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

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.