Evidence for cationic Group 4 zirconocene complexes with intramolecular phenyl co-ordination

Abstract

The mono- and bis-ring substituted zirconocenes with pendant phenyl groups [Zr(η-C₅H₅)(η-C₅H₄CMe₂Ph)Me₂] 2, [Zr(η-C₅H₅)(η-C₅H₄CMe₂C₆H₄Me-p)Me₂] 3, [Zr(η-C₅H₄CMe₂Ph)₂Me₂] 4, and [Zr(η-C₅H₄CMe₂C₆H₄Me-p)₂Me₂] 5 have been prepared. The crystal structures of 3 and 4 have been determined. Compounds 2–5 react with methyl abstracting reagents such as B(C₆F₅)₃ or [Ph₃C]⁺[B(C₆F₅)₄]^– to form cationic zirconocene complexes 6–9 as solvent separated ion pairs as shown by low temperature NMR spectroscopy. For the cationic complexes [Zr(η-C₅H₅)(η-C₅H₄CMe₂Ph)Me]⁺[RB(C₆F₅)₃]^– (R = Me 6a or C₆F₅6b) and [Zr(η-C₅H₅)(η-C₅H₄CMe₂C₆H₄Me-p)Me]⁺[RB(C₆F₅)₃]^– (R = Me 7a or C₆F₅7b) evidence for the co-ordination of a phenyl group to the zirconium centre via agostic C–H–M interaction was obtained by NMR spectroscopy. These cationic complexes can be considered as models for solvent adducts in Kaminsky catalysts. The cationic complexes [Zr(η-C₅H₄CMe₂Ph)₂Me]⁺[RB(C₆F₅)₃]^– (R = Me 8a or C₆F₅8b) (derived from 4) and [Zr(η-C₅H₄CMe₂C₆H₄Me-p)₂Me]⁺[RB(C₆F₅)₃]^– (R = Me 9a or C₆F₅9b) (derived from 5), respectively, exhibit more complex behaviour. These observations contrast with those from the previously published benzyl congener [Zr(η-C₅H₄CH₂Ph)₂Me₂] 1 which, with methyl abstracting agent, generates both a solvent separated cation/anion pair and a tight ion pair.