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

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  Feghali, Khalil, Harding, David J., Reardon, Damien, Gambarotta,   Sandro, Yap, Glenn, Wang, Qinyan Organometallics  2002 21 (5) 968-976

  Abstract: The dinuclear and trivalent complex   {[(Me3Si)NCH2CH2]2N(Me3Si)}2V2(?-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(?-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[(?-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.

  Address (URL): http://dx.doi.org/10.1021/om011040u