Linear Oligo(ferrocenyldimethylsilanes) with between Two and Nine Ferrocene Units: Electrochemical and Structural Models for Poly(ferrocenylsilane) High Polymers

Abstract

In order to gain insight into the electrochemical and conformational properties of the prototypical high polymeric poly(ferrocenylsilane), poly(ferrocenyldimethylsilane) [Fe(η-C5H4)2SiMe2]n 6, three series of oligo(ferrocenylsilanes) R-fc-[SiMe2-fc]n-1-R‘ (fc = Fe(η-C5H4)2) 72−79 (R = R‘ = H), 82−87 (R = H and R‘ = SiMe3), and 92−97 (R = R‘ = SiMe3) have been prepared and studied (the subscript n in the oligomer refers to the number of ferrocene units present). These species were prepared via the anionic ring-opening oligomerization of the silicon-bridged [1]ferrocenophane Fe(η-C5H4)2SiMe2 5. Initiation with ferrocenyllithium FcLi (Fc = Fe(η-C5H5)(η-C5H4)) followed by quenching with H2O or SiMe3Cl afforded H-fc-[SiMe2-fc]n-1-H (72-9) or H-fc-[SiMe2-fc]n-1-SiMe3 (82-7), respectively. Initiation with the dilithioferrocene complex fcLi·2/3TMEDA followed by quenching with H2O or SiMe3Cl similarly afforded the oligomers H-fc-[SiMe2-fc]n-1-H (72-9) or alternatively the bis(silyl)-capped species Me3Si-fc-[SiMe2-fc]n-1-SiMe3 (92-7), respectively. The individual molecular compounds of these three series of oligomers 72−79, 82−87, and 92−97 were isolated in pure form from the oligomeric mixtures by column chromatography and these were structurally characterized by 1H, 13C, and 29Si NMR spectroscopy, mass spectrometry, and in selected cases by elemental analysis. The structure of the linear pentamer 75 has also been determined by single crystal X-ray diffraction. The central portion of this species possesses a trans planar zigzag conformation in the solid state and appears to be a valuable model for the analogous conformation of the high polymer 6 in crystalline domains. The electrochemical behavior of each pure oligomer was studied by cyclic and differential pulse voltammetry and was found to depend on whether an odd or an even number of ferrocene units were present. For oligomer systems containing an odd number of iron centers two reversible redox processes of varying intensities at −0.02−0.00 V and 0.21−0.23 V (vs ferrocene) were observed with a redox splitting of 0.21−0.23 V. For oligomer systems containing an even number of iron centers larger then two, three reversible redox processes of varying intensities were observed at ca. 0.00, 0.13, and 0.24 V vs ferrocene. As the oligomer chain length increased, the electrochemical behavior for both the “odd” and “even” series approached that of the high polymer 6 for which two reversible redox processes at 0.00 and 0.24 V (vs ferrocene) of equal intensity exist. These results are completely consistent with the previously proposed theory that initial oxidation of 6 affords a product in which alternating iron sites are oxidized. Spectroelectrochemical experiments show an intervalence electron transfer absorption (1100−1350 nm, εmax ⩽ 150 M-1 cm-1) for partially oxidized oligo(ferrocenylsilanes) that is typical for class II mixed-valent compounds. Additionally, single crystals were obtained of the mixed-valent linear trimer, 732+, as an I3- salt and a single crystal X-ray diffraction study revealed the presence of terminal ferrocenium centers and a central ferrocene group consistent with the alternating iron oxidation concept.