Synthesis and Characterization of Salalen Lanthanide Complexes and Their Application in the Polymerization of rac-Lactide

Abstract

A series of neutral lanthanide complexes supported by ONNO Salalen-type ligands were synthesized, and their catalytic activity for the polymerization of rac-lactide (rac-LA) was explored. The amine elimination reactions of Ln[N(SiMe3)2]3(μ-Cl)Li(THF)3 with the ONNO Salalen-type ligand L1H2 (L1 = (2-O-C6H2-But 2-3,5)CHNCH2CH2N(Me)CH2(2-O-C6H2-But 2-3,5)) in a 1:1 molar ratio in tetrahydrofuran (THF) gave the neutral lanthanide amides L1Ln[N(SiMe3)2](THF) (Ln = Y (1), Sm (2), Nd (3)). Reaction of the lanthanide amides with benzyl alcohol produces the dimeric lanthanide alkoxo complex (L 1 LnOCH2Ph)2 (Ln = Y (4), Sm (5)) in high isolated yield. Y[N(SiMe3)2]3(μ-Cl)Li(THF)3 reacted with the Salalen-type ligand L2H2 (L2 = (2-O-C6H2-But 2-3,5)CHNCH2CH2N(Me)CH2{2-O-C6H2-(CPhMe2)2-3,5}) in a 1:l molar ratio in THF also gave the desired yttrium amide, but this complex could not be separated because of its very good solubility even in n-pentane. The proton exchange reactions of L1H2 and L2H2 with (C5H5)3Ln(THF) in a 1:1 molar ratio in THF and then with 1 equiv of benzyl alcohol gave the desired lanthanide alkoxides [L 1 Ln(OCH2Ph)]2 (Ln = Y (4), Sm (5), Yb (6)) and [L 2 Y(OCH2Ph)]2 (7), respectively. Complexes 1–7 were well characterized by elemental analyses, IR spectra, X-ray single-crystal structure determination, and NMR spectroscopy in the case of complexes 1, 4, and 7. Complexes 1–3 are isostructural and have a solvated monomeric structure. The coordination geometry around the lanthanide atom can be best described as a distorted trigonal bipyramid. Complexes 4–7 are dimeric species in the solid state. They all contain a Ln2O2 core bridging through the oxygen atoms of the two OCH2Ph groups. Each of the lanthanide atoms is also six-coordinated to form a distorted octahedron. It was found that all the complexes are efficient initiators for the ring-opening polymerization of rac-LA, giving PLA with good heterotacticity (P r up to 0.85). The observed order of increase in activity is in agreement with the order of the ionic radii, whereas the stereoselectivity is in reverse order. The steric bulkiness of the substituents on the phenol ring has no obvious impact on the rate and stereocontrolability of the polymerizations. The Ln–O species resulted in more controllable polymerization than the corresponding Ln–N species, and complex 4 can initiate rac-LA polymerization in a controlled manner.

Cubane‐Type Polynuclear Zinc Complexes Containing Tridentate Schiff Base Ligands: Synthesis, Characterization, and Ring‐Opening Polymerization Studies of rac‐Lactide and ε‐Caprolactone

“…ROP processes offer excel control in terms of molecular weight ( M n ), polydispersity (MWDs) and microstructure . Several complexes have been successfully employed in the ROP of lactide, including lithium, sodium, potassium, magnesium, calcium, strontium, aluminum, tin, zinc, some transition metals, and rare‐earth metals . Since complete elimination of catalyst rests from the polymer is virtually impossible, it is highly preferable to use non‐ or low toxic metals (for instance for biomedical applications).…”

Lanthanides and actinides: Annual survey of their organometallic chemistry covering the year 2013

“…Tris(cyclopentadienyl) lanthanide complexes Cp 3 Ln(THF) (Ln = Y, Sm, Yb) have been employed as starting materials in the synthesis of a series of lanthanide complexes comprising tetradentate Schiff-base ("Salalen" type) ligands according to Scheme 80 [57].…”

Group 4 Metal Complexes of Phenylene–Salalen Ligands in rac‐Lactide Polymerization Giving High Molecular Weight Stereoblock Poly(lactic acid)

“…[8] Salalens are tetradentate dianionic {ONNO}-type ligands that include two phenolate external donors and an imine and amine internal donors and may therefore be regarded as hybrids of the salan and salen ligands. [26] However, in all cases where isotacticity wasd emonstrated, the degree of stereocontrol was relatively low.T he variety of salalen ligandsd escribed herein-phenylene-salalen ligands-features a1 ,2-phenylene unit between the imine and amine donors. [21] Salalen complexes have been investigateda sc atalysts in rac-lactide polymerization as well:a luminium complexes of achiral salalen ligandsl ed to PLA of either heterotactically inclined or isotactically inclinedPLA depending on the phenolate substitution pattern; [9c, 17, 22] aluminium complexes of chiral aminomethylpyrrolidine-based salalen ligands led to PLA of the novel gradient isotactic multiblock PLA; [23] group 4c omplexes of salalenl igandsa lso led to PLA of either heterotactically inclined or isotactically inclined PLA; [24,25] and lanthanide salalen complexes led to heterotacticP LA.…”

“…[21] Salalen complexes have been investigateda sc atalysts in rac-lactide polymerization as well:a luminium complexes of achiral salalen ligandsl ed to PLA of either heterotactically inclined or isotactically inclinedPLA depending on the phenolate substitution pattern; [9c, 17, 22] aluminium complexes of chiral aminomethylpyrrolidine-based salalen ligands led to PLA of the novel gradient isotactic multiblock PLA; [23] group 4c omplexes of salalenl igandsa lso led to PLA of either heterotactically inclined or isotactically inclined PLA; [24,25] and lanthanide salalen complexes led to heterotacticP LA. [26] However, in all cases where isotacticity wasd emonstrated, the degree of stereocontrol was relatively low.T he variety of salalen ligandsd escribed herein-phenylene-salalen ligands-features a1 ,2-phenylene unit between the imine and amine donors. [27] Chromiumc omplexes of ap henylene-salalen ligand featuring tert-butyl phenolate substituents (Lig 1 H 2 of the current work) were previously introducedb yN ozaki and were found to be active in epoxide/carbon dioxide copolymerization.…”