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Structures and Molecular Properties of Charge-Perturbed Molecules, 54. - Pyridinium-Tetraphenylcyclopentadienide Betaine and the Single-Electron Oxidation to Its Radical Cation
Bock, Hans,Nick, Sabine,Naether, Christian,Goebel, Ilka,John, Andreas,Kleine, Markus
, p. 105 - 114 (2007/10/02)
The single-crystal structure of the dark-blue pyridinium-tetraphenylcyclopentadienide betaine shows the pyridine ring and the phenyl substituents twisted between 37 and 55 deg, indicating reduced ? interactions between cos2 ω = 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 ET solvent parameters.Cyclovoltammetry in H2CCl2 / 0.1M R4N(+)ClO4(-) 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 F3CCOO(-)Ag(+), 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. - Key Words: Pyridinium-cyclopentadienide betaine structure / Redox potentials / ESR-ENDOR spectra / Radical cation
