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Structure, Spectroscopy and Magnetism of 1,4-Dihydro-1,4-Diazine Radical Cations: Exceptionally Stable Intermediates Related To Methylviologen and Flavosemiquinone Radicals

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1,4-Dialkyl-1,4-dihydro-1,4-diazine radical cations 1-3 have been established in recent years as unusually stable intermediates of corresponding two-step redox systems. The stability is evident from large comproportionation constants Kc > 1012 and from the isolability of persistent radical cation salts with counter anions such as Br, I-, I3-, PF6-, BPh4-, or (TCNQ2)-. The structures of several crystalline derivatives have been determined, showing planar π systems and, in one instance, an anion-dependent tendency to form π/π dimers. Effects of dimerization are also evident from comparative magnetic susceptibility measurements of 1,4-diethyl-1,4-dihydroquinoxalinium iodide and tetraphenylborate. UV/Vis absorption spectra of the radical cations have been determined and interpreted with the help of molecular orbital calculations. The most simple member of the series, 1,4-diethyl-1,4-dihydropyrazinum radical cation 1, exhibits a long wavelength forbidden band (2B1u2Au) with a conspicuous vibrational fine structure. The results obtained for the small but very stable new radical cations 1 and 2 provide clues to the stability of flavosemiquinone oxidation states in pertinent oxidoreductase enzymes and show ways to new components for the design of materials with anisotropic physical properties.

Affiliations: 1: Institut fur Anorganische Chemie der Universität Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany; 2: Departement de Recherche Fondamentale sur la Matiere, Condensée/S.E.S.A.M/S.C.P.M., Centre d'Etudes Nucleaires de Grenoble, 85 X, F-38041 Grenoble Cedex, France; 3: J. Heyrovsky Institute of Physical Chemistry and Electrochemistry, Czech Academy of Sciences, Dolejškova 3, CR-18223 Prague, Czech Republic


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