Synthesis, Structure, and 1‑Hexene Polymerization Catalytic Ability of Group 5 Metal Complexes Incorporating an [OSSO]-Type Ligand

Group 5 metal complexes bearing an [OSSO]-type bis­(phenolate) ligand have been successfully prepared by the reaction of a bis­(phenol) compound <b>3</b> or its dilithio salt <b>6</b> with VO­(OPr)₃, NbCl₅, or TaCl₅. X-ray crystallographic analyses revealed the monomeric nature of the resulting complexes, [OSSO]­VO­(OPr) (<b>5</b>), [OSSO]­NbCl₃ (<b>7</b>), and [OSSO]­TaCl₃ (<b>8</b>), where the metal centers adopted cis-α configurations of the [OSSO] tetradentate ligand. The tantalum­(V) complex <b>8</b> with ^<i>i</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 ([<i>mmmm</i>] > 95%, <i>M</i>_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