Synthesis and characterization of borohydride rare-earth complexes supported by 2-pyridinemethanamido ligands and their application in the ring-opening polymerization of cyclic esters

Abstract

The synthesis of 2-pyridinemethanamido borohydride complexes of yttrium and neodymium was achieved through the in situ deprotonation of the protio-ligand 2-pyridinemethanamine C₅H₃R₁N-C(CH₃)R₂-NH(2,6-ⁱPr₂C₆H₃), denoted as PyAH (with PyAH¹: R₁ = R₂ = H; PyAH²: R₁ = CH₃, R₂ = H; PyAH³: R₁ = C(CH₃)N-(2,6-ⁱPr₂C₆H₃), R₂ = CH₃), in the presence of trisborohydride RE(BH₄)₃(THF)₃ (RE = Y and Nd) as a precursor and a base. The isolation of various molecular structures, nine of which were structurally characterized by X-ray diffraction analysis, was achieved and revealed to depend not only on (i) the nature of the 2-pyridinemethanamido ligand and (ii) the rare-earth element but also on (iii) the reaction conditions, notably the type of base used. These include seven mono-substituted species, eventually also comprising the cation derived from the base reagent, such as [(PyA¹)Y(BH₄)₃]₂[Mg(THF)₆] (1_Y), [(PyA¹)Nd(BH₄)₃Mg(PyA¹)](THF)₄ (1_Nd), (PyA¹)Nd(BH₄)₂(THF)₂ (1′_Nd), [(PyA¹)Nd(THF)(BH₄)(μ-BH₄)]₂ (1′′_Nd), [(PyA²)Nd(BH₄)₃]₂[Mg(THF)₆] (3_Nd), (PyA²)Nd(BH₄)₂(THF)₂ (3′_Nd) and (PyA³)Nd(BH₄)₂ (4_Nd), as well as two bis-substituted complexes (PyA¹)₂Y(BH₄) (2_Y) and (PyA¹)₂Nd(BH₄) (2_Nd). On the other hand, the unexpected amido/ene-amido derivative [(PyA(EA))Y(BH₄)₂][Li(THF)₄] (5_Y) (PyA(EA): R₁ = CCH₂-N(2,6-ⁱPr₂C₆H₃), R₂ = CH₃), where the PyAH³ protio-ligand underwent double deprotonation, was recovered from the reaction carried out with ⁿBuLi in the yttrium series. In some cases, the synthesis led to the isolation of borohydride 2-pyridinemethanamido-supported magnesium complexes (PyA²)Mg(BH₄)(THF) and (PyA³)Mg(BH₄)(THF). In parallel, the PyAH² pro-ligand could be structurally analyzed, and an unprecedented adduct of the type [KN(SiMe₃)₂·PyAH¹]₂ was isolated and characterized by X-ray diffraction analysis. Preliminary investigations of the ring-opening polymerization of L-lactide and ε-caprolactone with some of the complexes synthesized are finally presented, demonstrating moderate to high catalytic activities.