Reactive Cobalt⁻Oxo Complexes of Tetrapyrrolic Macrocycles and N-based Ligand in Oxidative Transformation Reactions

Electron transfer in biological systems

Examples of how metalloproteins feature in electron transfer processes in biological systems are reviewed. Attention is focused on the electron transport chains of cellular respiration and photosynthesis, and on metalloproteins that directly couple electron transfer to a chemical reaction. Brief mention is also made of extracellular electron transport. While covering highlights of the recent and the current literature, this review is aimed primarily at introducing the senior undergraduate and the novice postgraduate student to this important aspect of bioinorganic chemistry.

Room temperature C-O bond cleavage of vinyl cyclic synthons via a metallaphotoredox approach

Herein, we report visible-light induced C-O bond cleavage of vinyl-appended cyclic synthons via a Co(II)-photoredox dual catalytic approach operating at room temperature. This methodology exhibits a broad scope and is capable of accessing linear as well as branched allyl arenes simply by tuning the ring size of the cyclic motifs, in a mild and environmentally friendly protocol. Mechanistic studies unveil an interesting aspect of the reaction pathway involving a challenging homolytic cleavage of the Co(III)-O bond, 1,5-HAT of an unstable Co(II)-organometallic intermediate, and the key roles of O2 and the photocatalyst. The successful removal of the directing group further adds an important dimension to the methodology.

A synthetically useful catalytic system for aliphatic C-H oxidation with a nonheme cobalt complex and m-CPBA

We report herein a synthetically useful catalytic system for aliphatic C-H oxidation with a mononuclear nonheme cobalt(II) complex and m-chloroperbenzoic acid (m-CPBA). Preliminary mechanistic studies suggest that a high-valent cobalt-oxygen species (e.g., cobalt(IV)-oxo or cobalt(III)-oxyl) is the oxidant that effects C-H oxidation via a rate-determining hydrogen atom abstraction (HAA) step.

Utilizing the oxygen-atom trapping effect of Co3O4 with oxygen vacancies to promote chlorite activation for water decontamination

Heterogeneous high-valent cobalt-oxo [≡Co(IV)=O] is a widely focused reactive species in oxidant activation; however, the relationship between the catalyst interfacial defects and ≡Co(IV)=O formation remains poorly understood. Herein, photoexcited oxygen vacancies (OVs) were introduced into Co3O4 (OV-Co3O4) by a UV-induced modification method to facilitate chlorite (ClO2-) activation. Density functional theory calculations indicate that OVs result in low-coordinated Co atom, which can directionally anchor chlorite under the oxygen-atom trapping effect. Chlorite first undergoes homolytic O-Cl cleavage and transfers the dissociated O atom to the low-coordinated Co atom to form reactive ≡Co(IV)=O with a higher spin state. The reactive ≡Co(IV)=O rapidly extracts one electron from ClO2- to form chlorine dioxide (ClO2), accompanied by the Co atom returning a lower spin state. As a result of the oxygen-atom trapping effect, the OV-Co3O4/chlorite system achieved a 3.5 times higher efficiency of sulfamethoxazole degradation (~0.1331 min-1) than the pristine Co3O4/chlorite system. Besides, the refiled OVs can be easily restored by re-exposure to UV light, indicating the sustainability of the oxygen atom trap. The OV-Co3O4 was further fabricated on a polyacrylonitrile membrane for back-end water purification, achieving continuous flow degradation of pollutants with low cobalt leakage. This work presents an enhancement strategy for constructing OV as an oxygen-atom trapping site in heterogeneous advanced oxidation processes and provides insight into modulating the formation of ≡Co(IV)=O via defect engineering.

Synthesis of unsymmetrical ketones via dual catalysed cross-coupling of α,β-unsaturated carboxylic acids with aryldiazonium salts

A visible light-enabled synthesis of unsymmetrical ketones has been accomplished by the cross-coupling of α,β-unsaturated carboxylic acids and aryldiazonium salts embracing a synergistic eosin Y and Co(OAc)2·4H2O catalysis. The reaction involves decarboxylative aerobic CC bond cleavage, and is endowed with the creation of new C-C and C-O bonds with good substrate scope.

Low-Voltage Hydrogen Production via Hydrogen Peroxide Oxidation Facilitated by Oxo Ligand Axially Coordinated to Cobalt in Phthalocyanine Moiety

A cobalt phthalocyanine having an electron-poor CoN4 (+δ) in its phthalocyanine moiety was presented as an electrocatalyst for hydrogen peroxide oxidation reaction (HPOR). We suggested that hydrogen peroxide as an electrolysis medium for hydrogen production and therefore as a hydrogen carrier, demonstrating that the electrocatalyst guaranteed high hydrogen production rate by hydrogen peroxide splitting. The electron deficiency of cobalt allows CoN4 to have the highly HPOR-active monovalent oxidation state and facilitates HPOR at small overpotentials range around the onset potential. The strong interaction between the electron-deficient cobalt and oxygen of peroxide adsorbates in Co─OOH- encourages an axially coordinated cobalt oxo complex (O═CoN4 ) to form, the O═CoN4 facilitating the HPOR efficiently at high overpotentials. Low-voltage oxygen evolution reaction guaranteeing low-voltage hydrogen production is successfully demonstrated in the presence of the metal-oxo complex having electron-deficient CoN4 . Hydrogen production by 391 mA cm-2 at 1 V and 870 mA cm-2 at 1.5 V is obtained. Also, the techno-economic benefit of hydrogen peroxide as a hydrogen carrier is evaluated by comparing hydrogen peroxide with other hydrogen carriers such as ammonia and liquid organic hydrogen carriers.

Supramolecular Enhancement of Electrochemical Nitrate Reduction Catalyzed by Cobalt Porphyrin Organic Cages for Ammonia Electrosynthesis in Water

The electrochemical nitrate (NO3 - ) reduction reaction (NO3 RR) to ammonia (NH3 ) represents a sustainable approach for denitrification to balance global nitrogen cycles and an alternative to traditional thermal Haber-Bosch processes. Here, we present a supramolecular strategy for promoting NH3 production in water from NO3 RR by integrating two-dimensional (2D) molecular cobalt porphyrin (CoTPP) units into a three-dimensional (3D) porous organic cage architecture. The porphyrin box CoPB-C8 enhances electrochemical active site exposure, facilitates substrate-catalyst interactions, and improves catalyst stability, leading to turnover numbers and frequencies for NH3 production exceeding 200,000 and 56 s-1 , respectively. These values represent a 15-fold increase in NO3 RR activity and 200-mV improvement in overpotential for the 3D CoPB-C8 box structure compared to its 2D CoTPP counterpart. Synthetic tuning of peripheral alkyl substituents highlights the importance of supramolecular porosity and cavity size on electrochemical NO3 RR activity. These findings establish the incorporation of 2D molecular units into 3D confined space microenvironments as an effective supramolecular design strategy for enhancing electrocatalysis.

Asymmetrically Coordinated CoB1 N3 Moieties for Selective Generation of High-Valence Co-Oxo Species via Coupled Electron-Proton Transfer in Fenton-like Reactions

High-valence metal species generated in peroxymonosulfate (PMS)-based Fenton-like processes are promising candidates for selective degradation of contaminants in water, the formation of which necessitates the cleavage of OH and OO bonds as well as efficient electron transfer. However, the high dissociation energy of OH bond makes its cleavage quite challenging, largely hampering the selective generation of reactive oxygen species. Herein, an asymmetrical configuration characterized by a single cobalt atom coordinated with boron and nitrogen (CoB₁ N₃ ) is established to offer a strong local electric field, upon which the cleavage of OH bond is thermodynamically favored via a promoted coupled electron-proton transfer process, which serves an essential step to further allow OO bond cleavage and efficient electron transfer. Accordingly, the selective formation of Co(IV)O in a single-atom Co/PMS system enables highly efficient removal performance toward various organic pollutants. The proposed strategy also holds true in other heteroatom doping systems to configure asymmetric coordination, thus paving alternative pathways for specific reactive species conversion by rationalized design of catalysts at atomic level toward environmental applications and more.

Revealing the Generation of High-Valent Cobalt Species and Chlorine Dioxide in the Co3O4-Activated Chlorite Process: Insight into the Proton Enhancement Effect

A Co₃O₄-activated chlorite (Co₃O₄/chlorite) process was developed to enable the simultaneous generation of high-valent cobalt species [Co(IV)] and ClO₂ for efficient oxidation of organic contaminants. The formation of Co(IV) in the Co₃O₄/chlorite process was demonstrated through phenylmethyl sulfoxide (PMSO) probe and ¹⁸O-isotope-labeling tests. Both experiments and theoretical calculations revealed that chlorite activation involved oxygen atom transfer (OAT) during Co(IV) formation and proton-coupled electron transfer (PCET) in the Co(IV)-mediated ClO₂ generation. Protons not only promoted the generation of Co(IV) and ClO₂ by lowering the energy barrier but also strengthened the resistance of the Co₃O₄/chlorite process to coexisting anions, which we termed a proton enhancement effect. Although both Co(IV) and ClO₂ exhibited direct oxidation of contaminants, their contributions varied with pH changes. When pH increased from 3 to 5, the deprotonation of contaminants facilitated the electrophilic attack of ClO₂, while as pH increased from 5 to 8, Co(IV) gradually became the main contributor to contaminant degradation owing to its higher stability than ClO₂. Moreover, ClO₂^- was transformed into nontoxic Cl^- rather than ClO₃^- after the reaction, thus greatly reducing possible environmental risks. This work described a Co(IV)-involved chlorite activation process for efficient removal of organic contaminants, and a proton enhancement mechanism was revealed.

Seminal trace elements and their relationship with sperm parameters

Male reproductive problems may derive from many reasons including the environmental toxicants which may either intaken by occupational exposure, nutrition or bad air quality. The increased exposure to these substances due to rapid industrial development and technology has raised the questions: Is there a relationship between sperm parameters and these substances, and if so, in what extent? Results of studies on the subject reported conflicting results, many of which were not investigated in the seminal plasma. The aim of the current study was to evaluate the relationship between 23 metals and trace elements in human seminal plasma and semen parameters many of which were not investigated before. Levels of 23 metals in human seminal plasma were assessed by inductively coupled plasma mass spectrometry (ICP-MS). We examined the differences between subjects with normal ejaculate (normozoospermia) and pathologic ejaculate (with at least one abnormal semen parameter) according to the WHO criteria. The only significant difference was detected for Se while the other element's difference was not statistically significant. Se was statistically significantly increased in normal semen group suggesting the positive effect of this element on semen parameters.

Role of "S" Substitution on C-H Activation Reactivity of Iron(IV)-Oxo Cyclam Complexes: a Computational Investigation

A comprehensive density functional theory (DFT) investigation has been presented in this article to address the role of equatorial sulfur ligation in C-H activation. A non-heme iron-oxo compound with four nitrogen atoms constituting the equatorially connected macrocyclic framework (represented as N₄) [Fe(IV)═O(THC)(CH₃CN)]²⁺(THC = 1,4,8,11-tetrahydro1,4,8,11-tetraazacyclotetradecane) has been considered as the base compound. Other complexes have been anticipated by the sequential replacement of this nitrogen by sulfur, that is, N₄, N₃S₁, N₂S₂, N₁S₃, and S₄. Counterions, as always, have been considered to avoid the self-interaction error in DFT. Generally, the anti-conformers (with respect to equatorial N-H and Fe═O) turned out to be the most stable. It was found that with the enrichment of the equatorial sulfur atom, reactivity increases successively, that is, we get the trend N₄ < N₃S₁ < N₂S₂ < N₁S₃ < S_4. Our investigations have also verified the available experimental results where it has been reported that N₂S₂ is more reactive than N₄ in their mixed conformation. In search of insights into this typical pattern of reactivity, the interplay of several factors has been recognized, such as the distortion energy which decreases for the transition states with the addition of sulfur; the spin density on the oxygen atom which increases implying that the radical character of abstractor increases on sulfur ligation; the energy of the electron acceptor orbital (the lowest unoccupied molecular orbital (σ_z²*)) which decreases continuously with the sulfur substitution; and the triplet-quintet oxidant energy gap which decreases consistently with S enrichment in the equatorial position. The computational predictions reported here, if further validated by experiments, will definitely encourage the synthesis of sulfur-ligated bio-inspired complexes instead of the ones constituting nitrogen exclusively.

Soybean Oil Epoxidation Catalyzed by a Functionalized Metal–Organic Framework with Active Dioxo-Molybdenum (VI) Centers

In this work, a functionalized gallium metal–organic framework with active dioxo-molybdenum (VI) centers was evaluated as a catalyst in the epoxidation of soybean oil using tert-butyl-hydroperoxide as an oxidizing agent. The influence of the reaction time, temperature, and concentration of the oxidizing agent was studied, and it was demonstrated that the highest epoxide selectivity was obtained at 110 °C after 4 h of reaction (29% conversion and 91% selectivity) using a soybean oil/oxidizing agent ratio of 1/2. The stability of the metal–organic framework was confirmed by infrared spectroscopy, X-ray powder diffraction, thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy EDS. The stability tests demonstrated that the catalyst could be reused in the catalytic process for the recovery of vegetable oils.

Graphical Abstract

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

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

Efficient degradation of aqueous organic contaminants in manganese(II)/peroxymonosulfate system assisted by pyridine organic ligands

Although manganese(II) is known to have no role in peroxymonosulfate (PMS) activation, through a series of sulfamethoxazole (SMX) oxidation experiments, we found that the addition of pyridine organic ligands can improve the catalytic activity and accelerate SMX oxidation. For the organic ligands to be effective: the stability constant of the Mn(III) complex should be higher than that of the Mn(II) complex. A positive correlation was observed between the SMX oxidation rate and Mn(II) concentration, and the maximum PMS utilization efficiency was achieved. Many shreds of evidence verified that neither •SO₄^- nor •OH was associated with SMX oxidation. The enhanced effect of phenanthroline on the Mn(II)/PMS system was attributed to the highly oxidative intermediate manganese species (Mn(V)), originating from the two-electron transfer reaction of complexed Mn(III) and PMS. Notably, the main oxidizing species did not change (η-(PMSO₂) ∼ 100%) regardless of the initial PMSO concentration or pH value. Additionally, the analysis of SMX degradation products revealed that the oxygen transfer oxidation pathway was dominant in the Mn(II)/phenanthroline/PMS system, while the N radical coupling pathway also contributed significantly to SMX oxidation. This work offers new insights into the formation of high-valent manganese species and provides a potential strategy for applying low-concentration Mn(II) to wastewater treatment.

The Role of Porphyrinoid Photosensitizers for Skin Wound Healing

Microorganisms, usually bacteria and fungi, grow and spread in skin wounds, causing infections. These infections trigger the immune system and cause inflammation and tissue damage within the skin or wound, slowing down the healing process. The use of photodynamic therapy (PDT) to eradicate microorganisms has been regarded as a promising alternative to anti-infective therapies, such as those based on antibiotics, and more recently, is being considered for skin wound-healing, namely for infected wounds. Among the several molecules exploited as photosensitizers (PS), porphyrinoids exhibit suitable features for achieving those goals efficiently. The capability that these macrocycles display to generate reactive oxygen species (ROS) gives a significant contribution to the regenerative process. ROS are responsible for avoiding the development of infections by inactivating microorganisms such as bacteria but also by promoting cell proliferation through the activation of stem cells which regulates inflammatory factors and collagen remodeling. The PS can act solo or combined with several materials, such as polymers, hydrogels, nanotubes, or metal-organic frameworks (MOF), keeping both the microbial photoinactivation and healing/regenerative processes' effectiveness. This review highlights the developments on the combination of PDT approach and skin wound healing using natural and synthetic porphyrinoids, such as porphyrins, chlorins and phthalocyanines, as PS, as well as the prodrug 5-aminolevulinic acid (5-ALA), the natural precursor of protoporphyrin-IX (PP-IX).

Cationic Pyrrolidine/Pyrroline-Substituted Porphyrins as Efficient Photosensitizers against E. coli

New porphyrin–pyrrolidine/pyrroline conjugates were prepared by revisiting 1,3-dipolar cycloaddition reactions between a porphyrinic azomethine ylide and a series of dipolarophiles. Cationic conjugates obtained by alkylation of the pyrrolidine/pyrroline cycloadducts showed ability to generate singlet oxygen and to produce iodine in presence of KI when irradiated with visible light. Some of the cationic derivatives showed photobactericidal properties towards a Gram-negative bioluminescent E. coli. In all cases, these features were significantly improved using KI as coadjutant, allowing, under the tested conditions, the photoinactivation of the bacterium until the detection limit of the method with a drastic reduction of the required photosensitizer concentration and irradiation time. The obtained results showed a high correlation between the ability of the cationic porphyrin derivative to produce singlet oxygen and iodine and its E. coli photoinactivation profile.

Unraveling the Overlooked Involvement of High-Valent Cobalt-Oxo Species Generated from the Cobalt(II)-Activated Peroxymonosulfate Process

Sulfate radical (SO₄^•-) is widely recognized as the predominant species generated from the cobalt(II)-activated peroxymonosulfate (PMS) process. However, in this study, it was surprisingly found that methyl phenyl sulfoxide (PMSO) was readily oxidized to the corresponding sulfone (PMSO₂) with a transformation ratio of ∼100% under acidic conditions, which strongly implied the generation of high-valent cobalt-oxo species [Co(IV)] instead of SO₄^•- in the Co(II)/PMS process. Scavenging experiments using methanol (MeOH), tert-butyl alcohol, and dimethyl sulfoxide further suggested the negligible role of SO₄^•- and hydroxyl radical (^•OH) but favored the generation of Co(IV). By employing ¹⁸O isotope-labeling technique, the formation of Co(IV) was conclusively verified and the oxygen atom exchange reaction between Co(IV) and H₂O was revealed. Density functional theory calculation determined that the formation of Co(IV) was thermodynamically favorable than that of SO₄^•- and ^•OH in the Co(II)/PMS process. The generated Co(IV) species was indicated to be highly reactive due to the existence of oxo-wall and capable of oxidizing the organic pollutant that is rather recalcitrant to SO₄^•- attack, for example, nitrobenzene. Additionally, the degradation intermediates of sulfamethoxazole (SMX) in the Co(II)/PMS process under acidic conditions were identified to further understand the interaction between Co(IV) and the representative contaminant. The developed kinetic model successfully simulated PMSO loss, PMSO₂ production, SMX degradation, and/or PMS decomposition under varying conditions, which further supported the proposed mechanism. This study might shed new light on the Co(II)/PMS process.

Concerted Proton-Electron Transfer Reactivity at a Multimetallic Co4O4 Cubane Cluster

High-valent oxocobalt(IV) species have been invoked as key intermediates in oxidative catalysis, but investigations into the chemistry of proton-coupled redox reactions of such species have been limited. Herein, the reactivity of an established water oxidation catalyst, [Co₄O₄(OAc)₄(py)₄][PF₆], toward H-atom abstraction reactions is described. Mechanistic analyses and density functional theory (DFT) calculations support a concerted proton-electron transfer (CPET) pathway in which the high energy intermediates formed in stepwise pathways are bypassed. Natural bond orbital (NBO) calculations point to cooperative donor-acceptor σ interactions at the transition state, whereby the H-atom of the substrate is transferred to an orbital delocalized over a Co₃(μ₃-O) fragment. The mechanistic insights provide design principles for the development of catalytic C-H activation processes mediated by a multimetallic oxo metal cluster.

Tetrapyrrolic Macrocycles: Synthesis, Functionalization and Applications 2018

Natural and synthetic macrocycles like porphyrins, corroles and phthalocyanines are considered strong candidates to be used in different fields, such as catalysis, sensing, medicine, materials science, or in the development of advanced biomimetic models. All these applications are strongly dependent on the availability of compounds with adequate and specific structural features. This Special Issue has collected 13 contributions which consolidate and expand our knowledge on the application of these macrocycles in different fields accompanied by innovative synthetic methodologies to afford and to functionalize this type of compounds.

A cobalt-substituted Keggin-type polyoxometalate for catalysis of oxidative aromatic cracking reactions in water

Oxidative aromatic cracking reactions in water were achieved using a catalytic system with a cobalt-substituted Keggin-type polyoxometalate as a catalyst, an Oxone® as a sacrificial oxidant and sodium bicarbonate as an additive under mild conditions.

The mechanistic role of a support-catalyst interface in electrocatalytic water reduction by Co3O4 supported nanocarbon florets

Comprehending the mechanistic involvement of a support-catalyst interface is critical for effective design of industrially relevant electrocatalytic processes such as the alkaline hydrogen evolution reaction (alHER). The understanding of the kinetically sluggish alHER exhibited by both Pt and Pt-group-metal-free catalysts is primarily derived from indirect electrochemical parameters such as the Tafel slope. To address these issues, we establish the critical role of a nanocarbon floret (NCF) based electrochemical support in generating a key cobalt-oxohydroxo (OH-Co[double bond, length as m-dash]O) intermediate during the alHER through operando Raman spectro-electrochemistry. Specifically, interfacial nano-engineering of a newly designed carbon support (NCF) with a spinel Co₃O₄ nanocube catalyst is demonstrated to achieve a facile alHER (-0.46 V@10 mA cm^-2). Such an efficient alHER is mainly attributed to the unique lamellar morphology with a high mesoporous surface area (936 m² g^-1) of the NCF which catalyses the rate-determining water dissociation step and facilitates rapid ion diffusion. The dissociated water drives the formation of the OH-Co[double bond, length as m-dash]O intermediate, spectroscopically captured for the first time through the emergence of a νOH-Co[double bond, length as m-dash]O Raman peak (1074 cm^-1). The subsequent alHER proceeds through the Volmer-Heyrovsky route (119 mV dec^-1) via the T_d Co²⁺↔ Co³⁺↔ Co⁴⁺ oxidative pathway. Concomitant graphitization of the NCF through the disappearance of νsp³C-H (2946 cm^-1) supports the co-operative dynamics at the Co₃O₄-NCF interface. Thus, the NCF positively contributes towards the lowering of the overpotential with a low charge-transfer resistance (R_ct = 35.8 Ω) and high double layer capacitance (C_dl = 410 mF cm^-2).