Electron paramagnetic resonance studies of valence tautomer anion radicals and ion-pairing in semiquinones

Abstract

The epr spectral parameters of the monohomocyclooctatetraene anion radical (XXXV) have been found to be relatively insensitive to changes in solvent, temperature, and counterion. The similar geometry of the anion radical, XXXV, and the monohomocyclooctatetraene dianion (XXXVI) is noted. A disproportionation equilibrium between XXXVI, XXXV, and cis-fused bicycle(6.1.0)nona-2,4,6-triene (XXXIV) is apparent. For the systems studied, the anion radical XXXV is favored over any of its valence tautomeric forms. The anion radicals formed during the alkali metal reduction of XXXIV in liquid ammonia have been investigated. Potassium metal reduction of XXXIV in liquid ammonia produced solutions of the anion radical, XXXV. Similar results were obtained from the potassium reduction of cis-fused bicycle(6.1.0)nona-2,4,6-triene-9.9'-d₂ (XLV) in liquid ammonia. In contrast, lithium or sodium reduction of XXXIV in liquid ammonia produced diamagnetic solution of XXXIV in liquid ammonia produced diamagnetic solutions which upon warming to +25� produced the methylcyclooctatetraene anion radical (XI. I). Similar reductions of XLV in liquid ammonia produced solutions of the anion radical, R₂, whose structure is not known. The anion radicals formed during the electrolytic and alkali metal reductions of bullvalene (VIII) have been investigated. Alkali metal reduction (lithium, sodium, or potassium) of VIII in DME and THF produced solutions of the naphthalene anion radical (XV). Within experimental error the formation of LIV and XV in the systems studied is not due to impurities present in VIII. Temperature dependent epr studies have been conducted on semiquinones produced by alkali metal reduction (lithium, sodium, or potassium) of octahydro-9, 10-anthraquinone (LV) and 2.6-dimethylbenzoquinone (LVIII) in DME, THF, and liquid ammonia. Ion-pairs exhibiting different degrees of association have been observed, the degree of association depending on the temperature, solvent, counterion, and mode of anion radical formation. The results demonstrate a marked perturbation of the spin density distribution, in the respective quinoidal systems studied, as a consequence of ion-pair association between the semiquinone and alkali metal cation. Certain of the systems studied exhibited line width alternation phenomena. Possible mechanisms to explain the observed line width phenomena are discussed.