Dinucleating Amino-Phenolate Platform for Zinc Catalysts: Impact on Lactide Polymerization

Abstract

We report imine-and amine-based dinucleating ligands bearing a bisphenol backbone and explore their coordination chemistry with zinc to form zinc alkyl, alkoxide, acetate, and amide complexes. Full characterization of the complexes shows that this ligand framework can support dinuclear and trinuclear complexes. We explore the reactivity of the zinc alkyl and alkoxide complexes as catalysts for the ring opening polymerization of lactide and compared this reactivity to analogous mononuclear complexes. We show that 1) The amine-based complexes are more reactive than the imine-based analogues; 2) The trinuclear zinc alkyl species show unusual control and reproducibility for lactide polymerization; and 3) The extent of bimetallic cooperation is hampered by the ability of the ligand framework to form trinuclear clusters.

Cooperative Effect in Binuclear Zinc Catalysts in the ROP of Lactide

“…developed di- or trinuclear zinc alkyl, alkoxide, acetate, and amide complexes based on imine- and amine-substituted dinuclear phenolate ligands containing a bisphenol backbone. Amine-based complexes were more active in the ROP of lactide than imine-based ones, and the activity of the alkyl complexes was found to differ, most likely due to the cooperative effects of the metal centers …”

“…Amine-based complexes were more active in the ROP of lactide than imine-based ones, and the activity of the alkyl complexes was found to differ, most likely due to the cooperative effects of the metal centers. 28 The ligand design in catalytically active binuclear metal catalysts is crucial since the intermetallic metal•••metal distance plays a key role in the activity and selectivity of the catalyst as was demonstrated for weakly and strongly bound dimeric zinc β-diketiminate catalysts. 29,30 Metal complexes of bridged bis(amidinato), 31−38 bis(diketiminato), 39,40 and bis-(indenyl) 41,42 ligands are of potential interest due to the ease with which their steric and electronic properties can be modified, and some showed higher catalytic activities and selectivities in organic transformations 17,19,43,44 and polymerization reactions 20,21,45−48 than their mononuclear counterparts.…”

Highly Active Homoleptic Zinc and Magnesium Complexes Supported by Constrained Reduced Schiff Base Ligands for the Ring-Opening Polymerization of Lactide

“…To tackle these problems, polylactide (PLA) derived from natural renewable resources is one of the most promising alternative and sustainable biodegradable materials used in packaging applications and biomedical applications such as drug delivery, implants, wound management, and tissue engineering. − The most common and effective route to obtaining well-defined PLA is the ring-opening polymerization (ROP) of lactide (LA) catalyzed by well-defined ligated metal complexes. − Among several available ligand options, the Schiff base ligand is one of the most promising supporting ligands due to the ease of ligand synthesis and modification. Many Schiff base metal complexes , such as Ca, Ti, Fe, Zn, Mg, Al, Ga, and In were reported as efficient catalysts for the ROP of cyclic esters where Zn and Mg metals have received significant attention, notably for their high reactivity and good polymerization controls as reported by several excellent works. − …”

Binucleating Bis(pyrazolyl)alkane Ligands and Their Cationic Dizinc Complexes: Modular, Bimetallic Catalysts for Ring-Opening Polymerization

“…In order to control and understand multimetallic cooperativity in main-group-catalyzed polymerization, a wide variety of bimetallic main group complexes have been prepared using binucleating ligands. − These examples include some of the most active lactide polymerization catalysts known , and have shown increasing amenability toward bimetallic structure–activity analysis and optimization. − However, the modularity of main group bimetallic catalysts still lags behind their transition metal analogues, whose structures can be optimized systematically and widely. − These transition metal bimetallics typically rely on flat coordination geometries and metal–metal bonding, features that are more difficult to maintain in main group metal coordination chemistry. − …”