Tetrathia[7]helicene-Based Complexes of Ferrocene and (η5-Cyclohexadienyl)tricarbonylmanganese: Synthesis and Electrochemical Studies

Tetrathia[7]helicene ([7]TH)-based complexes substituted at the thienyl ring ends by a ferrocenyl group (Fc) or by a (eta(5)-cyclohexadienyl)Mn(CO)(3) derivative have been prepared by Sonogashira coupling reactions starting from the mono- or diiodo [7]TH compounds. The molecular structure of one of the diferrocenyl [7]TH complexes was established by X-ray analysis. Electrochemical investigation on the Fc-[7]TH systems show that the Fc groups are significantly electron poorer with respect to Pc (Delta E degrees approximate to 0.15 V), due to the effective conjugation of the Fc redox moiety with the triple bond + helicene system, as also confirmed by spectroscopic data. Potential cycling around the second oxidation peak, assigned to the thiahelicene moiety, affords fast, regular growth of electrodeposited conducting films, provided that one terminal alpha-thiophene position be available for coupling; on the other hand, long alkyl chains hamper film formation. The conducting films feature a broad oxidation wave resulting from the merging of several redox peaks, having its onset at the Fc oxidation. Since conducting films obtained by electrooligomerization of parent tetrathiahelicene have their onset potentials 0.45 V more positive than the Fc redox sites in this studied Fc-[7]TH conjugates, the above continuity could point to some coupling between Fc redox centers and conjugated pi systems, favored by solid-state stacking.