Observation of Discrete Valence Tautomers in Crystalline Cyclopentadienyl Radicals

Single crystal X-ray (sc XRD) analyses of three symmetrically substituted cyclopentadienyl radicals (1, 2, 5) containing sterically demanding aryl groups showed that they crystallize as discrete valence tautomers (Jahn–Teller distortion) in the solid state with the unpaired electron either located in the b₁ orbital (type I, state ²B₁), resulting in a localized radical with two adjacent double bonds, or the a₂ orbital (type II, state ²A₂), leading to an allyl-type radical. Their properties in solution were examined by EPR spectroscopy as well as cyclovoltammetry and UV/vis spectroscopy including two additional cyclopentadienyl radicals (1–5). The electronic nature of 1–5 was further investigated by quantum chemical calculations.