Highly Active, Thermally Stable, Ethylene‐Polymerisation Pre‐Catalysts Based on Niobium/TantalumImine Systems
Authors: Redshaw Carl , Walton Mark , Clowes Lucy , Hughes David L. , Fuller Anna‐Marie , Chao Yimin , Walton Alex , Sumerin Victor , Elo Pertti , Soshnikov Igor , Zhao Weizhen , Sun Wen‐Hua 
Published: May 2013

The reactions of MCl5 or MOCl3 with imidazole‐based pro‐ligand L1H, 3,5‐tBu2‐2‐OH‐C6H2‐(4,5‐Ph2‐1H‐)imidazole, or oxazole‐based ligand L2H, 3,5‐tBu2‐2‐OH‐C6H2(1H‐phenanthro[9,10‐d])oxazole, following work‐up, afforded octahedral complexes [MX(L1, 2)], where MX=NbCl4 (L1, 1 a; L2, 2 a), [NbOCl2(NCMe)] (L1, 1 b; L2, 2 b), TaCl4 (L1, 1 c; L2, 2 c), or [TaOCl2(NCMe)] (L1, 1 d). The treatment of α‐diimine ligand L3, (2,6‐iPr2C6H3NCH)2, with [MCl4(thf)2] (M=Nb, Ta) afforded [MCl4(L3)] (M=Nb, 3 a; Ta, 3 b). The reaction of [MCl3(dme)] (dme=1,2‐dimethoxyethane; M=Nb, Ta) with bis(imino)pyridine ligand L4, 2,6‐[2,6‐iPr2C6H3N(Me)C]2C5H3N, afforded known complexes of the type [MCl3(L4)] (M=Nb, 4 a; Ta, 4 b), whereas the reaction of 2‐acetyl‐6‐iminopyridine ligand L5, 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 L6H–L10H, 3,5‐(R1)2‐2‐OH‐C6H2CHN(2‐OR2‐C6H4), (L6H: R1=tBu, R2=Ph; L7H: R1=tBu, R2=Me; L8H: R1=Cl, R2=Ph; L9H: R1=Cl, R2=Me; L10H: R1=Cl, R2=CF3) afforded [MCl4(L6–10)] complexes (M=Nb, 6 a–10 a; M=Ta, 6 b–9 b). In the case of compound 8 b, the corresponding zwitterion was also synthesised, namely [Ta−Cl5(L8H)+]⋅MeCN (8 c). Unexpectedly, the reaction of L7H with TaCl5 at reflux in toluene led to the removal of the methyl group and the formation of trichloride 7 c [TaCl3(L7‐Me)]; conducting the reaction at room temperature led to the formation of the expected methoxy compound (7 b). 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 (1H NMR, 13C NMR and 1H1H and 1H13C correlations) on the reactions of compounds 4 a/4 b with either MAO(50) or AlMe3/[CPh3]+[B(C6F5)4]− were consistent with the formation of a diamagnetic cation of the form [L4AlMe2]+ (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 4 a/4 b systems with MAO(50) were active. A parallel pressure reactor (PPR)‐based homogeneous polymerisation screening by using pre‐catalysts 1 b, 1 c, 2 a, 3 a and 6 a, 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 1 a–1 c, 2 b, 5, 6 a, 6 b, 7 a, 8 a and 8 c.