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

Abstract

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 (ΔH⁰_ROP = −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 (ΔH⁰_ROP = −8.9 kJ mol⁻¹, ΔG⁰_ROP = −3.1 kJ mol⁻¹). Compared to the previously described ROP of the unsubstituted analogue [Ni(η⁵-C₅H₄)₂(CH₂)₃] (1) (ΔH⁰_ROP = −10 kJ mol⁻¹, ΔG⁰_ROP = −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.