Synthesis and structure of amine-bridged bis(phenolate) lanthanide complexes and their application in the polymerization of ε-caprolactone

Rare-Earth Complexes Supported by Tripodal Tetradentate Bis(phenolate) Ligands: A Privileged Class of Catalysts for Ring-Opening Polymerization of Cyclic Esters

“…For the large La (ionic radius for the six-coordinated center 1.032 Å), a dinuclear structure with μ-bridging O­(phenolates) and six-coordinated metal centers has been most often observed (Scheme ). Such a peculiar dimeric structure was also established for Y­{ON­(OMe)­O Cl,Cl }­(N­(SiHMe 2 ) 2 ), despite the small Y center, possibly as a result of limited crowding provided by the chloro substituents . Alternatively, a few amido complexes have been prepared by salt metathesis from chloro or borohydride precursors (vide infra) (Scheme ); , the yields were just slightly lower than those from the amine elimination route.…”

“…Amido complexes of the type [Ln­{ONXO R1,R2 }­(NR 2 ) (donor) n ] m offer an ideal compromise between ease of preparation, stability for isolation/handling, and reactivity as a catalyst (the amido acting as the initiating group) or a catalyst precursor (the amido being transformed in situ in an alkoxide; vide infra). They constitute the largest subclass developed in this chemistry, with many variations in terms of substituents (R 1 and R 2 ) on the phenolate rings, fourth pendant donor (X), amido group, and rare-earth metal. − ,− They are most conveniently prepared by amine elimination, via the protonolysis of two bulky amido ligands in the homoleptic precursors Ln­(N­(SiRMe 2 ) 2 ) 3 (THF) n (R = H, Me) by the acidic bis­(phenol) {ONXO R1,R2 }­H 2 proligands (Scheme ). This is actually the technique that was employed by Kerton and co-workers in 2004 for the fast construction of libraries of complexes, by a combination of six [Ln­{N­(SiMe 3 ) 2 } 3 ] reagents (Ln = Y, La, Pr, Sm, Gd, Yb) and eight tripodal {ONXO R1,R2 }­H 2 proligands (among others), in a search for the best combination in ROP catalysis (vide infra) …”

Synthesis of Poly(δ-caprolactone) via Bis(phenolate) Rare-Earth Metal Complexes Mediated Ring-Opening Polymerization and Its Chemical Recycling

“…Rare-earth metal complexes supported by tripodal dianionic diamino- or amino-alkoxy-bis­(phenolate) ligands, regarded as a “privileged class of catalysts”, have been widely used in the ROP of cyclic esters, such as β-butyrolactone, − cyclohexyl-fused γ-butyrolactone, , δ-valerolactone, lactide, − dioxan-2-one, ε-caprolactone, − 6-oxabicyclo[3.2.1]­oct-3-en-7-one, and phenyl-fused seven-membered-ring esters . The type of complexes exhibited high activity and good control over molecular weight and distribution, even excellent stereoselectivity, benefiting from easy modulation of electric and steric properties by variation of substituents on the phenolate ring and the side arm.…”

(Benz)Imidazolin-2-iminato Aluminum, Zinc, and Magnesium Complexes and Their Applications in Ring Opening Polymerization of ε-Caprolactone

“…In the past decades, aliphatic esters prepared from renewable sources have received considerable attention as important biodegradable materials for packaging, drug delivery, and medical devices. − For example, poly­(ε-caprolactone) (P­(ε-CL)) is suited for a delivery medium for the controlled release of drugs and for the delivery of antibodies and genes. − Up to date, although various methods have been exploited to prepare polyester products, the most efficient approach is the ring opening polymerization (ROP) of the corresponding monomeric cyclic esters. Currently established effective initiators for ROP of ε-CL include various kinds of structurally well-defined main group elements, , transition metals, , rare earth metals, − and actinides, ,, in which the Al, Zn, and Mg based systems seem to be the best candidates in the preparation of P­(ε-CL)’s due to their abundance in the Earth’s crust, low cost, and ease of manipulation. The appeal of these metals is further complemented by a rich structural chemistry: diversified Al, Mg, and Zn based complexes supported by a vast range of ligands, including salen-type, − imino, − phenolato, ,− porphyrin, etc., that have been described as homogeneous precatalysts in the ROP of ε-CL, which allows control of the molecular weight and molecular weight distribution of the polymer products.…”