Highly active, thermally stable, ethylene-polymerisation pre-catalysts based on niobium/tantalum-imine systems.

The reactions of MCl5 or MOCl3 with imidazole-based pro-ligand L(1)H, 3,5-tBu2-2-OH-C6H2-(4,5-Ph2-1H-)imidazole, or oxazole-based ligand L(2)H, 3,5-tBu2-2-OH-C6H2 (1H-phenanthro[9,10-d])oxazole, following work-up, afforded octahedral complexes [MX(L(1,2))], where MX=NbCl4 (L(1), 1a; L(2), 2a), [NbOCl2(NCMe)] (L(1), 1b; L(2), 2b), TaCl4 (L(1), 1c; L(2), 2c), or [TaOCl2(NCMe)] (L(1), 1d). The treatment of α-diimine ligand L(3), (2,6-iPr2C6H3N=CH)2, with [MCl4(thf)2] (M=Nb, Ta) afforded [MCl4(L(3))] (M=Nb, 3a; Ta, 3b). The reaction of [MCl3(dme)] (dme=1,2-dimethoxyethane; M=Nb, Ta) with bis(imino)pyridine ligand L(4), 2,6-[2,6-iPr2C6H3N=(Me)C]2C5H3N, afforded known complexes of the type [MCl3(L(4))] (M=Nb, 4a; Ta, 4b), whereas the reaction of 2-acetyl-6-iminopyridine ligand L(5), 2-[2,6-iPr2C6H3N=(Me)C]-6-Ac-C5H3N, with the niobium precursor afforded the coupled product [({2-Ac-6-(2,6-iPr2C6H3N=(Me)C)C5H3N}NbOCl2)2] (5). The reaction of MCl5 with Schiff-base pro-ligands L(6)H-L(10)H, 3,5-(R(1))2-2-OH-C6H2CH=N(2-OR(2)-C6H4), (L(6)H: R(1)=tBu, R(2)=Ph; L(7)H: R(1)=tBu, R(2)=Me; L(8)H: R(1)=Cl, R(2)=Ph; L(9)H: R(1)=Cl, R(2)=Me; L(10)H: R(1)=Cl, R(2)=CF3) afforded [MCl4(L(6-10))] complexes (M=Nb, 6a-10a; M=Ta, 6b-9b). In the case of compound 8b, the corresponding zwitterion was also synthesised, namely [Ta(-)Cl5(L(8)H)(+)]·MeCN (8c). Unexpectedly, the reaction of L(7)H with TaCl5 at reflux in toluene led to the removal of the methyl group and the formation of trichloride 7c [TaCl3(L(7-Me))]; conducting the reaction at room temperature led to the formation of the expected methoxy compound (7b). Upon activation with methylaluminoxane (MAO), these complexes displayed poor activities for the homogeneous polymerisation of ethylene. However, the use of chloroalkylaluminium reagents, such as dimethylaluminium chloride (DMAC) and methylaluminium dichloride (MADC), as co-catalysts in the presence of the reactivator ethyl trichloroacetate (ETA) generated thermally stable catalysts with, in the case of niobium, catalytic activities that were two orders of magnitude higher than those previously observed. The effects of steric hindrance and electronic configuration on the polymerisation activity of these tantalum and niobium pre-catalysts were investigated. Spectroscopic studies ((1)H NMR, (13)C NMR and (1)H-(1)H and (1)H-(13)C correlations) on the reactions of compounds 4a/4b with either MAO(50) or AlMe3/[CPh3](+)[B(C6F5)4](-) were consistent with the formation of a diamagnetic cation of the form [L(4)AlMe2](+) (MAO(50) is the product of the vacuum distillation of commercial MAO at +50 °C and contains only 1 mol% of Al in the form of free AlMe3). In the presence of MAO, this cationic aluminium complex was not capable of initiating the ROMP (ring opening metathesis polymerisation) of norbornene, whereas the 4a/4b systems with MAO(50) were active. A parallel pressure reactor (PPR)-based homogeneous polymerisation screening by using pre-catalysts 1b, 1c, 2a, 3a and 6a, in combination with MAO, revealed only moderate-to-good activities for the homo-polymerisation of ethylene and the co-polymerisation of ethylene/1-hexene. The molecular structures are reported for complexes 1a-1c, 2b, 5, 6a, 6b, 7a, 8a and 8c.

Vanadium-based pro-catalysts bearing depleted 1,3-calix[4]arenes for ethylene or ε-caprolactone polymerization.

Reaction of [V(X)(OR)(3)] (X = O, Np-tolyl, R = nPr, tBu) with 5,11,17,23-tBu-25,27-dihydroxycalix[4]arene (LH(2)) led to the formation of [V(X)(OR)L](2) X = O, R = nPr (1); X = Np-tolyl, R = nPr (2); X = Np-tolyl, R = tBu (3) as the major product. In the case of X = O, the minor hydrolysis product {[VO(OnPr)](2)(µ-O)L}(2) (4) has also been characterized. Complexes (1)-(4), in the presence of the co-catalyst dimethylaluminum chloride and the reactivator ethyltrichloroacetate, are highly active (≤16,400 g/mmol h bar), thermally stable pro-catalysts for the polymerization of ethylene. The use of silica supports with (1) and (2) under slurry conditions yielded polymer with activities ≤30 g/mmol h. Complexes (1)-(3) have also been screened as pro-catalysts for the ring-opening polymerization of ε-caprolactone; the conversion rate order (1) (94%) > (2) (46%) > (3) (20%) was observed at 80 °C over 72 h.