A series of 2,5-di- and 2,3,4,5-tetraferrocenyl-substituted thiophenes, furans, and pyrroles were synthesized using the Negishi C,C cross-coupling protocol. The electronic and electrochemical properties of these compounds were investigated by cyclic voltammetry (CV), square wave voltammetry (SWV), and in situ UV-vis/NIR spectroscopy. The molecular structures of 2,5-diferrocenyl furan and 2,3,4,5-tetraferrocenyl-1-methyl-1H-pyrrole in the solid state are discussed. The ferrocenyls could sequentially be oxidized giving two or four reversible responses for the appropriate di- or tetraferrocenyl-substituted heterocyclic molecules. The observed ΔE°' values range between 186 and 450 mV. The NIR measurements confirm electronic communication as intervalence charge transfer (IVCT) absorptions were found in the corresponding mono- and in case of the tetraferrocenyl compounds also in the dicationic species. All compounds, except tetraferrocenyl thiophene (a class I system), were classified as class II systems according to Robin and Day. They show a linear relationship between ΔE°' and the IVCT oscillator strength f which could be shown for the first time in organometallic chemistry. This was possible because the series of molecules exhibit analogous geometries and hence, similar electrostatic properties. This correlation was confirmed by electro- and spectro-electrochemical measurements. Within these studies a new approach for the estimation of the effective electron transfer distances r(ab) is discussed.

The ferrocenylthiophenes 2,3-Fc 2 -c C 4 H 2 S (9), 2,4-Fc 2 -c C 4 H 2 S (10), and 2,3,4-Fc 3 -c C 4 HS (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(η 5 -C 5 H 4 )(η 5 -C 5 H 5 )) in the presence of either [Pd(PPh 3 ) 4 ] or [Pd(CH 2 CMe 2 P t Bu 2 )(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- 12), and 2,3,4,5-Fc 4 -c C 4 S (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 [N n Bu 4 ][B(C 6 F 5 ) 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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