Arriving at an Experimental Estimate of the Intrinsic Activation Barrier of Olefin Insertion into the Zr−C Bond of an Active Metallocene Ziegler Catalyst

Bis(η-methylcyclopentadienyl)(η 4 -butadiene)zirconium adds 1 equiv of the organometallic Lewis acid B(C 6 F 5 ) 3 to yield the metallocene−(μ-C 4 H 6 )−borate betaine 5, which is an active, homogeneous, one-component Ziegler catalyst for the polymerization of 1-alkenes. The metallacyclic metallocene betaine 5 undergoes a degenerate π ⇌ σ ⇌ π-allyl interconversion on the NMR time scale in toluene solution (Δ G ⧧ m(obs) (toluene) = 19.8 kcal mol - 1 ) which becomes markedly faster in the presence of added reactive 1-alkenes (Δ G ⧧ m(obs) = 18.9 (1-hexene), 17.7 (1-pentene), 17.5 (1-butene), 17.2 kcal mol - 1 (propene)). This lowering of the activation barrier is probably due to an increased stabilization of the (σ-alkyl)(π-alkene)metallocene betaine intermediate 7, which at the same time is passed as the essential intermediate stage of the insertion of these alkenes into the reactive zirconium carbon bond of 5 to yield the mono olefin insertion products 9 . The Gibbs activation energies of this chemical insertion reaction (Δ G ⧧ chem = 20.1 (1-hexene), 18.8 (1-pentene), 18.5 (1-butene), 17.3 kcal mol - 1 (propene)) are similar in magnitude to the activation energies of magnetical exchange (Δ G ⧧ m(obs) ). This creates the interesting situation that in the presence of these reactive 1-alkenes, the (reversible) magnetical exchange rate ( k m (obs) ), as determined by the dynamic NMR experiment, is dependent on the rate of the competing (irreversible) overall chemical addition reaction ( k chem ). The rate constants k m (obs) and k chem were measured in the presence of these 1-alkenes, which allowed for a determination of the height of the first, i.e., the complexation barrier (Δ G ⧧ 1 = 18.5 (1-hexene), 17.3 (1-pentene), 17.1 (1-butene), 16.4 kcal mol - 1 (propene); standard state c ⊖ = 1 mol L -1 ) and its difference (ΔΔ G ⧧ 2 ) to the top of the actual insertion barrier (ΔΔ G ⧧ 2 = 1.6 (1-hexene), 1.5 (1-pentene), 1.3 (1-butene), 0.7 kcal mol - 1 (propene)). These values, together with the activation energy of the degenerate allyl ligand interconversion of the model system (σ-allyl)(π-allyl)zirconocene ( 10 ) (Δ G ⧧ 3 = 7 ± 0.5 kcal mol - 1 ), allowed for a good experimentally based estimate of the intrinsic activation energy (reaction 7 → 9 ) of the insertion of these 1-alkenes into the zirconium carbon bond at this group 4 bent metallocene unit. The thus obtained insertion barrier is Δ G ⧧ ins ≈ 10−11 kcal mol - 1 for the 1-alkenes used in this study. The alkene decomplexation barrier (reaction 7 → 5 ) is lower by ca. 1−2 kcal mol - 1 .