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 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] (1Y), [(PyA1)Nd(BH4)3Mg(PyA1)](THF)4 (1Nd), (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] (3Nd), (PyA2)Nd(BH4)2(THF)2 (3'Nd) and (PyA3)Nd(BH4)2 (4Nd), as well as two bis-substituted complexes (PyA1)2Y(BH4) (2Y) and (PyA1)2Nd(BH4) (2Nd). On the other hand, the unexpected amido/ene-amido derivative [(PyA(EA))Y(BH4)2][Li(THF)4] (5Y) (PyA(EA): R1 = CCH2-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.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

s-Block metal complexes of PC(H)P-bridged chalcogen-centred methanides: comparisons with isoelectronic PNP-bridged monoanions

This Perspective compares the chemistries of s-block metal complexes of isoelectronic PC(H)P- and PNP-bridged, chalcogen-centred monoanions with a focus on the unique behavior of the C(H)-bridged systems.