(Iminophosphoranyl)(thiophosphoranyl)methane Rare-Earth Borohydride Complexes: Synthesis, Structures and Polymerization Catalysis

The (iminophosphoranyl)(thiophosphoranyl)methanide {CH(PPh₂=NSiMe₃)(PPh₂=S)}- ligand has been used for the synthesis of divalent and trivalent rare-earth borohydride complexes. The salt metathesis of the potassium reagent [K{CH(PPh₂=NSiMe₃)(PPh₂=S)}]₂ with [Yb(BH₄)₂(THF)₂] resulted in the divalent monoborohydride ytterbium complex [{CH(PPh₂=NSiMe₃)-(PPh₂=S)}Yb(BH₄)(THF)₂]. The 2D ³¹P/¹⁷¹Yb HMQC-NMR spectrum clearly showed the coupling between both nuclei. The trivalent bisborohydrides [{CH(PPh₂=NSiMe₃)-(PPh₂=S)}Ln(BH₄)₂(THF)] (Ln = Y, Sm, Tb, Dy, Er, Yb and Lu) were obtained by reaction of [K{CH(PPh₂=NSiMe₃)(PPh₂=S)}]₂ with [Ln(BH₄)₃(THF)₃]. All new compounds were characterized by single X-ray diffraction. The divalent and trivalent compounds were next used as initiators in the ring-opening polymerization (ROP) of ε-caprolactone (CL) and trimethylene carbonate (TMC). All complexes afforded a generally well-controlled ROP of both of these cyclic esters. High molar mass poly(ε-caprolactone) diols (M_n,NMR < 101 300 g/mol, ÐM = 1.44) and α-hydroxy,ω-formate telechelic poly(trimethylene carbonate)s (M_n,NMR < 20 000 g/mol, ÐM = 1.61) were thus synthesized under mild operating conditions.