The electro- and spectroelectrochemical behavior of diverse (multi)­ferrocenyl five-membered heterocyclic compounds, including furan, thiophene, pyrroles, phospholes, etc. is discussed, giving a close insight into the electron transfer processes of these organometallic compounds in their mixed-valence state, whereby electronic and structural modification of the heterocyclic connecting units and/or of the redox-active ferrocenyl termini directly influences the electron transfer properties. In addition, the structural features of these compounds can be correlated with their electrochemical behavior, allowing calculation of the effective electron transfer distance within a specific series of molecules. Tendencies in molecular wire molecules based on bi-, ter-, quarter-, quinque-, and sexithiophene connecting building blocks and the appropriate pyrrole derivatives are discussed as well. The consequences of introducing an additional redox-active transition-metal building block, such as a titanocene or a zirconocene moiety, respectively, into the heterocyclic ring, on the electrochemical behavior of the resulting five-membered heterocycles are also highlighted.

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.

Novel 2,5-diferrocenyl-1-phenyl-1H-pyrrole (4) and 2,3,4,5-tetraferrocenyl-1-phenyl-1H-pyrrole (6) have been prepared by a 2- or 4-fold Negishi cross-coupling reaction of 2,5-dibromo-1-phenyl-1H-pyrrole (3) and 2,3,4,5-tetrabromo-1-phenyl-1H-pyrrole (5), respectively, with FcZnCl (2) (Fc = Fe(η5-C5H4)(η5-C5H5)) in the presence of [(Ph3P)4Pd] as catalyst. The electronic and structural properties of 4 and 6 were investigated with UV−vis spectroscopy and single-crystal X-ray diffraction (6). Comparison of the appropriate bond distances in the pyrrole core system of 6 demonstrates considerable electron delocalization. Cyclic, square wave, and linear sweep voltammetry as well as in situ NIR spectro-electrochemistry highlight the electrochemical properties of both compounds. Molecules 4 and 6 display two (4) or four (6) electrochemically reversible one-electron transfer processes with remarkably high ΔE 1/2 values and reduction potentials of E 0 ′ = −238 and E 0 ′ = 212 mV for 4 (ΔE 1/2 = 450 mV) and E 0 ′ = −280, E 0 ′ = 51, E 0 ′ = 323, and E 0 ′ = 550 mV for 6 (ΔE 1/2 = 322, 264, and 233 mV) using [NBu4][B(C6F5)4] as the supporting electrolyte. The pyrroles could be classified as class II systems according to Robin and Day. Additionally, 4[PF 6 ] n (n = 1, 2) were synthesized and studied, giving CV responses and NIR spectra identical to those obtained for 4 from electrochemical oxidations.

2,5-Diferrocenyl-1-phenyl-1H-phosphole (3) has successfully been prepared by a cyclization reaction of phenylphosphine with 1,4-diferrocenylbutadiyne. Subsequent reaction with elemental sulfur and selenium, respectively, leads to the formation of the appropriate phosphole sulfide (4) or selenide (5). Molecules 4 and 5 have structurally been characterized by single crystal X-ray diffraction. Despite the tetrahedral environment at the phosphorus atom, the c C 4 P ring itself is planar and coplanar to the cyclopentadienyl rings of the ferrocenyl termini. Electrochemical measurements revealed that the two ferrocenyl groups could be oxidized at discrete potentials, with separation of the individual redox waves of 280 mV (3), 240 mV (4) and 235 mV (5), respectively. These values agree with other examples of heterocyclic-bridged diferrocenyl compounds such as diferrocenyl thiophene (260 mV) and diferrocenyl furan (290 mV). Compounds [3] + , [4] + and [5] + exhibit IVCT absorptions of weak to moderate strength which conforms well to the predictions of the Hush two-state model for weakly coupled mixedvalence systems. These conclusions are supported by DFT and TD-DFT results, which satisfactorily model the observed structural and spectroscopic parameters. The computational work assists in assigning the various low energy (LF, IVCT) electronic transitions and also highlights the key role of the unsaturated cis-diene-like C 4 H 2 building block of the heterocycle in promoting the Fc à Fc + electron-transfer transition.