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 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(L^{1, 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(L^6–10)] 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(L^7‐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 .