Formation Of Precursor Complex Research Articles

Proton-Coupled Electron-Transfer Reduction of Dioxygen: The Importance of Precursor Complex Formation between Electron Donor and Proton Donor.

The proton-coupled electron transfer (PCET) reaction has drawn extensive attention for its widespread occurrence in chemistry, biology, and materials science. The mechanistic studies via model systems such as tyrosine and phenol oxidation have gradually deepened the understanding of PCET reactions, which was widely accepted and applied to bond activation and transformation. However, direct PCET activation of nonpolar bonds such as the C-H bond, O2, and N2 has yet to be explored. Herein, we report that the interaction between electron donor and proton donor could overcome the barrier of direct O2 activation via a concerted electron-proton transfer mechanism. This work provides a new strategy for developing direct PCET activation of nonpolar bonds.

Cyclic degradation of thiocyanate in cyanide barren solution by manganese oxides

This study describes the degradation of thiocyanate by manganese oxides. With NaOH and CaO as precipitating agents, two manganese oxides, i.e. MnOx/Na and MnOx/Ca, were prepared using MnSO4·H2O as Mn resource and air as oxidant. The influence of various reaction parameters like pH, manganese oxide dosage and initial thiocyanate concentration were systematically investigated with MnO2 as a reference. Based on an entire process dynamics model and the initial kinetics analysis, the rate-limiting step (formation of the precursor complexes) and reaction orders were derived independently. The reaction pathway was detected and the formation of the precursor complexes between surface positively charged manganese oxides and thiocyanate was the first step of thiocyanate degradation, after which electron transfer occurred within the precursor complexes and intermediate products were formed, which were rapidly released to produce SO42− and CN−, and some of the CN− would be further oxidized to HCO3− and NH4+. The formation of precursor complexes is related to the charge distribution on the surface of the manganese oxide and the protonation of thiocyanate in solution, which makes the lower pH favorable. The results of cyclic treatment of cyanide barren solution showed good regenerability of MnOx/Ca and gradual reduction of undesired secondary salts (especially SO42− ions) in the degraded liquid, which made it a potentially reliable technique to regulate the thiocyanate concentration during the cyanidation process of gold ores economically.

PH-Dependent mechanisms and kinetics of the removal of acetaminophen by manganese dioxide

Acetaminophen is commonly found in aquatic environments because it is widely used as an analgesic. In this study, the extent of transformation and adsorption of acetaminophen by manganese dioxide (MnO2) at different pH conditions was systematically investigated. The adsorption of acetaminophen by MnO2 was significant under acidic conditions but negligible under neutral and alkaline conditions. This indicates that the rate-determining steps in the removal of acetaminophen are electron transfer under acidic conditions and precursor complex formation under neutral and alkaline conditions. The kinetic simulation result is consistent with the above results, and the kinetics can be described using a surface complexation and transformation kinetic model if the initial concentration of acetaminophen exceeds the total concentration of the reactive surface sites on MnO2. Otherwise, the Michaelis–Menten model and the pseudo-first-order kinetic model can be used to describe the removal kinetics under acidic conditions and under neutral and alkaline conditions, respectively. The main transformation products were p-benzoquinone and a dimer of 4-aminophenol and acetaminophen under acidic and alkaline conditions, respectively. This indicates that dehydration–oxidation is the main pathway only under acidic conditions. Under alkaline conditions, the main transformation mechanism is dimerization–hydrolysis.

PH-Dependent Degradation of Diclofenac by a Tunnel-Structured Manganese Oxide

The mechanism of diclofenac (DIC) degradation by tunnel-structured γ-MnO2, with superior oxidative and catalytic abilities, was determined in terms of solution pH. High-performance liquid chromatography with mass spectroscopy (HPLC–MS) was used to identify intermediates and final products of DIC degradation. DIC can be efficiently oxidized by γ-MnO2 in an acidic medium, and the removal rate decreased significantly under neutral and alkaline conditions. The developed model can successfully fit DIC degradation kinetics and demonstrates electron transfer control under acidic conditions and precursor complex formation control mechanism under neutral to alkaline conditions, in which the pH extent for two mechanisms exactly corresponds to the distribution percentage of ionized species of DIC. We also found surface reactive sites (Srxn), a key parameter in the kinetic model for mechanism determination, to be exactly a function of solution pH and MnO2 dosage. The main products of oxidation with a highly active hydroxylation pathway on the tunnel-structured Mn-oxide are 5-iminoquinone DIC, hydroxyl-DIC, and 2,6-dichloro-N-o-tolylbenzenamine.

Transformation of m-aminophenol by birnessite (δ-MnO2) mediated oxidative processes: Reaction kinetics, pathways and toxicity assessment

The m-aminophenol (m-AP) is a widely used industrial chemical, which enters water, soils, and sediments with waste emissions. A common soil metal oxide, birnessite (δ-MnO2), was found to mediate the transformation of m-AP with fast rates under acidic conditions. Because of the highly complexity of the m-AP transformation, mechanism-based models were taken to fit the transformation kinetic process of m-AP. The results indicated that the transformation of m-AP with δ-MnO2 could be described by precursor complex formation rate-limiting model. The oxidative transformation of m-AP on the surface of δ-MnO2 was highly dependent on reactant concentrations, pH, temperature, and other co-solutes. The UV-VIS absorbance and mass spectra analysis indicated that the pathway leading to m-AP transformation may be the polymerization through the coupling reaction. The m-AP radicals were likely to be coupled by the covalent bonding between unsubstituted C2, C4 or C6 atoms in the m-AP aromatic rings to form oligomers as revealed by the results of activation energy and mass spectra. Furthermore, the toxicity assessment of the transformation productions indicated that the toxicity of m-AP to the E. coli K-12 could be reduced by MnO2 mediated transformation. The results are helpful for understanding the environmental behavior and potential risk of m-AP in natural environment.

Mixed-halide perovskite synthesis by chemical reaction and crystal nucleation under an optical potential

The development of new methods to engineer lead halide perovskite crystals with a controlled band gap and emission properties is an active subject in materials science and chemistry. We present the preparation of mixed-halide lead perovskites by spatially- and temporally- controlled chemical reactions and crystal growth under an optical potential in unsaturated precursor solutions. The crystals are characterized by transmission and photoluminescence spectral measurements and X-ray diffraction analysis. When compared with the spontaneous formation of multiple crystals in saturated precursor solutions, the optical potential creates large single crystals with a high chloride composition, providing distinct blue and green fluorescent crystals of chloride–bromide lead perovskites. We discuss the formation of mixed-halide perovskites from the viewpoints of an increased rate of chemical reaction via the formation and desolvation of precursor complexes and a decreased free energy potential.

Geochemistry, zircon U-Pb and Lu-Hf systematics of high-grade metasedimentary sequences from the South Muya block (northeastern Central Asian Orogenic Belt): Reconnaissance of polymetamorphism and accretion of Neoproterozoic exotic blocks in southern Siberia

High-grade metasedimentary rocks of the South Muya block (Baikal-Muya Foldbelt) have been studied with respect to whole-rock elemental and Nd isotopic compositions, as well as the U-Pb and Lu-Hf systematics of detrital zircon and in-situ grown metamorphic grains in order to reveal the initial source provenance for sedimentation and the age of major metamorphic modification. Based predominantly on trace-elemental compositions, the sedimentary protolith is most likely a poorly sorted and immature greywacke, such as those formed in island arc or active continental margin settings. The island arc-related chemical signature coupled with zircon U-Pb age data supports a Neoproterozoic (>760 Ma) sedimentation age, whereas Nd-Hf isotope data of precursor sediments are linked to a relatively immature Tonian (∼940–780 Ma) continental arc involving Paleoproterozoic crustal basement. The limited provenance information and metamorphic history of the Muya block suggests it was exotic to Siberia and originally related to another continent, e.g. – subduction-accretion complexes of South China. The polyphase metasediments experienced high-grade metamorphism in the Tonian during the 764–754 Ma arc accretion to an enigmatic continental block and then in the Ediacaran during the 622–608 Ma accretion of the Baikal-Muya belt to the arc situated at the southern Siberia margin. Metamorphic overprinting during the introduction of supracrustal rocks to deeper-crustal levels led to protracted recrystallization of primary igneous zircon population and growth of new granulite-facies grains that did not involve notable juvenile input. Tonian sedimentary provenance and two metamorphic events suggest a protracted evolution for the Baikal-Muya belt associated with the formation of precursor complexes beyond the southern margin of Siberia and their later accretion to Late Cryogenian – Ediacarian arc complexes of the southern Siberian, which proceeded only after the opening of the Paleoasian Ocean (670–630 Ma).

Outer-Sphere 2 e- /2 H+ Transfer Reactions of Ruthenium(II)-Amine and Ruthenium(IV)-Amido Complexes.

A diverse set of 2 e- /2 H+ reactions are described that interconvert [RuII (bpy)(en*)2 ]2+ and [RuIV (bpy)(en-H*)2 ]2+ (bpy=2,2'-bipyridine, en*=H2 NCMe2 CMe2 NH2 , en*-H=H2 NCMe2 CMe2 NH- ), forming or cleaving different O-H, N-H, S-H, and C-H bonds. The reactions involve quinones, hydrazines, thiols, and 1,3-cyclohexadiene. These proton-coupled electron transfer reactions occur without substrate binding to the ruthenium center, but instead with precursor complex formation by hydrogen bonding. The free energies of the reactions vary over more than 90 kcal mol-1 , but the rates are more dependent on the type of X-H bond involved than the associated ΔG°. There is a kinetic preference for substrates that have the transferring hydrogen atoms in close proximity, such as ortho-tetrachlorobenzoquinone over its para-isomer and 1,3-cyclohexadiene over its 1,4-isomer, perhaps hinting at the potential for concerted 2 e- /2 H+ transfers.

Outer‐Sphere 2 e−/2 H+ Transfer Reactions of Ruthenium(II)‐Amine and Ruthenium(IV)‐Amido Complexes

AbstractA diverse set of 2 e−/2 H+ reactions are described that interconvert [RuII(bpy)(en*)2]2+ and [RuIV(bpy)(en‐H*)2]2+ (bpy=2,2′‐bipyridine, en*=H2NCMe2CMe2NH2, en*‐H=H2NCMe2CMe2NH−), forming or cleaving different O−H, N−H, S−H, and C−H bonds. The reactions involve quinones, hydrazines, thiols, and 1,3‐cyclohexadiene. These proton‐coupled electron transfer reactions occur without substrate binding to the ruthenium center, but instead with precursor complex formation by hydrogen bonding. The free energies of the reactions vary over more than 90 kcal mol−1, but the rates are more dependent on the type of X−H bond involved than the associated ΔG°. There is a kinetic preference for substrates that have the transferring hydrogen atoms in close proximity, such as ortho‐tetrachlorobenzoquinone over its para‐isomer and 1,3‐cyclohexadiene over its 1,4‐isomer, perhaps hinting at the potential for concerted 2 e−/2 H+ transfers.

Oxidative Degradation of Methylene Blue Using Mn3O4 Nanoparticles

Mn3O4 nanoparticles were synthesized from one-step reduction of KMnO4 with glycerol at 80 °C. The structural and surface morphological characterizations were carried out using FT-IR, XRD, and FESEM analyses. The elemental composition was evidenced from EDX analysis. XRD analysis showed the tetragonal crystal geometry of Mn3O4 nanoparticles with an average crystallite size of ∼20 nm. The surface morphology of the Mn3O4 nanoparticles was found to be spherical from the FESEM image. The Mn3O4 nanoparticles were then tested as a potential oxidant for the decolorization of methylene blue (MB) and found to be capable of N-demethylation of MB forming thionine as the final product, and removing 80% of the dye in approximately 1 h. The decolorization of MB by Mn3O4 occurred through a surface mechanism, i.e., formation of surface precursor complex between MB and surface-bound Mn (II, III), where, electron transfer occurs within the surface complex. The effect of suspension pH (3–4 < pHpzc; 5–10 > pHpzc) on MB decolorization was assessed. Suspension pH exerted double-edged effects on MB decolorization by influencing the formation of surface precursor complex, and reducing potential of the system.

Caffeine metabolites not caffeine protect against riboflavin photosensitized oxidative damage related to skin and eye health

Caffeine metabolites were found to bind riboflavin with dissociation constant in the millimolar region by an exothermic process with positive entropy of reaction, which was found by 1H NMR and fluorescence spectroscopy to occur predominantly by hydrogen bonding with water being released from riboflavin solvation shell upon caffeine metabolite binding to riboflavin. The caffeine metabolites 1-methyl uric acid and 1,7-dimethyl uric acid were shown by transient absorption laser flash photolysis to be efficient as quenchers of triplet riboflavin with second-order rate constant of 1.4 108Lmol−1s−1 and 1.0 108Lmol−1s−1, respectively, in aqueous solution of pH6.4 at 25°C and more efficient than the other caffeine metabolite 1,7-dimethyl xanthine with second-order rate constant of 4.2 107Lmol−1s−1. Caffeine was in contrast found to be non-reactive towards triplet riboflavin. Caffeine metabolites rather than caffeine seem accordingly important for the observed protective effect against cutaneous melanoma identified for drinkers of regular but not of decaffeinated coffee. The caffeine metabolites, but not caffeine, were by time resolved single photon counting found to quench singlet excited riboflavin through exothermic formation of ground-state precursor complexes indicating importance of hydrogen bounding through keto-enol tautomer's for protection of oxidizable substrates and sensitive structures against riboflavin photosensitization.

Birnessite (δ-MnO2) mediated degradation of organoarsenic feed additive p-arsanilic acid.

p-Arsanilic acid (p-ASA), is a widely used animal feed additive in many developing countries, and is often introduced to agricultural soils with animal wastes. A common soil metal oxide, birnessite (δ-MnO2), was found to mediate its degradation with fast rates under acidic conditions. Experimental results indicate that adsorption and degradation of p-ASA on the surface of δ-MnO2 were highly pH dependent, and the overall kinetics for p-ASA degradation and formation of precursor complex could be described by a retarded first-order rate model. For the reaction occurring between pH 4.0 and 6.2, the initial rate equation was determined to be rinit=2.36×10(-5)[ASA]0.8[MnO2]0.9[H+]0.7. p-ASA first forms a surface precursor complex on δ-MnO2 during degradation, followed by formation of p-ASA radicals through single-electron transfer to δ-MnO2. The p-ASA radicals subsequently undergo cleavage of arsenite group (which is further oxidized to arsenate) or radical-radical self-coupling. Instead of full mineralization (with respect to arsenic only), about one-fifth of the p-ASA "couples" to form an arsenic-bearing azo compound that binds strongly on δ-MnO2. The fast transformation of p-ASA to arsenite and arsenate mediated by δ-MnO2 significantly increases the risk of soil arsenic pollution and deserves significant attention in the animal farming zones still using this feed additive.

NMR characterization of PAMAM_G5.NH2 entrapped atomic and molecular assemblies.

High resolution NMR spectroscopy, NMR diffusiometry, and NMR cryoporometry have been used to investigate aqueous solution (D2O) of PAMAM_G5.NH2-(Au)(25-100) and PAMAM_G5.NH2-(H2O)1000-(H2O)4000 systems. In the case of dendrimer entrapped gold nanoparticles, the detailed analysis of high resolution NMR spectra has shown that no precursor complex formation happens under the circumstances applied for reduction. Further PGSE results verify that gold nanoparticles of 1.9-2.6 nm size are entrapped in the outermost part of the dendrimers and probably more than one dendrimer molecule takes part in the stabilization process. This system looks like a transition state between dendrimer encapsulated nanoparticles (DENs) and dendrimer stabilized nanoparticles (DSNs), and we deal with it in details for what this means. NMR cryoporometry experiments were performed to detect the encapsulation of water molecules. The results show that, in the swelling PAMAM_G5.NH2 dendrimers, by adding water step by step, there are specific cavities for water with diameters of 3.6 and 5.2 nm. These cavities have a penetrable wall for water molecules and probably exist very close to the terminal groups. The permeability of the cavities is increasing with the increase of the water content. In dilute solution, the formation of nanoparticles is determined by the ratio of the rate of nucleation and aggregation and the latter is affected by the PAMAM_G5.NH2.

Theoretical Advances in Aromatic Nitration

In this paper we want to present together our theoretical findings on aromatic nitration, involving both homocyclic and heterocyclic compounds. Our theoretical proposals have been introduced in recent years via several research communications. These theories convey new concepts in Organic Chemistry, such as Electric Hindrance, Autogenic Electromeric Effect, the formation of Precursor Complexes, as well as, short- and long-distance electric interactions in the reactive intermediates. These concepts explain the regiochemistry and the reaction yields, clearing up some obscurities found in Organic Chemistry. Thus, these theoretical considerations are filling gaps in the theory of supposedly well known chemical reactions, improving and updating the Chemical Education.

Brønsted Acid-Promoted C–H Bond Cleavage via Electron Transfer from Toluene Derivatives to a Protonated Nonheme Iron(IV)-Oxo Complex with No Kinetic Isotope Effect

The reactivity of a nonheme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), was markedly enhanced by perchloric acid (70% HClO4) in the oxidation of toluene derivatives. Toluene, which has a high one-electron oxidation potential (Eox = 2.20 V vs SCE), was oxidized by [(N4Py)Fe(IV)(O)](2+) in the presence of HClO4 in acetonitrile (MeCN) to yield a stoichiometric amount of benzyl alcohol, in which [(N4Py)Fe(IV)(O)](2+) was reduced to [(N4Py)Fe(III)(OH2)](3+). The second-order rate constant (kobs) of the oxidation of toluene derivatives by [(N4Py)Fe(IV)(O)](2+) increased with increasing concentration of HClO4, showing the first-order dependence on [HClO4]. A significant kinetic isotope effect (KIE) was observed when mesitylene was replaced by mesitylene-d12 in the oxidation with [(N4Py)Fe(IV)(O)](2+) in the absence of HClO4 in MeCN at 298 K. The KIE value drastically decreased from KIE = 31 in the absence of HClO4 to KIE = 1.0 with increasing concentration of HClO4, accompanied by the large acceleration of the oxidation rate. The absence of KIE suggests that electron transfer from a toluene derivative to the protonated iron(IV)-oxo complex ([(N4Py)Fe(IV)(OH)](3+)) is the rate-determining step in the acid-promoted oxidation reaction. The detailed kinetic analysis in light of the Marcus theory of electron transfer has revealed that the acid-promoted C-H bond cleavage proceeds via the rate-determining electron transfer from toluene derivatives to [(N4Py)Fe(IV)(OH)](3+) through formation of strong precursor complexes between toluene derivatives and [(N4Py)Fe(IV)(OH)](3+).

Influence of the mixed micelles on the electron transfer reaction [Co(NH3)5Cl]2+ + [Fe(CN)6]4−

Electron transfer reaction between chloropentaamminecobalt (III) and hexacyanoferrate (II) has been studied in the mixed surfactant aqueous solutions of sodium dodecyl sulfate (SDS) and octylphenol (ethylene oxide)9.5 ether (TX-100). The observed rate constants kobs have been determined for various micellar media with different compositions. A pseudophase model together with an outer-sphere electron transfer reaction mechanism has been used to analyze the experimental results to obtain the precursor complex formation equilibrium constant KIP, the electron transfer rate constant ket, and the pseudophase equilibrium constant K. The dependence of those parameters on the micellar concentration has been discussed.

Elucidation of the mechanism of surface-initiated atom transfer radical polymerization from a diazonium-based initiator layer

This study elucidates the influence of the atom trans- fer radical polymerization initiator structure, monolayer versus disordered multilayer, on the growth kinetics and the structural transition of poly(methyl methacrylate) (PMMA) brush layers. The multilayer initiator film, prepared by acylation of the electro- grafted 2-phenylethanol layer using 2-bromoisobutyryl bromide, consists of � 4.6 times more tert-butyl bromide groups compared to monolayer initiator prepared by self assembly technique. The results demonstrate the formation of precursor complex between Cu I catalyst and the bromine initiator as a prerequisite step before the onset of polymerization. Furthermore, the PMMA brushes formed by the polymerization from the multilayered initiator layer at 50 � C are 20-fold thicker compared to the polymerization at 25 � C due to the swelling of the multilayered initiator film. In con- trast, the thickness of the PMMA layer on the monolayer initiator is less affected by the polymerization temperature. By varying the initiator density on the surface, the solvent content in the PMMA layer is shown to vary from 15% to 94%, resulting in the transition from concentrated over semidiluted to diluted brushes. V C 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 4465- 4475, 2012

Kinetics and pathways for the debromination of polybrominated diphenyl ethers by bimetallic and nanoscale zerovalent iron: Effects of particle properties and catalyst

Polybrominated diphenyl ethers (PBDEs) are recognized as a new class of widely-distributed and persistent contaminants for which effective treatment and remediation technologies are needed. In this study, two kinds of commercially available nanoscale Fe0 slurries (Nanofer N25 and N25S), a freeze-dried laboratory-synthesized Fe0 nanoparticle (nZVI), and their palladized forms were used to investigate the effect of particle properties and catalyst on PBDE debromination kinetics and pathways. Nanofers and their palladized forms were found to debrominate PBDEs effectively. The laboratory-synthesized Fe0 nanoparticles also debrominated PBDEs, but were slower due to deactivation by the freeze-drying and stabilization processes in the laboratory synthesis. An organic modifier, polyacrylic acid (PAA), bound on N25S slowed PBDE debromination by a factor of three to four compared to N25. The activity of palladized nZVI (nZVI/Pd) was optimized at 0.3Pd/Fewt% in our system. N25 could debrominate selected environmentally-abundant PBDEs, including BDE 209, 183, 153, 99, and 47, to end products di-BDEs, mono-BDEs and diphenyl ether (DE) in one week, while nZVI/Pd (0.3Pd/Fewt%) mainly resulted in DE as a final product. Step-wise major PBDE debromination pathways by unamended and palladized Fe0 are described and compared. Surface precursor complex formation is an important limiting factor for palladized Fe0 reduction as demonstrated by PBDE pathways where steric hindrance and rapid sequential debromination of adjacent bromines play an important role.

Using pyridine as nitrogen-rich precursor to synthesize Co-N-S/C non-noble metal electrocatalysts for oxygen reduction reaction

The development of non-noble metal catalysts is of great interest due to their significant potential application in both fuel cell systems and metal–air batteries, particularly when considering long term commercial deployment. In this regard, novel Co-N-S/C non-noble metal catalysts supported on carbon, are synthesized in this study using a solvent-milling method followed by heat-treatment at elevated temperatures. Pyridine is used as the nitrogen-rich ligand for Co-Nx precursor complex formation. The morphology and composition of the catalyst are characterized by X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM) as well as X-ray photoelectron spectroscopy (XPS). Several catalysts containing different amounts of Co are also synthesized. The optimal Co content is found to be in the range of 10–15wt% nominal, in terms of catalytic oxygen reduction reaction (ORR) activity. This catalyst shows high electroactivity for the ORR with a high stability in alkaline media. Using rotating disk electrode measurements, and Koutechy–Levich analysis, the overall electron transfer number in the catalyzed ORR is found to be 3.8–4.0, suggesting the catalyzed ORR is a four-electron transfer process from O2 to H2O.

Electrochemical Reduction of Oxygen on Nano-Structured M (M = Fe, Co, Ce, W)Py/C in Alkaline Electrolyte

This work reports the preparation and characterization of novel non-precious metal catalysts metal catalysts supported on carbon, M (M = Fe, Co, Ce, W)Py/C, using a solvent-milling method followed by a heat-treatment at elevated temperature, where pyridine is used as the N-rich ligand for precursor complex formation. The morphology and composition of the catalysts were characterized by Fourier-transform infrared (FT-IR) spectra and thermal analysis (TGA). The catalytic activity on oxygen reduction reaction (ORR) was found to depend strongly on the heat-treatment temperatures and the metal pressures for catalyst synthesis. The resulting MPy/C of 40/60 ratio heat-treated at 800oC shows the best catalytic performance in terms of ORR and follows the order: Co > Fe > Ce > W, where the catalyst composed of Co10wt% -Py30wt%/C shows the highest electroactivity for ORR in alkaline media. Using Rotating disk electrode measurements and Koutechy-Levich analysis, the overall electron transfer number in the catalyzed ORR on Co10wt% -Py30wt%/C was found to be 3.8, suggesting the catalyzed ORR is a 4-electron transfer process from O2 to H2O.