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

Reaction of LnCl₃ (Ln = Y, Yb) with 2 equiv of NaL^2,6‑ipr2_Ph (L^2,6‑ipr2_Ph = [(2,6-^<i>i</i>Pr₂C₆H₃)­NC­(Me)­CHC­(Me)­N­(C₆H₅)]⁻) afforded the chlorides (L^2,6‑ipr2_Ph)₂YCl (<b>1</b>) and (L^2,6‑ipr2_Ph)₂YbCl (<b>2</b>). Crystal structure analysis revealed <b>2</b> to be the unsolvated monomer. Treatment of the chlorides <b>1</b> and <b>2</b> with NaBH₄ in a 1/1 molar ratio in THF led to the preparation of the monoborohydrides (L^2,6‑ipr2_Ph)₂LnBH₄ (Ln = Y (<b>3</b>), Yb (<b>4</b>)) in good yields. Reaction of LnCl₃ (Ln = Y, Yb) with 2 equiv of NaL^2‑Me (L^2‑Me = [N­(2-MeC₆H₄)­C­(Me)]₂CH^–) in THF, followed by treatment with 1 equiv of NaBH₄, afforded the monoborohydrides (L^2‑Me)₂LnBH₄ (Ln = Y (<b>5</b>), Yb (<b>6</b>)). Complexes <b>3</b>–<b>6</b> were fully characterized, including X-ray crystal structure analyses. Complexes <b>3</b>–<b>6</b> 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 <b>3</b>–<b>6</b> 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 <b>6</b>. The <i>M</i>_n(obsd) values (<i>M</i>_n = the number-average molar mass) of the resulting PCL are in good agreement with <i>M</i>_n(calcd), with a ratio of monomer to <b>6</b> of up to 1000. The polymerization kinetics of l-LA in THF at 20 °C by complex <b>6</b> displays a first-order dependence on the monomer concentration. Notably, the binary <b>6</b>/^<i>i</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>i</i>PrOH per metal initiator. All the obtained PLAs showed monomodal, narrow distributions (<i>M</i>_w/<i>M</i>_n = 1.06–1.11), with the <i>M</i>_n values decreasing proportionally with an increasing amount of ^<i>i</i>PrOH. Complex <b>4</b> can also initiate the polymerization of methyl methacrylate (MMA) at −40 °C with high activity, affording the PMMA with 83.3% syndiotacticity