Redox-active metallomacrocycles and cyclic metallopolymers: photocontrolled ring-opening oligomerization and polymerization of silicon-bridged [1]ferrocenophanes using substitutionally-labile Lewis bases as initiators.

Irradiation of silicon-bridged [1]ferrocenophane [Fe(eta-C(5)H(4))(2)SiMe(2)] (1) in the presence of substitutionally labile Lewis bases such as 4,4'-dimethyl-2,2'-bipyridine (Me(2)bpy) initiates ring-opening polymerization and oligomerization via cleavage of an iron-cyclopentadienyl bond. A distribution of cyclic polyferrocenylsilane c-PFS (PFS = [Fe(eta-C(5)H(4))(2)SiMe(2)](n)) and a series of cyclic oligomers (2(2)-2(7)) were isolated by column chromatography and fully characterized. Varying temperature and concentration were found to influence the molecular weight distribution and the ratio of polymer to oligomer products, enabling the formation of c-PFS with molecular weights >100 kDa. Cyclic polymer samples were found to possess lower hydrodynamic radii and viscosity and higher glass transition temperatures than those of their linear PFS counterparts (l-PFS) of comparable molecular weight. Compared with crystalline samples of l-PFS of similar molecular weights, c-PFS formed smaller spherulites, as observed by polarizing optical microscopy. While the wide-angle X-ray scattering (WAXS) patterns from lower molecular weight l-PFS were found to differ from those from higher molecular weight samples, those obtained for lower and higher molecular weight samples of c-PFS are identical and resemble diffraction patterns of high molecular weight l-PFS. The electrochemical behavior of each cyclic oligomer 2(2)-2(7) was studied by cyclic and differential pulse voltammetry and was found to depend on whether the oligomer contains an odd or even number of ferrocene units. In contrast to linear analogs, two reversible redox processes of varying intensities were observed for cyclic oligomers containing an even number of iron centers, while three reversible redox processes of varying intensities were observed for cyclic oligomers containing an odd number of iron centers. As the oligomer chain length increased, the electrochemical behavior of all cyclic oligomers approached that of both cyclic and linear high molecular weight polymers.