Role of torsional strain in the ring-opening polymerisation of low strain []nickelocenophanes.

Ring-opening polymerisation (ROP) of strained [1]- and [2]metallocenophanes and related species is well-established, and the monomer ring-strain is manifest in a substantial tilting of the cyclopentadienyl ligands, giving angles of ∼14-32°. Surprisingly, tetracarba[4]nickelocenophane [Ni(η-CH)(CH)] () undergoes ROP (pyridine, 20 °C, 5 days) to give primarily insoluble poly(nickelocenylbutylene) [Ni(η-CH)(CH)] (), despite the lack of significant ring-tilt. The exoenthalpic nature of the ROP was confirmed by DFT calculations involving the cyclic precursor and model oligomers (Δ0ROP = -14 ± 2 kJ mol), and is proposed to be a consequence of torsional strain present in the bridge of . The similarly untilted disila-2-oxa[3]nickelocenophanes [Ni(η-CH)(SiMe)O] () and [Ni(η-CH)(SiMePh)O] () were found to lack similar torsional strain and to be resistant to ROP under the same conditions. In contrast, 1-methyltricarba[3]nickelocenophane {Ni(η-CH)(CH)[CH(CH)]} () with a significant tilt angle ( ∼ 16°) was found to undergo ROP to give soluble polymer {Ni(η-CH)(CH)[CH(CH)]} (). The reversibility of the process in this case allowed for the effects of temperature and reaction concentration on the monomer-polymer equilibrium to be explored and thereby thermodynamic data to be elucidated (Δ0ROP = -8.9 kJ mol, Δ0ROP = -3.1 kJ mol). Compared to the previously described ROP of the unsubstituted analogue [Ni(η-CH)(CH)] () (Δ0ROP = -10 kJ mol, Δ0ROP = -4.0 kJ mol), the presence of the additional methyl substituent in the bridge appears to marginally disfavour ROP and leads to a corresponding decrease in the equilibrium polymer yield.