Synthesis and X-ray Structures of New Titanium(IV) Aryloxides and Their Exploitation for the Ring Opening Polymerization of ε-Caprolactone

A series of titanium catecholates have been prepared and characterized by single-crystal X-ray diffraction studies. Complexes 1a-7a were synthesized by the reaction of 1 equiv of ligand with Ti(OiPr)4. All are dimers in the solid-state, in which a catechol bridges between two titanium centers. Electronic (nitro and methoxy groups) and steric (tert-butyl groups) effects of the ligand have been investigated. Complex 1b was synthesized by reaction of 2 equiv of ligand with Ti(OiPr)4. A dimer is again observed with the same bridging ligand together with a terminal catechol moiety. All complexes contain a coordinated 2-propanol ligand on each titanium center forming pseudo-octahedral metal centers. All complexes were tested for the ring-opening-polymerization of epsilon-caprolactone to afford polycaprolactone (PCL). Reasonable yields (up to 79%) were obtained at room temperature, and narrow molecular weight distributions (1.13-1.27) were observed for the PCL produced. The most active complex was found to be complex 1a, containing unsubsituted catechol ligands.