An exploration of the coupling reactions of epoxides and carbon dioxide catalyzed by tetramethyltetraazaannulene chromium(III) derivatives: formation of copolymers versus cyclic carbonates.
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abstract
The high catalytic activity of a tetramethyltetraazaannulene (tmtaa) chromium complex toward the copolymerization of cyclohexene oxide and carbon dioxide to discriminatively provide poly(cyclohexylene carbonate) has directed further studies into the capabilities of the catalyst system. Various [PPN]X (PPN(+) = bis(triphenylphosphoranylidene)ammonium) cocatalysts, where X = Cl, N(3), Br, CN, and OBzF(5), in the presence of (tmtaa)CrCl were examined for catalytic reactivity and selectivity for polycarbonate formation, achieving turnover frequencies of 1500 h(-1) at 80 degrees C in the case of PPNCl. The catalyst system was examined under varied pressures and found to be active even at 1 bar of CO(2) pressure. In addition to cyclohexene oxide, the (tmtaa)CrCl complex was investigated for catalytic activity toward the coupling of carbon dioxide with propylene oxide, isobutylene oxide, 1,2-epoxyhexane, styrene oxide, and 4-vinyl cyclohexene oxide. Activation energies were found for the copolymerization reaction between cyclohexene oxide and carbon dioxide utilizing the tetramethyltetraazaannulene catalyst system to be 67.1 +/- 4.2 kJ.mol(-1) and 65.2 +/- 2.5 kJ.mol(-1) in neat epoxide and with methylene chloride cosolvent, respectively, upon monitoring these processes by in situ infrared spectroscopy. Supplementary to the studies involving (tmtaa)CrCl, electronic effects at the metal center on catalytic activity were examined through derivatization of the tmtaa ligand, resulting in increased activity as electron-donating substituents were added.
