Synthesis and structural diversity of lanthanide amidate complexes and their catalytic activities for the ring-opening polymerization of rac-lactide

Anionic lanthanide amidate complexes, Li(THF)Ln[C6H5C(O)NC6H3(iPr)2]4(THF)n·(sol)m (Ln = La (1), Nd (2), Sm (3), Yb (4), Y (5), n = 0 or 1; sol = THF or n-Hex), were synthesized for the first time by direct protonolysis reaction of LiLn(NiPr2)4(THF) with benzamide proligand, C6H5CONHC6H3(iPr)2 (HL). It was confirmed by their characterization data that complexes 1–5 presented rich and varied structures with changeable coordination modes of the amidate ligands. On the other hand, similar reactions of HL with neutral homoleptic lanthanide amides, Ln[N(SiMe3)2]3, cleanly yielded the neutral lanthanide amidate complexes, {PhC(O)NC6H3(iPr)2}2Ln{OC(Ph)(C6H3(iPr)2)N–H–N(C6H3(iPr)2)C(Ph)O}(THF)2 (Ln = La (6), Nd (7), Sm (8)). Single-crystal X-ray structural determination revealed that complexes 6–8 featured an unprecedented N–H–N bond between two of the amidate ligands, making it the most notable difference from normal neutral analogs. Complexes 1–8 were characterized by elemental analysis and IR spectroscopy, with additional 1H NMR spectroscopy in the case of complexes 1, 5 and 6. Furthermore, both the anionic and neutral lanthanide amidate complexes proved to be efficient catalysts for the ring-opening polymerization of rac-lactide, affording heterotactic-rich polylactides. The anionic lanthanide amidate complexes exhibited significantly higher activities than their neutral counterparts due to the cooperative effect.