Vanadyl complexes bearing bi-dentate phenoxyimine ligands: synthesis, structural studies and ethylene polymerization capability†
Reaction of [VO(OnPr)3] with the Schiff bases 3,5-(tBu)2-2-OH-C6H2CH(N(x-OR-C6H4)) (R = Me; x = 2, L1H; x = 3, L2H; x = 4, L3H; R = Et (L4H), CF3 (L5H), Ph (L6H)) or 4-methyl-3-(R)-2-(OH)-C6H4CN(2′-(2′′-(OR1)C6H4)C6H4) (R = adamantyl, R1 = Ph (L7H) or R = C(Me)2Ph, R1 = Ph (L8H)) afforded the bis(chelate) vanadium(IV) complexes [VO(Ln)2] (n = 1 (1·2MeCN); n = 2 (2); n = 3 (3·2MeCN); n = 4 (4); n = 5 (5); n = 6 (6); n = 6, (7·1.5MeCN); n = 7, (8); n = 8, (9)); in the case of L6H, the oxo-bridged vanadium(V) complexes [VO(μ-O)(L6)]2 (10) was also isolated. By contrast, interaction of 4-methyl-3-(R)-2-(OH)-C6H4CN(2′-(2′′-(OR1)C6H4)C6H4) (R = adamantyl, R1 = Me (L9H); R = tBu, R1 = Me (L10H); R = C(Me)2Ph, R1 = Me (L11H)) with [VO(OnPr)3] led to the isolation of the dinuclear complexes [VO(μ-OH)(μ-OnPr)(Ln)]2 (n = 9, (11·4MeCN); 10, (12); 11, (13)), respectively. The molecular structures of 1 to 13 are reported. All complexes have been screened as pre-catalysts for the polymerization of ethylene in the presence of the co-catalyst diethylaluminium chloride (DEAC) with or without ethyltrichloroacetate (ETA) present at 1 or 10 bar of ethylene. Under high pressure, all pre-catalysts exhibited high activity and afforded high molecular weight (Mw ≈ 200 000 to 675 000 g mol−1), linear polyethylene with activities (in the presence of ETA) in the range 4960–16 400 g mmol−1 h−1; at one bar, the products were generally of lower molecular weight. The use of methylaluminoxane (MAO) or modified MAO (MMAO) as co-catalyst led to trace or poor (≤110 g mmol−1 h−1) activity, respectively.