Zirconium and Hafnium Complexes with Cycloheptane- or Cyclononane-Fused [OSSO]-Type Bis(phenolato) Ligands: Synthesis, Structure, and Highly Active 1‑Hexene Polymerization and Ring-Size Effects of Fused Cycloalkanes on the Activity

Zirconium and hafnium complexes bearing cycloheptane- or cyclononane-fused [OSSO]-type bis­(phenolato) ligands ([C7] and [C9], respectively) were prepared and subjected to the polymerization of 1-hexene as the precatalyst. The polymerizations produced poly­(1-hexene)­s with high activities and high isospecificity, where complexes bearing [C9] were more reactive than those bearing [C7]. Their activities were compared with those of the corresponding complexes bearing cyclohexane- and cyclooctane-fused ligands ([C6] and [C8], respectively), which we reported previously, to show the order of activity [C8] > [C9] > [C7] > [C6]. The ring-size effect on the activity was investigated with the help of DFT calculations on active and dormant cationic zirconium species, π complexes of the active species with propene, and transition states for propene insertion into the Zr–C­(iBu) bond. The order of activity speculated from the activation energy, that is the energy difference between the π complex and the corresponding transition state, was [C8] > [C7] > [C9] ≈ [C6]. However, calculations on active and dormant cationic zirconium complexes including [B­(C6F5)4]− as the counteranion revealed that the active species are more stable than the dormant species by 9.1 kcal mol–1 for [C8] followed by 7.4 kcal mol–1 for [C9] and 3.1 kcal mol–1 for [C7] and, in contrast, that the active species with [C6] is less stable by 1.0 kcal mol–1 than the corresponding dormant species. Thus, the abundances of active species bearing [C6] and [C7] are reduced, which leads to the reversal of the order of [C7] and [C9] on the basis of activation energy to reproduce the order observed experimentally.