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

Abstract

Bis(η-methylcyclopentadienyl)(η4-butadiene)zirconium adds 1 equiv of the organometallic Lewis acid B(C6F5)3 to yield the metallocene−(μ-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 π ⇌ σ ⇌ π-allyl interconversion on the NMR time scale in toluene solution (ΔG ⧧ m(obs)(toluene) = 19.8 kcal mol-1) which becomes markedly faster in the presence of added reactive 1-alkenes (ΔG ⧧ 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 (ΔG ⧧ 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 (ΔG ⧧ 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 (ΔG ⧧ 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 (ΔΔG ⧧ 2) to the top of the actual insertion barrier (ΔΔG ⧧ 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) (ΔG ⧧ 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 ΔG ⧧ 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.

The (butadiene)zirconocene route to active homogeneous olefin polymerization catalysts

“…Scheme 10. in each olefin insertion step and an equivalent chiral auxiliary is built up concertedly from the CH 2 -terminus of the inserted 1-alkene. A very detailed mechanistic study [23] has revealed that the olefin addition step must probably be regarded as a (reversible) preequilibrium step in the overall sequence, so that stereochemistry is actually controlled in the subsequent olefin-insertion step (i.e. at the stage of the transition state 43 à ).…”

Cyclometalated group 4 complexes supported by tridentate pyridine-2-phenolate-6-(σ-aryl) ligands: Catalysts for ethylene polymerization and comparisons with fluorinated analogues

“…Yield: 0.21 g, 52%. 1 H NMR (500 MHz, CD 2 Cl 2 ): d 1.36 (s, 9H, 5-t Bu), 1.84 (s, 9H, 3-t Bu), 4.20 (d, J = 8.2 Hz, 2H, CH 2 ), 4.47 (d, J = 8.2 Hz, 2H, CH 2 ), 6.31 (t, J = 7.3 Hz, 2H, pPh), 6.45 (t, J = 7.8 Hz, 4H, m-Ph), 6.63 (d,J = 7.3 Hz,4H, o-Ph), 6.71 (t, J = 8.0 Hz, 1H, H 9 ), 6.98 (d,J = 8.3 Hz,1H,H 14 128.35,128.49,136.74,136.90,139.15,141.91,142.96,156.95,158.05,161.74,195.04. Anal.…”

“…Yield: 0.18 g, 48%. 1 H NMR (500 MHz, C 6 D 6 ): d 1.35 (s, 9H, 5-t Bu), 1.70 (s, 9H, 3-t Bu), 3.42 (d, J = 9.4 Hz, 2H, CH 2 ), 3.56 (d, J = 9.4 Hz, 2H, CH 2 ), 6.26 (t, J = 7.3 Hz, 2H, p-Ph), 6.37 (t, J = 7.7 Hz, 4H, m-Ph), 6.49 (d,J = 7.7 Hz,1H,H 8 ),6.75 (t,J = 8.0 Hz,1H,H 9 ), 6.80 (d,J = 7.5 Hz,4H,7.16 (d,J = 4.4 Hz,1H,H 10 ),7.32 (d,J = 0.8 Hz,1H,H 6 ),7.37 (d,J = 2.2 Hz,1H,H 13 ),7.60 (d,J = 1.2 Hz,1H,H 4 ),7.66 (d,J = 2.3 Hz,1H,H 15 ). 13 C NMR (126 MHz, C 6 D 6 ): 08,126.63,132.39,136.22,137.54,142.30,145.81,154.99,158.85,161.42,187.37.…”

“…Yield: 0.20 g, 49%. 1 H NMR (500 MHz, C 6 D 6 ): d 1.36 (s, 9H, 5-t Bu), 1.81 (s, 9H, 3-t Bu), 2.15 (s, 3H, Me 18 ), 2.980 (d, J = 10.8 Hz, 2H, CH 2 ), 3.05 (s, 3H, Me 19 ), 3.09 (d, J = 10.8 Hz, 2H, CH 2 ), 6.35 (t, J = 7.4 Hz, 2H, p-Ph), 6.47 (t, J = 7.7 Hz, 4H, m-Ph), 6.70 (d, J = 7.6 Hz, 4H, o-Ph), 6.75 (dd, J = 7.9, 1.0 Hz, 1H, H 10 ), 6.83 (t, J = 7.9 Hz, 1H, H 9 ), 6.97 (d,J = 7.6 Hz,1H,H 14 ),7.16 (m,2H,H 15 126.59,130.66,136.01,137.90,141.68,142.11,145.59,156.27,159.43,165.65,203.01. Anal.…”

“…Filtration and concentration of the mixture gave a pale yellow solid at À78°C. Yield: 0.14 g, 39%.1 H NMR (500 MHz, C 6 D 6 ): d 1.35 (s, 9H, 5-t Bu), 1.74 (s, 9H, 3-t Bu), 3.21 (virtual s, 4H, CH 2 ), 6.31 (t, J = 7.3 Hz, 2H, p-Ph), 6.39 (t, J = 7.5 Hz, 4H, m-Ph), 6.43 (d, J = 7.4 Hz, 1H, H 8 ), 6.70 (t, J = 8.0 Hz, 1H, H 9 ), 6.76 (d, J = 8.0 Hz, 4H, o-Ph), 7.19(d, J = 7.7 Hz, 1H, H 10 ), 7.33 (d, J = 2.3 Hz, 1H, H 13 ), 7.36 (d, J = 1.3 Hz, 1H, H 6 ), 7.69 (d, J = 2.4 Hz, 1H, H 15 ), 7.76 (d, J = 1.3 Hz, 1H, H 4 ). 13 C NMR (126 MHz, C 6 D 6 ): d 30.77 (3-CMe 3 ), 31.79 (5-CMe 3 ), 34.57 (CMe 3 ), 35.64 (CMe 3 ), 79.08 (CH 2 ), 117.55 (C 8 ), 123.61 (C 10 ), 123.76 (p-Ph), 124.91 (C 13 ), 125.65 (C 6 ), 127.26 (C 15 ), 128.47 (m-Ph), 129.85 (oPh), 135.09 (C 4 ), 139.22 (C 9 ); 4°carbons: 122.48, 126.…”