Azo Radical Research Articles

Radical reactivity of antiaromatic Ni(II) norcorroles with azo radical initiators.

Norcorrole is a stable 16π-antiaromatic porphyrinoid that exhibits characteristic reactivities and physical properties. Here, we disclose the reaction of Ni(II) norcorroles with alkyl radicals derived from azo radical initiators. The radical selectively attacked the distal α-position relative to the meso-position to construct a nonaromatic bowl-shaped structure. The photophysical and electrochemical properties of the obtained radical adducts were compared to those of the parent Ni(II) norcorrole. The radical reactivity of Ni(II) norcorroles was investigated by density functional theory (DFT) calculations.

Aggregation of positively charged poly(methyl methacrylate) latex particles in the presence of trifluoromethanesulfonate anion

Abstract We investigated the unique aggregation behavior of 3 types of positively charged poly(methyl methacrylate) (PMMA) latex particles in the presence of a trifluoromethanesulfonate (triflate) anion. The particles were prepared based on surfactant-free emulsion polymerization using azo radical initiators. The triflate anion was strongly adsorbed to surfaces of positively charged PMMA latex particles, as confirmed by electrophoresis. Additionally, its presence significantly reduced the critical coagulation concentration of the cationic PMMA latex particles compared with other anions.

Thermal hazards assessment of three azo nitrile compounds

Commercially available azo radical compounds provide efficient initiation of many chemical reactions. However, such energetic azo group initiators have thermal stability issues at ambient or even sub-ambient temperatures. Azo decomposition initiated by heat and/or light can generate significant amounts of nitrogen gas with rapid pressure increases, presenting a safety challenge for shipping, storage and usage. In this study, three azo nitrile compounds were examined using various thermal calorimetry techniques with only 5 mg to 1 g scale samples. We obtained calorimetric data on exothermic activity and gas generation. The testing results were subsequently analyzed by using differential iso-conversional methods to obtain key decomposition kinetics parameters, including activation energy and pre-exponential factor. The model was then validated and utilized for prediction of several key safety parameters for scale-up applications, including Self-Accelerating Decomposition Temperature (SADT), Time to Maximum Rate (TMR), temperature profiles at iso- or non-isothermal conditions for shelf-life evaluation, adiabatic temperature rise, and rate of pressure increase. For compounds such as 2,2’-azobis-(2,4-dimethylvaleronitrile) and 1,1’-azobis(cyclohexane-1-carbonitrile) which went through phase transitions associated with thermal decomposition, the thermal decomposition behavior of liquid and solid state was found to differ in characteristic. This methodology for thermal decomposition kinetics assessment enables safe storage, handling, and scale-up process preparation.

Mechanistic Elucidation of an Alcohol Oxidation Reaction Promoted by a Nickel Azophenolate Complex

Borrowing hydrogen (BH) reactions by base metals have attracted significant focus with a perspective in green and sustainable chemistry. In a BH reaction, dehydrogenation of the alcohol motifs is a fundamental step, where several well-established catalysts exhibit metal–ligand cooperativity. We showcase here that the redox noninnocence of the azophenolate backbone in an earth-abundant nickel catalyst drives the alcohol dehydrogenation by a radical pathway. Detailed mechanistic probation including saturation kinetics supports the binding of the substrate alcohol to the nickel center. Thermodynamic data derived from such analysis also proves the azophenolate hemilability to pave the further steps. An Eyring analysis quantifies the kinetic barrier associated with a crucial hydrogen atom transfer step that is driven by the azo radical. Furthermore, both Hammett correlation and linearity in Evans–Polanyi plot implicate that C–H abstraction by radical mechanism as the rate-determining step. The overall mechanism offered by the azophenolate catalyst closely resembles the galactose oxidase-based copper phenoxide chemistry.

Transient Stabilization Effect of CO2 in the Electrochemical Hydrogenation of Azo Compounds and the Reductive Coupling of α‐Ketoesters

AbstractThe carbon dioxide (CO2) capture and utilization has attracted a great attention in organic synthesis. Herein, an unpresented transient stabilization effect (TSE) of CO2 is disclosed and well applied to the electrochemical hydrogenation of azo compounds to hydrazine derivatives. Mechanistic experiments and computational studies imply that CO2 can capture azo radical anion intermediates to protect the hydrogenation from potential degradation reactions, and is finally released through decarboxylation. The promotion effect of CO2 was further demonstrated to work in the preliminary study of electrochemical reductive coupling of α‐ketoesters to vicinal diol derivatives. For the electrochemical reductive reactions mentioned above, CO2 is indispensable. The presented results shed light on a different usage of CO2 and could inspire novel experimental design by using CO2 as a transient protecting group.

Transient Stabilization Effect of CO2 in the Electrochemical Hydrogenation of Azo Compounds and the Reductive Coupling of α-Ketoesters.

The carbon dioxide (CO2 ) capture and utilization has attracted a great attention in organic synthesis. Herein, an unpresented transient stabilization effect (TSE) of CO2 is disclosed and well applied to the electrochemical hydrogenation of azo compounds to hydrazine derivatives. Mechanistic experiments and computational studies imply that CO2 can capture azo radical anion intermediates to protect the hydrogenation from potential degradation reactions, and is finally released through decarboxylation. The promotion effect of CO2 was further demonstrated to work in the preliminary study of electrochemical reductive coupling of α-ketoesters to vicinal diol derivatives. For the electrochemical reductive reactions mentioned above, CO2 is indispensable. The presented results shed light on a different usage of CO2 and could inspire novel experimental design by using CO2 as a transient protecting group.

Azo‐Initiator‐Induced Cascade Defect Passivation for Efficient and Stable Planar Perovskite Solar Cells

Remarkable progress in perovskite solar cells (PSCs) has been made by virtue of surface and bulk modification for defect control of perovskite layers, however, the interior defects in the perovskite layer can be hardly passivated by means of conventional passivation strategies, which makes perovskites prone to decomposition with inferior device performance. Here, the authors propose a cascade defect passivation strategy to doubly reduce perovskite defects by introducing a series of azo radical initiators. The primary passivation process of the azo compounds is realized on the perovskite surfaces through strong coordination interactions between Pb2+ and carbonyl/cyano groups. The secondary passivation occurs after thermal treatment, and the decomposed products diffuse into the interior defects of the initial passivated perovskite films through grain boundaries to accomplish the cascade defect passivation. Excitingly, this passivation strategy inhibits unwanted defect‐assisted recombination effectively, and thus CH3NH3PbI3 (MAPbI3)‐based inverted PSCs exhibit a significant increase in power conversion efficiency (PCE) from 16.92% to 19.69%. Moreover, the dual‐passivated PSCs show only 10% PCE loss after 3000 h in an inert atmosphere. Overall, the first proposed cascade defect passivation strategy is highly efficient in promoting defect healing of perovskites, demonstrating a new way to prepare high‐quality perovskite films for high‐performance PSCs.

Photo-Electroswitchable Arylaminoazobenzenes.

Azobenzenes appended with a redox-active arylamino group (redox auxiliary, RA) are prepared and shown to undergo fast, complete, and catalytic Z→E azo isomerization upon electron loss from the RA unit of the azobenzene. The RA-azo structures can be reversibly (E→Z→E)n cycled by sequential photo- and electrostimulation. Due to the robust nature of the RA•+-azo radical cation chain carrying species, initiation of electron transfer (ET) catalysis occurs at low levels (1.0-0.04 mol %) of catalytic loading and is effective even at Z-RA-azo concentrations of 10-4-10-5 M, yielding TONs (turnover numbers) of 100-2300 under such dilute conditions. The RA-azo Z→E conversion is demonstrated using chemical oxidation (redox switching), electrochemical oxidation (electro switching), and photochemical oxidation (photoredox switching). The Z→E acceleration is shown to be at least 2 × 109-fold for RA-azo 5. DFT calculations on methyl yellow suggest that a N-centered radical cation of the RA group stabilizes the Z→E N-N twist transition state of the RA•+-azo, yielding a large reduction in the barrier for RA•+-azo compared to neutral RA-azo. The RA-azo structure class has nanomechanical features that can be toggled with photo- and electrostimulation, the latter offering a quick switch for complete Z→E conversion.

Controlled soap-free emulsion polymerization stability using a novel cationic azo radical initiator with chloride or triflate counter anion

In our previous work, we developed a novel cationic azo radical initiator, 2,2′-azobis-[2-(1,3-dimethyl-4,5-dihydro-1H-imidazol-3-ium-2-yl)]propane triflate (ADIP) to prepare cationic and hydrophilic polymer particles for incorporation into cells. To apply this cationic radical initiator to hydrophobic polymer particles for a broader range of industrial applications, we attempted to synthesize polymethylmethacrylate (PMMA) latex via surfactant-free emulsion polymerization (SFEP). However, we found that the synthesized emulsion coagulated and contained a wide-size distribution of the particles, resulting in the failure of stable emulsion polymerization. In this study, we have successfully synthesized cationic PMMA particles with a narrow size distribution without any coagulation using ADIP-Cl which is an ADIP derivative in which the triflate counter anion was replaced with chloride. Compared to other halides and polyatomic anions, the triflate anion caused a significant aggregation of the PMMA particles. Furthermore, we confirmed that ADIP-Cl can be also applicable for the size-controlled synthesis of PMMA nanoparticles with sub-100 nm size in an acetone-water mixture. We envision that polymer particles prepared by ADIP-Cl via SFEP can contribute to the design of various functional materials.

Stable, yet "naked", azo radical anion ArNNAr- and dianion ArNNAr2- (Ar = 4-CN-2,6-iPr2-C6H2) with selective CO2 activation.

Azo radical anion 1˙- and dianion 12- have been isolated by one- and two-electron reduction of the azo compound 1 (ArNNAr, Ar = 4-CN-2,6-iPr2-C6H2) with alkali metals, respectively. The reduced species have been characterized by single-crystal X-ray analysis, EPR, UV and FT-IR spectroscopy, as well as SQUID measurements. The filling of one and two electrons in the π* orbital of the N-N double bond of 1 leads to a half-double N-N bond in 1˙- and a single N-N bond in 12-. The uncoordinated nature of these reduced species enables them to activate CO2. The exposure of 1˙- solution to CO2 led to the formation of oxalate anion C2O42-, while that of 12- solution to CO2 afforded the hydrazine dicarboxylate dianion [1-2CO2]2-, which reversibly dissociated back to 1 and CO2 upon oxidation.

Hydrogen and Atom Transfer Activity of Saffron Extracts by Square Wave Voltammetry

AbstractSaffron is an edible spice with highly appreciated sensory and antioxidant properties. One of the most representative redox species found in saffron extracts is crocin, whose content is used to evaluate the quality and value of the resulting spice. In this study, a voltammetry method based on the direct detection of crocin at a bare glassy carbon electrode is presented. The principle of the method is based on the monitoring of the anodic wave exhibited by crocin (0.1–1.0 mM) after its mixing with the azo radical initiator AAPH (20 mM) in ethanol:acetonitrile (1 : 1) solution. The decay rate of the anodic peak (E=+434 mV vs. Ag/Ag+), as a result of the consumption of crocin by AAPH, was used as index of the hydrogen transfer capacity and, thus, of its antioxidant activity. With a decay rate of k=0.02 h−1, crocin exhibits only a weak antioxidant activity in comparison with tocopherols (k=0.13 h−1), but still sufficient to protect against the oxidation of safranal, a further redox species found in saffron extracts and mainly responsible for its flavor. The proposed approach was finally applied to discriminate saffron extract samples from different geographical origins. The proposed approach is suitable to characterize the quality of saffron extracts and estimate its antioxidant properties.

One-step amino-functionalization of milled carbon fibre for enhancement of thermo-physical properties of epoxy composites

Development of new chemical approaches for preparation of engineered carbon-based fillers is critical for high-performance applications. Herein, an efficient method for covalent functionalization of polyacrylonitrile-based carbon fibre through azo radical addition under mild condition is demonstrated. In this way, isobutyronitrile radicals in situ produced from thermal decomposition of 2,2′-azobisisobutyronitrile (AIBN), were covalently grafted on milled carbon fibre (MCF) surface, assisted by microwave irradiation, as evidenced by FTIR, Raman, and TGA analysis. The grafted isobutyronitriles on MCF surface (n-MCF) were applied for further MCF amino-functionalization (a-MCF) via nucleophilic reaction of an amine-rich compound. Then, both pure MCF and a-MCF were incorporated into epoxy matrix; and its curing process and thermo-physical properties were investigated using DSC, rheometry, DMA, TGA, and flexural analysis. The Tg and flexural strength of epoxy/a-MCF composites, compared to epoxy/MCF, increased by ∼3.5% and∼10.2%, resulting from good adhesion between a-MCF and epoxy matrix which confirmed by SEM observations.

Effect of Silver Nanomaterials on the Activity of Thiol-Containing Antioxidants

The use of nanomaterials in consumer products is rapidly expanding. In most studies, nanomaterials are examined as isolated ingredients. However, consumer products such as foods, cosmetics, and dietary supplements are complex chemical matrixes. Therefore, interactions between nanomaterials and other components of the product must be investigated to ensure the product's performance and safety. Silver nanomaterials are increasingly being used in food packaging as antimicrobial agents. Thiol-containing compounds, such as reduced glutathione (GSH), cysteine, and dihydrolipoic acid, are used as antioxidants in many consumer products. In the current study, we have investigated the interaction between silver nanomaterials and thiol-containing antioxidants. The selected Ag nanomaterials were Ag coated with citrate, Ag coated with poly(vinylpyrrolidone), and Au nanorods coated with Ag in a core/shell structure. We observed direct quenching of the 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) by all three Ag nanomaterials to varying degrees. The Ag nanomaterials also reduced the quenching of DPPH by GSH to varying degrees. In addition, we determined that the mixture of GSH and Au@Ag nanorods held at 37 °C was less effective at quenching azo radical than at ambient temperature. Furthermore, we determined that Au@Ag nanorods significantly reduced the ability of GSH and cysteine to quench hydroxyl and superoxide radicals. The work presented here demonstrates the importance of examining the chemical interactions between nanomaterials used in products and physiologically important antioxidants.

ChemInform Abstract: The Oxidation of Ni(II) N‐Confused Porphyrins (NCPs) with Azo Radical Initiators and an Unexpected Intramolecular Nucleophilic Substitution Reaction via a Proposed Ni(III) NCP Intermediate.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

The oxidation of Ni(ii) N-confused porphyrins (NCPs) with azo radical initiators and an unexpected intramolecular nucleophilic substitution reaction via a proposed Ni(iii) NCP intermediate

The oxidation of Ni(II) N-confused porphyrins (NCPs) with azo radical initiators resulted in an unexpected intramolecular nucleophilic substitution reaction via a proposed Ni(III) NCP intermediate, which could be detected by HRMS.

Surface Modification of Silicon Carbide Nanoparticles by Azo Radical Initiators

The surface of silicon carbide (SiC) nanoparticles was modified with three types of azo radical initiators: 2,2′-azobisisobutyronitrile (AIBN), 2,2′-azobis(2-methylpropionamidine)dihydrochloride (AMPA), and 2,2′-azobis[N-(2-carboxyethyl)-2-methylpropionamidine)n-hydrate (ACMPA). The radical species generated from the azo initiators successfully reacted with the unsaturated hydrocarbons on the surface of SiC nanoparticles. Consequently, the hydrophobic SiC surface became hydrophilic, and the dispersion stability of SiC nanoparticles in aqueous solution improved significantly. Further, it was found that the stability of SiC nanoparticles in aqueous solution under various pH values could be controlled by the structure of the azo radical initiators. When SiC nanoparticles reacted with AIBN and were further hydrolyzed by NaOH aqueous solution, since carboxyl groups were introduced on their surface, their stability improved at pH ≥5. Contrarily, when SiC nanoparticles were modified with AMPA, amine groups were ...

Organo-Cobalt Mediated Living Radical Polymerization of Vinyl Acetate

Vinyl acetate polymerization initiated by azo radical sources in the presence of cobalt(II) tetramesitylporphyrin ((TMP)CoII) shows an induction period followed by an organo-cobalt mediated living radical polymerization (LRP). The induction period corresponds to converting (TMP)CoII to an organo-cobalt porphyrin derivative (organo-Co(TMP)). Living character at low vinyl acetate conversion is demonstrated by a linear increase in molecular weight with conversion, relatively low polydispersity homopolymers, and formation of block copolymers with methyl acrylate ((TMP)Co-PVAc-b-PMA). Deviations from ideal LRP occur by radical termination and chain transfer events at moderate conversion of vinyl acetate (VAc). Mechanistic studies demonstrate that the VAc radical polymerization is controlled by a degenerative transfer mechanism that utilizes organo-cobalt complexes as the transfer agent. Kinetic studies are utilized in comparing radical polymerization of vinyl acetate and methyl acrylate that are mediated by or...

Singlet Diradical Complexes of Chromium, Molybdenum, and Tungsten with Azo Anion Radical Ligands from M(CO)6Precursors

The homoleptic diamagnetic complexes M(mer-L)(2), M = Cr, Mo,W (1a,b, 2a,b, and 4a,b), were obtained by reacting the hexacarbonyls M(CO)(6) with the tridentate ligands 2-[(2-N-arylamino)phenylazo]pyridine (HL = NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(H) (HL(a)) or NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(CH(3)) (HL(b))) in refluxing n-octane. In the case of M = Mo, the dinuclear compounds [Mo(L)(pap)](2)(mu-O) (3a,b) (pap = 2-(phenylazo)pyridine), were obtained as second products in moist solvent. X-ray diffraction analysis for Cr(L(b))(2) (1b), Mo(L(a))(2) (2a), and W(L(a))(2) (4a) reveals considerably distorted-octahedral structures with trans-positioned azo-N atoms and cis-positioned 2-pyridyl-N and anilido nitrogen atoms. Whereas the N(azo)-M-N(azo) angle is larger than 170 degrees, the other two trans angles are smaller, at about 155 degrees (M = Cr, 1b) or 146 degrees (M = Mo, W; 2a, 4a), due to the overarching bite of the mer-tridentate ligands. The bonds from M to the neutral 2-pyridyl-N atoms are distinctly longer by more than 0.08 A than those to the anilido or azo nitrogen atoms, reflecting negative charge on the latter. The N-N bond distances vary between 1.339(2) A for 1b and 1.373(3) A for 4a, clearly indicating the azo radical anion oxidation state. Considering the additional negative charge on anilido-N, the mononuclear complexes are thus formulated as M(IV)(L*(2-))(2). The diamagnetism of the complexes as shown by magnetic susceptibility and (1)H NMR experiments is believed to result from spin-spin coupling between the trans-positioned azo radical functions, resulting in a singlet diradical situation. The experimental structures are well reproduced by density functional theory calculations, which also support the overall electronic structure indicated. The dinuclear 3a with N-N distances of 1.348(10) A for L(a) and 1.340(9) A for pap is also formulated as an azo anion radical-containing molybdenum(IV) species, i.e., [Mo(IV)(L*(2-))(pap*-)](2)(mu-O). All compounds can be reversibly reduced; the Cr complexes 1a,b are also reversibly oxidized in two steps. Electron paramagnetic resonance spectroscopy indicates metal-centered spin for 1a+ and 1a- and g approximately 2 signals for 2a-, 3a+, 3a-, and 4a-. Spectroelectrochemistry in the UV-vis-NIR region showed small changes for the reduction of 2a, 3a, and 4a but extensive spectral changes for the reduction and oxidation of 1a.

Organocobalt Mediated Radical Polymerization of Acrylic Acid in Water

Low polydispersity high molecular weight poly(acrylic acid) (PAA) is formed rapidly by the aqueous polymerization of acrylic acid (AA) initiated by an azo radical source (V-70) in the presence of cobalt complexes of tetra(3,5-disulfonatomesityl) porphyrin. Water-soluble cobalt(II) porphyrins in conjunction with an azo radical source and acrylic acid react to produce organocobalt complexes that mediate the radical polymerization of AA in water. Linear increase in number-average molecular weight with monomer conversion indicates the living character of the polymerization. Kinetic−mechanistic studies demonstrate that organocobalt complexes mediate the living radical polymerization by both a degenerative transfer radical interchange polymerization (RIP) mechanism and cobalt(II) stable free radical reversible termination.

Phenolic Glycosides with Antioxidant Activity from the Stem Bark of Populus davidiana

Phytochemical study on the EtOAc-soluble fraction of the stem bark of Populus davidiana resulted in the isolation of 10 phenolic glycosides (1-10), which were identified on the basis of physicochemical and spectroscopic analyses. Among these, three new compounds, populosides A-C (1-3), were determined to be 2-coumaroylmethyl-4-hydroxyphenyl-beta-D-glucopyranoside, 2-coumaroylmethylphenyl-beta-D-glucopyranoside, and 2-feruoylmethylphenyl-beta-D-glucopyranoside, respectively. Compounds 1-10 were tested for their radical scavenging activity against an azo radical, ABTS*+. Of these, populosides A-C (1-3), populoside (4), grandidentatin (8), salireposide (9), and coumaroyl-beta-D-glucoside (10) exhibited antioxidant activity in this assay.