A series
of rare-earth (RE) metal complexes (Y, Sm, Nd, and La)
stabilized by polydentate N-methylethylenediamine-bridged
tris(phenolato) ligands was synthesized and characterized. Lanthanum
complexes showed good activity in catalyzing the cycloaddition reaction
of terminal epoxides with CO2 under ambient conditions
(i.e., room temperature, 1 bar CO2), giving rise to cyclic
carbonates in 49–99% yields. More importantly, generally challenging
internal epoxides were also transformed into cyclic carbonates in
70–99% yields in the presence of 1 bar CO2 at 60
°C. This is the first RE-based catalyst for efficient cycloaddition
of CO2 and epoxides under ambient conditions and is among
the most active catalysts for this important transformation. The lanthanum
complex was also recycled six times. Kinetic study of the cycloaddition
of cyclohexene oxide and CO2 was conducted, and the kinetic
equation was determined as follows: rate = k[epoxide]1[CO2]0[cat.]1.26[TBAI]0.4. The Gibbs activation energy (333 K) was determined to
be 29.8 kcal/mol.
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