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

The synthesis of a series of divalent and trivalent beta-diketiminate borohydrides [(dipp)2NacNacLn(BH4)(THF)(2)] ((dipp)(2)NacNac = (2,6-C(6)H(3)iPr(2))NC(Me)CHC(Me)N(2,6-C6H(3)iPr(2)); 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)(2)NacNacK] 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 ?kappa(2)(H) mode to the metal. Only in the lutetium complex [(dipp)2NacNacLu(BH4)(2)(THF)] does one BH4 group coordinate in a kappa(3)(H) mode, whereas the other one coordinates as kappa(2)/(H). This kind of mixed kappa(2)/kappa(3)(H) coordination mode is rare. The application of the divalent and trivalent compounds as initiators in the ring-opening polymerization (ROP) of epsilon-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(?-caprolactone) diols (Mn,NMR < 92 700 g mol(1), D-M = 1.51) and alpha-hydroxy,omega-formate telechelic poly(trimethylene carbonate)s (Mn,NMR < 16 000 g mol(1), D-M = 1.59) were thus synthesized under mild operating conditions.