Zinc complexes of fluorous alkoxide-imino ligands: Synthesis, structure, and use in ring-opening polymerization of lactide and β-butyrolactone

The reaction of fluorous imino-alcohols {ONR1,R2} H (R-1 = Me, Ph; R-2 = Ph, CH2Ph, cyclohexyl, piperidinyl) with Zn(N(SiMe3)(2))(2) (1 or 0.5 equiv) systematically led to bis(ligand) complexes Zn{ONR1,R2}(2) (1-3), which were isolated in 84-88% yield. Imino-alcohols {ON(R1,R)2} H react selectively with ZnEt2 to give the corresponding complexes {ONR1,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 {ONPh,Bn} Zn(O/Pr) (8) in modest yield, due to rapid ligand redistribution. Complexes 1-8 were authenticated by elemental analysis, H-1, F-19 and C-13 NMR spectroscopy in solution, and single-crystal X-ray diffraction studies for 1, 3, 5, 7 and 8. In situ combinations of {ONPh,Bn} ZnEt (5)/BnOH and Zn{ONPh,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.