Nucleophilically Assisted and Cationic Ring-Opening Polymerization of Tin-Bridged [1]Ferrocenophanes

To obtain mechanistic insight, detailed studies of the intriguing “spontaneous” ambient temperature
ring-opening polymerization (ROP) of tin-bridged [1]ferrocenophanes Fe(η-C5H4)2SnR2 3a (R = t-Bu) and
3b (R = Mes) in solution have been performed. The investigations explored the influence of non-nucleophilic
additives such as radicals and radical traps, neutral and anionic nucleophiles, Lewis acids, protic species,
and other cationic electrophiles. Significantly, two novel methodologies and mechanisms for the ROP of
strained [1]ferrocenophanes are proposed based on this study. First, as the addition of amine nucleophiles
such as pyridine was found to strongly accelerate the polymerization rate in solution, a new nucleophilically
assisted ROP methodology was proposed. This operates at ambient temperature in solution even in the
presence of chlorosilanes but, unlike the anionic polymerization of ferrocenophanes, does not involve
cyclopentadienyl anions. Second, the addition of small quantities of the electrophilic species H+ and
Bu3Sn+ was found to lead to a cationic ROP process. These studies suggest that the “spontaneous” ROP
of tin-bridged [1]ferrocenophanes may be a consequence of the presence of spurious, trace quantities of
Lewis basic or acidic impurities. The new ROP mechanisms reported are likely to be of general significance
for the ROP of other metallocenophanes (e.g., for thermal ROP in the melt) and for other metallacycles
containing group 14 elements.