Stepwise vs. concerted pathways in scandium ion-coupled electron transfer from superoxide ion to p-benzoquinone derivatives

Superoxide ion (O2˙-) forms a stable 1 : 1 complex with scandium hexamethylphosphoric triamide complex [Sc(HMPA)(3)(3+)], which can be detected in solution by ESR spectroscopy. Electron transfer from O2˙- -Sc(HMPA)(3)(3+) complex to a series of p-benzoquinone derivatives occurs, accompanied by binding of Sc(HMPA)(3)(3+) to the corresponding semiquinone radical anion complex to produce the semiquinone radical anion-Sc(HMPA)(3)(3+) complexes. The 1 : 1 and 1 : 2 complexes between semiquinone radical anions and Sc(HMPA)(3)(3+) depending on the type of semiquinone radical anions were detected by ESR measurements. This is defined as Sc(HMPA)(3)(3+)-coupled electron transfer. There are two reaction pathways in the Sc(HMPA)(3)(3+)-coupled electron transfer. One is a stepwise pathway in which the binding of Sc(HMPA)(3)(3+) to semiquinone radical anions occurs after the electron transfer, when the rate of electron transfer remains constant with the change in concentration of Sc(HMPA)(3)(3+). The other is a concerted pathway in which electron transfer and the binding of Sc(HMPA)(3)(3+) occurs in a concerted manner, when the rates of electron transfer exhibit first-order and second-order dependence on the concentration of Sc(HMPA)(3)(3+) depending the number of Sc(HMPA)(3)(3+) (one and two) bound to semiquinone radical anions. The contribution of two pathways changes depending on the substituents on p-benzoquinone derivatives. The present study provides the first example to clarify the kinetics and mechanism of metal ion-coupled electron-transfer reactions of the superoxide ion.     

New perspective of electron transfer chemistry

A new perspective of electron transfer chemistry is described for fine control of electron transfer reactions including back electron transfer in the charge separated state of artificial photosynthetic compounds and its synthetic application. Fundamental electron transfer properties of suitable components of efficient electron transfer systems are described in light of the Marcus theory of electron transfer, in particular focusing on the Marcus inverted region, and they are applied to design multi-step electron transfer systems which can well mimic the function of a photosynthetic reaction center. Both intermolecular and intramolecular electron transfer processes are finely controlled by complexation of radical anions, produced in the electron transfer, with metal ions which act as Lewis acids. Quantitative measures to determine the Lewis acidity of a variety of metal ions are given in relation to the promoting effects of metal ions on the electron transfer reactions. The mechanistic viability of metal ion catalysis in electron transfer reactions is demonstrated by a variety of examples of chemical transformations involving metal ion-promoted electron transfer processes as the rate-determining steps, which are made possible by complexation of radical anions with metal ions.     

The Radical Anions of Heptalene and 1, 7-Methano-[12]annulene

The radical anions of heptalene and of 1, 7-methano-[12]annulene are generated by metal reduction and characterized by means of their ESR-spectra. Whereas the neutral hydrocarbons are π-bond localized their corresponding radical anions turn out to be π-bond delocalized. This could be deduced from an interpretation of the different hyperfine splittings using a simple MO-model.     

1,2,5-Thiadiazole 1,1-dioxides and Their Radical Anions: Structure,Properties, Reactivity,and Potential Use in the Construction of Functional Molecular Materials

This work provides a summary of the preparation, structure, reactivity, physicochemical properties, and main uses of 1,2,5-thiadiazole 1,1-dioxides in chemistry and material sciences. An overview of all currently known structures containing the 1,2,5-thiadiazole 1,1-dioxide motif (including the anions radical species) is provided according to the Cambridge Structural Database search. The analysis of the bond lengths typical for neutral and anion radical species is performed, providing a useful tool for unambiguous assessment of the valence state of the dioxothiadiazole-based compounds based solely on the structural data. Theoretical methodologies used in the literature to describe the dioxothiadiazoles are also shortly discussed, together with the typical ‘fingerprint’ of the dioxothiadiazole ring reported by means of various spectroscopic techniques (NMR, IR, UV-Vis). The second part describes the synthetic strategies leading to 1,2,5-thiadiazole 1,1-dioxides followed by the discussion of their electrochemistry and reactivity including mainly the chemical methods for the successful reduction of dioxothiadiazoles to their anion radical forms and the ability to form coordination compounds. Finally, the magnetic properties of dioxothiadiazole radical anions and the metal complexes involving dioxothiadiazoles as ligands are discussed, including simple alkali metal salts and d-block coordination compounds. The last section is a prospect of other uses of dioxothiadiazole-containing molecules reported in the literature followed by the perspectives and possible future research directions involving these compounds.     

Isostructural metal-anion radical coordination polymers with tunable phosphorescent colors (deep blue, blue, yellow, and white) induced by terminal anions and metal cations

Five phosphorescent metal-anion radical coordination polymers based on a new anion radical ligand generated by in situ deprotonation of a stable zwitterionic radical are described. The N,O,N-tripodal anion radical ligand links metal cations, which leads to five isostructural coordination polymers, [M(3)(bipo(-.))(4)(L)(2)](n) (M=Cd or Mn, Hbipo(-.)=2,3'-biimidazo[1,2-a]pyridin-2'-one, L=Cl(-), HCOO(-) or SCN(-)). The isostructural coordination polymers exhibit novel one-dimensional spirocycle-like structures. Three isostructural Cd(II) coordination polymers display unusual phosphorescent color changes (blue, yellow, and white) induced by terminal anions. Significantly, the Cd(II) coordination polymer with terminal Cl(-) possesses moderate quantum yield, and shows a bright white-light phosphorescence emission, which is independent of excitation wavelength and can even be excited by visible light. Upon adjusting the metal cation to Mn(II), two isostructural Mn(II) coordination polymers reveal deep-blue-light phosphorescence emissions that are independent of terminal anions. As radical-based coordination polymers, some of them show antiferromagnetic interactions between radical species or radical and metal center.     

Reactions of simple and peptidic alpha-carboxylate radical anions with dioxygen in the gas phase

α-Carboxylate radical anions are potential reactive intermediates in the free radical oxidation of biological molecules (e.g., fatty acids, peptides and proteins). We have synthesised well-defined α-carboxylate radical anions in the gas phase by UV laser photolysis of halogenated precursors in an ion-trap mass spectrometer. Reactions of isolated acetate (˙CH(2)CO(2)(-)) and 1-carboxylatobutyl (CH(3)CH(2)CH(2)˙CHCO(2)(-)) radical anions with dioxygen yield carbonate (CO(3)˙(-)) radical anions and this chemistry is shown to be a hallmark of oxidation in simple and alkyl-substituted cross-conjugated species. Previous solution phase studies have shown that C(α)-radicals in peptides, formed from free radical damage, combine with dioxygen to form peroxyl radicals that subsequently decompose into imine and keto acid products. Here, we demonstrate that a novel alternative pathway exists for two α-carboxylate C(α)-radical anions: the acetylglycinate radical anion (CH(3)C(O)NH˙CHCO(2)(-)) and the model peptide radical anion, YGGFG˙(-). Reaction of these radical anions with dioxygen results in concerted loss of carbon dioxide and hydroxyl radical. The reaction of the acetylglycinate radical anion with dioxygen reveals a two-stage process involving a slow, followed by a fast kinetic regime. Computational modelling suggests the reversible formation of the C(α) peroxyl radical facilitates proton transfer from the amide to the carboxylate group, a process reminiscent of, but distinctive from, classical proton-transfer catalysis. Interestingly, inclusion of this isomerization step in the RRKM/ME modelling of a G3SX level potential energy surface enables recapitulation of the experimentally observed two-stage kinetics.     

Anionen in der Gasphase

Important general structural and energetic characteristics are provided for atomic and molecular anions, before spectroscopic quantities such as electron affinity are defined to characterize stable anions. Prepared in this way one encounters atomic anions in the first scope. After a short prelude with diatomic molecular anions an attempt is made to juxtapose anions of alkanes and silanes and the corresponding olefinic systems. Associated with these species there is the phenomenon of short-lived resonances on the one hand, while on the other hand the anions can be stabilized by appropriate geometry distortions or substituents. Distonic radical anions, where the radical-center does not coincide with the charge center, or dipolarly-bound anions, where the extra electron is very feebly bound, are not very well known hitherto, but have a wide field of application in organic chemistry. The presentation of solvated anions with a special focus on S_N2-reactions completes this area. Only a few examples document the unique double-Rydherg anions, which are characterized by a pair of electrons bound loosely to a cationic core. The multiply-charged anions are shown to have a very wide field of application in the sequel. The initial “disappointment” that the well-known small textbook-polyanions do not exist in the gas phase is followed by the apothegm “more space for charge separation”. With this key- note a wide field for organic (carbonic acids, carbon clusters) as well as inorganic compounds (complexes) is opened.     

Alkali Metal Adducts of Aromatic Nitro Compounds—Reactions and Use in Polymerization of Vinyl Monomers

Abstract

The reaction of alkali metals with nitrobenzene and p-nitro-toluene in THF at various molar ratios was found to lead to the formation of radical ions, dianions, and alkali metal adducts of reduction derivatives of the nitro compounds such as azo- and azoxybenzene. The anionic polymerization of styrene, methyl methacrylate, methacrylonitrile, and acrylo-nitrile by these anions was investigated. All the initiators did not polymerize styrene while the least reactive radical-anion was found to polymerize acrylonitrile completely, methacrylonitrile to a small extent, but not methyl methacrylate.

The order of reactivity of those adducts toward organic halides was similar to that found in polymerization. Metalla-tion of polynitrostyrene by lithium biphenyl solution led only to partial conversion of the nitro groups to radical-anions which were not reactive.     

Synthesis,Structures, and Properties of Crystalline Salts with Radical Anions of Metal‐Containing and Metal‐Free Phthalocyanines

Radical anion salts of metal‐containing and metal‐free phthalocyanines [MPc(3?)]^.?, where M=Cu^II, Ni^II, H₂, Sn^II, Pb^II, Ti^IVO, and V^IVO ( 1 – 10 ) with tetraalkylammonium cations have been obtained as single crystals by phthalocyanine reduction with sodium fluorenone ketyl. Their formation is accompanied by the Pc ligand reduction and affects the molecular structure of metal phthalocyanine radical anions as well as their optical and magnetic properties. Radical anions are characterized by the alternation of short and long C?N_imine bonds in the Pc ligand owing to the disruption of its aromaticity. Salts 1 – 10 show new bands at 833–1041 nm in the NIR range, whereas the Q‐ and Soret bands are blue‐shifted by 0.13–0.25 eV (38‐92 nm) and 0.04–0.07 eV (4–13 nm), respectively. Radical anions with Ni^II, Sn^II, Pb^II, and Ti^IVO have S=1/2 spin state, whereas [Cu^IIPc(3?)]^.? and [V^IVOPc(3?)]^.? containing paramagnetic Cu^II and V^IVO have two S=1/2 spins per radical anion. Central metal atoms strongly affect EPR spectra of phthalocyanine radical anions. Instead of narrow EPR signals characteristic of metal‐free phthalocyanine radical anions [H₂Pc(3?)]^.? (linewidth of 0.08–0.24 mT), broad EPR signals are manifested (linewidth of 2–70 mT) with g‐factors and linewidths that are strongly temperature‐dependent. Salt 11 containing the [Na^IPc(2?)]^? anions as well as previously studied [Fe^IPc(2?)]^? and [Co^IPc(2?)]^? anions that are formed without reduction of the Pc ligand do not show changes in molecular structure or optical and magnetic properties characteristic of [MPc(3?)]^.? in 1 – 10 .     

Electron spin resonance studies of electron addition to 2-chloro- and 2-bromo-2-nitropropane

The products of electron addition to 2-chloro- and 2-bromo-2-nitropropane, which include their radical anions, have been detected by electron spin resonance spectroscopy.Radical anions of various aliphatic α-substituted nitro compounds have been proposed as reactive intermediates in radical-anion chain substitution reactions (S_RN1) (Scheme 1) and other related reactions.     

Carbon-halogen Bond Cleavage Energies of Alkyl Halides and Haloalkyl Radical Anions in Solution

Radical anions are intermediates in a variety of processes:dissolving metal reductions, cathodic reductions⁽¹⁾,homogeneous redox reactions, S_RN² reactions⁽²⁾ and nucleophilic substitions⁽³⁾. Several techniques ranging from pulse rediolysis, near-infrared spectroscopy, derivative linear-sweep voltammetry, and electron spin resonance spectroscopy have been employed to study the characteristic properties of the radical anions. The most characteristic property for the radical anions derived from organic halides, in particular, has been known to be the severe weakening of the carbon-halogen bond and consequently the ease of its dissociative cleavage to form halide ion and radical (eq.l) or to form halogen atom and anion (eq.2) as seen in the key step of the S_RN¹ mechanism.     

Gold clusters: reactions and deuterium uptake

We have examined the reactivity and saturation of small gold clusters (cations, neutrals and anions) towards several molecules and find that specific small gold clusters exhibit a pronounced variation in their reactivity towards hydrogen, methane and oxygen. The reactivity not only depends strongly on cluster size but also on the cluster charge state. For example, small (n<15) gold cations react readily with D₂, but no evidence of reaction is observed for the anions under our experimental conditions. Similar behavior is seen for methane. With oxygen only even atom (odd electron) anions are reactive, and Au ₁₀ ⁺ is the only cation which exhibits evidence of reaction. The global features (small cluster cations reactive towards H₂, CH₄, but large ones not reactive, odd electron anions reactive towards O₂) are qualitatively explained by appealing to a simple frontier orbital picture. The uptake of deuterium and methane on gold clusters also exhibits a pronounced size dependence with D/Au varying from a high of 3 for the dimer to zero for clusters containing more than 15 Au atoms. Comparison of the methane and deuterium saturation behavior leads us to suggest that methane is dissociated and bound as CH₃ and H.     

Redox Processes in Amidinylcyclopentadiene Compounds and Thallium Complexes

Cyclic voltammetry at platinum and dropping mercury electrodes in acetonitrile suggests that the reduction of amidinylcyclopentadiene ligands involves generation of corresponding radical anions and the oxidation, radical cations, whereas the reduction of thallium complexes occurs through the deposition of the free metal (the process is affected by the complexing agent) and their oxidation leads to the ligand oxidation (the process is affected by the metal ions). Given cyclic voltammetric data on model compounds of rigid structure in solutions, amidinylcyclopentadiene ligands and thallium complexes exist in acetonitrile in a nondissociated covalent form with a bidentate metal (hydrogen) coordination with the complexing agent.     

Effect of the concentration of Pt(IV) and Pd(II) chloro complexes on the proportions of their ion-exchanged and coordinatively bound species on the γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> surface

The interaction of platinum(IV) and palladium(II) chloro complexes with the γ-Al₂O₃ surface in a wide range of surface metal concentrations is reported. Varying the concentration of the adsorbed metal complex on the alumina surface causes changes both in the proportions of weakly and strongly bound desorbable platinum species and in the proportions of desorbable (ion-exchanged) and nondesorbable (coordinatively bound) complexes. The adsorbed palladium complexes are more uniform in chemical composition and binding strength and consist largely of desorbable species removable from the surface by competitive sorption of anions. The absolute amount of coordinatively bound platinum and palladium species increases as the total metal content of the sample is raised to 1.0% and remains almost invariable at higher metal contents.     

Methods of nonenzymatic determination of hydrogen peroxide and related reactive oxygen species

Contemporary nonenzymatic methods for the qualitative and quantitative determination of hydrogen peroxide and reactive oxygen species, preceding to hydrogen peroxide or resulting from it, are reviewed. Many of these procedures can be applied to the detection and determination of reactive oxygen species, for example, anions, peroxide radical anions, hydroxide radicals, etc., in both model and real samples that are of practical importance in biochemistry and medicine. The main direction of development in this area includes the target formation of a surface layer of a sensing element at the nanoscale level, including using nanoparticles. In some cases, higher selectivity can be achieved, and the analytical and performance characteristics of the procedures, such as minimum detectable concentration, analytical range, or sensitivity, can be improved. Most of the cited papers were published after 2010.     

The synthesis of diene and of vinyl copolymers containing predetermined quantities of polynuclear hydrocarbon or heterocyclic adducts—I. The formation and stability toward tetrahydrofuran of alkali metal salts of polynuclear hydrocarbons

The formation and stabilities of complexes formed in THF between various polynuclear hydrocarbons and excess sodium and lithium metal have been studied. Anthracene and acenaphthylene, which possess high electron affinities, form dianions with either metal whilst phenanthrene forms the dianion only with lithium. Both phenanthrene and naphthalene give solely radical ions on reaction with sodium; it is found that the formation of the naphthalene dianion with lithium is inversely dependent on the naphthalene concentration.The radical anions of all four polynuclear hydrocarbons are relatively stable to the THF solvent whereas the dianions react appreciably in a matter of days to form a variety of adducts and derivatives which have been isolated and identified by NMR spectroscopy.     

Generation of naphthoquinone radical anions by electrospray ionization: solution, gas-phase, and computational chemistry studies

Radical anions are present in several chemical processes, and understanding the reactivity of these species may be described by their thermodynamic properties. Over the last years, the formation of radical ions in the gas phase has been an important issue concerning electrospray ionization mass spectrometry studies. In this work, we report on the generation of radical anions of quinonoid compounds (Q) by electrospray ionization mass spectrometry. The balance between radical anion formation and the deprotonated molecule is also analyzed by influence of the experimental parameters (gas-phase acidity, electron affinity, and reduction potential) and solvent system employed. The gas-phase parameters for formation of radical species and deprotonated species were achieved on the basis of computational thermochemistry. The solution effects on the formation of radical anion (Q(?-)) and dianion (Q(2-)) were evaluated on the basis of cyclic voltammetry analysis and the reduction potentials compared with calculated electron affinities. The occurrence of unexpected ions [Q+15](-) was described as being a reaction between the solvent system and the radical anion, Q(?-). The gas-phase chemistry of the electrosprayed radical anions was obtained by collisional-induced dissociation and compared to the relative energy calculations. These results are important for understanding the formation and reactivity of radical anions and to establish their correlation with the reducing properties by electrospray ionization analyses.     

Anthralin: primary products of its redox reactions

One-electron reduction significantly enhances the ability of anthralin, 1, to act as a hydrogen atom donor. On annealing of an MTHF glass in which the radical anion of anthralin, 1*-, is generated radiolytically, this species decays mainly by loss of H* to give the anthralyl anion, 2- . On the other hand, radicals formed on radiolysis of matrices that are suitable for the generation of radical anions or cations are capable to abstract H* from anthralin to give the anthralyl radical, 2* . Both 2- and 2* are obtained simultaneously by mesolytic cleavage of the radical anion of the anthralin dimer. Contrary to general assumptions, the anthralyl radical is found to be much more reactive toward oxygen than the anion. All intermediates are characterized spectroscopically and by reference to quantum chemical calculations. Attempts to generate the radical cation of anthralin by X-irradiation of an Ar matrix containing anthralin led also to significant formation of its radical anion, i.e., anthralin acts apparently as an efficient electron trap in such experiments.     

A rare earth metallocene containing a 2,2′-azopyridyl radical anion

Introducing spin onto organic ligands that are coordinated to rare earth metal ions allows direct exchange with metal spin centres. This is particularly relevant for the deeply buried 4f-orbitals of the lanthanide ions that can give rise to unparalleled magnetic properties. For efficacy of exchange coupling, the donor atoms of the radical ligand require high-spin density. Such molecules are extremely rare owing to their reactive nature that renders isolation and purification difficult. Here, we demonstrate that a 2,2′-azopyridyl (abpy) radical (S = 1/2) bound to the rare earth metal yttrium can be realized. This molecule represents the first rare earth metal complex containing an abpy radical and is unambigously characterized by X-ray crystallography, NMR, UV-Vis-NIR, and IR spectroscopy. In addition, the most stable isotope ⁸⁹Y with a natural abundance of 100% and a nuclear spin of ½ allows an in-depth analysis of the yttrium–radical complex via EPR and HYSCORE spectroscopy. Further insight into the electronic ground state of the radical azobispyridine-coordinated metal complex was realized through unrestricted DFT calculations, which suggests that the unpaired spin density of the SOMO is heavily localized on the azo and pyridyl nitrogen atoms. The experimental results are supported by NBO calculations and give a comprehensive picture of the spin density of the azopyridyl ancillary ligand. This unexplored azopyridyl radical anion in heavy element chemistry bears crucial implications for the design of molecule-based magnets particularly comprising anisotropic lanthanide ions.

Unambiguous characterization of the first 2,2′-azobispyridine radical-containing rare earth metal complex through X-ray crystallography, DFT computations, EPR and HYSCORE spectroscopy.     

Characterization of potassium and sodium-potassium alloy solutions containing metal anions and complexed cations by means of NMR and ESR techniques

The influence of a complexing agent, kind of solvent and temperature on the stability and ionic composition of potassium and sodium-potassium alloy solutions containing metal anions and complexed cations as well as solvated electrons are discussed basing on the analysis of alkali metal NMR and ESR spectra. Surprisingly it seems that the stability of metal solutions in tetrahydrofuran at ambient temperature is inversely proportional to the durability of K+ complex in the case of five studied ligands. The most stable metal solutions were obtained using 15-crown-5. It was shown that the characteristic blue colour of metal solutions is not connected with the presence of solvated electrons but with metal anions.