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

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Abstract

Reaction of LnCl3 (Ln = Y, Yb) with 2 equiv of NaL2,6-ipr2Ph (L2,6-ipr2Ph = [(2,6-iPr2C6H3)NC(Me)CHC(Me)N(C6H5)]−) afforded the chlorides (L2,6-ipr2Ph)2YCl (1) and (L2,6-ipr2Ph)2YbCl (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 (L2,6-ipr2Ph)2LnBH4 (Ln = Y (3), Yb (4)) in good yields. Reaction of LnCl3 (Ln = Y, Yb) with 2 equiv of NaL2-Me (L2-Me = [N(2-MeC6H4)C(Me)]2CH–) in THF, followed by treatment with 1 equiv of NaBH4, afforded the monoborohydrides (L2-Me)2LnBH4 (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 β-diketiminate ligands and one η3-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 ε-caprolactone (ε-CL) to give polymers with relatively narrow molar mass distributions. The activity depends on both the central metal and the ligand (Y > Yb and L2,6-ipr2Ph > L2-Me). The best control over the molar mass was found for complex 6. The Mn(obsd) values (Mn = the number-average molar mass) of the resulting PCL are in good agreement with Mn(calcd), with a ratio of monomer to 6 of up to 1000. The polymerization kinetics of l-LA in THF at 20 °C by complex 6 displays a first-order dependence on the monomer concentration. Notably, the binary 6/iPrOH system exhibited an “immortal” nature and proved able to quantitatively convert 10 000 equiv of l-LA with up to 200 equiv of iPrOH per metal initiator. All the obtained PLAs showed monomodal, narrow distributions (Mw/Mn = 1.06–1.11), with the Mn values decreasing proportionally with an increasing amount of iPrOH. Complex 4 can also initiate the polymerization of methyl methacrylate (MMA) at −40 °C with high activity, affording the PMMA with 83.3% syndiotacticity.

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