Synthesis and characterization of group 4 metal alkoxide complexes containing imine based bis-bidentate ligands: effective catalysts for the ring opening polymerization of lactides, epoxides and polymerization of ethylene

Abstract

A series of Ti(IV), Zr(IV) and Hf(IV) complexes containing imine based bis-bidentate ligands were synthesized and characterized by various spectroscopic techniques, elemental analysis and X-ray crystallography. The ligands m-xysal-(^tBu)₄ (L¹(^tBu)₄), m-xysal-(Me)₂(^tBu)₂ (L²Me₂(^tBu)₂) and m-xysal-(Cl)₄ (L³Cl₄) were reacted with Ti(OⁱPr)₄, Zr(OⁱPr)₄·ⁱPrOH and Hf(O^tBu)₄ in a 1 : 1 stoichiometric ratio to form complexes 1–3 (L₂M₂(OR)₄, where L = m-xysal-(^tBu)₄, m-xysal-(Me)₂(^tBu)₂ and m-xysal-(Cl)₄, M = Ti and R = ⁱPr, 4–6 (L₂M₂(OR)₄, where L = m-xysal-(^tBu)₄, m-xysal-(Me)₂(^tBu)₂ and m-xysal-(Cl)₄, M = Zr and R = ⁱPr and 7–9 (L₃M₃(OR)₆, where L = m-xysal-(^tBu)₄, m-xysal-(Me)₂(^tBu)₂ and m-xysal-(Cl)₄, M = Hf and R = ^tBu respectively. Complex 5 was crystallized from a 1 : 1 : 1 mixture of chloroform, ethanol and toluene to yield an ethoxy substituted complex 5a (L₂M₂(OR)₄, L = m-xysal-(Me)₂(^tBu)₂, M = Zr and R = Et. The X-ray structures of 1, 5a and 7 illustrate that 1 and 5a are binuclear helical complexes, whereas 7 is trinuclear. These complexes were found to be active for the ring opening polymerization (ROP) of lactides (rac-LA, L-LA) and epoxides. All the complexes produced atactic poly(lactic acid) (PLA) with good number average molecular weight (M_n) and narrow molecular weight distributions (MWDs). The magnetic isotropic shielding constants were calculated using the GIAO/B3LYP/LANL2DZ approach and correlated with the experimental values. The HOMO–LUMO energy band gaps and Mulliken charges were calculated using the DFT method to explain the reactivity of these complexes according to the relationship between chemical hardness and reactivity established by Pearson. In addition, complexes 1–9, activated by methylaluminoxane (MAO), were used and found to be moderately active for ethylene polymerization.