Vanadium-catalyzed ethylene-propylene copolymerization: The question of the metal oxidation state in Ziegler-Natta polymerization promoted by (beta-diketonate)(3)V

Four different (beta-diketonate)(3)V complexes [beta-diketonate = 2,4-penta nedione (acac), 2-acetylcyclohexanone (Cy-acac), 2,2,6,6-tetramethyl-3,5-he ptanedione (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 copoly merization with the aim of gaining insights into the structure of the activ e species. Data on polymer composition and catalyst activity indicate that one of the roles of the Al cocatalyst is to trigger a major ligand scrambli ng around the transition metal. Attempts to isolate the catalytically activ e species afforded a compound formulated as {[(beta-diketonate)AlCl2][VCl2] [(beta-diketonate)(2)AlCl]}. This species, which displayed only a minor cat alytic activity, arises from a parasite process (catalyst deactivation). Th e formulation was supported by chemical degradation experiments with THF, w hich afforded a mixture of [V2Cl3(THF)(6)][AlCl4] and [(acac)(2)Al(THF)(2)] [Al Cl-4] (5). Two unprecedented V(II) complexes (R-acac)(2)V(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 rol e of reactivating substances, commonly employed in the industrial processes , is to reoxidize V(II) to the trivalent state. The reaction formed the cat alyst precursor (R-acac)(2)VCl(TMEDA), which was characterized on the basis of analytical and spectroscopic data. In agreement with this proposal, a t rapping experiment carried out with ZnCl2 or oxidation with CuCl allowed th e isolation and characterization of (t-Bu-acac)(2)V(TMEDA)] [X] [X = ZnCl4- (8), CuCl2- (9)]. The structures of 5, 7a, 8, and 9 have been elucidated b y X-ray diffraction. Crystal data are as follows. 5: triclinic space group P (1) over bar, a = 8.916(3) Angstrom, b = 9.802(3) Angstrom, c = 15:390(5) Angstrom, alpha = 88.156(4)degrees, beta = 86.041(4)degrees, gamma = 82.71 0(4)degrees, Z = 2. 7a: triclinic space group P (1) over bar, a = 7.896(1) Angstrom, b = 10.032(2) Angstrom, c = 13.134(2) Angstrom, alpha = 75.265(2) degrees, beta = 88.520(2)degrees, gamma = 71.445(2)degrees, Z = 2. 8: ortho rhombic space group Pbcn, a = 18.723(7) Angstrom, b = 19.675(7) Angstrom, c = 18.836(7) Angstrom, Z = 4. 9: monoclinic space group C2/c, a = 13.676(5) Angstrom, b = 19.521(3) Angstrom, c = 13.206(3) Angstrom, beta = 98.47(3)d egrees, Z = 4.