All lined up: C1‐symmetric octahedral titanium complexes (see structure, Ti dark gray, N blue, O red, I purple) whose labile positions reside in different electronic environments were designed using the readily available salalen ligands. With methylalumoxane as co‐catalyst, highly active catalysts were obtained, which yielded high‐molecular‐weight polypropylene with ultra‐high isotacticities (see 13C NMR spectrum) and melting transitions.

A straightforward synthesis of a new type of tetradentate dianionic [OSSO]-type ligand is described. This ligand features an ethylenedithiol core bridged via methylene groups to substituted phenols, thus representing an S analogue of the [ONNO]-type Salan ligands. The [OSSO]H2 ligand precursor reacted with titanium(IV) isopropoxide and with zirconium(IV) tert-butoxide to give the corresponding [OSSO]-M(OR)2 complexes, which formed as single C2-symmetric isomers but were fluxional according to variable-temperature NMR. An X-ray structure of [OSSO]-Zr(O-t-Bu)2 supported the fac-fac wrapping mode of the ligand. The dibenzyl complex [OSSO]-Zr(bn)2 that was obtained by a reaction between the ligand precursor and tetrabenzylzirconium was found to be an active 1-hexene polymerization catalyst upon activation with B(C6F5)3, leading to a stereoirregular polymer despite its C2 symmetry.

New Salan ligand precursors that include differently substituted phenol arms were prepared by a twostep 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)ZrX 2 ]-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 ditert-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(1-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 catalystsLig 10 ZrBn 2 shaving 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%.

The first synthesis of achiral and chiral [ONNO']-type Salan ligands featuring two different phenol arms, and the diastereoselective formation of the corresponding octahedral C1-symmetric zirconium complexes is described; the activity and isospecificity induction of the [ONNO']Zr(bn)2 complexes in 1-hexene polymerisation reflected those of the parent symmetric compounds.

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