Ladislav Omelka

Redox-initiated radical decomposition of triazenes and their platinum complexes studied by cyclic voltammetry and EPR spectroscopy

Triazenes p-R1–C6H4–NN–NR2R3(R1= H, butyl, CH3O, CN and NO2, R2= CH3, cyclohexyl, butyl or longer chains, R3= H, OH) are irreversibly cathodically reduced at –1.5 to –2.7 V (vs. saturated calomel electrode) and oxidised from 0.8 to 1.7 V. The reduction peak potentials became more negative and the first oxidation peak potentials less positive if R3 was OH. Using spin-trapping, radicals p-R1–C6H4˙ and ˙R2 were identified as fragmentation products in the electrolytic and peroxy-initiated decomposition of triazenes. Radicals p–R1C6H4–NO˙–R2, representing the oxidised cage products after N2 elimination, were observed in the oxidation of triazenes with peroxy acid. An NO2-centred radical anion was found in the cathodic reduction of a Pt complex with R1= NO2. The above specified decomposition route of triazenes is modified if these are coordinated in Pt complexes. The formation of radicals is discussed assuming two tautomeric forms of triazenes.

Electron Spin Resonance and Electrochemical Studies of Some Mesoionic and Neutral 1,2,3,4‐Oxa‐ and ‐Thiatriazoles and Their Alkylation Products

Voltammetric studies of mesoionic 1,2,3,4‐oxa‐ and ‐thiatriazoles confirmed that the former may be reduced irreversibly at potentials between ‐1.19 and ‐1.73 V vs. ferrocene in acetonitrile at scan rates between 0.05 and 1000 mV s‐1, whereas the latter are reduced reversibly under the same conditions. The corresponding radical anions were examined by ESR spectroscopy. Cationic alkylation products of the above compounds showed different electrochemical behaviour depending on the character of the heterocyclic ring. 1,2,3,4‐Thiatriazolium ions were reduced reversibly. All coupling constants of the resulting neutral radicals were fully assigned using isotopic labelling. A cation with an 1,2,3,4‐oxatriazolium ring reacted by cleavaging of an exocyclic C—S bond on electron uptake. The resulting alkyl radical was spin‐trapped.

EPR study of radical products generated from N-alkylbenzylamines by the action of some peroxo and lead(IV) agents

To understand the behavior of secondary amines in the role of antioxidants, in more detail, the radical transformations of N-alkylbenzylamines in the presence of different agents were investigated using EPR spectroscopy. The aminoxyl radicals prepared by the reaction with 3-chloroperbenzoic acid undergo the transformations at alkyl substituent by the addition of t-BuO2· radicals, secondary aminoxyls being the main radical product. The reaction with redox agents like PbO2 and/or Pb(OAc)4 also mostly proceeds at the alkyl substituents and results in the generation of C-centered radicals detectable by spin trapping method.Graphical abstract.

ELECTROCHEMICAL AND ELECTRON SPIN RESONANCE INVESTIGATIONS OF SOME 1,2,3-OXA- AND -THIADIAZOLES

Voltammetric studies on mesoionic 1,2,3‐oxa‐ and ‐thiadiazoles confirmed that the former may be reduced irreversibly at potentials between ‐1.48 and ‐2.12 V vs. Fc/Fc+ in acetonitrile at a scan rate of 200 mV s‐1, whereas the latter are reduced reversibly under the same conditions. The corresponding anion radicals were examined by ESR spectroscopy. Cationic alkylation products showed different electrochemical behaviour depending on the character of the heterocyclic ring. 1,2,3‐Thiadiazolium ions were reduced reversibly. All coupling constants of the resulting neutral radicals were fully assigned. 1,2,3‐Oxadiazolium ions were all reduced irreversibly.

Radical products of the oxidation of 2,2-diacyl-1-arylhydrazines with the system Pb(OAc)4-CF3COOH-CH2Cl2

SummaryThe oxidation of 2,2-Diacyl-1-arylhydrazines1a–l with the system Pb(OAc)4-CF3COOH-CH2Cl2 led to the formation of two types of radical products. Phenazinium cation radicals5a–h were formed from 2,2-diacyl-1-arylhydrazines1a–h, while hydrazyl radicals2i–l were observed to be the only radical products of the oxidation of1i–l. The generated radicals are characterized by their EPR parameters.ZusammenfassungDie Oxidation der 2,2-Diacyl-1-arylhydrazine1a–l mit dem System Pb(OAc)4-CF3COOH-CH2Cl2 führte zu zwei unterschiedlichen Typen von Radikalen. Aus den 2,2-Diacyl-1-arylhydrazinen1a–h werden die Phenaziniumkationenradikale5a–h gebildet, während nach Oxidation von1i–l die entsprechenden Hydrazyle2i–l als einzige Radikalprodukte nachweisbar waren. Die erzeugten Radikale wurden durch ihre EPR-Parameter charakterisiert.