Electron Transfer Studieson 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 bromothiophenes 5–7with FcZnCl (8; Fc = Fe(η5-C5H4)(η5-C5H5)) in the presence of either [Pd(PPh3)4] or [Pd(CH2CMe2PtBu2)(μ-Cl)]2as catalyst.Concerning electron transfer studies on ferrocenyl-substituted aromaticheterocycles, the electrochemistry as well as in situ UV–vis–near-IRspectroelectrochemistry highlight the electrochemical properties ofthese 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 organometalliccompounds display one (1, 2), two (3, 4, 9, 10), three(11, 12), or four (13) well-resolvedelectrochemically reversible one-electron-transfer processes using[NnBu4][B(C6F5)4] as the supporting electrolyte. The spectroelectrochemicalstudies reveal that ferrocenyl units placed in the α-positionof the thiophene ring interact more strongly with the heterocyclethan those in the β-position. Thus, the intensity of the ligand-to-metalcharge transfer (LMCT) absorptions, caused by interactions betweenthe thiophene core and the ferrocenyl moieties, decreases from 1 2. Furthermore, in the series of diferrocenylthiophenes theinteraction between the iron centers in the mono-oxidized compoundsdecreases in the series 3> 9> 10> 4. Thestructural properties of 10were investigated by single-crystalX-ray diffraction studies, indicating that 10possessesa syn conformation in the solid state with respect to the orientationof the two ferrocenyl units along the central thiophene core. Compound 10is isomorphic with 3. [ABSTRACT FROM AUTHOR]