Synthesis of bis-2H and 4H-chalcogenapyrans and benzochalcogenapyrans via Pd0 catalyzed dimerization of Fischer type carbene complexes:: redox properties and electronic structure of these new extended electron rich molecules

Extended electron rich bis-chalcogenapyrans and bis-benzochalcogenapyrans have been synthesized by Pd-0 catalyzed dimerization of alpha-and gamma-methylene chalcogenapyran and benzochalcogenapyran Fischer type carbene complexes. Voltammetric studies performed on these molecules show a single two-electron wave around 0 V versus SCE, which is ascribed to the oxidation of the neutral form in radical and dipyrylium cations. DFT calculations show that the oxidation leads to a rotating movement around the central C-C bond and suggest that the solvent plays a major role in the observation of the two one-electron systems. Furthermore, according to the structure, these molecules are likely to be reduced at very low potential (E= -1.5 V vs SCE) via a two-electron transfer reaction. (C) 2007 Elsevier Ltd. All rights reserved.