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

The synthesis of 2-pyridinemethanamido borohydride complexes of yttrium and neodymium was achieved through the deprotonation of the protio-ligand 2-pyridinemethanamine CHRN-C(CH)R-NH(2,6-PrCH), denoted as PyAH (with PyAH1: R = R = H; PyAH2: R = CH, R = H; PyAH3: R = C(CH)N-(2,6-PrCH), R = CH), in the presence of trisborohydride RE(BH)(THF) (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(BH)][Mg(THF)] (1Y), [(PyA1)Nd(BH)Mg(PyA1)](THF) (1Nd), (PyA1)Nd(BH)(THF) (1'Nd), [(PyA1)Nd(THF)(BH)(μ-BH)] (1''Nd), [(PyA2)Nd(BH)][Mg(THF)] (3Nd), (PyA2)Nd(BH)(THF) (3'Nd) and (PyA3)Nd(BH) (4Nd), as well as two bis-substituted complexes (PyA1)Y(BH) (2Y) and (PyA1)Nd(BH) (2Nd). On the other hand, the unexpected amido/ene-amido derivative [(PyA(EA))Y(BH)][Li(THF)] (5Y) (PyA(EA): R = CCH-N(2,6-PrCH), R = CH), where the PyAH3 protio-ligand underwent double deprotonation, was recovered from the reaction carried out with BuLi in the yttrium series. In some cases, the synthesis led to the isolation of borohydride 2-pyridinemethanamido-supported magnesium complexes (PyA2)Mg(BH)(THF) and (PyA3)Mg(BH)(THF). In parallel, the PyAH2 pro-ligand could be structurally analyzed, and an unprecedented adduct of the type [KN(SiMe3)2·PyAH1] 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.