Synthesis, Properties, and Electron Transfer Studies of Ferrocenyl Thiophenes

For a series of ferrocenyl thiophenes of type Fe((5)-C5H4-(4-R-(C4H2S)-C-c-3-yl))((5)-C5H4-(C6H3-3,5-(CF3)(2)) [R = H (3a), OMe (4a)], Fe((5)-C5H4-(4-R-(C4H2S)-C-c-3-yl)((5)-C5H4-CHO) [R = H (3b), OMe (4b)], and Fe((5)-C5H4-(4-R-(C4H2S)-C-c-3-yl)((5)-C5H4-CN) [R = H (3c), OMe (4c)], the influence of electron-withdrawing substituents at the ferrocenyl moiety and electron-donating groups at the thiophene unit on the electronic behavior of 3a-c and 4a-c is reported. The coupling of the ferrocenyl and the thiophene moieties has been realized using the Negishi C,C cross-coupling reaction protocol. Compounds 3a and 4c were structurally characterized by single-crystal X-ray diffraction studies. In electrochemical measurements the ferrocenyl redox potential depends on the particular substitution at the ferrocenyl and the thiophene unit. Moreover, UV/Vis/NIR studies showed ligand-to-metal charge transfer (LMCT) interactions, which occur after oxidation and are shifted bathochromically as the donor-acceptor energy gap decreases. Using different substituents, possessing electron-withdrawing or donating capabilities, allows adjusting the energy difference between the ferrocenium-acceptor unit and the donating thiophene system.