β‑Diketiminate Rare Earth Borohydride Complexes: Synthesis, Structure, and Catalytic Activity in the Ring-Opening Polymerization of ε‑Caprolactone and Trimethylene Carbonate

The synthesis of a series of divalent
and trivalent β-diketiminate
borohydrides [(dipp)2NacNacLn­(BH4)­(THF)2] ((dipp)2NacNac = (2,6-C6H3iPr2)­NC­(Me)­CHC­(Me)­N­(2,6-C6H3iPr2); Ln = Sm, Eu,
Yb) and [(dipp)2NacNac­Ln­(BH4)2­(THF)] (Ln = Sc, Sm, Dy, Yb, Lu) is reported. All compounds
were obtained by salt metathesis in THF from [(dipp)2NacNacK]
and the corresponding homoleptic divalent and trivalent borohydrides
[Ln­(BH4)2(THF)2] (Ln = Sm, Eu, Yb),
[Sc­(BH4)3(THF)2], and [Ln­(BH4)3(THF)3] (Ln = Sm, Dy, Yb, Lu), respectively.
The complexes were fully characterized, and their solid-state structures
were established by single-crystal X-ray diffraction. In both the
divalent and trivalent compounds, the BH4– groups coordinate in a κ3(H) mode to the metal.
Only in the lutetium complex [(dipp)2NacNacLu­(BH4)2(THF)] does one BH4– group
coordinate in a κ3(H) mode, whereas the other one
coordinates as κ2(H). This kind of mixed κ2/κ3(H) coordination mode is rare. The application
of the divalent and trivalent compounds as initiators in the ring-opening
polymerization (ROP) of ε-caprolactone (CL) and trimethylene
carbonate (TMC) was investigated. All complexes afforded a generally
well-controlled ROP of both of these cyclic esters. High molar mass
poly­(ε-caprolactone) diols (Mn,NMR < 92 700 g mol–1, ĐM = 1.51) and α-hydroxy,ω-formate
telechelic poly­(trimethylene carbonate)­s (Mn,NMR < 16 000 g mol–1, ĐM = 1.59) were thus synthesized under mild operating
conditions.