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 alpha angles of similar to 14-32 degrees. Surprisingly, tetracarba[4]nickelocenophane [Ni(eta(5)-C5H4)(2)(CH2)(4)] (2) undergoes ROP (pyridine, 20 degrees C, 5 days) to give primarily insoluble poly(nickelocenylbutylene) [Ni(eta(5)-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 (Delta H0ROP = -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(eta(5)-C5H4)(2)(SiMe2)(2)O] (13) and [Ni(eta(5)-C5H4)(2)(SiMePh)(2)O] (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(eta(5)-C5H4)(2)(CH2)(2)[CH(CH3)]} (15) with a significant tilt angle (alpha similar to 16 degrees) was found to undergo ROP to give soluble polymer {Ni(eta(5)-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 (Delta H0ROP = -8.9 kJ mol(-1), Delta G0ROP = -3.1 kJ mol(-1)). Compared to the previously described ROP of the unsubstituted analogue [Ni(eta(5)-C5H4)(2)(CH2)(3)] (1) (Delta H0ROP = -10 kJ mol(-1), Delta G0ROP = -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.