Bis(guanidinate) Alkoxide Complexes of Lanthanides: Synthesis, Structures and Use in Immortal and Stereoselective Ring-Opening Polymerization of Cyclic Esters

• Published in: Chemistry : a European journal 2008-06, Vol.14 (18), p.5440-5448
• Main Authors: Ajellal, Noureddine, Lyubov, Dmitrii M., Sinenkov, Mikhail A., Fukin, Georgii K., Cherkasov, Anton V., Thomas, Christophe M., Carpentier, Jean-François, Trifonov, Alexander A.
• Format: Article
• Language: English
• Subjects: Catalysis; Chemical Sciences; guanidinate ligands; lactides; lanthanides; polymerization; ring-opening polymerization
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200800288

A series of new bis(guanidinate) alkoxide Group 3 metal complexes [Ln{(Me3Si)2NC(NiPr)2}2(OR)] (R=OtBu, Ln=Y, Nd, Sm, Lu; R=OiPr, Ln=Y, Nd, Lu) has been synthesized. X‐ray structural determinations revealed that bis(guanidinate) tert‐butoxides are monomeric complexes. The isopropoxide complex [Y{(Me3Si)2NC(NiPr)2}2(OiPr)] undergoes slow decomposition in solution, to afford the unusual dimeric amido complex [(Y{(Me3Si)2NC(NiPr)2}2{μ‐N(iPr)CN})2]. Complexes [Ln{(Me3Si)2NC(NiPr)2}2(OR)] (R=OtBu, Ln=Y, Nd, Sm, Lu; R=OiPr, Ln=Y, Nd, Lu) are active catalysts/initiators for the ROP of rac‐lactide and rac‐β‐butyrolactone under mild conditions. Most of those polymerizations proceed with a significant degree of control. Bis(guanidinate) alkoxides appear to be well suited for achieving immortal polymerization of lactide, through the introduction of large amounts of isopropanol as a chain‐transfer agent. The synthesized complexes are able to promote the stereoselective ROP of rac‐β‐butyrolactone to afford syndiotactic poly(hydrobutyrate) through a chain‐end control mechanism, while they are surprisingly non‐stereoselective for the ROP of lactide under strictly similar conditions. Chain‐end control: Bis(guanidinate) alkoxide Group 3 metal complexes (OR=OtBu, OiPr; Ln=Y, Nd, Sm, Lu) promote the stereoselective ring‐opening polymerization (ROP) of rac‐β‐butyrolactone to form syndiotactic poly(hydroxybutyrate) through a chain‐end control mechanism (see scheme), while these complexes are essentially non‐stereoselective for the ROP of lactide (PLA=polylactide, PHB=poly(hydroxy‐3‐butyrate)).