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[C 4H6B(C6F5)(3)] betaine 6. Complex 6 is a homogeneous single component Ziegl er catalyst that actively polymerizes ethene and propene, respectively. Wit h the olefins ethene, propene, 1-butene, 1-pentene, 1-hexene, or 1,5-hexadi ene complex 6 undergoes a stoichiometric insertion reaction at -20 degrees C to generate the metallacyclic carbon-carbon coupling products 9a-9f, whic h 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 co nstant of the degenerate allyl inversion process of the starting material ( 6 --> ent-6, k(m(obs))). This allows for a mathematical kinetic deconvoluti on of the two-step reaction sequence, namely the initial alkene addition pr ocess to 6 to generate the reactive (pi-alkene)(sigma-allyl)metallocene-typ e intermediate 8 (rate of formation (k(1)[alkene], rate of alkene dissociat ion: k(-1)) and the subsequent insertion reaction 8 - 9 (k(2)) to give k(1) (= k(chem) + 2 km(obs)) and the ratio k(2)/k(-1) (= k(chem)/2 km(obs)) Thi s procedure quantitatively determines the two transition states involved. I n 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 actua l insertion step is rate-determining and is preceded by the alkene addition /alkene dissociation preequilibrium. For example, the rate of 1-pentene dis sociation at the stage of the intermediate 8 to reform, the starting materi al 6 is similar to 70 times higher than the competing actually product form ing alkene insertion reaction to yield 9d. The difference of transition-sta te energies ranges from Delta Delta G(2)(double dagger) = 1.7 +/- 0.4 kcal mol(-1) for 1-butene insertion to Delta Delta G(2)(double dagger) = 2.1 +/- 0.4 kcal mol(-1) for 1-pentene and 1-hexene insertion, respectively. The k inetic analysis of the alkene insertion reaction at the single component "c onstrained geometry" catalyst [Me2Si(C5H4)(NBu)-Bu-t]Zr[C4H6B(C6F5)(3)] 11 was carried out analogously. B(C6F5)(3) addition to [Me2Si(C5H4)(NBu)-Bu-t] 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 to give the mon o-alkene insertion products 14a-f, each found in solution as a single diast ereoisomer. The kinetic analysis shows an even more pronounced alkene addit ion/alkene dissociation preequilibrium step (k(-1)/k(2) approximate to 130 for 1-pentene) followed by rate-determining insertion (Delta Delta G(2)(dou ble dagger) ranging from +2.2 to +3.0 kcal mol(-1)). Reaction profiles feat uring the actual alkene insertion step as the kinetically controlling activ ation barrier could be characteristic for group 4 metallocene Ziegler catal ysts and related systems.