Heteroscorpionate Rare-Earth Catalysts for the Low-Pressure Coupling Reaction of CO2 and Cyclohexene Oxide

Novel heteroscorpionate rare-earth-metal complexes (Y, La, and Lu) have been developed for the coupling reaction of CO2 and cyclohexene oxide under mild conditions. The reaction of an alcohol heteroscorpionate ligand precursor (bpzbdmapeH) with 1 equiv of the tris(silylamide) and tris(alkyl) precursors [M{N(SiHMe2)(2)}(3)(thf)(2)] (M = Y, La, Lu) and [M(CH2SiMe3)(3)(thf)(2)] ( M = Y, Lu) proceed to give good yields of the neutral heteroscorpionate mononuclear bis(dimethylsilyl)amide complexes [M{N(SiHMe2}(2)(kappa(3)-bpzbdmape)(thf)] (M = Y (1), La (2), Lu (3)) and the alkyl complexes [M(CH2SiMe3)(2)(kappa(3)-bpzbdmape)(thf)] (M = Y (4), Lu (5)). These compounds were isolated with a distorted-octahedral geometry due to the coordination of a THF molecule. The molecular structures of these compounds were determined by spectroscopic methods, and the X-ray crystal structure of 1 was also established. Complexes 1-5 were efficient catalysts for the coupling reaction of CO2 and cyclohexene oxide, and complex 4 was found to be the most active and selective catalyst to produce poly(cyclohexene carbonate) in the absence of a cocatalyst. Furthermore, in combination with the appropriate amounts of cocatalytic additives, complex 4 allowed the highly selective synthesis of cyclic carbonate or polycarbonate at only 1 bar of CO2 pressure. Under these conditions, poly(cyclohexene carbonate) was obtained when PPNCl was used as the nucleophilic cocatalyst, while the use of Bu4NBr led to the selective production of cis-cyclohexene carbonate.