Life and Death of an Active Ethylene Polymerization Catalyst. Ligand Involvement in Catalyst Activation and Deactivation. Isolation and Characterization of Two Unprecedented Neutral and Anionic Vanadium(I) Alkyls

Abstract

Reaction of {2,6-bis[2,6-(i-Pr) 2 PhN C(Me)] 2 (C 5 H 3 N)}VCl 3 ·1.3(CH 2 Cl 2 ) ( 1 ) with stoichiometric amount of methyl alumoxane (PMAO) in toluene resulted in the methylation of the pyridine ring ortho position affording {2,6-bis[2,6-(i-Pr) 2 PhN C(Me)] 2 (2-MeC 5 H 3 N)}VCl 2 ·0.5 (toluene) ( 2 ). In the process the ligand became an anionic amide, one chlorine atom was eliminated by the metal center, and the vanadium coordination number decreased by one unit. This new trivalent compound is a potent ethylene polymerization precatalyst, and polymers produced by the complex activated with PMAO showed a bimodal character in the GPC. Its bimodality is tentatively explained with the existence of two catalytically active species (mono and dialkylation of the vanadium center). Further attack of a strong alkylating agent such as MeLi occurs at the pyridine ring to either remove the methyl group, or to place an additional methyl group on the ring meta position. Both processes imply two-electron reduction of the metal center and formation of the corresponding V(I) derivatives {2,6-bis[2,6-(i-Pr) 2 PhN C (Me)] 2 (C 5 H 3 N)}V(CH 3 )(μ-CH 3 )Li(Et 2 O) 3 ( 3 ) and the ionic [{2,6-bis[2,6-(i-Pr) 2 PhN C(Me)] 2 (2,3-Me 2 C 5 H 3 N)}V(CH 3 ) 2 ][Li(THF) 2 (TMEDA) 2 ]·0.5(Et 2 O) ( 4 ) which were isolated in crystalline form. Crystal data are as follows. 1: monoclinic, space group P2 1 /n, a = 18.267(5) Å, b = 16.643(4) Å, c = 38.195(9) Å, β = 96.061(5)°, Z = 12; 2: monoclinic, space group P2 1 /c, a = 15.741(2) Å, b = 16.565(2) Å, c = 15.849(2) Å, β = 100.316(3)°, Z = 4; 3: orthorhombic space group Pnma, a = 19.724(6) Å, b = 22.001(7) Å, c = 11.035(3) Å, Z = 4; 4: monoclinic space group P2 1 /n, a = 11.932(3) Å, b = 25.363(6) Å, c = 18.957(4) Å, β = 96.587(4)°, Z = 4.