Vanadium-Catalyzed Ethylene−Propylene Copolymerization:  The Question of the Metal Oxidation State in Ziegler−Natta Polymerization Promoted by (β-diketonate)3V

Four different (β-diketonate)3V complexes [β-diketonate = 2,4-pentanedione (acac), 2-acetylcyclohexanone (Cy-acac), 2,2,6,6-tetramethyl-3,5-heptanedione (t-Bu-acac), and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (F-acac)] have been prepared and tested as catalysts for propylene−ethylene copolymerization with the aim of gaining insights into the structure of the active species. Data on polymer composition and catalyst activity indicate that one of the roles of the Al cocatalyst is to trigger a major ligand scrambling around the transition metal. Attempts to isolate the catalytically active species afforded a compound formulated as {[(β-diketonate)AlCl2][VCl2][(β-diketonate)2AlCl]}. This species, which displayed only a minor catalytic activity, arises from a parasite process (catalyst deactivation). The formulation was supported by chemical degradation experiments with THF, which afforded a mixture of [V2Cl3(THF)6][AlCl4] and [(acac)2Al(THF)2][Al Cl4] (5). Two unprecedented V(II) complexes (R-acac)2V(TMEDA) [R = H(7a), Cy(7b), t-Bu (7c)] have been prepared and reacted with halocarbons to model the reactivation process. The results indicated that the primary role of reactivating substances, commonly employed in the industrial processes, is to reoxidize V(II) to the trivalent state. The reaction formed the catalyst precursor (R-acac)2VCl(TMEDA), which was characterized on the basis of analytical and spectroscopic data. In agreement with this proposal, a trapping experiment carried out with ZnCl2 or oxidation with CuCl allowed the isolation and characterization of (t-Bu-acac)2V(TMEDA)][X] [X = ZnCl4 - (8), CuCl2 - (9)]. The structures of 5, 7a, 8, and 9 have been elucidated by X-ray diffraction. Crystal data are as follows. 5: triclinic space group P1̄, a = 8.916(3) Å, b = 9.802(3) Å, c = 15.390(5) Å, α = 88.156(4)°, β = 86.041(4)°, γ = 82.710(4)°, Z = 2. 7a: triclinic space group P1̄, a = 7.896(1) Å, b = 10.032(2) Å, c = 13.134(2) Å, α = 75.265(2)°, β = 88.520(2)°, γ = 71.445(2)°, Z = 2. 8: orthorhombic space group Pbcn, a = 18.723(7) Å, b = 19.675(7) Å, c = 18.836(7) Å, Z = 4. 9: monoclinic space group C2/c, a = 13.676(5) Å, b = 19.521(3) Å, c = 13.206(3) Å, β = 98.47(3)°, Z = 4.