Electron Transfer Studies on Ferrocenylthiophenes: Synthesis, Properties, and Electrochemistry

The ferrocenylthiophenes 2,3-Fc(2)-(C4H2S)-C-c (9), 2,4-Fc(2)-(t)(C4H2S (10), and 2,3,4-Fc(3)-(C4HS)-C-t (11) have been prepared by a 2- or 3-fold Negishi cross-coupling reaction of the appropriate bromo thiophenes 5-7 with FcZnCl (8; Fc = Fe(eta(5)-C5H5)(eta(5)-C5H5)) in the presence of either [Pd(PPh3)(4)] or [Pd((CH2CMe2PBu2)-Bu-t)(mu-Cl)](2) as catalyst. Concerning electron transfer studies on ferrocenyl-substituted aromatic heterocycles, the electro-chemistry as well as in situ UV-vis-near-IR spectroelectrochemistry highlight the electrochemical properties of these compounds in a series of mono-, di-, tri-, and tetraferrocenylthiophenes, including 2-Fc-(C4H3S)-C-t (1), 3-Fc-(C4H3S)-C-c (2), 2,5-Fc(2)-(C4H2S)-C-c (3), 3,4-Fc(2)-(C4H2S)-C-c (4), 2,3,5-Fc(3)-(C4HS)-C-t (12), and 2,3,4,5-Fc(4)-(C4S)-C-c (13). These organometallic compounds display one (1, 2), two (3, 4, 9, 10), three (11, 12), or four (13) well-resolved electrochemically reversible one-electron-transfer processes using [(NBu4)-Bu-n][B(C6F5)(4)] as the supporting electrolyte. The spectroelectrochemical studies reveal that ferrocenyl units placed in the a-position of the thiophene ring interact more strongly with the heterocycle than those in the beta-position. Thus, the intensity of the ligand-to-metal charge transfer (LMCT) absorptions, caused by interactions between the thiophene core and the ferrocenyl moieties, decreases from 1(+) to 2(+). Furthermore, in the series of diferrocenylthiophenes the interaction between the iron centers in the mono-oxidized compounds decreases in the series 3(+) > 9(+) > 10(+) > 4(+). The structural properties of 10 were investigated by single-crystal X-ray diffraction studies, indicating that 10 possesses a syn conformation in the solid state with respect to the orientation of the two ferrocenyl units along the central thiophene core. Compound 10 is isomorphic with 3.