ansa-zirconocene ester enolates:: Synthesis, structure, reaction with organo-Lewis acids, and application to polymerization of methacrylates

The synthesis, structural characterization, and abstraction chemistry of ansa-zirconocene ester enolate complexes relevant to the isospecific polymerization of methacrylates are reported. Reactions of rac-(EBI)ZrMe(OTf) and rac-(EBI)Zr(OTf)(2) [EBI = C2H4(Ind)(2)] with 1 and 2 equiv of lithium isopropylisobutyrate in toluene produce the first examples of ansa-zirconocene mono- and diester enolate complexes: rac-(EBI)ZrMe[OC((OPr)-Pr-i)=CMe2] (1) and rac-(EBI)Zr[OC(OiPr)=CMe2](2) (2) in 89% and 50% isolated yields, respectively. The reaction of 1 with B(C6F5)(3) Was investigated in six different organic solvents; in THF at ambient temperature, this reaction cleanly produces the isolable cationic ansa-zirconocene ester enolate complex rac-(EBI)Zr+(THF)[OC((OPr)-Pr-i)=CMe2][MeB(C6F5)(3)](-) (3) in quantitative yield. The analogous reaction of 1 with Al(C6F5)(3) in toluene, however, proceeds through a proposed novel, intramolecular proton transfer process in which propylene is eliminated from the isopropoxy group, subsequently producing a carboxylate-bridged tight ion pair rac-(EBI)Zr+(Me)OC(Pr-i)OAl(C6F5)(3-) (4). In addition to standard spectroscopic and analytical characterizations for the isolated complexes 1-4, complexes 2 and 4 have also been structurally characterized by X-ray diffraction studies. Polymerization of methyl methacrylate (MMA) and n-butyl methacrylate (BMA) has been investigated using complexes 1, 3, and 4. Both the isolated cationic 3 and neutral 1 (the latter combined with B(C6F5)(3) in situ) are highly active (10 min for quantitative MMA conversion) and highly isospecific ([mm] >95% for PMMA; [mm] >99% for PBMA) via enantiomorphic-site control, producing polymethacrylates with extremely narrow molecular weight distributions (M-w/M-n = 1.03). The aluminate complex 4, however, produces syndiotactic PMMA predominantly via chain-end control.