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 Na-2-O2NL (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 THE in vacua, 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 Sm-2(mu-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(epsilon-CL). Selected amide initiators were also assessed but gave poorer control, as judged by broad PDI (M-w/M-n) values and significant amounts of cyclic poly(epsilon-CL)s. Of the borohydrides, only 2-4 were active for the ROP of rac-LA, and activity increased in the order O2NL = O2NOMe approximate to O2Npy < O2NNme2. The latter ligand also gave the best control of the ROP, as judged by the PDIs and M-n, 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 alpha,omega-dihydroxy-terminated polyesters with -CH(Me)CH2OH and -CH(Me)OH end groups. DFT calculations on Eu(O-2'N-NMe2)(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 alpha,omega-dihydroxy-terminated polymers was the most favored, in agreement with experiment. (Ligand abbreviations: O2NL = RCH2N(CH2-2-O-3,5-(C6H2Bu2)-Bu-t)2 where R = CH2OMe, CH2NMe2, py, or Et for L = OMe, NMe2, py, or Pr, respectively; O-2'/N-NMe2 = Me-2-NCH2CH2N(CH2-2-O-C6H4)(2).)