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

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-iPr, NSSNCy, 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(NMe2) 4 or Hf(NMe2)(4) [(NSSN-iPr)Zr(NMe2)(2) (1), (NSSN-Cy)Zr(NMe2)(2) (2), (NSSN-Mes)Zr(NMe2)(2) (3), (NSSN-iPr)Hf(NMe2)(2) (4), and (NSSN-Cy)Hf(NMe2)(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 was superior to those generally displayed by zirconium complexes featuring OSSO ligands and compared well with those of the most active group 4 complexes operating in a toluene solution.

Automated summary

A new class of zirconium and hafnium complexes coordinated by linear dianonic tetradentate NSSN ligands have been reported. These complexes exhibit octahedral coordination geometry and show significant catalytic activity in the ring-opening polymerization of cyclic esters, with complex 1 displaying the highest activity among the complexes studied.