyde and catalysts such as Fe, Cu(OH)₂ , and CuCl₂ -18-crown-6 complex. Moreover, extremely high turnover numbers (>27,000) have been obtained for the C-H oxidation of saturated hydrocarbons with molecular oxygen (1 atm) using a combination of Cu(OAc)₂ and acetonitrile at 70 °C. Related oxidation reactions such as epoxidation, Baeyer-Villiger reaction, acetoxylation of β-lactams with molecular oxygen and a suitable aldehyde at room temperature are described.

Design of molecular water oxidation catalysts with earth-abundant metal ions

The four-electron oxidation of water (2H2O → O2 + 4H+ + 4e-) is considered the main bottleneck in artificial photosynthesis. In nature, this reaction is catalysed by a Mn4CaO5 cluster embedded in the oxygen-evolving complex of photosystem II. Ruthenium-based complexes have been successful artificial molecular catalysts for mimicking this reaction. However, for practical and large-scale applications in the future, molecular catalysts that contain earth-abundant first-row transition metal ions are preferred owing to their high natural abundance, low risk of depletion, and low costs. In this review, the frontier of water oxidation reactions mediated by first-row transition metal complexes is described. Special attention is paid towards the design of molecular structures of the catalysts and their reaction mechanisms, and these factors are expected to serve as guiding principles for creating efficient and robust molecular catalysts for water oxidation using ubiquitous elements.

Controlling the Reactivity of Copper(II) Acylperoxide Complexes

The redox state of the metallomonooxygenases is finely tuned by imposing specific coordination environments on the metal center to reduce the activation energy for the generation of active-oxygen species and subsequent substrate oxygenation reactions. In this study, copper(II) complexes supported by a series of linear tetradentate ligands consisting of a rigid 6-, 7-, or 8-membered cyclic diamine with two pyridylmethyl (-CH₂Py) side arms (L6^Pym2, L7^Pym2, and L8^Pym2) are employed to examine the effects of the coordination environment on the reactivity of their acylperoxide adduct complexes. The UV-vis and electron paramagnetic resonance spectroscopic data indicate that the ligand-field splitting between the d_x²-y² and d_z² orbitals of the starting copper(II) complexes increase with an increase of the ring size of the diamine moiety (L6^Pym2 → L7^Pym2 → L8^Pym2). In the reaction of these copper(II) complexes with m-chloroperbenzoic acid (m-CPBA), the L6^Pym2 complex gives a stable m-CPBA adduct complex, whereas the L7^Pym2 and L8^Pym2 complexes are immediately converted to the corresponding m-chlorobenzoic acid (m-CBA) adducts, indicating that the reactivity of the copper(II) acylperoxide complexes largely depends on the coordination environment induced by the supporting ligands. Density functional theory (DFT) calculations on the m-CPBA adduct complexes show that the ligand-field-splitting energy increases with an increase of the ring size of the diamine moiety, as in the case of the starting copper(II) complexes, which enhances the reactivity of the m-CPBA adduct complexes. The reasons for such different reactivities of the m-CPBA adduct complexes are evaluated by using DFT calculations.

Ligand Radical Mediated Water Oxidation by a Family of Copper o-Phenylene Bis-oxamidate Complexes

Understanding the reactivity landscape for the activation of water until the formation of the O-O bond and O₂ release in molecular chemistry is a decisive step in guiding the elaboration of cost-effective catalysts for the oxygen-evolving reaction (OER). Copper(II) complexes have recently caught the attention of chemists as catalysts for the 4e^-/4H⁺ water oxidation process. While a copper(IV) intermediate has been proposed as the reactive intermediate species, no spectroscopic signature has been reported so far. Copper(III) ligand radical species have also been formulated and supported by theoretical studies. We found, herein, that the reactivity sequence for the water oxidation with a family of Copper(II) o-phenylene bis-oxamidate complexes is a function of the substitution pattern on the periphery of the aromatic ring. In-situ EPR, FTIR, and rR spectroelectrochemical studies helped to sequence the elementary electrochemical and chemical events leading toward the O₂ formation selectively at the copper center. EPR and FTIR spectroelectrochemistry suggests that ligand-centered oxidations are preferred over metal-centered oxidations. rR spectroelectrochemical study revealed the accumulation of a bis-imine bound copper(II) superoxide species, as the reactive intermediate, under catalytic turnover, which provides the evidence for the O-O bond formation during OER.

Substituent Effects in Carbon-Nanotube-Supported Copper Phenolato Complexes for Oxygen Reduction Reaction

Unprotected mononuclear pyrene-modified (bispyridylaminomethyl)methylphenol copper complexes were designed to be immobilized at multiwalled carbon nanotube (MWCNT) electrodes and form dinuclear bis(μ-phenolato) complexes on the surface. These complexes exhibit a high oxygen reduction reaction activity of 12.7 mA cm^-2 and an onset potential of 0.78 V versus reversible hydrogen electrode. The higher activity of these complexes compared to that of mononuclear complexes with bulkier groups is induced by the favorable early formation of a dinuclear catalytic species on MWCNT.

Sulfur-Containing Analogues of the Reactive [CuOH]2+ Core

With the aim of drawing comparisons to the highly reactive complex LCuOH (L = bis(2,6-diisopropylphenylcarboxamido)pyridine), the complexes [Bu₄N][LCuSR] (R = H or Ph) were prepared, characterized by spectroscopy and X-ray crystallography, and oxidized at low temperature to generate the species assigned as LCuSR on the basis of spectroscopy and theory. Consistent with the smaller electronegativity of S versus O, redox potentials for the LCuSR^-/0 couples were ∼50 mV lower than for LCuOH^-/0, and the rates of the proton-coupled electron transfer reactions of LCuSR with anhydrous 1-hydroxy-2,2,6,6-tetramethyl-piperidine at -80 °C were significantly slower (by more than 100 times) than the same reaction of LCuOH. Density functional theory (DFT) and time-dependent DFT calculations on LCuZ (Z = OH, SH, SPh) revealed subtle differences in structural and UV-visible parameters. Further comparison to complexes with Z = F, Cl, and Br using complete active space (CAS) self-consistent field and localized orbital CAS configuration interaction calculations along with a valence-bond-like interpretation of the wave functions showed differences with previously reported results ( J. Am. Chem. Soc. 2020, 142, 8514), and argue for a consistent electronic structure across the entire series of complexes, rather than a change in the nature of the ligand field arrangement for Z = F.

New Polyporphyrin Arrays with Controlled Fluorescence Obtained by Diaxial Sn(IV)-Porphyrin Phenolates Chelation with Cu2+ Cation

New coordination oligomers and polymers of Sn(IV)-tetra(4-sulfonatophenyl)porphyrin have been constructed by the chelation reaction of its diaxialphenolates with Cu2+. The structure and properties of the synthesized polyporphyrin arrays were investigated by 1H Nuclear Magnetic Resonance (1H NMR), Infra Red (IR), Ultra Violet - Visible (UV-Vis) and fluorescence spectroscopy, mass spectrometry, Powder X-Rays Diffraction (PXRD), Electron Paramagnetic Resonance (EPR), thermal gravimetric, elemental analysis, and quantum chemical calculations. The results show that the diaxial coordination of bidentate organic ligands (L-tyrazine and diaminohydroquinone) leads to the quenching of the tetrapyrrole chromophore fluorescence, while the chelation of the porphyrinate diaxial complexes with Cu2+ is accompanied by an increase in the fluorescence in the organo-inorganic hybrid polymers formed. The obtained results are of particular interest to those involved in creating new 'chemo-responsive' (i.e., selectively interacting with other chemical species as receptors, sensors, or photocatalysts) materials, the optoelectronic properties of which can be controlled by varying the number and connection type of monomeric fragments in the polyporphyrin arrays.

Proton Switch in the Secondary Coordination Sphere to Control Catalytic Events at the Metal Center: Biomimetic Oxo Transfer Chemistry of Nickel Amidate Complex

High-valent metal-oxo species are key intermediates for the oxygen atom transfer step in the catalytic cycles of many metalloenzymes. While the redox-active metal centers of such enzymes are typically supported by anionic amino acid side chains or porphyrin rings, peptide backbones might function as strong electron-donating ligands to stabilize high oxidation states. To test the feasibility of this idea in synthetic settings, we have prepared a nickel(II) complex of new amido multidentate ligand. The mononuclear nickel complex of this N5 ligand catalyzes epoxidation reactions of a wide range of olefins by using mCPBA as a terminal oxidant. Notably, a remarkably high catalytic efficiency and selectivity were observed for terminal olefin substrates. We found that protonation of the secondary coordination sphere serves as the entry point to the catalytic cycle, in which high-valent nickel species is subsequently formed to carry out oxo-transfer reactions. A conceptually parallel process might allow metalloenzymes to control the catalytic cycle in the primary coordination sphere by using proton switch in the secondary coordination sphere.

New Polyporphyrin Arrays with Controlled Fluorescence Obtained by Diaxial Sn(IV)-Porphyrin Phenolates Chelation with Cu2+ Cation

New coordination oligomers and polymers of Sn(IV)-tetra(4-sulfonatophenyl)porphyrin have been constructed by the chelation reaction of its diaxialphenolates with Cu2+. The structure and properties of the synthesized polyporphyrin arrays were investigated by 1H Nuclear Magnetic Resonance (1H NMR), Infra Red (IR), Ultra Violet - Visible (UV-Vis) and fluorescence spectroscopy, mass spectrometry, Powder X-Rays Diffraction (PXRD), Electron Paramagnetic Resonance (EPR), thermal gravimetric, elemental analysis, and quantum chemical calculations. The results show that the diaxial coordination of bidentate organic ligands (L-tyrazine and diaminohydroquinone) leads to the quenching of the tetrapyrrole chromophore fluorescence, while the chelation of the porphyrinate diaxial complexes with Cu2+ is accompanied by an increase in the fluorescence in the organo-inorganic hybrid polymers formed. The obtained results are of particular interest to those involved in creating new ‘chemo-responsive’ (i.e., selectively interacting with other chemical species as receptors, sensors, or photocatalysts) materials, the optoelectronic properties of which can be controlled by varying the number and connection type of monomeric fragments in the polyporphyrin arrays.

Structural Characterization of the [CuOR]2+ Core

Formal Cu(III) complexes bearing an oxygen-based auxiliary ligand ([CuOR]²⁺, R = H or CH₂CF₃) were stabilized by modulating the donor character of supporting ligand L^Y (L^Y = 4-Y, N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Y = H or OMe) and/or the basicity of the auxiliary ligand, enabling the first characterization of these typically highly reactive cores by NMR spectroscopy and X-ray crystallography. Enhanced lifetimes in solution and slowed rates of PCET with a phenol substrate were observed. NMR spectra corroborate the S = 0 ground states of the complexes, and X-ray structures reveal shortened Cu-ligand bond distances that match well with theory.

Homogeneous Catalysts Based on First-Row Transition-Metals for Electrochemical Water Oxidation

Strategies that enable the renewable production of storable fuels (i. e. hydrogen or hydrocarbons) through electrocatalysis continue to generate interest in the scientific community. Of central importance to this pursuit is obtaining the requisite chemical (H+ ) and electronic (e- ) inputs for fuel-forming reduction reactions, which can be met sustainably by water oxidation catalysis. Further possibility exists to couple these redox transformations to renewable energy sources (i. e. solar), thus creating a carbon neutral solution for long-term energy storage. Nature uses a Mn-Ca cluster for water oxidation catalysis via multiple proton-coupled electron-transfers (PCETs) with a photogenerated bias to perform this process with TOF 100∼300 s-1 . Synthetic molecular catalysts that efficiently perform this conversion commonly utilize rare metals (e. g., Ru, Ir), whose low abundance are associated to higher costs and scalability limitations. Inspired by nature's use of 1st row transition metal (TM) complexes for water oxidation catalysts (WOCs), attempts to use these abundant metals have been intensively explored but met with limited success. The smaller atomic size of 1st row TM ions lowers its ability to accommodate the oxidative equivalents required in the 4e- /4H+ water oxidation catalysis process, unlike noble metal catalysts that perform single-site electrocatalysis at lower overpotentials (η). Overcoming the limitations of 1st row TMs requires developing molecular catalysts that exploit biomimetic phenomena - multiple-metal redox-cooperativity, PCET and second-sphere interactions - to lower the overpotential, preorganize substrates and maintain stability. Thus, the ultimate goal of developing efficient, robust and scalable WOCs remains a challenge. This Review provides a summary of previous research works highlighting 1st row TM-based homogeneous WOCs, catalytic mechanisms, followed by strategies for catalytic activity improvements, before closing with a future outlook for this field.

Influence of Ligand Denticity and Flexibility on the Molecular Copper Mediated Oxygen Reduction Reaction

To date, the copper complex with the tris(2-pyridylmethyl)amine (tmpa) ligand (Cu-tmpa) catalyzes the ORR with the highest reported turnover frequency (TOF) for any molecular copper catalyst. To gain insight into the importance of the tetradentate nature and high flexibility of the tmpa ligand for efficient four-electron ORR catalysis, the redox and electrocatalytic ORR behavior of the copper complexes of 2,2′:6′,2″-terpyridine (terpy) and bis(2-pyridylmethyl)amine (bmpa) (Cu-terpy and Cu-bmpa, respectively) were investigated in the present study. With a combination of cyclic voltammetry and rotating ring disk electrode measurements, we demonstrate that the presence of the terpy and bmpa ligands results in a decrease in catalytic ORR activity and an increase in Faradaic efficiency for H₂O₂ production. The lower catalytic activity is shown to be the result of a stabilization of the Cu^I state of the complex compared to the earlier reported Cu-tmpa catalyst. This stabilization is most likely caused by the lower electron donating character of the tridentate terpy and bmpa ligands compared to the tetradentate tmpa ligand. The Laviron plots of the redox behavior of Cu-terpy and Cu-bmpa indicated that the formation of the ORR active catalyst involves relatively slow electron transfer kinetics which is caused by the inability of Cu-terpy and Cu-bmpa to form the preferred tetrahedral coordination geometry for a Cu^I complex easily. Our study illustrates that both the tetradentate nature of the tmpa ligand and the ability of Cu-tmpa to form the preferred tetrahedral coordination geometry for a Cu^I complex are of utmost importance for ORR catalysis with very high catalytic rates.

Redox and electrocatalytic ORR behavior of the mononuclear copper complexes of 2,2′:6′,2″-terpyridine (terpy) and bis(2-pyridylmethyl)amine (bmpa) in neutral aqueous buffer solution: High Faradaic efficiencies for H₂O₂ production were revealed along the ORR active potential window using the rotating ring disk electrode (RRDE), and the foot-of-the-wave analysis (FOWA) was applied to describe the catalytic activity quantitatively. Additionally, the stability of the catalysts under operating conditions receives considerable attention.

Aerobic intramolecular carbon-hydrogen bond oxidation promoted by Cu(I) complexes

The oxidation of C-H bonds by copper centres in enzymes with molecular oxygen takes place in nature under ambient conditions. Herein we report a similar transformation in which under ambient pressure and temperature (1 atm, 25 °C) the complex TpMsCu(THF) (TpMs = hydrotris(3-mesityl-pyrazol-1-yl)borate) undergoes the intramolecular oxidation of an alkylic C-H bond with O2, leading to the formation of a trinuclear compound where alkoxy and hydroxyl ligands are bonded to the copper centres, as inferred from X-ray studies. The presence of adventitious Cu(0) derived from the partial decomposition of initial TpMsCu(THF) facilitates the formation of such a trinuclear compound. DFT studies support the reaction taking place through a Cu(iii) alkoxy-hydroxyl copper intermediate.

Selective Oxidation of Simple Aromatics Catalyzed by Nano-Biomimetic Metal Oxide Catalysts: A Mini Review

The process of selective oxy-functionalization of hydrocarbons using peroxide, O3, H2O2, O2, and transition metals can be carried out by the reactive oxygen species such as hydroxyl/hydroperoxyl radical and/or metal oxygenated species generated in the catalytic reaction. Thus, a variety of mechanisms have been proposed for the selective catalytic oxidation of various hydrocarbons including light alkanes, olefins, and simple aromatics by the biological metalloproteins and their biomimetics either in their homogeneous or heterogeneous platforms. Most studies involving these metalloproteins are Fe or Cu monooxygenases. The pathways carried out by these metalloenzymes in the oxidation of C-H bonds invoke either radical reaction mechanisms including Fenton's chemistry and hydrogen atom transfer followed by radical rebound reaction mechanism or electrophilic oxygenation/O-atom transfer by metal-oxygen species. In this review, we discuss the metal oxide nano-catalysts obtained from metal salts/molecular precursors (M = Cu, Fe, and V) that can easily form in situ through the oxidation of substrates using H2O2(aq) in CH3CN, and be facilely separated from the reaction mixtures as well as recycled for several times with comparable catalytic efficiency for the highly selective conversion from hydrocarbons including aromatics to oxygenates. The mechanistic insights revealed from the oxy-functionalization of simple aromatics mediated by the novel biomimetic metal oxide materials can pave the way toward developing facile, cost-effective, and highly efficient nano-catalysts for the selective partial oxidation of simple aromatics.

A facile biomimetic catalytic activity through hydrogen atom abstraction by the secondary coordination sphere in manganese(III) complexes

This paper describes the synthesis and structural characterization of four new manganese(iii) complexes (1-4) derived from N3O donor Schiff base ligands and their biomimetic catalytic activities related to catechol oxidase and phenoxazinone synthase. X-ray crystallography reveals that the Schiff bases coordinate the metal centre in a tridentate fashion, leaving the pendant tertiary amine nitrogen atom either protonated or free to balance the charge of the system, and these pendant triamines participate in strong hydrogen bonding interactions in the solid state. The hydrogen bonding ability of the pendant triamines at the second coordination sphere plays a crucial role in the substrate recognition and the stability of the complex-substrate intermediates. The effect of substitution at the phenolate ring towards the redox potential of the metal centre and the catalytic activity of these complexes has been observed. Detailed kinetic studies further disclose the deuterium kinetic isotope effect in which the transfer of the proton along the hydrogen bond from the substrates to the pendant triamine group at the secondary coordination sphere occurs at the key step in the catalytic reaction. The present reactivity nicely resembles the biochemical reactivities in the natural system in which a concerted electron and proton transfer to different species is usually observed. Remarkably, although some sort of influence of the secondary coordination sphere on catalytic activity has been reported mimicking the function of these metalloenzymes, such a direct participation of the secondary coordination sphere, particularly in modelling phenoxazinone synthase, has not been observed to date.

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.

•

A dioxygen-derived mononuclear osmium-peroxo complex was characterized

•

The peroxo ligand is stabilized by internally aromatic metallacycle

•

O₂ activation involves the reversible aromatization-dearomatization

•

A concerted double-hydrogen transfer mechanism for alcohol dehydrogenation

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.

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.