Ytterbium amides of linked bis(amidinate): synthesis, molecular structures, and reactivity for the polymerization of l-lactide

Abstract

The steric effect of an amide group on the synthesis, molecular structures and reactivity of ytterbium amides supported by linked bis(amidinate) L (L = [Me₃SiNC(Ph)N(CH₂)₃NC(Ph)NSiMe₃]) is reported. Reaction of LYbCl(THF)₂ with equimolar NaNHAr′ and NaNHAr (Ar′ = 2,6-Me₂C₆H₃; Ar = 2,6-ⁱPr₂C₆H₃), respectively, gave the corresponding monometallic amide complexes LYb(NHAr′)(DME) 1 and LYb(NHAr)(DME) 2, in which the linked bis(amidinate) is coordinated to the metal center as a chelating ligand. The similar reaction with NaN(SiMe₃)₂ afforded a bimetallic amide complex (TMS)₂NYb(L)₂YbN(TMS)₂3 formed through the rearrangement reaction of L induced by the bulky N(SiMe₃)₂ group. In complex 3 the two linked bis(amidinate)s act as bridging ancillary ligands to link two YbN(TMS)₂ species in one molecule. The definite molecular structures of 1–3 were provided by single-crystal X-ray analysis. Complexes 1–3 are efficient initiators for the polymerization of L-lactide, and their catalytic performance is highly dependent on the amido groups and molecular structures. The polymerizations initiated by complexes 1 and 2 proceeded in a living fashion as evidenced by the narrow polydispersities of the resulting polymers, together with the linear natures of the number average molecular weight versus conversion plots, while the polymerization system with complex 3 provided polymers with rather broad molecular weight distributions.