New highly active and selective ethylene oligomerization catalysts based on cationic diamide zirconium complexes

Group 4 complexes based on the diamide ligand, [Me3CN(SiMe2)(2)NCMe3](2-), have been investigated as possible catalysts for olefin conversions. The dialkyl complexes, (Me2SiNCMe3)(2)ZrR2 (R = CH2Ph, Me), were converted into zwitterionic (X-ray: {Me2SiNCMe3}(2)Zr(eta (1)-Bz) {eta (n)-BzB(C6F5)(3)}), or cationic mono-alkyl compounds by reaction with B(C6F5)(3) or borate reagents. These electrophilic species catalyzed ethylene oligomerization, affording highly linear alpha-olefins under mild conditions (C-6 fraction > 99.5 % 1-hexene at 50 degreesC, 6 bar). The highest activities (290 Kg/g Zr.h) were obtained in toluene solvent using tri-iso-butylaluminium (TIBA) as scavenger. The catalyst exhibited activity decay in alkane solvent, which was countered by replacement of TIBA by more crowded trialkylaluminium scavengers or branched alkylaluminoxane scavengers. The product consisted primarily of linear alpha-olefins (LAO) with low levels of remote-branched AO's. These are formed by successive insertion of higher olefin and one or more ethylenes in a growing chain, followed by chain transfer.