title: Stable radical trianions from reversibly formed sigma-dimers of selenadiazoloquinolones studied by in situ EPR/UV-vis spectroelectrochemistry and quantum chemical calculations.
abstract: The redox behavior of the series of 7-substituted 6-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-h]quinolines and 8-substituted 9-oxo-6,9-dihydro[1,2,5]selenadiazolo[3,4-f]quinolines with R(7), R(8) = H, COOC(2)H(5), COOCH(3), COOH, COCH(3), and CN has been studied by in situ EPR and EPR/UV-vis spectroelectrochemistry in dimethylsulfoxide. All selenadiazoloquinolones undergo a one-electron reduction process to form the corresponding radical anions. Their stability strongly depends on substitution at the nitrogen atom of the 4-pyridone ring. The primary generated radical anions from N-ethyl-substituted quinolones are stable, whereas for the quinolones with imino hydrogen, the initial radical anions rapidly dimerize to produce unusually stable sigma-dimer (σ-dimer) dianions. These are reversibly oxidized to the initial compounds at potentials considerably less negative than the original reduction process in the back voltammetric scan. The dimer dianion can be further reduced to the stable paramagnetic dimer radical trianion in the region of the second reversible reduction step. The proposed complex reaction mechanism was confirmed by in situ EPR/UV-vis cyclovoltammetric experiments. The site of the dimerization in the σ-dimer and the mapping of the unpaired spin density both for radical anions and σ-dimer radical trianions with unusual unpaired spin distribution have been assigned by means of density functional theory calculations.
authors.affiliation: Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava, Radlinského 9, SK-812 37 Bratislava, Slovak Republic.
authors.forename: Andrej
authors.initials: A
authors.lastname: Staško
authors.forename: Karol
authors.initials: K
authors.lastname: Lušpai
authors.forename: Zuzana
authors.initials: Z
authors.lastname: Barbieriková
authors.forename: Ján
authors.initials: J
authors.lastname: Rimarčík
authors.forename: Adam
authors.initials: A
authors.lastname: Vagánek
authors.forename: Vladimír
authors.initials: V
authors.lastname: Lukeš
authors.forename: Maroš
authors.initials: M
authors.lastname: Bella
authors.forename: Viktor
authors.initials: V
authors.lastname: Milata
authors.forename: Michal
authors.initials: M
authors.lastname: Zalibera
authors.forename: Peter
authors.initials: P
authors.lastname: Rapta
authors.forename: Vlasta
authors.initials: V
authors.lastname: Brezová
mesh_terms: D000838:Anions / Q000737:chemistry; D019281:Dimerization; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D015394:Molecular Structure; D016566:Organoselenium Compounds / Q000737:chemistry*; D011789:Quantum Theory; D015363:Quinolones / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D000838:Anions; D005609:Free Radicals; D016566:Organoselenium Compounds; D015363:Quinolones