Ring-opening polymerization of rac-lactide by bis(phenolate)amine-supported samarium borohydride complexes: An experimental and DFT study

The synthesis and ring-opening polymerization (ROP) capability of bis(phenolate)amine-supported samarium borohydride and amide complexes are reported, together with a DFT study. Reaction of Na2O2NL (L = OMe, NMe2, py, or Pr) with Sm(BH4)3(THF)3 gave the borohydride complexes Sm(O2NL)(BH4)(THF) (L = OMe (2), NMe2 (3), or py (4)) or Sm(O2NPr)(BH4)(THF)2 (5). Compounds 4 and 5 lost THF in vacuo, forming phenolate O-bridged dimers 1 and 6, respectively. Reaction of H2O2NL with Sm{N(SiHMe2)2}3(THF)2 formed monomeric Sm(O2NL){N(SiHMe2)2}(THF) (L = OMe (7), NMe2 (8), or py (9)) with tetradentate O2NL ligands, but dimeric Sm2(µ-O2NPr)2(O2NPr)(THF) (10) with tridentate O2NPr. Reaction of Sm{N(SiMe3)2}3 with H2O2NL (L = OMe or NMe2) led to zwitterionic products Sm(O2NL)(HO2NL). The bulkier amide compounds Sm(O2NL){N(SiMe3)2}(OEt2)n (n = 1, L = OMe (12) or py (13); n = 0, L = NMe2 (14)) were prepared by reaction of Sm(O2NL)(BH4)(THF) with KN(SiMe3)2. The X-ray structures of 2, 5, 6, 7, 10, 13, and 14 were determined. The borohydrides 2-5 were very efficient initiators for the ROP of e-CL, giving linear dihydroxytelechelic poly(e-CL). Selected amide initiators were also assessed but gave poorer control, as judged by broad PDI (Mw/Mn) values and significant amounts of cyclic poly(e-CL)s. Of the borohydrides, only 2-4 were active for the ROP of rac-LA, and activity increased in the order O2NL = O2NOMe ˜ O2Npy <O2NNMe2. The latter ligand also gave the best control of the ROP, as judged by the PDIs and Mn values. All gave heterotactically enriched poly(rac-LA) with Pr values in the range 0.82-0.84. The ROP of rac-LA with the amides 7, 9, and 12 was faster but much less well controlled. Overall, the borohydride initiators were superior for the ROP of both e-CL and rac-LA when compared to otherwise identical amide initiators. MALDI-ToF MS analysis of the poly(rac-LA) formed with 3 showed both -CH(Me)CHO and -CH(Me)CH2OH end groups originating from the insertion of the first LA monomer into the Sm-BH4 moiety of 3. In contrast, 2 and 4 formed only a,¿-dihydroxy-terminated polyesters with -CH(Me)CH2OH and -CH(Me)OH end groups. DFT calculations on Eu(O2'NNMe2)(BH4) found two mechanisms for the initial ring-opening step of LA by the borohydride group, giving pathways leading to either aldehyde- or alcohol-terminated poly(lactide)s. Of these two pathways, the one giving a,¿-dihydroxy-terminated polymers was the most favored, in agreement with experiment. (Ligand abbreviations: O2NL = RCH2N(CH2-2-O-3,5-C6H2tBu2)2 where R = CH2OMe, CH2NMe2, py, or Et for L = OMe, NMe2, py, or Pr, respectively; O2'NNMe2 = Me2NCH2CH2N(CH2-2-O-C6H4)2.