Zinc complexes of fluorous alkoxide-imino ligands: synthesis, structure, and use in ring-opening polymerization of lactide and beta-butyrolactone.
The reaction of fluorous imino-alcohols {ON(R1,R2)}H (R(1) = Me, Ph; R(2) = Ph, CH(2)Ph, cyclohexyl, piperidinyl) with Zn(N(SiMe(3))(2))(2) (1 or 0.5 equiv) systematically led to bis(ligand) complexes Zn{ON(R1,R2)}(2) (1-9), which were isolated in 84-88% yield. Imino-alcohols {ON(R1,R2)}H react selectively with ZnEt(2) to give the corresponding complexes {ON(R1,R2)}ZnEt (4-7), in 74-78% yield. The stability of complexes 4-7 appears to be controlled by the nature of the substituent at the imine N atom: a phenyl group leads to slow ligand redistribution while alkyl-type substituents (benzyl, cyclohexyl, piperidinyl) prevent this disproportionation. Treatment of 5 with isopropanol afforded {ON(Ph,Bn)}Zn(OiPr) (8) in modest yield, due to rapid ligand redistribution. Complexes 1-8 were authenticated by elemental analysis, (1)H, (19)F and (13)C NMR spectroscopy in solution, and single-crystal X-ray diffraction studies for 1, 3, 5, 7 and 8. In situ combinations of {ON(Ph,Bn)}ZnEt (5)/BnOH and Zn{ON(Ph,Bn)}(2) (2)/BnOH are active systems for the ROP of racemic lactide and beta-butyrolactone at 20-50 degrees C, yielding atactic polylactide and poly(3-hydroxybutyrate) with good molecular weight control (M(n) up to 20,700 g/mol) and relatively narrow molecular weight distributions (M(w)/M(n) = 1.06-1.57). These binary systems also allow the immortal ROP of lactide and beta-butyrolactone, using excess alcohol (up to 5 equiv) vs the Zn catalyst.
Grunova
Ekaterina
E
Catalysis and Organometallics, UMR 6226 Sciences Chimiques de Rennes, CNRS-University of Rennes 1, 35042, Rennes Cedex, France.
Roisnel
Thierry
T
Carpentier
Jean-François
JF