Stereorigid OSSO-Type Group 4 Metal Complexes in the Ring-Opening Polymerization of rac-Lactide

The synthesis and characterization of a series of group 4 metal complexes of general formula {OSSO_X}M(OR)₂ (X = R = ^tBu, M = Zr (1); X = cumyl, M = Zr, R = ^tBu (2); X = cumyl, M = Ti, R = ⁱPr (4); X = cumyl, M = Hf, R = ^tBu (5)) and {OSSO_X}₂Zr (X = Cl (3)) supported by o-phenylene-bridged bis(phenolato) ligands (OSSO_tBu-H = 6,6'-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-di-tert-butyphenol); OSSO_Cum-H = 6,6'-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-bis(2-phenylpropan-2-yl)phenol); OSSO_Cl-H = 6,6'-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-dichlorophenol)) are described herein. Complexes 1-5 were readily obtained by σ-bond metathesis reactions between the proligand and the appropriate homoleptic metal precursor. The reaction with OSSO_Cl yielded the bis-ligand complex{OSSO_Cl}₂Zr (3) regardless of the OSSO_Cl-H/Zr(O^tBu)₄ molar ratio or experimental conditions. All complexes were characterized in solution using NMR spectroscopy and, in the case of 2, by single-crystal X-ray diffraction experiments. These complexes show a fac-fac ligand wrapping and a cis relationship between the other two monodentate ligands; zirconium and hafnium complexes 1-3 and 5 are configurationally stable, whereas titanium complex 4 is fluxional in solution at room temperature. The complexes tested in the ring-opening polymerization (ROP) of racemic-lactide showed, except in the case of 3, moderate rates and good levels of polymerization control. Upon addition of an exogenous alcohol (isopropyl alcohol or tert-butyl alcohol) efficient binary catalytic systems were achieved. Polymerizations were well-controlled, as testified by the linear growth of the molecular weight as polymerization proceeded, narrow polydispersity indices, and molecular weights close to those expected on the basis of added alcohol amounts. Experimental and theoretical evidence is provided that ROP reactions operate according to an activated monomer mechanism.