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

The dinuclear and trivalent complex {[(Me3Si)NCH2CH2]2N(Me3Si)}2V2(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 {[(Me3Si)NCH2CH2]2N(Me3Si)}V(CH3)(pyridine) (2). However, reaction of 1 with Me2AlCl, AlMe3, or PMAO-IP yields the tetrametallic species {[(Me3Si)NCH2CH2]2N(Me3Si)}2V2(μ-Cl)2(AlMe2)2 (3), where the central core of 1 was preserved except for the vanadium centers, which were reduced to the divalent state. The two Me2Al 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 AlCl3 resulted in disproportionation forming the tetravalent complex {[(Me3Si)NCH2CH2]2N(Me3Si)}VCl2AlCl3 (4) and the pentanuclear mixed-valent V(II)/V(III) species [(AlCl2){[(Me3Si)NCH2CH2]2N(Me3Si)}V]2[(μ-Cl)6V]·(toluene)2 (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.