4.5 Article

Bis(β-diketiminate) Rare-Earth-Metal Borohydrides: Syntheses, Structures, and Catalysis for the Polymerizations of L-Lactide, ε-Caprolactone, and Methyl Methacrylate

Journal

ORGANOMETALLICS
Volume 31, Issue 17, Pages 6222-6230

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/om3005299

Keywords

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Funding

  1. National Natural Science Foundation of China [21132002, 20972107]
  2. Priority Academic Program Development of Jiangsu Higher Education Institutions

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Reaction of LnCl(3) (Ln = Y, Yb) with 2 equiv of NaLPh2,6-ipr2 (L-Ph(2,6-ipr2) = [(2,6-(Pr2C6H3)-Pr-i)NC(Me)CHC(Me)N(C6H5)](-)) afforded the chlorides (L-Ph(2,6-ipr2))(2)YCl (1) and (L-Ph(2,6-ipr2))(2)YbCl (2). Crystal structure analysis revealed 2 to be the unsolvated monomer. Treatment of the chlorides 1 and 2 with NaBH4 in a 1/1 molar ratio in THF led to the preparation of the monoborohydrides (L-Ph(2,6-ipr2))(2)LnBH(4) (Ln = Y (3), Yb (4)) in good yields. Reaction of LnCl(3) (Ln = Y, Yb) with 2 equiv of NaL2-Me (L2-Me = [N(2-MeC6H4)C(Me)](2)CH-) in THF, followed by treatment with 1 equiv of NaBH4, afforded the monoborohydrides (L2-Me)(2)LnBH(4) (Ln = Y (5), Yb (6)). Complexes 3-6 were fully characterized, including X-ray crystal structure analyses. Complexes 3-6 are isostructural. The central metal in each complex is ligated by two beta-diketiminate ligands and one eta(3)-BH4- group in a distorted trigonal bipyramid. Complexes 3-6 were found to be highly active in the ring-opening polymerization of L-lactide (L-LA) and epsilon-caprolactone (epsilon-CL) to give polymers with relatively narrow molar mass distributions. The activity depends on both the central metal and the ligand (Y > Yb and L-Ph(2,6-ipr2) > L2-Me). The best control over the molar mass was. found for complex 6. The M-n(obsd) values (M-n = the number-average molar mass) of the resulting PCL are in good agreement with M-n(calcd), with a ratio of monomer to 6 of up to 1000. The polymerization kinetics of L-LA in THF at 20 degrees C by complex 6 displays a first-order dependence on the monomer concentration. Notably, the binary 6/(PrOH)-Pr-i system exhibited an immortal nature and proved able to quantitatively convert 10 000 equiv of L-LA with up to 200 equiv of (PrOH)-Pr-i per metal initiator. All the obtained PLAs showed monomodal, narrow distributions (M-w/M-n = 1.06-1.11), with the M-n values decreasing proportionally with an increasing amount of (PrOH)-Pr-i. Complex 4 can also initiate the polymerization of methyl methacrylate (MMA) at -40 degrees C with high activity, affording the PMMA with 83.3% syndiotacticity.

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