Iron(II) Bis-CNN Pincer Complex-Catalyzed Cyclic Carbonate Synthesis at Room Temperature

A series
of unsymmetrical pyridine-bridged bis-pincer-type iron­(II)
complexes were synthesized, and the complexes were used as catalysts
for the cycloaddition of CO2 and epoxides. At ambient temperature,
the combined use of iron complexes and tetrabutyl ammonium bromide
(TBAB) resulted in an efficient catalytic system for the synthesis
of cyclic carbonates under low CO2 pressure (0.5 MPa) and
solvent-free conditions. More importantly, in the absence of TBAB,
propylene oxide was also easily converted to the target products when
the temperature was increased to 100 °C, and a turnover frequency
(TOF) value of 7900 h–1 was achieved at 120 °C.
An elevated reaction temperature (80 °C) was required to achieve
high conversion of challenging internal epoxides and oxetanes to their
respective carbonates with good yields. Moreover, the iron catalyst
can be easily recycled six times via simple filtration without any significant loss of activity.