Role of torsional strain in the ring-opening polymerisation of low strain [n]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 α angles of ∼14-32°. Surprisingly, tetracarba[4]nickelocenophane [Ni(η5-C5H4)2(CH2)4] (2) undergoes ROP (pyridine, 20 °C, 5 days) to give primarily insoluble poly(nickelocenylbutylene) [Ni(η5-C5H4)2(CH2)4] n (12), 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 (ΔH0ROP = -14 ± 2 kJ mol-1), and is proposed to be a consequence of torsional strain present in the ansa bridge of 2. The similarly untilted disila-2-oxa[3]nickelocenophanes [Ni(η5-C5H4)2(SiMe2)2O] (13) and [Ni(η5-C5H4)2(SiMePh)2O] (14) were found to lack similar torsional strain and to be resistant to ROP under the same conditions. In contrast, 1-methyltricarba[3]nickelocenophane {Ni(η5-C5H4)2(CH2)2[CH(CH3)]} (15) with a significant tilt angle (α ∼ 16°) was found to undergo ROP to give soluble polymer {Ni(η5-C5H4)2(CH2)2[CH(CH3)]} n (18). 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 (ΔH0ROP = -8.9 kJ mol-1, ΔG0ROP = -3.1 kJ mol-1). Compared to the previously described ROP of the unsubstituted analogue [Ni(η5-C5H4)2(CH2)3] (1) (ΔH0ROP = -10 kJ mol-1, ΔG0ROP = -4.0 kJ mol-1), the presence of the additional methyl substituent in the ansa bridge appears to marginally disfavour ROP and leads to a corresponding decrease in the equilibrium polymer yield.