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-iPr, 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-iPr)Zr(NMe 2 ) 2 ( 1), (NSSN-Cy)Zr(NMe 2 ) 2 ( 2), (NSSN-Mes)-Zr(NMe 2 ) 2 ( 3), (NSSN-iPr)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 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.

Metal Catalyzed Polymerization: From Stereoregular Poly(α‐olefins) to Tailor‐Made Biodegradable/Biorenewable Polymers and Copolymers

“…[153] With the aim of exploring new coordinative environments, zirconium and hafnium complexes coordinated by tetradentate tioether- amide ligands (Figure 14, 84-86) were prepared and tested in the ring opening polymerization of cyclic esters. [154] The complexes were highly active, their activities were superior to those displayed by analogous complexes coordinated by OSSO ligands and well compare with that of the most active group 4 complexes. Finally, titanium amidinate complexes (Figure 14, 87-91) were used as catalysts for the controlled polymerization of cyclic esters under industrially attractive melt conditions.…”

“…In absence of exogenous alcohol, the activities were moderate, while in the presence of exogenous alcohol higher activities were obtained, but the reaction proceeds according to an activated monomer mechanism [153] . With the aim of exploring new coordinative environments, zirconium and hafnium complexes coordinated by tetradentate tioether‐amide ligands (Figure 14, 84 – 86 ) were prepared and tested in the ring opening polymerization of cyclic esters [154] . The complexes were highly active, their activities were superior to those displayed by analogous complexes coordinated by OSSO ligands and well compare with that of the most active group 4 complexes.…”

Electronic and Steric Effects on L‐Lactide Ring‐Opening Polymerization with NSSN‐type Zr(IV) Complexes

“…In the effort to seek for alternative coordination environment, [7] we recently reported the synthesis and the characterization of new Group IV complexes supported by linear tetradentate ligands that exhibit two amide functions coupled with two thioether groups (NSSN). [8] The main reason that prompted us to investigate these complexes is that Group IV complexes supported by amide ligands were barely applied in the ring opening polymerization of cyclic esters. It worth citing the complexes with tetradentate bis(sulfonamide)amine ligands [9] and the complexes with dianionic cyclam ligands, [10] these complexes were found fairly active and exerted a good control over the polymerization process affording polylactide with narrow polydispersity indexes (PDIs) and predictable molecular weights.…”

Metallocene catalysts for the ring-opening co-polymerisation of epoxides and cyclic anhydrides

“…27 Many different catalysts have been used to effect epoxideanhydride ROCOP, including organocatalysts, [28][29][30] and complexes of aluminium, [31][32][33] cobalt, 10,12,34 chromium, 35,36 iron, 16,17 magnesium, 27,37,38 and zinc. [39][40][41] However, there are only limited reports of the Group 4 metals (Ti, Zr, Hf ) being used in this transformation; perhaps surprising given there are multiple studies detailing their efficacy in the ROP of cyclic esters [42][43][44][45][46] and the ROCOP of epoxides and CO 2 . [47][48][49][50][51][52][53][54][55] In 1999, Endo and co-workers reported the use of titanium alkoxide catalysts in the ROCOP of oxetane and anhydrides, whilst heterogeneous zirconium phosphate materials with intercalated amines have also been demonstrated to catalyse the ROCOP of glycidyl phenyl ether and hexahydro-4-methylphthalic anhydride.…”