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.