Mononuclear salen-gallium complexes for iso-selective ring-opening polymerization (ROP) of rac-lactide

Abstract

A series of mononuclear salen-supported gallium amido/alkoxide derivatives were prepared and structurally characterized and subsequently used as initiators in rac-lactide ring-opening polymerisation (ROP). The reaction of variously substituted salen ligands (1a-1f) with 0.5 equiv. of Ga(NMe) allowed the isolation of the corresponding (salen)Ga-NMe chelates (2b-2d, 2f) via an amine elimination route, as poorly soluble compounds in common solvents. The (salen)Ga-OBn derivatives (3a-3e) may be readily accessed by an amine-elimination/alcoholysis sequence and the molecular structures of 3a, 3d and 3e were confirmed through X-ray crystallography diffraction analysis. The present (salen)Ga-X species may effectively mediate the iso-selective ROP of rac-LA in a controlled manner (P up to 0.77 using a 1/1 2f/BnOH mixture as ROP initiator), with a ROP activity greatly dependent upon steric hindrance and geometrical constraints imposed by the variously substituted salen ligands. Based on the present study, salen ligands with limited steric hindrance and a certain degree of flexibility appear best suited for iso-selective ROP by (salen)Ga chelates. The salen-gallium complex 3a is also effective for the controlled ROP of ε-caprolactone (CL) and the production of PCL-b-PLA copolymers.

Synthesis and Characterization of (pyNO−)2GaCl: A Redox-Active Gallium Complex

“…The Ga-Cl bond is in the range of other non-bridging Ga-Cl bonds reported for structurally characterized five-coordinate gallium(III) complexes. For example, the bis(o-iminosemiquinonate) gallium(III) chloride complex reported by Piskunov et al has a Ga-Cl of 2.1875(5) Å [23] and the gallium salen complex reported by the Dagorne group has a Ga-Cl distance of 2.2255(6) Å [24] The bis(β-ketoiminate) gallium(III) chloride complex prepared by the Carmalt group has a Ga-Cl bond length of distance 2.231(7) Å [25]. The Ga-N py bond distances are also similar to other Ga-N distances reported for gallium complexes supporting pyridine ligands: The pyGaX 3 (X = Cl and Br) complexes have an average Ga-N distance of 1.98 Å [26] while the {pyGaCl 2 H} 2 dimer has Ga-N distances of 2.000(2) and 1.998(2) Å [27].…”

Synthesis of Biodegradable Polymers: A Review on the Use of Schiff-Base Metal Complexes as Catalysts for the Ring Opening Polymerization (ROP) of Cyclic Esters

“…Unlike their Al counterparts, Ga(III) catalysts for the ROP of cyclic esters remain widely unexplored. Mononuclear Salen complexes 326-331 ( Figure 50) proved active in the ROP of r-LA, both in toluene and in bulk [141]. Regardless of the reaction conditions, the activity trend was found to be 331 ~ 326 > 330 > 328 > 329 > 327, suggesting that less sterically demanding substituents and flexible linkers are beneficial.…”

Metal Complexes as Catalysts/Moderators for Polymerization Reactions

“…311 Salen-supported Ga III species 4.25 (Figure 4.8) were shown to be effective catalysts for the controlled and moderately iso-selective ROP of rac-lactide. 312…”