Group 3 metal complexes supported by tridentate pyridine- and thiophene-linked bis(naphtholate) ligands: synthesis, structure, and use in stereoselective ring-opening polymerization of racemic lactide and β-butyrolactone

Abstract

Scandium, yttrium and lanthanum amido complexes supported by tridentate bis(ortho-silyl-substituted naphtholate)-donor ligands ({ONO(SiPh3)}(2-) and {ONO(SitBuMe2)}(2-), donor = 2,6-pyridine; {OSO(SiPh3)}(2-), donor = 2,5-thiophene) have been prepared in high yields (72-96%) by reaction of the corresponding pro-ligand {OZO(SiR3)}H(2) and Ln[N(SiHMe(2))(2)](3)(THF)(n) precursor. The solid-state structures of {ONO(SiPh3)}La[N(SiHMe(2))(2)](THF) (3), {ONO(SitBuMe2)}Ln[N(SiHMe(2))(2)](THF) (Ln = Sc, 4; Y, 5) and {OSO(SiPh3)}Ln[N(SiHMe(2))(2)](THF) (Ln = Sc, 7; La, 9) have been determined by single-crystal X-ray diffraction studies. In all five complexes, the naphtholate rings twist in the same direction from the plane of the pyridine or thiophene linker, to give rise to C(s)-symmetric (non crystallographic) structures. Compounds are 1-9 single-site initiators for the ring-opening polymerization (ROP) of racemic lactide (rac-LA) at 20 degrees C, affording poly(lactides)s (PLAs) with relatively narrow polydispersities and molecular weights in good agreement with calculated values. When carried out in THF, the polymerizations afforded heterotactic-enriched PLAs (P(r) up to 0.93), while atactic polymers are formed in toluene. Compounds 1-3 and 7-9, having o-SiPh(3) substituents on the naphtholate rings, are also active for the ROP of racemic beta-butyrolactone at 20-50 degrees C, to form syndiotactic-enriched poly(3-hydroxybutyrate)s (PHBs) (P(r) up to 0.87) when using toluene as the solvent, whereas atactic polymers were obtained in THF. The nature of the metal center (Sc, Y, La), the central linker in the ligand framework (pyridine, thiophene), and the ortho-silyl substituent (SiPh(3), SiMe(2)tBu) significantly affect the degree of stereocontrol in those polymerizations.

Discrete Metal Catalysts for Stereoselective Ring‐Opening Polymerization of Chiral Racemic β‐Lactones

“…We thus recently prepared Group 3 metal complexes based on sterically demanding silyl ortho-substituted tridentate 2, 6- [30] Syndiotactic-enriched PHBs, with P r up to 0.87, were formed when using toluene as the solvent, whereas atactic polymers were obtained in THF; the same solventdependence of stereoselectivity was observed in ROP of rac-BL with amino-alkoxy-bis(phenolate) yttrium systems (vide supra).…”

Variation of structural motifs in lanthanoid hydroxo clusters by ligand modification

“…The interchangeable nature of the ligand was confirmed when the tetranuclear and pentanuclear [Ln 4 (OH) 2 (7) 10 ] (Ln = Nd, Eu) and [Er 5 (OH) 5 (7) 10 ] motifs were isolated, of which both were structurally similar to the motifs shown inFigures 3 and 5 46. Only a single crystallographically characterised example of a lanthanoid-thiophene interaction has been isolated to date which required tethering of the thiophene group at a capping position by rigid naphthalenol arms to effect, hence this result was not surprising 47.…”

Rare Earth Pincer Complexes: Synthesis, Reaction Chemistry, and Catalysis

“…Relatively low PDI values indicated a good level of polymerization control. Finally, high stereoselectivities were observed in all cases, especially with 123-Y and 123-La [113].…”

“…Conversely, poor solubility of lactide in noncoordinating solvents can decrease control over the polymerization, producing broader molecular weight distribution polymers (vide supra). [113] (continued)…”

“…Notably, the trend was not uniform as it was inversed with complexes 123-Ln, limiting definitive conclusions regarding the factors affecting stereochemistry. In addition to polymerization of LA, complexes 123-Ln and 124-Ln were also active in the ROP of rac-β-butyrolactone (BBL) (vide infra) [113].…”