NSSN-Type Group 4 Metal Complexes in the Ring-Opening Polymerization of l-Lactide

Abstract

A new class of zirconium and hafnium complexes coordinated by linear dianonic tetradentate NSSN ligands is reported. The ligands feature two amide functions coupled with two thioether groups linked by a central flexible ethane bridge and two lateral rigid phenylene bridges and differ for the substituents on the aniline nitrogen atoms, i.e., isopropyl, cyclohexyl, or mesityl substituents: NSSN- i Pr, NSSN-Cy, or NSSN-Mes. They were prepared by reacting 2-aminothiophenol with dibromoethane to afford the NSSN ligands without substituents on the aniline nitrogen atoms, which were subsequently alkylated through a reductive amination of acetone or cyclohexanone or palladium-catalyzed cross-coupling reaction with mesityl bromide. The corresponding zirconium and hafnium complexes 1 - 5 were obtained through a transamination reaction between the neutral ligands and Zr(NMe 2 ) 4 or Hf(NMe 2 ) 4 [(NSSN- i Pr)Zr(NMe 2 ) 2 ( 1 ), (NSSN-Cy)Zr(NMe 2 ) 2 ( 2 ), (NSSN-Mes)Zr(NMe 2 ) 2 ( 3 ), (NSSN- i Pr)Hf(NMe 2 ) 2 ( 4 ), and (NSSN-Cy)Hf(NMe 2 ) 2 ( 5 )]. They were characterized in solution by NMR spectroscopy and in solid state by X-ray diffraction analysis (except for 3 ). All complexes present an octahedral coordination geometry with a fac - fac ligand wrapping and a cis relationship between the other two monodentate ligands. The catalytic performances of 1 - 5 in the ring-opening polymerization of cyclic esters were investigated. Complex 1 was the most active: its polymerization activity…