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-C₆H₃<i>i</i>Pr₂)­NC­(Me)­CHC­(Me)­N­(2,6-C₆H₃<i>i</i>Pr₂); 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 (<i>M</i>_n,NMR < 92 700 g mol^–1, <i><i>Đ</i></i>_M = 1.51) and α-hydroxy,ω-formate telechelic poly­(trimethylene carbonate)­s (<i>M</i>_n,NMR < 16 000 g mol^–1, <i><i>Đ</i></i>_M = 1.59) were thus synthesized under mild operating conditions