Six-coordinated dibenzyl Zr complex bearing an [OSSO]-type tetradentate ligand was synthesized, and the structure was fully characterized by NMR spectroscopy and X-ray crystallography. The zirconium complex has C(2) symmetry in solution. Owing to an effect of the cyclooctane ring structure, the combination of the zirconium complex and B(C(6)F(5))(3) or (Ph(3)C)[B(C(6)F(5))(4)] as the activator exhibited high activity toward polymerization of 1-hexene to yield highly isotactic poly(1-hexene).

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Group 5 metal complexes bearing an [OSSO]-type bis­(phenolate) ligand have been successfully prepared by the reaction of a bis­(phenol) compound 3 or its dilithio salt 6 with VO­(OPr)3, NbCl5, or TaCl5. X-ray crystallographic analyses revealed the monomeric nature of the resulting complexes, [OSSO]­VO­(OPr) (5), [OSSO]­NbCl3 (7), and [OSSO]­TaCl3 (8), where the metal centers adopted cis-α configurations of the [OSSO] tetradentate ligand. The tantalum­(V) complex 8 with i Bu3Al/(Ph3C)­[B­(C6F5)4] as an activator catalyzed the polymerization of 1-hexene to provide completely isotactic poly­(1-hexene), with a narrow molecular weight distribution ([mmmm] > 95%, M w = 9100 g mol–1, PDI = 1.6); this is the first group 5 metal catalyst for α-olefin polymerization to afford a highly isospecific polymer.

trans-Cyclooctanediyl-bridged [OSSO]-type ligand 4 reacts with TiCl(4)(thf)(2) in toluene to produce the corresponding titanium(IV) dichloro complexes as an inseparable mixture of cis-α isomer 6a and cis-β isomer 6b in a ratio of 2:1, whereas treatment of dilithio salt of 4 with TiCl(3)(thf)(3) in Et(2)O afforded chloride-bridged dimeric titanium(III) complex 8, which indicated the antiferromagnetic character in a nonpolar solvent solution. Di(isopropoxy) titanium(IV) complex 10 having a C(2)-symmetric cis-α configuration was synthesized by the reaction of 4 with Ti(O(i)Pr)(4) in toluene as yellow crystals. Moreover, the reaction of 4 with Ti(NEt(2))(4) in toluene resulted in the unexpected formation of [OSSO]-type bis(phenolato)-bridged dinuclear diamido titanium(IV) complex 11, which adopted a distorted tetrahedral geometry on the titanium center. These titanium complexes were characterized on the basis of their NMR spectroscopic data, and the molecular structures of complexes 8, 10, and 11 were established by single crystal X-ray diffraction. The titanium(IV) and (III) complexes 6 and 8 upon activation with a cocatalyst in toluene polymerized 1-hexene isospecifically to produce poly(1-hexene) having high molecular weight (M(w) = 22,000-52,000 g mol(-1)) and relatively narrow polydispersity (M(w)/M(n) = 1.7-1.8), albeit with low activity [0.27-1.0 g mmol(cat)(-1) h(-1)].