Experimental Characterization of the Alkene-Addition/-Insertion Energy Profile at Homogeneous Group 4 Metal Ziegler-Type Catalysts

Treatment of the ansa-metallocene complex [Me2Si(C5H4)2]Zr(butadiene) (s-cis-/s-trans-4) with B(C6F5)3 yields the corresponding ansa-zirconocene[C4H6B(C6F5)3] betaine 6. Complex 6 is a homogeneous single component Ziegler catalyst that actively polymerizes ethene and propene, respectively. With the olefins ethene, propene, 1-butene, 1-pentene, 1-hexene, or 1,5-hexadiene complex 6 undergoes a stoichiometric insertion reaction at −20 °C to generate the metallacyclic carbon−carbon coupling products 9a−9f, which feature an internal C4C5 alkene coordination to the metal center and an intramolecular C6−Zr ion pair interaction. The rate of the overall 1-alkene insertion process 6 → 9 (k chem) was measured, and the observed rate constant of the degenerate allyl inversion process of the starting material (6 → ent-6, k m(obs)). This allows for a mathematical kinetic deconvolution of the two-step reaction sequence, namely the initial alkene addition process to 6 to generate the reactive (π-alkene)(σ-allyl)metallocene-type intermediate 8 (rate of formation (k 1[alkene], rate of alkene dissociation: k - 1) and the subsequent insertion reaction 8 → 9 (k 2) to give k 1 (= k chem + 2 k m(obs)) and the ratio k 2/k - 1 (= k chem/2 k m(obs)). This procedure quantitatively determines the two transition states involved. In each case of the 6 → 9b−f series the second transition state is higher than the first one: a general energy profile is observed in which the actual insertion step is rate-determining and is preceded by the alkene addition/alkene dissociation preequilibrium. For example, the rate of 1-pentene dissociation at the stage of the intermediate 8 to reform the starting material 6 is ∼70 times higher than the competing actually product forming alkene insertion reaction to yield 9d. The difference of transition-state energies ranges from ΔΔG ⧧ 2 = 1.7 ± 0.4 kcal mol-1 for 1-butene insertion to ΔΔG ⧧ 2 = 2.1 ± 0.4 kcal mol-1 for 1-pentene and 1-hexene insertion, respectively. The kinetic analysis of the alkene insertion reaction at the single component “constrained geometry” catalyst [Me2Si(C5H4)NtBu]Zr[C4H6B(C6F5)3] 11 was carried out analogously. B(C6F5)3 addition to [Me2Si(C5H4)NtBu]Zr(butadiene) 10 yields a 1.8:1 mixture of the stereoisomeric betaines 11A/11B (“supine/prone” orientation of the ligand). The 11A/11B complex mixture actively polymerizes ethene. At low temperature the 11A/11B mixture reacts stoichiometrically with the series of olefins listed above