Achiral Lanthanide Alkyl Complexes Bearing N,O Multidentate Ligands. Synthesis and Catalysis of Highly Heteroselective Ring-Opening Polymerization of rac-Lactide

Abstract

Alkane elimination reactions of amino-amino-bis(phenols) H 2 L 1-4 , Salan H 2 L 5 , and methoxy--diimines HL 6,7 with lanthanide tris(alkyl)s, Ln(CH 2 SiMe 3 ) 3 (THF) 2 (Ln ) Y, Lu), respectively, afforded a series of lanthanide alkyl complexes 1-8 with the release of tetramethylsilane. Complexes 1-6 are THFsolvated mono(alkyl)s stabilized by O,N,N,O-tetradentate ligands. Complexes 1-3 and 5 adopt twisted octahedral geometry, whereas 4 contains a tetragonal bipyramidal core. Bearing a monoanionic moiety L 6 (L 7 ), complex 7 (8) is a THF-free bis(alkyl). In complex 7, the O,N,N-tridentate ligand combined with two alkyl species forms a tetrahedral coordination core. Complexes 1, 2, and 3 displayed modest activity but high stereoselectivity for the polymerization of rac-lactide to give heterotactic polylactide with the racemic enchainment of monomer units P r ranging from 0.95 to 0.99, the highest value reached to date. Complex 5 exhibited almost the same level of activity albeit with relatively low selectivity. In contrast, dramatic decreases in activity and stereoselectivity were found for complex 4. The Salan yttrium alkyl complex 6 was active but nonselective. Bis(alkyl) complexes 7 and 8 were more active than 1-3 toward polymerization of rac-LA, however, to afford atactic polylactides due to di-active sites. The ligand framework, especially the "bridge" between the two nitrogen atoms, played a significant role in governing the selectivity of the corresponding complexes via changing the geometry of the metal center. The NMR spectrum of the active species of the rac-lactide oligomer attached to complex 1 demonstrated a coordination-insertion mechanism. In addition it also confirmed that the geometry of the metal center of complex 1 in the solid state was retained in solution (THF) during the polymerization, which contributed significantly to the high selectivity of the complex.

Exploring Electronic versus Steric Effects in Stereoselective Ring‐Opening Polymerization of Lactide and β‐Butyrolactone with Amino‐alkoxy‐bis(phenolate)–Yttrium Complexes

“…[10] The solution structure of [Y(ONOO )- 6 ]benzene at different temperatures showed many resonances and proved uninformative due to their complexity caused by severe broadening and overlapping. We assume that this may reflect the coexistence of mononuclear and dinuclear (and possibly higher aggregated) species in solution; [11] however, in the context of the present study directed toward stereoselectivity of ROP reactions, this aspect was not investigated further.…”

“…[4] Amido or alkoxide complexes of this class proved to be particularly active and productive for the controlled ROP of racemic lactide (rac-LA) [5,6] and racemic bbutyrolactone (rac-BBL), [7] which afforded highly heterotactic polylactides (PLA) and highly syndiotactic poly(3-hydroxybutyrate)s (PHB), respectively (Scheme 1). Detailed microstructural NMR spectroscopy investigations [5b, 7b] revealed that those reactions proceed by means of a "chainend control mechanism" (CEM), in which the stereogenic center of the last inserted monomer in the growing polymer chain biases the catalyst to enchain a monomer of opposite stereochemistry.…”

Yttrium‐Amidopyridinate Complexes: Synthesis and Characterization of Yttrium‐Alkyl and Yttrium‐Hydrido Derivatives

“…The Y-amidopyridinate fragment is planar (average deviation from the plane is 0.0156 Å) and the Y-C(alkyl) bond length [2.455(2) Å] is comparable to the values reported for related six-coordinate yttrium-monoalkyl compounds. [18,20] The Y- (4) 3.6917 (14) 3.53(11) C (28)…”

Neodymium borohydride complexes supported by diamino-bis(phenoxide) ligands: diversity of synthetic and structural chemistry, and catalytic activity in ring-opening polymerization of cyclic esters

“…9 These ligands have been successfully employed in organolanthanide chemistry to synthesize diverse types of complexes. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] In particular, alkoxy, amido and alkyl lanthanide species supported by tri-and tetradentate bis(phenoxide) ligands proved to be efficient initiators/catalysts for ROP of cyclic esters.…”