An EPR study of the vanadium species formed upon interaction of vanadyl N and C-capped tris(phenolate) complexes with AlEt3 and AlEt2Cl

Reactions of vanadyl N and C-capped tris(phenolate) complexes 1 and 2 (Scheme 1) with AlEt3 and AlEt2Cl, in the absence and in the presence of ethyltrichloroacetate (ETA), were monitored by EPR. It was shown, that vanadium(IV) complexes with proposed structures LNVIV(Et)(AlEt3) and LNVIV(Et)(AlEt2Cl) are formed at the initial stage of interaction of 1 with AlEt3 and AlEt2Cl, respectively (LN are unidentified products of interaction of the initial N-capped ligand and VO moiety with aluminum alkyls). In the absence of re-activator (ETA), LNVIV(Et)(AlEt3) and LNVIV(Et)(AlEt2Cl) rapidly convert at room temperature to V(III) species, which relatively slowly transform to complexes LNVIIEt2 and LNVII(Et)Cl. Concentration of LNVIV(Et)(AlEt2Cl) dramatically increases in the presence of ETA, that correlates with the increase of the polymerization activity of the catalyst system 1/AlEt2Cl. The complex LNVIV(Et)(AlEt2Cl) can be precursor of the active species of polymerization. Similarly, ETA increases concentration of vanadium(IV) complex LCVIV(Et)(AlEt2Cl) in the catalyst system 2/AlEt2Cl (LC are unidentified products of interaction of the initial C-capped ligand and VO moiety with aluminum alkyls). The complex LCVIV(Et)(AlEt2Cl) can be precursor of the active species of polymerization.

The vanadium(V) complexes of N and C-capped tripodal ligands (complexes 1 and 2, respectively) serve as active, thermally robust pro-catalysts for ethylene homo- and ethylene/propylene copolymerization. EPR spectroscopic studies of the systems 1(2)/AlEt2Cl, and 1(2)/AlEt2Cl/ETA (ETA = trichloroacetate) show, that the concentration of V(IV) species in the reaction solution correlates with the polymerization activity of the 1(2)/AlEt2Cl catalyst systems and complexes of the type LVIV(Et)(AlEt2Cl) can be precursors of the active species of polymerization of the catalyst systems 1(2)/AlEt2Cl.Figure optionsDownload as PowerPoint slide