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)2PhNC(Me)]2(C5H3N)}VCl3·1.3(CH2Cl2) (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)2PhNC(Me)]2(2-MeC5H3N)}VCl2·0.5 (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)2PhNC (Me)]2(C5H3N)}V(CH3)(μ-CH3)Li(Et2O)3 (3) and the ionic [{2,6-bis[2,6-(i-Pr)2PhNC(Me)]2 (2,3-Me2C5H3N)}V(CH3)2][Li(THF)2(TMEDA)2]·0.5(Et2O) (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.

Systematics and Future Projections Concerning Redox‐Noninnocent Amide/Imine Ligands

“…ortho-Iminopyridines (IMPY, Scheme 3) were originally sampled as ligands for olefin polymerization catalysis, without any recognition of their redox potential. [19,20] However, there has been growing recognition that certain of the examples in Scheme 3 can be redox-active, [21] and others probably await exploitation in this regard. For example, the ortho-iminopyridines have been shown to be richly redox-active.…”

Homo- and Co-polymerization of Ethylene with 1-Hexene Using Heterogeneous Catalysts Consisting of Bis(imino)pyridinevanadium Complex Supported on Clay Minerals and Alkyl Aluminum Compounds

“…When the temperature was raised from 60 to 70 , the PDI value decreased with the decreasing low molecular weight polyethylene (Run 12). The active species produced from the bis(imino)pyridinevanadium complex was investigated by Reardon et al 6) . They reported that the in-situ reaction of 2,6-[2,6-(i-Pr)2PhN C(CH3)]2C5H3NVCl3 and MAO on the pyridine ring resulted in the formation of the methylated complex, 2,6-[2,6-(i-Pr)2PhN C(CH3)]2(2-CH3C5H3N)VCl2, through elimination of one chlorine atom.…”

“…Among the transition metal complexes having a bis(imino) pyridine backbone, the bis(imino)pyridinevanadium(III) complexes have been investigated, because these combined with MAO show a significant activity for the polymerization or oligomerization of ethylene 6)～16) . Moreover, as remarkable properties, these complexes promoted the copolymerization of ethylene and α-olefin 6) , and the supported catalysts using a modified magnesium chloride can produce high-molecular weight polyethylene 13) . When metal complexes are used for most of the industrial polymerization processes, they must be immobilized on carriers.…”

Radical Chemistry of Iminepyridine Ligands

“…In particular, there is no correlation of selectivity with product stability. The C3 and C4 adducts are usually similar in energy and much more stable than any of the other adducts, but so far C3 addition has only been observed in two rather exceptional cases (Ca, [8] V [9] ). In contrast, the much less stable C2-alkylation product seems to form easily, and the pyridine N-alkylation product has been observed for Li, Mg and Zn.…”