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 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.