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(eta-methylcyclopentadienyl)(eta(4)-butadiene)zirconium adds 1 equiv of the organometallic Lewis acid B(C6F5)(3) to yield the metallocene-(mu-C4H6)-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 pi reversible arrow sigma reversible arrow pi-allyl interconversion on the NMR time scale in toluene solution (Delta G double dagger m(obs)-(toluene) = 19.8 kcal mol(-1)) which becomes markedly faster in the presence of added reactive 1-alkenes (Delta G(m(obs)double dagger ) = 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 (sigma-alkyl)(pi-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 (Delta G(chem)double dagger = 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 ((Delta G(m(obs))(double dagger)(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 (Delta G(1)double dagger = 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 (Delta Delta G(2)double dagger) to the top of the actual insertion barrier (Delta Delta G(2)double dagger = 1.6 (1-hexene), 1.5 (1-pentene), 1.3 (l-butene), 0.7 kcal mol(-1) (propene)). These values, together with the activation energy of the degenerate allyl ligand interconversion of the model system (sigma-allyl)(pi-allyl)zirconocene (10) (Delta G(3)double dagger = 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 Delta G(ins)double dagger approximate to 10-11 kcal mol(-1) for the l-alkenes used in this study. The alkene decomplexation barrier (reaction 7 --> 5) is lower by ca. 1-2 kcal mol(-1).