Zirconium and hafnium complexes bearing pyrrolidine derived salalen-type {ONNO} ligands and their application for ring-opening polymerization of lactides

Abstract

The reactions of pyrrolidine derived salalen-type {ONNO} ligands (S)-L-H with 1 equiv. M(OPr)(HOPr) (M = Zr or Hf) in diethyl ether yielded complexes [LM(OPr)] (L = L, M = Zr (1); L = L, M = Zr (2), Hf (3); L = L, M = Zr (4), Hf (5)). All of these complexes were well characterized by NMR spectroscopy, elemental analyses and single-crystal X-ray analysis in the case of complexes 1 and 3-5. X-ray structural determination revealed that these complexes were analogous mononuclear species and had a similar structure in which the metal centers were six-coordinated to two oxygen atoms and two nitrogen atoms of one ligand and two oxygen atoms of two isopropoxy groups. All of these complexes efficiently initialized the ring-opening polymerization of lactides to afford polymers with controlled molecular weight and narrow polydispersity. Furthermore, the ring-opening polymerization of rac-lactide catalyzed by complexes 1-5 afforded isotactic-enriched polymers in solution (P = 0.74-0.80) and under melt conditions (P = 0.63-0.72).

Simple Zn(ii) complexes for the production and degradation of polyesters

“…However, the stereoselective ROP of rac-lactide can improve the material properties of the polymer and thus it is oen studied in academic research. A diverse range of metals has been applied to lactide ROP including Mg(II), 15,16 group IV, [17][18][19][20][21][22][23][24] Fe(II/III), [25][26][27][28][29][30][31][32] Al(III), [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] and In(III). [50][51][52] Initiators based on zinc(II) have consistently shown high activity under solvent-free conditions and, in some cases, stereocontrol is observed.…”

Hydroboration of Aldehydes, Ketones, and Carbodiimides Promoted by Mono(imidazolin‐2‐iminato) Hafnium Complexes

“…Complexes of hafnium have been predominantly used as Lewis acid catalysts in various organic transformations, such as the polymerization of cyclic esters, and as catalysts for the homo‐ and copolymerization of α‐olefins . Recently, Hf‐MOF compounds have been used as active catalysts for the activation of small molecules .…”

Benzimidazolin-2-iminato Hafnium Complexes: Synthesis, Characterization, and Catalytic Addition of Alcohols to Carbodiimides

“…Hafnium complexes have mainly been used for the catalytic homo- and copolymerization of α-olefins − and as Lewis acid catalysts in various organic transformations including the polymerization of cyclic esters. − More recently, Hf–MOF complexes have been utilized in the catalytic activation of small molecules. − Among the group-IV metals, hafnium is considered to be the most oxophilic metal when compared with zirconium and titanium, as illustrated by their corresponding metal–oxygen bond dissociation energies (Ti–O = 666 kJ/mol, Zr–O = 766 kJ/mol, and Hf–O = 801 kJ/mol) . Hence, the strong metal–oxygen bond and the high electrophilicity of the metal have precluded the use of organometallic complexes of hafnium in processes involving oxygen-containing moieties with acidic protons. − We have postulated that if, during a stoichiometric or a catalytic process, we start with a metal–oxygen bond and after the transformation we end up with a different metal–oxygen bond, then, theoretically, we are not paying thermodynamically for the M–O bond; however, we need to have the energy of activation to be able to execute the process. − …”

“…moieties to carbodiimides have received substantial interest in recent years, and various types of catalysts have been used such as main-group metal centers, copper, lanthanides, actinides, ,,,, and titanium . Inspired by the observation that hafnium–oxygen moieties can be an active motif in catalytic processes, ,,− ,,− ,,,− we started to design our ligands toward the synthesis of hafnium complexes. On the one hand, we need a ligand that is highly nucleophilic to allow the stabilization of the metal complex and to ensure that the ligand will remain attached to the metal after the catalytic process.…”