LINEAR OLIGO(FERROCENYLDIMETHYLSILANES) WITH BETWEEN 2 AND 9 FERROCENE UNITS - ELECTROCHEMICAL AND STRUCTURAL MODELS FOR POLY(FERROCENYLSILANE) HIGH POLYMERS

In order to gain insight into the electrochemical and conformational p roperties of the prototypical high polymeric poly(ferrocenylsilane), p oly(ferrocenyldimethylsilane) [Fe(eta-C5H4)(2)SiMe(2)](n) 6, three ser ies of oligo(ferrocenylsilanes) R-fc-[SiMe(2)-fc](n-1)-R' (fc = Fe(eta -C5H4)(2)) 7(2)-7(9) (R = R' = H), 8(2)-8(7) (R = H and R' = SiMe(3)), and 9(2)-9(7) (R = R' = SiMe(3)) have been prepared and studied (the subscript n in the oligomer refers to the number of ferrocene units pr esent). These species were prepared via the anionic ring-opening oligo merization of the silicon-bridged [1]ferrocenophane Fe(eta-C5H4)(2)SiM e(2) 5. Initiation with ferrocenyllithium FcLi (Fc = Fe(eta-C5H5)(eta- C5H4)) followed by quenching with H2O or SiMe(3)Cl afforded H-fc-[SiMe 2-fc](n-1)-H (7(2-9)) or H-fc-[SiMe(2)-fc](n-1)-SiMe(3) (8(2-7)), resp ectively. Initiation with the dilithioferrocene complex fcLi . 2/3TMED A followed by quenching with H2O or SiMe(3)Cl similarly afforded the o ligomers H-fc-[SiMe(2)-fc](n-1)-H (7(2-9)) or alternatively the bis(si lyl)-capped species Me(3)Si-fc-[SiMe(2)-fc](n-1)-SiMe(3) (9(2-7)), res pectively. The individual molecular compounds of these three series of oligomers 7(2)-7(9), 8(2)-8(7), and 9(2)-9(7) were isolated in pure f orm from the oligomeric mixtures by column chromatography and these we re structurally characterized by H-1, C-13, and Si-29 NMR spectroscopy , mass spectrometry, and in selected cases by elemental analysis. The structure of the linear pentamer 7(5) has also been-determined by sing le crystal X-ray diffraction. The central portion of this species poss esses a trans planar zigzag conformation in the solid state and appear s to be a valuable model for the analogous conformation of the high po lymer 6 in crystalline domains. The electrochemical behavior of each p ure oligomer was studied by cyclic and differential pulse voltammetry and was found to depend on whether an odd or an even number of ferroce ne units were present. For oligomer systems containing an odd number o f iron centers two reversible redox processes of varying intensities a t -0.02-0.00 V and 0.21-0.23 V (vs ferrocene) were observed with a red ox splitting of 0.21-0.23 V. For oligomer systems containing an even n umber of iron centers larger then two, three reversible redox processe s of varying intensities were observed at ca. 0.00, 0.13, and 0.24 V v s ferrocene. As the oligomer chain length increased, the electrochemic al behavior for both the ''odd'' and ''even'' series approached that o f 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 elec tron transfer absorption (1100-1350 nm, epsilon(max) less than or equa l to 150 M(-1) cm(-1)) for partially oxidized oligo(ferrocenysilanes) that is typical for class II mixed-valent compounds. Additionally, sin gle crystals were obtained of the mixed-valent linear trimer, 7(3)(2+) , as an I-3(-) salt and a single crystal X-ray diffraction study revea led the presence of terminal ferrocenium centers and a central ferroce ne group consistent with the alternating iron oxidation concept.