Comparative NMR study on the reactions of Hf(IV) organometallic complexes with Al/Zn alkyls

NMR spectroscopy has been exploited to investigate the reactions of Hf(IV) organometallic complexes with trialkylaluminum and dialkylzinc, with the aim of obtaining insights into the elementary steps of coordinative chain transfer polymerization (CCTP). Bis(cyclopentadienyl)hafnium dimethyl (Cp₂HfMe₂, 1Me₂) and [N-(2,6-diisopropylphenyl)-α-(2-isopropylphenyl)-6-(1-naphthalenyl)-2-pyridinemethanaminato]hafnium dimethyl (2Me₂) complexes have been chosen as case studies for understanding the differences between poorly performing and highly active CCTP catalysts, in an attempt to assess the effect of the ancillary ligand on the transalkylation rate. 2Me₂ was found to react much more quickly with both AlEt₃ and ZnEt₂ in comparison to 1Me₂, mainly due to a remarkably lower activation enthalpy. In addition, while the ethylation rate was found to depend on the nature of the alkylating agent for 1Me2, it does not for 2Me₂. This difference in reactivity was observed also in the case of the ion pairs obtained by reacting 1Me₂ and 2Me₂ with [CPh₃][B(C6F₅)4]. For the latter species, NMR indicated that two main deactivation pathways, namely anion decomposition and σ-bond methatesis of Hf–alkyl groups, occur.