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Bis(β‐diketiminato) lanthanide amides: synthesis, structure and catalysis for the polymerization of l‐lactide and ε‐caprolactone

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Treatment of the chlorides (L2,6‐iPr2 Ph)2LnCl (L2,6‐iPr2 Ph = [(2,6‐ i Pr2C6H3)NC(Me)CHC(Me)N(C6H5)]) with 1 equiv. of NaNH(2,6‐ i Pr2C6H3) afforded the monoamides (L2,6‐iPr2 Ph)2LnNH(2,6‐ i Pr2C6H3) (Ln = Y (1), Yb (2)) in good yields. Anhydrous LnCl3 reacted with 2 equiv. of NaL2,6‐iPr2 Ph in THF, followed by treatment with 1 equiv. of NaNH(2,6‐ i Pr2C6H3), giving the analogues (L2,6‐iPr2 Ph)2LnNH(2,6‐ i Pr2C6H3) (Ln = Sm (3), Nd (4)). Two monoamido complexes stabilized by two L2‐Me ligands, (L2‐Me)2LnNH(2,6‐ i Pr2C6H3) (L2‐Me = [N(2‐MeC6H4)C(Me)]2CH); Ln = Y (5), Yb (6)), were also synthesized by the latter route. Complexes 1, 2, 3, 4, 5, 6 were fully characterized, including X‐ray crystal structure analyses. Complexes 1, 2, 3, 4, 5, 6 are isostructural. The central metal in each complex is ligated by two β‐diketiminato ligands and one amido group in a distorted trigonal bipyramid. All the complexes were found to be highly active in the ring‐opening polymerization of L‐lactide (L‐LA) and ε‐caprolactone (ε‐CL) to give polymers with relatively narrow molar mass distributions. The activity depends on both the central metal and the ligand (Yb < Y < Sm ≈ Nd and L2‐Me < L2,6‐iPr2 Ph). Remarkably, the binary 3/benzyl alcohol (BnOH) system exhibited a striking ‘immortal’ nature and proved able to quantitatively convert 5000 equiv. of L‐LA with up to 100 equiv. of BnOH per metal initiator. All the resulting PLAs showed monomodal, narrow distributions (M w/M n = 1.06 − 1.08), with molar mass (M n) decreasing proportionally with an increasing amount of BnOH. The binary 4/BnOH system also exhibited an ‘immortal’ nature in the polymerization of ε‐CL in toluene. Copyright © 2014 John Wiley & Sons, Ltd. Bis(β‐diketiminato) lanthanide amides are highly active initiators for the ring‐opening polymerization of ε‐caprolactone and L‐Lactide, and their binary systems with BuOH exhibited an “immortal” nature for the controlled ROP of cyclic esters.
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Keywords: controlled polymerization; cyclic ester; diketiminato; lanthanide; monoamido complex

Document Type: Research Article

Publication date: June 1, 2014

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