Mixed Allyl Rare-Earth Borohydride Complexes: Synthesis, Structure, and Application in (Co-)Polymerization Catalysis of Cyclic Esters.

A series of new trivalent rare-earth allyl-borohydride complexes with the formula [RE(BH₄)₂(C₃H₅)(thf) _x] (RE=Sc ( 1), x=2; RE=Y ( 2) and La ( 3), x=3) were synthesized by reaction of the corresponding rare-earth trisborohydrides [RE(BH₄)₃(thf) _x] with half an equivalent of bis(allyl)magnesium. The complexes were fully characterized by determining their X-ray structure. Similar to their previously described Nd ( 4) and Sm ( 5) analogues, these complexes display a monomeric structure with two terminal trihapto BH₄ groups, one π-η³ allyl ligand, three THF molecules for complexes 2 and 3, and two THF molecules for complex 1. The catalytic behavior of complexes 1- 5 toward the ring-opening polymerization (ROP) of l-lactide ( l-LA) and ϵ-caprolactone (ϵ-CL) was assessed. The Nd complex featured the best activity toward l-LA (turnover frequency (TOF)=1300 h⁻¹) and the order was Nd>La>Sm>Y>Sc. Complexes 1- 3 were found very active for the ROP of ϵ-CL (TOF=166 000 h⁻¹), which is in line with the already established exceptionnally high performance of complexes 4 and 5. With both monomers, it was shown that the borohydride moiety was the preferentially initiating group, rather than the allyl one. The random copolymerization of l-LA and ϵ-CL was performed with complexes 1- 5, in the absence or in the presence of benzyl alcohol as a chain-transfer agent, affording copolymers with ϵ-caprolactone up to 62 % inserted. The copolymers synthesized display a variety of microstructures, that is, blocky, random, or quasi-alternating. [ABSTRACT FROM AUTHOR]