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

Reaction of LnCl 3 (Ln = Y, Yb) with 2 equiv of NaL 2,6-ipr2 Ph (L 2,6-ipr2 Ph = [(2,6- i Pr 2 C 6 H 3 )NC(Me)CHC(Me)N(C 6 H 5 )] − ) afforded the chlorides (L 2,6-ipr2 Ph ) 2 YCl ( 1 ) and (L 2,6-ipr2 Ph ) 2 YbCl ( 2 ). Crystal structure analysis revealed 2 to be the unsolvated monomer. Treatment of the chlorides 1 and 2 with NaBH 4 in a 1/1 molar ratio in THF led to the preparation of the monoborohydrides (L 2,6-ipr2 Ph ) 2 LnBH 4 (Ln = Y ( 3 ), Yb ( 4 )) in good yields. Reaction of LnCl 3 (Ln = Y, Yb) with 2 equiv of NaL 2-Me (L 2-Me = [N(2-MeC 6 H 4 )C(Me)] 2 CH – ) in THF, followed by treatment with 1 equiv of NaBH 4, afforded the monoborohydrides (L 2-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 β-diketiminate ligands and one η 3 -BH 4 – 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 L 2,6-ipr2 Ph > L 2-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 °C by complex 6 displays a first-order dependence on the monomer concentration. Notably, the binary 6 / i PrOH system exhibited an “immortal” nature and proved able to quantitatively convert 10 000 equiv of l -LA with up to 200 equiv of i PrOH 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 i PrOH. Complex 4 can also initiate the polymerization of methyl methacrylate (MMA) at −40 °C with high activity, affording the PMMA with 83.3% syndiotacticity.