C1-Symmetric Zirconium Complexes of [ONNO′]-Type Salan Ligands: Accurate Control of Catalyst Activity, Isospecificity, and Molecular Weight in 1-Hexene Polymerization

New Salan ligand precursors that include differently substituted phenol arms were prepared by a two-step synthetic pathway. The ligand precursors included activity-enhancing electron-withdrawing groups on one phenol ring, stereodirecting bulky groups on the other phenol ring, and either diaminoethane or trans-1,2-diaminocyclohexane as the diamine skeleton. Reacting the ligand precursors with either tetra-(O-tert-Bu)zirconium or tetrabenzylzirconium led to [(Salan)ZrX2]-type complexes (X = O-tert-Bu, Bn) of octahedral geometry and C-1-symmetry as single stereoisomers in all cases. Crystal structure studies indicated that the fac-fac isomers had formed. The chiral Salan ligands were found to wrap in a fully diastereoselective manner. All the dibenzylzirconium complexes polymerized 1-hexene upon activation with tris(pentafluorophenl)borane. The electron-withdrawing ability of the substituents was the dominant factor in enhancing the catalysts' activity; thus Lig(2)ZrBn(2), featuring a dichlorophenolate arm and a di-tert-Bu-phenolate arm, led to an activity of 1300 g mmol(-1) h(-1), whereas Lig(4)ZrBn(2), featuring a diiodophenolate arm and a di-tert-Bu-phenolate arm, led to an activity of 180 g mmol(-1) h(-1). Pentad analysis showed that despite the catalysts' C-1-symmetry, hemiisotactic polymers had not formed. Instead, isotactic polymers were produced, the extent of isotacticity depending on the bulk of substituents on both rings, and independent of the rigidity of the diamine skeleton; thus Lig(2)ZrBn(2) led to poly(1-hexene) of [mmmm] = 54%, and Lig(4)ZrBn(2) led to poly(I-hexene) of [mmmm] = 76%. The Salan complexes of the more rigid diaminocyclohexane skeleton were less active, their typical activities being ca. 55 g mmol(-1) h-1, and led to polymers of higher molecular weights. Unraveling these trends enabled the development of a catalyst-Lig(10)ZrBn(2)-having a high activity of 300 g mmol(-1) h(-1) and leading to high a molecular weight polymer of almost perfect isotacticity of [mmmm] = 95%.