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

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Abstract

The ferrocenylthiophenes 2,3-Fc2-cC4H2S (9), 2,4-Fc2-cC4H2S (10), and 2,3,4-Fc3-cC4HS (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(η5-C5H4)(η5-C5H5)) in the presence of either [Pd(PPh3)4] or [Pd(CH2CMe2PtBu2)(μ-Cl)]2 as catalyst. Concerning electron transfer studies on ferrocenyl-substituted aromatic heterocycles, the electrochemistry 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-cC4H3S (1), 3-Fc-cC4H3S (2), 2,5-Fc2-cC4H2S (3), 3,4-Fc2-cC4H2S (4), 2,3,5-Fc3-cC4HS (12), and 2,3,4,5-Fc4-cC4S (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 [NnBu4][B(C6F5)4] as the supporting electrolyte. The spectroelectrochemical studies reveal that ferrocenyl units placed in the α-position of the thiophene ring interact more strongly with the heterocycle than those in the β-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.

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