Oxo-Bridged Bimetallic Group 4 Complexes Bearing Amine-Bis(benzotriazole phenolate) Derivatives as Bifunctional Catalysts for Ring-Opening Polymerization of Lactide and Copolymerization of Carbon Dioxide with Cyclohexene Oxide
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We report the synthesis, crystal structure, and catalytic studies for ring-opening polymerization (ROP) of group 4 metal alkoxides based on amine-BiBTP derivatives (amine-BiBTP = amine-bis(benzotriazole phenolate)). Dinuclear group 4 metal alkoxides [{(amine-BiBTP)Mt(OiPr)}2(μ-O)] (4–7, Mt = Ti, Zr, Hf) resulted from treatment of amine-BiBTP-H2 as the ligand precursor with 1.0 molar equiv of metal precursor (Ti(OiPr)4, Zr(OiPr)4(iPrOH), or Hf(OiPr)4(iPrOH)), followed by the addition of H2O (0.5 equiv) in good yields. The solid-state structure of 4–7 reveals a bimetallic BiBTP-ligated metal(IV) alkoxide with an oxo ligand chelating two metal atoms, and the bonding mode of the metal-O(μ-oxo)-metal moiety assumes a linear type. Catalysis of lactide polymerization and carbon dioxide/cyclohexene oxide copolymerization of oxo-bridged bimetallic metal complexes was systematically examined. Zirconium alkoxide 6 shows an effective catalyst in the ROP of lactide with “living” and “immortal” fashions, yielding poly(lactide)s with the predicted molecular weights and narrow polydispersity indices (PDIs <1.25) as well as an isotactic-enriched predominance (Pm up to 0.71). Bimetallic BiBTP-featuring Zr complex 6 was also able to couple carbon dioxide and cyclohexene oxide to give poly(cyclohexene carbonate-co-cyclohexene oxide), having a high copolymer selectivity and good carbonate-linkage contents under the optimal conditions. This is a successful example of catalysis for both processes using one well-defined zirconium(IV) alkoxide complex as a bifunctional catalyst.
