Radical Dimerization of 5,5'-Diphenyl-3,3',4,4'-tetramethoxy-2,2'-bipyrrole: π Dimer in the Crystal, σ Dimer in Solution.

The crystalline, spinless π dimer (1.+ ⋅PF6 )2 was obtained by the oxidation of bipyrrole 1 with ferrocenium hexafluorophosphate. The diamagnetic species generated electrochemically in solution was identified as the σ dimer by NMR spectroscopy.