Stability of Metal−Carbon Bond versus Metal Reduction during Ethylene Polymerization Promoted by a Vanadium Complex:  The Role of the Aluminum Cocatalyst

Abstract

The dinuclear and trivalent complex {[(Me 3 Si)NCH 2 CH 2 ] 2 N(Me 3 Si)} 2 V 2 (µ-Cl) 2 (1) is the precursor to mono-and dinuclear alkyl derivatives that are thermally stable. For example, treatment with MeLi gives a stable methyl derivative, probably isostructural with 1, which upon further treatment with pyridine affords the mononuclear complex {[(Me 3 Si)NCH 2 -CH 2 ] 2 N(Me 3 Si)}V(CH 3 )(pyridine) (2). However, reaction of 1 with Me 2 AlCl, AlMe 3 , or PMAO-IP yields the tetrametallic species { 3), where the central core of 1 was preserved except for the vanadium centers, which were reduced to the divalent state. The two Me 2 Al residues remained coordinated to the amido ligand. The reduction of vanadium to the divalent state relates to the relatively short life of 1 as an ethylene polymerization catalyst. A similar reaction of 1 with AlCl 3 resulted in disproportionation forming the tetravalent complex {[(Me 3 Si)NCH 2 CH 2 ] 2 N(Me 3 Si)}VCl 2 AlCl 3 (4) and the pentanuclear mixed 5). The fact that complex 5 contains a divalent vanadium atom stripped of its ligand system indicates that two different reaction mechanisms are operating to reduce the vanadium center and that the differing Lewis acidity of the two aluminum species is the determining factor.

Bis(pyrazolyl)pyridine vanadium(III) complexes as highly active ethylene polymerization catalysts

“…Studies of the oxidation state of vanadium catalysts for ethylene polymerization have generally revealed that V(III) and V(IV) precursors are often reduced to inactive V(II) species [9][10][11][12]. Alkyl vanadium(III) complexes bearing amidinate [13,14] and amide [15,16] ligands produce shortlived ethylene polymerization catalysts and are inactive in propene polymerization.…”

Olefin Polymerization with Non-metallocene Catalysts (Early Transition Metals)

“…Taking into account the above reports (vanadium complexes using chelate anionic and neutral donor ligands) [91][92][93][94][95][96][97][98], as described above, control, stabilization of the oxidation state, catalytically active species) by an appropriate ligand modification seems to be a key for the efficient catalyst with vanadium(III), …”

“…The reaction of 19 with AlCl 3 afforded disproportionated compounds, (tetravalent) 19c and (one divalent and two trivalent) 19d, clearly indicating that disproportionation was the basis of a reduction of the vanadium center. These results suggested a possibility of the reduction of 19 affording 19b and that the addition intermediate 19a, which is likely to be the catalytically-active species, has the intrinsic instability of a vanadium-carbon bond [91].…”

“…Cl 2 Al AlCl 2 Scheme 3.9 Reaction of dinuclear vanadium(III) chloride complex containing bis(amido)amine ligand [91] although the actual active species including the activation/polymerization mechanism are still not clear at the present stage. A series of vanadium(III) complexes containing b-enaminoketonato [99, 100], phenoxy-imine [101][102][103][104], imino-pyrrolide [105] ligands (exemplified in Scheme 3.11) were tested as olefin polymerization catalyst in the presence of Et 2 AlCl and Cl 3 CCO 2 Et.…”

Non-Cp type homogeneous catalytic systems for olefin polymerization

“…The compound 47 with M = Zr and D = N could be activated by B(C 6 F 5 ) 3 for ethylene and propylene polymerization [132] but the compound 47 with M = V and D = N required Me 2 AlCl for ethylene polymerization [140].…”