The single-crystal structure of the dark-blue pyridinium-tetraphenylcyclopentadienide betaine shows the pyridine ring and the phenyl substituents twisted between 37 and 55 degrees, indicating reduced pi interactions between cos(2) omega = 0.6 and 0.4. According to AM1 calculations based on the structure coordinates, the total charges of the pyridinium cation acceptor and the cyclopentadienyl anion donor amount to +0.62 and -0.53. This pronounced betaine zwitterionic character causes a 0.5 eV (!) bathochromic shift from 22000 cm(-1) in aqueous to 18000 cm(-1) in n-heptane solution, which can be correlated with the respective E(T) solvent parameters. Cyclo-voltammetry in H2CCl2/0.1 M R(4)N(circle plus)ClO(4)(circle minus) yields an irreversible reduction potential at -1.5 V and two oxidation potentials at +0.36 and +1.28 V, of which the lower one is reversible. With F3CCOOcircle minus Ag-circle plus, therefore, a radical cation can be generated and is characterized by ESR/ENDOR spectra. The coupling constants are satisfactorily reproduced by McLachlan calculations, which also suggest that most of the spin population should be localized within the five-membered ring.
