2,2'-Bithienyl Derivatives: EPR Investigation of Their Radical Ions in Solution, Electrochemical Properties, and Crystal Structure

Article abstract of DOI:10.1021/jo00063a031

5,5'-Dimethyl-, -diisopropyl-, -di-tert-butyl-, and -bis(trimethylsilyl)-2,2'-bithienyl have been prepared (1, 2, 3, and 4 respectively).The radical cations of 1, 2, and 3 have been generated in fluid solution.The electron paramagnetic resonance (EPR) spectra have been analyzed with the help of Hueckel/McLachlan and MINDO/3 calculations and show the presence of cis and trans isomers, although X-ray diffraction studies of 1 and 4 have identified only the trans isomer in solid state.MINDO/3 predicts the observation of both isomers of 1(*,1+) on the EPR timescale by estimating the barrier to ring rotation.This value is much higher than in the parent molecule, which fits in with the observation of presumably only one isomer by nuclear magnetic resonance (NMR).If the 2- and 5-positions are not blocked, oligomerization occurs: 2-methylthiophene gives the 5,5'-dimethyl-2,2'-bithienyl radical cation (1(*,1+), and 2,2'-bithienyl itself gives the poly-2,2'-bithienyl radical cation.The radical anion of 4 (and of its 3,3'-dideuterated derivative) has been generated, but the lines in the EPR spectrum are broad, and only one isomer can be detected.Oxidation potentials corresponding to the anodic peak potentials of the irreversible oxidation peaks for 1, 2, and 3 are lower than that of 2,2'-bithienyl as expected, while that of 4 is only slightly higher, which is consistent with the weak electron-withdrawing effect of the trimethylsilyl substituent.On the other hand, a quasireversible redox couple (E⁰ = -2.34 V vs SCE) has been found for the reduction of 4.