Five octahedral eilatin complexes of the type [M(L-L)(2)(eilatin)](2+) (M = Ru, Os; L-L = bipyridyl-type ligands) were synthesized, and their dimerization via pi-pi stacking was studied by crystallography and (1)H NMR techniques. The X-ray structures of these racemic complexes were solved and revealed that the eilatin complexes are organized as discrete dimers in which the eilatin residues of each complex are stacked in centrosymmetric packing. Chemical shift dependence on concentration in the (1)H NMR spectra support fast dimer-monomer equilibrium, and the structures of the dimers in acetonitrile solution are proposed to be analogous to their solid-state structures. Dimerization constants in acetonitrile were measured for the five racemic eilatin complexes that exhibit different structural parameters, as well as for the two enantiomeric forms of one of these complexes. They were found to be independent of the metal (Ru vs Os), strongly dependent on the steric effects introduced by the L-L ligands (2,2'-bipyridine, 1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline, and 2,2'-biquinoline), and dependent on the optical purity of the complexes. A clear preference for heterochiral over homochiral dimer formation was demonstrated. This is the first report of chiral recognition in solution, exhibited by simple chemical systems held solely by pi-stacking interactions.

Phenylenediamine bis(phenolate) binds in the reduced dianionic form to titanium and zirconium alkoxides giving either mononuclear or bridging dinuclear complexes depending on the metal precursors; the fluxional mononuclear titanium complex is found to exhibit extremely high activity in polymerisation of rac-lactide in the melt.

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 Salan ligands assembled around the chiral 2,2'-bipyrrolidine backbone is described; as chelation to a metal can only occur via specific faces of the two pyrrolidine nitrogens, these ligands lead to predetermined chirality at metal centres of octahedral titanium and zirconium complexes.

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