Di- and Tri-Zinc Catalysts for the Low-Pressure Copolymerization of CO₂ and Cyclohexene Oxide

The syntheses and characterization of three new macrocyclic proligands, with variation of the para aryl ring substituent, are reported. Dizinc and trizinc acetate complexes are prepared using these ligands and are characterized using infrared and nuclear magnetic resonance spectroscopies, mass spectrometry, elemental analysis, and, for the three trizinc complexes, single-crystal X-ray diffraction. The X-ray crystallographic and spectroscopic data indicate bridging and terminal acetate coordination modes, both in the solid state and in solution, for the trizinc complexes. All of the complexes show good turnover numbers and frequencies, under 1 atm of pressure of carbon dioxide, for the copolymerization of CO₂ and cyclohexene oxide to produce poly(cyclohexene carbonate). The electronic nature of the ancillary ligands’ substituents influences the catalytic activity of the complex, with the electron-donating substituent reducing the activity. The dizinc catalysts show markedly higher activities than the trizinc analogues, suggesting that the coordination environment within the macrocycle is crucial to controlling the catalytic activity.