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.