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

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.