Controlled Syntheses, Characterization, and Reactivity of Neutral and Anionic Lanthanide Amides Supported by Methylene-Linked Bis(phenolate) Ligands

A series of neutral and anionic bis(phenolate) lanthanide amides were synthesized by general metathesis reactions,
and their reactivity was explored. Protolytic ligand exchange reactions of MBMPH2 (MBMP = 2,2‘-methylene bis(6-tert-butyl-4-methyl-phenolate)) with [Ln{N(TMS)2}2(μ-Cl)(THF)]2 (TMS = SiMe3) afforded the desired bridged
bis(phenolate) lanthanide chlorides [(MBMP)Ln(μ-Cl)(THF)2]2 [Ln = Nd (1), Yb (2)] in high isolated yields. These
lanthanide chlorides were found to be useful precursors for the synthesis of the corresponding lanthanide derivatives.
Reactions of 1 and 2 with 2 equiv of NaN(TMS)2 in THF produced the expected neutral bis(phenolate) lanthanide
amido complexes (MBMP)Ln[N(TMS)2](THF)2 [Ln = Nd (3), Yb (4)] in high yields. Whereas the reactions of 1 and
2 with LiN(TMS)2 in a 1:4 molar ratio gave the anionic bis(phenolate) lanthanide amides as discrete ion-pair complexes
[Li(THF)4][(MBMP)Ln{N(TMS)2}2] [Ln = Nd (5), Yb (6)] in high isolated yields. Further study revealed that 5 and
6 can also be conveniently synthesized in high yields by the direct reactions of MBMPH2 with [Ln{N(TMS)2}2(μ-Cl)(THF)]2 in a 2:1 molar ratio, and then with 4 equiv of nBuLi. The reactivity of the neutral and anionic bis(phenolate) lanthanide amides was comparatively investigated. It was found that the insertion reactions of carbodiimide
into the Ln−N bond of neutral lanthanide amido complexes 3 and 4 gave the anticipated bis(phenolate) lanthanide
guanidinate complexes [(μ-O−MBMP)Nd{(iPrN)2CN(TMS)2}]2 (7) and (MBMP)Yb[(iPrN)2CN(TMS)2] (8), respectively,
in high yields, whereas the similar reaction of carbodiimide with anionic amido complex 5 provided the unexpected
ligand-redistributed products, and the homoleptic ion-pair bis(phenolate) neodymium complex [Li(DME)2(THF)][(MBMP)2Nd(THF)2] (9) was finally isolated as one of the products. Furthermore, the anionic bis(phenolate) lanthanide
amides showed higher catalytic activity for the polymerization of ε-caprolactone than the neutral ones. All of the
complexes were characterized with elemental analysis and IR spectra, and the definitive molecular structures of
1−3 and 5−9 were provided by single-crystal X-ray analyses.